Study On Flow And Heat Transfer Process Of Droplet Collisions In Seawater Cooling System

Droplet collision as a complex flow process involving multi-scale,non-linear problems,is ubiquitous in nature and human life,such as raining,inking and spray of liquid fuel in internal combustion engines.Seawater droplets collision in cooling tower will change the atomization and affect the contact area between air and sea water.During the operation of the cooling tower,the collision process and the heat transfer of droplets will further affect the performance of shower cooling towers.Study on flow and heat transfer process of droplet collisions in seawater cooling system will provide theoretical basis for the design of cooling tower.In the present work,flow and heat transfer process of droplet collisions was simulated using volume of fluid and adaptive dynamic grid by Fluent.First,collisions of tetradecane droplet were simulated which validates the accuracy of numerical method.After,simulations of head-on and off-center binary collision of seawater droplets with equal-sized were carried out and three different types of outcomes were covered.Then,simulations of head-on collision of seawater droplets with unequal-sized were carried out and two different types of outcomes were covered.Regions of various collision regimes is captured.Schematics of Energy during collision are obtained.Finally,the numerical simulations of different temperatures were carried out.The heat transfer law of seawater droplet was summarized.The dissertation contents and conclusions are as follows:?1?For equal-sized seawater droplets collisions under the conditions of D₀=100?m,We:10?150,Re:287.48?1113.40,x=0?0.55,Oh=0.011.Three kinds of outcomes are obtained,which are coalescence,reflexive separation and stretching separation.Reflexive separation occurs under low collision parameters while stretching separation occurs under high collision parameters.The rotation motion exists in the deformation,and the deformed droplet is small at high collision parameters.The coalescence mainly occur in the low-We region and the droplet polymerization area covers We the widest when x=0.35.As x increases,the coalescence area first expand and then shrink.?2?The collision process is accompanied by the mutual conversion of the kinetic energy,surface energy and viscous dissipation energy.The total energy of the droplet drops rapidly and slightly at the moment of initial contact,viscosity dissipation energy increases throughout the process,and kinetic energy is zero at the end of collision process.The rotation of the droplet generates centrifugal force in the off-centre collision,which increases the kinetic energy of the droplet and promotes the separation.?3?For unequal-sized seawater droplets head-on collisions with?=1.5,2.Two kinds of outcomes are obtained,which are coalescence and reflexive separation.The mass of small droplet flows into the large droplet during the collision,and deformation of the small droplets is significantly greater than the large.As?increases,it is more difficult for droplets to undergo reflexive separation,and the reflexive separation area expands towards high We.?4?For equal-sized seawater droplets head-on collisions at seawater temperature are 300K,320K and 340K,the critical Weber number of coalescence and reflexive separation are 25.8,23.5and 20 separately.As the droplet temperature increases,the Oh decreases,the stability of the droplet decreases.The critical We moves to the low We region,the reflexive separation area expands,and the droplet is more likely to undergo reflexive separation.?5?For equal-sized seawater droplets head-on collisions with different temperatures:320K and300K,the temperature difference between two droplets can cause the self-propelled migration of the coalesced droplet as a consequence of the pressure difference and the induced thermocapillary flow.The coalesced droplet migrates to the side towards the high-temperature droplet.Late in the process,slight retracement of the droplet migration occurs,but overall migration direction remains the same.