M.Ing. / This study involves the mathematical modeling and experimental verification of a heat exchanger accumulator. The study was initiated with a literature survey which, according to the author, revealed that there was no published material that described how heat exchanger accumulators are designed to ensure that they are correctly sized according to the operating system and conditions. The heat exchange process that takes place within the heat accumulator was studied and a mathematical model of a heat exchanger accumulator developed. This model was used to develop a universal design procedure that correctly sized the heat exchanger accumulator according to various requirements identified by the author. The model was then verified by conducting experimental tests and it was concluded that the model could be used to design heat exchanger accumulators.

Thermal energy storage via the latent heat of suitable phase change materials has the advantages of higher energy storage density and relatively isothermal behaviour compared to sensible heat storage systems. Glauber's salt (Na₂S0₄∙10H₂0) is one of the most extensively studied phase change materials for solar energy systems because of its low price, suitable phase change temperature and high latent heat. However, segregation due to incongruent melting behaviour leading to loss in the heat storage efficiency upon repeated melting-freezing cycling is a serious problem which has severely limited application of Glauber's salt. In this study Glauber's salt was encapsulated in 25 mm diameter hollow spheres and agitated in different systems including a liquid fluidized bed, rotating drum and rotating tube to reduce or eliminate the Toss in its heat storage efficiency. The encapsulated mixture consisted of 96% Glauber's salt and 4% borax by weight with 5% by volume air space in the capsules. Some capsules containing 25%, 15% and 5% by weight excess sodium sulfate and 10% by weight excess water were also prepared, to test the effect of sodium sulfate concentration under different agitation conditions. The heat storage capacity of 5756 capsules, agitated by fluidizing with water in a pilot plant size (0.34 m diameter) column, showed a decrease over the first three cycles to about 60% of that theoretically possible, but there was no further decrease over the next 93 cycles under fluidization conditions. The heat storage efficiency was found to be improved by increasing the superficial water velocity and by decreasing the cooling rate. Heating rate had little or no effect. The fluidized capsules provide enhanced heat transfer rates to or from the heat storage medium, enabling the energy to be charged or discharged in about one hour with realistic inlet and outlet temperatures. The high heat transfer rates are an important advantage for the system and may open new areas of applications for thermal energy storage by encapsulated phase change material. Economic analysis of the liquid fluidized bed heat storage system shows that operating costs are almost negligible compared to fixed capital costs. The heat storage efficiency of capsules decreased to 38.4% of the theoretical capacity or 67% of the corresponding agitated (fluidized) system in only 7 cycles under fixed bed conditions, and the efficiency decreased with further cycling. 97.5% of the original heat storage-capacity was recovered within three cycles when these capsules were refluidized. Performances of the regular and different composition capsules were tested in the rotating tube, with rotation around a fixed horizontal axis passing through the capsules' centers, and in the rotating drum, with impact due to collisions in addition to rotation. The results showed that full rotation of a capsule around a horizontal axis improves the heat storage efficiency. However, full recovery of the theoretical capacity was not possible, even under vigorous mixing conditions. The efficiencies in the rotating tube were similar to those in the rotating drum for capsules subject to the same number of rotations around a horizontal axis. At high rotation speeds centrifugal force had a negative influence, especially in the rotating tube. On the basis of heat storage capacity per unit volume or weight of phase change material, 47% by weight sodium sulfate concentration was found to be optimal for the rotating drum and the rotating tube cases. Some small scale experiments were performed to determine the relative importance of different factors in the loss of heat storage capacity. Sodium sulfate concentration gradients in the capsules with different thermal cycling histories were found by thermogravimetric analysis. The results showed that bulk segregation of anhydrous sodium sulfate is not the only reason for the loss of heat storage capacity in systems using Glauber's salt. Microencapsulation of anhydrous sodium sulfate beneath a layer of Glauber's salt crystals is at least as important. Experiments to determine the degree of subcooling, believed to be another factor in the loss of heat storage capacity, showed that a mixture of 96% Glauber's salt and 4% borax by weight undergoes subcooling of about 5 K in gently agitated capsules. Nucleation and crystallization temperatures both increase with increased agitation. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

