Fundamentals of ThermalFluid Sciences 3 rd Edition Yunus

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Fundamentals of Thermal-Fluid Sciences, 3 rd Edition Yunus A. Cengel, Robert H. Turner, John

Fundamentals of Thermal-Fluid Sciences, 3 rd Edition Yunus A. Cengel, Robert H. Turner, John M. Cimbala Mc. Graw-Hill, 2008 Chapter 16 MECHANISMS OF HEAT TRANSFER Mehmet Kanoglu Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

Objectives • Understand the basic mechanisms of heat transfer, which are conduction, convection, and

Objectives • Understand the basic mechanisms of heat transfer, which are conduction, convection, and radiation, and Fourier's law of heat conduction, Newton's law of cooling, and the Stefan– Boltzmann law of radiation • Identify the mechanisms of heat transfer that occur simultaneously in practice • Develop an awareness of the cost associated with heat losses • Solve various heat transfer problems encountered in practice 2

INTRODUCTION • Heat: The form of energy that can be transferred from one system

INTRODUCTION • Heat: The form of energy that can be transferred from one system to another as a result of temperature difference. • Heat Transfer deals with the determination of the rates of such energy transfers as well as variation of temperature. • Higher-temperature to the lower-temperature medium. • Heat transfer stops when the two mediums reach the same temperature. • Heat transfer modes: 3

CONDUCTION Conduction: The transfer of energy from the more energetic particles of a substance

CONDUCTION Conduction: The transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles. In gases and liquids, conduction is due to the collisions and diffusion of the molecules during their random motion. In solids, it is due to the combination of vibrations of the molecules in a lattice and the energy transport by free electrons. Heat conduction through a large plane wall of thickness x and area A. 4

STEADY HEAT CONDUCTION IN PLANE WALLS Steady Operation: The rate of heat transfer through

STEADY HEAT CONDUCTION IN PLANE WALLS Steady Operation: The rate of heat transfer through wall must be constant. Heat transfer through a wall is one-dimensional when the temperature of the wall varies in one direction only. Fourier’s law of heat conduction. 5

Under steady conditions, the temperature distribution in a plane wall is a straight line:

Under steady conditions, the temperature distribution in a plane wall is a straight line: d. T/dx = const. In heat conduction analysis, A represents the area normal to the direction of heat transfer. The rate of heat conduction through a plane wall is proportional to the average thermal conductivity, the wall area, and the temperature difference, but is inversely proportional to the wall thickness. k: A measure of the ability of a material to conduct heat. Negative sign in the equation ensures that heat transfer in the positive x direction is a positive quantity. 6

Thermal Conductivity The thermal conductivity of a material is a measure of the ability

Thermal Conductivity The thermal conductivity of a material is a measure of the ability of the material to conduct heat. 7

Thermal Diffusivity cp Specific heat, J/kg · °C: Heat capacity per unit mass cp

Thermal Diffusivity cp Specific heat, J/kg · °C: Heat capacity per unit mass cp Heat capacity, J/m 3 · °C: Heat capacity per unit volume Thermal diffusivity, m 2/s: Represents how fast heat diffuses through a material 8

Ex-17 -1, pg 660 Heat loss through wall. Determine rate of heat loss. 9

Ex-17 -1, pg 660 Heat loss through wall. Determine rate of heat loss. 9

CONVECTION Convection: • The mode of energy transfer between a solid surface and the

CONVECTION Convection: • The mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion. • Involves the combined effects of conduction and fluid motion. • The faster the fluid motion, the greater the convection heat transfer. Heat transfer from a hot surface to air by convection. 10

Forced convection: If the fluid is forced to flow over the surface by external

Forced convection: If the fluid is forced to flow over the surface by external means (a fan, pump, or the wind) Natural (or free) convection: If the fluid motion is caused by buoyancy forces. The cooling of a boiled egg by forced and natural convection. 11

Newton’s law of cooling h As Ts T convection heat transfer coefficient, W/m 2

Newton’s law of cooling h As Ts T convection heat transfer coefficient, W/m 2 · °C the surface area through which convection heat transfer takes place the surface temperature the temperature of the fluid sufficiently far from the surface. h is not a property of the fluid. It is an experimentally determined parameter whose value depends on all the variables influencing convection such as - the surface geometry - the nature of fluid motion - the properties of the fluid - the bulk fluid velocity 12

