Dissertation (Ph. D.)--University of Stellenbosch, 1992. / Bibliography. Also available via the Internet.

An investigation was conducted to determine the feasibility of using the technique of ultrasonic nebulization of water into the anode gas stream for evaporative cooling of a Proton Exchange Membrane (PEM) fuel cell. The basic concept of this form of internal evaporative cooling of the PEM fuel cell is to introduce finely atomized liquid water into the anode gas stream, so that the finely atomized liquid water adsorbs onto the anode and then moves to the cathode via electro-osmotic drag, where this water then evaporates into the relatively dry cathode gas stream, carrying with it the waste thermal energy generated within the fuel cell. The thermal and electrical performance of a 50 cm2 PEM fuel cell utilizing this technique was compared to the performance obtained with conventional water management. Both techniques were compared over a range of humidification chamber temperatures for both the anode and cathode gas streams so as to determine the robustness of the proposed method. The proposed method produced only meager levels of evaporative cooling (at best 2 watts, for which a minimum of 30 watts was required for adequate cooling), but the average cell voltage increased considerably (as much as a 10% gain), and the technique increased the fault tolerance of the fuel cell (the Nafion™ membrane did not dry out even if cell temperature went well in excess of 70° C despite both anode and cathode humidification temperatures of 55° C). An interesting phenomena was also observed wherein the fuel cell voltage oscillated regularly with a period of tens of seconds, and that the amplitude of this oscillation corresponded inversely with the level of humidification received by the fuel cell.

This thesis presents a thermodynamic model for a screw chiller and cooling tower system for the purpose of developing an optimized control algorithm for the chiller plant. The thermodynamic chiller model is drawn from the thermodynamic models developed by Gordon and Ng (1996). However, the entropy production in the compressor is empirically related to the pressure difference measured across the compressor. The thermodynamic cooling tower model is the Baker & Shryock cooling tower model that is presented in ASHRAE Handbook - HVAC Systems and Equipment (1992). The models are coupled to form a chiller plant model which can be used to determine the optimal performance. Two correlations are then required to optimize the system: a wet-bulb/setpoint correlation and a fan speed/pump speed correlation. Using these correlations, a "quasi-optimal" operation can be achieved which will save 17% of the energy consumed by the chiller plant.

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.

En esta tesis se analiza de manera detallada los sistemas de refrigeración solar con equipos de absorción de doble efecto con captadores solares de concentración en términos de energía primaria consumida. El análisis de estos sistemas permite compararlos con sistemas de refrigeración por absorción de simple efecto accionados con energía solar térmica de baja temperatura y con sistemas convencionales de compresión. Estos tres tipos de sistemas han sido modelizados mediante el software de simulación TRNSYS para cubrir la demanda frigorífica de un edificio genérico de oficinas localizado en Sevilla (España). Un aspecto importante en esta tesis es la modelización de las enfriadoras de absorción de simple y doble efecto mediante el método de la ecuación característica. Este método permite modelizar las prestaciones térmicas del equipo de absorción de manera sencilla y robusta, además que debido a su estructura puede ser aplicado a cualquier sistema de refrigeración por absorción. / This thesis presents a detailed analysis of solar cooling systems, with double-effect absorption chillers activated by concentrating solar collectors, in terms of primary energy savings. This analysis allows comparing this type of systems with solar assisted air-conditioning installations composed by single-effect absorption chillers and flat plate solar collectors, and conventional compression systems. These three types of systems have been modeled in a TRNSYS simulation environment and applied to a generic building located in the city of Seville (Spain) to cover its cooling demand. An important aspect of this thesis is the modeling of single and double-effect absorption chillers by means of the characteristic equation method (ChEM). This method permits modeling of thermal performance of the absorption machine in a simple and robust manner, and due to its structure, the method can be applied to any absorption refrigeration system.

No description available.

Abstract There is much rich solar energy in Taiwan situated at the subtropics;Therefore it¡¦s suitable for solar energy is utilized as the driving energy for absorption cooling system. Simultaneously the use of solar energyreduces our dependence on fossil fuel ,and reaches the goal of energy conservation and reduction of carbon footprint deeply.After establishing absorption cooling system in TRNSYS.16 platform, climate data TMY2 form for several cities of Taiwan are input to the simulation program. As a result of Taiwan climate variation, summer is getting more and more longer ; winter is getting shorter gradually .Hence we simulated program from March to October .Then system operating the parameters of solar collector area and the volume of the storage tank are varied to research those effects on the system performance. The research focuses on the simulation data for monthly solar fraction(f),seasonal solar fraction(F) as well as how much solar energy saved could transform carbon dioxide content for single effect and double effect absorption cooling system operated under the climate of the major cities in Taiwan .Those data could provide the design needs.

