Designing buildings to use energy more efficiently can lead to lower energy costs, while maintaining comfort for occupants. Computational fluid dynamics (CFD) can be utilized to visualize and simulate expected flows in buildings and structures. CFD gives architects and designers the ability to calculate the velocity, pressure, and heat transfer within a building. Previous research has not modeled natural ventilation situations that challenge common design rules of thumb used for cross-ventilation and single-sided ventilation. The current study uses a commercial code (FLUENT) to simulate cross-ventilation in simple structures and analyzes the flow patterns and heat transfer in the rooms. In the Casa Giuliana apartment and the Affleck house, this study simulates passive cooling in spaces well-designed for natural ventilation. Heat loads, human models, and electronics are included in the apartment to expand on prior research into natural ventilation in a full-scale building. Two different cases were simulated. The first had a volume flow rate similar to the ambient conditions, while the second had a much lower flow rate that had an ACH of 5, near the minimum recommended value Passive cooling in the Affleck house is simulated and has an unorthodox ventilation method; a window in the floor that opens to an exterior basement is opened along with windows and doors of the main floor to create a pressure difference. In the Affleck house, two different combinations of window and door openings are simulated to model different scenarios. Temperature contours, flow patterns, and the air changes per hour (ACH) are explored to analyze the ventilation of these structures. / Master of Science

No description available.

This work can be viewed in three separate sections, each of which build off of the prior. The first part of this study examined the flow in a 1/16th scale calandria test section based on a typical CANDU moderator layout. The experiments utilized forced flow supplied to the vessel and electrical heated rods to mimic the heat flow from calandria tubes. The size of the vessel, flow rates, and power levels were used to scale the experiments such that the provided representative temperature fields. The temperature field inside the vessel was measured and shown to compare well with CFD predictions over a wide range of inlet conditions and power levels. Additionally, this work addressed the scaling distortions in the experiment which occurred due to physical limitations when performing experiments at 1/16 scale (e.g., a smaller number of heater rods with a larger diameter were used in the experiment because at 1/16-scale direct fabrication of 390 fuel channel simulators is not feasible). The work proposed the H factor addition to the Ar. This additional scaling criteria was shown to better maintain the flow regimes expected CANDU moderators by taking into account distortions introduced by surface heating instead of volumetric heating in addition to the reduction in total number of tubes. While this work involved forced convective flows at the inlet of the vessel, in some regions of the calandria buoyancy induced forces were sufficiently high such that these phenomena altered the direction and magnitude of the flows as compared to purely forced convective behavior. Hence further work, discussed below, was initiated to better understand and measure these local phenomena where buoyancy forces are of similar magnitude as those of forced convection. Such local conditions we have terms mixed convection regime for the purposes of this thesis. The second part of this work further examined the mixed convection between a subset of the CANDU calandria tubes, namely how does a lower tube effect the mixed convection heat transfer of the upper tube in an inline arrangement. To isolate and measure the phenomena with sufficient detail, a small number of tubes was studied and advanced diagnostics such as Particle Image Velocimetry (PIV) and Laser Induced Fluorescence (LIF) were employed. This study combined fluid velocity, temperature and wall temperature measurements with CFD simulations to develop a mechanistic model and understanding of the effect of natural convection plumes from lower elevations on the natural circulation phenomena on an upper cylinder. Superposition of the natural convection phenomena combined with pseudo forced convection effects from the lower elevation cylinder’s plume was used to model the mixed convection phenomena. This model was shown to perform well, with nearly all data being predicted to with +-20% for experiments performed in this work, and experiments in literature. A major finding from the preceding discussion is the importance of the lower elevation plume velocity on the local phenomena on the upper cylinder. The third section further expanded upon the prior two by replacing the lower cylinder with a diffuser nozzle which could provide a forced convective component with accurately defined velocities. Such measurements allow for accurate definition of the local Ri number and allowed full access for instrumentation to observe the velocity fields. The major contribution of this work was a flow regime map that defined the phenomena around a heated cylinder under mixed convection conditions. Additionally, the establishment of a database of fluid temperature and velocity measurements for a wide range of Ri was also developed and used to further validate CFD predictions. / Thesis / Doctor of Philosophy (PhD)

