1. A thermocouple junction in the form of 4 mm radius sphere is to be used to measure the temperature of a gas stream. The junction is initially at 35 degree Celsius and is placed in a gas stream which is at 300 degree Celsius. The thermocouple is removed from the hot gas stream after 10 seconds and kept in still air at 25 degree Celsius with convective coefficient 10 W/m2 K. Find out the time constant of the thermocouple. Assume the thermos-physical properties as given below
h = 37.5 W/m2 K
p = 7500 kg/m3
c = 400 J/kg K
a) 6.67 seconds
b) 106.67 seconds
c) 206.67 seconds
d) 306.67 seconds
Explanation: T = p V c/h A = p r c/3 h = 106.67 seconds.
2. What percentage of water an average human body can have?
a) 52%
b) 62%
c) 72%
d) 82%
Explanation: Average human body is 72% water by mass.
3. Heisler charts are valid if
a) Fourier number is equal to 0.2
b) Fourier number is less than 0.2
c) Fourier number is greater than 0.2
d) Fourier number is equal to 0.4
Explanation: The solution to the transient heat flow in infinite flat plates are available in the form of these charts.
4. The transient response of a solid can be determined by the equation. (Where, P is density, V is volume, c is specific heat and A is area)
a) – 4 p V c = h A (t – t0)
b) – 3 p V c = h A (t – t0)
c) – 2 p V c = h A (t – t0)
d) – p V c = h A (t – t0)
Explanation: It can be determined by relating rate of change of internal energy with conductive heat exchange at the surface.
5. A 2 cm thick steel slab heated to 525 degree Celsius is held in air stream having a mean temperature of 25 degree Celsius. Estimate the time interval when the slab temperature would not depart from the mean value of 25 degree Celsius by more than 0.5 degree Celsius at any point in the slab. The steel plate has the following thermal physical properties
Density = 7950 kg/m3
C P = 455 J/kg K
K = 46 W/m K
a) 6548 s
b) 6941 s
c) 4876 s
d) 8760 s
Explanation: t – t a/t I – t a = exponential (- h A T/p V c). Now A/V = 100 per meter.
6. An average convective heat transfer coefficient for flow of air over a sphere has been measured by observing the temperature-time history of a 12 mm diameter copper sphere (density = 9000 kg/m3 and c = 0.4 k J/kg K) exposed to air at 30 degree Celsius. The temperature of the sphere was measured by two thermocouples one located at the center and the other near the surface. The initial temperature of the ball was 75 degree Celsius and it decreased by 10 degree Celsius in 1.2 minutes. Find the heat transfer coefficient
a) 27.46 W/m2 K
b) 21.76 W/m2 K
c) 29.37 W/m2 K
d) 25.13 W/m2 K
Explanation: t – t a/t I – t a = exponential (- h A T/p V c). So, h = 25.13 W/m2 K.
7. Transient condition means
a) Conduction when temperature at a point varies with time
b) Very little heat transfer
c) Heat transfer with a very little temperature difference
d) Heat transfer for a short time
Explanation: The term transient or unsteady state designates a phenomenon which is time dependent.
8. Which of the following is not correct in a transient flow process?
a) The state of matter inside the control volume varies with time
b) There can be work and heat interactions across the control volume
c) There is no accumulation of energy inside the control volume
d) The rate of inflow and outflow of mass are different
Explanation: In transient heat conduction there is accumulation of energy inside the control volume.
9. The curve for unsteady state cooling or heating of bodies is
a) Hyperbolic curve asymptotic both to time and temperature axis
b) Exponential curve asymptotic both to time and temperature axis
c) Parabolic curve asymptotic to time axis
d) Exponential curve asymptotic to time axis
Explanation: α/α 0 = exponential [- h A T/p c V], which represents an exponential curve.
10. What is the wavelength band for TV rays?
a) 1 * 10 3 to 34 * 10 10 micron meter
b) 1 * 10 3 to 2 * 10 10 micron meter
c) 1 * 10 3 to 3 * 10 10 micron meter
d) 1 * 10 3 to 56 * 10 10 micron meter
Explanation: This is the maximum and minimum wavelength for TV rays.