Theoretical And Experimental Study Of Condensation Flow Stability In Horizontal Pipe And Simulation Of Condensation Film Characteristics Outside Pipe

Rapid progress in industrial application,space exploration has promoted the evolution of heat dissipation system.Phase-change heat transfer has become one of the most effective thermal management technologies.However,the complexity and instability of the system will increase greatly.In the present thesis,the instability of condensing flow will be revealed.One the one hand,condensing flow outside horizontal pipes is studied by numerical simulation,the fluctuation of the liquid film thickness outside the pipe and heat transfer performance are studied.One the other hand,theroeical analysis and experimental investigations are performed to study the condensing flow instability,and the factors affecting the fluctuation of condensation pressure drop are discussed.First,a numerical model to describe the water vapor condensation at phase interface is established,and the shell side condensation of shell-andtube heat exchanger is simulated.The growth and flow of liquid flim outside the pipe are calculated.The results show that vertical and lateral drainage are two main ways for condensing droplets at the bottom of pipe to fall into the lower pipe,which resulting a wavy liquid film with uneven thickness distribution.The average liquid film thickness outside the tube fluctuates all the time.In addition,the average liquid film around the tube increases with the number of tubes,resulting in the deterioration of heat transfer.Secondly,a model to describe the thermodynamic process of R134 a condensation in a horizontal tube is established.The mechanism of pressure drop oscillation of condensing flow was analyzed based on Lyapunov instability principle.The flow drift would occur and trigger the flow instability in the condenser channel,when the working fluid operates on the negative slope region of pressure-drop flow curve.The results indicated that the onset-instability of condensing flow with the outlet quality by 1,and flow instability would be ended for the outlet quality of working fluid with the quality of 0.8.In addition,the much higher inlet super,smaller pipe diameter and lower heat flux would be easier to generate the negative slope on the pressure-drop flow curve of the condenser.As a result,the pressure drop oscillation and the unstable operation would be occurred in the condensing flow system.The pressure drop oscillation curve is obtained by numerically solving the dynamic model of the condensate section.Finally,a two-phase closed-loop experiment is built to investigate the condensing flow pressure drop in the horizontal pipe.The experimental result shows that the working medium pressure drop between the inlet and outlet pipe fluctuates obviously.The frequency of pressure drop fluctuation signal caused by the flow pulsation characteristic of gear pump is concentrated in 70 Hz and 95 Hz,and the frequency of pressure drop vibration generated by the condensing system is about 10 Hz.When the heating power in the evaporation section is relatively low,the frequency of pressrue drop fluctuation signal concentrates at 10 Hz,and the maximum amplitude accounts for about 20% of the average pressure.The influence of pump flow pulsation on pressure drop increases greatly with the increase of heating power in the evaporation section.With the increase of inlet steam flow,the influence of vibration characteristics of the condensing system is strengthened.