Publication Date

2024

Document Type

Thesis

Committee Members

Mitch Wolff, Ph.D. (Advisor); Abdeel Román, Ph.D. (Committee Member); C. Taber Wanstall, Ph.D. (Committee Member)

Degree Name

Master of Science in Mechanical Engineering (MSME)

Abstract

Aircraft platforms are continually upgraded with increasingly high quantities of high-powered electronics. As such, efficient thermal management systems are crucially important to overcome system instabilities and support pulsed power profiles. To create effective thermal control systems, it is imperative to thoroughly examine and account for a variety of potential system behaviors caused by transient changes in the flow regime. If left unconstrained, these changes will create thermal instabilities, which can severely damage electronics and hurt the overall reliability of the aircraft. These instabilities can be described by both void fraction and quality. Electrical Capacitance Tomography (ECT) allows for the collection of capacitance data from the fluid moving through a channel. The vapor, liquid, and the transition between the two, will have different permittivity values depending on the state of the fluid. These permittivity values can be found and recorded by the ECT sensor and easily translated to void fraction measurements. A series of ramped profiles with varying conditions were tested to see what types of system behaviors were present between the maximum and minimum values of the pulse profile, which in this case, resembles a square sine wave. Using data recorded by the ECT sensor, a physical understanding of what is occurring during transient conditions between pulses or throughout instabilities can be constructed. Determining the quality of the refrigerant while instabilities are present or for pulsed loads becomes more difficult, especially when trying to apply traditional methods using the Energy Balance Equation. The Energy Balance Equation provides unrealistic results for quality during transient conditions due to its dependence on the quantity of power input. Additionally, the Energy Balance Equation is unable to properly model instabilities present in the flow, particularly those present due to the flow regime. The Energy Balance Equation wrongly assumes that the system can react instantaneously to changes in conditions. In contrast, the values of void fraction are not susceptible to the same issues induced by the time lag, as present in the Energy Balance Equation, because the values of void fraction are measured from the ECT sensor. Using correlations between void fraction and quality will provide a more accurate and realistic representation of the quality of the R-134a refrigerant at a given instant in time. For this research, only the Separated Flow Model was considered. This model has six eminent correlations that each share the same basic equation but contain their own unique coefficients. The models were validated against the results obtained through the ECT sensor to identify situations in which they were optimal. A custom correlation was then calibrated using the experimental data from the ECT sensor. The general Separated Flow Model Equation, which relates quality to void fraction, was then manipulated algebraically to solve for quality in terms of void fraction. This allowed for the calculation of quality based off the experimentally measured data from the ECT sensor. This achievement proved significant because it made it possible to find quality during transient conditions. A further series of tests were then conducted to find the practical bounds of this new quality prediction method. After validating this new model over varying operating conditions, the model proved to be a robust method to accurately predict quality under a large range of operating conditions. The results of this experiment will significantly enhance the ability of future thermal control systems to swiftly and efficiently adapt to instabilities and other abrupt fluctuations in the thermal loads they are required to manage.

Page Count

96

Department or Program

Department of Mechanical and Materials Engineering

Year Degree Awarded

2024

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