D.Ing. (Mechanical Engineering) / In the past two decades the refrigeration, air-conditioning and heat pump industries began the conversion from chlorofluorocarbon (CFC) refrigerants to hydrochlorofluorocarbons (HCFCs) and to natural refrigerants. This changeover not only involves redesigning, re-optimizing and re-testing all new original equipment but also involves retrofitting many large existing systems. Combining this process with the goal of developing more accurate design methods and more energy-efficient cycles, heat transfer and, specifically, heat transfer enhancement, has become a very active research field and will probably continue to boom in the next decades as the HCFCs are also phased out of use. The most prominent alternative refrigerants are R134a and R407C to replace the present market dominating refrigerant R22. Many heat transfer enhanced techniques have simultaneously been developed for the improvement of energy consumption, material saving, size reduction and pumping power reduction. Helical wire inserts in tubes are a typical technique that offers a higher heat transfer increase and, at the same time, only a mild pressure drop penalty. This study investigates the heat transfer characteristics of a horizontal tube-in-tube heat exchanger with a helical wire inserted in the inner tube. The influence of the pitch (or helix angle) of such geometry on the heat transfer performance and pressure drop during condensation (having all other geometric parameters the same) was investigated experimentally. Firstly, three refrigerants were tested in three helical wire-inserted tubes with different pitches of 5, 7.77 and 11 mm. The local and average heat transfer coefficients, and semi-local and average pressure drops were studied systematically. The experimental results were compared not only with the referenced experimental data of the smooth tubes, but also with the results of micro-fin tubes. The heat transfer enhancement factors, pressure drop loss penalty factors and overall efficiencies of the tested condensers with helical wire-inserted geometry were calculated. The tube with a helical wire pitch of 5 mm inserts was found to have the highest enhancement factor and overall efficiency. Secondly, the heat transfer enhancement mechanism was studied and explained. It was found that the extension of the annular flow regime contributed mainly to this enhancement. The transitional qualities from annular flow to intermittent flow were derived and incorporated in a flow regime map. Thirdly, heat transfer coefficient and pressure drop correlations for this special heat transfer enhancement geometry were deduced, and they predicted the experiment data to within 80% and 78% respectively, within a deviation of  20%. Finally, the water flowing through helical wire-inserted tubes (glass) was demonstrated, providing a visual understanding of the heat transfer enhancement mechanism. / Prof. J.P. Meyer Prof. L. Liebenberg

Some conductive and convective heat transfer problems with radiative boundary conditions are analysed theoretically. Three specific problems have been analysed. The study has, therefore, been divided into three parts. In Part I the temperature distribution produced in-long, solid circular and rectangular cylinders and a solid sphere in interplanetary space is studied. The solid bodies receive parallel radiation flux on one side and emanate radiant energy to their surroundings at zero degree Rankine. Steady state, constant physical and surface properties, and no heat loss by convection are assumed. Solution of the linear conduction equation with nonlinear boundary conditions is obtained by two approximate, semi-analytical methods, (i) point matching and (ii) least-squares fitting. The results are compared with earlier results obtained by a variational method. The least-squares fit method appears to be most suitable regarding accuracy and simplicity in computation. Its accuracy does not appear to depend appreciably either on the radiation-conduction parameter or on the surface absorptivity. The effect of semi-grayness of the receiving surface is analysed. In Part II the heat transfer characteristics of a circular fin dissipating heat from its surface by convection and radiation are analysed. The temperature is assumed uniform along the base of the fin and constant physical and surface properties are assumed. There is radiant interaction between the fin and its base. Two separate situations are considered. In the first situation heat transfer from the end of the fin is neglected. Solution of the linear conduction equation with nonlinear boundary conditions has been obtained by the least-squares fit method. A solution has also been obtained by the finite difference method and the results compared. Results are presented for a wide range of environmental conditions and physical and surface properties of the fin. In the second situation heat transfer from the end of the fin is also included in the analysis. The solution is obtained by a finite difference procedure. It is shown that neglecting heat transfer from the end is a good approximation for long fins or for fins of high thermal conductivity material. In Part III the problem of laminar heat transfer in a circular tube under radiant heat flux boundary conditions has been analysed. Fully developed velocity profile is assumed and the tube is considered stationary. A steady radiant energy flux is being incident on one half of the tube circumference while the fluid emanates heat through the wall on all sides by radiation to a zero degree temperature environment. A solution by finite difference procedure has been obtained. The temperature distribution and the Nusselt number variation are presented for a wide range of the governing physical parameters. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

