In gas quenching, metal parts are rapidly cooled from hightemperatures, and the convection heat transfer coefficientdistributions are of importance for the hardness and thedistortion (the shape nonuniformities) of the quenched parts.Thermochromic liquid crystals (TLC) and a thin foil techniques,were investi- gated and used for studies of a circular cylinderin axial flows, affected and not affected by upstream owmodifying inserts. Quadratic prisms in cross ows were alsostudied, a single prism, two prisms arranged in-line, and forfour prisms arranged in a square pattern. In this study,particle image velocime- try (PIV) was used for visualizationof the flow, giving physical insight to the convection heattransfer data. Further, relations of the typeNu=CReewere established. The TLC and thin foil techniques werealso used to indicate the dimensions of separated flowregions. Descriptors:Fluid mechanics, wind-tunnel, turbulence,gas quenching, con- vection heat transfer, thermochromic liquidcrystals, calibration, temperature measurement errors, thinfoils, particle image velocimetry, cylinder in axial flow, flowmodifying inserts, quadratic prisms in cross flow

Falling film evaporators that operate in the nucleate boiling regime in the refrigeration industry offer a number of advantages over their flooded counterparts such as lower refrigerant charge and at times improved heat transfer. Existing literature has not characterised the influence of surface characteristics on the falling film boiling process, and they are poorly understood for the pool boiling process. The purpose of this study was therefore to experimentally measure the influence of roughness, material and nanostructures on the heat transfer of falling film boiling and pool boiling of saturated refrigerants on the outside of horizontal tubes. The critical heat flux point was measured if it occurred, and the falling film heat transfer enhancement ratio, critical dryout threshold and general dryout characteristics were investigated in the study. The tubes tested consisted of plain copper, stainless steel and mild steel tubes that were polished and roughened with various grades of sandpaper. Furthermore, three types of nanostructured surfaces were applied to polished copper tubes, namely a layer-by-layer (LbL) coating of silica nanoparticles, a copper oxide (CuO) nanostructure coating and a commercial nanocoating process termed nanoFLUX. The nanoFLUX tube had the highest heat transfer coefficients of tubes tested under both pool boiling and falling film conditions, with between 40 and 200% higher heat transfer coefficients than those of a polished copper tube. The nanoFLUX surface outperformed the other surfaces due to a combination of rougher microstructure and a unique heat transfer mechanism, possibly linked to capillary wicking of liquid inside the nanochannels of the porous coating. The falling film heat transfer enhancement ratio was found to increases as surface roughness was increased on plain tubes, suggested to be as a result of enhanced microlayer evaporation from the trapped sliding bubbles in the thin flowing film. The nanoFLUX and CuO surfaces experienced lower critical heat flux as a result of departure from nucleate boiling under pool boiling and falling film boiling conditions compared with plain surfaces.. However, the nanoFLUX and CuO tubes performed well in terms of critical dryout at lower heat fluxes. The wicking capabilities of the nanoFLUX and CuO surfaces were thought to be the cause of their improved dryout capabilities at lower heat fluxes, but increased heat fluxes possibly led to dryout of the nanostructures resulting in operation in the Cassie-Baxter state and subsequent reduced wettability. / Thesis (PhD)--University of Pretoria, 2020. / Mechanical and Aeronautical Engineering / PhD (Mechanical Engineering) / Unrestricted