The survival and germination of Clostridium perfringens spores in different heating media and at different heating rates was studied to determine the fate of C. perfringens spores during abusive cooking and cooling of pork products. The heat resistance (HR) of C. perfringens spores from three strains that were either previously heat shocked (HS) or non-heat shocked (NHS) was determined individually and as a cocktail in phosphate buffer solution (pH 7.4) (PBS), beef gravy (BG), ground pork (GP) and cured ground pork (CGP) at 75ºC. The effect of the heating rate on HR, germination and outgrowth of C. perfringens spores in CGP was determined by increasing the temperature from 20 to 75ºC at a rate of 4, 8, and 12ºC/h prior to heating and holding at 75ºC for 48 h. Heating rates at 4ºC/h in GP and CGP were repeated with additional cooling from 54.4 to 7.2ºC within 20 h (temperature abuse). Linear survival curves were observed on NHS spores in the four heating media, whereas HS spores showed linear curves when heated in PBS and BG, and biphasic curves when heated in GP and CGP. In general, HS spores were more heat sensitive than NHS spores. NHS spores heated in GP had greater HR than spores heated in CGP, BG or PBS. There were no significant differences (P>0.05) on the HR of C. perfringens spores in CGP heated from 20 to 75ºC at 4, 8, or 12ºC/h. Heating rates of 8 and 12ºC/h showed no difference in germination and outgrowth of inoculated spores, whereas at 4ºC/h, growth of C. perfringens occurred between 44 and 56ºC. Temperature abuse during cooling of GP resulted in 2.8 log CFU/g increase of C. perfringens counts. In CGP, C. perfringens counts decreased by 1.1 log CFU/g during cooling from 54.4 to 36.3ºC and then increased by 1 log CFU/g until the product reached 7.2ºC. However, with an initial inoculum in raw CGP of 5 log CFU C. perfringens spores/g, C. perfringens counts did not exceed 3.4 log CFU/g during a 20 h abusive cooling. These results suggest there is no risk associated with C. perfringens in cured pork products under the conditions tested. Results from the present study indicate that different behavior may be expected with different meat products.

The gas turbine blade/vane internal cooling is achieved by circulating the compressed air through the cooling passages inside the turbine blade. Leading edge and trailing edge of the turbine blade are two critical regions which need to be properly cooled. Leading edge region receives extremely hot mainstream flow and high heat transfer enhancement is required. Trailing edge region usually has narrow shaped geometry and applicable cooling techniques are restricted. Heat transfer will be investigated in the leading edge and trailing edge cooling channels at high rotation numbers close to the engine condition. Heat transfer and pressure drop has been investigated in an equilateral triangular channel (Dh=1.83cm) to simulate the cooling channel near the leading edge of the gas turbine blade. Three different rib configurations (45°, inverted 45°, and 90°) were tested at four different Reynolds numbers (10000-40000), each with five different rotational speeds (0-400 rpm). By varying the Reynolds numbers (10000-40000) and the rotational speeds (0-400 rpm), the rotation number and buoyancy parameter reached in this study were 0-0.58 and 0-2.3, respectively. 45° angled ribs show the highest thermal performance at stationary condition. 90° ribs have the highest thermal performance at the highest rotation number of 0.58. Heat transfer coefficients are also experimentally measured in a wedge-shaped cooling channel (Dh =2.22cm, Ac=7.62cm2) to model an internal cooling passage near the trailing edge of a gas turbine blade where the coolant discharges through the slot to the mainstream flow. Tapered ribs are put on the leading and trailing surfaces with an angle of attack of 45°. The ribs are parallel with staggered arrangement on opposite walls. The inlet Reynolds number of the coolant varies from 10,000 to 40,000 and the rotational speeds varies from 0 to 500 rpm. The inlet rotation number is from 0 - 1.0. The local rotation number and buoyancy parameter are determined by the rotational speeds and the local Reynolds number at each region. Results show that heat transfer is high near the regions where strong slot ejection exists. Both the rotation number and buoyancy parameter have been correlated to predict the rotational heat transfer enhancement.

Air compressor systems play a large role in modern industry. These compressors can account for a significant portion of a manufacturing facility’s electric consumption and any increase in efficiency can lead to economic benefits. Air compressors are sensitive to ambient conditions, as evidenced by the fact that compressing cooler and drier air decreases the amount of work required to compress the air. A thermodynamic model of an air compressor system was developed and several cases were run by using both vapor compression and absorption cycle chillers to cool and dehumidify the inlet air. The results show that the performance increases as much as 8 percent for the compressor system with absorption inlet cooling and as much as 5 percent when using vapor compression inlet cooling. Climates with higher humidity and temperatures can see the most benefits from inlet air cooling and dehumidification.