Master of Science / Department of Mechanical and Nuclear Engineering / Mohammad H. Hosni / A traditional cooling/refrigeration cycle has four main system components which are an evaporator, a compressor, a condenser, and an expansion valve. This type of cycle requires use of refrigerants which have been found to be harmful to the environment, including causing damage to the atmospheric ozone layer. The main objective of the project was to investigate a water-based non-vapor compression cooling system. Water as a working fluid has the advantages of being inexpensive and environmentally safe for use, as compared to commercially available chemical refrigerants. The water-based cooling system investigated employed cavitation phenomena in converging-diverging glass nozzles. Cavitation is an important phenomenon in fluids, and is common occurring in many devices such as pumps, refrigeration expansion valves, and capillary tubes. It occurs when the static pressure of the fluid falls below the vapor pressure, into a metastable liquid state. Cavitation can be in the form of traveling bubble cavitation, vortex cavitation, cloud cavitation, or attached wall cavitation. In this thesis, the focus was first on visualizing cavitation for water flowing through converging- diverging glass nozzles. These nozzles had throat diameters between 2 mm and 4 mm. Two systems were used: (1) a continuous flow system, where water was driven by a centrifugal pump, and (2) a transient blow down system, where water flow was initiated using a suction pump. A high-speed camera was used to record videos and images of the associated cavitation phenomena. A thermal infrared camera was used in an attempt to measure temperature drop in the nozzle while the system was running The second part of this thesis focused on the understanding of the fundamental thermodynamics phenomena and on the development of practical knowledge relevant to the cavitation process. Two equations of state were used in the analysis, the van der walls equation of state, and the Peng Robinson equation of state. Equations of state were used to predict the transition from vapor to liquid. At a given temperature, the equations were solved for a pressure value corresponding to saturated liquid and saturated vapor specific volume values. Then, the equations were used to determine the spinodal liquid and vapor lines, which represent the metastabillity limits for the liquid and vapor. The characteristic equations of state, combined with implementation of the Law of Corresponding States and thermodynamic theory, were used to estimate the temperature reduction available for refrigeration.

Master of Science / Department of Mechanical and Nuclear Engineering / B. Terry Beck / Mohammad H. Hosni / Cavitation is the formation of vapor phase from the liquid phase by reduction in its absolute pressure below the saturation pressure. Unlike boiling, where the temperature of the liquid is increased to cause vaporization, the reduction in the pressure alone can cause the liquid to turn into vapor. Cavitation is undesirable in many engineering applications as it is associated with reduction in efficiency and is known to cause damage to pump and propeller components. However, the endothermic nature of cavitation could be utilized to create a region of low temperature that could be utilized to develop a new refrigeration cycle. The work presented in this thesis is part of ongoing research into the potential cooling capacity of cavitation phenomena, where the cavitation in a converging-diverging nozzle is being investigated. Due to the constricting nature of the throat of the converging-diverging nozzle, the liquid velocity at the throat is increased, obeying the continuity law. With an increase in velocity, a reduction in absolute pressure is accompanied at the throat of the nozzle according to the Bernoulli’s principle. The local absolute pressure at the throat can go lower than the saturation vapor pressure, thereby causing the fluid to cavitate. The effect of water temperature on the flowrates, the onset of cavitation within the nozzle, and the resulting length of the cavitation region within the nozzle are the subject of this thesis. Experimental results and analysis are presented which also show that near the onset of cavitation, the flowrate can go beyond the choked flowrate, causing the local pressure in the throat to go well below zero for an extended amount of time in the metastable state, before nucleating (cavitating) into a stable state. Flow visualization using a high speed digital camera under different operating conditions was aimed at investigating the region of cavitation onset, which appears to be associated with boundary layer separation just downstream of the nozzle throat. In order to delay the boundary layer separation point in the downstream section of the nozzle, the diffuser region of the nozzle was modified to enable two flow paths, where one path would suck the flow near the inner walls of the nozzle and the other would allow the bulk of the flow to pass through. This was achieved with the use of inserts. Various inserts were tested in an attempt to capture the effect of inserts on the cavitation phenomena. Their effect on the flowrates, length of two phase region, and cavitation onset are presented in this thesis.

The master´s thesis deals with the flow induced by rotation of cavitating fluid in converging-diverging nozzle, which simulates the vortex rope in impeller of water turbines. Measurement is performed on an experimental circuit in laboratory. Results from experimental measurements are compared with CFD simulation of single and two-phase flow. The main focus is to compare the difference of hydraulic losses and shapes of cavitating structures identified in the experiment and in the simulation.

Master of Science / Department of Mechanical and Nuclear Engineering / Steven Eckels / Both cavitating and flashing flows are important phenomena in fluid flow. Cavitating flow, a common consideration in valves, orifices, and metering devices, is also a concern in loss of coolant accidents for liquid water in power plants when saturation pressures are below atmospheric pressure. Flashing flow is a common consideration for devices such as relief and expansion valves and fluid injectors as well as for loss of coolant accidents in which the coolant’s saturation pressure is above atmospheric. Of the two phenomena, flashing flow has received greater interest due to its applicability to safety concerns, though cavitating flow is perhaps of greater interest in terms of energy efficiency. It is possible for cavitating and flashing flow to actually become sonic. That is, the local velocity of a fluid can exceed the local speed of sound due to the unique properties of two-phase mixtures. When a flow becomes sonic, it is possible for the flow to accelerate and impose additional energy losses that would not otherwise occur. Models of this aspect of two-phase flow are not well developed, typically only being presented for the case of constant area ducts. In this paper two models for cavitating sonic flow are developed and described by applying the integral forms of the mass, momentum, and energy equations to a control volume of variable cross-sectional area. These models, based on the homogeneous equilibrium model (HEM) and separated flow model, are then applied to experimental data taken by the author with R-134a as the fluid of interest. Experimental data were taken with four instrumented converging-diverging nozzles of various geometries using a custom testing rig that allowed for precise control and measurement of flow parameters such as mass flow, temperature, and pressure. The resultant data from the models are then examined, focusing on the resultant velocities, Mach numbers, quality, and shear stresses.

[pt] Neste trabalho foi analisado o escoamento de fluidos não Newtonianos através de canais axisimétricos convergentes divergentes. A solução da conservação de massa e de conservação de momento foi obtida numericamente via volumes finitos utilizando o programa de computador Fluent. A equação constitutiva de fluidos Newtonianos generalizados foi utilizada para modelar o comportamento não Newtoniano, utilizando a equação constitucional de Shunk-Scriven para cálculo da viscosidade, que assume como sendo a média geométrica ponderada pelo classificador de escoamento R entre a viscosidade de cisalhamento e a viscosidade de extensão. Os resultados de perda de pressão e vazão são comparados com os resultados calculados pela relação simplificada proposta por Souza Mendes e Naccache, 2002 entre a perda de carga e vazão de fluidos viscoelásticos fluindo através do meio poroso, para analisar a sua performance. / [en] In this work, the flow of non-Newtonian fluids through axisimmetric convergingdiverging channels is analyzed. The solution of mass and momentum conservation equations is obtained numerically via finite volume technique using the Fluent software. The Generalized Newtonian Fluid constitutive equation was used to model the non- Newtonian fluid behavior, using the Shunk-Scriven model for the viscosity, where a weighted geometric mean by the flow classifier R between shear and extensional viscosities is assumed. The results of pressure drop and flow rate are compared to the ones predicted by a previously proposed simplified relation (Souza Mendes and Naccache, 2002) between pressure drop and flow rate, for viscoelastic fluids flow through porous media, in order to analyze its performance.

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