abstract: The heat transfer enhancements available from expanding the cross-section of a boiling microchannel are explored analytically and experimentally. Evaluation of the literature on critical heat flux in flow boiling and associated pressure drop behavior is presented with predictive critical heat flux (CHF) and pressure drop correlations. An optimum channel configuration allowing maximum CHF while reducing pressure drop is sought. A perturbation of the channel diameter is employed to examine CHF and pressure drop relationships from the literature with the aim of identifying those adequately general and suitable for use in a scenario with an expanding channel. Several CHF criteria are identified which predict an optimizable channel expansion, though many do not. Pressure drop relationships admit improvement with expansion, and no optimum presents itself. The relevant physical phenomena surrounding flow boiling pressure drop are considered, and a balance of dimensionless numbers is presented that may be of qualitative use. The design, fabrication, inspection, and experimental evaluation of four copper microchannel arrays of different channel expansion rates with R-134a refrigerant is presented. Optimum rates of expansion which maximize the critical heat flux are considered at multiple flow rates, and experimental results are presented demonstrating optima. The effect of expansion on the boiling number is considered, and experiments demonstrate that expansion produces a notable increase in the boiling number in the region explored, though no optima are observed. Significant decrease in the pressure drop across the evaporator is observed with the expanding channels, and no optima appear. Discussion of the significance of this finding is presented, along with possible avenues for future work. / Dissertation/Thesis / Ph.D. Mechanical Engineering 2013

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Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Melanie M. Derby / Power plants are significant water users, accounting for 15% of water withdrawals worldwide. To reduce water usage, compact condensers are required to enable air-cooled condensers and reduce infrastructure costs. Steam flow condensation was studied in 0.952-mm and 1.82-mm hydraulic diameter mini-gaps in an open loop experimental apparatus. The apparatus was validated with single-phase flow. Flow condensation experiments were conducted for a wide range of steam mass fluxes (i.e., 35–100 kg/m²s) and qualities (i.e., 0.2–0.9) in hydrophilic copper and hydrophobic Teflon-coated channels. Water contact angles were 70° and 110° on copper and Teflon, respectively, and in general, filmwise condensation was the primary condensation mode in the hydrophilic channel and dropwise condensation was the primary mode observed in the hydrophobic channel. Pressure drops were reduced by 50–80% in the hydrophobic channels. Condensation heat transfer was enhanced by 200–350% in hydrophobic mini-gaps over hydrophilic mini-gap due to dropwise condensation. Droplet dynamics (e.g., nucleation, coalescence and departure) were quantified during dropwise condensation. A model was created which includes droplet adhesion and drag forces for droplet departure diameters which were then correlated to heat transfer coefficients. An overall mean absolute error of 9.6% was achieved without curve fitting. Noncondensable gases can reduce heat transfer in industrial systems, such as power plants due to the additional layer of thermal resistance from the gas. Condensing steam-nitrogen experiments were conducted for nitrogen mass fractions of 0–30%; the addition of nitrogen reduced heat transfer coefficients by up to 59% and 30% in hydrophilic and hydrophobic mini-gaps, respectively. It was found that during dropwise condensation, the noncondensable layer was perturbed by cyclical droplet motion, and therefore heat transfer coefficients were increased by 2–5 times compared with filmwise condensation of the same mass fraction of nitrogen.

