Spray Cooling Dynamics And Its Application In Gas Turbine Blade Cooling

In order to improve the performance of the gas turbine,it is necessary to increase the gas temperature in the turbine,which needs higher capacity for turbine blade cooling.The main cooling methods of gas turbine blades are internal cooling and external cooling.The compressed air from the compressor cools the inner and outer surfaces of the blades through the channels inside the blades.In order to further improve the cooling performance of the air,some mist can be added into the air,and the mist evaporation can be used to absorb heat,thus significantly reducing the amount of compressed air and improving the efficiency of gas turbine.Under the extreme conditions of high temperature,high pressure and high rotation speed of gas turbine blades,the interaction mechanism of droplets and blades in spray cooling is still unclear.In this thesis,with gas turbine blade cooling as the background,the numerical simulation method is used to study the morphological variations of the interaction between droplet and solid wall,and between droplet and droplet,and its influence on heat transfer during the collision between droplet and blade.In droplet impact cooling,the surrounding air can be entrained into the droplet.The dynamics of bubble breakup and migration in the droplet are also studied in this thesis.For blade internal cooling under rotation condition,a novel internal cooling channel is proposed,the novel channel can take advantage of the Coriolis force due to rotation,push the coolant towards the leading edge and trailing edge,result in the heat transfer enhancement on the both edges.Finally,mist cooling of the internal cooling channel is studied in this thesis,the dynamics of droplet impact cooling is analyzed,and provide guidance for the cooling of high temperature gas turbine blades.The main research content of this thesis consists of the following aspects:1.Dynamics and heat transfer of binary droplets impacting on a hot substrate:The interaction between droplet and droplet can be ignored in dilute phase spray impact cooling,thus the spray impact cooling can be simplified to the interaction between a single droplet and the solid substrate.However,at high heat flux,spray cooling with dense phase is needed,and the interaction among droplets cannot be ignored.In this thesis,the dynamics and heat transfer characteristics of binary droplets impact are investigated through non-dimensional parameters,and compared with those of the single droplet impact.It is found that the binary droplet impact morphologies can be divided into three types:(1)a liquid bridge is formed and jump up from the substrate;(2)the droplet is flattened and jump up from the substrate with breakup;(3)the droplet is flattened and jump up from the substrate without breakup.The spreading factor and contact time increase with increasing Weber number;the spreading factor and contact time of binary droplets impact are larger than that of single droplet impact under the same Reynolds number and Weber number.For a given Reynolds number and Weber number,the transient heat transfer rate increases and then decreases with time upon impact on the substrate.The transient heat transfer rates of a single droplet impact are larger than that of binary droplets impact,so are the total input heat except at large Weber number.For a single droplet impact,the transient heat transfer rates reach the maximum upon the maximum spreading.For binary droplets impact,it depends on the Reynolds number and the contact angle.For binary droplets impact on a hot super-hydrophobic surface,the hot substrate can promote the spreading and retard the receding due to thermo-capillary effect,and thus will enhance heat transfer between the droplet and the hot substrate.2.The dynamics of gas filament contraction in liquid:In multiple droplets impacting on the substrate,bubbles will be generated at the bottom of the droplet or inside the droplet due to entrainment,which will have great influence on the dynamics of the droplet.In order to quantitatively investigate the dynamics of bubbles,bubbles are simplified into gas filament.The propagation of capillary waves on the gas-liquid surface and its effect on breakup dynamics of gas filament is studied.It is found that the gas filaments will contract under the effect of surface tension when it is subjected to a small disturbance,as a result it will break into several bubbles or contracts into a single bubble.When the initial aspect ratio is large,the gas filament will break into two bubbles from both ends by end-pinching mechanism;when the initial aspect ratio is decreased,the gas filament will break up from middle plane;when the initial aspect ratio is small,the gas filament contracts into a single bubble without breakup.Unlike the contraction of liquid filament in air,there is only one critical initial aspect ratio for each Ohnesorge number,below which a gas filament will not break up.By studying the mechanism of gas filament contraction dynamics,it is found that the fate of gas filament contraction depends on the propagation of capillary waves at the gas-liquid interface,and a one-dimensional capillary superposition theoretical model is developed to predict the fate of gas filament contraction.Besides,it is found that the critical aspect ratios decrease with decreasing viscosity ratio.3.The migration of bubbles in liquids at high temperature gradients:When the droplets with bubbles impact the high-temperature hot substrate,a large temperature gradient will be generated inside the droplets,so the effect of thermal capillary force on the bubbles cannot be ignored.In this thesis,the migration of a single bubble under the effect of thermal capillary force and buoyancy is studied deeply,the effects of density ratio,viscosity ratio,thermal conductivity ratio,Reynolds number,Froude number,Peclet number and Marangoni number between bubble and liquid are analyzed.The buoyant effect is dominant at low Froude number,leading to upward migration of the bubble.The thermal capillary effect is dominant at high Froude number,leading to downward migration of the bubble.The Marangoni number has negligible effect on the bubble migration at low Reynolds number or Peclet number,while at high Reynolds number or Peclet number,increasing Marangoni number can reduce the upward migration velocity.Increasing the density ratio can increase the upward buoyant force due to the large density difference between the inner gas and the outer liquid.At low Reynolds number,thermal capillary effect can be enhanced by increasing the viscosity ratio and the thermal conductivity ratio,thus the downward driving force can be increased.While at high Reynolds number,the above ratios have no effect on the driving force.4.Single-phase cooling of the blade internal channel under rotating condition:For the traditional U channel,it will cause uneven heat transfer between the leading edge and trailing edge due to the Coriolis force during rotation.Therefore,in this thesis,a novel internal cooling channel with an internal partition is proposed,which can take advantage of the Coriolis force due to rotation to enhance the heat transfer of both the leading edge and trailing edge simultaneously.The novel channel and the traditional U channel are simulated in detail.It is found that the heat transfer performance of a novel channel is superior to that of the U channel.The heat transfer of the novel channel can be enhanced simultaneously at the leading edge and the trailing edge by taking advantage of the Coriolis force due to rotation.Heat transfer on both the trailing edge and the leading edge of the novel channel increases with increasing rotation number.From this study,it is observed that the novel channel has an optimal relative tip wall distance around 0.5 and an optimal relative partition wall distance around 0.375 from the trailing edge,at which the higher heat transfer level and relatively lower pressure penalty is achieved.In contrast to the U channel,the pressure drop of novel channel increases faster with increasing mass flow rate.Compared to the U channel under elevated conditions,the optimal novel channel has about 20.1%higher heat transfer on the trailing wall and 56.6%on the leading wall.5.Mist cooling of the blade internal channel under rotating condition:In this study,spray cooling is applied to the traditional internal channel cooling,the mist cooling performance of the blade internal channel under the high-speed rotating condition is studied,and the mist cooling performance of the internal channel is analyzed in details.It is found that the global average Nusselt number of mist cooling in the U channel increases with increasing Reynolds number.Mist cooling is superior to that of the air cooling.Compared to the air cooling,the global average Nusselt number of mist cooling is increased by about 70-80%.The heat transfer performance of mist cooling in the U channel increases with increasing mist mass ratio,decreases with decreasing mist diameter.Mist initial temperature has negligible influence on the mist cooling performance of the U channel.