<p>The present work relates to a new procedure, to perform temperature measurements with unprecedented accuracy. The new approach relies on a correction based on a two-wire probe thermocouple that enables a precise estimation of the conduction error. The difference between measured temperature by a thermocouple and total gas temperature for steady conditions can be decomposed into three main contributions: velocity error, conduction error and radiation error. Radiation error can be considered negligible for temperatures lower than 800K. The velocity error can be corrected using dedicated experimental calibrations to measure the recovery factor. However, the conduction error, remains an unresolved challenge in the aerospace and power-energy community. The proposed method includes a comprehensive correction with different options for the postprocessing. The method has been demonstrated using high-fidelity aero-structural computational simulations.</p>

<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>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)

The emergence of metal Additive Manufacturing (AM) over the last two decades has opened venues to mitigate the challenges associated with stochastic open-cell metal foams manufactured through the traditional foaming process. Regular lattices with user-defined unit cell topologies have been reported to exhibit better mechanical properties in comparison to metal foams which extend their applicability to multifunctional heat exchangers subjected to both thermal and mechanical loads. The current study aims at investigating the thermal-hydraulic characteristics of promising novel unit cell topologies realizable through AM technologies. Experimental investigation was conducted on four different topologies, viz (a) Octet, (b) Face-diagonal (FD) cube, (c) Tetrakaidecahedron, and (d) Cube, printed in single-cell thick sandwich type configuration in 420 stainless steel via Binder Jetting technology at same intended porosity. The effective thermal conductivity of the samples was found to be strongly dependent on the lattice porosity, however, no significant dependence on the unit-cell topology was demonstrated. Face-diagonal cube lattice exhibited the highest heat transfer coefficient and pressure drop, and consequently provided the lowest thermal-hydraulic performance. A procedure to incorporate the manufacturing-induced random roughness effects in the samples during numerical modelling is introduced. The numerical simulations were conducted on samples exhibiting the roughness profiles having statistically same mean roughness as the additively manufactured coupons and the results were compared to that obtained from the intended smooth-profiled CAD models that were fed into the printing machines. The analysis showed that inclusion of roughness effects in computational models can significantly improve the thermal performance predictions. Through this study, we demonstrate that additively manufactured ordered lattices exhibit superior thermal transport characteristics and future developmental efforts would require extensive experimentations to characterize their thermal and flow performance as well as local surface quality and AM-induced defect recognition. Experimental findings would also need to be supported by computational efforts where configurations which closely mimic the real AM parts could be modeled. A combined experimental-numerical framework is recommended for advancements in metal additive manufacturing-enabled enhanced heat transfer concepts.

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 numerical study presents investigation of impinging jets cooling effect on a hot flat plate. Different configuration of single jet, 5-cross and 9-square setups have been studied computationally in order to understand about their behaviour and differences behind their physics. Moreover, a specific confined wall was designed to increase two crucial parameters of the cooling effect of impinging jets; average heat transfer coefficient of impingement wall and average air temperature difference of outlet the domain and jet inlet. The 2-D simulation has been performed to design the confined wall to optimise the domain geometry  to achieve project goals contains highest average heat transfer coefficient of hot plate in parallel to highest average air temperature difference of outlet. Different effective parameters were chosen after 2-D simulation study and literature review; Jet to wall distance H/D = 5, Radial distance from centre of plate R/D = 20, jet diameter D = 10 mm. The 3-D computational study was performed on single jet, 5-cross and 9-square configurations to investigate the differences of results and find best setup for the specific boundary condition in this project. Single jet geometry reveals high temperature level in the outlet, but very low average heat transfer coefficient due to performance of a single jet in a domain (Re= 17,232). In the other side, 5-cross setup has been studied for Reynolds number of 9,828, 11,466, 17,232 and 20,000 and it was found that range of 11,466 to 17,232 performs very well to achieve the purposes in this study. Moreover, turbulence models of ,  and  have been used to verify the models (Re=17,232) with available experimental data for fully developed profile of the jets inlets and wall jet velocity and Reynolds stress components near the wall boundary condition. All three turbulence models predict well   the velocity components for jets fully developed profile and for wall boundary condition of the target plate. But since  model has been validated with the Reynolds stress components by experimental data, therefore is more reliable to continue the study with verified simulation. Finally 9-square configuration was investigated (Re=17,232) and the result compared with other setups. It was concluded that 5-cross multiple jets is best design for this project while 9-square multiple impinging jets also fulfils the project purpose, but for extended application in industry each setup is suitable for specific conditions. 5-cross multiple jets is good choice for large cooling area which can be used in number of packages to cover the area, while 9-square jets setup performs well where very high local heat transfer is needed in a limited area.