yes / The coupling between natural convection and conduction within rectangular enclosure was investigated numerically. Three separate heat sources flush mounted on a vertical wall and an isoflux condition was applied at the back of heat sources. Continuity, momentum and energy conservation equations were solved by using control volume formulation and the coupling of velocity and pressure was treated by using the “SIMPLE” algorithm. The modified Rayleigh number and the substrate/fluid thermal conductivity ratio were used in the range 𝑹𝒂𝒍𝒛∗=𝟏𝟎^𝟒−𝟏𝟎^𝟕 and 𝑹𝒔=𝟏𝟎−𝟏𝟎𝟎𝟎 respectively. The investigation was extended to compare results of FC-77 with Air and also for high values of 𝑹𝒔>𝟏𝟎𝟎𝟎. The results illustrated that, when the modified Rayleigh number increases, dimensionless heat flux and local Nusselt number increases for both fluids. Opposite behaviour for the thermal spreading in the substrate and the dimensionless temperature 𝜽, they were decreased when 𝑹𝒂𝒍𝒛∗ is increased. Also with increasing the substrate/fluid thermal conductivity ratio for a given value of the modified Rayleigh number the thermal spreading in the substrate increased which is the reason of the decrease in the maximum temperature value. The present study concluded that, for high values of 𝑹𝒔>𝟏𝟓𝟎𝟎, the effect of the substrate is negligible.

No description available.

The Division for Electricity of Uppsala University is developing a wave power concept. The energy of the ocean waves is harvested with wave energy converters, consisting of one buoy and one linear generator. The units are connected in a submerged substation. The mechanical design is kept as simple as possible to ensure reliability. The submerged substation includes power electronics and different types of electrical power components. Due to the high cost of maintenance operations at sea, the reliability of electrical systems for offshore renewable energy is a major issue in the pursuit of making the electricity production economically viable. Therefore, proper thermal management is essential to avoid the components being damaged by excessive temperature increases. The chosen cooling strategy is fully passive, and includes no fans. It has been applied in the second substation prototype with curved heatsinks mounted on the inner wall of the pressurized vessel. This strategy has been evaluated with a thermal model for the completed substation. First of all, 3D-CFD models were implemented for selected components of the electrical conversion system. The results from these submodels were used to build a lumped parameter model at the system level. The comprehensive thermal study of the substation indicates that the rated power in the present configuration is around 170 kW. The critical components were identified. The transformers and the inverters are the limiting components for high DC-voltage and low DC-voltage respectively. The DC-voltage—an important parameter in the control strategy for the WEC—was shown to have the most significant effect on the temperature limitation. As power diodes are the first step of conversion, they are subject to large power fluctuations. Therefore, we studied thermal cycling for these components. The results indicated that the junction undergoes repeated temperature cycles, where the amplitude increased with the square root of the absorbed power. Finally, an array of generic heat sources was optimized. We designed an experimental setup to investigate conjugate natural convection on a vertical plate with flush-mounted heat sources. The influence of the heaters distribution was evaluated for different dissipated powers. Measurements were used for validation of a CFD model. We proposed optimal distributions for up to 36 heat sources. The cooling capacity was maximized while the used area was minimized.

Conventional heat transfer enhancement methods have focused on the surface characteristics of the heat-exchanger. The enhancement of heat transfer through altering the characteristics of the working fluid has become a new subject of interest. Micro-encapsulated phase change material (MEPCM) slurries show improved heat transfer abilities compared to single phase heat transfer fluids such as water due to their higher specific heat values in their phase change temperature range. The present work is a numerical and experimental study towards fundamental understanding of the impact of using PCM on thermal and fluid flow characteristics of different single-phase and two-phase heat transfer applications. The mathematical formulation to represent the presence of single and multi-component MEPCM is developed and incorporated into the numerical model for single-phase and two-phase fluid flow systems. In particular, the use of PCM in its encapsulated form for heat transfer enhancement of liquid flow in the presence of evaporation is explored. In addition, an experimental study is conducted to validate the numerical model in a setting of natural convection flow. Finally, the application of PCM in its layered form on the effectiveness of drying of moist porous materials (e.g. paper) is investigated.

This work aims to develop a theoretical power operations envelope for the PUR-1 reactor. Given the bulk coolant temperature, the reactor’s power level is limited primarily by the Onset of Nucleate Boiling. Additional limitations to the reactor power are explored including the dose rate at the top of the pool due to shine and the airborne effluent of argon and nitrogen. Operations in excess of the facility cooling capacity will be proposed and are already permitted at other US research reactor facilities, provided temperature limitations are met. The MCNP and NATCON code packages have been implemented to assist in power limitation measurement. A brief discussion on the licensing considerations is included to provide some framework for pursuit of these higher power levels. The maximum power consideration ensures continued full use of the facility while maximizing its effectiveness in the teaching laboratories and access to researchers. The final power level is limited by the administrative dose limit at the top of the reactor pool as well as the Onset of Nucleate Boiling power level as a function of bulk pool temperature. The result is an operational envelope which would allow operators to have the maximum neutron flux without changing the facility or creating phase transition within the light water coolant.

