CONVECTION HEAT TRANSFER Heat TransferCONVECTION HEAT TRANSFERHeat transfer by the motion and mixing of the molecules of a liquid or gas iscalled convection.EO 1.9 Given the formula for heat transfer and the operatingconditions of the system, CALCULATE the rate of heattransfer by convection.ConvectionConvection involves the transfer of heat by the motion and mixing of "macroscopic" portions ofa fluid (that is, the flow of a fluid past a solid boundary). The term natural convection is usedif this motion and mixing is caused by density variations resulting from temperature differenceswithin the fluid. The term forced convection is used if this motion and mixing is caused by anoutside force, such as a pump. The transfer of heat from a hot water radiator to a room is anexample of heat transfer by natural convection. The transfer of heat from the surface of a heatexchanger to the bulk of a fluid being pumped through the heat exchanger is an example offorced convection.Heat transfer by convection is more difficult to analyze than heat transfer by conduction becauseno single property of the heat transfer medium, such as thermal conductivity, can be defined todescribe the mechanism. Heat transfer by convection varies from situation to situation (upon thefluid flow conditions), and it is frequently coupled with the mode of fluid flow. In practice,analysis of heat transfer by convection is treated empirically (by direct observation).Convection heat transfer is treated empirically because of the factors that affect the stagnant filmthickness:Fluid velocityFluid viscosityHeat fluxSurface roughnessType of flow (single-phase/two-phase)Convection involves the transfer of heat between a surface at a given temperature (T_{s}) and fluidat a bulk temperature (T_{b}). The exact definition of the bulk temperature (T_{b}) varies dependingon the details of the situation. For flow adjacent to a hot or cold surface, T_{b} is the temperatureof the fluid "far" from the surface. For boiling or condensation, T_{b} is the saturation temperatureof the fluid. For flow in a pipe, T_{b} is the average temperature measured at a particular cross-section of the pipe.HT-02 Page 18 Rev. 0