A radiative heat transfer problem

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Discussion Overview

The discussion revolves around a radiative heat transfer problem involving a firefighter facing a large fire. Participants explore the calculation of net radiative heat fluxes on the firefighter's front and back, considering various factors such as view factors, emissivity, and the configuration of surfaces involved in the heat exchange.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a homework statement involving a firefighter and a fire, seeking help with identifying the view factor.
  • Multiple participants seek clarification on the concept of a semi-infinite surface, with one suggesting it extends infinitely in width and height, while another emphasizes the importance of thickness in the context of gas radiation.
  • There is a discussion about determining the configuration factor between the firefighter and the ground, with one participant expressing uncertainty about how to treat the sky in this context.
  • One participant suggests that the firefighter's side exposed to the flames and ground has a shape factor of unity, while the other side only sees the ground and sky.
  • Another participant proposes using a textbook for shape factor formula tables and discusses the geometry involved in calculating shape factors.
  • There is mention of heat exchange with the ground affecting the firefighter's front, with one participant noting that parameterized equations predict a shape factor of 0.5 for the flames.

Areas of Agreement / Disagreement

Participants express differing views on the importance of thickness in radiation calculations and the determination of view factors. There is no consensus on the exact values of shape factors or the treatment of the sky, indicating that multiple competing views remain.

Contextual Notes

Some participants reference the need for specific formulas and tables for shape factors, indicating potential limitations in their current understanding or resources. The discussion also highlights the complexity of the problem, with unresolved aspects regarding the configuration factors and assumptions about the surfaces involved.

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Homework Statement



A fire fighter (approximated by a two-sided black surface at 310 K 180 cm long and 40 cm wide) is facing a large fire at a distance of 10 m (approximated by a semi-infinite black surface at 1500 K). Ground and sky are at 0 °C (and may also be approximated as black). What are the net radiative heat fluxes on the front and back of the fire fighter?

Homework Equations



q = ε.σ.T4.A.Fij

The Attempt at a Solution



I have problem identifying the view factor in this case. Can anyone help me? Thanks.
 
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What is a semi-infinite surface dimensionwise?
 
LawrenceC said:
What is a semi-infinite surface dimensionwise?

my understanding is the wall is extending to infinity both in width and in height. But as the wall is standing on the ground, that's where the 'semi' comes from. By the way, the thickness doesn't make a difference as radiation is mainly concerned about surface.
 
"By the way, the thickness doesn't make a difference as radiation is mainly concerned about surface."

Actually thickness is quite important when it is a sheet of flame (gas radiation) with no solid wall behind it. The emissivity is a function of flame thickness. CO2 and H20 are good emitters.

Can you determine the configuration factor between the fireman and the ground in front of him, then use factor algebra to determine the factor for the flame?
 
LawrenceC said:
"By the way, the thickness doesn't make a difference as radiation is mainly concerned about surface."

Actually thickness is quite important when it is a sheet of flame (gas radiation) with no solid wall behind it. The emissivity is a function of flame thickness. CO2 and H20 are good emitters.

Can you determine the configuration factor between the fireman and the ground in front of him, then use factor algebra to determine the factor for the flame?

I'm sorry but I don't know how to determine the configuration factor between the fireman and the ground. I was also wondering how to treat the sky?
 
One side of him is exposed to the flames and ground, both of which are semi-infinite planes. The other side sees only the ground and sky so its shape factor would be unity.

Do you have a textbook that provides shape factor formula tables for differing geometries? For instance, a small area dA (infintesimal) at a distance from a finite area either looking directly at it (flame) or looking 90 degrees from it (ground). Dimensions of the finite rectangle would be 'a' wide and 'b' long. The distance dA from the rectangle would be 'c'. The shape factors are then provided in terms of ratios of the dimensions.
 
LawrenceC said:
One side of him is exposed to the flames and ground, both of which are semi-infinite planes. The other side sees only the ground and sky so its shape factor would be unity.

Do you have a textbook that provides shape factor formula tables for differing geometries? For instance, a small area dA (infintesimal) at a distance from a finite area either looking directly at it (flame) or looking 90 degrees from it (ground). Dimensions of the finite rectangle would be 'a' wide and 'b' long. The distance dA from the rectangle would be 'c'. The shape factors are then provided in terms of ratios of the dimensions.

Thank you for pointing this one out. I do have a textbook for reference. But I was wondering if I assume the view factor of fireman to wall be 0.5 as if the wall is extending in the opposite direction, this would make it 'infinite' and the factor would be 1, so dividing by two according to symmetry the original factor would be 0.5.
 
Don't forget about the heat exchange with the ground. Some thermal energy is lost on that exchange (fireman's front).

The parameterized equations for the factors I have access to predict a factor of 0.5 to the flames.
 
LawrenceC said:
Don't forget about the heat exchange with the ground. Some thermal energy is lost on that exchange (fireman's front).

The parameterized equations for the factors I have access to predict a factor of 0.5 to the flames.

Thank you so much for the answer!
 

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