Browsing by Subject "two-phase"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
Item Experimental and Numerical Investigation of Pool Boiling Heat Transfer on Engineered Nano-Finned Surfaces(2014-08-10) Yang, HongjooPool boiling experiments for nanocoatings (or nanostructured surfaces) show that despite the lower thermal conductivity values than carbon, silicon yielded higher values of CHF (critical heat flux). Subsequently numerical studies showed that the interfacial thermal resistance (Kapitza resistance or ?R_(k)?) between a nanofin and fluid molecules is the dominant component of the thermal impedance network. The values of R_(k) for silicon were predicted to be ~1000 times smaller than that of carbon in these numerical simulations. Since the total thermal impedance of silicon nanofins is lower than that of carbon they cause higher levels of enhancement of CHF. Surface adsorption of the liquid molecules on a nanofin results in the formation of dense ?compressed phase? which in turn induces thermal capacitance and diodic behavior. This is termed as the ?nanofin effect?: which implies that CHF is more sensitive to R_(k) than the thermal conductivity of the nanofin. Hence, the objective of this study was to verify the nanofin effect. Experimental and numerical investigation of transport phenomena during pool boiling were performed in this study for liquid subcooling of 0 ?, 5 ? and 10 ? on horizontal planar heater configuration. Surface temperature was measured using nanosensor (Thin Film thermocouple or ?TFT?) arrays. Heater surfaces (with or without nanofins of different heights) were composed of ceramic, oxide and metal surfaces. The nanofins were fabricated using Step and Flash Imprint Lithography (SFIL). Contact angle was measured both before and after the experiments. Nucleate pool boiling heat transfer was enhanced with increase in pillar height. Numerical predictions for R_(k) obtained from Molecular Dynamics (MD) simulations were found to be consistent with the level of heat flux enhancement observed in the experiments for the different nanofin configurations. Hence this study demonstrates that R_(k) is the more dominant parameter for heat transfer enhancement during pool boiling ? compared to the thermal conduction resistance (or material properties) of the nanofin itself. As an outcome of these investigations future topics of research are also proposed (such as, using temperature nano-sensors for the investigation of controlled fouling on pool boiling phenomena for heaters with micro/nano-structured surfaces).Item Microgravity Flow Regime Transition Modeling(2010-07-14) Shephard, Adam M.Flow regime transitions and the modeling thereof underlie the design of microgravity two-phase systems. Through the use of the zero-g laboratory, microgravity two-phase flows can be studied. Because microgravity two-phase flows exhibit essentially no accelerations (i.e. no buoyancy or gravitational forces), the effects of acceleration on two-phase flow can be decoupled from the effects of other fluid phenomenon. Two-phase systems on earth are understood mostly through empiricisms. Through microgravity two-phase research, a fundamental understanding of two-phase systems can be obtained and applied to both terrestrial systems in space applications. Physically based bubbly-bubbly/slug and bubbly/slug-slug flow regime transition models are introduced in this study. The physical nature of the models demonstrates a new understanding of the governing relationships between coalescence, turbulence, void fraction, boundary layer affects, and the inlet bubble size distribution. Significantly, the new models are dimensionless in addition to being physically derived. New and previous models are evaluated against zero-g data sets. Previous models are not accurate enough for design use. The new models proposed in this study are far more detailed than existing models and are within the precision necessary for most design purposes. Because of the limited data available, further experimental validation is necessary to formally vet the model. Zero-g data set qualification and flight experiment design have not been standardized and as a result, much of the data in the literature can be shown not to represent microgravity conditions. In this study, a set of zero-g quality criteria are developed and used to qualify the data sets available in the literature. The zero-g quality criteria include limitations on buoyancy forces relative to surface tension and inertial forces as well as requirements on acceleration monitoring and flow development length and time. The resulting evaluation of the data sets available in the literature unveils several experiment design shortfalls, which have resulted in data sets being misrepresented as zero-g data sets. The quality standards developed in this study should continue to be improved upon and used in the design of future zero-g fluid experiments. The use of one-g single-phase models in approximating zero-g two-phase experimental data was successfully performed in this study. Specifically the models for pressure drop, friction factor, wall shear, and velocity profile are demonstrated. It is recognized that the mixing apparatus will affect the flow regime transitions, specifically the distribution of bubble sizes that exit the mixing apparatus. Unfortunately, little-to-no information regarding the mixing apparatus used in past experiments can be found in the literature. This will be an area for further developmental research. In summary, the approach to understanding and modeling two-phase phenomenon demonstrated in this study provides tools to future researchers and engineers. Special attention to data qualification and experiment standardization provides a different prospective and interpretation of the currently available data. The physically based and dimensionless modeling demonstrated in this study can be extended to other studies in the field as well as providing a basis for the application of heat transfer modeling to microgravity two-phase systems, specifically boiling and condensation.