Maharashi Dayanand University (MDU) 2009-6th Sem B.E Mechanical Engineering HEAT TRANSFER- - Question Paper
2309
B.E. sixth SEM (MECH. ENGG)
exam - MAY, 2009
HEAT TRANSFER
PAPER : ME-306-E
At the mean film temp tF = 60C the properties are
thermal diffusivity a = 7.2 x 10-3 m2/sec. Thermal conductivity k = 0.213 w/ink kinematic viscosity v =
0.65 x 10 m2/sec; densityp = 956.8 kg/m3.
7. Saturated steam at atmospheric pressure condenses
on .the outer surface of a vertical tube of length lm
and outer diameter 75 mm. The tube wall is maintained at a uniform surface temp of 40C -by the flow of cooling water inside the tube. Estimate the steam condensation rate and the heat transfer rate to the .tube. What water flow rate will result in a 5C temp difference of water between the outlet and inlet of pipe ? Also calculate the flow Reynolds number to check the assumption .of laminar flow conditions. 20
8. A one-shell, two tube pass heat exchanger having 3000 thin wall brass tubes of 20 mm diameter has been installed in a steam power plant with a heat load of 2.3 * 101w. The steam condenses at 50C and the cooling water enters the tubes at 20C at the rate of 3000 kg/s. Calculate the overall heat transfer coefficient, the tube length per pass, and the rate of condensation of steam. Take the heat transfer
coefficient for condensation on the outer surfaces of .the tubes as 15500 w/mk and the latent heat of steam as 2380 kj/kg Further presume the following fluid properties: 20
c = 4180J/Kgk, vl = 855- x 10*Ns/m*, k = 0.613 w/mk, Pr = 5.83.
B. E. 6th Sem. (Mech. Engg.) Examination - May, 2009
Heat Transfer Paper; ME-306-E
Time : Three hours]_[ Maximum Marks: 100
Before answering the question, candidates should ensure that they have been supplied the correct and complete question paper. No complaint in tins regard, will be entertained after exannnation.
Note : Attempt any five questions.
1. (a) A person who sits in front of a fireplace feels warm. Through what processes of heat transfer docs he receive heat ? 4
(b) A solar plane 1 m x 1.25 m, receives solar radiation 1500 watts. Calculate surface temperature of the plane if the ambient temperature is 25C and the convective heat transfer coefficient of the air film over the surface of plane is 1*2.5 w/m2 deg. 12
(c) Write the rate equations for the three modes of heat transfer. 4
2. Prove that heat loss Q per square of outside surface area of a hollow sphere heated from within is equal to
q = (/lh)Symbols have theii usual meanings.
(r2~n)* n
A spherical furnace with 25 cm internal diameter and 30 cm external diameter is made of refractory material of mean thermal conductivity 0.45 kcal/m-hr-deg. Find the hourly loss of heat if the temperatures at the inner and outer surfaces are 400C and 25C respectively. What would be the reduction in heat loss when the furnace is covered with 20 mm thick insulation of conductivity k, 17.45 w/m deg, 20
3. An infinite slab of 20 cm thickness and thermal conductivity 20 w/mk separates two fluid having temperatures 35 C and 25C respectively, 'lhe heat transfer coefficient on the hot fluid side is 25w/m2k and that on the cold fluid side is 50 w/m2k, if the heat generation in the slab is at uniform rate of 6 kw/m3 set up an expression for the temperature distribution in the slab Proceed to determine : 20
(a) maximum temperature-in the slab and its location
(b) temperature at the centre of slab and at the two surfaces,
(c) Heat transferred from each surface.
4. The steel ball bearings of 40 mm diameter and initially at uniform temperature of 600C are quenched in an oil bath maintained at 50C temperature. The heat transfer coefficient between the bail bearings and oil is 325 w/m2K and the thermodynamic properties of bearings can be taken as : K = 45w/mk, a = 1.25 xlO*5 m2/s. Determine:
(a) time duration for which bearings must remain in oil to attain 225C temp.
(b) amount of heat removed from bearings during this time
(c) the instantaneous heat transfer rate from the * bearings when they are first immersed on. oil and when the)'' reach 225C. 20
5. (a) Define Lambert's cosine law of radiation and
prove that the intensity of radiation is . always constant at any angle of emission for a diffused surface. .12
(b) Define the following terms as applied to radiation heat transfer: 8
(i) Black, gray and real surface
(ii) Spcctral and special distribution of energy
(iii) Specular and diffused reflection
(iv) Emissive power and intensity of radiation.
6. In a certain pharmaceutical process, castor oil at 35C flows over a flat plate at 6cm/s. The plate is 6 m. long, is heated uniformly and maintained at a surface temp of 95C. Make calculations*for: .f , 20
(a) hydrodynamic and thermal boundary layer thickness at the trailing edge of the plate .
(b) total drag per unit width on one side of the plate
(c) Local heat transfer coefficient at the end of the* plate
(d) total heat flux from the surface per unit width.
23094**(P-4)(Q-a)(09) ( 3 ) P. T. O.
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