Mechanical Engineering
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Item Burning Emulations of Condensed Phase Fuels Aboard The International Space Station(2022) Dehghani, Parham; Sunderland, Peter B; Quintiere, James G; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Little is known about the fire hazards of solids and liquids in microgravity. Ground-based tests are too short to overcome ignition transients and testing dozens of condensed fuels in orbit is prohibitively expensive. Burning rate emulation is one way to address this gap. It involves emulating condensed fuels with gases using a porous burner with embedded heat flux gages. This is a study of microgravity burning rate emulation aboard the International Space Station. The burner had porous round surfaces with a diameter of 25 mm. The fuel mixture was gaseous ethylene, and it was diluted with various amounts of nitrogen. The resulting heats of combustion were 15 – 47.2 kJ/g. The flow rate, oxygen concentration in the ambient, and pressure were varied. Heat flux to the burner was measured with two embedded heat flux gages and a slug calorimeter. The effective heat of gasification was determined from the heat flux divided by the fuel flow rate. Radiometers provided the radiative loss fractions. A dimensional analysis based on radiation theory yielded a relationship for radiative loss fraction. RADCAL, a narrow-band radiation model, yielded flame emissivities from the product concentrations, temperature, flame length, and pressure. Previously published analytical solutions to these flames allowed prediction of flame heights and radius, and when combined with the radiation empirical relationship led to corrections of total heat release rate from the flames due to radiative loss. Average convective and radiative heat flux were obtained from the analytical solution and a model based on the geometrical view factor of the burner surface with respect to the flame sheet, that was used to calculate the heat of gasification. All flames burning in 21% by volume oxygen self-extinguished within 40 s. However, steady flames were observed at 26.5, 34, and 40% oxygen. The analytical solution was used to quantify flame steadiness just before extinction. The steadiest flames reached more than 94% of their steady-state heat fluxes and heights. A flammability map as a plot of the heat of gasification versus heat of combustion was developed based on the measurement and theory for nominal ambient oxygen mole fractions of 0.265, 0.34, and 0.4.Item The Effects of Gravity on Flow Boiling Heat Transfer(2021) Hammer, Caleb Franklin; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Flow boiling is a method of phase change heat transfer used widely in electronics cooling, refrigeration, air conditioning, and other areas where stable temperatures are needed. An area of interest is spaceflight systems, where efficient heat transfer is desired to minimize mass, power requirements, and cost. When compared to terrestrial gravity conditions, the heat transfer of flow boiling in microgravity typically depreciates. This depreciation has been documented across multiple experimental studies performed by teams using different fluids, tube geometries, and flow regimes over the past three decades. Though select experimental microgravity flow boiling heat transfer data are available in the literature, holistic results are sparse due to the cost and limited availability of microgravity research. The two-phase heat transfer mechanisms responsible for the depreciation are therefore not well known, and so heat transfer models for variable gravity flow boiling do not exist. The goal of the proposed study is to develop models for flow boiling heat transfer through a tube as a function of gravity by identifying the effect of gravity on different heat transfer mechanisms. The scope of this proposal involves modeling three microgravity flow regimes (bubbly, slug, and annular flow) to serve as baseline predictions for flow boiling heat transfer without the influence of gravity. Additional gravity effects can be identified using partial and hyper-gravity data. Experiments have been performed aboard parabolic flights and on the ground at various flow rates, heating rates, and inlet subcoolings in microgravity, hyper-gravity, Lunar gravity, Martian gravity, and terrestrial gravity. Results from the experiments showed that negligible slip velocity plays an important role in modeling flow boiling heat transfer. Simulations using modified single-phase models of an accelerating flow were performed which predicted microgravity flow boiling heat transfer well in the nucleate boiling regime.Additional experiments concerning terrestrial gravity quenching heat transfer have been performed to address research gaps in microgravity