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A pool boiling experimental facility has been designed and built to investigate nucleate pool boiling in water under sub atmospheric pressure. Liquid crystal thermography, a non intrusive technique, is used for the determination of surface temperature distributions. This technique uses encapsulated liquid crystals that reflect definite colors at specific temperatures and viewing angle. Design of the test section is important in this experimental study. Since a new TLC is required for every new set of test conditions, a permanently sealed test section is not an option. The real challenge is to design a leak proof test section which is flexible so that it can be taken apart easily. A plexiglass test section, including a top chamber with an internal volume of 60.9 x 60.9 x 66.4 mm and a bottom plate of 5.5mm thickness is designed and assembled together using quick grips. In the test section, water is boiled using 85.0mm x 16.0mm and 0.050mm thick Fecralloy® as the heating element. The TLC sheet is attached to the bottom plate and the heating element is placed on top of TLC so that the temperature distribution of the heating element during boiling can be interpreted from TLC. A camera system fast enough to capture the thermal response of the TLC and an arrangement to capture both hue of the TLC and growth of the bubble on the same frame has been designed and successfully used. This system allowed recording of position, size and shape of the bubble with synchronized surface temperature. In order to get hue vs. temperature relation, in-situ calibration of the TLC is performed for each test condition with the present experimental setup and lighting conditions. It is found that the calibration curve of the TLC at atmospheric pressure is different from the calibration curve of the same TLC at subatmospheric pressures. The maximum temperature difference between the two curves for the same hue is found to be only 0.6°C. The experiment is run at four different test conditions of subatmospheric pressure and low heat flux. It is run at system pressures of 6.2kPa (0.89Psi) and 8.0kPa (1.16Psi) with a constant heat flux of 1.88kW/m2 and 2.70kW/m2, and a constant heat flux of 2.70kW/m2, 3.662kW/m2 and 4.50 kW/m2 respectively. Analysis of nucleating surface temperatures using thermochromic liquid crystal technique is performed for these test conditions and the bubble dynamics is studied. The temperature distribution is quite varied in each case and the temperature is at its maximum value at the center of the bubble and it decreases radially from the center. The dry spot observed during the experiments indicates that the process of evaporation of the microlayer is dominant at subatmospheric pressures. It is observed that at very low pressure and heat flux the bubble growth is accompanied by the neck formation. Boiling parameters such as bubble frequency, bubble size and contact are also analyzed and a summary of these results for four different test conditions is presented and the relevant differences between the cases are discussed and the effect of increase in pressure and heat flux is noted.

