<p>Experiments were performed to study the effect of surface finish and jet velocity on the boiling performance at the stagnation point under a free liquid planar jet. A rectangular jet with dimensions 9 mm x 1 mm was used to impinge subcooled water on the center of a copper surface 8 mm width x 20 mm length. Jet velocities ranged from 0.9 to 2.5 m/s while the degree of subcooling was kept constant at 10 °C.</p> <p>Three surfaces were prepared using emery paper #1200, #500 and #320 and the arithmetic mean square of the roughness <strong>Ra</strong> = 18.72, 401.65 and 533.53 nm.</p> <p>Increasing the jet velocity has shown to increase the heat flux slightly in the single phase regime. Also by increasing the jet velocity, boiling was found to start at higher surface superheat achieving higher values of burn out heat flux BOF for jet velocities V_j ≤ 1.5 m/s. This trend agrees with studies reported in literature. Some contradicting results occurred at higher jet velocities which is attributed to the flow profile.</p> <p>For jet velocities lower than 2 m/s, the surface with higher <strong>Ra </strong>was found to have a delayed Onset of Nucleate Boiling ONB, higher Burn out Heat Flux BOF, and lower rate of heat transfer in the single phase regime. Surface finish did not show significant effect on boiling performance at higher jet velocities. The contradictions observed at jet velocities higher than 1.5 m/s were attributed to the flow profile. Results regarding the effect of surface finish on heat transfer in the single phase regime under liquid jet impingement were compared to literature and a reasonable agreement was found. More studies are needed to explain the contradictions found for higher jet velocities.</p> / Master of Applied Science (MASc)

<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>In an effort to better understand the physical mechanisms responsible for pool boiling heat transfer, an analytical model is developed that better describes the changing shape and size of a growing bubble. Indeed, any analysis of thermal transport due to nucleate pool boiling requires bubble frequency predictions which are intimately linked to bubble volume. The model is developed and validated for quasi-static bubble growth due to gas injection and for bubble growth due to vaporization within the heat-transfer controlled growth regime; it highlights the need to include the asymmetric nature of growing bubbles when modeling bubble growth.</p> <p>In addition, a numerical study of quasi-static bubble shape for both adiabatic bubble growth and vapour bubble growth provides insight into the dependence the bubble shape evolution has on the Bond number. In so doing, bubble profiles generated from a numerical treatment of the Capillary equation are benchmarked to quasi-static gas injected bubble formations and to heat-transfer controlled vapour bubble formations.</p> <p>The numerical treatment of bubble shape evolution leads to a simplifying bubble geometry for low Bond number applications. The geometric model accounts for bubble shape transformation throughout the bubble growth cycle including the necking phenomenon. An analytical model of quasi-static adiabatic bubble growth is accordingly developed based on the proposed low Bond number geometric model; it is coupled with a geometric detachment relation and a force balance detachment criterion that are dependent on the Bond number. The resulting predicted bubble growth characteristics, such as profile, volume, centre of gravity and aspect ratio, are validated with the benchmarked numerical treatment of the problem.</p> <p>Furthermore, the low Bond number geometric model is applied to bubble growth due to vaporization. In order to solve the mass-energy balance at the vapour bubble interface, a spherical surface area is commonly assumed. This leads to the need for correction factors and provides little insight into the physical mechanism responsible for bubble shape. In this study, the transitioning shape of a vapour bubble is considered in the integral analysis of the interfacial mass-energy balance. The model predicts the following bubble growth characteristics: profile, volume, centre of gravity, and aspect ratio.</p> / Doctor of Philosophy (PhD)

<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)

<p>Explosive boiling is a phenomenon encountered in severe nuclear reactor accidents during quench cooling, core relocation or through fuel-coolant interactions. The mitigation of accident conditions is important from a safety standpoint since explosive boiling is potentially capable of destructive forces. Explosive boiling occurs when coolant water encounters a hot solid surface and absorbs a high degree of superheat. The resultant boiling mode is violent and features the rapid decomposition of liquid on a microsecond timescale with liquid atomization and ejection. In this study, the explosive boiling force of a single water droplet impacting hot solid surfaces was estimated with secondary droplet analyses using high-speed imaging.