energies
Article
Investigation on the Thermal Condition
of a Traditional Cold-Lane in Summer
in Subtropical Humid Climate Region of China
Hui Chen 1,2 , Yin Wei 1,3, * , Yaolin Lin 4, *, Wei Yang 5, * , Xiaoming Chen 1 ,
Maria Kolokotroni 6 , Xiaohong Liu 1 and Guoqiang Zhang 1,2
1
2
3
4
5
6
*
College of Architecture, Hunan University, Changsha 410082, China; chenhui2012@hnu.edu.cn (H.C.);
jjchenxiaoming@163.com (X.C.); widerose@126.com (X.L.); gqzhang@hnu.edu.cn (G.Z.)
National Center for International Research Collaboration in Building Safety and Environment,
College of Civil Engineering, Hunan University, Changsha 410082, China
School of Civil Engineering, Hunan University of Science and Technology, Xiangtan 411201, China
School of Environment and Architecture, University of Shanghai for Science and
Technology, Shanghai 200093, China
Faculty of Architecture, Building and Planning, The University of Melbourne, Melbourne 3010, Australia
Mechanical and Aerospace Engineering Department, Brunel University London, Brunel UB8 3PH, UK;
maria.kolokotroni@brunel.ac.uk
Correspondence: yinwei@hnust.edu.cn (Y.W.); ylin@usst.edu.cn (Y.L.); wei.yang@unimelb.edu.au (W.Y.)
Received: 26 October 2020; Accepted: 10 December 2020; Published: 14 December 2020
Abstract: A Chinese traditional narrow street, named Cold-Lane, can create a microclimatic zone that
provides pedestrian thermal comfort under hot and humid climate conditions. This phenomenon was
observed through experimental measurement during the summer of 2016. The heat transfer rate over
the pedestrian body surface was calculated to reveal why pedestrians experience a cool sensation,
and computational flow dynamics (CFD) simulation was carried out to study the influence of the
street aspect ratio on the shading effect. It was found that the perception of thermal comfort can be
attributed mainly to the radiation between the relatively cool surrounding walls and the human body,
and the wind velocity has little effect on sensible heat dissipation. The cool horizontal and vertical
surfaces in the street canyon are mainly due to the shading effect as a result of the small aspect ratio,
which is a typical characteristic of the traditional Chinese street. The shading effect of the high walls
on both sides creates the cooling effect of this narrow street.
Keywords: street canyon; pedestrian thermal comfort; radiant cooling; solar shading; night ventilation;
traditional Chinese street
1. Introduction
The street microclimate has a significant impact on pedestrian thermal comfort and many
studies have focused on the street canyon [1–3]. Nagaraet et al. [4] performed measurements on the
physical–thermal environment on a street in Osaka, Japan and found that the thermal sensation of
pedestrians was generally consistent with the ambient conditions, and a buffer zone between the exit and
the outdoor space is important to improve the thermal comfort of the pedestrians. Santamouris et al. [5]
carried out a field experiment in a deep pedestrian canyon in Athens, measuring air temperature inside
and outside the canyon, in addition to the surface temperature for seven continuous days. They found
that air temperature stratification was significant, with up to 3 ◦ C difference, and the traffic area in the
canyon was hotter than that in the pedestrian area by about 2 ◦ C.
Pearlmutter et al. [6] used a semi-empirical model to predict the effect of street geometry on
pedestrian thermal comfort under trans-seasonal weather conditions. The results from the model
Energies 2020, 13, 6602; doi:10.3390/en13246602
www.mdpi.com/journal/energies
Energies 2020, 13, x FOR PEER REVIEW
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Pearlmutter
Energies
2020, 13, 6602et al. [6] used a semi-empirical model to predict the effect of street geometry
2 ofon
21
pedestrian thermal comfort under trans-seasonal weather conditions. The results from the model
indicated that a suitable street orientation can substantially increase the pedestrian thermal comfort
indicated
that a suitable
orientation
cansurvey
substantially
increase the of
pedestrian
thermalparameters
comfort level.
level.
Pantavou
et al. [7]street
carried
out a field
and measurement
environmental
to
Pantavou
et
al.
[7]
carried
out
a
field
survey
and
measurement
of
environmental
parameters
to assess
the
assess the relationship between pedestrian thermal comfort and environmental parameters
in three
relationship
between
pedestrian
thermal
comfortthat
andair
environmental
in threeare
urban
areas
urban
areas in
Athens,
Greece. They
concluded
temperature parameters
and wind velocity
the main
in
Athens,
Greece.
They
concluded
that
air
temperature
and
wind
velocity
are
the
main
meteorological
meteorological parameters affecting pedestrian thermal comfort sensation, but metabolic rate has no
parameterseffect.
affecting pedestrian thermal comfort sensation, but metabolic rate has no significant effect.
significant
Hong and
and Lin
Lin [8]
Hong
[8] found
found that
that tree
tree arrangement,
arrangement, building
building layout
layout patterns,
patterns, and
and orientations
orientations with
with
respect
to
the
prevailing
wind
direction
have
a
significant
impact
on
the
outdoor
wind
environment
respect to the prevailing wind direction have a significant impact on the outdoor wind environment
and pedestrian
level,
andand
a square
central
spacespace
can offer
to air currents.
and
pedestrianthermal
thermalcomfort
comfort
level,
a square
central
cangood
offerexposure
good exposure
to air
Rosso
et
al.
[9]
evaluated
the
application
of
sustainable
and
cost-effective
materials,
i.e.,
grassland,
currents. Rosso et al. [9] evaluated the application of sustainable and cost-effective materials, i.e.,
asphalt, andasphalt,
natural stones,
as roofing
and as
paving
materials,
and demonstrated
howdemonstrated
thermal perception
grassland,
and natural
stones,
roofing
and paving
materials, and
how
is positively
influenced
by high influenced
albedo surfaces
because
are more
sensitive are
to lower
thermal
perception
is positively
by high
albedopedestrians
surfaces because
pedestrians
more
surface temperature.
sensitive
to lower surface temperature.
Jamei and
and Rajagopalan
Rajagopalan [10]
[10] showed
showed that
that deeper
deeper canyons,
canyons, higher
higher height-to-width
height-to-width aspect
aspect ratios,
ratios,
Jamei
and lower
lower sky
sky view
view factors,
in future
contributed to
to lower
lower levels
levels of
of mean
and
factors, in
future climate
climate scenarios,
scenarios, contributed
mean radiant
radiant
temperatures,
and
increasing
the
tree
canopy
coverage
from
40%
to
50%
leads
to
a
more
effective
temperatures, and increasing the tree canopy coverage from 40% to 50% leads to a more effective
cooling effect than
than vegetation.
vegetation. Zhang et al. [11] investigated street canyons in the hot and humid area
China and evaluated
evaluated the
the effects of different design
in Guangzhou, China
design factors
factors and
and their
their interactions
interactions on
pedestrian thermal comfort through numerical simulations.
simulations.
central China,
China,traditional
traditionalnarrow
narrowstreets
streets
have
been
developed
over
a period
of centuries,
In central
have
been
developed
over
a period
of centuries,
and
◦ C.
and
provide
a
cool
sensation
to
pedestrians
even
when
the
outdoor
air
temperature
is
above
30
provide a cool sensation to pedestrians even when the outdoor air temperature is above 30 °C. Chen
ChenZhong
and Zhong
conducted
a survey
a traditional
street
which
resemblesananurban
urbangorge
gorge in
and
[12,13][12,13]
conducted
a survey
on aontraditional
street
which
resembles
◦C
Zhejiang province and found that the temperature inside the street was 5 °C
lower than that outside
and more stable. Ma
Ma [14]
[14] investigated
investigated aa narrow
narrow street at Guangzhou, and concluded that radiation is
the most prominent influence on pedestrian thermal comfort.
comfort.
