Unstart Margin Characterization Method of Scramjet Considering Isolator–Combustor Interactions

Abstract

Characterizing the unstart margin is very important for the hypersonic inlet to achieve maximum performance within safety range. A novel approach for characterizing the unstart margin when combustor–isolator interactions are taken into account is presented. First, experiments with four Mach numbers were conducted in a direct-connect supersonic isolator–combustor. The start-to-unstart process and the unstart-to-restart process were analyzed, and it is found that they should be discussed separately due to the hysteresis and different conditions of the isolator. A good way of indicating the unstart margin should be that the unstart margin maintains a linear relationship with the equivalence ratio. For the shock-train leading edge, it is concluded that, when combustion is considered, its behavior is rather complicated, which makes it inappropriate for characterizing the unstart margin. Then, the backpressure method is discussed, and its nonlinearity with equivalence ratio makes it not an ideal way to indicate the unstart margin. A new approach based on pressure distribution area integral is proposed to characterize unstart margin. Results show that unstart margin indicated by the pressure distribution area integral has a better linear relationship with equivalence ratio. However, the pressure distribution area integral method will need many sensors. To use as few sensors as possible, the genetic algorithm is introduced for sensor selection and reduction. The simulation and test at multi-operating conditions show its validity.