"Quenching is the rapid cooling process from an elevated temperature. Compared to water and oil quench medium, high pressure and velocity gas is preferred to quench medium and high hardenability steel, with the potential to reduce distortion, stress and cracks. Currently, no standard test exists to characterize the gas quench steel hardenability and measure the performance of industrial gas quench furnaces. In this thesis, the fundamental difference between the liquid and gas quenching, heat transfer coefficient, was emphasized. It has been proven that gas quenching with constant HTC cannot generate the similar cooling curves compared to liquid quenching. Limitations on current gas quench steel hardenability tests were reviewed. Critical HTC, a concept like critical diameter, was successfully proved to describe the gas quench hardenability of steel. An attempt to use critical HTC test bar and measure the HTC distribution of gas quench furnace was made. Gas quenching, usually with slow cooling rate, may reduce hardness and Charpy impact toughness, compared to water and oil quenching. Lattice parameter and c/a ratio of as-quenched martensite in steel was measured using high resolution X-ray diffraction and Rietveld refinement. For AISI 4140, Charpy impact toughness decreases when the cooling rate decreases after quenching and tempering. Austenite percentage and carbon content in austenite is proposed as the dominated mechanism."

"The importance of two-phase heat transfer for thermal management of aerospace avionic systems has become increasingly important as these systems have become miniaturized. Embedded active cooling systems are used to remove heat from processors and other electronic components and transferring this heat to radiators or other heat exchangers. As the characteristic dimension of flow channels for two-phase flow becomes comparable to bubble size, the mini-channels (< 3 mm) used to direct the cooling fluid can complicate nucleate boiling heat transfer. Bubbles can encounter other heated walls, rapidly expanding and greatly reducing heat transfer as well as causing pressure oscillations and flow instabilities. The use of eletrohydrodynamic (EHD) effects, through the introduction of non-uniform electric fields, can help mitigate this problem by altering the behavior of nucleating bubbles. A combined experimental and computational study was undertaken using HFE-7100, an engineered fluid used in heat transfer applications, to investigate the potential for enhancement of nucleate boiling using EHD effects induced by applying a non-uniform electric field. In the experimental study, a minichannel was constructed consisting of an upper and lower copper electrode and glass side walls to allow visualization. The channel height and width were 3mm and 4.76 mm respectively, representative of the minichannel regime. The upper electrode was grounded while the lower electrode was heated and biased to high voltage. Optical imaging combined with post-processing and statistical analysis was used to quantify the effect of EHD on the bubble behavior. Bubbles were found to form preferentially on nucleation sites resulting from imperfections in the heated copper surface over artificially created nucleation sites. When a high voltage is applied across the electrodes, the electric field enhancement along the rim of the nucleation site is believed to influence the force balance on the forming bubble and thereby influence the bubble departure size and frequency. EHD forces also act on the bubble surface as a result of the variation in permittivity between the liquid and vapor phases, altering its shape as has been previously reported in the literature. Test results are presented that demonstrate that the application of EHD increases the nucleation site density on the heated surface and increase the bubble departure frequency from individual sites. In addition, test results are presented to show that EHD forces alter the shape of bubbles during growth and the vertical position of the detached bubbles as they are carried along in the cross flow. To better understand the underlying phenomena affecting the bubble shape and departure frequency, a numerical simulation of the bubble growth and departure was performed using COMSOL multiphysics software customized to incorporate a user-defined body force based on the Maxwell Stress Tensor. Tracking of the bubble surface, including coalescence and breakup was incorporated using the phase field variable method in which the Navier-Stokes and heat transfer equations are solved for each phase of the fluid. Results from the simulations confirmed the sensitivity of the bubble elongation and neck formation to the nucleation site geometry, specifically the angle along the rim where field enhancement occurs. The enhanced constriction of the bubble neck resulted in early detachment of bubbles when compared to simulations in which EHD was not applied. This finding provides some insight into the higher bubble departure frequency and nucleation site density observed in the experiment. The results from the combined experimental and numerical study suggest that EHD enhancement may provide a mechanism for extending the use of nucleate heat transfer to minichannels, thereby enabling additional options for cooling in compact, embedded systems. "

Gas quench technology has been rapidly developed recently with the intent to replace water and oil quench for medium and high hardenability steel. One of the significant advantages is to reduce the distortion and stress, compared to water and oil quench. However, not like liquid quench, no gas quench steel hardenability test standard exists. The fundamental difference between liquid quench and gas quench is heat transfer coefficient. The workpiece with the same hardness after liquid and gas quench process may have different microstructure due to different cooling curves. The concept of equivalent gas quench heat transfer coefficient (HTC) is proposed to have the same cooling curve, microstructure and hardness when compared with liquid quench. Several influencing factors on steel hardenability have been discussed, such as austenizing temperature, heating rate, holding time, composition variation and grain size difference. The phase quantification by X-ray Diffraction and Rietveld Refinement method is developed to measure phase percentage for steel microstructure, including martensite, ferrite and carbides. The limitations and improvements of modified Jominy gas quench test are discussed. The fundamental limitation of Jominy gas quench test is that one gas quench condition cannot be used for both low hardenability steel and high hardenability steel at the same time. The same steel grade would have different hardenability curves under different gas quench conditions, which made it difficult to compare the hardenability among different steels. The critical HTC test based on Grossmann test is proposed to overcome the limitations. In the test, different gas quench HTC conditions are applied to the sample with the same geometry. After sectioning each bar at mid-length, the bar that has 50% martensite at its center is selected, and the applied gas quench HTC of this bar is designated as the critical HTC. This test has many advantages to take the place of modified Jominy gas quench test. Since one of the advantages of gas quench is greater process flexibility to vary cooling rates, gas marquenching technology is proposed to obtain martensite with less sever cooling rate and reduce the distortion and stress.

