Experimental results are reported on the heat transfer and fluid friction of heated hydrogen and helium gas flows undergoing transition from turbulent to laminar flow in a circular tube. The entering Reynolds numbers range from 2350 to 12,500 and the nondimensional heat-flux parameter ranges from 0.0021 to 0.0061. Local heat-transfer coefficients and friction factors are obtained, and the flow transition, which is evident in these results, is verified at small heat fluxes by measuring directly the turbulence intensity at the center line with a hot-wire anemometer. At large heat fluxes, laminarization is found to occur at local bulk Reynolds numbers well in excess of the minimum number for fully turbulent adiabatic flow, and the resulting heat-transfer coefficients are much lower than those associated with fully turbulent flow at the same Reynolds number. The relation between laminarization in heated tubes and in severely accelerated external boundary layers is investigated and some similarities are noted. The acceleration and pressure-gradient parameters used to predict boundary-layer laminarization are modified for tube flow and used to correlate the initiation and completion of laminarization in the heated tube.
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.
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