Vapor Flow in Cylindrical Heat Pipes

Solutions of the complete axisymmetric Navier–Stokes equations for steady, laminar vapor flow in circular heat pipes with various lengths of evaporator and condenser have been obtained by finite-difference methods. In addition, a new series solution for the slow-motion case was obtained that is valid for arbitrary distributions of evaporation and condensation and that confirms the numerical result in the limit of low Reynolds number. For uniform evaporation and condensation, the motion in the evaporator is found to be described adequately by similar solutions in both limits, and in the transition from low to high Re, the flow is completely determined by the evaporator Reynolds number. The evaporator is very weakly coupled to the condenser. The conditions in the condenser are decidedly more complex, and similar solutions are of value only for small Reynolds numbers and long tubes. Reverse flows occur for condenser Reynolds numbers greater than two and occupy a substantial fraction of the condenser length. Complete flow descriptions for symmetrical and asymmetrical heat pipes were obtained, and practical results for the calculation of pressure losses in low-speed heat-pipe vapor flows are given.