Mechanical Engineering
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Item TWO-PHASE HEAT TRANSFER MECHANISMS WITHIN PLATE HEAT EXCHANGERS: EXPERIMENTS AND MODELING(2016) Solotych, Valentin; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Two-phase flow heat exchangers have been shown to have very high efficiencies, but the lack of a dependable model and data precludes them from use in many cases. Herein a new method for the measurement of local convective heat transfer coefficients from the outside of a heat transferring wall has been developed, which results in accurate local measurements of heat flux during two-phase flow. This novel technique uses a chevron-pattern corrugated plate heat exchanger consisting of a specially machined Calcium Fluoride plate and the refrigerant HFE7100, with heat flux values up to 1 W cm-2 and flow rates up to 300 kg m-2s-1. As Calcium Fluoride is largely transparent to infra-red radiation, the measurement of the surface temperature of PHE that is in direct contact with the liquid is accomplished through use of a mid-range (3.0-5.1 µm) infra-red camera. The objective of this study is to develop, validate, and use a unique infrared thermometry method to quantify the heat transfer characteristics of flow boiling within different Plate Heat Exchanger geometries. This new method allows high spatial and temporal resolution measurements. Furthermore quasi-local pressure measurements enable us to characterize the performance of each geometry. Validation of this technique will be demonstrated by comparison to accepted single and two-phase data. The results can be used to come up with new heat transfer correlations and optimization tools for heat exchanger designers. The scientific contribution of this thesis is, to give PHE developers further tools to allow them to identify the heat transfer and pressure drop performance of any corrugated plate pattern directly without the need to account for typical error sources due to inlet and outlet distribution systems. Furthermore, the designers will now gain information on the local heat transfer distribution within one plate heat exchanger cell which will help to choose the correct corrugation geometry for a given task.Item High Reynolds Number Vertical Up-Flow Parameters For Cryogenic Two-Phase Helium I(2014) Mustafi, Shuvo; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The two phase flow characteristics of helium I are of interest since under most operational scenarios this cryogenic fluid exists in both liquid and vapor form because of its extremely low boiling point and latent heat of vaporization. There is a significant knowledge gap in the flow boiling parameters of helium (heat transfer coefficient, pressure drop and dryout heat flux) for high Reynolds number vertical up-flows (Re =10^5-10^6). This dissertation fills this gap and helps to expand the use of helium as an inert simulant for hydrogen. Since no prior correlations for the flow boiling parameters existed for vertical up-flows of helium at these Reynolds numbers, any predictions of these parameters were dependent on correlations that were tested at lower Reynolds numbers, or correlations based on other fluids. The thermophysical properties of helium I are significantly different from most other fluids; therefore the capability of prior correlations in predicting experimental observations was limited. As part of this research new correlations are proposed for the flow boiling parameters. This research begins the investigation of a new regime for two-phase helium I flows at Reynolds numbers above 3e5. The techniques described will enable future work to address other gaps in knowledge for helium I flows that still remain. The prior heat transfer coefficient correlation over-predicted the data that was collected for this research. The new correlation improves the agreement with data by a factor of 98. Two prior models for pressure drop, the separated flow model and the homogeneous flow model, under-predict the observed pressure drop. The newer versions of the separated flow and the homogeneous flow correlations improve agreement with the data by about a factor of 3 and by more than a factor of 2 respectively. The previous dryout heat flux correlation considerably over predicts the observed dryout heat flux. The new correlation improves agreement with the data by a factor of 21. Significant cryogenic challenges were overcome to collect the research data. The strategies described for surmounting the diverse challenges such as thermal acoustic oscillations and low dryout heat flux could be used by future two-phase cryogenic flow researchers.Item LIMITS OF THIN FILM COOLING IN MICROGAP CHANNELS(2011) Rahim, Emil; Bar-Cohen, Avram; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The forced flow of dielectric liquids, undergoing phase change while flowing in a narrow channel, is a promising candidate for the thermal management of advanced semiconductor devices. Such channels may be created