Heat Transfer Practice Test 3

1. The following data pertains to the junction of a thermocouple wire used to measure the temperature of the gas stream.
Density = 8500 kg/m³, specific heat = 325 J/kg K, thermal conductivity = 40 W/m K and the heat transfer coefficient between the junction and gas = 215 W/m² K.
If thermocouple junction can be approximated as 1 mm diameter sphere, determine how long it will take for the thermocouple to read 99% of the initial temperature difference

2. Choose the correct option regarding fin efficiency and fin effectiveness

3. A thermocouple junction of spherical form is to be used to measure the temperature of the gas stream. The junction is at 20 degree Celsius and is placed in a gas stream which is at 200 degree Celsius. Make calculations for junction diameter needed for the thermocouple to have thermal time constant of one second. Assume the thermos-physical properties as given below
k = 20 W/ m K
h = 350 W/m² K
c = 0.4 k J/kg K
p = 8000 kg/m³

4. “Fin is insulated at the tip”. What does that mean?

5. Let us assume there are two pieces of copper wire 0.1625 cm in diameter with a device that melts it at 195 degree Celsius. The wires are positioned vertically in air at 24 degree Celsius and the heat transfer coefficient of the wire is 17 W/ square m K. Let us say k = 335W/m K i.e. of wire. Find out the energy input?

6. In heat dissipation from an infinitely long fin, the boundary conditions are

7. Two long rods A and B of the same diameter have thermal conductivities k and 4k and have one of their end inserted into a furnace at 400 K. At 9.5 m away section from the furnace, the temperature of rod B is120 degree Celsius. So find out at what end from the furnace end, the same temperature would be reached in the rod A?

8. The temperature distribution in case of infinitely long fin is

9. Heisler charts are valid if

10. The relevant boundary conditions in case of heat dissipation from a fin insulated at the tip are