The exploration of bypass matching limitation and mechanisms in a double bypass engine compression system

Abstract In order to clarify the influence of bypass configuration on the variation range of the bypass ratio in a variable cycle engine, as well as revealing the effect of bypass ratio adjustment on the engine aerodynamic performance, an integrated zero-dimensional bypass model is established based on a double bypass engine compression system. The bypass model employs the control volume method and is validated by the bypass flow database which is established using a novel high-accuracy simplified bypass system computational method. By relating the model parameters to the engine component operation points, evaluation of the bypass aerodynamic performance in the real engine environment is realized, and the feature parameters decisive for the bypass matching characteristic are proposed. The operating range of the bypass system is then disclosed using the bypass model, which is instructive for the aerodynamic design of the whole compression system. Results show that to increase the total bypass ratio, one can decrease bypass backpressure, reduce the core driven fan stage (CDFS) total pressure ratio, increase the mode selector valve (MSV) opening angle or open up the forward variable area bypass injector (FVBAI). Different methods tend to influence the bypass matching characteristics differently. The non-monotonicity characteristic of the dividing flow loss around the MSV will induce a multi-solution zone for the double bypass configuration and make it difficult to diagnose the system operating state. Moreover, the inner bypass will reach the critical condition before the outer bypass, yet the supercritical condition is not recommended as it requires unique bypass geometry and will bring about high flow loss.