Research On Flow Heat Transfer Characteristics And System Application Of R290 Working Fluid Direct Contact Condenser

With the continuous transformation of the energy structure in the new century,green and low-carbon development has gradually become the primary issue facing various industries.My country’s energy structure is still dominated by coal,and it is still facing greater energy and environmental pressure.The development of the refrigeration industry is focusing on two aspects.On the one hand,it continuously emphasizes the impact on the environment by reducing the ozone layer depletion and greenhouse effect caused by refrigerants.On the other hand,it strives to improve system performance and reduce energy consumption in the industry.The traditional vapor compression Freon refrigeration cycle has been unable to meet the development needs of the industry due to its poor environmental characteristics and system performance.The limitation of the heat transfer performance of the intermediate wall heat exchanger in the traditional refrigeration cycle also limits the further improvement of the system performance.With the advantage of the strong heat transfer capacity of the direct contact condenser,the performance of the direct contact condensation refrigeration cycle system is better than that of the conventional vapor compression refrigeration cycle,and has a good application prospect.To further investigate the impact of condensation performance on refrigeration cycles with a direct contact condenser,this study utilizes numerical simulation software to analyze the internal flow and heat transfer characteristics of the direct contact condenser.The inlet boundary conditions for the condenser are obtained based on theoretical cycle calculations.Numerical simulations are conducted for the direct contact condenser with added flow disturbance elements under multiple operating conditions.The condenser’s outlet temperature,heat transfer area,condensation capacity,volumetric heat transfer coefficient,and comprehensive performance evaluation indicators are analyzed from multiple perspectives to assess the heat and mass transfer characteristics inside the direct contact condenser.A comparison is made among different types of direct contact condensers under the same operating conditions.Finally,based on the numerical simulation results,the system performance of direct contact condensation refrigeration cycles is explored for different types of direct contact condensers.The main conclusions are as follows:(1)There are annular flow,slug flow and bubbly flow in the direct contact condenser condensation process.The streamlines of the annular flow are relatively straight and the heat transfer capacity is weak;the heat transfer of the bubbly flow is the most intense and the heat transfer effect is the best.The larger the volume of the condenser under the bubble flow pattern,the larger the heat transfer area,the lower the outlet temperature,and the better the heat and mass transfer performance.Under the same condensation heat load and temperature difference,the heat transfer area of a direct contact condenser is only 8% to14% of the heat transfer area of a surface condenser.(2)Adding baffles and bypass columns in direct contact condensers will change the flow field in the heat exchange process of the condenser,enhance the mixing degree of the gas-liquid two-phase fluid,and improve the condensation performance of the condenser.The condensing performance of the direct contact condenser with 3 baffles is the best.the bypass column divides the superheated steam bubble area,which increases the degree of disturbance in the condenser and improves the condensing performance.The comprehensive performance evaluation index η is more suitable for direct contact condensers with baffles than ζ.(3)In the comparative analysis of condensation performance among different types of direct contact condensers,it was observed that the three-pass flow condenser demonstrates the highest pressure drop and volumetric heat transfer coefficient.The cylindrical condenser exhibits a slightly higher pressure drop and volumetric heat transfer coefficient compared to the droplet column condenser.While the droplet column condenser showcases the highest overall performance index,indicating superior performance,it also incurs the highest production cost among the various condenser types.(4)In the comparative analysis of condensation performance among different types of direct contact condensers,it was observed that the three-pass flow condenser has the lowest power consumption for the main cycle compressor in the refrigeration cycle,and this value is lower than both the cross-flow cylindrical condenser and the empty tube condenser.Conversely,the five-pass flow condenser exhibits the highest power consumption for the main cycle compressor.Furthermore,the three-pass flow condenser consistently achieves the highest coefficient of performance(COP)for the refrigeration cycle system,surpassing both the cross-flow cylindrical condenser and the empty tube condenser.On the other hand,the five-pass flow condenser demonstrates the lowest COP among the various condenser types in the refrigeration cycle system.