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Friday, 6 February 2015

Elements of heat and mass transfer- Model question paper for B.E/B.Tech Mechanical Engineering



1(a) Write a note on respiratory gas exchange in lungs (6 Marks)
(b) Differentiate between thermal conductivity and thermal diffusivity. (6 Marks)
(c) Explain why a quilt is a better insulator than a woolen blanket of the same thickness. (4 Marks)
(d) Identify the heat transfer processes that determine the temperature on the inside of a room on a hot summer day. (4 Marks)
2(a)What is Biot number? Explain its significance. (4 Marks)
(b) What are Fins? Explain in detail transfer of heat from pin-fins of constant cross-section. (8 Marks)
(c) Write a note on thermal sterilization of canned goods. (4 Marks)
(d) A 200 g slab of butter at 300 K is put in a refrigerator at 277K. It takes 120 minutes for the centre of the slab to reach 281K. How long would it take for the centre of the slab to reach 279K? (4 Marks)
3(a) Using a neat figure, explain nodal mesh. (6 Marks)
(b) Explain method of energy balance for an interior node. (8 Marks)
(c) A 4mm brass plate is initially at 00C. One side of the plate is suddenly brought to a temperature of 1000C, with the other side maintained at 00C. Develop a finite difference formulation for obtaining the temperature distribution across the plate for various times. (6 Marks)
4(a) What is transpiration cooling? Write a note on reduction of evaporative losses in liquid nitrogen flask by transpiration. (10 Marks)
(b) State and explain Reynolds transport theorem. (6 Marks)
(c) Find the rate of heat transfer from a spherical water tank of diameter 10m, when exposed to quiescent air at 200C, when its surface temperature is 350C. (4 Marks)
5 (a) With special relevance to velocity profile and temperature profile, explain heat transfer in Couette flow. (6 Marks)
(b) Deduce the governing equations for free convection flows. (6 Marks)
(c) Explain why the surface of a bridge always freezes faster than the surface of a road. (4 Marks)
(d) Saturated steam at 540C condenses on a 2cm diameter vertical tube of length 1m with the surface temperature of 400C. Determine the rate of condensation of stem assuming film condensation. (4 Marks)
6 (a) What is free molecular transfer? Explain. (6Marks)
(b) Explain Fick law. (4 Marks)
(c) A mixture of inert gases contains equal mass fractions of helium, argon and xenon. Determine the composition in terms of the mole fractions. (4 Marks)
(d) Calculate the fraction of gases remaining at the centre of a 1cm thick slab of steel after 10 hours of vacuum out-gassing treatment. Assume the initial distribution of gas in steel to be uniform and diffusivity to be 10-9 m2/s. (6 Marks)
7 (a) Write a note on absorption of gas by a falling film of liquid. (8 Marks)
(b) Consider a turbulent flow over a flat plate. Determine the percentage change in the mass transfer rate, if the Schmidt number is halved by (a) reducing the viscosity by a factor of two, or (b) doubling the binary diffusivity. (6 Marks)
(c) Sketch the temperature versus length curves for a counter-flow and a parallel-flow heat exchanger for the case when Cc = Ch. (6 Marks)
8(a) What is a blackbody? Calculate the total emission from a black surface in terms of intensity of radiation. (6 Marks)
(b) What is configuration factor? Explain the role of reciprocity relation in the evaluation of configuration factors. (6 Marks)
(c) An isothermal furnace with a small hole is used to calibrate radiometric devices. If the radiation intensity is to lie within 0.5% of the nominal, determine the allowable variation in the temperature of the furnace when it is operating at 300K. (4 Marks)
(d) Consider two concentric spheres, 25 and 50 mm in diameter. The outer surface of the inner sphere is a blackbody and is at 1000K, while the inner surface of the outer sphere is gray with emissivity of 0.8 and is at a temperature of 500K. Determine the radiative exchange between the two surfaces. (4 Marks)

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