NUMERICAL SIMULATION ON CONVECTIVE HEAT TRANSFER IN A TWO-PHASE CLOSED THERMOSYPHON

摘要

After a historical review on the recent development of the study on the condensation heat transfer in a thermosyphon, a new idea on covering the effect of interfacial shear stress and subcooling on condensation heat transfer is proposed in this paper. Since the laminar filmwise condensation model in the condensation section and the filmwise boiling on the surface of liquid pool in evaporation section are both based on the Nusselt theory, the calculation of a single thermosyphon is rather approximate. Under a certain condition, what entering in the condensation section is not a pure saturated vapor, but a mixture of vapor and liquid which can spoil the mechanism of the filmwise condensation. Secondary, the vapor enters in the condensation section with a certain velocity and the shear stress acting on the liquid film is in the opposite direction of vapor, so the thickness of the condensing liquid film would be increased, which violates the hypothesis of the Nusselt theory. Finally, it is the basic of theory analysis and numerical simulation of heat pipe heat exchanger that the condensation heat transfer can be expressed accurately. So, it is important to analyze the process of heat transfer in condensation section of the heat pipe. The flow and heat transfer model in of the condensation section in a gravitational heat pipe is developed. A correlation among the falling height of the liquid film, the interfacial shear stress and the thickness of the liquid film in axis-symmetrical coordinates is derived. Based on the discretized height of the falling liquid film, the model equations are solved by utilizing both the numerical integration and the numerical differentiation. The temperature on the inner wall of the pipe is calculated according to the thermal equilibrium, which is accurate within the error of 3% compared with the experimental data. The numerical results are compared with both the experimental data and the values calculated from Nusselt theory. It is shown that the analysis of the heat transfer characteristics for a thermosyphon based on a dual effect of interfacial shear stress and subcooling is reasonable. It is of significance for theoretical study and practical application.