## MCQs

Total Questions : 71 | Page 1 of 8 pages
Question 1. Glass spheres of 2 mm radius and at 500 degree Celsius are to be cooled by exposing them to an air stream at 25 degree Celsius. Find maximum value of convective coefficient that is permissible. Assume the following property values
Density = 2250 kg/m³
Specific heat = 850 J/kg K
Conductivity = 1.5 W/m K
1.    245 W/m²K
2.    235 W/m²K
3.    225 W/m²K
4.    215 W/m²K
Answer: Option C. -> 225 W/m²K
Question 2. 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)
1.    – 4 p V c = h A (t – t0)
2.    – 3 p V c = h A (t – t0)
3.    – 2 p V c = h A (t – t0)
4.    – p V c = h A (t – t0)
Answer: Option D. -> – p V c = h A (t – t0)
Answer: (d).– p V c = h A (t – t0)
Question 3. Transient condition means
1.    Conduction when temperature at a point varies with time
2.    Very little heat transfer
3.    Heat transfer with a very little temperature difference
4.    Heat transfer for a short time
Answer: Option A. -> Conduction when temperature at a point varies with time
Answer: (a).Conduction when temperature at a point varies with time
Question 4. 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
1.    27.46 W/m² K
2.    21.76 W/m² K
3.    29.37 W/m² K
4.    25.13 W/m² K
Answer: Option D. -> 25.13 W/m² K
Question 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
1.    6548 s
2.    6941 s
3.    4876 s
4.    8760 s
Answer: Option B. -> 6941 s
Question 6. Which of the following dimensionless number gives an indication of the ratio of internal (conduction) resistance to the surface (convective) resistance?
1.    Biot number
2.    Fourier number
3.    Stanton number
4.    Nusselt number
Answer: Option A. -> Biot number
Question 7. In the lumped system parameter model, the variation of temperature with time is
1.    Linear
2.    Exponential
3.    Sinusoidal
4.    Cube
Question 8. A gold ring (k = 65 W/m K) measuring 15 * 10 * 60 cm is exposed to a surface where h = 11.5 W/m² K. Find the value of biot number
1.    0.68
2.    0.58
3.    0.48
4.    0.38
Question 9. Lumped parameter analysis for transient heat conduction is essentially valid for
1.    B I < 0.1
2.    1 < B I < 10
3.    0.1 < B I < 0.5
4.    It tends to infinity
Answer: Option A. -> B I < 0.1