The friction sawing of wood by a high speed rotating disk has been studied. The present experiments were carried out by sawing two commonly used timbers; namely, Douglas fir and Western red cedar of different moisture Contents. A 14 inch diameter, 14 gauge steel disk with a smooth edge, driven at a rotating speed of 4,620 rpm was used for the research. A theoretical analysis of heat transmission characteristics and temperature distribution in the sawing disk is presented. Experimental results showed that the frictional forces, power consumed and cutting temperatures increased as feed speeds increased. The results also showed that the moisture content of the wood had no noticeable influence on the sawing action. A narrow kerf, straight, smooth and polished cut surfaces are some of the advantageous features of the process. At low feed speeds the calculated cutting edge temperatures were well below the ignition temperature of the wood specimen. However at high feed speeds the experimental horse power values increased and high calculated cutting edge temperatures consequently obtained. Excessive power consumption and high cutting edge temperatures were believed to be related to the difficulty of disposition of cut material with a smooth edge disk. Reasonably high feed speeds were evident in cutting plywood and veneer. For these materials clean, smooth and polished cut surfaces were evident. The method may be advantageous in the cutting of plastic sheets. When cutting thicker lumber with this method the feed speed was confined to an impractical low level and power consumed was far higher than that required for ordinary sawing. Thus, whether this method of sawing can be put into practical use or not is determined by the possibility of having an effective means to dispose of cut material. In this connection several methods of modifying the disk for more efficient cutting and waste disposal are presented. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

A single-drop study using motion picture photography was used to predict the heat transfer with a change of phase (vaporization) between two immiscible liquids. In all, three systems were studied using furan, isopentane, and cyclopentane as the dispersed phase liquids and distilled water as the continuous phase liquid. The correlation which predicted the overall heat transfer coefficient for all the three systems was Nu = 0. 0505 (Pe') [formula omitted], where the Nusselt number and the modified Peclet number were based on the dispersed phase liquid properties. This correlation was developed from the experimental data only up to ten percent evaporation and was not found to hold well for the total evaporation range, when the total evaporation time from this correlation was compared with that obtained by a dilatometric method. Individual equations for each system, however, gave good agreement between experimental and theoretical total evaporation time. The average rate of heat transfer for all the three systems is given by the equation, [formula omitted] where 'C' is a constant which is different for each system. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

