CFD Analysis of Supersonic Exhaust in a Scramjet Engine – Mechanical Projects

Abstract:

When pressures and temperatures become so high in supersonic flight that it is no longer efficient to slow the oncoming flow to subsonic speeds for combustion, a scramjet (supersonic combustion ramjet) is used in place of a ramjet. This paper is aimed at modeling the supersonic flow inside Scramjet engine using the Computational Fluid
Dynamics ANSYS Fluent. The purpose of this test is to validate FLUENT’s ability to predict reflecting shock waves and their effect on wall pressure distribution and heat transfer. Supersonic flow from a nozzle that represents the exhaust nozzle of a supersonic combustion ramjet (SCRAMJET) is modeled. Jet from the nozzle is issued into a domain which is bounded on one side by an afterbody wall which is parallel to the centerline of the nozzle. Shocks propagating from the nozzle exit reflect from the afterbody. Measured values of the distribution of wall pressure and heat transfer rate along the afterbody are used to validate the CFD simulation.
In this study, k-ε model has been used to examine supersonic flow in a model scramjet exhaust. The configuration used is similar to the DLR (German Aerospace Center) scramjet model and it is consists of a one-sided divergent channel with wedge-shaped and without wedge shaped. For the purpose of validation, the k-ε results are compared with experimental data for temperature at the bottom wall. In addition, qualitative comparisons are also made between predicted and measured shadowgraph images. The k-ε computations are capable of predicting flow simulations well and good.

INTRODUCTION
Scramjets are engines designed to operate at high speeds usually only associated with rockets and are typically powered by hydrogen fuel. Scramjet is an acronym for Supersonic combustion ramjet. A ramjet has no moving parts. Air entering the intake is compressed using the forward speed of the aircraft. The intake air is then slowed from a high subsonic or supersonic speed to a low subsonic speed by aerodynamic diffusion created by the inlet and diffuser. Fuel is then injected into the combustion chamber where burning takes place. The expansion of hot gases then accelerates the subsonic exhaust air to a supersonic speed. This results in a forward velocity. Scramjets on the other hand do not slow the free stream air down through the combustion chamber rather keeping it at some supersonic speed. This may appear mechanically simple however it is immensely more aerodynamically complex than a jet engine.
Keeping the free stream flow supersonic enables the scramjet to fly at much higher speeds. Supersonic flow is needed at higher speeds to maximize efficiency through the combustion process. Scramjet top speeds have been estimated between Mach 15 to Mach 24, however at this early stage Mach 9.6 is the fastest recorded flight achieved during the third and final flight of the X-43A flown by NASA. This is three times the speed of the SR-71, officially the fastest jetpowered aircraft which achieved Mach 3.2.

GEOMETRIC MODEL
Supersonic flow from a nozzle that represents the exhaust nozzle of a supersonic combustion ramjet (SCRAMJET) is modeled. Jet from the nozzle is issued into a domain which is bounded on one side by an after body wall which is parallel to the centerline of the nozzle. Shocks propagating from the nozzle exit reflect from the after body. Measured values of the distribution of wall pressure and heat transfer rate along the after body are used to validate the CFD simulation.The flow is considered to be two-dimensional, because the span of the experimental outlet is considerably larger than the height. Both geometries are shown in Fig. 1 and Fig. 2. The flow enters the exhaust section at a Mach
number of 1.66. In each case, the cowl wall opposite the after body angles initially upward. This is followed by a wedge, inducing a shock that reflects off of the after body.