Experiment And Study On The Performance Of Airborne Spray Cooling

With the increasing requirements for motility performance,stealing performance and defense performance of aircraft,technologies of high power laser beam and highly integrated micro electronic components have developed rapidly and been applied in fields such as material chemistry and airspace exploring.So how to effectively and reliably solve the problem of rapid cooling under high heat flux has significant research meaning.Compared with traditional cooling methods,spray cooling has the advantage of good heat transfer performance,small temperature difference,good surface temperature uniformity and low requirements for mass flux as well.However in the application of aircraft cooling not only the basic heat transfer requirements should be satisfied but also the adaptability under the flight envelope should be considered.In this paper high power spray cooling experiment bench is set up and then the influence of flight altitude,heating surface structure,nozzle parameters and different additives is investigated.Breakthroughs are achieved in the fields of heat transfer mechanism,mathematical model and application range and common conclusions are achieved to provide experimental and theoretical reference for principled sample machine of aircraft spray cooling system.Contents are as follows:(1)Numerical analysis on flow characteristics of spray coolingMathematical model of spray chamber is established based on the FLUENT software.The influence of spray height,spray angle and spray flow rate to liquid film flow characteristics of the heating surface is studied using the DPM solver.Conclusions are as follows: In the center area the film height is low and increases with the radius to the maximum value and then begins to decrease.The minimum film velocity appears in the center area and has a waving trend.Along the radius direction the velocity increases and reaches the maximum value when approaching to the edge of the heating surface.Besides with the increase of spray height the maximum value of film height decreases and the location of which is off the center area.At this time the waving trend of the velocity becomes severe.With the increase of spray angle,the maximum value of film height decreases and the location of which is off the center area.At this time the velocity decreases.With the increase of spray flow rate,the maximum value of film height increases while the film velocity remains unchanged.(2)Experimental research on spray cooling performance using water as cooling mediumSpray cooling heat transfer performance in non-boiling area is investigated using water as cooling medium.The influence of nozzle parameters,spray height,heating power,spray inlet pressure and gravity angle is mainly studied.Conclusions are as follows: Spray flow rate participated in heat transfer is the main influencing factor to spray cooling performance.The more flow rate,the better cooling performance.However,the enhancing effect of spray flow rate is limited when the flow rate is oversized.At this time system efficiency can be improved by increasing the nozzle atomization effect and reducing the flow rate.Each type of nozzle has an optimal spray height which is slightly smaller than the critical height which can be expressed as H=R/tan(0.5θ).Under a certain inlet pressure spray cooling performance of non-boiling area is mainly influenced by Reynolds number Re,Weber number We and evaporation intensityε.Heat transfer performance increases with the growth of all these parameters.Meanwhile,with the growth of gravity angle from 0° to 90° heat transfer coefficient presents an increasing and decreasing trend;while with the growth of gravity angle from 90° to 180°,heat transfer coefficient presents the same trend.Heat transfer coefficient attains the maximum value under the gravity angle of 30° and 120°.The minimum value appears under the gravity angle of 180°.(3)Experimental research on the influence of different heating surface structures to spray cooling heat transfer performanceFour different surfaces which are smooth surface,drilling surface,straight fin surface and combined surface are designed and applied.By analyzing relationship curves obtained.It has been found that compared with other surfaces,drilling surface has higher heat transfer coefficient and combined surface has highest cooling efficiency.Therefore for aircraft spray cooling system combined surface should be applied for its advantage to get low surface temperature and high cooling efficiency.Meanwhile straight fin surface can be a good substitution.(4)Experimental research on the influence of different additives to spray cooling heat transfer performancePotassium chloride aqueous solutions and ethylene glycol aqueous solutions with different mass fraction are applied to investigate the influence of different additives to spray cooling system performance.Conclusions are as follows: under low mass fraction due to the ionization effect of potassium chloride solution,Prandtl number has dominant influence on spray cooling heat transfer performance which makes heat transfer coefficient increasing with the growth of mass fraction until the mass fraction reaches to 1%;with continuous increase of mass fraction,Reynolds number becomes the primary influencing factor due to the increase of viscosity and surface tension.Besides under low mass fraction of ethylene glycol due to the inexistence of ionization,the increasing rate of Weber number and Prandtl number is less than the decreasing rate of Reynolds number.Therefore heat transfer coefficient decreases with the growth of mass fraction.Viscosity still increases when mass fraction continues to increase,making the effect of Reynolds number more significantly.So heat transfer coefficient continues to decrease.(5)Research on dimensionless correlationsExperimental correlations of aircraft spray cooling system experiment in non-boiling area under different heating surfaces are investigated.On the basis of summarizing predecessors’ work and establishing experiment bench as well as completing the experiment,four non-dimensional numbers influencing spray cooling performance which are Reynolds number Re,Weber number We,Prantdl number Pr and evaporation intensity εare provided while non-dimensional correlations of Nu for different heating surfaces are calculated.The influence of each non-dimensional number to precision is also analyzed.For drilling surface,enhancing factor ξ is applied for correction of the correlation,the value range and value basis of which are provided in order to enable the predicted values obtained by correlations falling within the deviation of ±15%.On the other hand the experimental correlation for straight fin surface is proposed which is not influenced by fin structure and fin number.The correlation above is also suitable for water,salt solution as well as alcohol solution.Comparing with correlations proposed before,the experimental correlations provided in this paper have wider application range and could reflect the influence of non-dimensional numbers to spray cooling performance more sufficiently.