Convective heat transfer from a
spherical particle suspended in air
EGEE 520 Term Project
Nari Soundarrajan
Background
2

Fluidized bed combustion (FBC) Ash
Management

Heat transfer to air and cooling of ash

Air flow (pressure drop), particle size
distribution of ash
Nari Soundarrajan EGEE 520 Fall 2005
Problem



3
Isolated hot particle
falling down in a
counter current of air
Particle falls down
slowly at terminal
velocity
Spherical particle
assumption
Nari Soundarrajan EGEE 520 Fall 2005
Energy Balance




4
Radiation exchange is considered εash ~ 0.95
ICs and BCs: Match conduction to convection in the air
boundary layer; hair averaged over the temperature range.
t=0: Particle temperature Tash=1473K, conduction in air is
negligible along flow axis.
Assume conduction inside particle is fast compared to
convection at boundary (lumped capacitance)
Nari Soundarrajan EGEE 520 Fall 2005
Formulation

3D model of “spherical” particle
in an air cylinder

Convective Heat Transfer
calculations for sphere in
immersed flow
using Nu = hR/kair
obtained k using
standard relations.

Cpair= 1005 (J/kg.K) at 298K
= 1090 (J/kg.K) at 1000K

5
Kair = 0.026 (W/m.K) at 298K
= 0.068 (W/m.K) at 1000K
Nari Soundarrajan EGEE 520 Fall 2005
FEM Solution
2 D Solution
6
3 D Solution
Nari Soundarrajan EGEE 520 Fall 2005
Results – 2D
r = R,
7
r = 10R
r = 20R
Nari Soundarrajan EGEE 520 Fall 2005
Results – 3D
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Nari Soundarrajan EGEE 520 Fall 2005
Validation: Absence of convection
x axis at z=1
x axis at z=0.98
along…
flux
Conductive
Biot no
hairR/kash < 0.1
Z axis at x=0, y=0;
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Nari Soundarrajan EGEE 520 Fall 2005
Findings & Future Work
10

Particle size (diameter greatly increases) localized
temperature gradient and downstream convection

Radiation effects are significant

Multiple particle interaction needs to be evaluated

Effect of temperature air convection properties to be
evaluated thoroughly.
Nari Soundarrajan EGEE 520 Fall 2005
Acknowledgements


Dr. Elsworth for starting me off and the feedback
Peter Rozelle (DOE) for information on sphericity
and FBC parameters.
Key References
Weinell, C.E., DamJohansen, K. and Johnsson, J.E., 1997, "Single-particle
behaviour in circulating fluidized beds", Powder Technology, 92 (3), 241-252.
Mihalyko C., Lakatos B.G., Matejdesz A. and Blickle T., “Population balance model
for particle-to-particle heat transfer in gas-solid systems,” International Journal of
Heat and Mass Transfer, 47(6), pp. 1325-1334, 2004.
Bird, Stewart, Lightfoot, “Transport Phenomenon”, [Eastern Ed. Reprint 1994], John
Wiley and Sons, Singapore, pp. 409, 1960.
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Nari Soundarrajan EGEE 520 Fall 2005