</p> <p>A water droplet at 25°C with a Weber number of 432 impacted perpendicular to solid surfaces at temperatures from 30-700°C. Solid surfaces of copper, brass and stainless steel varied in thermal diffusivity from 3.48 x10^-6to 1.17 x10^-4m²/s. Curved and flat impact surfaces were also tested. Explosive boiling was most prominent when the instantaneous interface temperature attained the superheat limit temperature (300°C ±17°C). Maximum boiling force was encountered at the superheat limit with reduced force at surface temperatures in the nucleate boiling regime and near zero force in the film boiling regime. Thermal disintegration dominates over inertial break up of the droplet near the superheat limit region. Thermal diffusivity effects were only distinguishable in the 250-450°C region where increasing thermal diffusivity translated to larger boiling forces. Secondary droplet counts, size, trajectories were dependent on the boiling mode present at the interface with very strong variances caused by thermal break up of the initial droplet. Explosive boiling caused greater fragmentation creating more secondary droplets with smaller sizes and larger ejection trajectories. A curved surface showed slightly higher explosive boiling force in the superheat limit region but with negligible effects on secondary droplet properties.</p> / Master of Applied Science (MASc)

Study of quench cooling is very important in nuclear reactor safety for limiting the extent of core damage during the early stages of severe accidents after Loss of Coolant Accidents (LOCA). Quench of a hot dry surface involves the rapid decrease in surface temperature resulting from bringing the hot surface into sudden contact with a coolant at a lower temperature. The quench temperature is the onset of the rapid decrease in the surface temperature and corresponds to the onset of destabilization of a vapor film that exists between the hot surface and the coolant. Re-wetting the surface is the establishment of direct contact between the surface and the liquid at the so-called re-wetting temperature. Re-wetting is characterized by the formation of a wet patch on the surface which then spreads to cover the entire surface. Situations involving quench and re-wetting heat transfer are encountered in a number of postulated accidents in Canada Deuterium Uranium (CANDU) reactors, such as re-wetting of a hot dry calandria tube in a critical break LOCA. This accident results in high heat transfer from the calandria tube to the surrounding moderator liquid which can cause the calandria tube surface to experience dryout and a subsequent escalation in the surface temperature. If the calandria tube temperature is not reduced by initiation of quench heat transfer, then this may lead to subsequent fuel channel failure. In literature very limited knowledge is available on quench and re-wetting of hot curved surfaces like the calandria tubes. In this study, a Water Quench Facility (WQF) has been constructed and a series of experiments were conducted to investigate the quench and re-wetting of hot horizontal tubes by a vertical rectangular water multi-jet system. The tubes were heated to a temperature between 380-800°C in a controlled temperature furnace then cooled to the jet temperature. The temperature variation with time in the circumferential and the axial directions of the tubes has been measured. The twophase flow behavior and the propagation of the re-wetting front around and along the tubes were simultaneously observed by using a high-speed camera. The effects of several parameters on the cooling process have been investigated. These parameters include: initial surface temperature, water subcooling (in the range 15- 800C), jet velocity (in the range 0.15-1.60 m/s), tube solid material (brass, steel and Alumina), surface curvature, tube wall thickness, jet orientation and number of jets. The variables studied include the re-wetting delay time (time to quench after initiating the cooling process), there-wetting front propagation velocity, the quench and re-wetting temperatures, the quench cooling rates and the boiling region size. The quench and the re-wetting temperatures as well as the re-wetting delay time were found to be a strong function of water subcooling. The quench and re-wetting temperatures increase with increasing water subcooling. The rewetting delay time decreases with increasing the water subcooling, decreasing initial surface temperature, increasing liquid velocity and decreasing the surface curvature. There-wetting front velocity is mainly dependent on the initial surface temperature and water subcooling. The re-wetting velocity increases by decreasing the initial surface temperature and by increasing the water subcooling. Decreasing the surface curvature was found to also increase the re-wetting front velocity. Correlations of the phenomena studied have been developed and provided good prediction of the experimental data collected in this study and data available from literature. The. results of this study provide novel knowledge and an experimental database for mechanistic modeling of quench heat transfer on calandria tube surfaces that experience dryout and film boiling. / Thesis / Doctor of Philosophy (PhD)

