Heat Transfer Practice Test 3

1. Consider the following statements pertaining to heat transfer through fins
(i) They must be arranged at right angles to the direction of flow of working fluid
(ii) The temperature along the fin is variable and accordingly heat transfer rate varies along the fin elements
(iii) Fins are equally effective irrespective whether they are on the hot side or cold side of the fluid
(iv) Fins are made of materials that have thermal conductivity higher than that of wall
Identify the correct statements

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

3. The parameter m = (h P/k A _C) ^1/2 has been stated to increase in a long fin. If all other parameters are constant, then

4. For a fin of finite length with an insulated end, the fin efficiency is given as

5. An iron billet (k = 65 W/m K) measuring 20 * 15 * 80 cm is exposed to a convective flow resulting in convection coefficient of 11.5 W/m² K. Determine the Biot number

6. A rod of 10 mm diameter and 80 mm length with thermal conductivity 16W/ m K protrudes from a surface at 160 degree Celsius. The rod is exposed to air at 30 degree Celsius with a convective coefficient of 25W/square m K. How does the heat flow from this rod get affected if the same material volume is used for two fins of the same length? Assume short fin with insulated end

7. An electronic semiconductor device generates 0.16 k J/hr of heat. To keep the surface temperature at the upper safe limit of 75 degree Celsius, it is desired that the heat generated should be dissipated to the surrounding environment which is at 30 degree Celsius. The task is accomplished by attaching aluminum fins, 0.5 square mm and 10 mm to the surface. Work out the number of fins if thermal conductivity of fin material is 690W/m K and the heat transfer coefficient is 45k J/square m hr K. Neglect the heat loss from the tip of the fin

8. Mark the false statement regarding effectiveness of fin

9. Find the heat transfer rate from a hot surface for 6 fins of 10 cm length? The base temperature of the fin is maintained at 200 degree Celsius and the film is exposed to a convection environment at 15 degree Celsius with convective coefficient 25W/square m K. Each fin has cross-sectional area 2.5 square centimeter and is made of a material having thermal conductivity 250W/m K

10. 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