[15] and
and Höppe
Höppe [16]
[16] proposed
proposed similar
similar methodologies
methodologies to
to calculate
calculate body
bodyenergy
energy balance,
balance,
Zhu et al. [15]
based on
the
Munich
Energy
Balance
Model,
for
individuals
to
define
the
balance
equation
of
the
on the Munich Energy Balance Model, for individuals to define the balance equationhuman
of the
body considering
metabolic
rate, sensible
thermal
convection
rate, radiation
rate, evaporative
heat
human
body considering
metabolic
rate, sensible
thermal
convection
rate, radiation
rate, evaporative
dissipation
rate,
and
the
heat
storage
capacity
of
human
body.
heat dissipation rate, and the heat storage capacity of human body.
was conducted
conducted on
on aa traditional
traditional street
street in
in Yiyang,
Yiyang, aa city
city in
in central
central China.
China.
The survey in this study was
The studied street is narrower than those in previous studies, and located in the subtropical humid
climate zone,
referred
to asto
theashot-summer
and cold-winter
zone in China.
Historical
zone,which
whichis usually
is usually
referred
the hot-summer
and cold-winter
zone in
China.
typical temperature
profiles in this
city during
are presented
in Figure
1, which
shows1,that
the
Historical
typical temperature
profiles
in thissummer
city during
summer are
presented
in Figure
which
daily average
ranges
from 20 ranges
to 33 ◦ Cfrom
[17].20 to 33 °C [17].
shows
that thetemperature
daily average
temperature
Average T
Highest T
Lowest T
40
Temprature (°C)
35
30
25
20
15
June
July
August
September
Time
Figure 1. Local
Local typical
typical temperatures
temperatures from
from Yiyang meteorological station.
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Figure 2 shows two pictures of the traditional narrow pedestrian street, known as Cold-Lane,
Cold-Lane,
near a local temple named Wei
Gong
Miao.
The
walls
are
composed
of
bricks
and
clay
with gray
Wei Gong Miao.
plaster and the ground is covered
covered by moor stones.
stones. Local people report that it is cool and comfortable
while walking in this traditional
traditional street. It is an example of traditional urban development to keep
pedestrians comfortable.
comfortable.
(a)
(b)
Figure 2. A
A traditional
traditional gorge in central China. (a) Front
Front view of the temple; (b) Front view of the street.
To the
the best
best of
of the
the authors’
authors’ knowledge,
knowledge, no study has yet been conducted on such narrow street
canyons in
in this
thisclimate
climateregion.
region.TheThe
measurements
presented
in paper
this paper
measurements
presented
in this
were were
taken taken
duringduring
24–26
24–26
August
2016 the
withfirst
thetwo
firstdays
twobeing
days sunny
being sunny
theday
third
dayAn
rainy.
An analysis
of
August
2016 with
and theand
third
rainy.
analysis
of the heat
the
heat
transfer
rate
over
the
pedestrian
body
surface
was
conducted
based
on
the
data
from
the
transfer rate over the pedestrian body surface was conducted based on the data from the field
field
measurement.
measurement.
2.
Survey Methodology
Methodology
2. Survey
As
shown in
in Figure
Figure 3,
3, the
theold
oldstreet
streetisislocated
locatednear
nearaariver
river(marked
(markedwith
withwhite
whitecolor).
color).
The
city
As shown
The
city
is
◦
◦
is
located
at
28.6
N
latitude
and
112.3
E
longitude.
The
lane
is
around
80
m
long,
1
to
2
m
wide,
located at 28.6° N latitude and 112.3° E longitude. The lane is around 80 m long, 1 to 2 m wide, and
and
surrounded
tohigh
8 m walls
high walls
theand
eastwest.
and Therefore,
west. Therefore,
the width-to-height
ratio
surrounded
withwith
6 to 86m
on theon
east
the width-to-height
ratio ranges
ranges
from 4 from
to 6. 4 to 6.
Figure 4a illustrates the eight measuring points, of which points 1 and 8 were outside the lane,
and points 2 to 7 were inside the lane. At each point the dry-bulb air temperature, relative humidity,
and air velocity were recorded by a hygrothermograph and anemometer. The surface temperatures
inside the lane were measured by radiant thermometers, as shown in Figure 4b. During the three-day
investigation, the measurement time and weather conditions are listed in Table 1.
Figure 3. Cold-Lane seen from a satellite image (from Google Map).
As shown in Figure 3, the old street is located near a river (marked with white color). The city
cated at 28.6° N latitude and 112.3° E longitude. The lane is around 80 m long, 1 to 2 m wide, an
urrounded with
6 2020,
to 813,m6602
high walls on the east and west. Therefore, the width-to-height
ratio range
Energies
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om 4 to 6.
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Figure 4a illustrates the eight measuring points, of which points 1 and 8 were outside the lane,
and points 2 to 7 were inside the lane. At each point the dry-bulb air temperature, relative humidity,
and air velocity were recorded by a hygrothermograph and anemometer. The surface temperatures
inside the lane were measured by radiant thermometers, as shown in Figure 4b. During the threeFigure 3. Cold-Lane
seen
fromaa satellite
satellite image
(from(from
Map).
Figure 3.the
Cold-Lane
seen
from
image
Google
day investigation,
measurement
time
and
weather
conditions
areGoogle
listed in
Table 1. Map).
Road
8
7
6
North
5
4
East
3
2
Road
1
(a)
(b)
Figure
Figure4.4.Measuring
Measuringpoints:
points:(a)
(a)top
topview;
view;(b)
(b)front
frontview.
view.
Table 1. Measurement time and weather conditions during investigation [17].
Date
(y/m/d)
2016/8/24
2016/8/24
2016/8/25
2016/8/25
2016/8/25
2016/8/25
2016/8/26
2016/8/26
Weather
Time
Sunny
Sunny
Sunny
Sunny
Sunny
Sunny
Rainy
Rainy
15:00
19:30
7:00
11:30
15:30
19:10
7:30
13:00
Average
Temperature
35.0 °C
30.0 °C
28.0 °C
35.0 °C
35.0 °C
30.0 °C
26.0 °C
25.0 °C
Heat Index
40.3 °C
33.2 °C
31.9 °C
37.5 °C
37.5 °C
34.1 °C
-
Relative
Humidity
49%
62%
79%
41%
41%
66%
83%
89%
Wind
Direction
North
East
No wind
Unstable
East
East
Northeast
Northeast
Wind
Velocity
3 m/s
3 m/s
No wind
1 m/s
3 m/s
3 m/s
4 m/s
4 m/s
Energies 2020, 13, 6602
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Table 1. Measurement time and weather conditions during investigation [17].
Date
(y/m/d)
Weather
Time
Average
Temperature
Heat
Index
Relative
Humidity
Wind
Direction
Wind
Velocity
2016/8/24
2016/8/24
2016/8/25
2016/8/25
2016/8/25
2016/8/25
2016/8/26
2016/8/26
Sunny
Sunny
Sunny
Sunny
Sunny
Sunny
Rainy
Rainy
15:00
19:30
7:00
11:30
15:30
19:10
7:30
13:00
35.0 ◦ C
30.0 ◦ C
28.0 ◦ C
35.0 ◦ C
35.0 ◦ C
30.0 ◦ C
26.0 ◦ C
25.0 ◦ C
40.3 ◦ C
33.2 ◦ C
31.9 ◦ C
37.5 ◦ C
37.5 ◦ C
34.1 ◦ C
-
49%
62%
79%
41%
41%
66%
83%
89%
North
East
No wind
Unstable
East
East
Northeast
Northeast
3 m/s
3 m/s
No wind
1 m/s
3 m/s
3 m/s
4 m/s
4 m/s
Each measurement period lasted for one hour, starting from point 1 and reaching point 8 within
the hour.
