Heat Transfer (Solar Radiation)

  • Thread starter Chibus
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  • #1
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Homework Statement



This questions relates to solar radiation and heat balance. I'm having trouble with part (b) of the question, having solved (a) already.

The question is found in the following image:

xogvvb.jpg


Homework Equations





The Attempt at a Solution



Qconv=h*A*(T-Tambient)=420 W

Qconv=420 W
A=2m^2
Tambient=303 K

Given T=(The equation found in the link)^(1/4)

Emissivity=0.9
Ne=0.15
Gsn=1000 W/m^2
Aplate=2m^2
Stefan-Boltzman Constant=5.7E-8

The only thing I need is the abosrbance (alpha) of the surface.

I'm really unsure of how to calculate it, given I dont have the temperature of the sun.

Can anyone shed some light?



On a related note, regarding Kirchoffs law of thermal radiation:

Emissivity=Absorbance, given equilibrium temperature.

I'm having some trouble understanding where I can use this relationship. For example, in the given problem, I know that in the instant of time being looked at, the system is in equilibrium. Thus, I can say that the emissivity of the surface is equal to the absorbivity, however, this relationship doesnt apply when im considering radiation coming in from the sun?

The following is taken from a section of my lecture notes describing the radiation function and the solar absorbance:

i.e. I evalulate the absorbance of the surface at the temperature of the sun, but I evaluate the emissivity at the equilibrium temperature of the surface. I don't understand why this is so -

rlw8dd.png


Sorry fellas, just getting pretty confused here. Any help would be eternally appreciated.
 

Answers and Replies

  • #2
rock.freak667
Homework Helper
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If I remember my thermodynamics properly, for a grey body [itex]\alpha = \epsilon[/itex]. Which applies to any radiating surface I believe.
 
  • #3
374
1
The convective heat flux (q)=h*A*(Ts-Tinf) where Ts is the surface temp(a above), Tinf is the ambient air temp. h is the convection heat transfer coefficient. A is the area. You know q, A and both T's, looks like plug-and- chug to me.
 

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