Magnetic fluid flow by thermomagnetic convection with and without buoyancy was studied in experiments and computational simulations. A mineral oil based ferro magnetic fluid was subjected to varying magnetic fields to induce thermomagnetic convection. As such fluids are mainly developed to increase heat transfer for cooling the fundamental effects on magnetic fluid flow was investigated using various magnetic field distributions. Computational simulations of natural and thermomagnetic convection are based on a Finite-Element technique and considered a constant magnetic field gradient, a realistic magnetic field generated by a permanent magnet and alternating magnetic fields. The magnetic field within the fluid domain was calculated by the magneto-static Maxwell equations and considered in an additional magnetic body force known as the Kelvin body force by numerical simulations. The computational model coupled the solutions of the magnetic field equations with the heat and fluid flow equations. Experiments to investigate thermomagnetic convection in the presence of terrestrial gravity used infrared thermography to record temperature fields that are validated by a corresponding numerical analysis. All configurations were chosen to investigate the response of the magnetic fluid to the applied body forces and their competition by varying the magnetic field intensity and its spatial distribution. As both body forces are temperature dependent, situations were analysed numerically and experimentally to give an indication of the degree by which heat transfer may be enhanced or reduced. Results demonstrate that the Kelvin body force can be much stronger than buoyancy and can induce convection where buoyancy is not able to. This was evident in a transition area if parts of a fluid domain are not fully magnetically saturated. Results for the transition from natural convection to thermomagnetic convection suggest that the domain of influence of the Kelvin body force is aligned with the dominance of the respective body force. To characterise the transition a body force ratio of the Kelvin body force to buoyancy was developed that identified the respective driving forces of the convection cells. The effects on heat transfer was quantified by the Nusselt number and a suitable Rayleigh number. A modified Rayleigh number was used when both body forces were active to define an effective body force by taking the relative orientation of both forces into account. Results for the alternating magnetic field presented flow fields that altered with the frequency of the applied magnetic field but with varying amplitude. This affected the heat transfer that alternated with the frequency but failed to respond instantaneously and a phase lag was observed which was characterised by three different time scales.

This dissertation focuses on the effects of superhydrophobic (SHPo) surfaces on thermal transport. The work is divided into two main categories: thermal transport without phase change and thermal transport with phase change. Thermal transport without phase change is the topic of four stand-alone chapters. Three address jet impingement at SHPo surfaces and the fourth considers natural convection at a vertical, SHPo wall. Thermal transport with phase change is the topic of a single stand-alone chapter exploring pool boiling at SHPo surfaces. Two chapters examining jet impingement present analytical models for thermal transport; one considered an isothermal wall and the other considered an isoflux wall. The chapter considering the isothermal scenario has been archivally published. Conclusions are presented for both models. The models indicated that the Nusselt number decreased dramatically as the temperature jump length increased. Further, the influence of radial position, jet Reynolds number, Prandtl number and isoflux versus isothermal heating become negligible as temperature jump length increased. The final chapter concerning jet impingement reports an experimental exploration of jet impingement at post patterned SHPo surfaces with varying microfeature pitch and cavity fraction. The empirical results show a decrease in Nusselt number relative to smooth hydrophobic surfaces for small pitch and cavity fraction and the isoflux model agrees well with this data when the ratio of temperature jump length to slip length is 3.1. At larger pitch and cavity fractions, the empirical results have higher Nusselt numbers than the SHPo surfaces with small pitch and cavity fraction but remain smaller than the smooth hydrophobic surface. We attribute this to the influence of small wetting regions. The chapter addressing natural convection presents an analytical model for buoyant flow at a vertical SHPo surface. The Nusselt number decreased dramatically as temperature jump length increased, with greater decrease occurring near the lower edge and at higher Rayleigh number. Thermal transport with phase change is the topic of the final stand-alone chapter concerning pool boiling, which has been archivally published. Surface heat flux as a function of surface superheat was reported for SHPo surfaces with rib and post patterning at varying microfeature pitch, cavity fraction, and microfeature height. Nucleate boiling is more suppressed on post patterned surfaces than rib patterned surfaces. At rib patterned surfaces, transition superheat decreases as cavity fraction increases. Increasing microfeature height modestly increases the transition superheat. Once stable film boiling is achieved, changes in surface microstructure negligibly influence thermal transport.