Using Numerical Simulation to Obtain Length of Constant Area Section in Scramjet Combustor

Constant area section length downstream to the fuel injection point is a crucial dimension of scramjet duct geometry. It has a major contribution in creating the maximum effective pressure inside the combustor that is required for propulsion. The length is limited by the thermal choking phenomenon, which occurs when heat is added in a flow through constant area duct. As per theory, to avoid thermal choking the constant area section length depends upon the inlet conditions and the rate of heat addition. The complexity related to mixing and combustion process inside the supersonic stream makes it difficult to predict the rate of heat addition and in turn the length. Recent efforts of simulating the reacting flow inside scramjet combustors are encouraging and can be useful in this regard. The presented work attempts to use simulation results of scramjet combustion for predicting the constant area section length for a typical scramjet combustor. Though the technique appears to be simple, in general it can be applied in the design of combustor ducts where thermal chocking needs to be avoided. A typical scramjet combustor geometry with isolator, constant area section, and diverging section with an intrusive fuel injection scheme from the available experimental work is modeled for simulation. Reacting simulations are done and validated with the available experimental results. The inlet conditions being those corresponding to a Mach 6 scramjet flight. One-dimensional analysis of frictionless flow through constant area ducts is used to calculate maximum possible enthalpy addition in the flow. For the given injector configuration and equivalence ratio of 0.34, the limiting length of constant area section to avoid thermal choking was obtained as 59 mm. The value of this limiting length obtained from simulation is verified. Comments are made about the method.