D.Ing. / Many studies have been conducted in order to establish the respective influence of geometric parameters such as fins number, fin shape (apex angle), spiral angle, fin height, fin pitch etc. on the condensation heat transfer performance of the spiralled micro-fin tubes. However, the effect of the spiral angle could not be clearly established in those investigations, because other geometric parameters affecting the heat transfer performance such as fin height, fin thickness, apex angle were also varied. The influence of the spiral angle on the heat transfer performance during condensation inside spiralled micro-fin tubes having all other geometric parameters the same was experimentally investigated in this study. A new experimental-based predictive correlation was developed for practical design of this specific class of micro-fin tubes. Tests were conducted for condensation of R22, R134a and R407c inside a smooth and three micro-fin tubes having spiral angles of 10, 18 and 37 degrees. Experimental results indicated a heat transfer augmentation due to heat transfer area increase. As the spiral angle was increased, the heat transfer area increased causing a substantial heat transfer augmentation. Condensation inside the 10° spiralled micro-fins produced a heat transfer augmentation of about 170% for a heat transfer area increase of 1.87 when compared to condensation in ~he correspondent smooth tube while the 18° spiralled " micro-fins produced an augmentation of 180% for a heat transfer area increase of 1.94. The 37° spiralled micro-fins produced the highest enhancement of 220% for a heat transfer area increase of 2.13. Additional heat transfer augmentation was produced by: (a) the turbulence in the condensate film due to the presence of spiralled micro-fins (stronger effect at lower mass velocities and vapor qualities) and (b) the effect of surface tension forces (at higher vapor qualities). The proposed new correlation predicted the majority of experimental results of the present study within a deviation zone of ± 20 percent.

Driven by the importance of optimizing energy systems and technologies, the field of exergy analysis was developed to better illuminate process inefficiencies and evaluate performance. Exergy analysis provides important information and understanding that cannot be obtained from energy analysis. The field of exergy analysis is well formulated and understood except for thermal radiation (TR) heat transfer. The exergy flux, or maximum work obtainable, from TR has not been unambiguously determined. Moreover, many thermodynamic textbooks are misleading by incorrectly implying that the entropy and exergy transport with TR is calculated by using the same expressions that apply to heat conduction. Research on the exergy of TR was carried out by Petela. However, many researchers have considered Petela's analysis of the exergy of TR to be irrelevant to the conversion of TR fluxes. Petela's thermodynamic approach is considered irrelevant because, others argue, that it neglects fundamental issues that are specific to the conversion of fluxes, issues that are unusual in the context of exergy analysis. The purpose of the research in this thesis is to determine, using fundamental thermodynamic principles, the exergy flux of TR with an arbitrary spectrum and its relevance to solar radiation (SR) conversion. In this thesis it is shown that Petela's result can be used for the exergy flux of blackbody radiation (BR) and represents the upper limit to the conversion of SR approximated as BR. The thesis shows this by resolving a number of fundamental issues: (1) Inherent Irreversibility; (2) Definition of the Environment; (3) Inherent Emission; (4) Threshold Behaviour; (5) Effect of Concentrating TR. This thesis also provides a new expression, based on inherent irreversibility, for the exergy flux of TR with an arbitrary spectrum. Previous analysis by Karlsson assumes that reversible conversion of non-blackbody radiation (NBR) is theoretically possible, whereas this thesis presents evidence that NBR conversion is inherently irreversible. In addition the following conclusions and contributions are made in the thesis: (1) Re-stated the general entropy and exergy balance equations for thermodynamic systems so that they correctly apply to TR heat transfer. (2) Provided second-law efficiencies for common solar energy conversion processes such as single-cell Photovoltaics. (3) Showed that Omnicolor (infinite cell) conversion, the widely held ideal conversion process for SR, is not ideal by explaining its non-ideal behaviour in terms of exergy destruction and exergy losses. (4) Presented an ideal (reversible) infinite stage thermal conversion process for BR fluxes and presented two-stage thermal conversion as a practical alternative. (5) Showed that Prigogine's minimum entropy production principle cannot be used as a governing principle in atmospheric modeling, and that in general, it may have little significance. (6) Presented a graybody model of the planet that may prove useful in understanding the thermodynamics of the Earth system. (7) Showed that the expression derived from the Clausius equality for reversible processes is applicable, whereas the statement for irreversible processes is not applicable, when there is significant heat transfer by TR. (8) Showed that the 4/3 coefficient in the BR entropy expression can be obtained by simply using the concept of equilibrium and the experimentally observable relationship for BR energy (energy x T⁴). / Graduate

