Paper / Subject Code: 31008 / HEAT TRANSEEN. TLE. SEMV/ CREDIT BASE / MECH / MAY 2019 / 31.05.2019 Time : 3 Hours: Marks: 80 Note: - (1) Question No, 1 is compulsory (2) Answer any Three out of reaming Five (3) Make suitable assumption, if necessary QU. Solve any Five wey 4a) A steam pipe is insulated to reduce the heat loss. However, the measurement reveals that the rate of heat lost has increased instead of decreasing .Can you comment why? b) How does a fin enhance heat transfer at « surface? What are the various types of fins? List the assumptions made while analyzing the heat flow from a finned surface ‘¢) What is lump system analysis? What are the assumptions made in the lumped system analysis and when is it applicable? 4d) When heat transfer through a fluid layers is by conduction and when it is by convection? For what case, the rate of heat transfer is higher? ©) What are the limitations of LMTD method? How is Ftfe to LMTD method? State and explain Wien’s displac fe — NTU method superior ent law. Q.2 a) A thermocouple junction of spherical form is 8 mm diameter, ay Properties of the material are C= 420 Jikg-K, p = 8000 kgm’, k= 40 Wim-K and h = 40 Wem’ This junction is initially at 40°C and inserted in a stream of hot ait at 300°C. Find (i) Time constant of the thermocouple (Gi) The thermocouple is taken out from the hot air after 10 seconds and kept in air at 30°C. Assuming the heat transfer coefficient in air 10 W/m? K, find the temperature attained by the junction 20 seconds after removing from bot ait. by Derive Fo: ferential equation in the Cartesian Co-ordinate. cr) Q.3 a) A refrigerated truck is moving on a highway at 90 km/h in a desert area where the (10) ambient air temperature is $0 C. the body of the truck may be considered as a rectangular box measuring 10 m (Length) * 4 m (Width) « 3 m (Height). Assuming that the boundary layer on the four walls is turbulent, the heat transfer takes place only for the four surfaces and the wall surface of the truck is maintained at 10 C. Neglecting heat transferred from the front and back and assuming the flow to be parallel to 10 m long side, Calculate the following: 1, The heat loss from the four surfaces, 2. The power required to overcome the resistance acting on the four surfaces. Use the equation: Nu -0.036(Re)"*(Pr)"** if Re.>1*10* ‘The properties of air at 30°C p= 1.165 kgm’; Cp = 1,005 kickek; k = 0.02673 Wim-K: v= 16*10% mi 0.701 60312 Page 1 of 2Qa as Q6 60312 b) a) by a) a) b) Paper / Subject Code: 31005 / HEAT TRANSFER jernal resistance, with usual notations, For transient heat conduction, with negligibl @ show that: 2 = exp (-B,.F,) Also state the significations of *B,* and *F," Two stroke motor cycle cylinder consists of 15 fins. Ifthe outside and inside diameters of each fin are 200 mm and 100 mm respectively, the average fin surface temperature 475 C and atmospheric air temperature is 25°C, calculate the heat transfer rate from the fins for the following cases. i), When the motor eycle is stationary ii) When the motor cyele is running at the speed of 60 km/h The fin may be idealized as single horizontal flat plate of the same area, Use significant length L=0.9D to calculate Gr and Re. Use equation Nu = 0.54(GrePr)" if nature of flow is laminar ie. GrPr< 10” and Nu = 0.036(Rey?*(Pr)?" if nature of flow is turbulent. ‘The properties if air at 250C: k= 0.04266 Wim-K; v= 40.6110 m*/s; Pr= 0.677 Derive an expression for the effectiveness of a paralle! flow heat exchanger in terms of the number of transfer units, NTU, and the capacity ratio Civ Cm, A parallel flow heat exchanger has hot and cold water streams running through it and has the following data Mass flow rates of hot and cold water are 10 kg/min and 25 kg/min respectively. Inlet ‘and outlet temperatures of hot water are 70°C and 50°C respectively. Inlet temperature ‘of cold water is 25°C. Individual heat transfer coefficient on both sides = 60:W/m'K. Calculate: (i) Area of heat exchange, Gi) Exit temperatures of hot and cold if hot water flow rate is doubled, Define Shape factor and discuss its properties. Derive an expression for shape factor for (i) Hemispherical shape of radius R (ii) Two concentric eylinders. Consider two large parallel plates one at Ty = 727°C with emissivity €; =0.8 and other at 2= 227°C with emissivity 62 = 0.4. An aluminum radiation shield with an emissivity £, = 0.05 on both side is placed between the plates. Calculate the percentage reduction in heat transfer rate between two plates as a result of the shield. Use 6 = 5.6710" Wim'k* Write short note on any two of the following i) Heisler Charts. Boiling curves and various regimes of boiling. ) Heat Pipe. Page 2 of 2 (yy (10) a0) (a0) (10) 0) (a0)