No description available.

In an attempt to reduce the dependence on fossil fuels, a variety of research initiatives has focused on increasing the efficiency of conventional energy systems. One such approach is to use waste heat recovery to reclaim energy that is typically lost in the form of dissipative heat. An example of such reclamation is the use of waste heat recovery systems that take low-temperature heat and deliver cooling in space-conditioning applications. In this work, an ejector-based chiller driven by waste heat will be studied from the system to component to sub-component levels, with a specific focus on the ejector. The ejector is a passive device used to compress refrigerants in waste heat driven heat pumps without the use of high grade electricity or wear-prone complex moving parts. With such ejectors, the electrical input for the overall system can be reduced or eliminated entirely under certain conditions, and package sizes can be significantly reduced, allowing for a cooling system that can operate in off-grid, mobile, or remote applications. The performance of this system, measured typically as a coefficient of performance, is primarily dependent on the performance of the ejector pump. This work uses analytical and numerical modeling techniques combined with flow visualization to determine the exact mechanisms of ejector operation, and makes suggestions for ejector performance improvement. Specifically, forcing the presence of two-phase flow has been suggested as a potential tool for performance enhancement. This study determines the effect of two-phase flow on momentum transfer characteristics inside the ejector while operating with refrigerants R134a and R245fa. It is found that reducing the superheat at motive nozzle inlet results in a 12-13% increase in COP with a 14-16 K decrease in driving waste heat temperature. The mechanisms of this improvement are found to be a combination of two effects: the choice of operating fluid (wet vs. dry) and the effect of two-phase flow on the effectiveness of momentum transfer. It is recommended that ejector-based chillers be operated such that the motive nozzle inlet is near saturation, and environmentally friendly dry fluids such as R245fa be used to improve performance. This work provides critical methods for ejector modeling and validation through visualization, as well as guidance on measures to improve ejector design with commensurate beneficial effects on cooling system COP.

Slug velocity measurement and flow regime recognition using acoustic emission technology are presented. Two non-intrusive and three intrusive methods were employed to detect the slug regime and measure its velocity using AE sensors. For the non-intrusive methods, AE sensors were placed directly on the exterior of the steel pipe section of the test rig with and without clamps. The intrusive method involved using different waveguide configurations with the AE sensors flush with the inner wall of the pipe. The experimental study presented investigated the application of Acoustic Emission (AE) technology for detecting slug velocity in addition to differentiating flow regime in two-phase (gas/liquid) flow in horizontal pipes. It is concluded that the slug velocity can be determined with acoustic emission (AE) sensors. The results were compared to slug velocities measured using high speed camera (HSC) and Ultrasound Transit Time (UST) techniques with good agreement between the three techniques at low gas void fraction (GVF). However, at high GVF (up to 95%) where the UST technique has limitations in application, the AE and HSC offered a good agreement. Flow regimes were also differentiated by using a combination of AE technology and Kolmogorov–Smirnov test technique. Stratified, slug and bubble regimes were recognised differentiated.

No description available.

Bubble and cavitation effects phenomena can be encountered in two-phase gas-liquid systems in industry. In certain industries, particularly high-risk systems such as a nuclear reactor/plant, the detection of bubble dynamics, and the monitoring and measurement of their characteristics are necessary in controlling temperature. While in the petro-chemical engineering industry, such as oil transportation pipelines, the detection and monitoring of bubbles/cavitation phenomena are necessary to minimise surface erosion in fluid carrying components or downstream facilities. The high sensitivity of Acoustic Emission (AE) technology is feasible for the detection and monitoring of bubble phenomena in a two phase gas-liquid system and is practical for application within the industry. Underwater measurement of bubble oscillations has been widely studied using hydrophones and employing acoustic techniques in the audible range. However, the application of Acoustic Emission (AE) technology to monitor bubble size has hitherto not been attempted. This thesis presents an experimental investigation aimed at exploring AEs from gas bubble formation, motion and destruction. AE in this particular investigation covers the frequency range of between 100 kHz to 1000 kHz. The AE waveform analysis showed that the AE parameter from single bubble inception and burst events, i.e. AE amplitude, AE duration and AE energy, increased with the increase of bubble size and liquid viscosity. This finding significantly extends the potential use of AE technology for detecting the presence of bubbles in two-phase flow. It is concluded that bubble activity can be detected and monitored by AE technology both intrusively and non-intrusively. Furthermore, the bubble size can be determined by measurement of the AE and this forms the significant contribution of this thesis.