3. Results and Analysis from Investigation
3.1. Thermal Sensations Directly
The measurements were carried out by three architectural students and two teachers who stayed
in the street. During this time, they did not complain of being hot when in the Cold-Lane (at points 2
to 7 in Figure 4a). On the contrary, the researchers reported feeling unbearably hot while standing
Energies 2020,
13, x FOR PEER
5 of 20
outside
the Cold-Lane
(atREVIEW
points 1 and 8). The same feedback was reported by the local residents.
3.2. Dry-Bulb
Dry-Bulb Temperatures
Temperatures
3.2.
Atotal
totalofof
4 ONSET
Temp/RH
(relative
humidity)
were
used to
the air
A
4 ONSET
Temp/RH
(relative
humidity)
LoggersLoggers
were used
to measure
themeasure
air temperature
◦
temperature
(with
accuracy
of
±0.21
°C)
and
relative
humidity
(with
accuracy
of
±0.3.5%).
Figure
5a
(with accuracy of ±0.21 C) and relative humidity (with accuracy of ±0.3.5%). Figure 5a presents the
presents
the
variation
of
temperatures
of
every
measurement
point
at
different
times,
which
shows
variation of temperatures of every measurement point at different times, which shows the same trend.
the same
trend. that
Figure
5b shows between
that the the
difference
between
the
inside air
and
outside average
air
Figure
5b shows
the difference
inside and
outside
average
temperatures
was less
◦
temperatures
was
less
than
2
°C.
There
was
a
slight
time
lag
of
the
inside
temperatures
following
the
than 2 C. There was a slight time lag of the inside temperatures following the outside temperature.
outside
may cooling
be caused
radiant cooling
and through
night-time
ventilation,
through
This
maytemperature.
be caused byThis
radiant
andby
night-time
ventilation,
which
the walls
could
which theheat
walls
dissipate
at could
night. dissipate heat at night.
Temperature (°C)
35
30
Outside temperature
Inside temperature
Meteorological station
40
35
Temperature (°C)
Point 1
Point 2
Point 3
Point 4
Point 5
Point 6
Point 7
Point 8
40
30
25
25
20
20
15:00 19:30
7:00
11:30 15:30 19:10
7:30
Time
(a)
13:00
--
15:00 19:30
7:00
11:30 15:30 19:10
7:30
13:00
--
Time
(b)
Figure 5.
5. Measurement
variation
of of
dry-bulb
temperatures
with
time;
(b)
Figure
Measurement of
ofdry-bulb
dry-bulbtemperature:
temperature:(a)(a)
variation
dry-bulb
temperatures
with
time;
comparison
of dry-bulb
temperatures
inside
andand
outside
thethe
street.
(b)
comparison
of dry-bulb
temperatures
inside
outside
street.
3.3.
3.3. Relative
Relative Humidity
Humidity and
and Air
Air Velocity
Velocity
A
A hot
hot wire
wire anemometer
anemometer was
was used
used to
to measure
measure the
the air
airvelocity
velocitywith
withan
anaccuracy
accuracy of
of±0.1
±0.1 m/s
m/s and
and
average
inin
airair
velocity.
From
Figure
6,
average values
valueswith
withan
aninterval
intervalofof11min
mintotoaccount
accountfor
forthe
thefluctuations
fluctuations
velocity.
From
Figure
6, it can be observed that the relative air humidity inside and outside the lane are similar, and both
varied between 60% and 100%. As observed from Figure 7, the outside air velocity was higher than
that inside most of the time, with less fluctuation. Compared with the wind speed in Table 1, which
range from 0 to 4 m/s, the velocities measured are much lower than those from the local
meteorological station. This is due to the highly crowded buildings in this city, which lower the wind
comparison of dry-bulb temperatures inside and outside the street.
3.3. Relative Humidity and Air Velocity
A hot wire anemometer was used to measure the air velocity with an accuracy of ±0.1 m/s and
Energies 2020, 13, 6602
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average
values with an interval of 1 min to account for the fluctuations in air velocity. From Figure
6, it can be observed that the relative air humidity inside and outside the lane are similar, and both
varied
60%
Asair
observed
Figure
7, the outside
velocity
than
it can bebetween
observed
thatand
the100%.
relative
humidityfrom
inside
and outside
the laneair
are
similar,was
andhigher
both varied
that
inside
most
of
the
time,
with
less
fluctuation.
Compared
with
the
wind
speed
in
Table
1,
which
between 60% and 100%. As observed from Figure 7, the outside air velocity was higher than that inside
range
0 towith
4 m/s,
the velocities
measured
arethemuch
than
those
fromrange
the from
local
most offrom
the time,
less fluctuation.
Compared
with
wind lower
speed in
Table
1, which
meteorological
station.
This
is
due
to
the
highly
crowded
buildings
in
this
city,
which
lower
the
wind
0 to 4 m/s, the velocities measured are much lower than those from the local meteorological station.
speed
thethe
ground.
This isnear
due to
highly crowded buildings in this city, which lower the wind speed near the ground.
100
95
RH outside
RH inside
Meteorological station
90
Relative humidity (%)
85
80
75
70
65
60
55
50
45
40
35
15:00 19:30
7:00
11:30 15:30 19:10
7:30
13:00
--
Time
Energies 2020, 13, x FOR PEER REVIEW
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Relative air
air humidity.
humidity.
Figure 6. Relative
Air velocity inside
Air velocity outside
0.8
Air velocity (m/s)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
15:00 19:30
7:00
11:30 15:30 19:10
7:30
13:00
--
Time
Figure 7.
Air velocity.
velocity.
Figure
7. Air
3.4. Surface Temperatures at Each Position at Specific Time
3.4. Surface Temperatures at Each Position at Specific Time
Two rectified radiation thermometers were used to measure the wall surface temperature at the
Two rectified radiation thermometers were used to measure the wall surface temperature at the
same time, and the average value was used as the final reading. The accuracy of each thermometer is
same time, and the average value was used as the final reading. The accuracy of each thermometer is
±0.1 ◦ C. Figure 8 presents the profiles of surface temperatures at the ground surface, 1.5, 3, and 4 m
±0.1 °C. Figure 8 presents the profiles of surface temperatures at the ground surface, 1.5, 3, and 4 m
above the ground at positions 2 to 7 at different times. Figure 8a,c,d indicate strong solar radiation,
above the ground at positions 2 to 7 at different times. Figure 8a,c,d indicate strong solar radiation,
and show that the temperature in the center (positions 4 and 5) was lower than those at the two ends
and show that the temperature in the center (positions 4 and 5) was lower than those at the two ends
(positions 2 and 7). The reason for this is uncertain and perhaps due to a better shading effect or higher
(positions 2 and 7). The reason for this is uncertain and perhaps due to a better shading effect or
air velocity inside the lane.
higher air velocity inside the lane.
face temperature (°C)
38
36
34
32
30
28
Ground
1.5 m
3.0 m
4.0 m
40
38
ace temperature (°C)
40
36
34
32
30
28
Ground
1.5 m
3.0 m
4.0 m
Energies 2020, 13, 6602
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Figure 8. Temperature profiles at different heights: (a) 24 August, 15:00 (sunny); (b) 25 August,
7:00 (sunny); (c) 25 August, 15:00 (sunny); (d) 25 August, 19:10 (sunny); (e) 26 August, 7:30 (started
raining); (f) 26 August, 13:00 (rainy).
Energies 2020, 13, 6602
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Figure 8b shows that the temperature stratification at different sites was about the same and the
temperatures were less than 30 ◦ C in the morning. The low temperature is due to radiation heat loss
from the ground/wall surface and night-time ventilation.
When it began to rain and the solar radiation was low, the surface temperatures all fell continually
to approximately the same level, as shown in Figure 8e,f.
