Analysis And Experimental Study Of Evaporative Cooling Outside Horizontal Tube

Based on the cooling media type,condensation cooling equipment can be classified into air-cooled type,water-cooled type,and evaporative type,and the evaporative condensing cooling equipment represents a higher heat transfer coefficient compared with the air-cooled type and a less circulating water consumption compared with the water-cooled type.Meanwhile,it has the advantages of compact structure,low initial investment,and low energy consumption etc.Therefore,with the rapid development of the national economy and the growing demand of energy saving,the evaporative condensing cooling equipment is widely applied on various industrial and engineering fields such as petroleum,chemical industry,refrigeration,food processing field and construction etc.The heat transfer performance of an evaporative cooler is directly affected by the l contact area of liquid and air,since the heat and mass transfer process between the liquid film and the air is a main mechanism of the evaporative cooler to dissipate its heat.Nevertheless,the spreading of the liquid film is affected by many factors,such as the spray density of the water,the air flow rate,wettability of the substrates and the structures on the surface etc.For instance,small spray density of the cooling water can lead to the shrinkage of the flowing liquid film which will contribute to the appearance of dry spots and reduction of heat transfer efficiency especially when it’s heated.On the contrary,high spray density will increase the liquid film coverage,but it will thicken the liquid film,increase the heat transfer resistance,and deteriorate heat transfer at the same time.In this paper,the liquid film coverage and heat transfer of ribbed tube in the process of evaporative cooling heat transfer are studied experimentally,and the results were compared with the bare tube.The evaporative cooling process are a complex process in which heat and mass transfer are coupled to each other.To better characterize the heat transfer capacity of the evaporative cooling process,this paper conducted a theoretical analysis on the falling film evaporative cooling heat transfer process outside the horizontal tube.The heat was divided into several parts.Then a mathematical model was established based on the energy and mass balance of the air flow and the energy balance of the cooling water.The results of the falling film experiments shown that,the liquid film coverage rate f_wof the ribbed tube was higher than the f_w of the bare tube when the Reynolds number were the same.Moreover,at low Reynolds number,the liquid film coverage of the ribbed tube was 2.72times higher than that of the bare tube,which exhibits that the external rib of the tube can feasibly improve the effect of spreading of the liquid film and increase the liquid film coverage area.Meanwhile,Correlation formulas of the convection heat transfer coefficient between the tube-air and the liquid film-air,and the formula of the liquid film coverage rate were fitted from the experimental data obtained from the heat transfer experiments which was conducted under dry condition and wet condition.The accuracy of the mathematical model is verified by comparing the mathematical calculation results with the experimental data.Furthermore,a lot of calculation and analysis was carried by using mathematical models,and the results indicated that adding packing plates,increasing the velocity of the air flow,the flow rate of the cooling water and reducing the relative humidity of the inlet air can enhance the heat transfer.Among the factors that enhance heat transfer,velocity of the air flow has the most significant effect,while the flow of the cooling water has the minimal effect.From the perspective of mechanism,the packing plates can increase the mass transfer area.Increasing the velocity of the air flow can enhance the convective heat transfer and mass transfer process.Increasing the water flow rate can increase the liquid film coverage,and enhance the convective heat transfer between tube wall and liquid film.Relative humidity mainly affects the latent heat absorption capacity of the air,which is related to the driving force of the mass transfer.