Computational investigation of high pressure combustion mechanism in scram accelerator

Abstract

A computational study was carried out to model ,the high-pressure combustion mechanism in scram accelerator. Fluid dynamic modeling was based on RANS equations for reactive flows and it was solved in a fully coupled manner using fully implicit-upwind TVD scheme. For the accurate simulation of highpressure combustion in ram accelerator, g-species, 25step fully detailed reaction mechanism was incorporated with the existing CFD code previously used in the ram accelerator studies. The mechanism is based on GRI-Mech. 2.11 which includes pressuredependent rate formulation indispensable for the correct prediction of induction time in high-pressure environment. A real gas equation of state was also included to account for molecular interactions and real gas effects of high-pressure gases. The present combustion modeling is compared with previous ones using &step or 19-step mechanisms and ideal gas assumption. The result shows that mixture ignition characteristics are very sensitive to the combustion mechanisms, and the different mechanisms result in completely different reactive flow-field characteristics that have a significant relevance to the operation mode and the performance of scram accelerator. Introduction In scram accelerator, explosive gas mixture filled typically at 20-50 bar is compressed by shock waves and generates thrust force by high-speed combustion mechanisms such as shock-induced combustion or oblique detonation wave. Previous studies[l,2,3] show that ignition is presumed to occur in the boundary layer behind a impinging point of oblique shock wave, and the combustion characteristics has a great relevance to the operation mode and performance. Therefore, a :combustion mechanism must faithfully describe the induction time to capture the point of energy release and the rate of energy release to predict the combustion process including the generation of detonation wave. In previous scram accelerator studies, a 7-species and &step chemistry mechanism developed by Evans and Schexnaydert41 has been used for its simplicity,tl,2,5] and more detailed g-species, 19-step Jachimowski mechanism]61 has been used for low pressure cases where the mechanism is reliable.[3,7] However, the mechanisms used in the previous studies were developed under atmospheric condition and loose their accuracy at operating condition of ram accelerator. Because, these reaction mechanisms were based mostly on low pressure and high temperature data, whereas ram accelerators typically operate at pressures about tens and hundreds atm and its mixture ignition temperature is less than 1,400K[8]. As an effort to overcome these limits, Petersen et a1.[81 developed an g-species, 1% step reduced kinetics mechanism for the hydrogen based ram accelerator combustion.l91 This model is almost the subset of a 47-species, 279-step mechanism (RAMEC)[9] based on GRI-Mech. 1.2.1101 Since GRIMech. 2.11 comprised of 51-species, 277-reactions is available for the present,1111 a fully detailed, 9 species 25-step hydrogen/air reaction mechanism based on GRI-Mech. 2.11 was used in this research. This includes pressure dependent rate formulation that is indispensable for predicting induction time correctly in high pressure region. On the other hand, the ideal gas assumption loses its validity for ram accelerator flows since the combustion pressure in ram accelerator is extremely high. An assumption of ideal gas neglects intermolecular interactions and is generally valid for low pressure and/or high temperature systems. However, the typical fill pressure of ram accelerator is 20-50 atms, and the combustion in ram accelerator produces a pressure ratio of 20-40. Therefore, the pressure in the combustion zone is order of thousand atms. At this highly elevated pressure, it is no longer acceptable to neglect the intermolecular interactions, and the real gas effects become important. Recently, AIAA Paper 99-2263,35th AIAA&SMB/SAB/ASEE Joint Propulsion Conference and Exhibit, Los Angeles, CA, 20-24 June 1999. Copyright