This paper presents details of an experimental and computational investigation on the effect of misalignment between the combustor exit and nozzle guide vane endwall on the heat transfer distribution across an axisymmetric converging endwall. The axisymmetric converging endwall investigated was representative of that found on the shroud side of a first stage turbine nozzle section. The experiment was conducted at a nominal exit M of 0.85 and exit Re 1.5 x 10⁶ with an inlet turbulence intensity of 16%. The experiment was conducted in a blowdown transonic linear cascade wind tunnel. Two different inlet configurations were investigated. The first configuration, Case I, was representative of a combustor exit aligned to the nozzle platform, with a gap located at the interface of the tow components. The second configuration, Case II, the endwall platform was offset in the span-wise direction to create a backward facing step at the inlet. This step is representative of a misalignment between the combustor exit and the NGV platform. An infrared camera was used to capture the temperature history on the endwall, from which the endwall heat transfer distribution was determined. A numerical study was also conducted by solving RANS equations using ANSYS Fluent v.15. The numerical results provided insight into the passage flow field which explained the observed heat transfer characteristics. Case I showed the typical characteristics of transonic vane cascade flow, such as the separation line, saddle point, and horseshoe vortices. The presence of a gap at the combustor-nozzle interface facilitated the formation of a separated flow which propagated through the passage. This flow feature caused the passage vortex reattach to the SS vane at 0.44 x/C. The addition of the platform misalignment in Case II caused the flow reattachment region to occur near the vane LE plane. The separated flow which formed at the inlet step, merged with the recirculation region on the endwall platform, forming two counter-rotating auxiliary vortices. These vortices significantly delayed migration of the passage vortex, causing it to reattach on the SS vane at 0.85 x/C. These two flow features also had a significant effect on the endwall heat transfer characteristics. The heat transfer levels on the endwall platform, from -0.50 to +0.50 Cx relative to the vane LE, had an average increase of ~40%. However, downstream of the vane mid-passage, the heat transfer levels showed no appreciable heat transfer augmentation due to flow acceleration through the passage throat. / Master of Science

In a nuclear fuel cask, the heat generating spent fuel rods are packed in a housing and the resulting bundle is placed inside a cask of thick outer shell made of materials like lead or concrete. The cask presents a wide variation in geometrical dimensions ranging from the diameter of the rods to the diameter of the cask. To make the problem tractable, first the heat generating rod bundle alone is considered for analysis and the effective thermal conductance of the bundle is correlated in terms of the relevant parameters. In the second part, the bundle is represented as a solid of equivalent thermal conductance and the attention is focused on the modelling of the cask. The first part, dealing with the effective thermal conductance is solved using Fluent software, considering coupled conduction, natural convection and surface radiation in the heat generating rod bundle encased in a hexagonal sheath. Helium, argon, air and nitrogen are considered as working media inside the bundle. A correlation is obtained for the critical Rayleigh number which signifies the onset of natural convection. A correlation is also developed for the effective thermal conductance of the bundle, considering all the modes of transport, in terms of the maximum temperature in the rod bundle, pitch-to-diameter ratio, bundle dimension (or number of rods), heat generation rate and the sheath temperature. The correlation covers pitch-to-diameter ratios in the range 1.1-2, number of rods ranging from 19 to 217 and the heat generation rates encountered in practical applications. The second part deals with the heat transfer modeling of the cask with the bundle represented as a solid of effective (or equivalent) thermal conductance. The mathematical model describes two-dimensional conjugate natural convection and its interaction with surface radiation in the cask. Both Boussinesq and non-Boussinesq formulations have been considered for convection. Numerical solutions are obtained on a staggered mesh with a pressure correction method using a custom-made Fortran code. The surface radiation is coupled to the conduction and convection at the solid-fluid interfaces. Steady-state results are obtained using time-marching. Results for various quantities of interest, namely, the flow and temperature distributions, Nusselt numbers, and interface temperatures, are presented. The Grashof number based on the volumetric heat generation and gap width is varied from 105 to 5 ×109. The emissivities of the interfaces are varied from 0.2-0.8 for the radiative calculations. The solid-to-fluid thermal conductivity ratio for the inner cylinder is varied in the range 5-20 in the parametric studies. Simulations are also performed with thermal conductivity calculated in an iterative manner from bundle parameters. The dimensionless outer wall conductivity ratio is chosen to correspond to cask walls made of lead or concrete. The dimensionless thickness (with respect to gap width) of the outer shell is in the range of 0.0825-1, while the inner cylinder dimensionless radius is 0.2. Air is the working medium in the cask for which the Prandtl number is 0.71. Correlations are obtained for the average temperatures and Nusselt numbers at the inner interface in terms of the parameters. The radiation heat transfer is found to contribute significantly to the heat dissipation.