International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 6, June 2013
ISSN 2319 - 4847
Local heat transfer coefficient around a
horizontal heating element in
gas-solid fluidized bed
Anusaya M. Salwe
University of pune,Pune
Abstract
Experiments have been carried out in a laboratory gas-solid fluidized bed heat exchanger. Heat transfer coefficient between
immersed heated tube and bubbling fluidized bed is found experimentally around a tube at different angles. An experiment is
performed with three different particle diameter and ten different superficial gas velocities. The bed particles used were Geldart
B silica sand particles of diameter 200μm, 350μm and 500μm. Fluidizing media used was atmospheric air. The experimental
results showed that the local heat transfer coefficient is maximum at the top (180 0) and minimum at the bottom ( 00) from
direction of air. Heat transfer coefficient is increased with increasing the air velocity and it is found to decrease by increasing
the particle size for Geldart B group particles. In order to predict the heat transfer coefficient from a horizontally immersed
tube to fluidized bed directly, a correlation of Nusselt number has been proposed. The correlation predictions are compared
with the experimental data of this works. It is observed that they show a good agreement with each other.
Keywords: Fluidized bed, Heat transfer coefficient, Nusselt number
1. INTRODUCTION
The key objectives of this work are to study the bubbling Gas-Solid fluidized bed in order to find the suitability of sand
as a bed material in Gas-Solid fluidized bed and to find the local heat transfer coefficient around a horizontal heating
element immersed horizontally in bubbling fluidized bed. An experiment has been carried out in a laboratory fluidized
bed heat exchanger with atmospheric air as a fluidizing medium and silica sand of diameter 200μm, 350 μm and 500
μm diameter.
Many researchers have studied the effect of various parameters like fluidizing velocity and particle diameter on the
average heat transfer coefficient between the fluidizing bed and immersed heating element, but there is considerably
less literature available on the heat transfer coefficient around the horizontal tube and fluidized bed.L.M. Armstrong, S.
Gu and K.H. Luo [2] used two-fluid Eulerian–Eulerian formulation incorporating the KTGF was applied to a tube-tobed reactor with one immersed tube and compared with the results in the literature. They showed that the heat transfer
coefficient is maximum at the top of the tube.B.Stojanovic, J. Janevski and M. Stojiljkovic [3] experimentally
investigated the heat transfer by convection between an immersed tube and the fluidized bed on the laboratory scale
apparatus. They also noted that heat transfer coefficient is maximum at the top of the tube.U.S.Wankhede,
D.D.Adgulkar [7] carried out an experiment to find the heat transfer from the immersed body in a two dimensional
fluidized bed varying the gas velocity in the range of u/umf from 5 to 15. They compared the heat transfer behavior of
Geldart group A, B, C,D particles. The simulated values of the heat transfer coefficient calculated for three positions on
the circumference of the tube at the top (90), bottom (-90) and horizontal (0).They concluded, For the Group A material
the heat transfer coefficient varies drastically with the gas velocity. The variation for the heat transfer coefficient for
Group B with u/umf is linear. For Group C, because of cohesive nature of material shows increase in heat transfer
coefficient between 5 to 10 u/umf and a decrease for upper position. They used Eulerian approach for fluid dynamics and
heat transfer in fluidized beds and Gunn model with kinetic theory of granular flow is appropriate model for modeling
heat transfer between multiple phases. Sung Won Kim, Jung Yeul Ahn, Sang Done Kim, Dong Hyun Lee [11] were
carried out an experiments in a FBHE made of transparent acrylic plate. The effect of gas velocity on the average and
local heat transfer coefficients between a submerged horizontal tube and a fluidized bed has been determined in a
fluidized-bed-heat-exchanger of silica sand particles. They measured the heat transfer coefficient around the tube
circumference by thermocouples and an optical probe. They concluded that the average heat transfer coefficient exhibits
a maximum value with variation of gas velocity, the local heat transfer coefficient exhibits maximum values at the top
of the tube (00) and the average heat transfer coefficient increases with increasing gas velocity toward a maximum
value of the coefficient.