cryogen chilldown studies. Quenching heat transfer coefficients were recorded in the nucleate boiling regime and compared with correlations. The correlations were able to predict heat transfer for room temperature fluids much more accurately than for cryogenic fluids. Scaling parameters must be tuned to match cryogen data to examine the large disparity between cryogenic quenching heat transfer data and correlations observed in the literature.Item DESIGN AND ANALYSIS OF A NOVEL, ULTRA-LIGHT, CRYOGENIC DEWAR FOR BALLOON-BORNE OBSERVATORIES(2020) Denker, Samuel; diMarzo, Marino; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The deployment of cryogenic Dewars aboard high-altitude balloons is critical to astronomical observation at submillimeter wavelengths. Balloon-borne, cryogenically cooled telescopes are limited in size by weight restrictions of the balloons, which is dominated by the Dewars. This thesis presents a portion of the multi-phase BOBCAT project which reduces Dewar weight with the use of thin-walled designs and a novel multi-layer insulation (MLI) system. The BOBCAT-1 mission used conventional Dewar technology to demonstrate cryogen transfer at float altitude and establish baseline thermal performance of balloon-borne Dewars. Design and assembly of the BOBCAT-2 ultra-light Dewar showed successful fabrication of the thin-walled vessel and novel MLI system. Thermal modelling predicts that the BOBCAT-2 Dewar will experience an order of magnitude increase in heat transfer through the MLI, equivalent to a 60% increase through the Dewar in total, due to its larger volume and decreased number of radiation shields relative to the BOBCAT-1 Dewar.Item Experimental Investigation into the Heat Transfer Mechanism of Oscillating Heat Pipes using Temperature Sensitive Paints(2020) Francom, Matthew Brent; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Oscillating heat pipes (OHPs) represent a promising passive mechanism for the removal or spreading of heat. While simple to construct, the fluid and thermodynamics of these devices are still poorly understood. There is debate over whether the primary heat transfer mechanism is due to sensible heating of the liquid phase or due to latent heat transfer through phase change. To provide experimental data answering this question, an experimental apparatus was constructed to provide local temperature and heat transfer data across the face of an OHP during operation. This experiment utilized temperature sensitive paint alongside visual recording of the fluid motion in order to determine the relative latent and sensible contribution to the overall heat transfer. The OHP was tested with input powers ranging from 2.6 W to 10.1 W. It found that latent heat transfer was dominant, representing between 65% and 85% of the total heat transferred in all cases.Item THERMAL AND HYDRAULIC PERFORMANCE OF SPINE FIN TUBE HEAT EXCHANGERS AT LOW REYNOLDS NUMBER CONDITIONS(2017) Herrera, Carlos; Hwang, Yunho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The goal of this work is to present the air-side thermal and hydraulic performance of heat exchangers with spine-fin surface augmentation. Although not as common as plain / plate fin, spine-fin heat exchangers have been used for decades in household refrigeration evaporators and in the outdoor coils of household air-conditioning systems. Of particular interest in this study, was the performance at low air-side Reynolds numbers (500 – 900). Heat transfer coefficients for this geometry were evaluated for samples of varying fin pitch, fin height and tube diameter in both parallel and angled bank arrangements. Water was selected as the hot fluid operating in the turbulent regime with mass flow rates varying at each airflow rate test point. Static cold and hot stream temperatures were maintained for all tests. Air-side heat transfer coefficient (AHTC) is highest for the lower diameter tube heat exchangers and increases in fin pitch lowered the AHTC. This behavior is not seen in plain fin, microchannel and other heat exchangers.Item A STUDY OF HEAT TRANSFER AND FLOW CHARACTERISTICS OF RISING TAYLOR BUBBLES(2016) Scammell, Alexander; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Practical application of flow boiling to ground- and space-based thermal management systems hinges on the ability to predict the system’s heat removal capabilities under expected operating conditions. Research in this field has shown that the heat transfer coefficient within two-phase heat exchangers can be largely dependent on the experienced flow regime. This finding has inspired an effort to develop mechanistic heat transfer models for each flow pattern which are likely to outperform traditional empirical correlations. As a contribution to the effort, this work aimed to identify the