Increased power densities in VLSI chips have led to a need to develop cooling methods that can cope with the increased heat produced by such chips. Currently one of the more attractive methods to meet this goal is through the use of two phase flow of a fluid as changing phase of the material allows high heat transfer rates for a low temperature change. To bring this technology to commercialisation a greater understanding of the underlying physics involved at the microscale is required as there is much debate within literature as to what occurs during two phase flow heat transfer at these scales. The work conducted as part of this thesis is a step towards improving the understanding of the mechanisms involved with this process. This thesis describes the fabrication of a novel microchannel structure, which can be used to experimentally characterise two phase heat transfer as it occurs. The final process reported for these microchannels structures provides the basis of a technology for the fabrication of microchannels with increased sensor densities. Two types of microchannel devices have been fabricated for this project. The first device of these was an array of parallel microchannels formed by the reactive ion etching (RIE) of silicon, which was then bonded with Pyrex glass. These microchannels were simple in that sensors were not integrated for local measurement. However the production of these devices incorporated fabrication techniques such as anodic bonding and inductively coupled plasma RIE that were essential to the fabrication of more complex devices. The second device built was a single microchannel that contained an integrated heater and several temperature sensors. The use of wafer bonding enabled the device to take full advantage of both bulk and surface micromachining technology as the placement of the temperature sensors on the channel floor would not be possible with conventional bulk micromachining. The initial microchannel structures demonstrated that wafer bonding could be used to fabricate novel devices, but they highlighted the difficulty of achieving strong anodic bonds due to the presence of dielectric films throughout the fusion bonded wafer stack used in the channel fabrication. To improve the performance of the device the process was optimised through the use of insitu, non-destructive test structures. These structures enabled the uniformity and strength of the bonds to be optimised through visualisation over the whole wafer surface. The integrated sensors enabled temperature measurements to be taken along the channel with a sensitivity 3.60 ΩK-1 while the integrated heater has delivered a controllable and uniform heat flux of 264 kWm-2.

A careful review of the literature showed that there is a serious lack of information (experimental or analytical) on the pressure losses during two-phase flow in combining tee junctions. Pipe networks in industrial applications involve combining and dividing junctions and knowledge of the pressure losses at these junctions is essential for analysis of the flow distribution in the network. To this end, the pressure losses of air-water mixtures passing through a horizontal, combining tee junction with a 37.8 mm diameter were experimentally studied with annular, wavy, and slug flow regimes in the outlet. The test matrix independently varied the outlet flow rates, the outlet mixture qualities, the gas distribution between the inlets, and the liquid distribution between the inlets. All experiments were conducted at room temperature and a nominal absolute pressure at the centre of the junction of 150 kPa. The pressure distribution in all three legs of the tee was determined using up to 49 pressure taps distributed among the three sides and monitored using pressure transducers to produce accurate measurements of the pressure losses. Time-averaged pressure measurements with annular and wavy flows are reported, while pressure measurements with slug flows were not repeatable. A new model and empirical coefficients is presented that allows accurate prediction of pressure losses for flows with either an annular or wavy outlet. Time-varying pressure measurements are presented and analyzed using probability density functions. Different distributions were found for differential measurements depending on whether or not slugging was present in the system. The probability density functions for cases with annular or wavy flow in the outlet followed Gaussian distributions, while cases with slug flow had skewed distributions. Time-varying pressure signals showed a time lag between slug events based on pressure tap locations. A visual study with slug flow present in the system showed upstream travelling waves induced in a stratified inlet when slug flow was present in the other, which led to unexpected slugging under certain flow conditions. / October 2016