Energies 2020, 13, x FOR PEER REVIEW
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3.5. Comparison of Surface Temperatures between the West Wall and the East Wall
The result of the comparisonsTemperature
of the surface of
temperatures
the west wall and east wall at
West wall >between
East wall
the same height is presented in Figure
9, demonstrating
temperature
profiles were similar.
Temperature
of Westthat
walltheir
< East
wall
Energies 2020, 13, x FOR PEER REVIEW
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Temperature of West wall > East
45%wall
Temperature of West wall < East wall
45%
55%
55%
9. Percentage
in comparison
temperatures between
westwest
wall and
east
wall.
Figure
Percentage
in
of
between
wall
and
east wall.
wall.
Figure 9.
9.Figure
Percentage
in comparison
comparison
ofoftemperatures
temperatures
between
west
wall
and
east
Surface Temperatures
at Different Heights
with Time
3.6. Surface
Surface3.6.
Temperatures
at
3.6.
Temperatures
at Different
Different Heights
Heights with
with Time
Time
In Figure 10 it is observed that the surface temperatures increase when the positions are higher;
In Figure
Figure
10 it
it is
isgradient
observed
that
the
surface
temperatures
increase
when
the
positions
are higher;
higher;
In
10
observed
that
the
surface
increase
the
positions
are
the average
is 0.88
°C/m.
This
may betemperatures
caused by different
shadingwhen
schemes
that
resulted in
◦ C/m. This may be caused by different shading schemes that resulted in
the
average
gradient
is
0.88
the
variation
of
the
solar
radiation
intensity
on
the
wall
surface
at
different
heights.
All
of
the
surface
the average gradient is 0.88 °C/m. This may be caused by different shading schemes that resulted in
temperatures
dropped
to almost
the same on
value
the last
day with
overcast heights.
sky.
the variation
variation
of the
the solar
solar
radiation
intensity
theonwall
wall
surface
at an
different
All of
of the
the surface
surface
the
of
radiation
intensity
on
the
surface
at
different
heights.
All
temperatures dropped to almost the same value on the last
day
with
an
overcast
sky.
last day with an overcast sky.
Ground
1.5 m
3 m Ground
4m
Surface temperature (°C)
36
34
32
30
28
Surface temperature (°C)
36
34
1.5 m
3m
4m
32
30
28
26
24
22
26
15:00 19:30
7:00
11:30 15:30 19:10
7:30
13:00
Time
24
Figure 10. Temperatures at different heights.
3.7. Average Surface Temperatures Inside and Outside the Lane
22
15:00the19:30
11:30 15:30
Figure 11 reveals that
average7:00
temperature
at the 19:10
ground 7:30
surface13:00
of the outside road
(position 1 and 8) was about 5 °C higher than that inside the Cold-Lane (position 2 to 7). During the
Time
day, the outside road surface temperature was much higher than that inside the lane due to solar
radiation. At night the temperature
difference
was small
due to heights.
long-wave radiation between the
Figure 10. Temperatures
Temperatures
Figure
at different
road surface and the sky and night-time ventilation.
3.7. Average Surface Temperatures Inside and Outside the Lane
Figure 11 reveals that the average temperature at the ground surface of the outside road
(position 1 and 8) was about 5 °C higher than that inside the Cold-Lane (position 2 to 7). During the
day, the outside road surface temperature was much higher than that inside the lane due to solar
Energies 2020, 13, 6602
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3.7. Average Surface Temperatures Inside and Outside the Lane
Figure 11 reveals that the average temperature at the ground surface of the outside road (position
1 and 8) was about 5 ◦ C higher than that inside the Cold-Lane (position 2 to 7). During the day,
the outside road surface temperature was much higher than that inside the lane due to solar radiation.
At night the temperature difference was small due to long-wave radiation between the road surface
and the2020,
sky13,
and
night-time
ventilation.
Energies
x FOR
PEER REVIEW
9 of 20
45
Outside road surface
Inside surfaces（including ground,1.5 m, 3 m, 4 m)
Temperature (°C)
40
35
30
25
15:00 19:30
7:00
11:30 15:30 19:10
7:30
13:00
Time
Figure 11.
11. Average
Average surface
surface temperatures
temperatures inside and outside.
Figure
4. Calculation
Calculation of
of Heat
Heat Transfer
Transfer Rate
Rate on
on the
the Pedestrian
Pedestrian Body
Body
4.
As shown
shown in
in Figures
Figures 8,
8, 10,
10 and
11, the
the wall
wall surface
surface temperature
temperature at
at the
the bottom
bottom of
of the
the street
street canyon
canyon
As
and 11,
was
relatively
low.
In
order
to
evaluate
the
total
heat
gain,
and
the
impact
of
wall
temperature
on the
the
was relatively low. In order to evaluate the total heat gain, and the impact of wall temperature on
heat dissipation
dissipation of
of the
the human
human body,
body, the
the human
human body
body heat
heat balance
balance model
model was
was developed
developed to
to calculate
calculate
heat
the
heat
convection,
radiation
from
the
surrounding
walls,
and
latent
heat
dissipation
from
evaporation.
the heat convection, radiation from the surrounding walls, and latent heat dissipation from
Four cases areFour
considered
in considered
this section,ini.e.,
pedestrian
stillpedestrian
and no wind,
still and
windy,
evaporation.
cases are
this
section, i.e.,
stillpedestrian
and no wind,
pedestrian
pedestrian
walking,
and pedestrian
the
outside road.
still
and windy,
pedestrian
walking,on
and
pedestrian
on the outside road.
4.1. Parameter
4.1. Parameter
The survey was conducted by five people lasting for one hour each time. Human body heat
The survey was conducted by five people lasting for one hour each time. Human body heat
dissipation was calculated based on the heat transfer theory of Yang and Tao [18] and the human body
dissipation was calculated based on the heat transfer theory of Yang and Tao [18] and the human
heat balance model was drawn from Zhu [15]. Three key parameters, (a) sensible heat convection,
body heat balance model was drawn from Zhu [15]. Three key parameters, (a) sensible heat
(b) radiation, and (c) latent heat dissipation from evaporation, were calculated.
convection, (b) radiation, and (c) latent heat dissipation from evaporation,
were calculated.
We assume that the clothing insulation value is 0.5 clo. (0.08 m2 K/W), and
the surface temperature
We assume that the clothing insulation value is 0.5 clo. (0.08 m2 K/W), and the surface
of the clothes is close to that of the ambient air at 34.5 ◦ C (tcloth ) [15]; thus, the skin temperature is 37 ◦ C
temperature of the clothes is close to that of the ambient air at 34.5 °C (tcloth) [15]; thus, the skin
(tskin ) and approximately one-third of the human skin is exposed to air directly. For an adult with
temperature is 37 °C (tskin) and approximately one-third of the human skin is exposed to air directly.
height of 1.78 m and weight of 65 kg, the average body surface temperature can be estimated as [15]:
For an adult with height of 1.78 m and weight of 65 kg, the average body surface temperature can be
estimated as [15]:
2
1
(1)
th = × tcloth + × tskin = 35.3 ◦ C
32
31
(1)
t h = × t cloth + × t skin = 35.3 ℃
3 conditions were included for further calculations
Based on data from the field survey,3 the following
Based2).on data from the field survey, the following conditions were included for further
(see Table
calculations (see Table 2).
Energies 2020, 13, 6602
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Table 2. Parameters for calculation of the heat transfer rate over the pedestrian’s body.
