Thesis (M.S.)--Rochester Institute of Technology, 1995. / Typescript. Bibliography: leaves 86-89.

<p> Determination of boiling heat transfer rate during liquid jet impingement cooling (LJIC) depends on the intensity of bubble generation that is dependent on many flow and surface conditions such as jet velocity, liquid temperature, and surface superheat. Many empirical correlations have been developed previously to determine the total wall heat flux under various LJIC flow velocity, subcooling and surface superheat. However, only few studies have been able to model the governing heat transfer mechanisms associated with LJIC. In many industrial applications, there is a need to determine the effect of any changes in processing parameters on the total rate of heat transfer. Mechanistic heat transfer models can fulfill such need and allow for efficient model modifications at minimum cost and time.</p> <p> Three models have been developed in this study that address the underlying physics associated with jet impingement heat transfer in both single phase and nucleate boiling regimes. The first model accounts for the effect of bubble generation on the overall heat transfer rate at the jet stagnation by introducing a bubble-induced diffusivity (BID). The BID is added to molecular diffusivities in the momentum and energy Equations. The BID model adopts an analogous approach to the eddy diffusivity concept used in turbulence flow modeling. The BID model has been developed to provide a cost effective simulation tool of boiling heat transfer during LJIC by considering bubble generation effect on the overall heat transfer rate while avoiding the need to simulate extremely small time and length scales associated with phase change.</p> <p> The second model is a scenario identification procedure (SIP) that has been developed to predict the bubble growth termination (BGT) scenario. Considering the effect of jet velocity, water subcooling and surface superheat, the SIP identifies whether a bubble would locally collapse or slide by identifying the most probable equilibrium condition (thermal or dynamic) that the bubble would reach first. The main objective of the SIP is to avoid any inaccurate assumption of the probable BGT scenario. In this case, such procedure could improve the predictions of a more comprehensive wall heat flux model of the areas affected by various heat transfer mechanisms.</p> <p> The third model is a mechanistic wall flux partitioning (WFP) model that has been developed to predict the local wall heat transfer rate over the distance between jet stagnation and ten times the jet width. The WFP model assumes that primary heat transfer is due to sensible heating of liquid by forced convection and transient conduction. The WFP model incorporates a unified single-phase heat transfer model that is capable of capturing the observed secondary peaks downstream of stagnation. The WFP model also incorporates a sub-model that predicts abrupt changes in the liquid film thickness due to the formation of hydraulic jumps.</p> <p> The development of these three models have been carried out using experimental data obtained from a set of experiments that has been conducted to investigate the variation of the heat transfer rate and bubble dynamics under a planar free liquid jet. Investigation of bubble dynamics has been conducted using both intrusive optical probe and non-intrusive high speed imaging of the flow filed. The experiments have been conducted at atmospheric pressure, considering jet velocity of 0.4 to 1.7 m/s, degree of water subcooling of 10 to 28 °C, degree of wall superheat of -25 to 30 °C. Such variations have been studied along a heated surface between stagnation and ten times the jet width. Experiments were conducted using a 1 mm x 8 mm planar jet impinging on a heated horizontal flat copper surface. The distance between the jet and the heated surface was 10 mm. The experimental data have been used to develop a group of sub-models for single-phase heat transfer, bubble diameter, bubble population density, bubble release frequency, and bubble growth time. These sub-models have been used in the closure of the aforementioned models.</p> <p> The three models have been validated using independent experimental data. The BID model is capable of predicting stagnation heat flux within -15% and +30%. The SIP model was able to predict the right bubble growth termination scenario of 80% of the investigated cases. The WFP model is capable of predicting the local total wall heat flux within± 30%.</p> / Thesis / Doctor of Philosophy (PhD)

