CHAPTER 1 READING QUESTIONS 1.1 How are thermodynamics and the study of heat tra
ID: 1859395 • Letter: C
Question
CHAPTER 1 READING QUESTIONS
1.1 How are thermodynamics and the study of heat transfer related?
1.2 What are the symbol and the SI units for heat, rate of heat transfer, and heat flux?
1.3 Explain how the resistance method is used for a plane wall.
1.3.2 How does thermal conductivity play a part in heat transfer?
1.4 Explain how convection differs from conduction. What are the physical principles behind convection?
1.5 What are the physical principles behind radiation?
1.6 Explain how you would approach a parallel thermal circuit to determine a temperature change through a composite wall.
1.6.2 Explain contact resistance and provide one example on why it is important in heat transfer.
1.6.4 Draw a sample circuit for convection and radiation in parallel.
1.6.4 Explain how to determine the overall heat transfer coefficient, U.
1.7 How do we account for thermal insulation in a thermal circuit?
1.8 What is the difference between a closed and open system?
1.8.1. Explain each of the terms in equation 1.39 in words.
1.8.2 Draw the control surface in Figure 1.34 and explain the physical meaning / type of heat transfer for each term.
1.8.3 What would be sample boundary conditions for the control surface in figure 1.34
Explanation / Answer
1.1
Thermodynamics is a branch of natural science concerned with heat and its relation to energy and work. It defines macroscopic variables (such as temperature, internal energy, entropy, and pressure) that characterize materials and radiation, and explains how they are related and by what laws they change with time.
Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy and heat between physical systems.
1.2
Heat: Symbol = theta, Units = Kelvin
Rate of heat transfer: Symbol = Q, Units = Watts
Heat flux: Symbol = q, Units = W/m^2
1.3
Q =kA (dT/dx) can be re-written as Q = dT / [dx / (kA)].
Writing dx / (kA) = R we have Q = dT / R.
This eqn is similar to electrical resistance eqn I = V/R where I = current, V = voltage, R = resistance.
Just like electrical resistance resistance can be added in series to get equivalent resistance, we can thermal resistances dx / (kA) when in series to get equivalent thermal resistance for a composite wall.
1.3.2...As Thermal conductivity (k) increases, more heat transfer (Q) happens.
1.4.
Convection is caused by actual movement of fluid particles from hot region to cold region while conduction happens by vibration of molecules in their own location.
1.5
Radiation is caused by transfer of energy by means of photons in electromagnetic waves.
1.6
Get the equivalent thermal resistance Using the eqn 1/R = 1/R1 + 1/R2 + 1/R3 + ......
then use Q = (T2 - T1)/R
1.6.2
Contact resistance is caused because of surface roughness of the touching bodies. The effective area in contact increases because of roughness.
1.6.3
1.6.4
Using the eqn 1 / (UA) = Sigma [ 1/(hA) ] + Sigma [ R] where R = x / (kA)
1.7
Thermal insulation is accounted just like any other thermal resistance. For plane wall, x / (kA)
1.8
Closed system has no flow across its boundaries. Open system has flow across its boundaries.
1.8.1
Eqn 1.39 is missing. Please post it.
1.8.2
Fig 1.34 is missing. Please post it.
1.8.3
Fig 1.34 is missing. Please post it.