Study On Heat And Mass Transfer Characteristics And Particle Collection Mechanism Of Spray Heat Source Tower

The heat source tower heat pump?or reversibly used cooling tower heat pump?has great energy saving potential in subtropical region.Compared with the air source heat pump,the heat source heat pump absorbs both the sensible heat and the latent heat from ambient air,and the device does not need to defrost.The open heat source tower sprays the antifreeze solution on the surface of the packing material,and the ambient air is pumped from the bottom of the heat source tower.After running for a period of time,the surface of the packing material is easy to scale.The fouling of the packing not only affects the heat transfer,but also increases the air flow resistance of the device.In order to solve the fouling problem,a spray heat source tower without packing is proposed in this study.The spray heat source tower and the packed-bed heat source tower are quite different regarding the heat and mass transfer characteristics.The heat transfer performance of the spray heat source tower directly affects the working efficiency of the heat pump system.The development of heat and mass transfer model of spray heat source tower and the analysis of tower heat and mass transfer characteristics are the foundation for the evaluation,design and optimization of the spray heat source tower heat pump.The spray heat source tower could abate haze pollution for outdoor building environment during winter.The development of particle collection model is the foundation for the analysis and performance optimization of the particle scavenging performance of spray heat source towers.This study analyzed the heat and mass transfer process between liquid and gas in the spray heat source tower and developed the corresponding heat and mass transfer models utilizing conservation laws of mass,energy and momentum.The heat and mass transfer model was validated by experiments.The parametric study was conducted to reveal the heat and mass transfer characteristics of the spray heat source tower.An optimization method was proposed for the optimization of the tower thermal performance.The particle collection model was developed and the impacts of operating parameters on the particle scavenging efficiency was discussed.The detailed work is given as follows:?1?The heat source tower with downward spraying and upward flowing gas?HST-DU?was proposed.A heat and mass transfer mathematical model was developed and validated through experiments.The contact between liquid and gas is counter-current in HST-DU.A parametric study was conducted to study the impacts of initial solution temperature?-4 to-1??,gas velocity?2.5 to 4m/s?,initial droplet velocity?4 to 10m/s?,droplet diameter?0.65 to 1.2 mm?,solution mass concentration?16³0%?,tower width?0.5⁰.7m?,overall tower height?1.53⁴.53m?and gas inlet temperature?5¹1??on the heat rate,tower effectiveness and solution temperature distribution.The results showed that by decreasing droplet diameter or droplet initial velocity,the thermal performance of HST-DU is improved.The heat flow rate was enhanced with increasing gas velocity.However,when gas velocity increased from2.5m/s to 4m/s,the tower effectiveness first decreased from 37.15%to 33.03%,then increased to 36.15%.This was because the droplet detention time was significantly prolonged when the gas velocity was close to the critical gas velocity.The results of parametric study revealed the heat and mass transfer characteristics of HST-UD and provided a theoretical foundation for the optimization of tower thermal performance.?2?An optimization method established on the concepts of critical gas velocity and critical height was proposed for HST-DU.This method was based on multivariable analysis.Two operating parameters?gas velocity and droplet diameter?and one structural parameter?tower height?were concerned.The results indicated that when gas velocity and droplet diameter are simultaneously considered,using a smaller droplet diameter is not beneficial for improving the thermal performance and the power consumption would be higher.The optimized HST-DU could adopt larger droplet diameters and achieve better thermal performance simultaneously.Therefore,the optimization method could save the power consumption of water pump and realize better thermal performance.?3?To extend the droplet detention time,two types of upward spraying heat source towers were proposed:?1?the heat source tower with upward spraying and upward flowing gas?HST-UU?;?2?the heat source tower with upward spraying and downward flowing gas?HST-UD?.In upward spraying heat source tower,the sprayed droplets have to go through a rising stage and a falling stage.Two heat and mass transfer mathematical models considering rising and falling droplets simultaneously were developed based on conservation laws of mass,energy and momentum.Experiments were conducted to validate the accuracy of the models.The influences of different air velocities,droplet diameters and initial droplet velocities on the displacement,velocity and temperature distribution of the sprayed droplets were discussed in detail.The results showed that for HST-UU,increasing both the air velocity and the initial droplet velocity made the droplets rise higher.The droplet temperature variation in the descent stage is 1.5 to 2.4 times larger than that in the ascent stage.The droplet diameter had large impact on the droplet temperature distribution in both the ascent and descent stages.The initial droplet velocity and air velocity had small impacts on the droplet temperature distribution at the beginning period of the ascent stage.In the descent stage,the temperature of the droplet with small initial velocity rose quickly.For HST-UD,the overall calculation height performs an approximate linear relationship with the droplet initial velocity ranging from 6 to 18m/s.The droplet temperature variation at the droplet rising stage is larger than that at the droplet falling stage.Increasing gas flow rate properly could improve the heat and mass transfer performance.However,the heat and mass transfer performance would be affected if gas flow rate is too high.The research results reveal the heat and mass transfer characteristics of upward spraying heat source tower and provide a theoretical basis for the design and optimization.Three types of reversibly used cooling towers?HST-UD,HST-UU and HST-DU?were compared regarding thermal performance.HST-UU has the best thermal performance under the same gas speed,droplet diameter,droplet initial speed,initial gas temperature,initial humidity,liquid to gas ratio and initial solution temperature;HST-UD comes second and HST-DU comes third.?4?A particle collection model was developed to calculate the single droplet and the overall collection efficiencies.The impacts of air velocity,drop velocity and drop diameter on the particle scavenging performance were discussed in detail.The relationship between thermal efficiency and the overall collection efficiency was discussed.The results showed that the overall particle collection efficiency of the HST-DU reached up to 99.9%and 83%for PM₁₀ and PM_2.5,respectively.The main particle collection mechanism is the inertial impaction and the mechanisms of thermophoretic effect and diffusiophoretic effect were weak.The results showed that thermal efficiency and overall collection efficiency were inversely proportional to the drop diameter.Increasing air velocity could enhance the heat absorbing performance but affect the particle scavenging performance.The results reveal the effect of different particle collection mechanisms on the particle collection process,providing a theoretical basis for the optimization of the scavenging performance.