The heat transfer coefficients for oil-free liquid dichlorodifluoromethane (Preon-12 or Genetron-12) in nucleate boiling over a horizontal copper surface were measured at saturation temperatures of 0°F and 25°F and at heat flux densities between 6,000 and 63,000 Btu/hr-ft². The heating surfaces were finished with 400-A emery paper in two different patterns before each series of test-runs, while the geometric arrangement and the other parameters of the apparatus remained unchanged. Profilometer measurements of roughness and photomicrographs of the surface were taken. All the boiling curves calculated and plotted from the experimental results were of "S" shape, revealing a pronounced deviation from the conventional (normal) boiling curve, especially at lower saturation temperatures. The deviation of the boiling curve was probably due to the unpredictable nucleating characteristics of the heating surface and of the bubble population [1, 2]. This makes it impossible to correlate the experimental data by any of the existing empirical equations formulated by Zuber [3, 4, 5], Rohsenow [6, 7], Levy [16] and others. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Heat transfer characteristics associated with steam/air mixtures were studied in two pilot scale batch type steam/air retorts: a vertical positive flow retort and a horizontal forced circulation Lagarde retort. A method employing transient heat conduction into rectangular bricks of aluminum and stainless steel was developed to evaluate the surface heat transfer coefficient (h) of steam/air mixtures. A system was designed to facilitate an instantaneous drop of the test brick, from an insulated box inside the retort, into a specified steam/air medium after the come-up period. The influences of steam content, temperature, flow rate and flow direction of the heating medium and orientation of test bricks on the associated h values, temperature distribution and pressure stability in the retorts were studied. In addition, thermal processing efficacy was evaluated by measuring the rate of heat penetration into bricks of silicone rubber and rigid nylon which have thermal diffusivities in the range common for foods. In both retorts, steam content (S) of the mixture was found to be the major factor influencing h (p<0.05); however, temperature had no significant effect (p>0.05). Further, the flow direction and flow rate of the heating media in the positive flow retort, and brick orientation in the Lagarde retort also influenced h (p<0.05). The general relationship between h and S was exponential: h = a exp(bS). In the positive flow retort with the test brick in the vertical orientation, the values of a and b were 153 W/m²C and 0.0421 respectively, for steam/air media flowing in an upward direction, and were 337 W/m²C and 0.0355 respectively, for the media flowing downward. The surface heat transfer coefficient was also found to increase linearly with the medium flow rate. With the Lagarde retort, steam/air flow was always horizontal and flow rate was not adjustable. In this case, h was influenced by the test brick orientation. For bricks in the vertical orientation, the exponential parameters, a and b, were 1011 W/m²c and 0.0226 respectively, whereas in the horizontal orientation, these were 1669 W/m²C and 0.0132 Temperature distribution studies in the positive flow retort indicated that the overall standard deviation of the medium temperature at several locations during the cook period (excluding come-up) increased (p<0.05) with a decrease in the steam content and flow rate of the heating media. The effects of temperature and flow direction were nonsignificant (p>0.05). In the Lagarde retort, the temperature distribution was not influenced either by steam content or temperature of the steam/air medium. Pressure stability studies indicated that the air content and temperature of the medium increased (p<0.05) the standard deviations of retort pressure during the cook period. Based on the temperature and pressure deviations in the two retorts, steam/air mixtures with 86-90% steam contents were considered to provide satisfactory overriding air pressures for processing of retort pouches at 105-120°C. Heat penetration studies in the positive flow retort using nonpackaged test bricks of silicone rubber and nylon revealed an increase of up to 11% (p<0.05) in the heating rate index (f) of test bricks when the steam content of the media decreased from 100% to 50%. Heating of bricks at 120°C resulted in f values that were 5.5% larger (p<0.05) than those for bricks heated at 105°C. In the Lagarde retort, the effects of temperature and steam content of the media on f values were not significant. Heating bricks in the vertical orientation resulted in higher f values than in horizontal orientation in some tests, while a reverse trend was observed in others. The influence of entrapped air (15-30 mL per pouch) in retort pouches containing the bricks on f values was small when using a vertical rack that tightly constrained the bricks, whereas up to 260% higher values of f resulted when using an unconstraining horizontal rack while processing at 105-120°C in media of steam contents above 65%. These increases in f value could be prevented by using overriding air pressures of 70-100 kPa during the retort operation. The lag factor, j, was generally in the range of 0.5-1.0 for test bricks, with or without packaging, in the positive flow retort, and 0.8-1.1 in the Lagarde retort, when evaluated at 42% effectiveness for the come-up time. It was observed that in order for the j values to match the theoretical value of 1.27 for an infinite plate, the effectiveness was in the range of 60-90%. / Land and Food Systems, Faculty of / Graduate

