Regression Rate Improvement Of Paraffin-based Composite Grains In Hybrid Rocket Engine

With the advantages of high safety,low price,simple structure and adjustable thrust,hybrid rocket engines have broad application prospects in small launch vehicles,sounding rockets,commercial spaceflight and upper stage power systems.However,the inherent combustion characteristic of diffusion combustion leads to a lower fuel regression rate,which restricts the development of hybrid rocket engines.The composite paraffin-based grains can generate a like-grooves characteristic structure during combustion,which significantly improves the regression rate compared with the paraffin-based grains.In this paper,we analyze the flow field characteristics of the composite grains under the swirling injection and different size characteristic structures based on the numerical calculation,and the positive effect on the regression rate of composite grains was also verified by fire tests.In addition,3D scanning and radioluminescence imaging techniques were used to analyze the combustion characteristics of the composite grains.The research results can lay the foundation for the engineering application.The main work and conclusions are as follows:Firstly,paraffin-based grains were prepared by centrifugal casting method with paraffin wax as fuel matrix,and composite grains were prepared by combining 3D printing technology.A hybrid rocket engine ground test system was built.The regression rate of paraffin-based grains of different formulations was characterized by using ultrasonic method.The results showed that increasing the percentage of paraffin in the formulations could significantly improve the regression rate of grains,but it was not conducive to stable combustion of engines.Secondly,the flow field characteristics of the composite grains under different injection methods were studied,and the performance of the grains using swirling injection was analyzed by fire tests.The flow calculation results show that the highest swirling intensity is generated by the combination of composite grains and the same direction swirl injection,and the characteristic structure will weaken the swirl field generated by the opposite direction swirl injection due to the two opposing swirl fields in the combustion chamber.The fire tests show that the regression rate of the composite grains under the same direction swirl injection can be improved by up to 82.4% compared the axial flow injection,and the regression rate of the composite grains under the opposite direction swirl injection can be improved by up to50.6%.In addition,it was found that the oxygen-fuel ratio distribution was more concentrated in the combination of same direction swirl injection and composite grains,and the shift range was only 0.13.The results of 3D scanning showed that the composite grains formed the largest characteristic structure scale under the same direction swirl injection,followed by opposite direction swirl injection and the axial flow injection.Finally,the influence of the characteristic structure scale on the flow field in the combustion chamber was studied.And the chamber pressure,regression rate and combustion efficiency of the preset characteristic structure grains were characterized by fire tests.The influence of the characteristic structure on the flame structure was investigated by radiation luminescence technology.The calculation results show that the larger characteristic structure size leads the higher the swirling intensity.Fire tests show that the preset characteristic structure can further increase the regression rate of the composite grains by up to 36.7%,and the preset characteristic structure also has a positive effect on the combustion efficiency.The results of radioluminescence show that the characteristic structure is conducive to the more uniform distribution of the flame and increases the flame intensity.