Research On The Combustion Characteristics Of Hydrocarbon Fuels In Solid Fuel Ramjet With Swirl Flow

Solid fuel ramjet（SFRJ）draws attention throughout the world owing to its characteristics of high reliability,simple structure,and low cost.Many researchers have conducted a number of research on the combustion characteristics of SFRJ in the past 100 years.However,many problems that prevent the application of the SFRJ are still existed and hence need to be solved.At the end of the last century,swirling flow,which is regarded as a method that can enhance the performance of SFRJ are proposed and investigated by many researchers,but the mechanism of the effect of the swirling flow on the regression rate of different types of propellant is not clear,and a detailed analysis of the self-excited acoustic oscillation in the SFRJ combustor is needed.Thus,the aforementioned problems partially motivate this research.In the present paper,the types of fuel（including high density polyethylene,paraffin based-high-density polyethylene blended fuel）,inlet thermodynamics,and swirl intensities are varied independently to investigate the effect of combustion characteristics and performance of hydrocarbon polymer fueled SFRJ via numerical and experimental approaches.The research contents are summarized as follows.1)Firstly,a series of fairing tests are conducted via the connected pipe facility system.In these experiments,the influence of swirl intensities,chamber structures,and propellant types on engine performance is investigated.Many engine performance parameters including fuel regression rate,chamber pressure,chamber temperature,and engine thrust et al.are obtained.The results show that the use of swirling flow can enhance the regression rate significantly;increasing the concentration of paraffin in the propellant will increase the regression rate.According to the results of the scanning electron microscope,the microscopic morphology of the fuel can be changed with different fuel paraffin concentrations,and an over high fuel paraffin concentration will negatively affect the performance of the SFRJ.In addition,the engine performance can be sensitive to the engine structure,and sustained combustion cannot achieve when the combustor step height reduces to 10 mm and the sudden expansion ratio reaches 1.5.The analysis of the self excited thermoacoustic instability indicates that the dominant frequency of the engine is 318 Hz,and the amplitude could be enhanced with the increase of the swirl number and decrease of the fuel paraffin concentration.Moreover,a close correlation is observed between the system acoustical energy and the fuel combustion efficiency.2)A solver for two-dimensional turbulent chemical reaction flows has been developed,the solver is written in Fortran 90,which is mainly used to solve the two-dimensional axisymmetric turbulent flow in the SFRJ combustor.This solver uses the finite volume method to solve the Navier-Stokes equation of the flow field,the third-order MUSCL method is used as the reconstruction method,and the numerical flux is calculated by the AUSMPW+scheme.Moreover,the LU-SGS and the third-order Runge-Kutta method are adopted for the temporal discretization of the fluid domain and the solid domain,respectively.The chemical reaction is simulated using a seven components and three element reactions model,the finite rate method is utilized as the chemical model,and the solid fuel regression rate is calculated using the 0^th-order Arrhenius formula.The reliability and accuracy of the code are validated against many benchmark cases.3)The factors that influence the SFRJ combustion characteristics and fuel regression rate with swirling flow are investigated.Firstly,the influence of swirling flow on the fuel regression rate and the heat transfer of the solid fuel surface is numerically studied.The results indicate that at the reattachment point,the fuel pyrolysis and heat transfer are significantly enhanced with the introduction of the swirling flow.Secondly,the mechanism of the effect of combustor configuration on engine performance and self-sustained combustion characteristics with swirl flow is investigated,the results show that the reduction of the port diameter of the fuel can increase the turbulent viscosity,temperature,regression rate and other related parameters of the fuel surface,which indicates a smaller port diameter of the fuel may have a positive effect on the enhancement of engine performance.However,when the port diameter reduces to 60mm,the self-sustained combustion will not be established,owing to the instability of the recirculation zone.Finally,the effect of the incoming flow thermodynamic parameters on the combustion characteristics is analyzed in detail,it can be seen that increasing the swirl intensity,Mach number,mass flow rate and inlet temperature of incoming flow will increase the chamber temperature and regression rate of the fuel,this indicates that the pyrolysis characteristics of the fuel can be significantly affected by the incoming flow conditions.According to the analysis of engine performance,when the incoming mass flow rate is 0.5kg/s,the engine expressed the best performance.4)To investigate the main sources of the combustion instability and the flow features in the combustor when the instability occurs,a close investigation on the characteristics of self-excited combustion oscillation is conducted via numerical approach.The results show that when the self-excited combustion oscillation occurs,the flame surface expresses obvious periodic fluctuations.The engine combustion oscillation frequency increases with the increase of inlet mass flow rate,Mach number,and incoming temperature.The growth rate of the acoustic pressure is more sensitive to the changes in thermodynamic parameters of the incoming flow.As the inlet mass flow rate,Mach number,and temperature increases are set to 0.7 kg/s,2,and 605 K,the engine expresses the maximum growth rates,and the growth rates are 116.7 s^-1,86.2 s^-1,and 90 s^-1,respectively.The phase difference between pressure and heat release rate is analyzed,as the inlet temperatures are 540 K,605 K,and 670 K,the phase differences are 15°,27°,and 40°,respectively.According to the Rayleigh criterion,the phase differences are all within -90°～90°.Therefore,the thermal acoustical instability could be enhanced.In addition,the analysis of the combustion characteristics indicates that the regression rate and heat flux of the solid fuel surface can also exhibit periodic changes with the combustion oscillations,and the velocity and component distribution in the combustion chamber are also expressed a periodic oscillation with the same frequency of the acoustic pressure.5)Such periodic oscillations mentioned above are analyzed by conducting proper orthogonal（POD）and dynamic mode decomposition（DMD）methods,the eigen-frequency and the growth rate of each excited thermoacoustic mode are characterized.The POD studies reveal that the fluctuation energy of the first 6 modes contributes more than 99%of the total fluctuation energy of all modes.It means that the main features of the SFRJ combustors could be captured by using the first 6 modes.Unlike POD studies,DMD analysis captures the frequency information and corresponding spatial structures in the entire flow field.Examining the DMD growth rate reveals that all dominant DMD eigenmodes are stable（with growth rate=0）or marginally stable（positive growth rate）.