Thermocouple-Induced Perturbations
of the Flame Structure
P.A. Skovorodko*, A.G. Tereshchenko, D.A. Knyazkov, A.A.
Paletsky, O.P. Korobeinichev
* Institute of Thermophysics, Novosibirsk, 630090, Russia
Institute of Chemical Kinetics and Combustion, Novosibirsk,
630090, Russia
Background
• Measuring flame thermal structure with
appropriate accuracy is one of the most
important problems in combustion
research.
• Thermocouples
are
the
most
convenient and simple tool for
temperature measurements in flames of
gas and condensed systems.
Accuracy of thermocouple measurements
• It is well known that the accuracy of the
thermocouple measurements in the flame zone with
high temperature gradient is mainly influenced by
the following factors: (1) the thickness of
thermocouple, which leads to averaging the
temperature in the space occupied by the
thermocouple, (2) heat losses into the ends of the
thermocouple and (3) radiation heat losses at
temperatures higher than 1000 К.
• Taking into account the above mentioned factors,
accuracy of thermocouple measurements in the
flames declared in the majority of studies is about of
± 30-50 К.
Motivation
• In our laboratory, for many years temperature
profiles
in
one-dimensional
burner-stabilized
premixed flames at atmospheric pressure have been
measured by thermocouples and calculated using
the PREMIX code from the CHEMKIN II package with
the application of a detailed mechanism of chemical
reactions for each flame. It was found out that the
temperature distributions in the flames, obtained by
thermocouples, exceeded calculated values in the
area with temperature gradient and in the zone of
maximum concentration of radicals.
2000
T, K
Fig. 1. The methan
flame.
1–Т* experiment.
(Pt/Pt+10%Rh
with layer of SiO2,
circular 30 μm).
2 - ТСН4 – расчёт.
1750
1500
1
1250
2
1000
2
750
500
250
0
0.0
0.5
1.0
1.5
Distance from burner, mm
2.0
The first zone
from 0.1 to 0.5
mm.
The second
zone from 0.6 to
2 mm
Effect investigated. From 0.1 to 0.5 mm
• Circular and ribbon thermocouples present a certain
obstacle for the gas flow. It is usually assumed that,
due to a small size, the thermocouple produces
negligible perturbations of the flame structure. We
found out that the influence of this effect can be
significant. In the case of large mass flows in the
flame, which take place in combustion of condensed
systems, this effect becomes very significant.
• Goal of the work.
• To study the disturbances of gas flow, caused by
presence of the thermocouples in the flame, and to
define how these disturbances influence on the
results of thermocouple measurements. This effect
was studied both experimentally and numerically in
the gas flame at a pressure of 1 atm, and purely
numerically for the combustion of a condensed fuel
at pressure 20 atm.
Experimental Technique
• We
studied
the
temperature
profile
in
premixed
methane/oxygen/argon (CH4/O2/Ar – 6/15/79 vol. %) flame stabilized
on a flat burner at atmospheric pressure.
• The burner top was a porous brass disk 16 mm in diameter and 5
mm high. It was made of sintered spheres ~0.1 mm in diameter. The
relative porosity of the burner disk was ~ 40%. The surface
temperature of the burner was kept at 368 K. The flow rate of the
unburned mixture was 25 cm3/s. Temperature profiles were
measured using three types of Pt/Pt+10%Rh thermocouples coated
with a thin layer of SiO2 (~ 2–3 μm). One of the thermocouples
(TC1) had a circular cross section with a diameter of about 24 μm.
The other two thermocouples had rectangular cross sections, and
their dimensions were 10x110 μm (TC2) and 20x125 μm (TC3). The
error of the thermocouple measurements was within ±30 К.
Experimental uncertainty is determined from the scatter of
experimental data of 4–5 temperature profiles.
Thermocouple was located in parallel to the surface of
the burner
Photographs of thermocouples
TP1circular
а)
Workingучасток
section TP2-3
Рабочий
ТП2-3
б)
TP2 and TP3 ribbons
Ò, Ê
1800
1600
1400
1200
1
1000
2
800
600
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Distance from burner, mm
Fig.4.
