concept og heat transfer explained in simple terms

concept og heat transfer explained in simple terms

1. CONCEPT OF HEAT TRANSFER Lecture 1

2. Heat Transfer Definition : Heat transfer is the process of transfer of heat from high temperature system to a low temperature system. In the thermodynamic system, heat transfer is the movement of heat across the boundary of the system due to temperature difference between the system and surrounding. There are three modes of heat transfer : Conduction, convection and radiation. The process in which there is no transfer of heat between the system and its surrounding is called as adiabatic process.

3. Application : Evaporation : Heat is supplied in order to convert a liquid into vapour . The liquid present in material is evaporated with the help of heating to get a concentrated product. E.g. preparation of vegetable extracts. Distillation : Heat is supplied to liquid mixture for converting the liquid into vapour so that individual vapour components are condensed at another place. Drying : In the production of tablets, heat is passed through a carrier gas over a bed of wet solid mass for achieving drying. Crystallisation : S aturated solution is heated to bring about supersaturation, which promotes the crystallisation of drugs.

4. Cont.. Sterilisation : For the sterilisation of pharmaceuticals , autoclaves are used with steam as a heating medium. Dry heat is used for the sterilisation of glass apparatus and other containers.

5. Mechanism of Heat Transfer Heat flows from a region of high temperature to a region of low temperature. Heat may flow by one or more of the three basic mechanisms. Conduction Convection Radiation

6. Conduction: Conduction , is a process in which heat flows in a body is achieved by the transfer of the momentum of individual atoms or molecules without mixing . Or Transfer of heat from one atom to another within an object by in direct contact with each other. For example, flow of heat through metal shell of a boiler takes place by conduction as far as solid wall or shell is considered. The flow of heat is depends on the transfer of vibrational energy from one molecule to another , and in case of metal the movement of free electrons.

7. Convection ; Convection , is process in which heat flow is achieved by actual mixing of warmer portions with cooler portions of same material. It is the heat transfer due to bulk movement within fluid such as gases and liquids. For example , heating of water by hot surface is mainly by convection. Natural convection(or free convection) refers to a case where the fluid movement is created by the warm fluid itself. The density of fluid decreases as it is heated. Thus , hot fluids are lighter than cool fluid.

8. Cont .. Forced convection uses external means of producing fluid movement. Forced convection is what in winter days , heat loss from your body is increases due to the constant replenishment of cold air by the wind. Natural wind and fans are two most common sources of forced convection.

9. Radiation Radiation is a energy transfer process in which heat flows through space by means of electromagnetic waves. Radiative heat transfer occurs when the emitted radiation strikes another body and is absorbed .We all experience radiative heat transfer everyday ; solar radiation , absorbed by our skin, is why we feel warmer in the sun in the shadow. Solar water heaters, solar cookers, microwave ovens, microwave cookers, sonicator baths etc., are a few example in which radiation is utilized for producing heat.

10. Conduction Thermal conduction is the transfer of heat (internal energy) by microscopic collision of particles and movement of electrons within a body. The basic law of heat transfer by conduction can be written in the form of rate equation as follows: Rate = driving force resistance Driving force is the temperature drop across solid surfaces, the greater the temperature drop, the greater will be the rate of heat flow. The flow of heat will also depend on conductivity of material through which it is flowing. ------------- (1)

11. Cont .. For example , conduction of heat is faster through an iron rod than though wooden log. This factor is represented by the term resistance, which can be quantitatively expressed by Fourier's law. Resistance = Thickness of the surface (m) Proportionality constant Χ Area of surface = L Km. A This equation for resistance which obtained from Fourier’s law. (2)

12. Fourier’s law – Conduction of heat through Metal wall Fourier’s law states that rate of heat flow through a uniform material is proportional to area and temperature drop and inversely proportional to the length of path of flow. R ate of heat flow α Area (m2) Χ Temp difference ( Δ t) Thickness (m) q α A . Δ t L q = K. A. Δ t L Where, K =mean proportionality constant,W / m.K (3)

13. Derivation : Fourier law can be applied to a metal wall through which the conduction of heat taking place. Area of wall = A,m 2 Thickness of wall= L, m Face of wall (HH) is maintained at uniform, definite & higher temperature = t1, K Face of wall (CC) is maintained at a lower , but uniform temperature = t2 , K The heat flow will be at right angle to the plane A & is assumed to be in steady state.

14. Cont.. Consider thin section of thickness dL at an intermediate point in the wall. for this section , Fourier’s law may be applied as given : dQ = - k.A . dt d θ dL Where, Q= Heat transfer θ = Time,s K = Proportionality constant , W/ m.K t = Temperature ,K The ‘ minus ’ sign indicate the decrease in temperature in direction of flow. in equation ( dt / dL ) represents temperature gradient. (4)

15. Cont.. For the steady state heat transfer, this equation changes to : dQ /d θ = constant = q = - K.A.dt / dL ---------(5) Where , q= rate of heat transfer, J/s (or W) Rearranging equation (5) gives, q = -K.A. Δ t --------------------(6) L Where , Km= mean proportionality constant , W/ m.K in steady state heat transfer, ‘q’ remains constant. Rearranging equ ( 3 ) by comparing it with rate equ . q = Δ t L ...........................(7) K.A

16. Cont.. By Comparing above equation with rate expression , term Δ t indicate the driving force. Resistance = L K.A ----------- (8) Fourier’s law is thus used to define the resistance in quantitative term. The thermal conductivity (K) is the quantity of heat transmitted due to unit temperature gradient, in unit time under steady conditions in direction normal to a surface of the unit area. SI unit of thermal conductivity is watts per meter- kelvin (W/ m.K )