<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)

Two mathematical models (a one and a three-dimensional) were adopted to study, numerically, the thermal hydrodynamic behavior of flow inside a single cooling channel of a Nuclear Thermal Rocket (NTR) engine. The first model assumes the flow in the cooling channel to be one-dimensional, unsteady, compressible, turbulent, and subsonic. The working fluid (GH2) is assumed to be compressible. The governing equations of the 1-D model are discretized using a second order accurate finite difference scheme. Also, a commercial CFD code is used to study the same problem. Numerical experiments, using both codes, simulated the flow and heat transfer in a cooling channel of the reactor. The steady state predictions of both models were compared to the existing experimental results and it is concluded that both models successfully predict the steady state fluid temperature distribution in the NTR cooling channel.

In this thesis, combustion processes in a laboratory-scale methane based low pressure rocket motor (LPRM) is studied experimentally and numerically. Experiments are conducted to measure flame temperatures and chamber temperature and pressure. Single reaction-four species reacting flow of gaseous methane and gaseous oxygen in the combustion chamber is also simulated numerically using a commercial CFD solver based on 2-D, steady-state, viscous, turbulent and compressible flow assumptions. LPRM geometry is simplified to several configurations, i.e. Channel and Combustion Chamber with Nozzle and FWD. Flow in a Bunsen burner is simulated inside Channel geometry in order to validate the reaction model. Grid independence study is also conducted for reacting as well as non-reacting flows. Numerical model is calibrated based on experimental results. Results of the computational model are found in a good agreement with the experimental data after calibrating specific heats of the products. Parametric study is conducted in order to investigate the effects of different mass flow rates and chamber pressures on flow and combustion characteristics of a LPRM to provide insight to future studies.

The Purpose of this thesis is to experimentally demonstrate an inversion analysis technique, developed by Dr. Jay Frankel (UTK), that utilizes transient temperature data from probes embedded at known locations in a material. This allows one to determine thermal properties (thermal diffusivity and thermal conductivity) of the material, surface temperature, and the surface heat flux as they change with time. Dr. Frankel’s inversion method can be used to determine surface temperature and heat flux of a one-dimensional semi-infinite slab based on the transient data from one or two embedded probes, if the thermal conductivity and thermal diffusivity of the material are known. Frankel’s theory suggests that the thermal properties of the material can be determined if transient data from two thermocouple (TC) probes at known locations and the heat flux at the surface are known. This thesis investigates finding the thermal properties and surface temperature of materials using a two embedded thermocouple approach. As an initial check to the inversion analysis, the theoretical temperature solution for a one-dimensional semi-infinite slab was used. This validated that the analysis could converge to the constant thermal properties for the theoretical material. An experiment was run again to provide data for the materials copper and aluminum. Using a real material is fundamentally different from using theoretical determined (analytical) data, because the thermal properties for a real material vary with temperature. Since the inversion analysis converged to a constant solution for the theoretical temperatures, it was believed that the real material will converge to a solution. However, it was seen that the thermal diffusivity for the real materials never converged to the expected value. Although, when a constant handbook value for the thermal diffusivity is used to calculate the thermal conductivities from the experimental temperature data collected from the internal probes, the inversion analysis resulted in good agreement with experiment.

The Purpose of this thesis is to experimentally demonstrate an inversion analysis technique, developed by Dr. Jay Frankel (UTK), that utilizes transient temperature data from probes embedded at known locations in a material. This allows one to determine thermal properties (thermal diffusivity and thermal conductivity) of the material, surface temperature, and the surface heat flux as they change with time. Dr. Frankel’s inversion method can be used to determine surface temperature and heat flux of a one-dimensional semi-infinite slab based on the transient data from one or two embedded probes, if the thermal conductivity and thermal diffusivity of the material are known. Frankel’s theory suggests that the thermal properties of the material can be determined if transient data from two thermocouple (TC) probes at known locations and the heat flux at the surface are known. This thesis investigates finding the thermal properties and surface temperature of materials using a two embedded thermocouple approach. As an initial check to the inversion analysis, the theoretical temperature solution for a one-dimensional semi-infinite slab was used. This validated that the analysis could converge to the constant thermal properties for the theoretical material. An experiment was run again to provide data for the materials copper and aluminum. Using a real material is fundamentally different from using theoretical determined (analytical) data, because the thermal properties for a real material vary with temperature. Since the inversion analysis converged to a constant solution for the theoretical temperatures, it was believed that the real material will converge to a solution. However, it was seen that the thermal diffusivity for the real materials never converged to the expected value. Although, when a constant handbook value for the thermal diffusivity is used to calculate the thermal conductivities from the experimental temperature data collected from the internal probes, the inversion analysis resulted in good agreement with experiment.

Emissivity is an important parameter in calculating radiative cooling of a surface. In experiments at the NASA Ames hypervelocity ballistic range, one of the main errors indicated in temperature measurements is the uncertainty of emissivity for the materials under investigation. This thesis offers a method for measuring emissivity of materials at elevated temperatures at the University of Kentucky. A test specimen which consists of different sample materials under investigation and a blackbody cavity was heated in a furnace to an isothermal condition at known temperature. The emitted thermal radiation was measured and the comparison of sample and blackbody radiation yielded the desired emissivity. In addition to the furnace measurements, separate experiments were conducted in ambient air to determine how much irradiation is reflected back to the samples from the radiation shield used in the furnace to block undesired ambient radiation. Here, the sample heating was accomplished by applying a direct current across the samples. ANSYS simulations were performed to assist the design and analysis. Experiments were conducted in ambient air and a vacuum environment to verify these simulations.

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