Research On Interface Evolution And Heat Transfer Enhancement Mechanism Of Nitrogen Condensation

The condensation process of cryogenic fluid widely exists in the heat transfer equipment of many cryogenic systems,such as the condenser in the gas liquefaction system and the re-condenser in the cryogenic liquid storage tank.Improving the condensation heat transfer efficiency of cryogenic fluid is of great value for optimizing the design of cryogenic heat exchanger and realizing industrial energy saving.Due to the lack of experimental data of cryogenic fluid condensation,the mechanism of flow pattern and heat transfer performance during the cryogenic fluid condensation process is not clear.There are significant differences in physical properties between cryogenic fluids and normal temperature fluids.For example,film condensation plays a dominant role in the condensation process of nitrogen vapor,and the lower latent heat of nitrogen leads to a more intense phase change process at the gas-liquid interface,which makes the condensation mechanism of cryogenic fluid more complicated.The special physical properties of cryogenic fluid must be considered in the enhancement of condensation heat transfer performance.In this paper,starting from the gas-liquid and solid-liquid interfaces in the process of condensation,the work is carried out from three aspects: fluctuating liquid film condensation,condensation on the microstructure surface and mixture condensation,including theoretical analysis,numerical simulation and experimental verification.The specific research contents are as follows:1.Based on the visualization experimental set-up for the cryogenic condensation in liquid nitrogen temperature region,the flow characteristics and heat transfer performance of nitrogen condensation was obtained,and the internal relationship between the film fluctuation characteristics and the heat transfer enhancement was revealed through statistical analysis.Based on the wave theory of falling film,it is clarified that the inertial force plays a dominant role in the falling film flow during the process of nitrogen vapor condensation on a vertical plate,and the liquid nitrogen wave effect is more intense.The evolution characteristics of liquid film flow pattern were analyzed.The comparison of two kinds of heat transfer forms in a solitary wave reveals that the convection enhancement improves the local heat transfer performance.With the help of statistical tools,it is proved that the heat transfer enhancement also results from the thinning of liquid film and the increase of thinner film area.The visualization experimental study of the nitrogen condensation shows that the fluctuation effect of liquid film increases with the increase of condensing temperature difference.The liquid film experiences four typical flow patterns from laminar flow,two-dimensional fluctuating flow,transition flow to three-dimensional fluctuating flow,and the condensation heat transfer performance is gradually enhanced.2.A three-dimensional numerical model of the cryogenic condensation on the microstructure surface was established.The dominant role of surface tension in promoting the transverse migration of liquid film was clarified.The geometric enhancement factor of the condensation on the vertical microstructure surface was proposed,and the optimization direction of the high-efficiency microstructure surface was pointed out.The heat transfer characteristics and flow patterns of nitrogen vapor condensation on six groups of microstructure surfaces were measured.In the visualization experiment,it is observed that the liquid film velocity at the bottom of the micro fin is2-3 times of Nusselt’s prediction.It is proved that the microstructure can change the liquid film flat distribution characteristics and accelerate the drainage.The heat transfer coefficient of micro fin with 0.3 mm high and 1 mm pitch is 4 times higher than that of flat plate within the condensing temperature difference range of 3.5 K.The numerical simulation results of different pitches and heights show that the influence of the microstructure parameters on the condensate flow process and the heat transfer performance is mainly reflected in the surface curvature radius.The increase of fin pitch leads to the decrease of heat transfer enhancement degree,and the increase of fin height leads to the decrease of heat transfer enhancement area ratio.3.The experimental heat transfer correlation of mixture condensation was developed,and the accurate prediction of heat transfer performance during the cryogenic mixture condensation was realized.Based on the modified mass transfer source term,the methods to improve the mass transfer of cryogenic mixture condensation at the gas-liquid interface were proposed.Based on the heat and mass analogy theory,the main factors affecting the mixture condensation heat transfer performance were revealed.In the numerical simulation of nitrogen-neon condensation,the distribution characteristics of the species concentration field and the gas-liquid two-phase field in the mixture reveal the mechanism of the high concentration neon diffusion layer above the liquid film hindering the mass transfer process at the gas-liquid interface.The simulation results show that the mixture heat transfer performance is controlled by the condensation temperature difference and the concentration gradient in the neon diffusion layer.The condensation heat transfer performance of nitrogen-neon mixture can be improved by reducing the neon concentration in the mixture and increasing the vapor velocity.In the experimental study of nitrogen-neon mixture condensation,the direct measurement method of neon concentration was introduced and the flow characteristics and heat transfer performance was obtained.This paper can provide theoretical guidance and technical support for the study of condensation heat transfer and flow characteristics of cryogenic working fluids and the design of high efficiency heat transfer surface.