• Temperature
profiles in the
methane flame.
1- primary
experimental
data: ТTC1
profiles without
correction for
radiation heat
losses.
• 2- Tup
(unperturbed
flame, PREMIX),
Ò, K
1800
• Fig.5.
• Temperature
profiles in the
methane flame.
1- primary
experimental
data: ТTC3 profiles
without
correction for
radiation heat
losses.
1600
1400
1200
1
1000
800
2
600
0.0
0.2
0.4
0.6
0.8
1.0
Distance from burner, mm
1.2
2- Tup
(unperturbed
flame, PREMIX),
Ò, Ê
2000
1800
1600
1400
1200
1000
800
600
400
200
2 3
1
1400
2
1200
1
1000
0,5
4
800
4
0,0
3
0,2
1,0
1,5
0,3
2,0
Distance from burner, mm
0,4
2,5
3,0
Fig. 6.
Temperature
profiles in the
methane flame.
Curves 1, 2,
and 3:
smoothed
experimental
temperature
profiles with
correction for
radiation heat
losses,
1 - ТTC1,
2 - ТTC2,
3 - ТTC3.
Curve 4 - Тup,
300
• Fig. 7.
• Curves 1, 2, and 3:
thermocouplemeasured
temperature
elevation above the
unperturbed flame
temperature.
250
2
200
150
100
50
2
1
3
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
•
•
•
1 – ΔТ = T*ТC1 - ТСН4,
2 – ΔТ = Т*ТC2 - ТСН4,
3 – ΔТ = Т*ТC3 - ТСН4.
Distance from burner, mm
Exceedings at a distance of 0.2 mm reach the values of 130 -150 K,
which is considerably higher than the accuracy of measurements
(±30K).
Numerical Simulation of the Flow
• The plane external flow field over a thermocouple inserted in the
flow at some distance from the burner was simulated using the full
set of unsteady Navier–Stokes equations. A finite-difference
representation of the governing equations was made on a staggered
grid, which allowed the development of an effective algorithm for
simulating viscous flows. An approximate allowance for the heat
release due to chemical reactions was made by adding a source
term to the energy equation to provide a given temperature
distribution in the plane unperturbed isobaric flame, i. e., with no
thermocouple in the flame. The simulation domain was a rectangle
with dimensions of 2.4 mm in the longitudinal direction (x) and 1.5
mm in the transverse direction (y) with mesh sizes of the grid dx =
dy = 2.5 m. The initial distribution of the parameters in the
simulation domain was assumed to be the same as in the
undisturbed flame.
• In the presence of the thermocouple, this distribution is
violated due to the presence of solid surfaces in the flow
field and heat transfer between the gas and the
thermocouple surface. As a result, the flow begins to
change, and after some time, it reaches a steady state.
Calculations were performed in connection with the flame
of methane, which was used in our experiments, and also
to the flame of hexogen at a pressure 20 atm.
• The heat release was assigned with the aid of the onedimensional fixed sources, or with the aid of the source
term which value was determined by the local temperature
in the flow. The latter approach seems to be more adequate
from the point of view of the description of the nonlinear
effects of the mutual influence of chemical and gas
dynamic processes. The temperature distribution in the
undisturbed flame is reproduced well within the framework
any of these two approaches.
Temperature deviation in the flame of
methane at a pressure 1 atmosphere
T-Tup, K
Fig. 8. Deviation
numerical
200
1
100
(curves 1 – 4) and
experimental (curve
5)
temperature
profiles
from
unperturbed data
2
0
3
-100
1 - Q(T),  = 0;
5
2 - Q(x),  = 0;
-200
3 - Q(T),  = (T);
4
4 - Q(x),  = (T);
-300
0,0
of
0,5
1,0
1,5
Distance from burner, mm
2,0
5 – TTC3 – Tup.
