1. 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
a) 0.02376
b) 0.008974
c) 0.004563
d) 0.006846
Explanation: B = h L C/k = 0.006846.
2. A mercury thermometer with bulb idealized as a sphere of 1 mm radius is used for measuring the temperature of fluid whose temperature is varying at a fast rate. For mercury
If the time for the temperature change of the fluid is 3 second, what should be the radius of thermocouple to read the temperature of the fluid?
For the thermocouple material
k = 100 W/m K
α = 0.0012 m2/s
h = 18 W/m2 K
a) .864 mm
b) .764 mm
c) .664 mm
d) .564 mm
Explanation: T = k l/h α. So, radius is 0.864 mm.
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
a) 0.456 mm
b) 0.556 mm
c) 0.656 mm
d) 0.756 mm
Explanation: T = p V c/h A = p r c/3 h.
4. “The response of a thermocouple is defined as the time required for the thermocouple to reach the surrounding temperature when it is exposed to it”.
a) true
b) false
Explanation: It is the source temperature.
5. The sensitivity of thermocouple is defined as the time required by thermocouple to reach how much percentage of its steady state values?
a) 43.3
b) 53.2
c) 63.3
d) 73.3
Explanation: The time constant of a thermocouple represents the time required to attain 63.2% value.
6. The response time for different sizes and materials of thermocouple wires usually lie between
a) 0.04 to 2.5 seconds
b) 0.06 to 1.2 seconds
c) 0.02 to 0.04 seconds
d) 2.4 to 9.4 seconds
Explanation: Depending upon the type of fluid used, the response time for different sizes and materials of thermocouple wires usually lie between o.o4 to 2.5 seconds.
7. 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 time required by the thermocouple to reach 197 degree Celsius temperature. 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
a) 1.094 seconds
b) 2.094 seconds
c) 3.094 seconds
d) 4.094 seconds
Explanation: t – t a/t I – t a = exponential (- p V c T/h A).
8. An egg with mean diameter of 4 cm and initially at 25 degree Celsius is placed in an open boiling water container for 4 minutes and found to be boiled at a particular level. For how long should a similar egg boil at the same level, when refrigerator temperature is 5 degree Celsius? Use lumped parameter theory and assume following properties of egg
k = 12 W/m K
h = 125 W/m2 K
c = 2 k J/kg K
p = 1250 kg/m3
a) 251.49 seconds
b) 261.49 seconds
c) 271.49 seconds
d) 281.49 seconds
Explanation: T t – T INFINITY/T i – T INFINITY = e – b T.
9. A person is found dead at 5 pm in a room whose temperature is 20 degree Celsius. The temperature of body is measured to be 25 degree Celsius, when found and heat transfer coefficient is estimated to be 8 W/m2 K. Modeling the body as a 30 cm diameter, 1.7 cm long cylinder. Estimate the time of death of person.
a) 13.55 seconds
b) 12.55 seconds
c) 11.55 seconds
d) 10.55 seconds
Explanation: T t – T INFINITY/T i – T INFINITY = e – b T.
10. 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
a) 9.86 seconds
b) 8.86 seconds
c) 7.86 seconds
d) 6.86 seconds
Explanation: t – t a/t i – t a = exponential (- h A T/p V c).