Introduction to Convection:Flow and Thermal ConsiderationsChapter Six and Appendix ESections 6.1 through 6.8 and E.1 through E.3
Convection Heat Transfer Processes
Convection Heat TransferTABLE 1.1 Typical values of the convection heat transfer coefficient
Recall: Relation of convection to flow over a surface and developmentof velocity and thermal boundary layers:Newton’s law of cooling:h: Convection heat transfer coefficient Convection Heat TransferConvection heat transfer rate, qQuestion: How do we determine the value of h? )(TThqs)(TThAAqqsfluidk:Nu.
Dimensional AnalysisDefine the following dimensionless groups:Note, Nu (thus h)depends on location, x.Averaged Nusselt Number over length of plate will be of the form:Averaged convection heat transfer coefficient),,,,,,(fpkcVLxfhfpfkcVLLxfkhL,,1VLRe,NumberReynoldsvkckcfpfp)/(Pr,NumberPrandtlfkhLNu,NumberNusselt Lxx*Pr),Re,(*LxfNuPr),(ReLfNuNuLkhf
Convection Heat TransferExampleThe crankcase of an automobile is approximately 0.6 m long, 0.2 m wide, and 0.1 m deep (see Figure below). Assuming the surface temperature of the crankcase is 350 K, estimate the rate of heat flow from the crankcase to the atmospheric air at 276 K at a road speed of 30 m/s. Assume that the vibration of the engine and the chassis induce the transition from laminar to turbulent flow so near the leading edge that, for practical purposes the boundary layer is turbulent over the entire surface. Neglect radiation and use for the front and rear surfaces the same average convection coefficient as for the bottom and sides. (The surface area that dissipates heat is 0.28 m2.) For turbulent flow over a flat plate, you may use the following correlationReL= Reynolds number = VL/Pr = Prandtl number = v/v = kinematic viscosity ( v = /)= thermal diffusivity (=k/cp)V3/18.0PrRe036.0