Publication Date

2017

Document Type

Thesis

Committee Members

James Menart (Committee Member), Rory Roberts (Advisor), Mitch Wolff (Committee Member)

Degree Name

Master of Science in Mechanical Engineering (MSME)

Abstract

Two-phase heat exchangers offer the potential of significant energy transfer by taking advantage of the latent heat of vaporization as the working fluid changes phase. Unfortunately, the flow physics of the phase change process is very complex and there are significant gaps in the fundamental knowledge of how several key parameters are affected by the phase change process. Therefore, an initial investigation modeling a two-phase flow heat exchanger has been accomplished. Many key assumptions have been defined which are critical to modeling two-phase flows. This research lays an initial foundation on which further investigations can build upon. Two-phase heat exchangers will be a critical enabling technology for several key aerospace advancements in the 21st century. In this research, modeling two- phase flow heat exchangers to be used in modeling of NASA's next generation aircraft (N3- X) is accomplished. The heat exchanger model, which could be a condenser or an evaporator, currently accommodates two working fluids; kerosene (jet fuel) and a refrigerant (R134a). The primary goal is to obtain a dynamic, robust model by using numerical simulation tools (MATLAB/ SIMULINK) which can simulate the system efficiently and would be used in the conceptual aircraft (N3-X) model. The final goal of this project is to investigate the influence of pressure and enthalpy perturbations on the system. In other words, how quickly this system responds to change to perturbations, therefore the model will be transient. Two examples are used for demonstration of the transient response of a two- phase heat exchanger to a perturbation in pressure and enthalpy. Initially, pressure perturbation variation effects on how the quality of R134a effects the magnitude of the two- phase flow heat transfer coefficient, therefore the two- phase heat transfer rate calculated. This changing pressure approach used to provide a rapid thermal response to a rapid thermal load variation. Other conventional thermal methods (decreasing the temperature of the cold fluid or increasing the mass flow rate) results in slower response times than changing the pressure. For this analysis, a sample time of 0.000001 seconds was used. In addition, an enthalpy perturbation was investigated. Since, changing pressure suddenly from higher value (650 kPa) to the lower value (555 kPa) is not a real, physical scenario in life, the pressure change with transfer function would be employed to transform the system into first order system with two different time constants. Eventually, the time constant of the system plays a significant role in obtaining a quicker response.

Page Count

125

Department or Program

Department of Mechanical and Materials Engineering

Year Degree Awarded

2017

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