Temperature and heat transfer studies in a pilot-scale water immersion retort were performed. The temperature study investigated the temperature distribution and stability of the retort during the cook period. The investigation of heat transfer uniformity within the retort was based on heating and cooling parameters calculated from the heat penetration curves of food-simulating teflon transducers. The uniformity of sterilizing conditions within the retort was determined from process lethalities calculated for the transducers. Variable retort operating conditions consisted of two retort temperatures (115 and 125°C) and three weir heights (29.2, 31.2 and 34.6 cm). Mean standard deviations of thermocouple readings indicating temperature distribution during the cook period ranged from 0.19 to 0.22 C°. Slight temperature gradients were found between the upper and lower water channels and between the entrance and exit regions of water channels. The coldest locations (the exit regions of water channels 1 and 2) averaged approximately 0.6 C° lower than the hottest locations (the entrance and exit region of water channel 11 and the exit region of water channel 10). Mean standard deviations of thermocouple readings indicating temperature stability during the cook period ranged from 0.10 to 0.20 C°. Temperature stability was uniform between all water channels, except channel 11, which was less stable. The entrance and exit regions of water channels displayed similar stability. The existence of heat transfer variability within the water immersion retort was indicated. A retort temperature of 125°C produced smaller fh and fc values than 115°C. Variations in weir height influenced the distribution of fh values between trays. Weir height 2 (31.2 cm) exhibited uniform values between all trays. Weir height 1 (29.2 cm) exhibited uniform fh values between all trays, except for a significantly larger value for the very top tray. Although weir height 3 (34.6 cm) created the most variability between tray levels, weir height 1 displayed the widest range of fh values. More variability in fc values between trays was shown during the cool period. Weir height 2 displayed the most uniform fc values between trays, however, the range of fc values between trays was similar for all three weir heights. Within trays, a gradient of fh and fc values was found between the entrance, exit and middle positions, with the smallest values found in the entrance positions. In comparison, the largest fh values were found in the exit and middle positions of trays 1 and 10. The largest fc values were found in the middle positions of trays 1 and 3 and the middle and exit positions of tray 10. Weir heights 1 and 2 produced smaller jh values than weir height 3, variations in weir height had no influence on jc values. A gradient of jh values between tray levels was shown, with smaller values associated with upper trays and larger values with lower trays. Smaller jh and jc values were associated with the entrance positions of trays than with the middle and exit positions. A comparison with steam processing indicated larger fh values for the water immersion process and larger fc values for the cooling method used with the steam process. Calculation of process lethalities indicated variability of sterilizing conditions within the retort. Larger Fo values were associated with upper trays than with lower trays. Within trays, larger Fo values were found in the tray entrance positions than the middle and exit positions. The largest Fo values were exhibited in the entrance positions of the middle to upper trays, while the smallest values were found in the middle and exit positions of the bottom trays. Retort pressure studies indicated pressure stability during the cook period, however, during the initial minutes of the cool period, a significant pressure drop occurred, which the retort corrected. Pressure stability was maintained once the target pressure was re-established. / Land and Food Systems, Faculty of / Graduate

Forced convection heat transfer in the entrance region of tubes and ducts has been studied extensively since the pioneering work of Graetz. Most of the published solutions for axial flow heat transfer are based on the idealization of either fully established or uniform velocity profiles with developing temperature.

No description available.

With no universal criteria for classifying a heat-related death, the scope and magnitude of heat-related illnesses and deaths is underestimated. By using threshold temperatures based on one and two standard deviations from normal, a unique but universal classification for excessive/extreme heat was calculated. These threshold values were calculated for St. Louis, Missouri; New Orleans, Louisiana; Little Rock, Arkansas; Philadelphia, Pennsylvania; and Milwaukee, Wisconsin for June, July, and August from 1990 ? 1999. Statistical analysis was performed for each city on days with excessive/extreme maximum and/or minimum temperature values, death count, and death count one, two, and three days later in order to discover a possible strong and significant relationship between excessive/extreme heat and death.

The development and implementation of technologies that improves Heating Ventilation & Air Conditioning (HVAC) system efficiency, including unique waste heat recovery methods, are sought while considering financial constraints and benefits. Recent studies have found that through the use of advanced waste heat recovery systems, it is possible to reduce building’s energy consumption by 30%. Oscillating heat pipes (OHP) exists as a serpentine-arranged capillary tube, possesses a desirable aerodynamic form factor, and provides for relatively high heat transfer rates via cyclic evaporation and condensation of an encapsulated working fluid with no internal wicking structure required. In last two decade, it has been extensively investigated for its potential application in thermal management of electronic devices. This dissertation focuses on the application of OHP in waste heat recovery systems. To achieve the goal, first a feasibility study is conducted by experimentally assessing a nine turn copper-made bare tube OHP in a typical HVAC ducting system with adjacent air streams at different temperatures. Second, for a prescribed temperature difference and volumetric flow rate of air, a multi-row finned OHP based Heat Recovery Ventilator (OHP-HRV) is designed and analyzed for the task of pre-conditioning the intake air. Additionally, the energy and cost savings analysis is performed specifically for the designed OHP-HRV system and potential cost benefits are demonstrated for various geographical regions within the United States. Finally, an atypically long finned OHP is experimentally investigated (F-OHP) under above prescribed operating condition. Helical fins are added to capillary size OHP tubes at a rate of 12 fins per inch (12 FPI), thereby increasing the heat transfer area by 433%. The coupled effect of fins and oscillation on the thermal performance of F-OHP is examined. Also, F-OHP’s thermal performance is compared with that of bare tube OHP of similar dimension and operating under similar condition. It was determined that OHP can be an effective waste heat recovery device in terms of operational cost, manufacturability, thermal and aerodynamic performance. Moreover, it was also determined that OHP-HRV can significantly reduce energy consumption of a commercial building, especially in the winter operation.