A heater designed to monitor surface temperature fluctuations during pool boiling experiments while the bubbles were simultaneously being observed has been fabricated and tested. The heat source was a transparent indium tin oxide (ITO) layer commercially deposited on a fused quartz substrate. Four copper-nickel thin film thermocouples (TFTCs) on the heater surface measured the surface temperature, while a thin layer of sapphire or fused silica provided electrical insulation between the TFTCs and the ITO. The TFTCs were micro-fabricated using the liftoff process to deposit the nickel and copper metal films. The TFTC elements were 50 microns wide and overlapped to form a 25 micron by 25 micron junction. TFTC voltages were recorded by a DAQ at a sampling rate of 50 kHz. A high-speed CCD camera recorded bubble images from below the heater at 2000 frames/second. A trigger sent to the camera by the DAQ synchronized the bubble images and the surface temperature data. As the bubbles and their contact rings grew over the TFTC junction, correlations between bubble behavior and surface temperature changes were demonstrated. On the heaters with fused silica insulation layers, 1-2 C temperature drops on the order of 1 ms occurred as the contact ring moved over the TFTC junction during bubble growth and as the contact ring moved back over the TFTC junction during bubble departure. These temperature drops during bubble growth and departure were due to microlayer evaporation and liquid rewetting the heated surface, respectively. Microlayer evaporation was not distinguished as the primary method of heat removal from the surface. Heaters with sapphire insulation layers did not display the measurable temperature drops observed with the fused silica heaters. The large thermal diffusivity of the sapphire compared to the fused silica was determined as the reason for the absence of these temperature drops. These findings were confirmed by a comparison of temperature drops in a 2-D simulation of a bubble growing over the TFTC junction on both the sapphire and fused silica heater surfaces. When the fused silica heater produced a temperature drop of 1.4 C, the sapphire heater produced a drop of only 0.04 C under the same conditions. These results verified that the lack of temperature drops present in the sapphire data was due to the thermal properties of the sapphire layer. By observing the bubble departure frequency and site density on the heater, as well as the bubble departure diameter, the contribution of nucleate boiling to the overall heat removal from the surface could be calculated. These results showed that bubble vapor generation contributed to approximately 10% at 1 W/cm^2, 23% at 1.75 W/cm^2, and 35% at 2.9 W/cm^2 of the heat removed from a fused silica heater. Bubble growth and contact ring growth were observed and measured from images obtained with the high-speed camera. Bubble data recorded on a fused silica heater at 3 W/cm^2, 4 W/cm^2, and 5 W/cm^2 showed that bubble departure diameter and lifetime were negligibly affected by the increase in heat flux. Bubble and contact ring growth rates demonstrated significant differences when compared on the fused silica and sapphire heaters at 3 W/cm^2. The bubble departure diameters were smaller, the bubble lifetimes were longer, and the bubble departure frequency was larger on the sapphire heater, while microlayer evaporation was faster on the fused silica heater. Additional considerations revealed that these differences may be due to surface conditions as well as differing thermal properties. Nucleate boiling curves were recorded on the fused silica and sapphire heaters by adjusting the heat flux input and monitoring the local surface temperature with the TFTCs. The resulting curves showed a temperature drop at the onset of nucleate boiling due to the increase in heat transfer coefficient associated with bubble nucleation. One of the TFTC locations on the sapphire heater frequently experienced a second temperature drop at a higher heat flux. When the heat flux was started from 1 W/cm^2 instead of zero or returned to zero only momentarily, the temperature overshoot did not occur. In these cases sufficient vapor remained in the cavities to initiate boiling at a lower superheat.

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<p>An experimental investigation of the physical feasibility of thermal creep failure of the Calandria Vessel under a severe accident load is presented in this thesis. Thermal creep failure is postulated to occur if film boiling is instigated in the Shield Tank Water surrounding the Calandria Vessel. The objective of this experimental study is to measure the Critical Heat Flux (CHF) for a representative geometry in environmental conditions similar to those existing in the CANDU Calandria Vessel and Shield Tank Water.<br />Two geometries of downwards facing surfaces are studied. The first is termed the ‘confined’ study in which bubble motion is demarcated to the heated surface. The second is termed the ‘unconfined’ study where individual bubbles are free to move along the heated surface and vent in any direction.<br />The method used in the confined study is novel and involves the placement of a lip surrounding the heated surface. The level of confinement is adjusted by varying the inclination angle. Data has been obtained for Bond Numbers (Bo) 0, 1.5, 3, 3.6 and 11.8 with corresponding qCHF 596, 495, 295, 223, and 187 kW/m2, respectively. A correlation relating the CHF to level of confinement is stated. The CHF results are in good agreement with Theofanous et. al. (1994), as is the observation that a transition angle is observed in the correlation. The transition angle in this study is found to be ~5.5°. The obtained nucleate boiling curves are compared to Su et. al. (2008) data for similar Bo and excellent agreement is achieved in the medium to high heat flux regions.<br />The unconfined study consists of a downward facing plate in a pool of subcooled water. The obtained nucleate boiling curve is compared with the Stephan-Andelsalam correlation and agreement is not observed. There were visibly different trends in the convective heat transfer coefficient with a mean difference of 31%. The experimental data is compared to data obtained by Nishikawa et. al. (1984) and is found to be in acceptable agreement. The power requirement to instigate film boiling was not met, meaning that the CHF is greater than 1 MW/m2. Visual observations are made and an argument is based on the premise that the phenomenon of dryout for a downwards facing surface is similar to that of an upwards facing surface. The theory and current acceptance of CHF for an upwards facing surface is discussed—in particular Zuber’s “Hydrodynamic Limit” of 1.1 MW/m2, Dhir (1992) and recent experimental evidence from Theofanous et. al. (2002). These three studies were found to be in agreement with results presented here.<br />The experimental evidence presented herein supports the statement that thermal creep failure of the Calandria Vessel is physically unreasonable under analyzed severe accident loads.</p> / Master of Applied Science (MASc)