Duration and Weather
Air Characteristic
Mean Surface Temperature
15:00–16:00
Sunny
Heat index: 40.3 ◦ C
Wind direction: North
Wind velocity: 3 m/s
Dry-bulb temperature ta = 34.5 ◦ C
Relative humidity, 49%
Density ρ = 1.14 kg/m3
Specific heat capacity Cp = 1.005 kJ/(kg·K)
Thermal conductivity λ = 0.027 W/(m·K)
Kinematic viscosity υ = 16.5 × 10−6 m2 /s
Prandtl number Pr = 0.7
Inside the gorge wt = 30.2 ◦ C
Human body temperature th = 35.3 ◦ C
Outside the gorge (on the roads) tr = 34.8 ◦ C
4.2. Case 1—Pedestrian Still and no Wind
4.2.1. Sensible Heat Convection over Pedestrian Body to Air
The human body in the gorge can be assumed to be an isothermal cylinder, whose sensible heat
transfer rate can be calculated as follows, according to [18]:
Qc = hc × A × (th − ta ) = 1.72 W
(2)
λ × Nu
= 1.19 W/(m2 ·◦ C)
l
(3)
where
hc =
1
Nu = 0.11 × (Gr × Pr ) 3 = 78.89
(4)
g × a × ∆t × l3
= 5.27 × 108
(5)
v2
See Table 2 for the input values for Equations (2)–(5).
To validate the reliability of the above method, the calculated convective heat transfer coefficient
(hc ) was compared with those calculated by the method of Birkebak [19]:
Gr =
1
( th − ta ) 4
hc = 1.18 ×
= 1.12 W/(m2 ·◦ C)
4
(6)
The relative error was found to be 6.3%, indicating the models show good agreement.
4.2.2. Radiation Dissipation
While people are in the Cold-Lane, the view factor between the human body and the surface of
the ground and walls is close to 1.0. This is due to the high aspect ratio of 3 and the narrow width of
the street canyon. In addition, it was observed that there was no direct solar radiation on the human
body during the survey at 3:00 pm. Thus, it was assumed in this case that the heat transfer process is
similar to that of a body with a temperature of 35.3 ◦ C inside a cavity of 31.6 ◦ C, and the heat flux can
be calculated as follows [18]:
Qr = A × ε × σ × th 4 − tw 4 = 50.83 W
(7)
4.2.3. Latent Heat Transfer over the Human Body
The heat balance equation over the human body was used to evaluate the evaporative heating
transfer rate over the human body [15]:
M − W − Qc − Qr − QE − S = 0
(8)
Energies 2020, 13, 6602
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where M is the heat dissipation due to the metabolism, the metabolic rate (m) is at about 1.2 Met or
equal to 70 W/m2 over the human skin when standing still [15]. Therefore:
M = m × A = 124.6 W
(9)
where W is the mechanical output of the human body, which is about 0 W here; Qc is the sensible
convective heat transfer rate, taken as 1.72 W from Equation (2); Qr is the radiation heat transfer
rate, taken as 50.83 W from Equation (7); S is the heat storage of human body, assumed to be 0 W.
Therefore, for a human body with 1.78 m2 skin area, the evaporative heat dissipation rate can be
calculated as:
QE, still and no wind = M − Qc − Qr = 72.05 W
(10)
4.3. Case 2—Pedestrian Still and Windy
From field investigation, the average wind velocity v inside the street was found to be 0.29 m/s.
The sensible convective heat transfer coefficient can be calculated according to [19] as:
hc = 8.6 × v0.6 = 4.09 W/(m2 ·◦ C)
(11)
Based on Equations (2) and (7), the evaporative heat dissipation rate can be obtained:
QE, still and windy = M − Qc − Qr = 152.31 W
(12)
4.4. Case 3—Pedestrian Walking
This situation is similar to a pedestrian remaining still in a windy environment. Assuming walking
speed is 1.2 m/s, and that the internal heat-generation rate rises to 2.6 Met (150 W/m2 ) [15],
the evaporative heat dissipation rate can be calculated as:
QE, walking = M − Qc – Qr = 203.13 W
(13)
4.5. Case 4—Pedestrian on the Outside Road
Using the above method with the surface temperature on the ground of the outside road
(tr = 34.8 ◦ C) from Table 2, and assuming the same air temperatures inside and outside the Cold-Lane,
while the pedestrian stays still, the sensible long-wave radiative heat transfer rates are 1.73 W and
5.09 W, respectively. Therefore, the evaporative heat dissipation is:
QE, still and no win, outside road = Qsolar + M − Qc – Qr = 531.42 W
(14)
Because there is no shading outside the road, the heat gain due to solar radiation Qsolar is
calculated from the local monthly mean horizontal solar radiation intensity [20] and hypothetical
exposure of half of the human skin surface area exposed to the outside [15]. Qsolar is about 327.70 W.
Similarly, the evaporative heat dissipation rate while pedestrian remains still or is walking can be
obtained as:
QE, still and windy, outside road = Qsolar + M − Qc – Qr = 515.80 W
(15)
QE, walking, outside road = Qsolar + M − Qc –Qr = 576.57 W
(16)
4.6. Results Analysis
Clearly, all parameters in the actual situations are dynamic and the analysis provided here is
intended to show the heat transfer rates under typical situations. All results are displayed in Figure 12.
Energies 2020, 13, 6602
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Energies 2020, 13, x FOR PEER REVIEW
Sweat evaporative cooling by air
Radiant cooling by surfaces
Sensible heat convection by air
12 of 20
Outside road
600
Heat dissipation (W)
500
Inside gorge
400
300
200
100
0
Still+no wind
Still+windy
Walking
Still+no wind
Still+windy
Walking
Case
Figure 12. Total heat gain and dissipation of the human body.
Figure 12. Total heat gain and dissipation of the human body.
From Figure 12, it can be observed that:
From Figure
it can
observed
that: body inside the street is less than 50% of that gained
(1) The12,
total
heat be
gained
by the human
outside the street. This is mainly due to the shading effect of the high walls on both sides of the
(1) The total heat
gained by the human body inside the street is less than 50% of that gained outside
narrow street.
the street.
toheat
thedissipation
shadingaccounts
effect of
the high
walls
sides of the narrow
(2) This
Inside is
themainly
gorge, thedue
radiant
for about
30–40%
of the on
totalboth
heat dissipation,
which shows that the relatively low temperature of the ground and walls significantly cools the
street.
body. Evaporative cooling accounts for 50–70%, which shows that sweating is essential in
(2) Inside the human
gorge,
the radiant heat dissipation accounts for about 30–40% of the total heat
summer, even in the streets with shading. The proportion of sensible heat dissipation is very small.
dissipation,
which
theis no
relatively
low temperature
of 0.8%.
the When
ground
When
people shows
stand still that
and there
wind, heat dissipation
only accounts for
there and walls
significantly
cools
human
body.isEvaporative
coolingofaccounts
for 50–70%,
which
is wind
or the
when
the pedestrian
walking, the proportion
heat dissipation
rises to 8.2%
and shows that
6.4%,
respectively.
In
this
kind
of
street,
compared
to
standing,
the
heat
generated
by
walking
sweating is essential in summer, even in the streets with shading. The proportion of sensible
can help increase the total body heat gain by 29%.
heat dissipation is very small. When people stand still and there is no wind, heat dissipation
(3) When the pedestrian is outside the road, the radiant heat dissipation from surrounding surfaces and
only accounts
forheat
0.8%.
Whenbythere
windrelatively
or when
pedestrian
is walking,
the proportion
sensible
dissipation
the air is
become
verythe
small,
together accounting
for less than
4.5%. Evaporative
more than 95%. Correspondingly,
the street,
increase in
of heat dissipation
risesheat
to dissipation
8.2% andaccounts
6.4%,forrespectively.
In this kind of
compared to
heat gain due to walking decreased to 11% of the total heat gain.
standing, the heat generated by walking can help increase the total body heat gain by 29%.
5. The
Influence ofis
Street
Aspect
Ratio
on the
Effect
(3) When the
pedestrian
outside
the
road,
theShading
radiant
heat dissipation from surrounding surfaces
The heat
aspectdissipation
ratio of the Cold-Lane
about
3–4, so the
low temperature
the Cold-Lane
is clearly
and sensible
by theis air
become
relatively
veryofsmall,
together
accounting for
due
to
the
shading
effect
of
the
high
walls
on
both
sides.
