CONVECTIVE HEAT TRANSFER

Mohammad GoharkhahDepartment of Mechanical Engineering, Sahand Unversity of Technology,

Tabriz, Iran

Advanced Convection Heat TransferSubjects involved:

1- Basic Concepts 2- Differential Formulation of the Basic Laws3-Boundary layer4- Laminar Internal Flow5- Natural Convection6- Transition to Turbulence7- Turbulent Flows8- Convection with Phase Change9- Convection in Porous Media

Grading:1- Mid Term 6/202- Final Exam 10/203- Home works and Term Project 4/20

References1- A. Bejan, Convection Heat Transfer, John Wiley and Sons, 1995.2-M. Jiji Latif, Heat Convection, Springer- Verlog Berlin Heidelberg, 20063-P. H. Qosthuizen and D. Naylor, An Introduction to Convective Heat Transfer Analysis,WCB/McGraw Hill, 1999.4- W. M. Kays and M. E. Crawford, Convective Heat and Mass Transfer, McGraw- Hill,1993.

BASIC CONCEPTS

CHAPTER 1

CONVECTIVE HEAT TRANSFER- CHAPTER1By: M. Goharkhah

SAHAND UNIVERSITY OF TECHNOLOGY DEPARTMENT OF MECHANICAL ENGINEERING

What is convective heat transfer?Heat convection is the term applied tothe process involved when energy istransferred from a surface to a fluidflowing over it as a result of a differencebetween the temperatures of thesurface and the fluid. In convection,there is always a surface, a fluid flowingrelative to this surface and atemperature difference between thesurface and the fluid and the concern iswith the rate of heat transfer betweenthe surface and the fluid.

Convective heat transfer applications

CONVECTIVE HEAT TRANSFER- CHAPTER1By: M. Goharkhah

SAHAND UNIVERSITY OF TECHNOLOGY DEPARTMENT OF MECHANICAL ENGINEERING

History

Convective cooling of solid objects submerged into a moving single-phase fluid was addressed by Newton.

The boundary Iayer treatment of fluid flow and heat transfer wasaddressed by Prandtl.

The rigorously derived conservation equations and constitutiverelations developed for the single-phase fluid flow and heat transfer byFourier, Navier, and Stokes.

With time, the emphasis of convective heat transfer has expandedbeyond the single- and two-phase heat exchangers to the materials andmanufacturing processes such as the solidification of multicomponentliquids and the inter action of the weId pool and its surrounding plasma.Also, among these expansions are the reacting flows such as in the liquidand solid spray combustion.The heat exchange between electrons, heavier species, and suspendedsmall particles in flowing, low-pressure plasmas is being addressed.

CONVECTIVE HEAT TRANSFER- CHAPTER1By: M. Goharkhah

SAHAND UNIVERSITY OF TECHNOLOGY DEPARTMENT OF MECHANICAL ENGINEERING

Convection, Conduction and Radiation

• Convective heat transfer rates depend on the details of the flow fieldabout the surface involved as well as on the properties of the fluid. Thedetermination of convective heat transfer rates involves thedetermination of both the velocity and temperature fields.

• The transfer of heat by convection involves the transfer of energy fromthe surface to the fluid on a molecular scale and then the diffusion ofthis heat through the fluid by bulk mixing due to the fluid motion. Thebasic 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 andby conduction in the absence of it.

CONVECTIVE HEAT TRANSFER- CHAPTER1By: M. Goharkhah

SAHAND UNIVERSITY OF TECHNOLOGY DEPARTMENT OF MECHANICAL ENGINEERING

Convection, Conduction and Radiation

• Radiation energy exchange between two surfaces depends on thegeometry, shape, area, orientation, emissivity, and absorptivity of thetwo surfaces.

• Absorptivity is a surface property defined as the fraction of radiationenergy incident on a surface which is absorbed by the surface.

• The net heat exchange by radiation between two surfaces, q12, is givenby Stefan-Boltzmann radiation law: (Note that this is a simplifiedmodel for which the absorptivity is equal to the emissivity )

CONVECTIVE HEAT TRANSFER- CHAPTER1By: M. Goharkhah

SAHAND UNIVERSITY OF TECHNOLOGY DEPARTMENT OF MECHANICAL ENGINEERING

FORCED, FREE, AND COMBINED CONVECTION

In the case of forced convection, the fluid motion is caused by some externalmeans such as a fan or pump. In the case of free convection, the flow isgenerated by the body forces that occur as a result of the density changesarising from the temperature changes in the flow field.

In some flows in which a forced velocity exists, the effects of these buoyancyforces will, however. not be negligible and such flows are termed combined- ormixed free and forced convective flows.

CONVECTIVE HEAT TRANSFER- CHAPTER1By: M. Goharkhah

SAHAND UNIVERSITY OF TECHNOLOGY DEPARTMENT OF MECHANICAL ENGINEERING

Heat transfer involving a change of phase isclassified as convective heat transfer eventhough when the solid phase is involved, theoverall process involves combined andinterrelated convection and conduction. Heattransfer during boiling, condensation, andsolidification (freezing) all, thus, involveconvective heat transfer.

Heat transfer with Phase Change

Internal and External flows

CONVECTIVE HEAT TRANSFER- CHAPTER1By: M. Goharkhah

SAHAND UNIVERSITY OF TECHNOLOGY DEPARTMENT OF MECHANICAL ENGINEERING

Newton's Law of Cooling

• Heat transfer rate between a surface and an adjacent fluid in motion

Unlike thermal conductivity k, the heattransfer coefficient is not a materialproperty. it depends on geometry, fluidproperties, motion, and in some casestemperature difference

Analytical determination of h requiresknowledge of temperature distributionin a moving fluid

K: Thermal conductivity of the fluid

CONVECTIVE HEAT TRANSFER- CHAPTER1By: M. Goharkhah

SAHAND UNIVERSITY OF TECHNOLOGY DEPARTMENT OF MECHANICAL ENGINEERING

Newton's Law of Cooling

the definition used depends on the flow situation.For incompressible external flow over the outside of a body. Tf isconveniently taken as the temperature of the fluid in the free streamahead of the body.In the case of internal flows. such as flow through a pipe, the fluidtemperature will vary continuously across the whole flow area. In suchcases, fluid temperature is taken as some mean temperature. The mostcommonly used mean temperature is the so-called bulk temperature,Tb.

How the fluid temperature is defined???

CONVECTIVE HEAT TRANSFER- CHAPTER1By: M. Goharkhah

SAHAND UNIVERSITY OF TECHNOLOGY DEPARTMENT OF MECHANICAL ENGINEERING

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Heat transfer through a fluid layer of thickness L and temperature difference T.

Lc characteristic length

Nusselt NumberIn convection studies, it is common practice to nondimensionalize the governingequations and combine the variables, which group together into dimensionless numbersin order to reduce the number of total variables.Nusselt number: Dimensionless convection heat transfer coefficientThe 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.

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