Date of Award

May 2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Ryoichi S Amano

Committee Members

Deyang Qu, John R Reisel, Istvan Lauko, Robert M Cuzner

Keywords

air injection, cavitation, dilution, Gas turbine, Jet, STAR-CCM+

Abstract

Jet-in-cross-flow (JICF) is a well-known term in thermal flows field. Ranging from the normal phenomenon like the volcano ash and dust plumes to the designed film cooling and air fuel mixing for combustion, JICF is always studied to understand its nature at different conditions. Realizing the behavior of interacting flows and importance of many variables lead to the process of reiterating the shapes and running conditions for better outcomes or minimizing the losses. Summarizing the process under the name of optimization, two JICF applications are analyzed based on the principles of thermodynamics and fluid mechanics, then some redesigns are proposed to reach the optimal statuses for the goals sought. Correlations and recommendations are given between the input variables and the outputs.

In the first application, annular thermal mixing chamber, the cold stream penetrates the axial hot flow as circumferential inward jets. Thermal uniformity of the exit mixture is the target to maximize, and accordingly, a streamlined body is firstly suggested to be placed at the center of the chamber to divert the hot stream towards the cold one. Following the idea, the shape and dimensions (length and maximum diameter) are tested experimentally with four 3-D printed bodies expressing different aspect, blockage, and profile ratios. Later, an Analysis LED Design stage (numerical then experimental) checked the effect of adding swirlers on the best streamlined shape. Swirlers shape, number, and height are examined for the relation with the uniformity and pressure drop. By defining a decision-making variable (useful efficiency), the two contradicting variables were consolidated into one, and the swirlers performance was easier to be quantified and the most efficient one was nominated. At the final stage, a numerical study searched the optimal design(s) using design of experiment and optimization (Global and Hybrid) algorithms. The study sought the optimality of the dimensional aspects (diameter, length, and position) of the swirling streamlined body based on minimizing the contradicting objectives. The results were represented by Pareto curve, correlation matrix, parallel axes, and response surface model. It was understood that the optimization can offer improvement of 68% and 15% to the uniformity number and the pressure drop respectively.

On the other hand, aeration treatment for cavitating flow in axial Kaplan turbine was considered for the second engineering application. Using CFD models of a 7.5-cm hydro-turbine, cavitation situation was simulated, then air is injected from the housing to redistribute around the blades of the rotor. The value of the vapor fraction is tracked over the blades and the hub areas throughout the time of turbine cycles. Comparison is achieved by evaluating an average value for the vapor fraction at each case. Air mass flowrate and ports distribution are found to be effective in reducing the cavitation phenomena. Proposed linear aeration distributor on the housing presented a promising technology for spreading the air over the blade chord in a better way than the circumferential distribution. The study allowed the understanding of the flow behavior (in terms of air flow, liquid pressure, and cavitation formations) and turbine performance (i.e. mechanical power) at different air injection locations and turbine rotational speeds. A broader view of research investigated the functionality of linear aeration distributor on the hub with an air supply going through a hollow shaft. The invention of the hub air injection targets the marine industry (i.e. propellers) where the housing/shrouds do not exist, but it also can be a competitor to the housing air injection technology as well. For the two aeration approaches (housing and hub), the conducted numerical investigations were based on the vapor mitigation and power regain in the Kaplan turbine, meanwhile the experimentation looked for the vapor and motor power reduction for the propeller operation. A good agreement (qualitatively and quantitatively) was found between matching cases created for such purpose using tools for high-speed imaging, statistical analysis for turbulent flow, image processing and power measurements.

Finally, the dissertation sets some recommendations for the continuation of the researches on the two applications (thermal uniformity and aeration treatments) for better jets interaction with the cross flow by the consideration of the addition/orientation of guide vanes and the relocation of the jets on the turbine blades respectively.

Available for download on Monday, May 24, 2021

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