It is a well-known phenomenon that during Marangoni condensation of binary mixtures, a small concentration of more volatile constituent with smaller surface tension gives significant heat transfer enhancements. This is due to surface tension gradients causing instability in condensate film, resulting in a pseudo-dropwise mode of condensation which resembles closely to dropwise condensation of pure fluid on the hydrophobic surface, consequently, the film gets thinner with lower thermal resistance across the condensate film and thus higher heat transfer coefficient is achieved. Marangoni condensation of steam-ethanol mixtures has been widely investigated in the past. However, Marangoni condensation of self-rewetting fluids e.g. steam-butanol is yet to be investigated where the constituent in a small concentration is a less volatile component. Marangoni condensation of steam-ethanol, steam-butanol and steam-propanol mixtures has been investigated on a horizontal smooth tube at an atmospheric pressure. For all experiments, concentrations by mass in the boiler feed when cold prior to start of the experiment were 0.001%, 0.005%, 0.01%, 0.025%, 0.05%, 0.1%, 0.5% and 1.0%. The coolant temperature rise was measured accurately with a ten-junction thermopile. Tube wall temperature was measured using four thermocouples embedded in the test tube wall. Effects of pressure and vapour velocity over a wide range of vapour-to-surface temperature difference have been investigated. Care was taken to avoid error due to the presence of air in the vapour. Marangoni condensation of steam-butanol and steam propanol mixtures show significant heat transfer enhancements compared with that of steam-ethanol mixtures. Higher Heat flux and heat-transfer coefficients were observed. For the steam-ethanol mixtures, enhancement ratio (heat flux or heat-transfer coefficient divided by the corresponding value for pure steam condensation on a horizontal smooth tube for the same vapour-to-surface temperature difference and vapour velocity) of 5.5 was found at an ethanol concentration of 0.01%. For steam-butanol mixtures, the maximum enhancement ratio was found to be 11 at a concentration of 0.005% and 0.01%. For steam-propanol mixtures, the maximum enhancement ratio of 8.5 was found at the same mass concentrations as steam-butanol mixtures. Enhancement ratio was generally higher at lower ethanol concentrations, increases at first with increasing vapour-to-surface temperature difference and subsequently decreases at high vapour-to-surface temperature difference. Finally, a semi-empirical model was proposed to predict the Marangoni condensation of steam-ethanol mixtures based on the vapour phase diffusion theory of Sparrow and Marchall (1969) and pure steam dropwise theory of Rose (2002).

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Spray cooling is a promising technique which is used to remove large amounts of heat from surfaces. It is characterized by uniform heat removal, low droplet impact velocity and better cooling efficiency when compared to other cooling schemes. It can be used in electronic cooling, and other applications. However, due to the multiple impacts of droplets, the film fluid dynamics and morphology are quite complicated. Moreover, the effect of heat transfer under spray cooling is not well understood due to the large number of interdependent variables such as impact spacing, impact angle, droplet diameter, droplet velocity and droplet frequency to name a few. An experimental approach is proposed and used to minimize and control key independent variables to determine their effects on surface temperature and heat transfer cooling mode. The effects of droplet impact angle and spacing on different heat flux conditions are studied. The film thickness is also obtained to further investigate the relationship between the independent variable and the observed heat transfer mechanism. The study of coherent droplet impingement on an open surface is experimentally characterized using high speed imaging and infrared thermography. Single stream droplet impingent cooling with different impact angle is also studied. Temperature distribution and impact crater morphology are obtained under different heat flux conditions. Film thickness inside droplet impact craters is measured to understand the relationship between minimum surface temperature and film thickness. Next, double streams droplet impingement cooling with different spacings and impact angles are investigated. The optimum spacing is found to reduce the droplet-to-droplet collision and to minimize splashing, resulting in enhanced heat transfer and better use of the cooling fluid. The film thickness is also measured to understand the relationship between the heat transfer results and the controllable independent variables. The results and conclusions of this study are useful in understanding the physics of spray cooling and can be applied to design better spray cooling systems.

