EVALUATION OF THE PERFORMANCE OF SUPERSONIC EXHAUST DIFFUSER USING SCALED DOWN MODELS

Experiments were carried out on straight cylindrical supersonic exhaus t diffusers (SED) using cold nitrogen and hot rocket exhaust gases as driving fluids, in order to evaluate the effects of the ratios of the SED area to rocket nozzle throat area (A(d)/A(t)), SED area to rocket nozzle exit area (A(d)/A(e)), SED length to its diameter (LID) and spe cific heat ratio of the driving gases (k) on the minimum starting pres sure ratio, (P-o/P-a)st, of SED. The rocket nozzle and SED starting tr ansients were also simulated in the models. The study reveals that (P- o/P-2)st increases monotonically with increase in (A(d)/A(t)) and k. O ne-dimensional normal shock relations were used in predicting the (P-o /P-a)st since the compression in long ducts is basically a normal shoc k process. Predicted values of (P-o/P-a)st were validated with experim ental data. SED efficiency factors(eta(ns)) were arrived at based on o ne-dimensional normal shock relations, eta(ns) goes down at higher val ues of (A(d)/A(e)).(P-o/P-a)st is lower for lower k values for the sam e (A(d)/A(t)). Cylindrical SEDs exhibit no hysteresis. The results of this investigation were utilised in validating the design of high alti tude test (HAT) facility for testing the third stage motor (PS-3) of P olar Satellite Launch Vehicle (PSLV). The simulation of starting trans ients in the model revealed that the HAT facility shall not be operate d in the unstarted phase, because the rocket nozzle may fail due to vi olent oscillations of the vacuum chamber pressure. These experimental data were also utilised for designing a SED for PS-3 sub-scale motor, the results of which are covered in this paper. The accuracy of measur ements are within a range of +/-0.4%. Error analysis of the data were carried out and are presented in Appendix A. (C) 1998 Elsevier Science Inc. All rights reserved.