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guppygould
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Homework Statement
Hey all! So I have an exam coming up soon for my heat transfer module at uni and I'm fairly confident with the material, but in one of the tutorial sheets I have there are some questions which I don't know how to solve. I'm not really asking for a solution to these questions, I'd just like to know the relevant equations that I'd need to use to solve them. I've looked through my notes and used google but no such luck as of yet. Here is a copy of the tutorial sheet:
"Tutorial Sheet No. 2
Q1 The inside surface of an insulating layer is maintained at 290˚C, and the outside surface dissipates heat by convection to air at 20˚C. The insulation layer is 5cm thick and has a thermal conductivity of 1.25W/mK. Determine the minimum value of the heat transfer coefficient at the outer surface if the outside surface temperature should not exceed 75˚C.
Q2 A large wall of thickness 5cm and thermal conductivity k = 0.65W/mK has a front surface with an emissivity of 0.8. At this surface there is radiation exchange with the surroundings and convection heat transfer to the air. The air and surroundings are at 22˚C, and the convection heat transfer coefficient is 20W/m2K. If the front surface temperature of the wall is 152˚C what is the rear surface temperature?
Q3 A small, thin metal plate is insulated on one side and exposed to the sun on the other side. The plate absorbs solar energy at a rate of 600W/m2 and dissipates it by convection into the ambient air at 300K with a convection heat transfer coefficient 22W/m2K, and by radiation into the surrounding area which may be assumed to be a blackbody at Tsurr = 270K. The emissivity of the surface ε is 0.9. Determine the equilibrium surface temperature of the plate.
Q4 A cold room has walls constructed of 0.25m brick on the outside, 0.1m foam insulation material in the middle and 1.5cm wood on the inside. The air temperature on the outside is constant at 20˚C and the room is to be maintained at a temperature of -5˚C. The convection heat transfer coefficient for air to brick is 15W/m2K and for wood to air is 30 W/m2K. The thermal conductivities of wood, foam and brick are 0.15, 0.025 and 1 W/mK, respectively. Determine:
(a)the overall heat transfer coefficient
(b)the heat transfer rate per unit area of the wall surface
(c)the outside and inside surface temperatures of the composite wall
(d)the amount of energy to be removed by refrigeration in a 24 hour period if the total wall area is 75m2"
Homework Equations
This is what I need to know, it'll be the same equation for all of them or at least similar ones.
The Attempt at a Solution
I need the relevant equations to attempts the question.
Thanks in advance,
-Leo