In
the
following
section,
computational
flow
less than 4.5%. Evaporative heat dissipation accounts for more than 95%. Correspondingly, the
dynamics (CFD) simulation is conducted to investigate the temperature variation at the inner surface
increase
in heat gain due to walking decreased to 11% of the total heat gain.
of the street on 25 August with solar radiation and the different height-to-width ratios of the canyons.
5.1. CFD Simulation Setting and Validation
5. The Influence
of Street Aspect Ratio on the Shading Effect
5.1.1. Building Model and Flow Field
The aspect ratio of the Cold-Lane is about 3–4, so the low temperature of the Cold-Lane is clearly
Similar to Bottillo et al. [21], this study simulates solar ray tracing in the street canyon using Fluent
due to the shading
effect of the high walls on both sides. In the following section, computational flow
software licensed from Ansys (Canonsburg, USA); in addition, however, the current study focuses
dynamics (CFD) simulation is conducted to investigate the temperature variation at the inner surface
of the street on 25 August with solar radiation and the different height-to-width ratios of the canyons.
5.1. CFD Simulation Setting and Validation
Energies 2020, 13, 6602
13 of 21
on the inner surface temperature. To simplify the simulation, the Cold-Lane is considered to be a
straight street canyon, with walls on both sides of 2 m in width and 20 m in length. Because the goal
of this discussion is examination of the shading effect of walls with different heights, the heights of
the walls were set to be 2, 4, 6, and 8 m, respectively, corresponding to aspect ratios of 1, 2, 3, and 4.
The
building
and the
flow field are shown in the left schematic diagram in Figure 13, referring
to
Energies
2020, 13,model
x FOR PEER
REVIEW
13 of 20
the setting method from van Hooff et al. [22]. Based on the maximum height of the building (H = 8 m),
the building
mthe
(3 H)
away from
wind
incoming
entrance
boundary
inentrance
the northboundary
direction;
building
(H =is824
m),
building
is 24the
m (3
H) away
fromflow
the wind
incoming
flow
40 the
m (5
H) away
from 40
themwest
boundary,
east
boundary,
and skyeast
boundary;
andand
120 sky
m (15
H) from
in
north
direction;
(5 H)
away from
the
west boundary,
boundary,
boundary;
the south
As shown
the rightAs
schematic
Figure 13,diagram
following
and
120 mwind
(15 H)outlet
from boundary.
the south wind
outlet in
boundary.
shown indiagram
the rightinschematic
in
its trajectory,
the sunits
shines
on the the
eastsun
wall
in theon
morning,
onmorning,
the streetvertically
ground aton
noon,
Figure
13, following
trajectory,
shines
the eastvertically
wall in the
the
and onground
the west
wall inand
theon
afternoon.
street
at noon,
the west wall in the afternoon.
Figure 13.
13. Schematic
Schematic diagram
diagram of
of flow
flow field
field and
and sun
sun altitude angle.
Figure
5.1.2. Boundary Conditions and Solution Settings
5.1.2. Boundary Conditions and Solution Settings
The inflow boundary condition of the flow field is set to be velocity-inlet, and the wind velocity
The inflow boundary condition of the flow field is set to be velocity-inlet, and the wind velocity
is set to be 0.5 m/s at the pedestrian height of 1.5 m with a temperature of 300 K. The wind speed at
is set to be 0.5 m/s at the pedestrian height of 1.5 m with a temperature of 300 K. The wind speed at
the height of the building (assuming to be 8 m) comes from the distribution formula of the incoming
the height of the building (assuming to be 8 m) comes from the distribution formula of the incoming
air obtained
obtained from
from measurements
measurements in
in wind
wind tunnel
tunnel based
based on
on our
our previous
previous studies
studies [23,24],
[23,24], and
and can
can be
be
air
calculated as:
0.18
calculated as:
z
Uz
=
(17)
zH 0.18
UUHz
=( )
(17)
H
H
where Uz (m/s) and U (m/s) are the meanUwind
velocity at a height of z (m) and at the reference
H
height Uz
of the
building
H (m),are
respectively.
The turbulent
kinetic
energy
canand
be approximated
by
where
(m/s)
and Uof
H (m/s)
the mean wind
velocity at
a height
of zκ(m)
at the reference
Equation
(14):building of H (m), respectively. The turbulent kinetic energy κ can be approximated by
height
of the
k(z)
z
Equation (14):
(18)
= 0.095 × e−0.19 H
2
UH
z
k(z)
× e−0.19H
(18)
2 = 0.095
where κ(z) (m2 /s2 ) is the turbulent kinetic
at a height of z (m).
UHenergy
For Equation (13), the wind velocity at 8 m height is calculated from the wind speed at pedestrian
where κ(z) (m2/s2) is the turbulent kinetic energy at a height of z (m).
height, then compiled into a UDF (Universal Disk Format) file and imported into the velocity inlet
For Equation (13), the wind velocity at 8 m height is calculated from the wind speed at pedestrian
boundary condition. The exit boundary is set to be vent-outlet, the west and east boundary conditions
height, then compiled into a UDF (Universal Disk Format) file and imported into the velocity inlet
are set to be symmetric boundary, and the ground is adiabatic.
boundary condition. The exit boundary is set to be vent-outlet, the west and east boundary conditions3
The interior of the walls on both sides of the street are red brick walls with density of 1668 kg/m ,
are set to be symmetric boundary,
and the ground is adiabatic.
specific heat of 754 kJ/(kg ◦ C), thermal conductivity of 0.43 W/(m ◦ C), and radiation absorption rate
The interior of the walls on both sides of the street are red brick walls with density of 1668 kg/m 3,
of 0.67. The interface between the walls and the surrounding air is a coupled boundary.
specific heat of 754 kJ/(kg °C), thermal conductivity of 0.43 W/(m °C), and radiation absorption rate
For the simulation of solar radiation on 25 August, assuming that the wall surface temperature
of 0.67. The interface between the walls and the surrounding air is a coupled boundary.
increase due to solar radiation is a quasi-static process, the wall surface temperature at the beginning
For the simulation of solar radiation on 25 August, assuming that the wall surface temperature
of each hour was simulated. The radiation model adopts solar ray tracing in discrete ordinates (DO).
increase due to solar radiation◦ is a quasi-static process,
the wall surface temperature at the beginning
The building location is 112.3 E longitude and 28.6◦ N latitude, and the time period for simulation is
of each hour was simulated. The radiation model adopts solar ray tracing in discrete ordinates (DO).
7:00 AM to 18:00 AM. Both direct solar and diffuse solar radiation were calculated.
The building location is 112.3° E longitude and 28.6° N latitude, and the time period for simulation is
7:00 AM to 18:00 AM. Both direct solar and diffuse solar radiation were calculated.
The viscous model adopts the shear stress transport (SST) k-ω model, which was validated by
Fu et al. [24] using the default settings. Four levels of street canyon heights (2, 4, 6, and 8 m), with
one-hour intervals, from 7:00 to 18:00, were simulated. The modeling and CFD simulation time lasted
for around 20 days.
This study considered three types of equal density mesh with grid sizes of 0.35, 0.5, and 0.7 m.
Energies 2020, 13, 6602
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The viscous model adopts the shear stress transport (SST) k-ω model, which was validated by Fu
et al. [24] using the default settings. Four levels of street canyon heights (2, 4, 6, and 8 m), with one-hour
intervals, from 7:00 to 18:00, were simulated. The modeling and CFD simulation time lasted for around
20 days.
This study considered three types of equal density mesh with grid sizes of 0.35, 0.5, and 0.7 m.