by the spacing between silicon ribs in a microchannel cooler, between stacked silicon chips in a three-dimensional logic, RF, or heterogeneous microsystem, narrowly-spaced organic or ceramic substrates, or between a chip and a non-silicon polymer cover in a microgap cooler. These microgap configurations provide direct contact - and hence cooling - between a chemically-inert, dielectric liquid and the back surface of an active electronic component, thus eliminating the significant thermal resistance associated with a Thermal Interface Material (TIM) or the solid-solid contact resulting from the attachment of a microchannel cold plate to the chip. This dissertation explores the physics underpinning two-phase flow in miniature channels, through an extensive literature survey, and employs analytical, numerical, and experimental techniques to determine the thermal transport phenomena in microgap channels, with emphasis on the thermal limits of thin film heat transfer in annular flow. The applicability of several flow regime mapping methodologies has been examined. The predictions of these mapping methodologies have been compared to the visual observations of two-phase flow in microtubes and microchannels. The axial variation of two-phase heat transfer coefficients with local vapor qualities is reported, and the association of this variation with the dominant flow regime is discussed. The measured two-phase flow heat transfer coefficients are then sorted according to the dominant flow regime, and compared to the predictions of classical heat transfer correlations. Two-phase flow experiments were performed in a microgap cooler with the flow of HFE7100 and FC-87. The microgap cooler is 125 mm long, 14 mm wide, and was operated with three distinct gap sizes: 100, 200, and 500 micron. An instrumented Intel thermal test vehicle (TTV) flip-chip mounted via a BGA on an organic substrate, and equipped with 9 pre-calibrated temperature sensors, was used as the heated section of the microgap channel. Pressure drop across the channel, fluid inlet and exit temperature, and wall temperature were measured. Using commercial software, an "inverse" numerical technique was developed to identify the local heat flux and heat transfer coefficient. Local Annular heat transfer coefficients, for FC-87 flowing in the 100 micron channel, were found to display elements of the M-shaped variation with flow quality and reached a maximum value of 15 kW/m^2-K.Item ENHANCEMENT OF SPRAY COOLING HEAT TRANSFER USING EXTENDED SURFACES AND NANOFLUIDS(2007-11-05) Coursey, Johnathan Stuart; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Spray cooling is a powerful heat transfer technique in which an atomizing nozzle provides a flow of liquid droplets directed towards a hot surface. This dissertation explores two potentially powerful techniques capable of improving traditional spray cooling: nanofluids and extended surfaces. Nanofluids were experimentally studied in a pool boiling system to elucidate the underlying mechanisms of critical heat flux (CHF) enhancement. Dilute suspensions of nanoparticles were found to have a degrading or no effect on boiling performance. Greater concentrations (≥ 0.5 g/L) lead to modest (up to ~37%) increase in the CHF. The results were highly dependent on the working fluid/substrate combination, specifically wetting characteristics. Poorly wetting systems (e.g. water on copper) could be enhanced by nanofluids, whereas better wetting systems (e.g. ethanol on glass) showed no improvement. This conclusion was re-enforced when nanofouling caused by dryout of nanofluid was found to improve wetting as shown by a reduction in the advancing threephase contact angle. Interestingly, similar CHF enhancement was achieved without nanofluids using an oxidized surface, which is easily wetted with pure fluids. In fact, surface treatment alone resulted in similar CHF enhancement at ~20°C less wall superheat than required using nanofluids. Spray cooling was found to be adversely affected by the addition of nanoparticles due to changing thermophysical properties and/or nozzle clogging due to particle deposition. The addition of high aspect ratio open microchannels to the sprayed surface resulted in significant enhancement at all wall superheats and over 200% enhancement in the low temperature single-phase regime. The two-phase regime began at lower temperatures with microchannels, which lead to heat transfer enhancements of up to 181%. The onset of two-phase effects was found to be a strong function of channel depth. However, the onset of two-phase effects was found to occur at a temperature that was independent of nozzle pressure/mass flow rate. Therefore, nucleation and two-phase effects are likely triggered by the unique liquid distribution caused by the extended structures. Using high aspect ratio open microchannels, these mechanisms resulted in spray efficiencies approaching one, indicating almost complete utilization of the spray's ability to absorb heat.