ε – actual emissivity of
the thermocouple.
T(0) - T(), K
250
4
200
2
3
150
1
100
50
0
0,0
0,5
1,0
1,5
Distance from burner, mm
• Fig. 9. Data for heat
losses from
thermocouple TC3
due to radiation.
Numerical (curves 1,
2), and semiempirical (curves 3,
4)
• 1 - model with Q(T),
• 2 - model with Q(x),
2,0
• 3 – [Kaskan],
• 4 – [Wong].
40
30
400
200
0
0
200
10
20
20
y,m
50
60
70
600
400
600
Distance from burner, m
• Fig. 10.
Longitudinal
velocity field
(cm/s) for flow
around
thermocouple
TC3 predicted
by the model
with Q(T) and
 = 0.
• Fig. 11.
Isotherm field
(K) for the
same
conditions as
in Fig. 10.
1500
1200
900
400
600
y, m
600
200
0
0
200
400
Distance from burner,
600
m
Essence of the effect
• Thermocouple forms the deceleration zone and at
the same time it reflects the temperature in this
perturbed zone. Since the thermocouple stands in
the gas flow with the intensive heat release due to
the chemical reactions, in the region of the
thermocouple position is disrupted the balance
between heat release and removal of heat due to the
draining of products from the burning surface, and
also due to thermal conductivity. The disturbance of
this balance, typical of the undisturbed flame, leads
to the field distortion of isotherms in the vicinity of
the thermocouple.
The second zone of overheating
at a distance from 0.6 to 2 mm
2000
T, K
Fig. 1. The methan flame.
1–Т* experiment.
(Pt/Pt+10%Rh with
layer of SiO2, circular
30 μm).
2 - ТСН4 – расчёт.
1750
1500
1
1250
2
1000
2
750
500
250
0
0.0
0.5
1.0
1.5
Distance from burner, mm
2.0
The first zone from
0.1 to 0.5 mm.
The second zone
from 0.6 to 2 mm
Comparison of the zone of radicals with the second
region of overheating thermocouple
o
l
e
f
r
a
c
t
i
o
n




 M
5
0
0
0
,
0
0
3
Fig. 12. Elevation
thermocouple4
0
0
measured temperature
Î
Í
0
,
0
0
2
above the unperturbed
3
0
0
flame temperature
Í
(curves 1, 2, and 3) and
Н and ОН are radical
2
0
0
0
,
0
0
1
concentration profiles
2
in the unperturbed
1
0
0
3 flame.
1
2
0
0
,
0
0
0
1 - (TTC1- Tup ),
2 - (ТTC2 – Тup),
0
,
0
0
,
5
1
,
0
1
,
5
2
,
0
2
,
5
3
,
0
3 - (TTC3-Tup).
D
i
s
t
a
n
c
e
f
r
o
m
b
u
r
n
e
r
,
m
m
The effect of radical recombination on the
surface of quartz film on the thermocouple
• The number of radical collisions (N) per unit thermocouple
surface per unit time was calculated from the H and OH
concentration profiles. The heat flux to the thermocouple was
obtained using the known values of heat release (Н*) in a single
recombination event on the surface. A similar approach to
accounting for heat release on the thermocouple surface due to
the recombination of H radicals on this surface. The heat flux
data were transformed to temperature data by the procedure
used above to calculate the radiation correction:
•
T = Tc - Tg = [1.25d3/4 К·N·Н*·(/v)1/4]/ ,
• where К - is probability of radical recombination. In order that
the observed second maximum of the temperature elevation
may be explained by the radical recombination effect on the
thermocouple surface, the probability of recombination per
collision must be in the range from 10-3 to 10-2, which seems
quite realistic [Rubtsov and other J. Physical Chem. A, 2009, T.
83, № 10, pp. 1701-1704] for the anticatalytic SiO2 layer used.