SuperCritical-Water-cooled Reactors (SCWRs) are being developed as one of the Generation-IV nuclear-reactor concepts. Main objectives of the development are to increase thermal efficiency of a Nuclear Power Plant (NPP) and to decrease capital and operational costs. The first objective can be achieved by introducing nuclear steam reheat inside a reactor and utilizing regenerative feedwater heaters. The second objective can be achieved by designing a steam cycle that closely matches that of the mature supercritical fossil-fuelled power plants. The feasibility of these objectives is discussed. As a part of this discussion, heat-transfer calculations have been performed and analyzed for SuperCritical-Water (SCW) and SuperHeated-Steam (SHS) channels of the proposed reactor concept. In the calculations a uniform and three non-uniform Axial Heat Flux Profiles (AHFPs) were considered for six different fuels (UO2, ThO2, MOX, UC2, UC, and UN) and at average and maximum channel power. Bulk-fluid, sheath, and fuel centerline temperatures as well as the Heat Transfer Coefficient (HTC) profiles were obtained along the fuel-channel length. The HTC values are within a range of 4.7 – 20 kW/m2⋅K and 9.7 – 10 kW/m2⋅K for the SCW and SHS channels respectively. The main conclusion is that while all the mentioned fuels may be used for the SHS channel, only UC2, UC, or UN are suitable for a SCW channel, because their fuel centerline temperatures are at least 1000°C below melting point, while that of UO2, ThO2, and MOX may reach melting point. / UOIT

Current CANDU-type nuclear reactors use a once-through fuel-channel with an annulus gas insulating it from the moderator. The current reference design for a CANDU-type SuperCritical Water-Cooled Reactor (SCWR) is to eliminate the annulus gap and use a ceramic insert to insulate the coolant from the moderator. While such a design may work, alternative fuel-channel design concepts are under development to explore the optimum efficiency of SCWRs. One such alternative approach is called the Re-Entrant fuel-channel. The Re-Entrant fuel-channel consists of three tubes, the inner tube (flow tube), pressure tube and an outer tube. The fuel bundles are placed in the inner tube. An annulus is formed between the flow and pressure tubes, through which the primary coolant flows. A ceramic insulator is placed between the pressure tube and the outer tube. The coolant flows through the annulus receiving heat from the inner tube from one end of the channel to another. At the far end, the flow will reverse direction and enter the inner tube, and hence the fuel-string. At the inlet, the temperature is 350°C for a high-pressure coolant (pressure of 25 MPa), which is just below the pseudocritical point. At the outlet, the temperature is about 625ºC at the same pressure (the pressure drop is small and can be neglected). The objective of this work was to design the Re-Entrant channel and to estimate the heat loss to the moderator for the proposed new fuel-channel design. A numerical model was developed and MATLAB was used to calculate the heat loss from the insulated Re-Entrant fuel-channel along with the temperature profiles and the heat transfer coefficients for a given set of flow, pressure, temperature and power boundary conditions. Thermophysical properties were obtained from NIST REFPROP software. With the results from the numerical model, the design of the Re-Entrant fuelchannel was optimized to improve its efficiency / UOIT

Chosen as one of six Generation‒IV nuclear-reactor concepts, SuperCritical Water-cooled Reactors (SCWRs) are expected to have high thermal efficiencies within the range of 45 ‒ 50% owing to the reactor‟s high pressures and outlet temperatures. The behaviour of supercritical water however, is not well understood and most of the methods available to predict the effects of the heat transfer phenomena within the pseudocritical region are based on empirical one-directional correlations which do not capture the multi-dimensional effects and do not provide accurate results in regions such as the deteriorated heat transfer regime. Computational Fluid Dynamics (CFD) is a numerical approach to model fluids in multidimensional space using the Navier-Stokes equations and databases of fluid properties to arrive at a full simulation of a fluid dynamics and heat transfer system. In this work, the CFD code, FLUENT-12, is used with associated software such as Gambit and NIST REFPROP to predict the Heat Transfer Coefficients at the wall and corresponding wall temperature profiles inside vertical bare tubes with SuperCritical Water (SCW) as the cooling medium. The numerical results are compared with experimental data and 1-D models represented by existing empirical correlations. Analysis of the individual heat-transfer regimes is conducted using an axisymmetric 2-D model of tubes of various lengths and composed of different nodes count along the heated length. Wall temperatures and heat transfer coefficients were analyzed to select the best model for each region (below, at and above the pseudocritical region). To neutralize effects of the rest of the tube on that region, smaller meshes were used were possible. Two turbulent models were used in the process: k-ε and k-ω, with many variations in the sub-model parameters such as viscous heating, thermal effects, and low-Reynolds number correction. Results of the analysis show a fit of ±10% for the wall temperatures using the SST k-ω model in the deteriorated heat transfer regime and less than ±5% for the normal heat transfer regime. The accuracy of the model is higher than any empirical correlation tested in the mentioned regimes, and provides additional information about the multidimensional effects between the bulk-fluid and wall temperatures. Despite the improved prediction capability, the numerical solutions indicate that further work is necessary. Each region has a different numerical model and the CFD code cannot cover the entire range in one comprehensive model. Additionally, some of the trends and transitions predicted are difficult to accept as representation of the true physics of SCW flow conditions. While CFD can be used to develop preliminary design solutions for SCW type reactors, a significant effort in experimental work to measure the actual phenomena is important to make further advancements in CFD based analysis of SCW fluid behaviour. / UOIT