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Conjugate heat transfer is a crucial issue in a number of turbulent engineering fluidflow applications, particularly in nuclear engineering and heat exchanger equipment. Temperature fluctuations in the near-wall turbulent fluid lead to similar fluctuationsin the temperature of the solid wall, and these fluctuations in the solid cause thermalstress in the material which may lead to fatigue and finally damage. In the present study, the Reynolds Average Navier-Stokes (RANS) modelling approachhas been adopted, with four equation k−ε−θ2−εθ eddy viscosity based modelsemployed to account for the turbulence in the fluid region. Transport equations forthe mean temperature, temperature variance, θ2, and its dissipation rate, εθ, have beensimultaneously solved across the solid region, with suitable matching conditions forthe thermal fields at the fluid/solid interface. The study has started by examining the case of fully developed channel flow withheat transfer through a thick wall, for which Tiselj et al. [2001b] provide DNS dataat a range of thermal activity ratios (essentially a ratio of the fluid and solid thermalmaterial properties). Initial simulations were performed with the existing Hanjali´cet al. [1996] four-equation model, extended across the solid region as described above. However, this model was found not to produce the correct sensitivity to thermal activityratio of the near wall θ2 values in the fluid, or the decay rate of θ2 across the solid wall. Therefore, a number of model refinements are proposed in order to improve predictionsin both fluid and solid regions over a range of thermal activity ratios. These refinementsare based on elements from a three-equation non-linear EVM designed to bring aboutbetter profiles of the variables k, ε, θ2 and εθ near the wall , and their inclusion is shownto produce a good matching with the DNS data of Tiselj et al. [2001b].Thereafter, a further, more complex test case has been investigated, namely an opposedwall jet flow, in which a hot wall jet flows vertically downward into an ascendingcold flow. As in the channel flow case, the thermal field is also solved across the solidwalls. The modified model results are compared with results from the Hanjali´c modeland LES and experimental data of Addad et al. [2004] and He et al. [2002] respectively. In this test case, the modified model presents generally good agreement with the LESand experimental data in the dynamic flow field, particularly the penetration point ofthe jet flow. In the thermal field, the modified model also shows improvements in the θ2predictions, particularly in the decay of the θ2 across the wall, which is consistent withthe behaviour found in the simple channel flow case. Although the modified model hasshown significant improvements in the conjugate heat transfer predictions, in some instancesit was difficult to obtain fully-converged steady state numerical results. Thusthe particular investigation with the inlet jet location shows non-convergence numericalresults in this steady state assumption. Thus, unsteady flow calculations have beenperformed for this case. These show large scale unsteadiness in the jet penetration area. In the dynamic field, the total rms values of the modelled and mean fluctuations showgood agreement with the LES data. In the thermal field calculation, a range of the flowconditions and solid material properties have been considered, and the predicted conjugateheat transfer predicted performance is broadly in line with the behaviour shownin the channel flow.

