Heat Transfer Mechanical previous year questions with answer
Ques 21 GATE 2016 SET-2
A hollow cylinder has length L, inner radius r1, outer radius r2, and thermal conductivity k. The thermal resistance of the cylinder for radial conduction is
Ques 22 GATE 2016 SET-2
Consider the radiation heat exchange inside an annulus between two very long concentric cylinders. The radius of the outer cylinder is Ro and that of the inner cylinder is Ri. The radiation view factor of the outer cylinder onto itself is
Ques 23 GATE 2016 SET-2
Consider a parallel-flow heat exchanger with area Ap and a counter-flow heat exchanger with area Ac. In both the heat exchangers, the hot stream flowing at 1 kg/s cools from 80°C to 50°C. For the cold stream in both the heat exchangers, the flow rate and the inlet temperature are 2 kg/s and 10°C, respectively. The hot and cold streams in both the heat exchangers are of the same fluid. Also, both the heat exchangers have the same overall heat transfer coefficient. The ratio Ac/Ap is ________.
0.93 is the correct answer.
Ques 24 GATE 2016 SET-2
Two cylindrical shafts A and B at the same initial temperature are simultaneously placed in a furnace. The surfaces of the shafts remain at the furnace gas temperature at all times after they are introduced into the furnace. The temperature variation in the axial direction of the shafts can be assumed to be negligible. The data related to shafts A and B is given in the following Table.
| Quantity | Shaft A | Shaft B |
| Diameter (m) | 0.4 | 0.1 |
| Thermal conductivity (W/m-K) | 40 | 20 |
| Volumetric heat capacity (J/m3-K) | 2x106 | 2x107 |
The temperature at the centerline of the shaft A reaches 400°C after two hours. The time required (in hours) for the centerline of the shaft B to attain the temperature of 400°C is ________.
2.5 is the correct answer.
Ques 25 GATE 2016 SET-1
A plastic sleeve of outer radius ro = 1 mm covers a wire (radius r = 0.5 mm) carrying electric current. Thermal conductivity of the plastic is 0.15 W/m-K. The heat transfer coefficient on the outer surface of the sleeve exposed to air is 25 W/m2-K. Due to the addition of the plastic cover, the heat transfer from the wire to the ambient will
Ques 26 GATE 2016 SET-1
A steel ball of 10 mm diameter at 1000 K is required to be cooled to 350 K by immersing it in a water environment at 300 K. The convective heat transfer coefficient is 1000 W/m2-K. Thermal conductivity of steel is 40 W/m-K. The time constant for the cooling process τ is 16 s. The time required (in s) to reach the final temperature is ________.
42.2 is the correct answer.
Ques 27 GATE 2016 SET-1
An infinitely long furnace of 0.5 m x 0.4 m cross-section is shown in the figure. Consider all surfaces of the furnace to be black. The top and bottom walls are maintained at temperature T1 = T3 = 927°C while the side walls are at temperature T2 = T4 = 527°C. The view factor, F1-2 is 0.26. The net radiation heat loss or gain on side 1 is ________ W/m. Stefan-Boltzmann constant = 5.67x10-8 W/m2-K4.
24530 is the correct answer.
Ques 28 GATE 2016 SET-1
A fluid (Prandtl number, Pr = 1) at 500 K flows over a flat plate of 1.5 m length, maintained at 300 K. The velocity of the fluid is 10 m/s. Assuming kinematic viscosity, ν = 30x10-6 m2/s, the thermal boundary layer thickness (in mm) at 0.5 m from the leading edge is ________.
6.12 is the correct answer.
Ques 29 GATE 2015 SET-2
The total emissive power of a surface is 500 W/m2 at a temperature T1 and 1200 W/m2 at a temperature T2, where the temperatures are in Kelvin. Assuming the emissivity of the surface to be constant, the ratio of the temperatures T1/T2 is
Ques 30 GATE 2015 SET-2
A balanced counterflow heat exchanger has a surface area of 20 m2 and overall heat transfer coefficient of 20 W/m2-K. Air (Cp = 1000 J/kg-K) entering at 0.4 kg/s and 280 K is to be preheated by the air leaving the system at 0.4 kg/s and 300 K. The outlet temperature (in K) of the preheated air is