Thermal vibration analysis of scramjet engine structure based on the coupling combustion-thermal-structure approach

Abstract

Abstract A combustion-thermal-structure coupling approach for thermal vibration analysis of scramjet structure is proposed, and it can consider the influences of flight trajectory such as flight altitude, flight Mach number and combustion equivalence ratio on engine vibration characteristics. In this approach, the internal flow field parameters such as pressure and temperature of scramjet engine are determined by means of the quasi one-dimensional model, the combustion heat flow parameters of engine wall are determined by the reference enthalpy method, and the transient structure heat transfer and thermal structure analysis are determined by the finite element method. Based on this approach, the thermal vibration frequency of the scramjet engine and its variation with time are studied. Under the condition of the hypersonic flight, the structural frequency of the scramjet engine decreases gradually with time, and there are great differences in the reduction amplitude at each frequency due to the uneven distribution of structural temperature. In the inlet section with relatively lower temperature, the declining amplitude ratio of inlet breathing and bending modes is the smallest, and the declining in the first 200s is 11.28% and 14.16% respectively. The structure temperature of combustion chamber and nozzle is relatively high, and the corresponding third-order, fourth-order and fifth-order frequencies decrease more, which are 35.81%, 34.32% and 32.36% respectively. The variation of the free vibration frequency of the scramjet engine with flight trajectory such as flight Mach number and flight altitude is studied. With the increase of Mach number, the engine structure temperature increases faster, and each order frequency decreases faster. With the decrease of flight altitude, the heat flux on the inner wall of the scramjet engine increases, the structural temperature rises faster, and the frequency decreases more at the same time. In addition, because the total temperature in the scramjet thermal environment changes little, the transient temperature of the engine structure at different heights finally tends to the same value, and the structural modal frequency finally tends to the same. The combustion equivalence ratio of the scramjet engine owns a great influence on the temperature of combustion chamber and nozzle. With the increase of the engine combustion equivalence ratio, the temperature of engine combustion chamber and nozzle increases, and its first-order, third-order, fourth-order and fifth-order frequencies decrease faster, while the inlet is not affected by the equivalence ratio, thus the second-order and sixth-order frequencies change little. This study can provide a strategy for predicting the full flight trajectory vibration characteristics of the scramjet engine, and provide support for the structural optimization design and vibration response reduction of the scramjet engine.