heat transfer mechanisms for the slug flow regime through analysis of individual Taylor bubbles. An experimental apparatus was developed to inject single vapor Taylor bubbles into co-currently flowing liquid HFE 7100. The heat transfer was measured as the bubble rose through a 6 mm inner diameter heated tube using an infrared thermography technique. High-speed flow visualization was obtained and the bubble film thickness measured in an adiabatic section. Experiments were conducted at various liquid mass fluxes (43-200 kg/m2s) and gravity levels (0.01g-1.8g) to characterize the effect of bubble drift velocity on the heat transfer mechanisms. Variable gravity testing was conducted during a NASA parabolic flight campaign. Results from the experiments showed that the drift velocity strongly affects the hydrodynamics and heat transfer of single elongated bubbles. At low gravity levels, bubbles exhibited shapes characteristic of capillary flows and the heat transfer enhancement due to the bubble was dominated by conduction through the thin film. At moderate to high gravity, traditional Taylor bubbles provided small values of enhancement within the film, but large peaks in the wake heat transfer occurred due to turbulent vortices induced by the film plunging into the trailing liquid slug. Characteristics of the wake heat transfer profiles were analyzed and related to the predicted velocity field. Results were compared and shown to agree with numerical simulations of colleagues from EPFL, Switzerland. In addition, a preliminary study was completed on the effect of a Taylor bubble passing through nucleate flow boiling, showing that the thinning thermal boundary layer within the film suppressed nucleation, thereby decreasing the heat transfer coefficient.Item CHARACTERIZATION OF HEAT TRANSFER AND PRESSURE DROP OF NORMAL FLOW HEAT EXCHANGERS IN COUNTER FLOW CONFIGURATION(2014) Andhare, Rohit Subhash; Ohadi, Michael M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In today's times, successful technology advancement lies in making systems that are highly compact, offer superior energy efficiency, while sustainable and cost effective . There is interest in developing small heat exchangers having better flow distribution control rather than bulky heat exchangers which are energy intensive. Microchannels and microreactors controlled by microprocessors are slowly taking over energy conversion, transportation and process industry. The nature inspired - Fractal arrangement of manifold-microchannels has the potential to provide enormous heat transfer capabilities at an attractive coefficient of performance. However majority of such fractal flow manifolds are very short and operate with short counterpart microchannel. They have not been completely adopted for counter flow configuration required by majority of the industrial processes. The work covered under this thesis is focused on adopting of high performance fractal microchannel arrangement to counter flow configuration heat exchangers that are required by industrial processes. Two single phase solution heat exchangers were developed using this approach. The solution heat exchanger is an essential component in absorption refrigeration cycle to convert waste heat into cooling. The study also utilized the novel additive manufacturing process of 3D printing to develop a tubular manifold in order to promote the fractal normal flow on tubular surfaces. The heat exchangers developed as a part of this thesis show enhancement in the overall performance and demonstrate high potential of the proposed technology.Item THERMOPHYSICAL PROPERTIES AND BOILING HEAT TRANSFER OF SELF-ASSEMBLED NANOEMULSION FLUIDS(2013) Xu, Jiajun; YANG, BAO; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recently, society has witnessed a blossom of the development of electronics, communications, and auto-computing industries, and this trend is going to continue through this century. The power dissipation density has been increased drastically because of the continuous miniaturization and the multiplication of speed of operation and data transfer. Today, it is not unusual to see heat fluxes of 200 W/cm2 in a power module, a figure that is expected to increase up to 1000 W/cm2 in the near future. Thermal management of such high flux is quickly becoming the bottleneck to improvements in electronic and optoelectrical devices. Most efforts to improve thermal management technology in the past has been devoted to improving transport processes, such as jet impingement, and microchannels. Much less attention has been paid to the fact that the existing fluids themselves possess poor thermal transport properties. In this study, Nanoemulsion fluids have been developed to overcome barriers of state-of-the-art heat transfer fluids via forming