Protection of the thermocouple by anti-catalytic coating
•
•
Despite the fact that thermocouple is covered with
the anti-catalytic layer SiO2, due to the
recombination of the radicals of flame on the
surface of treatment, in the region of the maximum
of the concentration of radicals the significant
quantity of heat, which influences the indications
of thermocouple, can be separated.
Anti-catalytic coating does not guarantee 100% of
protection of thermocouple from the catalytic
reactions on its surface. Depending on the type of
coating, some catalytic effect with the heat
emission on the surface of thermocouple can
occur and in each specific case this must be borne
in mind.
RDX flame at 20 atm
•
Disturbances of thermal structure of the flame with high mass flow
rate, which is typical for combustion of CF at elevated pressure. To
study the influence of thermocouple shape on the temperature
profile, a flame of hexogen at 20 atm with the mass flow of 0.83
g/cm2·s [ Zenin, J. Prop. Pow., 1995, v.11, N.4, p.752-758] was
selected. Temperature profile in this flame was considered as
undisturbed (TRDX). The effect of disturbance by the thermocouple
of the flow structure, in which it is located, was studied for the
thermocouples of two forms: ribbon, with a cross-section of 3x60
μm - TP4, which was used in the work of Zenin, and that close to
square shape with a cross-section of 13x14 μm - TP5, which in
some approximation imitated round thermocouple.
Т, T, К
• Fig. 13. Temperature
profiles in the flame
of hexogen
calculated for two
thermocouples of
various forms with
similar crosssection. 1- TRDX; 2Q (T), 3- Q (x) for
TTP4 (3×60 μm ); 4- Q
(T) for TTP5 (13×14
μm); 5-7 - ∆T2-4,
respectively.
3000
3
2500
4
2000
1
1500
1000
2
5
500
6 7
0
0
10
20
30
40
Distance from burner, m
50
60
Vortex in the region after the thermocouple
Y, m
100
80
60
40
20
0
0
20
40
60
80
Distance from burner, m
100
Fig.14. Field
of isotherms
(K) in the
RDX flame
for the
thermocoupe
with a crosssection of
3x60 μm.
2400
120
2100
2400
1800
1500
60
1200
900
30
2700
Y, m
90
0
0
10
20
30
40
50
Distance from burner, m
60
• Fig.15. Field of
isotherms (K)
in the RDX
flame for the
thermocouple
with a crosssection of 3x60
μm.
Conclusions
•
•
•
1. The temperature profiles in the methane flame measured by
thermocouples of that covered with the anti-catalytic coating (film
SIO2) were found to be higher than the profile calculated by
Chemkin in the unperturbed flame in the region with a high
temperature gradient and in the region with maximum
concentrations of H and OH radicals.
2. The simulation of the arrangement of thermocouple in the flame
in the region of temperature gradient also confirmed the presence
of the effect of overheating thermocouple in this zone of flame.
This overheating can lead to an increase in the measured by
thermocouple gradient in comparison with its value in the
undisturbed flame.
3. Deceleration of the flow of the reacting gases in the
environment of the thermocouple in the region of temperature
gradient leads to a local increase in the heat release as a result of
the chemical reactions and the corresponding increase in the
temperature of thermocouple relative to unperturbed values.
Conclusions
•
•
•
4. The magnitude of effect of overheating grows with an increase of
the mass flow rate of gas in the flame and the width of
thermocouple. Ribbon thermocouples lead to the more essential
overstating of the measured temperature, than round with the same
cross-sectional area of the thermocouple.
5. At a pressure of 20 atmospheres the model of the disturbed flow
of the products of combustion of hexogen near the thermocouple
predicts overstating 1.5 - 2 times of the gradient of the measured
temperature in comparison with the same in the undisturbed flame.
There are foundations for assuming that the value of this
overstating will be grow with an increase of the pressure.
6. The developed model of perturbed flow around a thermocouple
predicts more intense heat transfer between the thermocouple and
chemically nonequilibrium reactive flow compared to the same
process in chemically inert flow (air).
Thanks for the attention!
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concerning the report.