Saturday, 14 July 2012

Heat Transfer

 Heat Transfer
 Heat Transfer
1. Introduction

Concept of heat transfer, Difference between the subject of “Heat Transfer” and its parent subject “Thermodynamics”. Different modes of heat transfer - conditions, convection, radiation.

2. Conduction

Fourier’s law of heat conduction, coefficient of thermal conductivity, effect of temperature and pressure on thermal conductivity of solids, liquids and gases and its measurement.
Three-dimensional general conduction equation in rectangular, cylindrical and spherical coordinates involving internal heat generation and unsteady state conditions. Derivation of equations for simple one dimensional steady state heat conduction from three dimensional equations for heat conduction though walls, cylinders and spherical shells (simple and composite), electrical analogy of the heat transfer phenomenon in the cases discussed above.
Equivalent areas, shape factor, conduction through edges and corners of walls and critical thickness of insulation layers on electric wires and pipes carrying hotfiuids. Internal generation cases along with some practical cases of heat conduction like heat transfer through underground electrical cables, simple model of heat conduction through piston crown and case of nuclear fuel rod with cladding. Influence of variable thermal conductivity on conduction through simple cases of walls I cylinders and spheres. Introduction to unsteady heat transfer, Newtonian heating and cooling of solids; definition and cxplanation of the term thermal diffusivity.

3. Theory of Fins

Straight rod type of fins of uniform cross-section; e.g. of circular, rectangular or any other cross-section). Straight fins with varying cross-sectional area and having triangular or trapezoidal profile area, circunerential find of rectangular cross section provided on the circumference of a cylinder.
Optimum design of straight find of rectangular and triangular profile cross-sections; fin effectiveness and fin efficiency for straight rod fins of rectangular and circular cross-section. Application of fins in temperature measurement of flow through pipes and determination of error in its measurement.
Convection Free and forced convection, derivation of three-dimensional mass, momentum and energy conservation equations (with introduction to Tensor notations)
Boundary layer formation, laminar and turbulent boundary layers (simple explanation only and no derivation)

4. Theory of dimensional analysis as applied to free and forced convective heat

Analytical formula for heat transfer in laminar and turbulent flow, flow over vertical and horizontal tubes and plates. Newton’s law of cooling. Overall coefficient of heat transfer. Different design criterion for heat exchangers. Log mean temperature difference for evaporator and condenser tubes, and parallel and counter flow heat exchangers. Calculation of number and length of tubes in a heat exchanger.
Convection with Phase Change (Boiling and Condensation)
Pool boiling, forced convection boiling, heat transfer during pool boiling of a liquid. Nucleation and different theories of nucleation, different theories accounting for the increased values of during nucleate phase of boiling of liquids; different phases of flow boiling (theory only)

5. Radiation

Process of heat flow, definition of emissivity, absorptivity, reflectivity and trarismissMty. Concept of black arid grey bodies, Plank’s law of non-chromatic radiation. Kirchoff’s law and Stefan Boltzmann law. Interchange factor. Lambert’s Cosine law and the geometric factor. Intensity of Radiation (Definition only), radiation density, irradiation, radiosity and radiation shields..
Derivation formula for radiation exchange between two bodies using the definition of radiosity and irradiation and its application to cases of radiation exchange between three or four bodies (e.g. boiler or other furnaces), simplification of the formula for its application to simple bodies like two parallel surfaces, concentric cylinders and a body enveloped by an other body etc.
Error in Temperature measurement by a thermocouple probe due to radiation losses.

1 comment:

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