Heat transfer from a flowing gas to a fixed bed of dried Dougas-fir wood chips has been studied by a transient method. Hot air at about 130°C flowed upward through 0.2 m dia x 1 m deep beds of commercially prepared wood chips which had been screened for thickness. Four different wood chip sizes were used, which varied in mean thickness from 2.44 to 7.26 mm. The thickest chips were 18.4 mm wide x 36.3 mm long. Gas temperatures were measured at a number of axial positions as the bed temperature rose from its initial temperature of about 20°C. Heat transfer coefficients were calculated by fitting the air temperature profiles to a transient mathemical model for plug flow of gas through a bed of slab-shaped particles with finite internal thermal resistance. The heat transfer model was solved analytically using an approach pioneered by Amundson (10) for fixed beds of spherical particles and based on Rosen's (6,7) function. This solution has not appeared elsewhere in the literature, and is shown to converge to that of Anzelius (1) if the Biot number for the particle approaches zero. Experiments were done at a series of air velocities with four wood-chip thicknesses and with spherical catalyst particles to provide a check on the technique. The effect on heating rate of 30% by volume steam in the incoming air was investigated. For selected experiments, solid temperatures within the wood chips were measured. A correlation of the heat transfer coefficients is presented. Pressure drop was measured as a function of air velocity for different sizes of wood chips at room temperature and the results are compared with predictions of the Ergun equation. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

The overall heat-transfer mechanism within a direct-fired rotary kiln has been examined theoretically. To accomplish this task, the work has been divided into three parts: (1) the characterization of radiative heat transfer within the freeboard area; (2) the overall heat transfer mechanism in the absence of freeboard flames; and, (3) the overall heat transfer mechanism in the presence of freeboard flames. The radiative heat transfer between a nongray freeboard gas and the interior surface of a rotary kiln has been studied by evaluating the fundamental radiative exchange integrals using numerical methods. Direct gas-to-surface exchange, reflection of the gas radiation by the kiln wall, and kiln wall-to-solids exchange have been considered. Graphical representations of the results have been developed which facilitate the determination of the gas mean beamlength and the total heat flux to the wall and to the solids. These charts can be used to account for both kiln size and solids fill ratio as well as composition and temperature of the gas. Calculations using these charts and an equimolal CO₂-h₂O mixture at 1110 K indicate that gas-to-surface exchange is a very localized phenomenon. Radiation to a surface element from gas more than half a kiln in diameter away is quite small and, as a result, even large axial gas temperature gradients have a negligible effect on total heat flux. Results are also presented which show that the radiant energy either reflected or emitted by a surface element is limited to regions less than 0.75 kiln diameters away. The radiative exchange integrals have been used, together with a modified reflection method, to develop a model for the net heat flux to the solids and to the kiln wall from a nongray gas. This model is compared to a simple resistive network/gray-gas model and it is shown that substantial errors may be incurred by the use of the simple models. To examine the overall heat-transfer mechanism in the absence of freeboard flames a mathematical model has been developed to determine the temperature distribution in the wall of a rotary kiln. The model, which incorporates a detailed formulation of the radiative and convective heat-transfer coefficients in a kiln, has been employed to examine the effect of different kiln variables on both the regenerative and the overall heat transfer to the solids. The variables include rotational speed, per cent loading, temperature of gas and solids, emissivity of wall and solids, convective heat-transfer coefficients at the exposed and covered wall, and thermal diffusivity of the wall. The model shows that the regenerative heat flow is most important in the cold end of a rotary kiln, but that generally the temperature distribution and heat flows are largely independent of these variables. Owing to this insensitivity it has been possible to simplify the model with the aid of a resistive analog. Calculations are presented indicating that both the shell loss and total heat flow to the bed may be estimated within 5 per cent using this simplified model. Finally, to examine the overall heat-transfer mechanism in the presence of freeboard flames a mathematical model has been developed to determine both the temperature and heat flux distributions within the flame zone of a rotary kiln. The model, which is based on the one-dimensional furnace approximation, has been employed to examine the effects of fuel type, firing rate, primary air, oxygen enrichment and secondary air temperature on the flame temperature, solids heat flux shell losses, and overall flame length. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

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