Srisha M. V. Rao

Observations on the non-mixed length and unsteady shock motion in a two dimensional supersonic ejector

Key features that drive the operation of a supersonic ejector are the complex gasdynamic interactions of the primary and secondary flows within a variable area duct and the phenomenon of compressible turbulent mixing between them, which have to be understood at a fundamental level. An experimental study has been carried out on the mixing characteristics of a two dimensional supersonic ejector with a supersonic primary flow (air) of Mach number 2.48 and the secondary flow (subsonic) which is induced from the ambient. The non-mixed length, which is the length within the ejector for which the primary and secondary flow remain visually distinct is used to characterize the mixing in the ejector. The operating pressures, flow rates and wall static pressures along the ejector have been measured. Two flow visualization tools have been implemented—time resolved schlieren and laser scattering flow visualization. An important contribution has been the development of in-house image processing algorithms on the MATLAB...

Effect of Primary Flow Mach Number on the Non-mixed Length in a Two-Dimensional Supersonic Ejector

Understanding the flow physics of confined supersonic jet is of primary importance in the design of the supersonic ejector [1] systems. Supersonic ejectors are reportedly used in gas dynamic lasers, wind tunnels, propulsive devices, and fuel cells. One important parameter of study in such device is non-mixed length [2] (NML). A schematic of the flow field observed in a supersonic ejector is shown in Fig. 1. Non-mixed length is defined as the zone where primary and secondary flows maintain distinct characteristics, visually [3]. Conventional techniques like pressure measurement have limitations in determining NML precisely due to constraints in placing the number of pressure sensors, spatially. Optical diagnostics tool like planar laser Mie scattering [4] (PLMS) helps to probe the flow better. This chapter explores the effect of primary flow Mach number (1.5, 2.0, 2.5, and 3.0) on the non-mixed length at different primary flow stagnation pressures (5–10 bar) using wall static pressure measurements and PLMS technique. For this experimentation, an existing supersonic gaseous ejector facility is utilized. Air is used as the working fluid in both primary and secondary flow.

Parametric experimental studies on mixing characteristics within a low area ratio rectangular supersonic gaseous ejector

We use the rectangular gaseous supersonic ejector as a platform to study the mixing characteristics of a confined supersonic jet. The entrainment ratio (ER) of the ejector, the non-mixed length (LNM), and potential core length (LPC) of the primary supersonic jet are measures to characterize mixing within the supersonic ejector. Experiments are carried out on a low area ratio rectangular supersonic ejector with air as the working fluid in both primary and secondary flows. The design Mach number of the nozzle (MPD = 1.5–3.0) and primary flow stagnation pressure (Pop = 4.89–9.89 bars) are the parameters that are varied during experimentation. Wall static pressure measurements are carried out to understand the performance of the ejector as well as to estimate the LNM (the spatial resolution is limited by the placement of pressure transducers). Well-resolved flow images (with a spatial resolution of 50 μm/pixel and temporal resolution of 1.25 ms) obtained through Planar Laser Mie Scattering (PLMS) show the flow dynamics within the ejector with clarity. The primary flow and secondary flow are seeded separately with acetone that makes the LNM and LPC clearly visible in the flow images. These parameters are extracted from the flow images using in-house image processing routines. A significant development in this work is the definition of new scaling parameters within the ejector. LNM, non-dimensionalized with respect to the fully expanded jet height hJ, is found to be a linear function of the Mach number ratio (Mach number ratio is defined as the ratio of design Mach number (MPD) and fully expanded Mach number (MPJ) of the primary jet). This definition also provides a clear demarcation of under-expanded and over-expanded regimes of operation according to [MPD/MPJ] > 1 and [MPD/MPJ] < 1, respectively. It is observed that the ER increased in over-expanded mode (to 120%) and decreased in under-expanded mode (to 68%). Similarly, LNM decreased (to 21.8%) in over-expanded mode and increased (to 20.4%) in under-expanded mode. Lengthening of LPC by 139% and a reduction of 50% in shock cell spacing have also been observed for specific flow conditions. The details regarding experimentation, analysis, and discussions are described in this article.

Studies in Free Jets from Supersonic ESTS Lobed Nozzles

The flow field associated with supersonic free jets can be understood better by using visualization techniques like schlieren in the laboratory. Supersonic jets are used in the fields of aerospace, refrigeration, and precision manufacturing. The supersonic free jets are normally characterized by a compressible flow field with complex shock structures at off-design conditions. Supersonic free jets from nozzles with exotic shapes like Elliptic Sharp Tipped Shallow (ESTS) lobed nozzle have three-dimensional flow fields that are difficult to completely understand using line-of-sight integrated techniques like schlieren. These nozzles present a challenge to the measurement of flow properties with novel flow diagnostic techniques. Density is one such flow property which is very informative yet difficult to obtain. The three-dimensional density fields can be obtained by the background-oriented schlieren (BOS) [1, 2]. Various nozzle configurations with non-axis symmetric profiles have been studied earlier [3] including the ESTS lobed nozzle [4, 5]. This is an attempt to capture the density flow field of an ESTS lobed nozzle free jet using BOS. In this study, a supersonic ESTS lobed nozzle with four lobes is operated to get a supersonic free jet. The density field of this jet is obtained at several azimuthal positions by turning the nozzle about its axis. The information from all the positions is used to reconstruct the three-dimensional flow field of the nozzle. From this density field, both the basic physics of the complex flow field can be understood as well as CFD modeling of the flow field can be validated.

Mixing Enhancement in Free Jets from Supersonic ESTS Lobed Nozzles

Mixing enhancement of two gaseous streams at high Mach numbers plays an important role in the design of supersonic ejectors and supersonic combustors. In order to obtain mixing enhancements in supersonic jets, many nozzles with exotic shapes have been developed [1].

Multi-Objective Optimization Of Ejectors For Hydrogen Fuel Cells

The use of Vector Evaluated Particle Swarm Optimization(VEPSO) technique to optimize ejectors is presented here. Two parameters, compression ratio and e‐ciency have been identifled as the objective functions to be optimized. Their relations to operating and design parameters of ejector is obtained by solving equations obtained from control volume based analysis using a constant area mixing approximation. The independent parameters considered are the area ratio and the exit mach number of the nozzle. The optimization is carried out at a particular entrainment ratio and results in a set of non-dominated solutions, the pareto front. The design parameters can be chosen from these curves based on the operating point that is required. Nomenclature A Area of cross-section [m 2 ] Greek AR Characteristic area ratio of the ejector ! Mass ∞ow ratio of secondary to primary A2=A1s ∞uid _ ms=mp P