As a result of climate change and urbanization, rising temperatures are causing increasing concern about extreme heat in cities worldwide. Urban extreme heat like other climate-related phenomena is a complex problem that requires expertise from a range of disciplines and multi-faceted solutions. Therefore, this study aims to develop a comprehensive understanding of urban heat issue by taking a multi-level approach that integrates science, technology, and policy. Throughout the three main papers of this dissertation, a variety of quantitative and qualitative methods, such as microclimate modeling, machine learning, statistical analysis, and policy content analysis, are used to analyze urban heat from different perspectives. The first paper of this dissertation focuses on the street canyon scale, aiming to identify the physical and vegetation parameters that have the greatest impact on changing thermal conditions in urban environments and to understand how these parameters interact with each other. Moving towards identifying applicable heat-related data and measurement techniques, the second paper assesses whether lower-resolution temperature data and novel sources of vulnerability indicators can effectively explain intra-urban heat variations. Lastly, the third paper of this dissertation reviews heat-related plans and policies at the Planning Districts level in Virginia, providing insights into how extreme heat is framed and addressed at the regional and local levels. This analysis is particularly important for states such as Virginia, which historically have not experienced multiple days of extreme heat during summers, as is common in southern and southwestern states of the United States. The results of this study provide insights into the contributing and mitigating factors associated with extreme heat exposure, novel heat-related data and measurement techniques, and the types of analysis and information that should be included in local climate-related plans to better address extreme heat. This dissertation explores new avenues for measuring, understanding, and planning extreme heat in cities, thereby contributing to the advancement of knowledge in this field. / Doctor of Philosophy / Due to climate change and fast city growth, temperatures are rising, and extreme heat is becoming a big worry in cities worldwide. Urban extreme heat is a challenging problem that needs expertise from different majors and diverse solutions. This dissertation aims to understand urban heat better by integrating science, technology, and policy. The three main research papers of this dissertation use various methods like modeling, statistics, and policy analysis to study urban heat from different angles. The first paper focuses on city streets and how certain physical features and vegetation affect citizens' thermal comfort. The second paper explores new ways to measure heat in urban areas, including using new sources of data and the application of lower-resolution data. Finally, the third paper reviews heat-related plans and policies in Virginia, helping us understand how extreme heat is addressed in areas that might not be accustomed to high temperatures. This dissertation's findings provide useful insights into why the severity of extreme heat is not the same in different parts of cities, present new ways to measure this difference and find solutions to lessen the negative impacts of exposure to heat. It also shows what information needs to be included in plans and policies to better deal with extreme hot weather at the local level such as in towns and cities. By exploring new ways to understand and handle extreme heat in cities, this research helps make progress in this important field. The goal of this research is to help cities prepare for and cope with urban extreme heat, keeping people safe and creating sustainable cities for the future.

Call number: LD2668 .T4 1984 V46 / Master of Science

This study presents the numerical analysis of steady laminar flow heat transfer in a horizontal rectangular duct with differential heating on the vertical walls. Three heating configurations: one uniform wall temperature (CS1) and two linearly varying wall temperature cases (CS2 and CS3) are analysed. The study considers the combined effects of natural convection, forced convection and radiation heat transfer on the overall heat transfer characteristics. Air, which is assumed to be a non-participating medium, is chosen as the working fluid. A computational fluid dynamics solver is used to solve a set of governing equations for a range of parameters.For chosen duct aspect ratios, the numerical model simulates the flow and heat transfer for two main effects: buoyancy and radiation heat transfer. Buoyancy effect is represented by Grashof number, which is varied from 2,000 to 1,000,000. The effect of radiation heat transfer is examined by choosing different wall surface emissivity values. The weak and strong radiation effect is represented by the emissivity values of 0.05 and 0.85 respectively. Three duct aspect ratios are considered - 0.5, 1 and 2. The heat transfer characteristics of all the above heating configurations - CS1, CS2, and CS3 are analysed and compared. The numerical results show that, for all heating configurations and duct aspect ratios, the overall heat transfer rate is enhanced when the buoyancy effect increases. Since buoyancy effect induces natural circulation, this circulation is therefore the main mechanism that enhances heat transfer. Radiation heat transfer is found to significantly influence convection heat transfer in high Grashof numbers.