Unsteady computational analysis of shrouded plug nozzle flows and reacting impinging jets
Abstract
The computations of a non-reacting nozzle-flow problem and a reacting impinging jet problem using a unified numerical methodology is presented. The nozzle problem is a shrouded plug configuration that operates at nozzle pressure ratio (NPR, ratio of inlet pressure to ambient pressure) between one to a design NPR of 6.23 for supersonic applications. An sub-scale model with extensive instrumentation is the basis of axisymmetric and three-dimensional computations done as both steady and unsteady problems with an aim to understand nozzle flow physics. The pressure distribution and shock structure predicted by steady computations not only detailed the shock physics but were also in close agreement with measured pressure data and visualization. The nozzle is observed to transition from normal shock at NPR's just above one to a lambda shock below NPR of 2.0 and then from a Mach reflection to a regular reflection within NPR range 2.25 to 3.1. A barrel oblique shock is observed above NPR of 3.1 before achieving perfect expansion at design NPR. During the shock transition the separation region behind the shock is observed to be fully attached for NPR's below 2.0, a regime called free shock separation (FSS), followed by reattached flow on plug wall called restricted shock separation (RSS) at higher NPR's. The unsteady computational analysis explained the shifts in frequencies observed in measurements. The unsteady computations at NPR of 1.26 show that the measured frequency of 170Hz is because of periodic choking and unchoking driven by large scale shock motion. In the FSS regime identified by computations the measured frequency remains constant at 200Hz. Following this the frequency shifts to above 300Hz and increases monotonically as the nozzle transitions from FSS to RSS observed to occur between NPR of 2.0 and approximately 2.25. Unsteady 3-D computations showed axisymmetric instantaneous flowfield at NPR of 1.26 while at NPR of 1.59 the dynamic flowfield was observed to be asymmetric. Time-averaged pressure distribution and oscillation frequency from three-dimensional unsteady computations were closer to the measurements than with axisymmetric assumption. An experimental impinging jet configuration involving MMH as fuel stream and RFNA as oxidizer stream, impinging at an included angle is studied to analyze combustion initiation, flame propagation, holding and sustenance. A newly devised reduced chemical mechanism is evaluated in this context for two background gases of argon and helium which showed different combustion behavior in recent experiments. Also, combustion is studied at two pressures corresponding to experiments (1atm) and applications (100 atm). Before the impinging jet configuration is studied, a simple homogeneous mixture constant volume combustion problem is formulated to understand chemical time scales. The chemical mechanism resulted in combustion in a millisecond time frame only for an initial mixture temperature of 800K with initial pressure of 100atm showing faster combustion than 1atm. The impinging configuration is studied first with a planar assumption before three-dimensional analysis is taken up to understand effect of background gas. In agreement with experiments, the helium and argon gases showed considerable differences. At low pressure of 1atm the combustion initiation, propagation and holding were analogous to experiments in both gases but the ignition event timings and flame sustenance were different in the two gases. Argon resulted in wider flame zone compared to helium. At high pressures the flame propagation behavior differed from low pressures with instantaneous combustion initiation but different flame behaviors in both gases. The flame propagation, holding and sustenance behavior has been explained in detail in the two back ground gases at both pressure extremes. The three-dimensional computations showed that the combustion behavior is different with predictions using planar assumption. The differences have been documented.
Degree
Ph.D.
Advisors
Merkle, Purdue University.
Subject Area
Aerospace engineering|Mechanical engineering
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.