The results show that the differences between these three grid sizes are very small, and finally 0.5 m
was adopted in the CFD simulation. The control equations were discretized by the second-order finite
element method. The Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm was
used for pressure-velocity coupling, and all residuals were required to be less than 10−5 before being
considered as converged. More information can be found in our previous research [23–25] in which
the grid sensitivity analysis was carried out.
5.1.3. Temperature Data Processing
Due to the fact that the entire model is simplified and many factors are excluded, some discrepancy
will exist between the outcomes of the temperature data from CFD simulation and the measured data.
However, according to the principle of similarity, the distribution of the surface temperature field on
the street ground and the inner wall are likely to be similar. Therefore, if the surface temperature data
obtained by CFD simulation can be converted reasonably, compared with the measured data, it is
possible to acquire reasonable results. In this study, it is assumed that the temperature distribution
of the CFD simulation result is similar to that of the field measurement result, and approximately
conforms to Equation (15):
TCFD − TCFD−average
TCFD−highest − TCFD−lowest
=
TField − TField−average
TField−highest − TField−lowest
(19)
where TCFD represents the surface temperature at any place obtained by CFD simulation; TCFD-lowest is
the lowest temperature at the wall surface; TField represents the temperature measured at the
same proportional position on site; and TCFD-lowest and TCFD-highest are the lowest and highest
temperature, respectively.
5.1.4. Validation
Based on Equation (15), the temperature data simulated by CFD was converted into measured
data at the corresponding position. Figure 14 shows the average of the inner surface temperature of
the Cold-Lane (east wall, ground, and west wall) at different heights at 15:00 on 25 August, based on
field measurements and CFD simulations. It can be observed from this figure that the difference in the
temperature between the CFD simulation and measurement is less than 1 K.
The comparison results at different times are shown in Figure 15. It can be seen that the CFD
simulation value and the measured value has a similar trend. The average temperature difference
between them is about 1.5 K with an error of 30%. Clearly, however, the measured temperature has a
time delay, which may be caused by the heat storage of the wall and night-time ventilation.
It can be seen from Figures 14 and 15 that the spatial distribution of the temperature simulated
and transformed by CFD is more accurate with a similar trend to the measured data. However, it has
a time lag and an error of 30%. Therefore, in the following analysis, only the average temperature
distribution on the surface of the block is analyzed, and the variations of temperatures over time are
not considered.
Energies 2020, 13, 6602
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Figure 14. Temperature at different heights by measurement and computational flow dynamics (CFD)
simulation
inPEER
the street
canyon.
Energies 2020,
13, x FOR
REVIEW
15 of 20
Field measured
CFD simulation converted by similarity
34
Field measured (oC)
33
32
31
30
29
28
27
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20
Time (o' clock)
Figure 15. Hourly temperature by measurement and CFD simulation in the street canyon.
Figure
15. Hourly temperature by measurement and CFD simulation in the street canyon.
5.2. Results Analysis
It can be seen from Figures 14 and 15 that the spatial distribution of the temperature simulated
5.2.1. The Average
Temperature
in the Street
Canyon
withtrend
Different
Aspect
Ratios data. However, it has
and transformed
by CFD
is more accurate
with
a similar
to the
measured
a time lag
andbe
anseen
error
of Figure
30%. Therefore,
in the following
analysis,
onlysurface
the average
temperature
It can
from
16 that the average
temperature
at the inner
of the canyon
in
distribution
the surface
blockinisaspect
analyzed,
variations
temperatures
over
time
the blockon
decreases
with of
thethe
increase
ratio. and
The the
average
groundof
temperature
drops
by 0.2
K are
with every 1 m increase in height; that is, with the aspect ratio of 1 as the benchmark, each time the
not considered.
aspect ratio increases by 100%, the average temperature drops by about 0.4 K.
5.2. Results Analysis
5.2.1. The Average Temperature in the Street Canyon with Different Aspect Ratios
It can be seen from Figure 16 that the average temperature at the inner surface of the canyon in
the block decreases with the increase in aspect ratio. The average ground temperature drops by 0.2 K
with every 1 m increase in height; that is, with the aspect ratio of 1 as the benchmark, each time the
aspect ratio increases by 100%, the average temperature drops by about 0.4 K.
5.2.1. The Average Temperature in the Street Canyon with Different Aspect Ratios
It can be seen from Figure 16 that the average temperature at the inner surface of the canyon in
the block decreases with the increase in aspect ratio. The average ground temperature drops by 0.2 K
withEnergies
every2020,
1 m13,increase
in height; that is, with the aspect ratio of 1 as the benchmark, each
time the
6602
16 of 21
aspect ratio increases by 100%, the average temperature drops by about 0.4 K.
Average temperature
Linear Fit of "Average temperature"
32.50
y = a + b*x
Equation
Average temperature (oC)
Average temperature
Plot
32.25
32.00
Weight
No Weighting
Intercept
32.64198 ± 0.09835
Slope
–0.42964 ± 0.03591
0.0129
Residual Sum of Squares
–0.99308592536010
Pearson's r
31.75
R-Square (COD)
0.98622
Adj. R-Square
0.97933
31.50
31.25
31.00
30.75
30.50
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Street aspect ratio (height/width)
Figure 16. Variation of the inner surface temperature with aspect ratio.
Energies 2020, 13, x FOR
PEER
Figure
16.REVIEW
Variation of the inner surface temperature with aspect ratio.
16 of 20
5.2.2. The Variation of Inner Surface Temperature in a Day
shown in Figure
17, in the morning
and evening
hours, the surface temperature of the block
5.2.2. TheAs
Variation
Inner
Temperature
in a hours,
Day
As
shown in of
Figure
17,Surface
in the morning
and evening
the surface temperature of the block (the
(the average value of the west wall, ground, and east wall) is relatively low for all cases. However, at
average value of the west wall, ground, and east wall) is relatively low for all cases. However, at noon,
noon, the temperature difference between the different aspect ratios is relatively high. When the
the temperature difference between the different aspect ratios is relatively high. When the aspect ratio
aspect ratio is 4, compared to an aspect ratio of 1, the average temperature is about 1.3 K higher. For
is 4, compared to an aspect ratio of 1, the average temperature is about 1.3 K higher. For all 12 h,
all 12 h, the average difference between the two adjacent aspect ratios is around 0.4 K. It should be
the average difference between the two adjacent aspect ratios is around 0.4 K. It should be noted that
noted that there should be a time lag for surface temperature if the heat storage effect of the wall
there should be a time lag for surface temperature if the heat storage effect of the wall material is taken
material is taken into account.
into account.
Aspect ratio = 4
Aspect ratio = 3
Aspect ratio = 2
Aspect ratio = 1
Average temperature of surfaces (°C)
36
35
34
33
32
31
30
29
28
27
26
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Time (o'clock)
Figure17.
17.Variation
Variationof
ofthe
thehourly
hourly average
average temperature
temperature under
under different
different aspect ratios during the day.
day.
Figure
5.2.3. The Temperature Differences between the West Wall, Street Ground, and East Wall
It can be observed from Figure 18 that the ground temperature of the street at noon is obviously
higher than that of the east wall and the west wall, which is caused by the direct solar radiation on
the ground. However, the ground temperature is the lowest in the morning and afternoon, which is
due to the shading effect from the walls on both sides of the street.
Energies 2020, 13, 6602
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5.2.3. The Temperature Differences between the West Wall, Street Ground, and East Wall
It can be observed from Figure 18 that the ground temperature of the street at noon is obviously
higher than that of the east wall and the west wall, which is caused by the direct solar radiation on the
ground. However, the ground temperature is the lowest in the morning and afternoon, which is due to
the shading effect from the walls on both sides of the street.
Figure 18. Variation of the hourly average temperature on the west wall, street ground, and east wall,
with an aspect ratio of 3.
The temperature on the west wall surface was the highest in the morning, which is caused by
direct solar radiation from the east in the morning, as shown in the right schematic diagram of Figure 13.
