<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 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)

The mechanism by which convective heat transfer is augmented by freestream turbulence in the stagnation region was studied experimentally. Previous work has suggested that the primary mechanism for the observed augmentation is the amplification of vorticity into strong vortices which dominate the flow field near the surface. Therefore, two separate experimental investigations were performed to further study this phenomenon. In the first, the spatiotemporal convection from a heated surface was measured during the normal collision of a vortex ring. The convection was observed to increase dramatically in areas where vortices forced outer fluid through the natural convection boundary layer to the surface. Regions where fluid was swept along the surface experienced much smaller increases in convection. These observations led to the development of a mechanistic model which predicted the heat transfer based on the amount of time that fluid remained within the thermal boundary layer prior to reaching the surface. In subsequent testing, the model was able to accurately predict the time-resolved convection based solely on the transient properties of the vortex present. In the second investigation, the model was applied to the vortices which form in a stagnating turbulent flow. Three turbulence conditions were tested which changed the properties of the vortices produced. Again, the model was successful in predicting the time-resolved convection over much of the experimental measurement time. The work of designing and calibrating the heat flux sensor used is also reported. A new sensor was developed specifically for the convection research performed herein as no existing sensor possessed the required spatiotemporal resolution and underwater capabilities. Utilizing spot-welded foils of thermoelectric alloys resulted in a very robust and sensitive sensing array which was thoroughly analyzed and calibrated. In the final section, the hybrid heat flux (HHF) method is presented which significantly increases the performance of existing heat flux sensors. It is shown (both numerically and experimentally) that by combining the spatial and temporal temperature measurements from a standard sensor, the time response increases by up to a factor of 28. Also, this method causes the sensor to be insensitive to the material to which it is mounted. / Ph. D.

A new gage which can measure skin friction and heat flux simultaneously was designed, constructed, and tested. This gage is the combination of a non-nulling type skin friction balance and a heat flux microsensor. By mounting the heat flux microsensor directly on the surface of the floating element of the skin friction balance, it was possible to perform simultaneous measurements of the skin friction and the heat flux. The total thickness of the heat flux microsensor is less than 2 μm, so the presence of this microsensor creates negligible disruption on the thermal and the mechanical characteristics of the air flow. Tests were conducted in the Virginia Tech supersonic wind tunnel. The nominal Mach number was 2.4, and Reynolds number per meter was 4.87 x 10⁷ with total pressure of 5.2 atm and total temperature of 300 °K. Results of the tests showed that this new gage was quite reliable and could be used repeatably in the supersonic flow. This gage also has an active heating system inside of the cantilever beam of the skin friction balance so that the surface temperature of the floating element can be controlled as desired. With these features, the effects of a temperature mismatch between the gage surface and the surrounding wall on the measurements of the skin friction and the heat flux were investigated. An infrared radiometer was used to measure the surface temperature distributions. Without the active heating, the amount of temperature mismatch generated by the gage itself was from 2.5 °K to 4.5 °K. The active heating produced the temperature mismatch of 18.7 °K. The largest temperature mismatch corresponds to the levels typically found in high heat flux cases when it is expressed in dimensionless terms. This temperature mismatch made sizable effects — a 24 % increase in the skin friction measurement and a 580 % increase in the heat flux measurements. These experimental results were compared with the computational results using the Computational Fluid Dynamics code GASP. The input flow conditions were obtained from the boundary layer measurements. The temperature mismatch was input by specifying the density and the pressure at each grid point on the wall. The Baldwin-Lomax algebraic turbulence model was used with the thin layer approximations. The comparison showed that the difference in the skin friction and heat flux was less than 10 % of the measured data when the temperature mismatch was less than 8.5 °K, but the difference was increased as the amount of the temperature mismatch increased. It is presumed that the disagreement between the measurements and the calculations was caused mainly by deficiencies in the turbulence model for this complex, developing viscous flow, because the Baldwin-Lomax model cannot account for the multiple length scale in this flow. / Ph. D.