<p> This investigation is an original study in nucleate pool boiling heat transfer, consisting of two parts: an experimental study and a theoretical study. The experimental study was performed with water boiling at atmospheric pressure on a single copper surface. Two different levels of heat flux were investigated. For the lower level of heat flux (92.21 kW/m2), three different levels of subcooling (0, 6.5, 12°C) were studied and for the higher level of heat flux (192.11 kW/m2), two different levels of subcooling (0, 7.5°C) were studied as well. </p> <p> The cross-spectral density function ·and the crosscorrelation function were used to determine the time elapsed (-r) between the start of bubble growth at two neighbouring active sites with separation (S). The experimental results indicate that for the lower level of heat flux at three different levels of subcooling, the separation (S) and the time elapsed (-r) are related. For the higher level of heat flux at 0°C subcooling it was not possible to detect any correlation, but for the 7.5°C subcooled condition a weak correlation was found to exist. For the lower level of heat flux, all the experimental data for the saturated and subcooled boiling conditions plotted as (S-Rd) versus (T-Tg) drew together into a single curve, indicating that a single relationship could fit all the data. </p> <p> Three different theoretical models were devised in an attempt to·explain the experimental observations. The first model involved heat diffusion in the water; the second model was based upon the disturbance caused by the propagation of a pressure pulse in a mixture of water and vapour and finally the third model involved heat diffusion in the solid. The first two models failed to give satisfactory agreement with the experimental results, but the theoretical predictions corresponding to heat diffusion through the solid gave good agreement with the experimental findings. </p> / Thesis / Doctor of Philosophy (PhD)

<p> An experimental study of temperature profiles in the near vicinity of a horizontal copper surface on which water was boiled is reported in this thesis. A series of three tests is reported for heat fluxes of 20,000, 50,000 and 100,000 BTU/HRFT^2 respectively. Four levels of subcooling were achieved in each series in the range of 0°F < θsub < 105°F. Using the superheat-layer thickness. as defined by Han and Griffith, results are reported indicating an increase in superheatlayer thickness for an increase in the degree of subcooling at a constant heat flux and a reverse effect for an increase in heat flux for a constant degree of subcooling. </p> <p> In addition, five tests are reported in which incipience of boiling was achieved. Using this data, Hsu's mathematical model for predicting bubble nucleation is tested. In general, good agreement is found between the Hsu model and the experimental data. </p> / Thesis / Master of Engineering (MEngr)

The boiling of water droplets on hot metal surfaces is studied experimentally and mathematically in order to establish the conditions necessary for droplets to enter a film boiling mode. The subsurface temperature history within a plate undergoing droplet boiling on the surface is measured. A numerical model of the heat transfer in the plate is then used to deduce from these data the following characteristics of droplet boiling: (1) the effective heat transfer coefficient between water droplet and plate during the initial transient forming the spherical droplet, (2) the apparent time period needed to establish the droplet in the film boiling mode, and (3) the minimum plate surface temperature reached during the initial formation of the boiling droplet. The effective heat transfer coefficient, formation time, and minimum surface temperature are sufficient to develop a calculation method which predicts the minimum initial plate temperature necessary for a water droplet to enter film boiling. This numerical conduction model accounts for the influence of plate material, plate thickness, oxidation of the plate surface, the boundary condition on the plate lower surface, and the size of the droplet. The prediction method is successfully used to estimate the minimum film boiling temperature for brass, graphite, Pyrex, copper, aluminum, stainless steel, and Zircalloy II. The findings of the experiments and numerical studies are applied to the rewetting phase of a loss-of-coolant-accident in a light water reactor. This application, in turn, provides explanations for some of the phenomena observed in studies of the prequench heat transfer within rod bundles including the effect of multiple droplet impacts, and suggests possible reasons for some of the difficulties experienced in attempts to establish the effective rewetting temperature on reactor fuel rod surfaces.

