5ME02 HEAT TRANSFER SECTION A UNIT 1 Introduction, heat transfer in engineering, modes of heat transfer, basic laws of heat transfer and their basic equations. Conduction thermal conductivity and thermal diffusivity effect of phase & temperature on thermal conductivity, one dimensional steady state heat conduction through slab, cylinder & sphere-simple and composite. Combined conduction- convection, overall heat transfer coefficient. General heat conduction differential equation. One dimensional steady state conduction with internal heat generation for infinite slab, wire & cylinder. (8 Hrs) UNIT 2 Insulations, critical radius of insulation, Conduction through extended surfaces, analysis of a uniform C.S. fin, fin efficiency, fin effectiveness, Biot number. Introduction to unsteady state heat conduction, Newton’s law of cooling, lumped heat capacity analysis. (8 Hrs) UNIT 3 Radiation- general concepts and definitions, black body & grey body concept. Laws of radiation- Kirchoff’s, Plank’s, Stefan- Boltzman’s, Wien’s law. Concept of shape factor, emissivity factor and radiation heat transfer equation. (No numericals). Radiation errors in temperature, measurement, radiation shield. (7 Hrs) SECTION B UNIT 4 Forced convection- heat convection, forced and natural convection, boundary layer theory, hydrodynamic & thermal boundary layers, boundary layer thickness. Laminar & turbulent flow over flat plate and through pipes & tubes (only concept, no derivation & analytical treatment). Dimensionless number and their physical significance Reynolds, Prandtl, Nusselt, Grashof number, empirical correlations for forced convection for flow over flat plate, through pipes & tubes & their applications in problem solving. (8 Hrs) UNIT 5 Free convection- velocity and thermal boundary layers for vertical plate, free convection over vertical cylinder and horizontal plate/cylinder (only concept, no derivation & analytical treatment). Use of empirical correlations in problem solving. Condensation & Boiling - introduction to condensation heat transfer, film & drop condensation. Boiling heat transfer, pool boiling curves. (7 Hrs) UNIT 6 Heat exchanger - applications, classification, overall heat transfer coefficient, fouling. L.M.T.D. & E.N.T.U. methods, temperature profiles, selection of heat exchangers. Introduction to working of heat pipe with and without wick. (7 Hrs) 2 HEAT TRANSFER __________________________________________________________________________________ HEAT TRANSFER 3 _________________________________________________________________________________ UNIT 1 INTRODUCTION In thermodynamics when two systems are brought into contact with each other some kind of wall, energy transfer such as heat and work take place between them. Work is transfer of energy to a principle which is evidence by changes in its position when acted upon by a force. Heat like work is energy in the process of being transferred. Energy is what is stored, and work and heat are two ways of transferring energy across the boundaries of system. The amount of energy transfer as heat can be determined from energy conservation considerations. Two system are said to be in thermal equilibrium with one another if no energy transfer such as heat occur between them in a finite period when they are connected through the diathermal wall. Temperature is property of matter which two bodies are in equilibrium having in common. Hot and cold are the adjectives used to describe high and low values of temperature. The energy transfer by heat will take place from body with higher temperature to the body with lower temperature, if they two are permitted to interact through a diathermal wall (second law of thermodynamics). The transfer and conversion of energy from one form to another is basic to all heat transfer processes and hence they are governed by the first as well as second law of thermodynamics. This does not and must not mean that the principles governing the transfer of heat transfer that can be derived from, or are more corollaries of, the basic law of thermodynamics. The major difference between the thermodynamics and heat transfer is that the former deals with the relation between heat and other forms of energy, whereas the latter is concerned with the analysis of the rate of heat transfer. Thermodynamics deals with the system in equilibrium so it cannot be expected to predict quantitatively the rate of change in process which results from non equilibrium states. Temperature is meant for heat transfer to take place. The knowledge of temperature distribution is essential in heat transfer studies because of the fact that heat flow takes place only wherever there is temperature gradient in a system. The heat flux (q) is defined as the amount of heat transfer per unit area per unit time, can be calculated from the physical law relating the temperature gradient in the heat flow. HEAT TRANSFER IN ENGINEERING The study of temperature distribution and heat transfer is of great importance to engineers because of its almost universal occurrence in many branches of science and engineering. The first step in optimal design of heat exchangers such as boiler, heater, refrigerator and radiator is a detailed analysis of heat transfer. This is essential to determine the feasibility and cost of the undertaking, as well as the size of equipment required to transfer specified amount of heat in given time. A through heat transfer analysis is most important for the proper sizing of fuel elements in the nuclear reactor core to prevent burnout. The performance of air craft is also depending on the ease with which structure and engine can be cooled. The design of chemical plant is usually done on the basis of heat transfer and the analogous mass transfer processes. An accurate heat transfer analysis is necessary in the refrigeration and air-conditioning applications to calculate the heat loads, and to determine the thickness of insulation to avoid excess in heat gains or losses. The utilization of solar 4 HEAT TRANSFER __________________________________________________________________________________ energy which is so abundantly available also requires a thorough knowledge of heat transfer for the proper design of the solar collector and associated equipment. These are only a few examples in to indicate the importance of heat transfer in engineering sciences. It is clear that engineers and scientist must have a thorough knowledge of science of heat transfer to be able to quantitatively analyse the problem involving the transfer of heat. MECHANISM OF HEAT TRANSFER Energy transfer as heat takes place by three distinct modes: conduction convection and radiation. The heat conduction is mode of heat transfer accomplish via two mechanism. By molecular interaction whereby the energy exchanges takes place by the kinetic motion or direct impact of molecule. Molecule at relatively high energy level. Molecule at a relatively high energy level imparts energy to adjacent molecule at lower energy levels. This types of energy transfer always exists so long as there is temperature gradient in a system comprising molecules of solid, liquid or gas. The drift of free electron as in the case of metallic solids. The metallic alloys have different concentration of free electrons, and their ability to conduct heat directly proportional to the concentration of free electron in them. The free electron concentration of non metals very low. Hence materials that are good conductor are good conductor of heat too. Pure conduction is found only in solid. CONVECTION It is possible only in the presence of fluid medium. When a fluid flows inside a duct or over a solid body and the temperature of the fluid and the solid surfaces are different the heat transfer between the fluid and the solid surface will take place. This is due to the motion of fluid relative to the surface. This type of heat transfer is called convection. The transport of fluid inseparably linked with the movement of fluid itself. If the fluid motion is set up by buoyancy effect resulting in from the density variation caused by the temperature difference in the fluid, the heat transfer is said to be free or natural HEAT TRANSFER 5 _________________________________________________________________________________ convection. On the other hand if the fluid motion is artificially created by means of an external agency like a blower or fan, the heat transfer is termed as forced convection. As the energy transferred between the solid surface and the fluid at the surface can take place only by conduction, the heat transfer by convection is always accompanied by conduction. RADIATION If two bodies are at different temperature are placed in an evacuated adiabatic enclosure so that they are not in contact through a solid or fluid medium, temperature of the two bodies will tend to become equal. The mode of heat transfer by which this equilibrium is achieved is called thermal radiation. Radiation is an electromagnetic wave phenomenon, and no mediums required for its propagation. In fact the energy transfer by radiation is maximum when the two bodies exchanging energy are separated by a perfect vacuum. Thermal radiation depends only on the temperature and on the optical properties of the emitter. Apart from the identification of different modes of heat transfer it is also important to determine whether a process is steady or unsteady. STEADY AND UNSTEADY PROCESS A steady process is a one which the rate of heat transfer does not vary with time. In steady process there can be