<p> In a postulated critical break LOCA in a CANDU reactor it is possible that heatup of a pressure tube (PT) causes ballooning contact with the calandria tube (CT). Stored heat in the PT is transferred out, yielding a high PT-CT heat flux, which can cause dry out of the CT and establishment of pool film boiling on the outer surface of the tube. The safety concern associated with this condition is that if the temperature of the CT experiencing film boiling gets sufficiently high then failure of the fuel channel may occur. However, quench heat transfer can limit the extent and duration of film boiling as has been experimentally observed. Current estimates of quench temperatures during pool film boiling are based primarily on experimental correlations. In this dissertation a novel mechanistic model of pool film boiling on the outside of a horizontal tube with diameter relevant to CT (approximately 130 mm) has been developed. The model is based in part upon characterizing the vapor film thickness for steady state film boiling under buoyancy driven natural convection flows around a tube located horizontally in a large liquid pool. Variations in steady state vapor film thickness as a function of the incident heat flux, the temperature of the CT outer wall, and the subcooling of the bulk liquid are analyzed. The calculated effective film boiling heat transfer coefficient is compared to available experimental data. Finally a transient equation is developed which quantifies the instability of the vapor film and a possible occurrence of rapid quench when a step change in governing parameters occurs, such as liquid subcooling. This mechanistic model can be employed in safety analysis to demarcate the conditions under which fuel channel failure will not occur in a postulated critical break LOCA.</p> / Thesis / Master of Applied Science (MASc)

Electrohydrodynamics in convective boiling heat exchangers has been studied since the early 1990’s and has been shown to result in a large variation in the average performance enhancement of these systems. The behaviour of EHD assisted convective boiling heat exchangers, is still largely unpredictable owing to a number of conflicting parameters which are rarely kept constant in empirical studies, i.e. flow pattern and heat flux. In this thesis, it is hypothesised that by reducing the number of confounding variables in the experimental test conditions, and understanding the behaviour of EHD in convective boiling systems from a flow pattern dependent point of view, this can allow for the development of flow pattern dependent experimental correlations & numerical models to develop a methodology for performance prediction, control strategies and system integration for an EHD assisted convective boiling heat exchange device. A 30 cm long, smooth, concentric, annular test section is used to analyse the effect of EHD on convective boiling performance under constant flow pattern, constant, low heat flux, and negligible free charge conditions. Saturated boiling conditions for flow-rates between 60 kg/m2s and 180 kg/m2s and thermodynamic quality range of 0.25 - 0.55 were tested. Heat transfer enhancement ranged from 0.95 to 2.3 fold and pressure drop penalty varied from 1.4-3 fold over these test conditions. The local EHD behaviour was found to be more consistent along the axial length of the test section compared to empirical data in the literature, which uses much longer test section lengths, where flow pattern can vary. An experimental database of EHD convective boiling data for horizontal annular electrode geometries was compiled to be used for analysis purposes. The performance of the heat exchanger in both free-field and high voltage conditions could be explained by looking at the flow patterns in each case. Electrostatic modelling was used to determine electric field strength distributions and interfacial stress due to the dielectrophoretic and electrostriction forces on the liquid vapour interface, which induce liquid extraction based flow pattern re-distribution in two phase dielectric flows. A fully coupled 2D, adiabatic numerical model for the effect of the electric body force on two phase flow pattern distribution was developed. Charge was neglected in this model. Two different models for the interface migration were used and compared; a moving mesh (MM) interface tracking model and a volume of fluid (VOF) interface tracking mode. Both were verified against published experimental data. For the liquid extraction verification case, the VOF model suffers interface stretching up to 300% resulting in a 42% slower extraction time and underestimated forces. However, it is useful to use the VOF model when simulating complex flow patterns which are subject to topological changes like bubble detachment or droplet coalescence as these cannot be simulated with the moving mesh model. The moving mesh model can be used to determine the error in forces and phase velocities when using the VOF model. A methodology for generating two-phase EHD flow pattern maps was developed by incorporating the electric Froude number