Today nucleate boiling is widely used in numerous industrial applications such as cooling processes because of the high achieved heat transfer rates for low temperature differences. It remains a possible cooling solution for the next generation of central processing units (CPU), which dissipate heat fluxes exceeding the capabilities of today’s conventional forced air cooling. However, nucleate boiling is a very complex and elusive process involving many mechanisms which are not fully understood yet and a comprehensive model is still missing. For this study a new experimental setup was designed, constructed and commissioned to investigate bubble nucleation, growth, departure and interaction during nucleate pool boiling from a silicon device fully immersed in fluorinert FC-72. The location of bubble nucleation is controlled by artificial cavities etched into the silicon substrate. Boiling is initiated with a heater integrated on the back and micro-sensors indicate the wall temperature at the bubble nucleation site. During this work three different silicon test section designs were fabricated and boiling experiments on these substrates successfully conducted. Bubble growth, bubble departure frequencies and bubble departure diameters for different dimensioned artificial cavities, varied pressure and increasing wall temperature were measured from high-speed imaging sequences. Bubble interactions like vertical and horizontal coalescence were visualised and their impact on the boiling heat transfer investigated. The influence of spacing between two neighbouring artificial cavities on bubble nucleation and departure frequencies, vertical coalescence frequencies and departure diameters was analysed. The acquired data are used as input for a numerical code developed by our collaborators (Brunel University, UK and Los Alamos National Laboratories, USA) and are a first step to validate the code. The code studies the interactions between bubble nucleation sites on solid surfaces as a network. The simulations will help design boiling substrates utilised for chip cooling applications with optimal artificial cavity distribution to maximise the cooling heat transfer.

Thesis. 1978. M.S. cn--Massachusetts Institute of Technology. Dept. of Chemical Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Bibliography: leaves 124-131. / / by Otto J. Solis. / M.S.cn

Thesis (M.S.)--University of Texas at El Paso, 2008. / Title from title screen. Vita. CD-ROM. Includes bibliographical references. Also available online.

Thesis (Ph.D.)--McMaster University, 2003. / Advisor: R. L. Judd. Includes bibliographical references (leaves 253-256). Also available via World Wide Web.

Modern nuclear reactors still use Zirconium-4 Alloy (Zircaloy®) as the cladding material for fuel elements. A substantial amount of research has been done to investigate the boiling heat transfer behind the cooling mechanism of the reactor. Boiling heat transfer is notoriously difficult to quantify in an acceptable manner and many empirical correlations have been derived in order to achieve some semblance of a mathematical model. It is well known that the surface conditions on the heat transfer surface plays a role in the formulation of the heat transfer coefficient but on the other hand it also has an effect on the pressure drop alongside the surface. It is therefore necessary to see whether there might be an optimum surface roughness that maximises heat transfer and still provides acceptably low pressure drop. The purpose of this study was to experimentally measure pressure drop and heat transfer associated with vertical heated tubes surrounded by flowing water in order to produce flow boiling heat transfer. The boiling heat transfer data was used to ascertain what surface roughness range would be best for everyday functioning of nuclear reactors. An experimental set-up was designed and built, which included a removable panel that could be used to secure a variety of rods with different surface roughnesses. The pressure drop, surface temperature, flow rate and heat input measurements were taken and captured in order to analyse the heat transfer and friction factors. Four rods were manufactured with different roughnesses along with a fifth rod, which remained standard. These rods were tested in the flow loop with water in the upward flow direction. Three different system mass flow rates were used: 0kg/s, 3.2kg/s and 6.4kg/s. Six repetitions were done on each rod for the tests; the first repetition was not used in the results since it served the purpose to deaerate the water in the flow loop. The full range of the power input was used for each repetition in the tests. For the heat transfer coefficient at a system mass flow rate of 3.2kg/s, satisfactory comparisons were made between the test results and those found in literature with an average deviation of 14.53%. At 6.4kg/s system mass flow rate the comparisons deviated on average 55.45%. The velocity of the fluid in the test section was calculated from the pressure drop and was validated using separate tests. The plain rod, with no added roughness, was found to be the optimal surface roughness which is what is used in industry today. The flow loop was in need of a couple of redesigns in order to produce more accurate results. Future work suggestions include adding more rods in the test section in order to investigate the nature of heat transfer in a rod bundle array as well as implementing all the suggested changes listed in the conclusion. / Dissertation (MEng)--University of Pretoria, 2017. / Mechanical and Aeronautical Engineering / MEng / Unrestricted