Example 16 -4 Power = VI Area of cable = 2*Pi*Radius 13

Example 16 -4 Power = VI Area of cable = 2*Pi*Radius 13

RADIATION • The energy emitted by matter in the form of electromagnetic waves as

RADIATION • The energy emitted by matter in the form of electromagnetic waves as a result of the changes in the electronic configurations of the atoms or molecules. • does not require the presence of an intervening medium. • heat transfer by radiation is fastest (at the speed of light) and it suffers no attenuation in a vacuum. This is how the energy of the sun reaches the earth. • In heat transfer studies we are interested in thermal radiation, i. e radiation emitted by bodies because of their temperature. • All bodies at a temperature above absolute zero emit thermal radiation. • Radiation is a volumetric phenomenon, and all solids, liquids, and gases emit, absorb, or transmit radiation to varying degrees. However, radiation is usually considered to be a surface phenomenon for solids. 14

Max. rate of radiation at T (K) of a body, Stefan–Boltzmann law = 5.

Max. rate of radiation at T (K) of a body, Stefan–Boltzmann law = 5. 670 10 8 W/m 2 · K 4 Stefan–Boltzmann constant Blackbody: The idealized surface that emits radiation at the maximum rate. Radiation emitted by real surfaces Emissivity : A measure of how closely a surface approximates a blackbody for which = 1 of the surface. 0 1. 15

Absorptivity : The fraction of the radiation energy incident on a surface that is

Absorptivity : The fraction of the radiation energy incident on a surface that is absorbed by the surface. 0 1 A blackbody absorbs the entire radiation incident on it ( = 1). Kirchhoff’s law: The emissivity and the absorptivity of a surface at a given temperature and wavelength are equal. The absorption of radiation incident on opaque surface of absorptivity α. 16

Net radiation heat transfer: The difference between the rates of radiation emitted by the

Net radiation heat transfer: The difference between the rates of radiation emitted by the surface and the radiation absorbed. Radiation heat transfer between a surface and the surfaces surrounding it. The determination of the net rate of heat transfer by radiation between two surfaces is a complicated matter since it depends on • the properties of the surfaces • their orientation relative to each other • the interaction of the medium between the surfaces with radiation When radiation and convection occur Radiation is usually significant relative to conduction or natural convection, but negligible relative to forced convection. simultaneously between a surface and a gas Combined heat transfer coefficient hcombined Includes the effects of both convection and radiation 17

Ex – 16 -5 Troom= 22 celcius, Inner surface of walls, floors, ceiling is

Ex – 16 -5 Troom= 22 celcius, Inner surface of walls, floors, ceiling is at 10 C (winter) and 25 C (summer). 18

SIMULTANEOUS HEAT TRANSFER exposed MECHANISMS Solid surfaces to a fluid Convection &/or radiation Solids

SIMULTANEOUS HEAT TRANSFER exposed MECHANISMS Solid surfaces to a fluid Convection &/or radiation Solids Conduction &/or radiation A still fluid (no fluid motion), heat transfer is by conduction and possibly by radiation. A flowing fluid, heat transfer is by convection and radiation. Most gases between two solid surfaces do not interfere with radiation. Liquids are usually strong absorbers of radiation. Although there are three mechanisms of heat transfer, a medium may involve only two of 19 them simultaneously.

Ex 16 -6 20

Ex 16 -6 20

Summary • Conduction ü Fourier’s law of heat conduction ü Thermal Conductivity ü Thermal

Summary • Conduction ü Fourier’s law of heat conduction ü Thermal Conductivity ü Thermal Diffusivity • Convection ü Newton’s law of cooling • Radiation ü Stefan–Boltzmann law • Simultaneous Heat Transfer Mechanisms 21

Thermal Resistance Concept Conduction resistance of the wall: Thermal resistance of the wall against

Thermal Resistance Concept Conduction resistance of the wall: Thermal resistance of the wall against heat conduction. Thermal resistance of a medium depends on the geometry and thermal properties of the medium. Analogy between thermal and electrical resistance concepts. rate of heat transfer electric current thermal resistance electrical resistance Electrical resistance temperature difference voltage difference 22