into each of the flow pattern transition equations. A semi-analytical model was developed to determine the maximum interfacial stress due to EHD for stratified flows to reduce the requirement of numerical modeling, and thus the flow pattern map generation methodology is fully equation based. Although transition equations developed by multiple researchers were used and compared, it is recommended that the Steiner transitions equations be used for EHD two-phase flow pattern mapping, until more fundamental experimental data can be gathered to modify the semi-empirical transition equations used in more state-of-the-art maps. EHD was found to significantly affect the “stratified-stratified wavy (SSW)” and “stratified wavy – intermittent/annular (SWIA)” transitions for concentric horizontal geometries, with minimal effect on the transition to dryout and no effect on the “intermittent dispersed bubbly (IB)” transition. The EHD flow pattern maps were generated and compared against data from the present study and a database of experimental EHD convective boiling studies. The regions where maximum enhancement were seen in the literature correlate well with those regions predicted by the maps. Performance correlations for the EHD convective boiling heat transfer and pressure drop were developed. They are based on the free-field Kandlikar correlation [1] for two-phase heat transfer and the Chisholm-Laird [2] correlations for two-phase pressure drop, respectively. The EHD flow pattern map is used to determine what the flow pattern for a given applied voltage will be, and flow pattern based enhancement linear multipliers are then used to determine the EHD performance above the free-field case. EHD is a form of active enhancement, i.e. it requires power. Thus, it would be used in systems that require performance control or regulation, in addition to some niche applications like space where it can be used instead of gravity. A method for EHD controller design was established and an EHD control algorithm was designed and implemented on the test section for the flow pattern and applied waveforms that were determined to be optimal to maximize enhancement in this geometry. System identification was performed empirically to determine the transfer function between EHD voltage and heat load to be controlled for. This resulted in a 1st order plus dead-time model to which proportional-integral controller constants were tuned. Two controllers were developed; a PID control system and a Smith model predictive control system and these were compared based on their ability to regulate the output quality of the heat exchanger when subject to dynamic heat loading. Regulation was achieved for a dynamic heat load within ±25% bound from the designed steady state load. These controllers operate on one flow pattern as the test section is 30 cm long. Flow pattern dependent controller design would be required for a full length convective boiling heat exchanger. / Thesis / Doctor of Philosophy (PhD) / Control of boiling heat transfer using electric fields is hard to predict. This thesis presents a set a design guidelines based on how the electric field enhances the flow pattern.

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)

Smart thermal management by which ultra-high heat fluxes (i.e., q''> 100 W/cm²) are dissipated efficiently, is increasingly desirable for many applications in aerospace, electronic packaging, metallurgy, as the existing cooling solutions are highly constrained. For example, the cooling strategy for aircraft must be executed in such a way that will operate independently of orientation while also screen out external heat loads coming from the neighboring electronic boxes and/or external sources. Therefore, it is crucial to develop heat transfer devices which could effectively dump heat away while additionally shield against external heat loads. Thermal diodes, by definition, accomplish this desirable unidirectional heat transfer functionality. Nonetheless, the existing thermal diodes are currently constrained by either a low diodicity (i.e., heat transfer ratio), gravitational dependence, a one-dimensional configuration, or poor durability. Further example for the necessity of smart thermal management would be in firefighting and nuclear reactor safety. Above a critical temperature referred to as the ``Leidenfrost temperature'', the highly effective nucleate boiling is completely replaced by insulating film boiling, causing a dramatic decrease in the essential cooling rate of water pool boiling and spray quenching. In chapter 2, after noting the mechanism and shortcomings of each existing solid-state and phase-change thermal diode, we develop a unique thermal diode, called bridging-droplet thermal diode, which operates independent of orientation, is planar and durable. Our diode is comprised of two opposing copper plates separated by an insulating gasket of micrometric thickness; one plate contains a superhydrophilic wick structure while the other is smooth and hydrophobic. In the forward mode of operation, water evaporates from the heated wicked plate and condenses on the