## HEAT TRANSFER JNTU previous years question papers

Time: 3 hours Max Marks: 80

Answer any FIVE Questions

All Questions carry equal marks

1. Heat is flowing through an annular pipe of inside radius r0 and outside radius r1. The thermal conductivity varies linearly with temperature from k0 at T0 to k1at T1. Develop an expression for heat flow through the pipe. Simplify if the expression assuming that r1-r0 is very small. Interpret the result physically. [16]

2. The outside and inside surface temperatures of a 20cm outer diameter and 18cm inner diameter pipe (k=40W/mK) are 400oC and 460oC respectively. Calculate the reduction in heat loss if a 5cm layer of insulation (k=0.06 W/mK) is put on the pipe. Assume that the inner and outer surface temperature of insulation is 390oC and 140oC . What is the inside surface temperature of this pipe in this case? [16]

3. (a) Define Reynolds number. Why is it important? [5]

(b) What is Dittus-Boelter equation? When does it apply? [5]

(c) A vertical plate at 1000C is 1 m wide and 20 cm high. It rests in still air at 1 atm and 200C. Determine the local heat transfer coefficient at 10 cm from the leading edge of the plate. The properties of the air at film temperature may be taken as: Thermal conductivity is 0.03 W/(m.K)

Viscosity is 2.03× 10-5 PaS

Density is 1.00 kg/m3.

Specific heat 1.01 kJ/(kg.K) [6]

4. (a) Discuss the problem of combined free and forced convection. [8]

(b) Air enters a rectangular duct measuring 0.3 m by 0.4 m with a velocity of 8.5 m/s and a temperature of 400C. The flowing air has a thermal conductivity 0.028 W/(m.K), kinematic viscosity 16.95 × 10-6 m2/s and from the empirical correlations the Nusselt number has been approximated to be 425. Work out the equivalent diameter of the flow passage, the Reynolds number and the convective heat film coefficient. [8]

5. (a) What is the effect of liquid pressure on boiling? [8]

(b) Estimate the heat flux which would occur in nucleate boiling of saturated water at 477oK upon platinum wire as submerged heating surface (491oK).Thermal properties of water

p = 968 kg/m3

k = 0.58 w/moC

Cp = 4180 j/kgoC

µ = 1.14 x 10-6 N.sec/m2 [8+8]

6. A polished metal pipe 5 cm outside diameter and 370 K temperature at the outer surface is exposed to ambient conditions at 295 K temperature. The emissivity of the surface is 0.2 and the convection coefficient of heat transfer is 15 W/m2.K. calculate the heat transfer by radiation and natural convection per meter length of the pipe. Take the thermal radiation constant s = 5.67 × 10-8 W/m2 K4. [16]

7. (a) Hot oil (Cp=5.2 kJ/kg.k) with a capacity rate of 2800 Kg/min flows through a double pipe heat exchanger. It enters at 380oC and leaves at 300oC. Cold oil (Cp = 4.8 kJ/kgk) enters at 30oC and leaves at 200oC. If the overall heat transfer coefficient is 1000 W/m2K, determine the heat transfer area required for

i. Parallel flow and

ii. Counter flow [4+4]

(b) Define overall heat transfer coefficient and write an expression for it by accounting fouling resistances at the inner and outer tube surfaces in shell and tube heat exchanger. [8]

8. (a) Discuss the different methods of feeding in evaporator with neat diagrams. [8]

(b) State and explain the Duhring?s rule. [8]