Optimal Analysis Of The Scramjet Thermodynamic Performance In Wide Mach Numbers

Operating in wide Mach numbers is the inevitable demand for the future development of scramjet. Scramjet at low Mach number is facing boundary of unstart, and at higher Mach number facing the ultimate temperature of the combustor material and gas dissociation. Scramjet should have enough thrust in the acceleration mode and good economic benefit in the cruise mode, therefore the research on how to make the engine run efficiently in wide Mach numbers is necessary.Characteristics of thermodynamic cycle performance of scramjet and ramjet on wide Mach numbers are analyzed, and the scramjet thermal chocking boundary and the max temperature limit of combustor is depicted by the compression ratio of temperature of the inlet and the fuel equivalent ratio of the combustion chamber. Under the two constraints scramjet has the optimized effective cycle work, which exists on the upper operation border. The chocking boundary determines the output performance of the scramjet at low freestream Mach number, and the dominant boundary turns gradually into the maximum temperature limit with the increase of Ma0. Under the limit of the maximum temperature of the flow in the combustor, the maximum effective cycle work of scramjet is limited, and in order to keep the combustion chamber running safely, there is no way to add more fuel into the combustor to get more effective work. Under this condition, we have to turn to isothermal heating process, which can decouple the compression process and the maximum temperature rise of the combustion process and keep the maximum temperature constant, for help. The best working mode is that the engine works on ramjet mode at low Mach number and on supersonic combustion mode at high Mach number, and it needs the constant Mach number and isothermal heating process to increase the engine thrust.Experiments and simulations research were made on the strut-cavity based scramjet combustor with fixed geometry. The results shows that the flight Mach number and fuel distribution have a great influence on the operation boundary of the scramjet. If the strut or the starting position of the combustion is close to the entrance of the isolator, it results in unstart of the inlet. The smaller flight Mach number is, the easier unstart phenomenon appears. At the same time, the fuel equivalent ratio or the heat addition that combustion chamber can hold is limited, and the fuel should be allocated to the downstream of the combustion chamber. Engine performance evaluation of ramjets adopting the "constant-divergent-constant" configuration were made. The lumped parameter of the scramjet performance evaluation model was established. It shows that for a fixed geometry scramjet, on the design point, can work in a large range of fuel equivalence ratio, and the biggest fuel equivalent ratio, constrained by the unstart boundary, decreases with the reduce of the flight Mach number. The bigger geometric expansion ratio of the scramjet combustor is, the greater the heat of combustion chamber can hold and the wider the range of scramjet can work normally, but at the same fuel equivalence ratio and Mach number, scramjet performance increase with the decrease of the expansion ratio of the combustion chamber. The expansion ratio of the combustion chamber should be adjusted to the minimum where the engine can work safely to obtain good performance.The scramjet working at constant dynamic pressure in wide Mach numbers, have to solve the issues of thrust trough under small Mach number, ensure that the scramjet has enough effective thrust in the acceleration mode and save fuel in the cruise mode. Increasing the area of the isolator and the combustion chamber under low Mach number, can increase the air mass flow rate to get more thrust. In order to ensure the acceleration performance, it needs to improve the combustion efficiency of the scramjet, reduce the combustor expansion ratio continuously in the accelerating process and increase the exhaust expansion ratio to ensure that the engine outputs the largest thrust. In the cruise mode, it needs to decreases fuel equivalence ratio to keep enough thrust to balance the flight resistance. The control system should adjust fuel equivalence ratio to the minimum to save fuel and extend flight range. A variable scramjet whose exhaust expansion ratio increase with decreased combustor expansion ratio is designed, and it can run form Mach number 2 to 7 safely.