Study On Cryogenic Condensation Heat Transfer Mechanism In Wavy Laminar Flow Region

Cryogenic condensation is defined as the condensation of working medium of boiling temperature below 123 K,such as methane,oxygen,nitrogen,hydrogen,and helium.Cryogenic condensation is widely occurred in the heat transfer equipment of many cryogenic systems,such as the condenser-evaporator in the air separation system,the condenser in the gas liquefaction system,the recondenser in the cryogenic liquid storage tank.The heat transfer equipment is a major investment in any cryogenic systems.Thus,deeply studying the cryogenic condensation mechanism is very important to improve the heat transfer performance and the design accuracy of the cryogenic heat transfer equipment.The condensate in the cryogenic heat exchanger is mainly in the wavy laminar flow region.However,because of the high quality environment needed by cryogenic condensation experiment and the difficulties in cryogenic measurements,the cryogenic condensation experimental data reported is still lacking,and some of the existing results contradict each other,which results that the cryogenic condensation characteristics and mechanism in wavy laminar flow region remains still not clear.Due to the lack of theoretical and experimental research of cryogenic condensation,the research conclusions and heat transfer correlations of condensation at room temperature are still the basis for the design of the cryogenic heat exchanger at present.However,the viscosity,the latent heat,the liquid-gas density ratio,the surface tension and the contact angle of the cryogenic fluid are much smaller than the room temperature fluid,and the lower the temperature,the greater the difference is.All these differences in fluid physical properties are the main factors that influence the condensate flow pattern and heat transfer characteristics.Thus it is necessary to validate existing heat transfer correlations based on room temperature fluids through implementing study on flow characteristics and heat transfer performance of cryogenic condensation.To better illuminate the physical mechanism in cryogenic condensation and obtain high quality heat transfer correlations of cryogenic condensation in wavy laminar flow region,the following work is carried out:1.A test platform for cryogenic condensation visualization was designed and built.A visualization method with light and high speed camera coaxially placed was proposed.Heat transfer data was extended to near turbulent zone based on the test platform built.Kutateladze heat transfer correlation,which was used to predicting the room temperature condensation heat transfer,was validated by the measured nitrogen condensation heat transfer results.The visualization method with light and high speed camera coaxially placed effectively avoided the spot and shadow of pictures filmed by existing visualization method.Visualization of cryogenic condensate flow fluctuation with different Reδ was implemented.The enhancing effect of liquid film fluctuation on condensation heat exchange was revealed.The relations between the interfacial wave velocity,frequency and Reδ was revealed through analyzing the statistical parameters of interfacial wave of the condensate film.2.A new condensation mass transfer model and a quantitative method of mass transfer coefficients was proposed.The new model effectively improved the temperature distribution in the liquid-vapor interface and also reduced the dependence of mass transfer coefficient on working medium and working conditions.The quantitative method solved the key problem that the mass transfer coefficient cannot be quantified in the existing mass transfer model.The model explained the reason why the interface temperature deviates from the saturation temperature when the coefficient is too small and why the calculation converges difficultly when the coefficient is too large.3.Based on the mass transfer model and the quantitative method of the mass transfer coefficient,the cryogenic condensation heat transfer simulation in wavy laminar flow region was realized.The cryogenic condensation heat transfer mechanism in wavy laminar flow region was illustrated from the microscopic view.The intermittent liquid film thinning of the liquid film is the main cause of reinforcement condensation heat transfer,and the emergence and strengthen of the internal convection inside the wave is the reason why the reinforcement of condensation heat transfer becomes more and more obvious as Reδ increases.This conclusion will be of great significance to the formulation of heat transfer enhancement method.