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The current trend in microelectronics is to manufacture devices with increased computational powers and reduced size. These devices with increased power densities are consequently subject to extreme thermal loads. Thermal management of these power loads is extremely challenging. The presence of the hotspots can make this challenge even more difficult. Jet impingement cooling is one of the top candidates for removing such extreme heat fluxes in microelectronics. Jet impingement cooling can achieve heat transfer coefficients (HTCs) due to its normal incident flow-field and ability to thin the local thermal boundary layer in the stagnation region. This dissertation presents the hotspot cooling performance for a confined jet impingement cooling configuration. This dissertation is divided into two parts. The first part presents the experimental data attained for single-phase water jet impingement cooling. Also investigated is the spatial dependence of the HTC relative to the offset between the jet/wall stagnation point and the center of the local hotspot. A theoretical model to predict the HTC as a function of jet-to-hotspot offset ratio and heating frequency is also derived. The second part presents hotspot cooling performance for the two-phase confined jet cooling performance. Electrically non-conductive fluids such as Novec 7100, Novec 7200, FC 72, and Ethanol were used as coolants for this part of the study. This study investigates the nucleate boiling regime as a function of the Reynolds number/Jet Velocity for these fluids. Additionally, this dissertation also presents the nucleate boiling regime as a function of the distance between the hotspot center and the jet stagnation point. Finally, a stagnation zone CHF prediction model is derived. Findings from this research will help thermal control engineers write active cooling algorithms to maintain the desired temperature at minimal pumping cost. This research will also help thermal designers to select appropriate coolants and design the device.

The present study experimentally investigates the heat transfer capability of supercritical carbon dioxide (sCO2) single-jet impingement. The evaluated jet Reynolds number range is between 80,000 and 1,000,000, with a non-dimensional jet-to-target surface spacing of 2.8. CO2-impinging jet stagnation conditions were maintained at approximately 20 MPa and a temperature of 673 K for most experiments. The goal is to understand how changes in the aforementioned parameters influence heat transfer between the working fluid and the heated surface. Additionally, due to the elevated Reynolds numbers and difference in thermodynamic properties between air and CO2, air-derived impingement correlations may not be appropriate for CO2 impingement; these correlations will be evaluated against experimental sCO2 impingement data. At the time of this study, no sCO2 impingement data was available relevant to sCO2 power cycles. The target surface is a 1.5-inch diameter copper block centered on the 3mm jet orifice. A mica heating element bolted to the bottom of the copper block provides a uniform heat flux. Thermocouples embedded in the copper block are used to determine the surface temperature. Nusselt numbers obtained from experimental sCO2 data are compared to area-averaged Nusselt numbers from air-derived correlations. The comparisons showed that air correlations drastically underpredict the heat transfer when sCO2 is used as the working fluid. A modified sCO2 correlation using experimental data at discussed conditions is derived based on an existing air correlation.

The continuous growth and demand of increased operating performances of electronics has brought about an increase in the chip power density posing threat to the thermal management of these devices. Although numerous thermal solutions ranging from passive to active cooling together with a variety of working fluids have been adopted, however, the question whether these available cooling methods could meet up with the ever-growing need for increased operating performances is a concerning one. Jet impingement cooling has been effectively used in many industrial applications due to its high heat transfer capability. The limited study at the micro scale suggests that it exhibits excellent heat transfer performance relative to conventional parallel flow in microchannels. Recently, Carbon dioxide in its supercritical state (304 K and 7.3 MPa) has been proven to be an excellent working fluid in dissipating high heat fluxes. Owing to the properties of this fluid (sCO2) and its high specific heat near the pseudocritical point, the heat transfer rate can be enhanced significantly compared traditional working fluids. However, knowledge about the heat transfer characteristics of micro jet impingement with Carbon dioxide in this state are lacking. In addition, flow boiling has been recognized to significantly enhance heat transfer rate due to its large thermal capacity giving an opportunity to further enhance the cooling ability of Carbon dioxide. In line of this continuous innovation the flow and the heat transfer characteristic of micro jet impingement with CO2 in both single-phase and two-phase were experimentally studied. A micro fluidic device was manufactured leveraging MEMS techniques. The micro device included a circular serpentine heater of diameter 2.01 mm and three resistance temperature detectors (RTDs) sputtered on a glass substrate made of fused silica, providing heating and temperature measurements, respectively. The heater, RTDs and their vias were sputtered with the calculated lengths, widths and thicknesses to achieve the desired resistances. The RTDs were arranged on the heater in a concentric manner to measure the average radial temperature distribution as the flow was assumed to be symmetric. The effects of the working fluid was investigated under governing parameters such as radial position, heat flux, mass flow rate, inlet temperature and inlet pressure. Results from the single-phase investigation showed a higher sensitivity of the heat transfer rate to the proximity to the pseudocritical temperature of the fluid with the optimum heat transfer rate recorded around the pseudocritical temperature subject to the increased specific heat around this region. By utilizing the flow boiling process, a further enhancement was observed pre the critical heat flux condition, suggesting a need to operating within the nucleate boiling region in its industrial adoption. It was recorded that the single jet performed better than the multi jet as a result of the interjet spacing which governs the effect of the colliding jets. Finally, several correlations with minimal mean absolute errors were introduced due to discrepancies from literatures.

