14.7 Radiation  (Page 5/14)

Career connection: energy conservation consultation

The cost of energy is generally believed to remain very high for the foreseeable future. Thus, passive control of heat loss in both commercial and domestic housing will become increasingly important. Energy consultants measure and analyze the flow of energy into and out of houses and ensure that a healthy exchange of air is maintained inside the house. The job prospects for an energy consultant are strong.

Problem-solving strategies for the methods of heat transfer

1. Examine the situation to determine what type of heat transfer is involved.
2. Identify the type(s) of heat transfer—conduction, convection, or radiation.
3. Identify exactly what needs to be determined in the problem (identify the unknowns). A written list is very useful.
4. Make a list of what is given or can be inferred from the problem as stated (identify the knowns).
5. Solve the appropriate equation for the quantity to be determined (the unknown).
6. For conduction, equation $\frac{Q}{t}=\frac{\text{kA}(T_2-T_1)}{d}$ is appropriate. [link] lists thermal conductivities. For convection, determine the amount of matter moved and use equation $Q=\text{mc}\text{Δ}T$ , to calculate the heat transfer involved in the temperature change of the fluid. If a phase change accompanies convection, equation $Q={\text{mL}}_{\text{f}}$ or $Q={\text{mL}}_{\text{v}}$ is appropriate to find the heat transfer involved in the phase change. [link] lists information relevant to phase change. For radiation, equation $\frac{Q_{\text{net}}}{t}=\sigma eA\left(T_2^4-T_1^4\right)$ gives the net heat transfer rate.
7. Insert the knowns along with their units into the appropriate equation and obtain numerical solutions complete with units.
8. Check the answer to see if it is reasonable. Does it make sense?