<div>The continued miniaturization and demand for improved performance of electronic devices has resulted in the need for transformative thermal management strategies. Flow boiling is an attractive approach for the thermal management of devices generating high heat fluxes. However, designing heat sinks for two-phase operation and predicting their performance is difficult because of, in part, commonly encountered flow boiling instabilities and a lack of experimentally validated physics-based phase change models. This work aims to advance the state of the art by furthering our understanding of flow boiling instabilities and their implications on the operating characteristics of electronic devices. This is of particular interest under transient and non-uniform heating conditions because of recent advancements in embedded cooling techniques, which exacerbate spatial non-uniformities, and the demand for cooling solutions for next-generation electronic devices. Additionally, this work aims to provide a high-fidelity experimental characterization technique for slug flow boiling to enable the validation of physics-based phase change models.</div><div><br></div><div>To provide a foundation for which the effects of transient and non-uniform heating can be studied, flow boiling instabilities are first studied experimentally in a single, 500 μm-diameter borosilicate glass microchannel. A thin layer of optically transparent and electrically conductive indium tin oxide coated on the outside surface of the microchannel provides a spatially uniform and temporally constant heat flux via Joule heating. The working fluid is degassed, dielectric HFE-7100. Simultaneous high-frequency measurement of reservoir, inlet, and outlet pressures, pressure drop, mass flux, inlet and outlet fluid temperatures, and wall temperature is synchronized to high-speed flow visualizations enabling transient characterization of the thermal-fluidic behavior.</div><div><br></div><div>The effect of flow inertia and inlet liquid subcooling on the rapid-bubble-growth instability at the onset of boiling is assessed first. The mechanisms underlying the rapid-bubble-growth instability, namely, a large liquid superheat and a large pressure spike, are quantified. This instability is shown to cause flow reversal and can result in large temperature spikes due to starving the heated channel of liquid, which is especially severe at low flow inertia.</div><div><br></div><div>Next, the effect of flow inertia, inlet liquid subcooling, and heat flux on the hydrodynamic and thermal oscillations and time-averaged performance is assessed. Two predominant dynamic instabilities are observed: a time-periodic series of rapid-bubble-growth instabilities and the pressure drop instability. The heat flux, ratio of flow inertia to upstream compressibility, and degree of inlet liquid subcooling significantly affect the thermal-fluidic characteristics. High inlet liquid subcoolings and low heat fluxes result in time-periodic transitions between single-phase flow and flow boiling that cause large-amplitude wall temperature oscillations and a time-periodic series of rapid-bubble-growth instabilities. Low inlet liquid subcoolings result in small-amplitude thermal-fluidic oscillations and the pressure drop instability. Low flow inertia exacerbates the pressure drop instability and results in large-amplitude thermal-fluidic oscillations whereas high flow inertia reduces their severity.</div><div><br></div><div>Flow boiling experiments are then performed in a parallel channel test section consisting of two thermally isolated, heated microchannels to study the Ledinegg instability. When the flow in both channels is in the single-phase regime, they have equal wall temperatures due to evenly distributed mass flux delivered to each channel. Boiling incipience in one of the channels triggers the Ledinegg instability which induces a temperature difference between the two channels due to flow maldistribution. The temperature difference between the two channels grows with increasing power. The experimentally observed temperature excursion between the channels due to the Ledinegg instability is reported here for the first time.</div><div><br></div><div>Time-resolved characterization of flow boiling in a single microchannel is then performed during transient heating conditions. For transient heating tests, three different heat flux levels are selected that exhibit highly contrasting flow behavior during constant heating conditions: a low heat flux corresponding to single-phase flow (15 kW/m²), an intermediate heat flux corresponding to continuous flow boiling (75 kW/m²), and a very high heat flux which would cause critical heat flux if operated at this heat flux continuously (150 kW/m²). Transient testing is first conducted using a single heat flux pulse between these heat flux levels and varying the pulse time. It is observed that any step up/down in the heat flux level that induces/ceases boiling, causes the temperature to temporarily over/under-shoot the eventual steady temperature. Following the single heat flux pulse experiments, a time-periodic series of heat flux pulses is applied. A square wave heating profile is used with pulse frequencies ranging from 0.1 to 100 Hz and three different heat fluxes levels (15, 75, and 150 kW/m²). Three different time-periodic flow boiling fluctuations are observed: flow regime transitions, pressure drop oscillations, and heating pulse propagation. For heating pulse frequencies between approximately 1 and 10 Hz, the thermal and flow fluctuations are heavily coupled to the heating characteristics, forcing the pressure drop instability frequency to match the heating frequency. For heating pulse frequencies above 25 Hz, the microchannel wall attenuates the transient heating profile and the fluid essentially experiences a constant heat flux.</div><div><br></div><div>To improve our ability to predict the performance of heat sinks for two-phase operation, high-fidelity characterization of key hydrodynamic and heat transfer parameters during microchannel slug flow boiling is performed using a novel experimental test facility that generates an archetypal flow regime, devoid of flow instabilities and flow regime transitions. High-speed flow visualization images are analyzed to quantify the uniformity of the vapor bubbles and liquid slugs generated, as well as the growth of vapor bubbles over a range of heat fluxes. A method is demonstrated for measuring liquid film thickness from the visualizations using a ray-tracing procedure to correct for optical distortions. Characterization of the slug flow boiling regime that is generated demonstrates the unique ability of the facility to precisely control and quantify hydrodynamic and heat transfer characteristics.</div><div><br></div><div>This work has advanced state-of-the-art technologies for the thermal management of high-heat-flux-dissipation devices by providing an improved understanding on the effects of transient and non-uniform heating on flow boiling and an experimental method for the validation of physics-based flow boiling modeling.</div>

Thesis. 1976. Ph.D.--Massachusetts Institute of Technology. Dept. of Chemical Engineering. / Microfiche copy available in Archives and Science. / Vita. / Bibliography: leaves 159-161. / Ph.D.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1979. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by Suzanne Marie Burzyk. / B.S.