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

The topic of heat transfer is traditionally taught as an upper level, lecture-style course to mechanical engineering students. Such courses do not provide students with ways to see and feel the important heat transfer concepts at hand. As a way to overcome this, novel, hands-on workshops have been designed and implemented into a heat transfer class taught to junior level mechanical engineering students. Two types of experimental workshops were created and used in two different years of a section of a heat transfer class. In the first year, twelve workshops were designed which included live demos so that the students could see and feel different modes of heat transfer while taking data and seeing real-time plots of temperature and heat flux in different experiments. The workshop introduced each topic the students would be learning in the lecture and was performed the week before the actual lecture on the topic. Each workshop included easily available materials, thermocouples, heat flux sensors, and data acquisition instruments for the students to use. The workshops also served replacements for what would be the third lecture of the week. Results from a concept inventory test given at the end of the first year showed a significant difference on certain question between an experimental group of students who had the workshops and a control group who took the traditional class lecture. However, there were still concepts and topics that the experimental group did not show improvement. They also showed a lack of improvement in their problem solving skills for quiz and test problems. For the second year of the experiment, the workshops were restructured quite a bit. The original 12 workshop format was cut down to only six in order to focus on the ones the students seemed to have benefited from the most. The workshops were also changed into a video-enhanced format where the students would watch a video of the experiment being done while also having the materials in front of them to place their hands on themselves. The students could therefore see and feel what was physically happening and still perform the experiment while watching real-time, pre-recorded plots of heat flux and temperature without worrying about making sure their setup was right and acquiring good results. The new video-enhanced workshops also included control volume and resistance diagrams for each experiment in order to help the students relate the workshops and concepts back to problems on their quizzes and tests. Results from these workshops seemed to show some statistical significance between the experimental and control groups on concept questions given on quizzes throughout the semester, but there was no difference on any questions from the ten concept questions given on the final exam. However, surveys taken by the students indicate that they believed the workshops did help them to understand the concepts in a real-world sense and that they helped them understand the class material better overall. Aside from the results of the workshops on the students learning, this study concludes with an analysis of important heat transfer concepts and how to test them. There is much debate about the underlying concepts in the topic of heat transfer and a thorough analysis on what specific concepts are important for students to know must be addressed. Many heat transfer concept questions on current concept inventories have more to do with thermodynamics and the mixing of the two topics is itself a misconception. / Master of Science

Instrumentation has been designed to process the signals from two types of skin friction gages and a microfabricated heat flux gage. Design changes for the skin friction gages are presented which will improve the performance of the two transducers. The instrumentation is simple in design and use and has been designed to maximize the performance of the skin friction and heat flux gages. The instrumentation is battery powered to minimize noise levels and to maintain instrumentation portability. A high-quality instrumentation amplifier, a voltage regulator, and a custom-designed circuit board have been combined to produce an instrumentation package which is stable and durable. The instrumentation has been specifically designed to handle low-level signals and can operate over a wide range of frequencies. Problems commonly associated with low-level signal conditioning like electrical noise, nonlinearities, and output drift are addressed. The performance specifications of the instrumentation are presented along with sample gage measurements. / Master of Science

This work focuses on the development of a system to noninvasively estimate blood perfusion using thermal methods. This is accomplished by the combination of a bioprobe, biothermal model, and parameter estimation techniques. The probe consists of a heat flux sensor and surface thermocouple placed in contact with tissue while the opposite side is cooled by jets of room temperature air. The biothermal model predicts the temperature and heat flux within tissue and probe based upon the input of blood perfusion and the thermal contact resistance between probe and tissue. Parameter estimation techniques are developed that use the model to simultaneously estimate blood perfusion and contact resistance based on experimental heat flux and/or temperature. A gradient based system minimizes a sum of squares error function based on either or both heat flux and temperature. This system is tested on human forearms and in controlled flow rate experiments using tissue phantoms. Blood perfusion estimates from the controlled experiments are positively correlated with experimental flow rate. Experimental measurements and statistical analysis show distinct variations in the heat flux signal and rises in perfusion estimates with increasing flow rate. This research validates the use of thermal and parameter estimation methods to develop a practical, noninvasive probe to clinically measure blood perfusion. / Master of Science

A fast response thin-film heat flux sensor was used to measure the time-resolved surface heat flux and temperature from a turbulent combustion flame impinging on a surface. Using the analytical semi-infinite conduction model, the unsteady surface heat flux was calculated from the transient temperature measurements and the surface temperature was calculated from the unsteady surface heat flux measurements. Methods of comparing time-resolved heat flux and temperature data were presented and discussed. The standard analytical method for converting surface temperature to heat flux was used. Two new analytical methods were developed for converting heat flux to surface temperature. The study is the first demonstration of time-resolved temperature signals generated from time-resolved heat flux measurements. The results graphically illustrate the effects of data processing on electrical noise present in the actual signal. The effect of flame unsteadiness is also shown, especially in the time-resolved heat flux measurements, which gives insight into the behavior of a propane torch. One application is for development of feed-forward control systems in industrial processes with fast transients. / Master of Science

A local heat-flux measurement system was built, calibrated and tested for use in unsteady flows. The system was designed to maintain constant wall temperature boundary conditions. The measuring element is a thin-film heat flux gage made by sputter-coating gold on a substrate. A constant-temperature anemometer is used to maintain the thin-film gage at a specified temperature under fluctuating conditions. A separate temperature control system maintains the surrounding boundary at the gage temperature. The system was calibrated for both steady and unsteady flows using a specially designed calibrator for local heat flux gages. The steady calibration was done with predominantly convective heat transfer . The unsteady calibration was achieved by adding oscillating radiant energy to the surface. Consequently, quantitative results can be obtained for both mean and fluctuating components of the heat transfer. The frequency response was good to 92 hertz. Sample results are presented for unsteady heat transfer caused by the vortex shedding from a cylinder in a steady crossflow. The shedding frequency was 82 hertz. / M.S.