Problem 1
Provide short answers to the following problems:
a) What the essential condition for a heat transfer process to occur?
b) What is a differentiation between the steady state conditions and the thermal equilibrium state?
c) Is it probable for the steady state temperature profile in a plane wall to be nonlinear at steady state? If yes, provide one ex.
d) Why do you feel cooler throughout a windy day than throughout a calm day even though the air temperature is the same?
e) Why it is more likely to get burn through putting a bare hand in boiling water at 100oC then into hot air at 200oC?
f) What is a temperature gradient in an object sitting in a room in that the temperature is maintained constant after a sufficiently long time?
g) Is it probable that the surface temperature of the object from part f) to be different than the room air temperature? If yes, please elucidate why?
h) What is the chief reason for a ground frost (air temperature is greater than zero)? For a given air temperature, is ground frost more likely to occur at windy or calm conditions?
i) What is a differentiation between and opaque and semitransparent object?
j) What is the chief reason for the surfaces of different objects to be dissimilar when they exposed to sunlight?
k) What is the differentiation between the closed and open systems?
l) What is the inherent differentiation between application of conservation of energy over a time interval and at an instantaneous of time?
m) Why the form of the energy balance for a surface of an object at stable and uneven state conditions is the same?
n) What is the main reason irreversible operation of the actual heat engines?
o) What is the result of the power output of the actual heat engine on its efficiency? Please describe.
Problem 2
An experiment to find out the convection coefficient allocated with airflow over the surface of a thick stainless steel casting includes insertion of thermocouples in the casting at distances of 10 mm and 20 mm from the surface. When the experiment was performed in a well insulated air duct so that the temperature of the walls surrounding the steel surface was the same as the air temperature, the thermocouples evaluated the temperatures of 50°C and 40°C in the steel, correspondingly, when the air and the surrounding wall temperatures were 200oC.
a) If the steel has a thermal conductivity of 15 W/m2K and surface emissivity of 0.95, what is the convection coefficient among the steel and hot air?
b) What are the contributions of the convective and radiation heat fluxes to the steel casting?
In the repeated experiments the duct insulation was removed and even although the hot air temperature was the same as before, i.e., 200oC, the surrounding walls temperature dropped to 20oC. As a consequence the steady state heat flux to the steel casting dropped by 13.3% compared to the first experiment.
c) Supposing the same convection coefficient as the value computed in part a), what would be the temperatures evaluated by the thermocouples?
d) Assume that the steel casting was well insulated from all sides except for surface from which the distance to thermocouples is evaluated. Supposing the same convection coefficient as the value computed in part a), what would be the temperatures of the thermocouples when:
i) The experiment is performed in the well insulated duct at T∞ = Tsurr = 200oC?
ii) The experiment is performed in non-insulated duct at T∞ = 100oC and Tsurr = 20oC?