CONVECTIVE HEAT TRANSFER

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1 CONVECTIVE HEAT TRANSFER Mohammad Goharkhah Department of Mechanical Engineering, Sahand Unversity of Technology, Tabriz, Iran

2 Advanced Convection Heat Transfer Subjects involved: 1- Basic Concepts 2- Differential Formulation of the Basic Laws 3-Boundary layer 4- Laminar Internal Flow 5- Natural Convection 6- Transition to Turbulence 7- Turbulent Flows 8- Convection with Phase Change 9- Convection in Porous Media Grading: 1- Mid Term 6/20 2- Final Exam 10/20 3- Home works and Term Project 4/20 References 1- A. Bejan, Convection Heat Transfer, John Wiley and Sons, M. Jiji Latif, Heat Convection, Springer- Verlog Berlin Heidelberg, P. H. Qosthuizen and D. Naylor, An Introduction to Convective Heat Transfer Analysis, WCB/McGraw Hill, W. M. Kays and M. E. Crawford, Convective Heat and Mass Transfer, McGraw- Hill, 1993.

3 CHAPTER 1 BASIC CONCEPTS

4 What is convective heat transfer? Heat convection is the term applied to the process involved when energy is transferred from a surface to a fluid flowing over it as a result of a difference between the temperatures of the surface and the fluid. In convection, there is always a surface, a fluid flowing relative to this surface and a temperature difference between the surface and the fluid and the concern is with the rate of heat transfer between the surface and the fluid. Convective heat transfer applications

5 History Convective cooling of solid objects submerged into a moving singlephase fluid was addressed by Newton. The boundary Iayer treatment of fluid flow and heat transfer was addressed by Prandtl. The rigorously derived conservation equations and constitutive relations developed for the single-phase fluid flow and heat transfer by Fourier, Navier, and Stokes. With time, the emphasis of convective heat transfer has expanded beyond the single- and two-phase heat exchangers to the materials and manufacturing processes such as the solidification of multicomponent liquids and the inter action of the weid pool and its surrounding plasma. Also, among these expansions are the reacting flows such as in the liquid and solid spray combustion. The heat exchange between electrons, heavier species, and suspended small particles in flowing, low-pressure plasmas is being addressed.

6 Convection, Conduction and Radiation Convective heat transfer rates depend on the details of the flow field about the surface involved as well as on the properties of the fluid. The determination of convective heat transfer rates involves the determination of both the velocity and temperature fields. The transfer of heat by convection involves the transfer of energy from the surface to the fluid on a molecular scale and then the diffusion of this heat through the fluid by bulk mixing due to the fluid motion. The basic heat transport mechanism in convection is still conduction Conduction is governed by Fourier's law. Conduction and convection both require the presence of a material medium. Heat transfer through a solid is always by conduction. Heat transfer through a fluid is by convection in the presence of bulk fluid motion and by conduction in the absence of it.

7 Convection, Conduction and Radiation Radiation energy exchange between two surfaces depends on the geometry, shape, area, orientation, emissivity, and absorptivity of the two surfaces. Absorptivity is a surface property defined as the fraction of radiation energy incident on a surface which is absorbed by the surface. The net heat exchange by radiation between two surfaces, q 12, is given by Stefan-Boltzmann radiation law: (Note that this is a simplified model for which the absorptivity is equal to the emissivity )

8 FORCED, FREE, AND COMBINED CONVECTION In the case of forced convection, the fluid motion is caused by some external means such as a fan or pump. In the case of free convection, the flow is generated by the body forces that occur as a result of the density changes arising from the temperature changes in the flow field. In some flows in which a forced velocity exists, the effects of these buoyancy forces will, however. not be negligible and such flows are termed combined- or mixed free and forced convective flows.

9 Heat transfer with Phase Change Heat transfer involving a change of phase is classified as convective heat transfer even though when the solid phase is involved, the overall process involves combined and interrelated convection and conduction. Heat transfer during boiling, condensation, and solidification (freezing) all, thus, involve convective heat transfer. Internal and External flows

10 Newton's Law of Cooling Heat transfer rate between a surface and an adjacent fluid in motion Unlike thermal conductivity k, the heat transfer coefficient is not a material property. it depends on geometry, fluid properties, motion, and in some cases temperature difference Analytical determination of h requires knowledge of temperature distribution in a moving fluid K: Thermal conductivity of the fluid

11 Newton's Law of Cooling How the fluid temperature is defined??? the definition used depends on the flow situation. For incompressible external flow over the outside of a body. T f is conveniently taken as the temperature of the fluid in the free stream ahead of the body. In the case of internal flows. such as flow through a pipe, the fluid temperature will vary continuously across the whole flow area. In such cases, fluid temperature is taken as some mean temperature. The most commonly used mean temperature is the so-called bulk temperature, Tb.

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13 Nusselt Number In convection studies, it is common practice to nondimensionalize the governing equations and combine the variables, which group together into dimensionless numbers in order to reduce the number of total variables. Nusselt number: Dimensionless convection heat transfer coefficient The Nusselt number represents the enhancement of heat transfer through a fluid layer as a result of convection relative to conduction across the same fluid layer. The larger the Nusselt number, the more effective the convection. A Nusselt number of Nu = 1 for a fluid layer represents heat transfer across the layer by pure conduction. L c characteristic length 13 Heat transfer through a fluid layer of thickness L and temperature difference T.

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