Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item A STUDY ON CRITICAL HEAT FLUX MECHANISMS AND THE TRANSITION TO FILM BOILING(2015) Thompson, Jason Christopher; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An infrared (IR) thermometry technique was used to study the critical heat flux (CHF) and the transition to film boiling during pool boiling and submerged jet impingement. Highly resolved temporal and spatial heat transfer measurements were obtained by measuring temperature distributions on the surfaces of an IR transparent test heater with a mid-range IR camera. Measurements were obtained for the nucleate boiling regime, CHF, early transition boiling regime, and through the transition to film boiling. The local heat flux, temperature, and dryout characteristics were used to compare the submerged jet and pool boiling conditions. It was found that similar mechanisms govern CHF and the transition to film boiling. This finding supports that the hydrodynamic models are incorrect, and CHF is governed by the surface characteristics and the dynamics of the microlayer.Item DEVELOPMENT OF A BOILING REGIME MAP AND GRAVITY SCALING PARAMETER FOR POOL BOILING HEAT TRANSFER(2010) Raj, Rishi; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Virtually all data to date regarding parametric effects of gravity on pool boiling have been inferred from experiments performed in low-g, 1g, and 1.8g conditions. The current work is based on observations of boiling heat transfer obtained over a continuous range of gravity levels (0g-1.8g) and varying heater sizes under subcooled liquid (FC-72/n-perfluorohexane) conditions. Variable gravity pool boiling heat transfer measurements were made during the parabolic flight campaigns organized by the European Space Agency (ESA) and NASA. Heater size was varied by using two (2.7x2.7 mm2 and 7.0x7.0 mm2) constant temperature microheater arrays consisting of 96 platinum resistance heaters deposited in a 10x10 configuration onto a quartz substrate. The ability to selectively power a subset of heater elements (1, 4, 9, 16, 25, 36, 64, and 96) in a square pattern out of the 10x10 configuration allowed a variation in heating area from 0.27x0.27 mm2 to 7.0x7.0 mm2. A parametric study on the effects of fluid properties, wall superheat, liquid subcooling, and dissolved gas concentration on boiling heat transfer was also performed. Based on the heater sizes and the gravity levels investigated, two pool boiling regimes were identified. For large heaters and/or higher gravity conditions, buoyancy dominated boiling and heat transfer results were heater size independent. Under low gravity conditions and/or for smaller heaters, surface tension forces dominated and heat transfer results were heater size dependent. A first ever pool boiling regime map differentiating buoyancy and surface tension dominated boiling regimes was developed. The non-dimensional ratio of heater size Lh and capillary length Lc was found suitable to differentiate between the boiling regimes. Transition between the regimes was observed to occur at a threshold value of Lh/Lc ~2.1. Pool boiling data in the buoyancy dominated boiling regime (Lh/Lc>2.1) was used to develop a gravity scaling parameter for pool boiling heat transfer. A non-dimensional temperature was defined in order to derive a gravity scaling parameter independent of dissolved gas concentrations and liquid subcooling. The power law coefficient for the gravity effect was observed to be a function of the non-dimensional wall temperature. The predicted results were found to be in good agreement with the heat transfer data over a wide range of gravity levels (0g-1.8g), dissolved gas concentrations, subcoolings, and heater surface morphologies. Use of this scaling parameter to obtain heat transfer at varying gravity levels is expected to save considerable experimental resources required to validate the performance of phase change based systems under different gravity conditions.Item Nucleate Pool Boiling Characteristics From a Horizontal Microheater Array(2005-12-14) Henry, Christopher Douglas; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Pool boiling heat transfer measurements from different heater sizes and shapes were obtained in low-g (0.01 g) and high-g (1.7 g) aboard the NASA operated KC-135 aircraft. Boiling on 4 square heater arrays of different size (0.65 mm2, 2.62 mm2, 7.29 mm2, 49 mm2) was investigated. The heater arrays consist of 96 independent square heaters that were maintained at an isothermal boundary condition using control circuitry. A fractional factorial experimental method was designed to investigate the effects of bulk liquid subcooling, wall superheat, gravitational level, heater size and aspect ratio, and dissolved gas concentration on pool boiling behavior. In high-g, pool boiling behavior was found to be consistent with classical models for nucleate pool boiling in 1-g. For heater sizes larger than the isolated bubble departure diameter predicted from the Fritz correlation, the transport process was dominated by the ebullition cycle and the primary mechanisms for heat transfer were transient conduction and microconvection to the rewetting liquid in addition to latent heat transfer. For heater sizes smaller than this value, the boiling process is dominated by surface tension effects which can cause the formation of a single primary bubble that does not depart the heater surface and a strong reduction in heat transfer. In low-g, pool boiling performance is always dominated by surface tension effects and two mechanisms were identified to dominate heat and mass transport: 1) satellite bubble coalescence with the primary bubble which tends to occur at lower wall superheats and 2) thermocapillary convection at higher wall superheats and higher bulk subcoolings. Satellite bubble coalescence was identified to be the CHF mechanism under certain conditions. Thermocapillary convection caused a dramatic enhancement in heat transfer at higher subcoolings and is modeled analytically. Lastly, lower dissolved gas concentrations were found to enhance the heat transfer in low-g. At higher dissolved gas concentrations, bubbles grow larger and dryout a larger portion of the heater surface.