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Seat No.: Enrolment
GUJARAT TECHNOLOGICAL UNIVERSITY
PDDC SEMESTER-VI EXAMINATION WINTER 2018
Subject Code:X61903 Date: 27/11/2018
Subject Name: Heat and Mass Transfer
Time: 02:30 PM TO 05:00 PM Total Marks: 70
Instructions:
1. Attempt all questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.
Q.1

Explain the terms: Fin Effectiveness and Efficiency of fin.
07

Explain the mechanism of convection heat transfer. Explain free and forced convection.
07
Q.2

What is critical thickness of insulation? Derive an expression for critical radius of insulation for cylinder.
07

State and explain Fick's law of diffusion.
07
OR

Define mass transfer. Explain different modes of mass transfer.
07
Q.3

State and explain Kirchoff's law.
07

What is boiling? Explain different regimes of boiling.
07
OR
Q.3

Explain the types of condensation.
07

Explain in brief: Planck's law and Wein's displacement law
07
Q.4

Using dimensional analysis show that Nusselt number is a function of Reynold number and Prandtl number.
07

Derive the expression for fin efficiency for a fin insulated at the tip.
07
OR
Q.4

Derive an equation of LMTD for counter flow heat exchanger.
07

A copper rod 30 mm in diameter extends from a wall at 150 0C. The surrounding air is at 20 0C. Assuming k 400 W/mK for copper and h 10 W/m2K for convection, Find:
Heat loss from rod assuming it to be infinitely long and
The minimum length of rod in order to consider it as infinitely long fin.
07
Q.5

Derive three dimensional general conduction equation in Cartesian coordinates for a homogenous material.
07

A heat exchanger is required to cool 55,000 kg/h of alcohol from 66 0C to 40 0C using 40,000 kg/h of water entering 5 0C. Calculate exit temperature of water, heat transfer rate and surface area required for parallel flow type heat exchanger. Take overall heat transfer coefficient U 580 W/m2K, Cp (alcohol) 3760 J/kgK and Cp (water) 4180 J/kgK.
07
OR
2
Q.5

A steam pipe of 5 cm inside diameter and 6.5 cm outer diameter is covered with a 2.75 cm radial thickness of high temperature insulation 1.1 W/mK). The surface heat transfer coefficient for inside and outside surfaces are 4650 W/m2K and 11.5 W/m2K respectively. The thermal conductivity of the pipe material is 45 W/m K. If the steam temperature is 200 0C and ambient air temperature is 25 0C, determine:
Heat loss per metre length of pipe
ii) Temperature at the interface
iii) Overall heat transfer coefficient
07

What are dimension less numbers? Explain physical significance of different dimensionless numbers.
07