Three Stream, Variable Density Mixing Layers under Transverse and Streamwise Pressure Gradients

by Sky Ellsworth

Funded by the National Science Foundation


The efficiency of the gas turbines used in power generation is of major importance to energy conservation. Nationwide, gas turbine and internal combustion engines produced 49 Billion Kilowatt-hours of energy for electric generation alone (Statistical Abstract of the United States, 1996). Thus, a small percentage increase in the efficiency of turbomachines used in power generation would have significant economic and environmental implications.

Gas turbine engine designers strive to increase engine efficiency, power to weight ratio, and range of operation. A thermodynamic analysis by Sirignano, Delaplanque, and Liu (1997), indicates that significant benefit can result from augmented burning in the turbine. The analysis shows that augmented combustion in the turbine, combined with regeneration, allows for an increase in:

In addition, an analysis by Sirignano and Kim (1996) indicates that some potential exists for reduction of NOx formation via combustion in the accelerating flow through the turbine.

These benefits are not easily achieved, however. The turbomachinery designer would have to account for many new factors when analyzing and constructing a powerplant with augmented burning in the turbine. These new factors that would have to be accounted for include:

A computational study is currently being conducted to better understand how to account for these factors. The computational research will consider a fully elliptic, turbulent, reacting multicomponent channel flow with streamwise acceleration and curvature.

The present experimental study is designed to provide qualitative, and some quantitative, validation of the computational results. Although this experiment is designed to verify computational results for the flow through a turbine, it will provide a much broader and general good. No previous experimental studies of three-stream, variable density mixing layers under streamwise and transverse pressure gradients have been performed. The main emphasis of the study will be placed on characterizing mixing and its inter-dependence on density ratio, flow velocities, and transverse and streamwise pressure gradients.

The experimental study is based on a three stream shear layer facility to model flow through a turbine with augmented burning. The facility is described as follows:

Compressors supply air to the control station at a pressure of approximately one MPa. Ten high pressure cylinders supply helium, which is regulated down to desired pressures.

Combinations of air and helium are used to simulate the high-temperature (low-density) region of combustion between two parallel streams found in a gas turbine engine with augmented combustion. Metering valves control precise mixtures of helium and air to the inner, upper and lower settling chambers, in addition to regulating pressures. Regulating the mass fraction of helium and air in this way allows adjustment of the densities and velocities (for fixed total pressures) to each of the three settling chambers. The settling chambers provide a low speed region to straighten the flow of each stream, before they are accelerated in the nozzle, and then mixed.

One pressure probe is located in each of the inner, upper, and lower settling chambers for total pressure measurements. The upper and lower streams, which we will now collectively call the outer streams, consist of air, while the middle stream consists of air-helium mixtures, of any composition desired, to achieve the low-density region.

The flow velocities at the nozzle exit, which is the test section entrance, range up to sonic values. The three streams come into contact in a planar configuration at the test section entrance, and proceed through the tunnel, encapsulated by two opposing glass walls, and two opposing flexible plates. The plates may be bent in the elastic region via a transverse load in a geometry described by common beam deflection theory.

Both Planar Laser Induced Fluorescence (PLIF) and nanosecond Schlieren photography will provide qualitative images of the flow.

Seven static pressure probes are located at 1.5-inch increments in each of the flexible plates. In addition, a total pressure profile may be taken at any of the static probe locations via a motorized traverse. The streams exhaust from the flexible plates into ambient conditions.

The control center, designed to synchronize and control of these processes, is composed of a combination of electrical and mechanical controls. The accurate synchronization required is made possible by a Pentium II computer, equipped with a National Instruments Data Acquisition Board, and Labview software.

Thus far, experimental data has been collected in the form of PLIF images, for the following cases:

The preceding cases were run under these approximate conditions:

Each of the preceding cases will be subject to pitot pressure profiles and nanosecond Schlieren photography in future tests.


I am currently a graduate student in the Mechanical and Aerospace Engineering Department at the University of California, Irvine. My research is conducted in the Supersonic Turbulence Laboratory, under the guidance of Dr. Dimitri Papamoschou.


University of California, Irvine
September, 1993 to June, 1998
Irvine, California

University of California, Irvine
June, 1998 to June, 2000
Irvine, California


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