2.EXPERIMENTAL PROGRAMME
Fig.1 shows an experimental set up. It consists of fluidizing column is of 59 mm internal diameter and 3 mm thickness
and 1 m height, made up of plexiglass to aid visualization. A fine mesh of copper is used as a distributor plate. A brass
tube of external diameter 10 mm and length 50 mm is fitted horizontally inside the column. The axis of brass tube is
Volume 2, Issue 6, June 2013
Page 344
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 6, June 2013
ISSN 2319 - 4847
perpendicular to the axis of fluidizing column. A Cartridge heater of 6 mm outer diameter and 45 mm long is inserted
inside the brass tube. Axis of brass tube is at the distance of 185 mm from the distributor plate. Fig 2 shows brass tube
and cartridge heater. Fixed bed height was 225 mm. A constant heat input of 18.2W is given to the heater. Two nylon
plugs of thickness 3mm each are fixed at the ends of brass tube to minimize the axial losses. It is assumed that heat
flow is in only radial direction of the brass tube. All the readings are taken at the steady state. For one sample size and
one velocity time taken was two hours.
1-Fluidizing column 2-Brass tube 3-Bed particles 4-Distributor plate 5-Orificemeter
6-Centrifugal blower 7-Inlet air flow 8-Manometer
Fig 1 : Experimental Set-up
2.1 Instrumentation:
A variable transformer and ammeter is used to give require input to the cartridge heater. Total Eight PT100thermocouples are used in the experimentation. Three of which are used to measure surface temperature on the
circumference of the tube at different angles. Four thermocouples are used to measure bed temperature. One
thermocouple is used to measure ambient temperature.Fig.3 shows position of Thermocouples. A blower of the capacity
1.5 m3/min. is used to supply compressed air to the fluidizing column through diffuser and distributor plate. A butterfly
valve mounted over an inlet pipe allows regulating the rate of flow of air. A differential manometer is connected across
an orificemeter. Readings of the manometer is calibrated to get the velocity of the inlet air. Another differential
manometer is connected to the fluidizing medium to find the pressure drop across the column.
Fig. 2: Brass tube and Cartridge heater
7
7
5
3
3
2
5
4
2
, 4 , 6
6
25
1
1
5
5
5
Fig.3: Position of Thermocouples
180
270
90
0
A ir F lo w
Fig.4: Side view of the brass tube showing angles at which temperature readings are taken
Volume 2, Issue 6, June 2013
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 6, June 2013
ISSN 2319 - 4847
2.2 Bed material: Bed/Particulate material-The bed material used in this work was silica particles that can be classified
as Group B according to Geldart’s classification. Standard Sieve analysis is carried out in laboratory for taking the bed
sample size of silica sand. Density of sand is measured directly using an Archimedes principle. Properties of bed
material are given in Table 1.
Table 1: Properties of bed material
Bed material
Sand 1
Sand 2
Sand 3
Properties
Dp (μm)
200
350
500
2600
2600
2600
Umf (m/s)
0.037
0.11
0.22
Umb (m/s)
0.047
0.0270
0.01890
Ut
3.63
11.12
22.69
ψ
0.6
0.6
0.6
εmf
0.49
0.49
0.49
ρ
3
(Kg/m )
(m/s)
3 RESULTS:
Fig.5: Distribution of Heat transfer coefficient at different angle and Dp=200 μm
Fig.6: Distribution of Heat transfer coefficient at different angle and Dp=350 μm
Fig.7: Distribution of Heat transfer coefficient at different angle and Dp=500 μm
Volume 2, Issue 6, June 2013
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 6, June 2013
ISSN 2319 - 4847
4 CONCLUSION:
1) Silica sand is having good fluidizing properties.
2) Heat transfer coefficient between in immersed heated tube to the fluidized bed of silica is measured experimentally at
different angles and from fig.5, 6 and 7, it is observed that it is maximum at 180 0 and minimum at 00.
3) Heat transfer coefficient for the granular material like Silica of the size range 200µm to 500µm (Geldart B) ranges
from 218 W/m2K to 692 W/m2K.
4) Heat transfer coefficient increases with decrease in particle diameter.
5) From the experimental data and linear regression method, a Correlation is proposed to find Nusselt number directly.
Nu = 123.4189507 Re0.038620
The Nusselt number calculated from experimentation and from correlation shows a good agreement with each other
and gives 15 % error.
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 6, June 2013
ISSN 2319 - 4847
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Author
Anusaya Salwe received her B.E.from Dr. Babasaheb Ambedkar marathwada university,Aurangabad in
2005.She is perusing her M.E. Heat Power from university of Pune. Her research interests include fluidized
bed heat transfer and fluidized bed combustion. She has seven years of teaching experience to UG students.
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