The laminar and turbulent flow regimes have been extensively investigated from as early as 1883, and research has been devoted to the transitional flow regime since the 1990s. However, there are several gaps in the mixed convection literature, especially when the flow is still developing. The purpose of the study was to experimentally investigate the heat transfer and pressure drop characteristics of developing and fully developed flow of low Prandtl number fluids in smooth horizontal tubes for forced and mixed convection conditions. An experimental set-up was designed and built, and results were validated against literature. Two smooth circular test sections with inner diameters of 4 mm and 11.5 mm were used, and the maximum length-to-diameter ratios were 1 373 and 872 respectively. Heat transfer measurements were taken at Reynolds numbers between 500 and 10 000 at different constant heat fluxes. A total of 648 mass flow rate measurements, 70 301 temperature measurements and 2 536 pressure drop measurements were taken. Water was used as the test fluid and the Prandtl number ranged between 3 and 7. It was found that a longer thermal entrance length was required for simultaneously hydrodynamically and thermally developing flow. Therefore, a coefficient of at least 0.12 (and not 0.05 as advised in most literature) was suggested. Because free convection effects decreased the thermal entrance length, correlations were also developed to calculate the thermal entrance length for mixed convection conditions. The boundaries between the flow regimes were defined mathematically, and terminology to define transitional flow characteristics was presented. For laminar flow, three different regions (forced convection developing, mixed convection developing and fully developed) were identified in the local heat transfer results and nomenclature and correlations were developed to define and quantify the boundaries of these regions. Correlations were also developed to calculate the local and average laminar Nusselt numbers of mixed convection developing flow. The laminar-turbulent transition along the tube length occurred faster with increasing Reynolds number, and was also influenced by free convection effects. As free convection effects became significant, the effect was first to disrupt the fluctuations inside the test section, leading to a slower laminar-turbulent transition along the tube length compared with forced convection conditions. However, as free convection effects were increased, the fluctuations inside the test section increased and caused the laminar-turbulent transition along the tube length to occur faster. The Reynolds number at which transition started was found to be independent of axial position for both developing and fully developed flow. However, the end of transition occurred earlier as the flow approached fully developed flow. When the flow was fully developed, the end of transition became independent of axial position. Furthermore, free convection effects affected both the start and end of the transitional flow regime, and caused the Reynolds number range of the transitional flow regime to decrease. Correlations were therefore developed to determine the start and end of the transitional flow regime for developing and fully developed flow in mixed convection conditions. The transitional flow regime across the tube length was divided into three regions. In the first region, the width of the transitional flow regime decreased significantly with axial position as the thermal boundary layer thickness increased, and free convection effects were negligible. In Region 2, the width of the transitional flow regime decreased with axial position, due to the development of the thermal boundary layer, as well as with increasing free convection effects. In the fully developed region (Region 3), the width of the transitional flow regime was independent of axial position, but decreased significantly with increasing free convection effects. At high Grashof numbers, free convection effects even caused the transitional flow regime of fully developed flow to become negligible. It was found that the boundaries of the different flow regimes were the same for pressure drop and heat transfer, and a relationship between pressure drop and heat transfer existed in all four flow regimes. In the laminar flow regime, this relationship was a function of Grashof number (thus free convection effects), while it was a function of Reynolds number in the other three flow regimes. Correlations to predict the average Nusselt numbers, as well as the friction factors as a function of average Nusselt number, for developing and fully developed flow in all flow regimes were developed. Finally, flow regime maps were developed to predict the convection flow regime for developing and fully developed flow for a wide range of tube diameters and Prandtl numbers, and these flow regime maps were unique for four reasons. Firstly, they contained contour lines that showed the Nusselt number enhancements due to the free convection effects. Secondly, they were valid for a wide range of tube diameters and Prandtl numbers. Thirdly, the flow regime maps were developed as a function of temperature difference (Grashof number) and heat flux (modified Grashof number). Finally, four of the six flow regime maps were not only valid for fully developed flow, but also for developing flow. / Thesis (PhD)--University of Pretoria, 2017. / NRF / TESP / Stellenbosch University/University of Pretoria / SANERI/SANEDI / CSIR / EEDSM Hub / NAC / Mechanical and Aeronautical Engineering / PhD / Unrestricted

Solar collector absorber tubes play a critical role in converting incident solar heat flux into absorbed thermal energy and transferring it to a heat transfer fluid. In this study a single horizontally orientated absorber tube was investigated numerically in terms of the influence of different circumferential spans of symmetrical and asymmetrical heat flux distributions on buoyancy-driven secondary flow, internal heat transfer and friction factor characteristics. Three types of circumferential heat flux boundaries were considered, namely fully uniform, partial uniform and sinusoidal non-uniform heat flux distributions. Both gravitational symmetry and asymmetry for non-uniform heat flux distributions were investigated to cover symmetry angles in terms of the gravitational field (g) of 0° (symmetrical case), 20°, 30°, 40° and 60°. Different sized stainless steel absorber tubes having a length of 10 m, and inner diameters of 62.7 mm, 52.5 mm, 40.9 mm and 35.1 mm were considered. Three dimensional steady-state simulations were performed for water as working fluid, covering laminar flow Reynolds numbers ranging from 130 to 2200, as well as for turbulent flow Reynolds numbers ranging from 3030 to 202 600. Buoyancy effects, temperature dependent fluid thermal properties, tube-wall heat conduction and the external wall heat losses by convection and radiation were taken into consideration. Average internal heat transfer coefficients, local internal heat transfer coefficients, Richardson numbers and overall friction factors were obtained for different angular spans of incident heat flux, inlet fluid temperatures, heat flux intensities and outer wall thermal conditions Laminar flow results indicated that the angular span, angular position, and intensity of the applied external heat flux all have significant influences on the buoyancy induced mixed convection inside the tube. This resulted in significant variations in the internal heat transfer coefficients and the friction factor which are not well described by classical empirical correlations. Buoyancy induced secondary flow significantly enhanced the internal heat transfer coefficient and significantly increased the friction factor compared to forced convection cases. Higher heat transfer coefficients and friction factors were obtained for non-uniform heat flux distributions compared to uniform heat flux distributions and were found to be dependent on the angle span and position of the heat flux. Higher inlet temperatures resulted in increased Nusselt numbers and lower friction factors, while higher external heat loss resulted in lower Nusselts numbers and lower friction factors. An increase in the asymmetry of the heat flux distribution resulted in a reduction of the Nusselt number and friction factor. Even though turbulent flow cases with a Reynolds number range of approximately 3000 to 9000 were also influenced by buoyancy driven secondary flow, and followed the same parameter trends, it occurred to a lesser extent compared to the laminar flow cases. Turbulent flow cases with Reynolds numbers higher than 9100, exhibited little dependence on secondary flow effects and indicates the suitability of classical fully uniform heat flux heat transfer and friction factor correlations for highly turbulent flow irrespective of the distribution or intensity of the heat flux. / Thesis (PhD)--University of Pretoria, 2017. / Advanced Engineering Centre of Excellence at the University of Pretoria, NRF, TESP, NAC, and SOLAR Hub with the Stellenbosch University, EEDSM Hub and CSIR is highly acknowledged and duly appreciated. / Mechanical and Aeronautical Engineering / PhD / Unrestricted