In the afternoon, the surface temperature of the east wall was the highest, which is caused by solar
radiation from the west. For the other three aspect ratios (1, 2, and 4), the same temperature trend can
be observed.
5.2.4. The Influence of Incoming Wind Speed on Wall Surface Temperature
It is assumed that that the wind speed at the pedestrian height of 1.5 m increases from 0.5 m/s to
1 m/s and to 1.5 m/s, and the air temperature is 300 K at 3:00 pm on 25 August. As shown in Figure 19,
for every 1 m/s increase in wind velocity, the surface temperature of the west wall, ground, and east
wall drop by an average of 2.4 K. This indicates that increasing the wind speed can effectively reduce
the wall temperature in the street canyon.
It is assumed that that the wind speed at the pedestrian height of 1.5 m increases from 0.5 m/s to
1 m/s and to 1.5 m/s, and the air temperature is 300 K at 3:00 pm on 25 August. As shown in Figure
19, for every 1 m/s increase in wind velocity, the surface temperature of the west wall, ground, and
east wall drop by an average of 2.4 K. This indicates that increasing the wind speed can effectively
Energies 2020, 13, 6602
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reduce the wall temperature in the street canyon.
y = a + b*x
Equation
West wall
Street ground
Intercept
33.5897 ± 0.34163
33.34764 ± 0.37738
33.91881 ± 0.32574
Slope
–2.39512 ± 0.31628
–2.49067 ± 0.34938
–2.33142 ± 0.30157
0.05002
0.06103
0.04547
Plot
34
Average temperature of surfaces (oC)
Residual Sum of Squares
Pearson's r
East wall
No Weighting
Weight
–0.99139333501358 –0.99030405533768 –0.991737609412082
R-Square (COD)
0.98286
0.9807
0.98354
Adj. R-Square
0.96572
0.9614
0.96709
33
West wall
Street ground
East wall
Linear Fit of "West wall"
Linear Fit of "Street ground"
Linear Fit of "East wall"
32
31
30
29
28
0.5
1.0
1.5
Wind velocity (m/s)
Figure
The average
average temperature
temperature of
of each
each inner
Figure 19.
19. The
inner surface
surface at
at different
different wind
wind velocities.
velocities.
6. Conclusions
6. Conclusions
The Cold-Lane in central China within a hot-summer and cold-winter region can make pedestrians
The Cold-Lane in central China within a hot-summer and cold-winter region can make
feel cool during a hot summer. The following conclusions can be made:
pedestrians feel cool during a hot summer. The following conclusions can be made:
1.
The air temperature difference between the inside and outside of the lane is less than 2 ◦ C,
1. The air temperature difference between the inside and outside of the lane is less than 2 °C, and
and the difference in relative humidity and air velocity are less than 10% and 0.5 m/s, respectively.
the difference in relative humidity and air velocity are less than 10% and 0.5 m/s, respectively.
The small differences indicate that they are clearly not the factors producing the cold sensation
The small differences indicate that they are clearly not the factors producing the cold sensation
felt by pedestrians.
felt by pedestrians.
2.
The highest
highestsurface
surfacetemperature
temperatureofofthe
the
inner
wall
is less
than
39 ◦which
C, which
is relatively
2. The
inner
wall
is less
than
39 °C,
is relatively
low. low.
The
The
walls
and
ground
are
cooled
by
radiative
heat
dissipation
and
ventilation
at
night.
walls and ground are cooled by radiative heat dissipation and ventilation at night.
3.
Based on
on the
the calculation
calculation of
of the
the specific
specific four
four cases
cases at
at 3:00
3:00 pm,
3. Based
pm, it
it is
is observed
observed that
that the
the proportion
proportion
of
sensible
heat
dissipation
is
small,
and
radiative
cooling
between
the
human
and
of sensible heat dissipation is small, and radiative cooling between the human bodybody
and lowlow-temperature
surfaces
of the
inner
wall
plays
important
role
coolingdown
downthe
thepedestrian.
pedestrian.
temperature
surfaces
of the
inner
wall
plays
an an
important
role
inincooling
This is similar to the conclusion reached by Rosso et al. [9] that pedestrians are sensitive to lower
surface temperatures.
4.
Under the conditions of this study, for every 1 m increase in the height of the walls on both sides,
the average temperature throughout the day decreases by 0.2 K. When the wind velocity across the
street increases by 0.5 m/s, the average surface temperature drops by 1.2 K. Thus, increasing the
aspect ratio of the street canyon and enhancing natural ventilation can both effectively reduce the
temperature of the inner surface of the street and improve the thermal comfort for pedestrians.
It should be noted that there are some limitations with this study. Due to the limited conditions,
this article is only a preliminary investigation. In addition, the quantity of data collected is insufficient
and many parameters were assumed during the calculation process. Firstly, the air temperature and
wall temperature were surveyed for only three days in the street canyon. Secondly, a large number of
Energies 2020, 13, 6602
19 of 21
assumptions were made in the calculation of pedestrian heat dissipation, e.g., in Section 4, the human
body and clothing were regarded as a whole. Thirdly, in the CFD simulation, the model was also
simplified, and only the situation on 25 August was analyzed. However, due to the similarity between
the field investigation and simulation in the flow and temperature fields, similar trends were exhibited
in the two sets of results, which revealed the basic characteristics of the Cold-Lane. More detailed
investigation of thermal comfort and CFD simulation in the street will be carried out in the future.
Author Contributions: Y.L., H.C., W.Y., M.K. and G.Z. contributed to the conception of the study, the development
of the methodology. Y.W. and X.C. developed computer models, and simulated and analyzed data. W.Y. and Y.L.
contributed to the whole revision process. X.L., G.Z., Y.L., Y.W. and G.Z. wrote the manuscript. All authors have
read and agreed to the published version of the manuscript.
Funding: This research was funded by Machinery Industry Innovation Platform Construction Project: Key
Laboratory of Healthy Environment and Energy Conservation of Large-space Plants (Grant No. 2019SA-10-07),
International Science and Technology Cooperation Program of China grant number 2014DFA72190, 201413370083,
201513370014, China Scholarship Council, Natural Science Foundation of China, grant number 51308206, China
Postdoctoral Science Foundation, Natural Science Foundation of Hunan, grant number 201631440729, IEA-EBC
Annex 62–Ventilative Cooling, and Annex80–Resilient Cooling.
Acknowledgments: Thanks for the work from Quan Tu, Liang Wei, Dong Cao and Qu Ye.
Conflicts of Interest: The authors declare no conflict of interest.
Nomenclature
human body surface area, taken as 1.78, m2
gravity of acceleration, m/s2
Grashof number, dimensionless
sensible convection heat transfer coefficient, W/(m2 ◦ C)
characteristic length taken as human height, assumed as 1.78 m
heat dissipation due to metabolism, W
metabolic rate, W/m2
Nusselt number, dimensionless
Prandtl number, dimensionless
sensible heat transfer rate of a human body, W
radiant heat flux, W
heat storage of human body, W.
air dry-bulb temperature, ◦ C
surface temperature of clothes, ◦ C
equivalent human body surface temperature, ◦ C
skin temperature, respectively, ◦ C
mean surface temperature of wall in the street, ◦ C
mechanical output of human body, W
expansion factor, equal to 1/(273 + tm ), in which tm is (th + ta )* 1/2, ◦ C−1
emissivity of human body, taken as 0.85
thermal conductivity of the air, W/(m K)
blackbody radiation constant from law of Stefan-Boltzmann, taken as 5.67×10−8 W/m2 ·K4
kinematic viscosity of the air, taken as 16.5 × 10−6 m2 /s
temperature difference between ambient air and human body, equal to (th −ta ), ◦ C
A
g
Gr
hc
l
M
m
Nu
Pr
Qc
Qr
S
ta
tcloth
th
tskin
tw
W
α
ε
λ
σ
υ
∆t
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