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Title: Convective heat transfer from a spherical particle suspended in air


1
Convective heat transfer from a spherical
particle suspended in air
  • EGEE 520 Term Project Nari Soundarrajan

2
Background
  • Fluidized bed combustion (FBC) Ash Management
  • Heat transfer to air and cooling of ash
  • Air flow (pressure drop), particle size
    distribution of ash

3
Problem
  • Isolated hot particle falling down in a counter
    current of air
  • Particle falls down slowly at terminal velocity
  • Spherical particle assumption

4
Energy Balance
  • Radiation exchange is considered eash 0.95
  • ICs and BCs Match conduction to convection in
    the air boundary layer hair averaged over the
    temperature range.
  • t0 Particle temperature Tash1473K, conduction
    in air is negligible along flow axis.
  • Assume conduction inside particle is fast
    compared to convection at boundary (lumped
    capacitance)

5
Formulation
  • 3D model of spherical particle in an air
    cylinder
  • Convective Heat Transfer calculations for sphere
    in immersed flowusing Nu hR/kair obtained k
    usingstandard relations.
  • Cpair 1005 (J/kg.K) at 298K 1090
    (J/kg.K) at 1000K
  • Kair 0.026 (W/m.K) at 298K 0.068
    (W/m.K) at 1000K

6
FEM Solution
2 D Solution
3 D Solution
7
Results 2D
r R, r 10R r 20R
8
Results 3D
9
Validation Absence of convection
x axis at z1
x axis at z0.98
along flux Conductive
Biot no hairR/kash lt 0.1
Z axis at x0, y0
10
Findings Future Work
  • 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.

11
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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