Thin-film silicon nitride (SiN) membranes mechanical resonators have been widely used for many fundamental opto-mechanical studies and sensing technologies due to their extremely low mechanical dissipation (high mechanical Q-factor). In this work, we experimentally demonstrate an opto-mechanical approach to perform thermal radiation sensing, using a SiN membrane resonator. An important aspect of this work is to develop a closed-form analytical heat transfer model for assessing the thermal coupling conditionbetween free-standing membranes and their environment. We also derive analytical expressions for other important intrinsic thermal quantities of the membrane, such as thethermal conductance, the heat capacity and the thermal time constant. Experimental results show good agreement with our theoretical prediction. Of central importance, we show that membranes of realistic dimensions can be coupled to their environment more strongly via radiation than by solid-state conduction. For example, membranes with 100nm thickness (frequently encountered size) are predicted to be radiation dominated when their side length exceeds 6 mm. Having radiation dominated thermal coupling is a key ingredient for reaching the fundamental detectivity limit of thermal detectors. Hence, our work proves that SiN membranes are attractive candidates for reaching the fundamental limit. We also experimentally exhibit the high temperature responsivity of the SiN membranes resonance, in which we shift a 88.7 KHz resonance by over 1 KHz when temperature increment on the membrane is approximately 2 K.

Multiscaled experimental investigations of heat transfer from nanoscales to macroscales are requisite to progress in energy technologies. In nuclear applications, material properties can undergo significant alteration due to destructive interaction with irradiating particles at microstructural levels that affect bulk properties. Correlating material microstructure to bulk material properties remains a crucial hurdle for obtaining first-principles-based, full-scale material property predictive capability. Ion-irradiated material studies provide valuable insight into material behavior under irradiation conditions that can be correlated to neutron irradiation effects. Through such studies, the need of costly (money and time) studies of neutron interaction with materials can be mitigated significantly. One of the challenges associated with studies of ion-irradiated materials is that the affected layer, or penetration depth, is typically very thin (~0.1-100μm for laboratory accelerators). Few investigations have been reported of ion-irradiation effects on thermal transport properties, in part, due to the challenge associated with measurements at the spatial scales of the zones of interest. This study expands the current knowledge base regarding thermal transport in ion-irradiated materials through the use of a multiscaled experimental approach using thermal wave methods. In a manner not previously explored, four thermal wave methods are used to characterize the proton-irradiated layer in ZrC including scanning thermal microscopy, spatial-scanning front-detection photothermal radiometry (PTR), lock-in IR thermography (lock-in IRT), and tomographic, frequency-based PTR. For the first time, the in-depth thermal conductivity profile of an ion-irradiated sample is measured directly. The profiles obtained by each of the spatial scanning methods are compared to each other and the numerical prediction of the ion-damage profile. The complementary nature of the various techniques validates the measured profile and the measured degradation of thermal conductivity in the ZrC sample showing the viability of such complementary studies.

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