Magister Chirurgiae Dentium - MChD / Objectives: Temperature changes along an implant body have not been widely studied. The objectives of this in vitro study were (i) to establish if the temperature of the abutment influences the temperature of the implant surface, (ii) to establish the temperature transmission from abutment to implant body, and (iii) to establish for what abutment temperature the critical time/temperature threshold of 47oC for 1 minute at implant level is reached. Materials and method: Eight K-type thermocouples were attached to an abutment/implant configuration, mounted in a thermostatically controlled environment. The abutment was exposed to hot water. The temperature at each thermocouple along the implant was logged over a maximum period of 10 minutes using appropriate software. The test was repeated 200 times. A logistic regression model was used for the analysis of the time/temperature databases. Results: There was a positive correlation between the temperatures of the implant and its abutment, albeit with a time delay. Critical threshold values for bone necrosis were reached. The effective dose 50 was estimated at 62.3oC (95% confidence interval estimate): for an abutment temperature of 62.3oC there is a 50% chance that 47oC for 1 minute at implant level is reached. Conclusion: The results of this in vitro study support the hypothesis that abutment temperature is transmitted to a dental implant body. Results of in vitro studies should be interpreted with caution. However, clinicians should be aware of temperature changes along implants and the potential risk associated with this. / South Africa

Heat transfer in circulating fluidized beds was studied in both a 7.3 m high, 152 x 152 mm square, pilot-scale combustor and a 9.3 m high, 152 mm ID transparent cold model unit. Results were obtained for particles of mean size 171-299 µm at superficial gas velocities from 4 to 9.5 m/s and for solids circulation rates up to 70 kg/m².s. For the combustor, results obtained by using membrane walls and a vertical tube as heat transfer surfaces show a strong influence of the cross-sectional area-averaged suspension density on time-averaged, length-averaged suspension-to-surface heat transfer coefficient. The influence of superficial gas velocity is found to be small. Radiation becomes significant at suspension temperatures higher than 400 C and at low suspension densities. Heat transfer coefficients were also found to vary with the lateral position of the tube. The vertical length of heat transfer surface is shown to be an important parameter, allowing seemingly discrepant published results to be reconciled. For the cold model unit, sudden and dramatic peaks in instantaneous heat transfer coefficients were measured using an instantaneous heat transfer probe. Simultaneous heat transfer and capacitance measurements suggest that these peaks are caused by the arrivals of particle strands at the heat transfer surface. Two-probe heat transfer measurements suggest the existence of a characteristic residence length for the strands at the wall in this column. A proposed heat transfer model, based on an overall core-annulus flow structure in the riser, and periodic formation, movement along the wall, and disintegration of strands in the annulus, gives reasonable agreement with a wide range of published data. It accounts successfully for the effects of heat transfer surface length and particle sizes. However, the effect of the heat transfer surface configuration on the flow pattern of particles must also be taken into account to give improved agreement with experimental data. / 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

Experimental investigations of the surface roughness effect on heat transfer have been extensively conducted since the initial work by Cope; however, they were mostly limited to internal flow geometries--circular tubes, annuli or rectangular ducts.

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.

We report the results of a series of experiments in three interrelated areas: light induced nucleation of vapor bubbles in superheated liquid helium 4, transient heat transfer in liquid helium isotopes 3 and 4, and homogeneous nucleation of vapor bubbles in superheated liquid helium 3. This work has resulted in significant contributions in each of these particular areas. Our transient heat transfer work has resulted in extremely high temperature measurements of the Kapitza thermal boundary conductance limits in helium 3 and 4, in measurements of the delay time to the onset of film boiling over a wide range of bath temperatures in helium 4, and in a determination of bubble growth rates in helium 4. These measurements have been compared with theory and have in some cases allowed the extension or elucidation of that theory. We have characterized the so called "light effect" and established photographically that small amounts of visible light will cause the formation of vapor bubbles at the interface of a solid and superheated liquid helium 4 and that this vapor can influence the quasi-steady-state heat flux vs temperature hysteresis curve. Finally, we have measured the homogeneous nucleation temperature of liquid helium 3 and found good agreement with the predictions of the Becker-Doring-Volmer-Zel'dovich Frenkel nucleation theory. This work is shown to have applications to practical cryogenic engineering, to further understanding of basic heat transfer and nucleation theory, and to practical and theoretical environmental and resource considerations.

A thesis submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 1990 / This work was carried out in response to the need for a simple engineering method for the thermal analysis of direct-contact air-water heat exchangers. A simple method of performance analysis is developed which is directly analogous and consistent with the fundamental approach used in conventional heat exchanger analysis and one in which the algebraic form of the overall equation and the grouping of each of the parameters are apparent. The range of conditions considered are air and water temperatures of between 0 and 50 DC and barometric pressures ranging from 80 to 120 kPa. The air conditions considered range from completely dry to completely satucated with water vapour. Both air cooling and water cooling processes are considered. [Abbreviated abstract. Open document to view full version]

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