self-assembled liquid nanodroplets in conventional heat transfer fluids to elevate their heat transfer capability. A systematic investigation on nanoemulsion fluids especially their applicability in thermal management of high heat flux devices was done on the following topics: (a) the preparation of several nanoemulsion heat transfer fluids and their inner structure characterization; (b) investigation of thermophysical and phase change heat transfer characteristics in both pool boiling and flow boiling conditions; (c) optimization of nanoemulsion fluids for better thermal performance and to identify the influence of different dispersed phase, base fluid and surfactants and their concentration, on (1) inner structure and thermophysical properties, and (2) on the phase change heat transfer characteristics; (d) analytical/numerical modeling and simulation of the nanoemulsion fluids and their enhanced thermophysical properties. Overall, nanoemulsion fluids with greatly enhanced heat transfer properties, especially, the phase change properties has been developed and demonstrated here. Potential applications and the future of nanoemulsion fluids are discussed tooItem Energy Efficient Two-Phase Cooling for Concentrated Photovoltaic Arrays(2013) Reeser, Alexander; Bar-Cohen, Avram; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Concentrated sunlight focused on the aperture of a photovoltaic solar cell, coupled with high efficiency, triple junction cells can produce much greater power densities than traditional 1 sun photovoltaic cells. However, the large concentration ratios will lead to very high cell temperatures if not efficiently cooled by a thermal management system. Two phase, flow boiling is an attractive cooling option for such CPV arrays. In this work, two phase flow boiling in mini/microchannels and micro pin fin arrays will be explored as a possible CPV cooling technique. The most energy efficient microchannel design is chosen based on a least-material, least-energy analysis. Heat transfer and pressure drop obtained in micro pin fins will be compared to data in the recent literature and new correlations for heat transfer coefficient and pressure drop will be presented. The work concludes with an energy efficiency comparison of micro pin fins with geometrically similar microchannel geometry.Item PERFORMANCE AND APPLICATIONS OF RESIDENTIAL BUILDING ENERGY GREY-BOX MODELS(2013) Siemann, Michael James; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The electricity market is in need of a method to accurately predict how much peak load is removable by directly controlling residential thermostats. Utilities have been experimenting with residential demand response programs for the last decade, but inconsistent forecasting is preventing them from becoming a dependent electricity grid management tool. This dissertation documents the use of building energy models to forecast both general residential energy consumption and removable air conditioning loads. In the models, complex buildings are represented as simple grey-box systems where the sensible energy of the entire indoor environment is balanced with the flow of energy through the envelope. When internet-connected thermostat and local weather data are inputs, twelve coefficients representing building parameters are used to non-dimensionalize the heat transfer equations governing this system. The model's performance was tested using 559 thermostats from 83 zip codes nationwide during both heating and cooling seasons. For this set, the average RMS error between the modeled and measured indoor air temperature was 0.44°C and the average daily ON time prediction was 1.9% higher than the data. When combined with smart power meter data from 250 homes in Houston, TX in the summer of 2012 these models outperformed the best traditional methods by 3.4 and 28.2% predicting daily and hourly energy consumption with RMS errors of 86 and 163 MWh. The second model that was developed used only smart meter and local weather data to predict loads. It operated by correlating an effective heat transfer metric to past energy data, and even further improvement forecasting loads were observed. During a demand response trial with Earth Networks and CenterPoint Energy in the summer of 2012, 206 internet-connected thermostats were controlled to reduce peak loads by an average of 1.13 kW. The thermostat building energy models averaged forecasting the load in the 2 hours before, during, and after these demand response tests to within 5.9%. These building energy models were also applied to generate thermostat setpoint schedules that improved the energy efficiency of homes, disaggregate loads for home efficiency scorecards and remote energy audits, and as simulation tools to test schedule changes and hardware upgrades.