Study On Heat And Mass Transfer Performance Of Counterflow Wet Seawater Cooling Towers

With the development of industry,the amount of waste heat generated in industrial production increases significantly,which needs to be discharged by cooling water systems.The shortage of freshwater resources is becoming increasingly serious,but the demand for cooling water is increasing for industrial production.Therefore,the development and utilization of seawater resources,especially using seawater instead of freshwater as industrial circulating cooling water can not only relieve the pressure of marine thermal pollution but could also be valuable in solving the freshwater shortage problem.Seawater circulation cooling technology is currently attracting increased attention in coastal areas.Seawater cooling towers are important components within recirculating cooling seawater systems.Ensuring its cooling effect is the key to the efficient operation of the system,so it is necessary to study the heat and mass transfer performance of seawater-air two-phase flow in seawater cooling towers.This paper focused on the counterflow wet seawater cooling tower(CWSCT),including two different tower types,seawater packed cooling tower(SPCT)and seawater shower cooling tower(SSCT).From the perspective of energy analysis and exergy analysis,detailed performance calculation models of SPCT and SSCT were established respectively.The accurate calculation method of thermal parameters and exergy parameters of seawater-air two-phase in heat and mass transfer process was expounded.The computer program developed under the VC++ framework was used for the numerical solution of the performance calculation model,and the reliability of the proposed model was validated by comparison with the experimental results obtained from literature.On the basis of that,the distribution of parameters in SPCT and SSCT under different salinity conditions was analyzed by numerical simulation,and the influence of single factor change on the heat and mass transfer performance of SPCT and SSCT under standard salinity conditions and high salinity conditions was studied.Because of the limitation of single factor research,the response surface method was used to analyze the influence of interaction of factors on the heat and mass transfer performance.On the premise of meeting the demand of heat dissipation,according to the current meteorological conditions and standard salinity conditions,the optimal operating conditions of SPCT and SSCT were determined by minimizing the exergy destruction and maximizing the cooling efficiency.The results of heat and mass transfer performance of SPCT show that:(1)The heat dissipation at the tower top is about 1.7 times higher than that at the tower bottom,which means the heat and mass transfer of the tower top is better.(2)The exergy destruction first decreases and then increases from the bottom to the top of SPCT,and the maximum value occurs at the tower top.The increase of salinity does not change the distribution trend of exergy destruction,but it will increase the total exergy destruction and decrease the exergy efficiency.(3)Under the same cooling task,with the increase of salinity,more air needs to be supplemented,and the higher the inlet water temperature and relative humidity,the greater the increase of air flow rate.(4)Under the fixed salinity,with the increase of water spraying density,the exergy efficiency and the heat dissipation increase,but the cooling efficiency decreases;when the driving force of heat and mass transfer changes(air wet-bulb temperature and inlet water temperature change),the variation trend of exergy efficiency and heat dissipation is consistent;when the air speed changes,the variation trend of exergy efficiency and heat dissipation is opposite.Among these parameters,the water temperature and air wet-bulb temperature are the significant parameters affecting exergy destruction,while these parameters have a similar influence on exergy efficiency.Compared with other parameters,the change of dry-bulb temperature has little effect on the heat and mass transfer performance.(5)Compared with the standard salinity condition,the cooling efficiency and exergy efficiency under the high salinity condition are significantly reduced,and the decreased degree at lower inlet water temperature and higher wet bulb temperature is significant.The results of heat and mass transfer performance of SSCT show that:(1)Under the same tower specifications and operating conditions,the cooling efficiency,exergy efficiency,air-side thermal efficiency,and heat dissipation of upward-spraying(USSCT)are about 15.59%,4.89%,8.62%,and 34.58% higher than that of downward-spraying(DSSCT).The effect of salinity on droplet velocity distribution in USSCT and DSSCT is less significant.(2)The critical diameter of droplets in USSCT decreases with the increase of salinity,and the decreased degree at higher air velocity is significant.Furthermore,the variation of salinity has little effect on droplet rise height in USSCT.(3)The droplet rise height is proportional to the droplet initial velocity and the air velocity,the higher the droplet initial velocity is,the greater the influence of air velocity is.The droplet rise height improves as the droplet diameter increases and gradually tends to be steady.However,when the ratio of droplet initial velocity to air velocity is less than 1.8,the increase of droplet diameter has little effect on droplet rise height.(4)Under the fixed salinity,reducing the droplet diameter and increasing the droplet initial velocity will have a positive impact on the heat and mass transfer performance of USSCT.Exergy destruction and exergy efficiency are normally sensitive to reasonable variation in droplet diameter,but the sensitivity decreases with the increase of droplet diameter.Furthermore,the exergy destruction is mainly governed by the droplet initial temperature and the air wet-bulb temperature.The droplet initial velocity and air velocity have a similar influence on exergy destruction.The influence of the air velocity,droplet initial velocity,droplet initial temperature,and air dry-bulb and wet-bulb temperature on exergy efficiency is not significant.(5)Compared with the standard salinity condition,the cooling efficiency and exergy efficiency under the high salinity condition are reduced,and the decreased degree at lower values of droplet diameter is significant.The response surface method was used to optimize the influencing factors of the heat and mass transfer performance of SPCT and SSCT.The combinations of operating parameters that result in the minimization of the exergy destruction,the maximization of the cooling efficiency,and satisfying the requisite heat dissipation were obtained.The results show that the respective cooling efficiency of SPCT and SSCT increases by 5.80% and 14.69% after optimization.The respective exergy destruction of them decreases by 11.17 % and 37.87 %.