<p>Nanofluids are engineered colloidal dispersions of nanoparticles in a liquid. The field of nanofluids has seen much interest due to reported heat transfer enhancements over the corresponding pure fluids at low particle concentrations. Particularly, a large increase in critical heat flux (CHF) has been widely reported along with modification of the boiling interface. Inconsistencies in reported impact on nucleate boiling heat transfer and the degree of CHF enhancement illustrate the need for further study.</p> <p>A pool boiling experiment has been designed and constructed at McMaster University to allow for the study the boiling of water-based nanofluids. The facility has been commissioned with saturated distilled water tests at atmospheric pressure, heat flux levels up to 1200 kW·m^-2, and at wall superheat levels up to 19.5^oC. Wall superheat and heat flux uncertainties were estimated to be ±0.6^oC and ±20 kW∙m^-2, respectively. For the installed test section, heat flux is limited to 2.62 ± 0.06 MW·m^-2. A high speed video system for the analysis of bubble dynamics was tested and used for qualitative comparisons between experimental runs. This system was tested at 2500 FPS and an imaging resolution of 39 pixels per mm, but is capable of up to 10 000 FPS at the same spatial resolution. Heat flux versus wall superheat data was compared to the Rohsenow correlation and found to qualitatively agree using surface factor <em>C_sf</em> = 0.011. Results were found to have a high degree of repeatability at heat flux levels higher than 600 kW·m^-2.</p> <p>The new facility will be used to conduct studies into the pool boiling of saturated water-based nanofluids at atmospheric pressure. Additional work will involve the control and characterization of heater surface conditions before and after boiling. Quantitative analysis of bubble dynamics will be possible using high speed video and particle image velocimetry.</p> / Master of Applied Science (MASc)

Heat transfer technologies based on boiling refer to applications like heat pumps, waste heat recovery systems, power plants and electronic components cooling. The widespread use of boiling as the heat transfer mode is due to high heat transfer coefficients associated with the phase change from liquid to vapor. Boiling heat transfer coefficients can be further enhanced by modifying the texture or chemical composition of the interface at which boiling occurs. The objective of this research is to fabricate textured surfaces with electrical discharge machining (EDM) and investigate the enhancement in pool boiling heat transfer, concerning machining and surface characterization parameters. It is complemented by a qualitative analysis of bubble dynamics with high-speed imaging, to provide insights into the differences in boiling performance associated with the changes in surface topography. Sink electrical discharge machined surfaces demonstrated ten times higher heat transfer coefficient compared to a polished surface during these studies. / Thesis / Master of Applied Science (MASc)

This thesis investigates pool boiling heat transfer from bare and graphene-coated NiCr wires in a saturated liquid of FC 770, a fluorocarbon fluid. Of particular interest was the effect of graphene-oxide platelets, dip-coated onto the heater surface, in enhancing the nucleate boiling heat transfer (BHT) rates and the critical heat flux (CHF) value. In the course of the pool boiling experiment, the primary focus was on the reduction mechanism of graphene oxide. The transition from hydrophilic to hydrophobic behavior of the graphene oxide-coated surface was captured, and the attendant effects on surface wettability, porosity and thermal activity were observed. A parametric sensitivity analysis of these surface factors was performed to understand the CHF and BHT enhancement mechanisms. In the presence of graphene-oxide coating, the data indicated an increase of 50% in CHF. As the experiment continued, a partial reduction of graphene oxide occurred, accompanied by (a) further enhancement in the CHF to 77% larger compared to the bare wire. It was shown that the reduction of graphene oxide progressively altered the porosity and thermal conductivity of the coating layer without changing the wettability of FC 770. Further enhancement in CHF was explained in terms of improved porosity and thermal activity that resulted from the partial reduction of graphene-oxide. An implication of these results is that a graphene-oxide coating is potentially a viable option for thermal management of high-power electronics by immersion cooling technology. / Master of Science