Experimental Study On Enhanced Heat And Mass Transfer Performance Of Elliptic Finned Tube Evaporative Condenser

With the rapid development of urban subway construction in China,the problems of high energy consumption and large investment have attracted increasing attention.The research shows that the system of traditional subway ventilation and air conditioning is a combination of water-cooled chiller and cooling tower,and its energy consumption accounts for about 30%of the total energy consumption of the subway system.Moreover,most subway stations are located in densely populated urban areas,so it is difficult to locate the cooling tower.In order to reduce the energy consumption of subway ventilation and air conditioning system and solve the layout problem of cooling tower,evaporative condenser has been applied in subway ventilation and air conditioning system.The principle is to use the latent heat of vaporization of water to absorb heat.It has the characteristics of compact overall structure,low water consumption and high heat exchange efficiency.Although the evaporative condenser has many advantages,in the actual operation process,the temperature of spray water gradually increases with the circulation use,and the heat transfer efficiency is greatly reduced.To solve this problem,and to further improve the performance of evaporative condenser,this paper proposes an elliptic finned tube evaporative condenser,in which the outer tube is elliptic tube,and the inner tube is round tube.the refrigerant flows in the similar annular region between the two tubes,the heat transfer with the cooling water in the round tube is an internal heat transfer process,and the heat transfer with the air and water film outside the elliptic tube is an external heat transfer process,the total heat transfer is the sum of internal and external heat transfer processes.In addition,an experimental system was built to analyze the effects of the changes of the faced air velocity,spray density,wet bulb temperature,circulating water temperature and cooling water flow rate on the heat and mass transfer performance,the main conclusions are as follows:（1）When the faced air velocity increases from 2.2 m/s to 3.1 m/s,the total heat flux increases continuously,when the faced air velocity is in the range of 3.1 m/s to 3.7 m/s,he total heat flux density shows a decreasing trend,and the peak value appears when the faced air velocity is 3.1 m/s.the optimum faced air velocity of the experimental system is 3.1 m/s.（2）When the spray density increases from 0.0014 kg/（m s）to 0.0056 kg/（m s）,the total heat flux increases by 34.8%,36.5%and 35.5%respectively under the faced air velocity of 2.8m/s,3.1 m/s and 3.4 m/s,when the spray density is greater than 0.0056 kg/（m s）,the total heat flux decreases.When the spray density is 0.0056 kg/（m s）,the total heat flux reaches the peak under the three conditions,the optimum spray density is 0.0056 kg/（m s）.（3）When the wet bulb temperature increases from 12.91°C to 26.89°C,the total heat transfer coefficient and heat flux of the external heat transfer process decrease by 52.6%and67.5%,while the total heat transfer coefficient and heat flux of the internal heat transfer process increase by 2.1%and 47.5%,respectively.Under the condition of high wet bulb temperature,the theoretical COP of the refrigeration system decreased from 5.69 to 5.18,only 8.9%lower,with the increase of inner tube heat transfer coefficient,and the overall heat transfer performance is still good.（4）When the circulating water temperature increases from 23°C to 35°C,the total heat transfer coefficient decreased from 328.56 W/（m²·K）to 108.58 W/（m²·K）,decreased by 66.9%;The total heat transfer coefficient of the internal heat transfer process decreases from 1448.2W/（m²·K）to 1413.62 W/（m²·K）,only 2.4%;Under the condition of high temperature circulating water,due to the existence of inner tube heat transfer process,the condenser designed in this paper has higher heat transfer efficiency than the conventional evaporative condenser with only outer tube heat transfer process.（5）When the cooling water flow rate increases from 0.036 m³/h to 0.216 m³/h,the total heat flux increases from 3411.0 W/m² to 4131.42 W/m²,increasing by 21.1%,and the theoretical COP increases from 5.13 to 6.67,increasing by 30%.（6）The correlation of heat transfer coefficient between water film and elliptic tube wall is:h_w=403.4（Γ/d）^0.584,the correlation of convective heat transfer coefficient between water film and air is:h_wa=0.0044（（λa）/d）（Re_a）^1.2761,the correlation of convective mass transfer coefficient between water film and air is:h_d=0.9097（m_a）^{1.1 0},the correlation of air flow resistance outside the tube of heat exchanger is:?P=0.364（（v2）/2）^2.4.Based on the theoretical analysis,experimental system design and performance experimental study of the elliptic tube evaporative condenser,this paper summarizes the variation of the heat and mass transfer performance with various influencing factors,these results provide a certain reference for the design and experimental study of tubular evaporative condenser,and are of great significance to promote the development of evaporative condenser towards a more energy-saving and efficient direction.