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B. Tech. (Sem. - 5,n)I{EATTRANSFER

SUBJECT CODE : ME - 303Paper ID : ['4,08151

lNote : Please fill subject code and paper ID on OMRI

Time : 03 Hours Maximum Marks : 60Instruction to Candidates:

l) Section - A is Compulsory.2) Attempt any Four questions from Section - B.3) Attempt any Tlvo questions from Section - C.

Roll No.

Total No. of Questions : 091

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b)

d)

e)

Section - A(10x2=20)

What is the importance of extended surfaces?

What is critical insulation thickness? Write the expression for criticalradius for cylindrical and spherical bodies.

What is time constant for thermocouples? Write the expression for thesame.

What are the assumptions for fin analysis?

What is overall heat transfer coefficient? Write the expression for thesame.

Define thermal diffrrsivity. Also give its unit.

Draw temperature - length curve for counter-current and parallel currentheat exchanger.

What is radiation shape factor? Write ageneric expression forradiationshape factor.

what is black body? write the significant of defining the black body.

What are the reasons for use of plan glass at flre top surface of solarcollectors? ^*.A.1o$."ot"

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Q2) What are different boundary conditions used for solution of

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problems? Write the expressions for each of them.

Q3) Byusingdimensional analysis develop ageneralizedempirical relationbetween

Nusselt number, Grashoff number and Prandtl number for free convectionheat transfer.

Q4) Derive an expression for one dimensional heattransfer and temperature profile

for straight rectangular fins.

Q5) Two slabs each 120 mm thick have thermal conductivities of 14.5 w/mk and

210 dmk. These are placed in contact but due to roughness only 30%o of

the area is in contact and the gap in the remaining area is 0.025 mm thick and

is filled with air. If the temperature of the face of the hot surface is at220"C

and the outside surface of the other slab is at 30oC. Determine (a) Heat flow

through composite system (b) The contact resistance and temperature drop' in contact. Assume that the conductivity of air is 0.032 w/mk and that half of

the contact (of the contact area) is due to either metal.

Q6) A flat furnace wall is constructed of a 10 qm layer of refractory brick with

K: 0.l2 kcallhr m C backed by a 20 cm layer of common brick with

K: 1 .2kcallhr m C. The temperature of the inner face of the wall is 760oC

and that of the outer face is 7 5oC. Calculate the heat flow through this way in

kcal/hr m2. Also draw the temperature network circuit.

Section - C(2x10=20)

Q7) A plane wall of fire clay brick ofthickness 25 cm has temperature of 1350oCand 50oC on its two sides. Thermal conductivity for fire clay brick is varying€ts:k (T) : 0.838*(1 +0.0007T), w/mk, where T is temperature in degreecentigrade. Evaluate (a) the rate of heat flow (b) the temperature at the midplane (c) distance of the plane at 400oC from LHS (d) sketch the profiledistribution in the wall.

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eS) A vertical plate, 0.4 m x 0.4 m in size and 3 mm thick, at the uniform

temperature of 180oC, is exposed to atmospheric air at 20'C. Find the

approximate time required for the plate to cool to 30oC, if the heat transfercoefficient in the natural convection for the vertical plate is given by:

h : I .42x (LTIL) r/a. For steel, P : 7800 kdtn' and Cp : 473 J/kgK.

eD Two parallel plates, 0.5 m x I m each, are spaced 0.5 m apart. Plates are at

temperature of 1000oC and 500oC are their emissivities are 0.2 and 0.5

respectively. The plates are located in a large room, the walls of which are

maintaine dat|7"C.The surfaces ofthe plate facing each other only exchange

heat by radiation. Determine the rated of heat lost by each plate and the heat

gained by the walls by radiation. Use the radiation network solution. Assume

the shape factor between the parallel plates as Frr : Fr, : 0.285.

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