Conjugate Heat Transfer In The Composite Cooling System Based On The Film/Transpiration

With the increase of the speed and endurance of high-speed aircraft,the heat loaded on the propulsion system and outer surface gets more and more severe which has exceeded the limit of current passive or part-passive thermal.Under this circumstance,active cooling is perhaps the most promising method to protect these key components from ablation and realize reusability.Since the insufficient cooling ability of film cooling with the inhaled air and regenerative cooling with fuels when the flight Mach number exceeds 6,it is of great importance and significance to develop a new multi-medium composite cooling system that could survive in the extreme thermal environment.In this paper,we studied the cooling mechanism of composite structures with liquid water as coolant and tried to explore the limitation of the composite cooling system,aimed at providing more references for the designers.The heat insulation effect in film cooling has been analyzed quantitatively from the point of working medium heat sinks,and the influences of parameters such as turbulent intensity,coolant temperature,injection angle,blowing ratio,the height of slot,and wall emissivity were studied.The results show that the heat sink of coolant is utilized to a poor extent due to the migration of coolant towards the mainstream core region.With the decrease of turbulent intensity,slot height,injection angle and wall emissivity,or the increase of coolant temperature,the range of the equivalent heat sink utilized could be expanded,and as a result,the coolant could be used more efficiently.A simplified one-dimensional heat transfer model with phase transition occurred in the cooling channel has been established,and the cooling performances of two composite structures where a tangential slot film and a partially shielded slot film were respectively combined with the internal cooling structure,were studied.Through the quantitative analysis of the heat transfer characteristics in the structures and systematic discussion of the influencing factors,it could be found that the flow and boiling process of the working medium in the internal channel is the main cause of the uneven wall temperature distribution and local high-temperature gradient.When the partial shield is applied,not only the efficiency of the coolant usage is higher but also the temperature distribution of the wall is more uniform.A simplified numerical model for the transpiration cooling with coolant phase change in the chamber was established.The heat transfer characteristics and resultant temperature distribution in the whole structure were studied,and the influences of some important parameters were discussed.The results show that about 10%～20%of the total heat flux entering the structure is removed by gaseous coolant through its convective heat transfer with the porous matrix,and the remaining more than 80%of heat flux is transferred into the chamber.When the heat transfer intensity in the chamber is reduced,the permeable wall is thicker,or the thermal conductivity of solid material is lower,the hot-side temperature of the structure will increase.In addition,a novel idea that using hydrogel as the coolant instead of liquid water was proposed,and some exploratory experiments were conducted.It is found that transpiration cooling with hydrogel as coolant behaves better than that with liquid water as coolant in the heating process when there is no further coolant supplement.A numerical strategy which could take the radiation effect from the external mainstream into account and achieve the coupled solution of transpiration cooling and mainstream was proposed,based on which the flow and heat transfer characteristics in different transpiration cooling structures under high-temperature fuel gas conditions were studied.The effects of wall emissivity,cold side heat transfer,and coolant mass flow rate were also analyzed.The results reveal that due to the distributed injection and accumulated effect of coolant along the flow direction,the net radiation absorption by the coolant is more obvious near the downstream wall.With the small coolant consumption,more than 90%of the total heat sinks usage is achieved in the porous matrix,and accordingly the insulation effect of outspread film is weak downstream.When another slot film is introduced at the upstream of the porous wall,the insufficient cooling problem at the front of the porous wall could be effectively improved,and a more uniform temperature distribution and reduce of coolant consumption could be achieved.