Hypersonic gaseous piston shock tunnel - Numerical and experimental results

Abstract

Recent experimental results have indicated that by inserting a thin section, previously filled with some kind of gas , in between the driver and driven sections of a shock tube/tunnel improves its performance. This technique has been able to produce higher reservoir enthalpy levels then usually obtained with a conventional shock tube/tunnel, when operating in the Equilibrium Interface Condition. This new technique, which is tentatively being called the Gaseous Piston Technique, also has been proven to increase the available test time by acting as a Separating Gas and, therefore, reducing the test gas contamination by the driver gas. An additional benefit of this reduced contamination is preventing combustion from taking place at the airhydrogen interface, when hydrogen is used as the driver gas and air as the test gas. The present numerical and experimental investigation accesses the influence of the Separating Gas nature and its initial fill pressure on the final Equilibrium Interface reservoir conditions. Results indicate that gases exhibiting a high molecular/atomic weight and a high value of the ratio of specific heats at high initial fill pressures perform the best. * Co!., Brazilian Air Force. Director of EAv CTA. f Capt, Brazilian Air Force. Presently visiting research scientist at the Rensselaer Polytechnic Institute, Troy, NY. AIAA Member. * Research Assistant. * Research Engineer. f Active Professor Emeritus of Aeronautical Engineering. AIAA Fellow. Copyright © 1998 by M.A.C. do Nascimento and M.A.S. Minucci. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. INTRODUCTION The active hypersonic gasdynamic research on physical phenomena in shock tube/tunnel flows has almost ceased from the mid fifties until late seventies. This, of course, followed worldwide governmental decisions on cutting funds for the development of a new generation of ballistic missiles and reentry spacecrafts. In the first years of the eighties, however, spurred by the development of air-breathing hypersonic vehicles, such as the American National Aerospace Plane, and more recently by the needs brought about by new advanced projects, such NASA's Hyper-X, research on hypervelocity/hypersonic flows was resumed. Due to the extremely high reservoir enthalpies required to duplicate hypervelocity/hypersonic real flights, a natural trend is to explore the possibilities offered by shock tunnel' operating in the so called Equilibrium Interface Condition. In this particular condition, high reservoir temperatures and pressures are obtained through successive shock wave reflections occurring between the moving interface, or contact surface, and the end of the driven tube. These reflections become successively weaker as the interface is continuously decelerated and eventually brought to rest. When the reflected shocks become weak enough, so that no appreciable changes in the reservoir temperature or pressure occur, an equilibrium situation or an Equilibrium Interface Condition is reached. As a consequence, the Equilibrium Interface Technique is capable of considerably improving both reservoir final temperature and pressure while still retaining most of the test time available for tailored operation. The same result is achieved in gun shock tunnels, by using a bulk free piston in the driven section, impulsively put in motion by the diaphragm 1 American Institute of Aeronautics and Astronautics Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc. rupture. Besides the fairly large temperature and pressure (that is the large enthalpy attained with the bulk free piston), also prevention of the test gas contamination by the driver gas, and consequently a larger testing time, have been observed. In a similar way, the Separating Gas acts as a Gaseous piston, not only heating and compressing the test gas, but also preventing the driven gas contamination by the driver gas. Although the reduced test gas contamination has been suggested by experiments conducted by Minucci and Nagamatsu9 and been successfully applied later by Holden in the CALSPAN LENS facility, further investigation in that area is needed. It is interesting to add, however, that, in the LENS facility, nitrogen is used as the Separating Gas in order to prevent combustion from taking place at the hydrogen/air interface Differently from the previous investigations conducted by the authors, the present research investigates not only experimentally but also numerically, some of the influencing parameters. The parameters are the SGS initial fill pressure, PG, the ratio of specific heats, y, and the atomic/molecular weight, R. These parameters, as it will be seen in the following sections, play important roles on the performance of this new and promising technique, which uses a Gaseous Piston instead of a bulk one. To carry out this study, a small Separating Gas Section, SGS, is inserted between the driver and the driven sections of conventional shock tube/tunnels and a computer code was developed as it will follow.