TAILIEUCHUNG - Heat Transfer Handbook part 122

Heat Transfer Handbook part 122. The Heat Transfer Handbook provides succinct hard data, formulas, and specifications for the critical aspects of heat transfer, offering a reliable, hands-on resource for solving day-to-day issues across a variety of applications. | TRANSPORT LIMITATIONS 1207 inertial vapor effects. Curve A represents the temperature distribution typical of a heat pipe with subsonic flow conditions and partial pressure recovery some inertial effects are present in the vapor flow . Increasing the heat rejection rate and lowering the condenser temperature will decrease the evaporator temperature as shown in curve B. Continued increases in the heat flux and reductions in the condenser temperature will result in a decrease in the overall average vapor temperature. However eventually the vapor velocity at the condenser inlet approaches the sonic velocity and a critical or choked flow condition exists. For this situation continued reductions in the condenser temperature only serve to decrease the temperature in the condenser region and have no effect on the vapor temperature in the evaporator. Unlike the heat transport limits discussed previously the sonic limitation actually serves as an upper bound to the axial heat transport capacity and does not necessarily result in dryout ofthe evaporator wick or total heat pipe failure. Attempts to exceed the sonic limit result in increasing both the evaporator temperature and the axial temperature gradient along the heat pipe thus reducing further the isothermal characteristics typically found in the vapor flow region. Levy 1968 developed a closed-form expression for the sonic limit derived from one-dimensional vapor flow theory. This analysis assumed that the frictional effects may be neglected thus inertial effects dominate and the vapor behaves as a perfect gas. Combining these assumptions with the energy and momentum equations results in expressions for the temperature and pressure ratios. Substituting the local Mach number and relating the axial heat flux to the density and velocity the relationship between the static and stagnation temperatures and pressure can be rewritten as T 1 ï 1 Ma. Tv 2 Po 9 -o 1 YvMa2 Pv where the subscripts o and v indicate the .

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