Heat Flux

Heat and Work

Convective Heat Transfer Coefficient

Thermodynamics Heat Transfer and Fluid Flow Volume 2 of 3
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Heat Transfer HEAT TRANSFER TERMINOLOGY Heat Flux The rate at which heat is transferred is represented by the symbol . Common units for heat Q transfer rate is Btu/hr. Sometimes it is important to determine the heat transfer rate per unit area, or heat flux, which has the symbol . Units for heat flux are Btu/hr-ft². The heat flux can be Q determined by dividing the heat transfer rate by the area through which the heat is being transferred. (2-1) Q Q A where: = heat flux (Btu/hr-ft²) Q = heat transfer rate (Btu/hr) Q A = area (ft²) Thermal Conductivity The heat transfer characteristics of a solid material are measured by a property called the thermal conductivity (k) measured in Btu/hr-ft-^oF. It is a measure of a substance’s ability to transfer heat through a solid by conduction. The thermal conductivity of most liquids and solids varies with temperature. For vapors, it depends upon pressure. Log Mean Temperature Difference In heat exchanger applications, the inlet and outlet temperatures are commonly specified based on the fluid in the tubes. The temperature change that takes place across the heat exchanger from the entrance to the exit is not linear. A precise temperature change between two fluids across the heat exchanger is best represented by the log mean temperature difference (LMTD or DT_lm), defined in Equation 2-2. (2-2) DT1m (DT₂ DT1) ln(DT₂/DT₁) where: DT2 = the larger temperature difference between the two fluid streams at either the entrance or the exit to the heat exchanger DT1 = the smaller temperature difference between the two fluid streams at either the entrance or the exit to the heat exchanger Rev. 0 Page 3 HT-02

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