(SEM- V) THEORY EXAMINATION 2021-22 HEAT AND MASS TRANSFER

HEAT AND MASS TRANSFER (KME-501)

B.Tech (Sem V) – Exam Notes & Solved Guide

SECTION A – Short Answer Type (2 Marks Each)

a. Difference between Thermodynamics and Heat Transfer
Thermodynamics deals with the amount of heat transfer between systems at equilibrium but does not consider the rate of heat transfer. Heat transfer deals with the rate of heat flow and temperature distribution during non-equilibrium conditions.

b. Thermal Conductivity
Thermal conductivity is the ability of a material to conduct heat.
Unit: W/m·°C.
It is defined as the heat conducted per unit area per unit time per unit temperature gradient.

c. Transient Heat Conduction
Transient heat conduction occurs when temperature within a body changes with time, such as heating or cooling processes.

d. Effectiveness and Efficiency of Fin
Fin effectiveness is the ratio of heat transfer with fin to heat transfer without fin.
Fin efficiency is the ratio of actual heat transfer to maximum possible heat transfer by the fin.

e. Turbulent Flow
Turbulent flow is characterized by irregular fluid motion, mixing, and eddies. It occurs at high Reynolds number.

f. Reynolds Number
Reynolds number is the ratio of inertial forces to viscous forces.
It helps predict whether the flow is laminar or turbulent.

g. Stefan–Boltzmann Law
The total radiation emitted by a black body is proportional to the fourth power of its absolute temperature:
q = σT⁴

h. Black, White, Grey and Opaque Body
A black body absorbs all radiation, a white body reflects all radiation, a grey body partially absorbs radiation, and an opaque body does not transmit radiation.

i. Classification of Heat Exchangers
Heat exchangers are classified based on flow arrangement, construction, and transfer process (parallel flow, counter flow, cross flow).

j. Modes of Mass Transfer
Mass transfer occurs by diffusion, convection, and phase change.

SECTION B – Descriptive Answers (10 Marks Each)

a. Heat Conduction Through a Composite Wall

For a composite wall consisting of different materials in series, the total heat transfer rate is given by:

Q = (T₁ − T₂) / (ΣR)

where ΣR is the sum of thermal resistances of individual layers.
Each layer contributes resistance equal to thickness divided by thermal conductivity.

b. Heat Transfer in a Frying Ladle Handle (Numerical)

This problem involves one-dimensional steady conduction with convection at the free end. Using Fourier’s law and boundary conditions, the convective heat transfer coefficient is determined so that the handle temperature remains safe.

c. Natural vs Forced Convection & Boundary Layers

Natural convection occurs due to buoyancy forces caused by temperature difference, while forced convection is due to external devices like fans or pumps.
Hydrodynamic boundary layer relates to velocity variation, while thermal boundary layer relates to temperature variation.

d. Radiation from Circular Hole

Radiative heat loss is calculated using Stefan–Boltzmann law assuming black body behavior. End surfaces are treated as parallel discs.

e. Effectiveness-NTU Method (Parallel Flow)

Effectiveness is defined as the ratio of actual heat transfer to maximum possible heat transfer. NTU method is useful when outlet temperatures are unknown.

SECTION C – Long Answer Type

a. General Heat Conduction Equation (Cartesian Coordinates)

Derived using energy balance on a differential control volume considering conduction in x, y, z directions and internal heat generation.
The temperature-thickness profile shows linear variation for steady-state conduction without heat generation.

b. Heat Loss from Mild Steel Tank (Numerical)

Thermal resistance network includes inside convection, conduction through steel wall, and outside convection.
Using:
Q = (Ti − Ta) / Rtotal
The outer surface temperature is obtained from conduction resistance relation.

c. Transient Cooling of Aluminium Plate

Using lumped capacitance method when Biot number < 0.1.
Temperature variation is given by:        (T − Ta)/(Ti − Ta) = exp(−Bi·Fo)

d. Free Convection from Vertical Plates

Using Nusselt number correlation given in the question, Grashof and Prandtl numbers are evaluated at film temperature to find maximum heat dissipation.

e. Forced Convection over Flat Plate

Heat transfer coefficient depends on flow direction, Reynolds number, and plate length.
Longer flow length increases boundary layer thickness and heat transfer rate.

f. Radiation Heat Exchange with Shield

Radiation shield reduces heat transfer by increasing overall radiation resistance.
Net heat exchange is calculated using emissivity and Stefan–Boltzmann law.

g. Parallel Flow Heat Exchanger (Numerical)

Using energy balance and LMTD method, area and heat transfer rate are calculated from given flow rates and temperatures.

h. Filmwise vs Dropwise Condensation

Filmwise condensation forms a continuous liquid film and has lower heat transfer.
Dropwise condensation forms droplets and provides much higher heat transfer rate.