opposing hydrophobic plate. The large contact angle of the dropwise condensate enables bridging across the gap to replenish the wicked evaporator, providing sustained phase-change heat transfer. Conversely, in the reverse mode the heat source is now on the hydrophobic plate, resulting in dryout and excellent thermal insulation across the gap. An orientation-independent heat transfer ratio (i.e. diodicity (η)) of approximately 85 was experimentally measured. In chapter 3, after highlighting that our experimental proof-of-concept discussed in chapter 2, was limited to only a narrow parameter space, we develop a comprehensive thermal circuit model for both the forward and reverse modes of operation to theoretically characterize the bridging-droplet thermal diode over a broad parameter space. Parameters that are varied include the gap height, input heat flux, effective thermal conductivity of the wetted wick structure, height of the wicking micropillars, wettability of the opposing smooth surface, and heat sink temperature. Our findings show that a vapor space height of Hᵥ≈ 250 μm, short and densely packed micropillars, a higher applied heat flux in the forward mode, and a hotter heat sink temperature result in optimal diodicities of η~ 100. In chapter 4, we discuss that the Leidenfrost effect has been a two-phase phenomenon thus far: either an evaporating liquid or a sublimating solid levitates on its vapor. Here, we demonstrate that an ice disk placed on a sufficiently hot surface exhibits a three-phase Leidenfrost effect, where both liquid and vapor films emanate from under the levitating ice. Curiously, the critical Leidenfrost temperature was over three times hotter for ice than for a water drop. As a result, the effective heat flux was an order of magnitude larger when quenching aluminum with ice rather than water over a wide temperature range of 150--550 °C. An analytical model reveals the mechanism for the delayed film boiling: the majority of the surface's heat is conducted across the levitating meltwater film due to its 100 °C temperature differential, leaving little heat for evaporation. In chapter 5, we note that nucleate boiling achieves dissipative heat fluxes as high as q''~ 100 W/cm² and is widely used for power plants, spray quenching metal alloys, desalination, and electronics cooling. However, above a Leidenfrost temperature of about 150 °C for water, an insulating vapor film massively degrades the heat flux by two orders of magnitude. Here, we demonstrate that robust nucleate boiling can be maintained even at temperatures as high as 400 °C by using ice particles in place of water droplets. Ice pellets are periodically released onto a superheated stage and compared to spray quenching at an equivalent mass flow rate. Ice quenching was twice as fast as spray quenching at low superheats, and at large superheats, only ice quenching is successful. Our results demonstrate that ice quenching can maintain groundbreaking heat fluxes of q''~ 100--1,000,W/cm² over a broad range of superheats, far superior than classical spray quenching. / Doctor of Philosophy / Smart thermal management by which enormous heat generated in avionics, electronic packaging, wildfire, etc are removed efficiently, is increasingly desirable as the current cooling solutions are highly constrained. For example, in the context of aircraft, equipment must be cooled down independent of aircraft orientation while also they are shielded from neighboring and/or external heat sources. In firefighting where the temperature of wildfire flames could get beyond 500 °C, dumping large volume of water from aircraft may not be adequate to quench the fire over a reasonable time frame as the liquid water loses its cooling effectiveness above a critical temperature. In chapter 2, after a brief review of existing cooling devices and their corresponding shortcomings commonly used in aircraft and electronic packaging, we develop a unique device for cooling of aircraft which operates independent of aircraft orientation, is durable over time, and can cool down surfaces irrespective of their dimensions. In chapter 3, after highlighting that our proof-of-concept of a new cooling device in chapter 2, was limited to only a finite number of experiments, we theoretically model the operating mechanism of our device to check for the criteria where our device works most efficient. In chapter 4, we discover that by placing an ice disk on a sufficiently hot surface, effective boiling where large amount of heat can be dumped away from the surface to the coolant, is extended to a very large surface temperature. To be specific, liquid water on smooth aluminum loses its cooling efficiency around 150 °C, while cooling the same surface with ice is still effective up to 550 °C. In chapter 5, we report that quenching with ice is twice as fast as quenching with liquid water at low surface temperatures (i.e., 150--300 °C), and at larger surface temperatures (i.e., beyond 300 °C), only ice quenching is successful. Comparing our ice quenching results against current cooling technologies, we note that ice quenching is superior.