Chapter 1: Introduction to the Three Heat Transfer Modes

1. Chapter 1: Introduction to the Three Heat Transfer Modes Heat Transfer: Energy in transient due to a temperature difference . Fluid Mechanics: Velocity Distribution  Shear Stress, Pressure drop. Heat Transfer: Temperature distribution  Heat transfer rate q. Thermodynamics: General principles, simple analyses, gross balance (global pictures of thermal systems). Heat transfer: More comprehensive analyses employing thermodynamic principles, more detailed calculations for engineering designs.

2. Chapter 1: Introduction to the Three Heat Transfer Modes (continued) Fig. 1-1 A case study illustrating the three heat transfer modes.

3. The Three Heat Transfer Modes - Conduction Different Types of Heat Transfer Processes (modes): Heat Conduction: Heat transfer in which energy exchange takes place in solid or fluids in rest from the region of high temperature to the region of low temperature due to the presence of a temperature gradient in the body – due to atomic and molecular activities. Fourier’s Law for One-dimensional Heat Conduction: For a linear temperature distribution

4. Conduction (continued) Thermal conductivity k: Metallic solids: 10 – 400 W/m-K Nonmetallic solids: 0.1 – 10 W/m-K Fluids: 0.01 – 10 W/m-K Insulation systems (a combination of nonmetallic solids/gases): 0.05 – 0.1 W/m-K Conduction is the most important heat transfer mode for solids

5. Convection Heat Transfer Convection Heat Transfer: Heat transfer by motion of medium in which the heat transfer takes place. The convection heat transfer mode is comprised of two mechanisms: bulk or macroscopic motion of fluid, and random molecular motion (diffusion or conduction). But the conduction effect is usually weak. Forced convection: when flow is caused by external means, such as by a fan, a pump, or atmospheric wind. Free or natural convection: the flow is induced by buoyancy forces which arise from density differences caused by temperature variations in the fluid. Latent heat exchange involving change of phase (hfg = 2.40  106 J/kg or 1040 Btu/lb for water). Boling: Liquid  Vapor; condensation: Liquid  Vapor Melting: Solid  Liquid, Freezing: Solid  Liquid (hsf = 3.34  105 J/kg for water)

6. Convection (continued)

7. Convection (continued) Newton’s Law of Cooling: where: Ts: the surface temperature, : the fluid temperature, and h: the heat transfer coefficient or film coefficient which encompasses all the parameters influencing convection heat transfer. In general: h (Boiling/condensation) > h (Forced convection) > h (Free convection)

8. Radiation Heat Transfer: Thermal radiation is energy emitted by matter that is at a finite temperature. Various Spectra of radiation exist (AM radio, FM radio, Microwave, Radar, X rays, …). From 50 – 6000 K, the radiant energy in the wave band 0.1 to 1000 m has a direct effect on the temperature of medium participating in radiation and on the radiation heat transfer – Thermal radiation. Radiation Heat Transfer • The phenomenon of radiation can be explained through the theory of photons or microscopic electromagnetic waves. • Sun light – visible radiation (0.4 – 0.7 m); most of thermal radiation – infrared radiation (0.8 – 25 m, invisible). • No transfer medium is required. Radiation transfer occurs most efficiently in a vacuum.

9. Radiation Heat Transfer (continued) where  is the Stefan-Boltzmann constant = 5.6710-8 W/m2-K4 or 0.171410-8Btu/h-ft2-oR4 The maximum heat flux at which radiation may be emitted from a surface is given by the Stefan-Boltzmann law for a blackbody or an ideal radiator: For a real surface, where  is called emissivity,

10. For a special case: a small surface and a much larger surface that completely surrounds the smaller one: Radiation Heat Transfer (continued) Fig. 1-6 Radiation exchange: (a) at a surface and (b) between a surface and large surroundings.

11. The Conservation of Energy Requirement The First Law of Thermodynamics - the law of conservation of energy: At any instant of time, - Surface phenomena, occurring at the control surface due to conduction, convection (fluid flow), or radiation. , - the rate of conversion from some other energy forms ( chemical, electrical, or nuclear) – a volumetric phenomenon. - associated with an increase ( > 0) or decrease ( < 0) in the energy of matter occupying the control volume. For a time intervalt

12. Relevance of Heat Transfer Power plants (boilers, steam turbines, combustors, gas turbines, and condensers) Engines (internal combustion engines and aircraft engines) that involve the combustion of fossil fuels Nuclear energy Chemical engineering Solar energy conversion systems Refrigeration and air conditioning systems The cooling of electronic equipment Air and water pollution