With the continuous miniaturization of integrated circuit chips over the last decade, there has been a steady increase in power density of electronic devices giving rise to the need for aggressive and effective cooling systems. The rapid growth of microfabrication technology has led to the development of microelectromechanical system (MEMS) based microchannels, which can be used as miniaturized heat exchangers capable of cooling high power electronic devices. Simultaneously, studies show that carbon dioxide in supercritical state (sCO2) has an excellent ability for cooling applications due to its exceptional thermophysical properties near critical point. Implementation of pin fins in heat transfer systems requires a thorough understanding of both fluid dynamics and heat transfer mechanisms. Thus, the aim of this research is to extend the current fundamental knowledge about both thermal and hydraulic performance of supercritical carbon dioxide (sCO2) in microchannel pin fin heat sinks. Microchannel devices with micro pin fin heat sinks and sCO2 have been designed and built in three generations. The devices were microfabricated at the Cornell Nanoscale Facility (CNF). First generation is microchannel array heat sinks, followed by circular pin fin heat sinks and airfoil pin fin heat sinks, as second and third generation devices, respectively. Heat transfer characterization including heat transfer coefficient, Nusselt number, and pressure drop was performed on the devices. In microchannel array devices (first generation) the capability of sCO2 and the effect of height to width ratio (H/W) on thermal performance of the microchannel heat sink was investigated. In circular micro pin fin heat sink device (second generation), a parametric study for the effects of fin height over diameter, and inline and staggered arrangement on heat transfer coefficient and pressure drop of the system were conducted. Heat transfer experiments were performed on airfoil pin fin heat sink devices, as well. In general, the heat transfer coefficient characteristics revealed the incredibly high values of heat transfer in such systems. Comparing thermal performance of the three generations, heat transfer coefficient obtained by the circular micro pin fin heat sink is higher than microchannel array heat sinks due to higher active surface area and better flow mixing creating turbulent flow. Micro pin fin configuration also prevents relaminarization followed by flow acceleration to be aggressively dominated in the entire micro channel. However, pressure drop in the microchannel array is less than that of circular micro pin fin heat sinks. It was observed that thermal performance of airfoil micro pin fin heat sinks is better than that of two microchannel array and circular pin fin heat sinks. Among airfoil micro heat sinks, staggered arrangement has higher thermal performance than inline arrangement. It is concluded that as airfoil thickness increases, thermal performance of the heat sink increases, but comprehensive performance of the heat sink decreases. It was also found that existing conventional scale correlations and flow maps did not predict well the corresponding characteristics in micro-scale systems, and thus, new correlations have been developed.

The main objective of this study is to fundamentally investigate the flow physics and the relationship to heat transfer in the presence of roughened surfaces undergoing jet impingement cooling mechanisms in a confined channel. Thermal and fluid dynamics characteristics are directly related to the surface condition. The surface roughness is proven to play a significant role in the surface heat transfer and skin friction coefficient. In combination with the surface finish, the flow condition plays a particular role in the turbulence behavior near the wall. The magnitude of these engineering quantities tends to deviate from a smooth surface compared to a rough surface scenario. The development of accurate lower and high-fidelity models is essential in the engineering world. Predicting the heat transfer and fluid mechanics behavior inside a component is essential for a designer, such as improving wall functions within the CFD community. Usually, literature only includes well-defined rough surfaces driven by some geometric parameters, non-uniform and irregular surfaces like the one found in additive manufacturing and other physics forming phenomena is somewhat lacking. The basic geometric configuration of a single jet impingement was chosen due to the ability to create a wall jet from a stagnation region. The experimental facility was designed under no crossflow configuration, where fluid enters from the plenum passing through the orifice hole (jet) and exiting in the radial direction of a confined channel. The current research investigated the fluid dynamics associated with jet impingement over rough surfaces using non-intrusive experimental methods. Multiple jet Reynolds numbers were investigated, ranging from 21,000 to 110,000 for three different jet diameter to roughness ratios. The current research study investigated heat transfer and fluid behavior using non-intrusive experimental methods. Temperature-sensitive paint (TSP) was utilized to obtain scalar temperature field over smooth and rough surfaces. These experimental results will be compared with available literature. The flow physics was investigated by performing stereoscopic Particle Image Velocimetry. The velocity fields were further analyzed using Proper Orthogonal Decomposition (POD) and tested versus the wall similarity theory. High accuracy microphones were utilized to obtain unsteady pressure values at different rough surfaces.

Recent advancements in material technology have led to the development of non-porous negative Poisson's ratio (NPR) materials (also called auxetic structures) by making spherical inline dimples on both sides of an elastic sheet. Manufacturing technologies such as 3-D printing and additive manufacturing paved the way to realize the complex shapes needed to achieve NPR behavior. These materials are desirable in many engineering applications, especially in the gas turbines hot-gas-path, due to their unique properties. In the current study, an effort is made to understand the flow physics and surface heat transfer mechanism for channel flow having one wall with spherical dimples and protrusions. An equivalent geometry, with dimples on both sides of the flat sheet with density beyond a certain threshold, would have NPR characteristics. Furthermore, dimples and protrusions are also studied in isolation to understand what key differences are brought by the combination of these two. Flow field measurements, in-and-around these features, are done using the stereoscopic PIV. The transient TLC method is used for local surface heat transfer measurement. Numerical simulations, steady RANS, URANS, and LES, are used in conjunction with experimental data to develop a detailed understanding of the flow field and surface heat transfer. A comparison of experimental measurements and numerical simulations identifies the areas for improvement in numerical modeling of the flow field and surface heat transfer in the presence of such geometries. The friction factor measurement along with heat transfer is used to characterize the thermal performance factor (TPF) of each geometry for a range of Reynolds number. The novelty of this work is the inline arrangement of features and first-of-its-kind PIV measurement for dimples-protrusions. The understanding developed from this work can easily be utilized for designing components involving NPR materials with dimples-protrusions.

Over the years methane and natural gas (NG) as a fuel have become an important source of energy in the power generation and transportation sectors, with the latter most prominently adopting propane used in public transportation shuttles. Notable interest in using liquid methane or NG fueled rocket engines has recently gained traction and are currently in development/production at SpaceX (raptor engine, full-flow staged combustion cycle) and Blue Origin (BE-4, staged combustion cycle). Given the variety of applications, sourcing and refinement of methane/NG may be completely different to ensure a certain purity is met, especially in an advanced rocket engine cycle like the raptor engine, that may be highly sensitive to minor fuel variances. The current study seeks to understand the ignition behavior of methane (with and without CO2 dilution) and NG surrogate mixtures, including CH4/C2H6/C3H8 and CH4/C2H6/C3H8/i-C4H10/n-C4H10, using a high-pressure shock tube facility at reflected shock conditions relevant to advanced rocket engines and gas turbine cycles. This included pressures near 16, 100, and 200 bar throughout a temperature range of 1000-1621 K. Ignition delay time data were compared with predictions of a chemical kinetic mechanism, and along with performed sensitivity and pathway analyses, further aided in the understanding of the observed ignition behavior. The combined effects of lower activation energy, increased concentration of OH and CH radicals, and increased heat of combustion all play some role in the observed promotion of ignition. These combined effects are primarily a function of fuel mixture and concentration, and combustion relevant pressure and temperature conditions (reflected shock conditions T5 and P5), which in addition dictate the propensity of preignition and deflagration-to-detonation transition behavior observed in the shock tube.