One of the basic goals in engineering is to formulate models which will provide a means for analytically predicting observed phenomenon. For some time, the partial differential equations describing the steady state and transient conduction of heat within a solid have been available. However, the straight forward use of these equations is often restricted due to the surface geometry of the solid. If the surface geometry is at all irregular, exact solutions will in general not exist. In that case, a solution is sought by some approximate numerical technique. The two techniques most often used are the finite difference method and the finite element method. The finite difference method is fairly simple to understand, but is difficult to apply to a problem with irregular boundaries. On the other hand, it is not a trivial matter to completely understand the finite element method, although it can handle irregular boundaries with greater ease than the finite difference method. To bridge the gap between these two methods, a third method is developed in this work which has the simplicity of the finite difference method, and can handle irregular boundaries with the ease of the finite element method.

This dissertation presents experimental data correlations for the spatially - averaged convective heat transfer coefficient for thin walled closed containers during and after gas injection. The different modes of heat transfer were identified, and correlations were made for each. Correlations are presented for the injection period, post-top injection, post-bottom injection, post- tangential, post-radial injection, and post-ejection heat transfer for various tank geometries. Of special significance are the very high heat transfer rates that were shown to be present in some cases immediately after injection. Heat transfer rates were shown to be, for a short period, up to almost two orders of magnitude higher than the natural convection predictions would indicate.

In the usual thermal stress analysis of a body, the stresses are found for a predetermined temperature distribution. In problems involving composite materials, thermal contact resistance between materials can significantly affect the temperature distribution. The contact resistance is normally a function of the contact pressure and thermally induced stresses can affect the temperature distribution. Thus in composite bodies the thermal stress problem and the heat transfer problem may be coupled and require a simultaneous solution of both problems.

Heat transfer in turbulent flow from external surfaces has been investigated extensively in the past. Most of the theoretical and experimental work deals with a two dimensional boundary layer on isothermal or uniform heat flux surfaces with longitudinal temperature variation. However, little attention has ever been paid to the case in which the temperature of the plate varies transversely. This is a three dimensional problem due to the spanwise variation of the thermal boundary layer. The most elementary example of this is the boundary formed by the sudden temperature impulse that acts transversely across a flat plate as shown in Fig. 1. Theoretically, this spanwise boundary increases the heat transfer coefficient. A knowledge of this is important in the field of heating and cooling system design. It is also important for the investigation of spot-heat flux measurement.

The purpose of this study was to develop and demonstrate a method for experimentally determining the convective heat transfer in a rigid vessel while air was being injected. The heat transfer took place between the air in the pressurized vessel and the surrounding walls which were maintained at a temperature of 32 F. with a circulating ice water bath. The study considers the effects of injection geometry and injection flow rate on the heat transfer process. The problem of heat transfer after injection has been considered by Means (1), and was responsible for establishing this study. The experimental technique used in this thesis provides a method by which Means may establish the heat transfer just after the injection period.

In recent years much attention has been devoted to the development of efficient heat exchangers. The regenerative or moving matrix heat exchanger is one type which has been considered. Most regenerative heat exchangers use one of the conventional types of high area matrices mounted in a drum or disk in such a manner that it can be moved in a continuous cycle from a cold fluid stream to a hot fluid stream and back again. A rather different approach to the design of a regenerative heat exchanger which has been patented in this country is a set of flexible moving bands which carry heat energy from one fluid stream to another in a continuous manner. Such a regenerator is illustrated in Figure 1. This is essentially a parallel plate heat exchanger with a variation in the temperature of the plates and the fluid in the direction of the length of the plate.

Green roofs have become an important urban mitigation technology due to their ability to address multiple environmental issues. One of the most common benefits attributed to green roofs is the reduction in heating and cooling loads in buildings by dissipating heat through evaporation. This study focuses on evaluating the effect that evaporative cooling has on the thermal performance of green roofs. Sponge and floral foam were used as porous media for their ability to retain water inside its body, transport it to the surface, evaporate it at a constant rate and for their different pore sizes. Test trays containing sponge or floral foam saturated with water were tested in a low speed wind tunnel equipped to measure weight, temperature and heat flux. Two types of experiments were conducted: one with evaporation at the surface, and the other with evaporation blocked by an impervious layer. The testing conditions for all tests were kept constant except for the ability of evaporation to happen. Evaporation rate for floral foam was 0.14 kg/m2hr and 0.29 kg/m2hr for sponge. Results of tests with evaporation show a decrease of 45-49% in heat conducted through the roof when compared to the tests without evaporation. For optimal thermal performance of green roofs, a material that enhances water transport and thus evaporation at the surface is necessary with large pores and low field capacity. Surface temperatures on test with evaporation were found to be between 3-7°C lower than those without evaporation. Applying a 2 sample t-test to the data, the relationship between heat flux and evaporation was found to be statistically significant.

With the advent of the new" high Tc superconductors as well as the increasing use of cryo-cooled conventional electronics, liquid nitrogen will be one of the preferred cryogens used to cool these materials. Consequently, a more thorough understanding of the heat transfer characteristics of liquid nitrogen is required. In these investigations the transient heating characteristics of liquid nitrogen to states of nucleate and film boiling under different liquid flow conditions are examined. Using a metal hot wire/plate technique, it is verified that there is a premature transition to film boiling in the transient case at power levels as much as 30 percent lower than under steady state nucleate boiling conditions. It is also shown that the premature transition can be reduced or eliminated depending on the flow velocity The second part of this research analyses the nucleation (boiling) process from a dynamical systems point of view. By observing how the boiling system variables evolve and fluctuate over time, it is hoped that physical insight and predictive information can be gained. One goal is to discover some indicator or signature in the data that anticipates the transition from nucleate boiling to film/boiling. Some of the important variables that make up the boiling system are the temperature of the heater and the heat flux through the heater surface into the liquid nitrogen. Results, gained by plotting the system’s trajectory in the heat flux-temperature plane, are that on average the system follows a counterclockwise trajectory. A physical model is constructed that explains this behavior. Also, as the applied heater power approaches levels at which the transition to film is known to occur, the area per unit time swept out in the heat flux-temperature plane is seen to reach a maximum. This could be of practical interest as the threshold to film boiling can be anticipated and possibly prevented.

Master of Science in Engineering by research: “A dissertation submitted to the Faculty of Engineering and Built Environment, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Master of Science in Engineering.” Johannesburg, 05 February 2018 / In most industrial processes, energy is an integral part of the production process; therefore, energy consumption has become an intensified area in chemical engineering research. Extensive work has been done on energy optimisation in continuous operations; unlike in batch operations because it was believed that due to the small scale nature of batch plants, small amounts of energy is consumed. Certain industries such as the brewing and dairy industries have shown to be as energy intensive as continuous processes. It is, therefore, necessary for energy minimisation techniques to be developed specifically for batch processes in which the inherent features of batch operations such as time and scheduling are taken into account accordingly. This can be achieved through process integration techniques where energy consumption can be reduced while economic feasibility is still maintained. Most of the work done on energy minimisation either focuses on direct heat integration, where cold and hot units operating simultaneously are integrated, or indirect heat integration, where units are integrated with heat storage. The schedules used in these models are, in most cases, predetermined which leads to suboptimal results. This work is aimed at minimising energy consumption in multipurpose batch plants by using direct heat integration together with multiple heat storage vessels through mathematical programming. The proposed approach does not use a predetermined scheduling framework. The focus lies on the heat storage vessels and the optimal number of heat storage vessels together with their design parameters, namely size and the temperature at which the vessels are initially maintained, are determined. The formulation developed is in the form of a mixed integer non-linear program (MINLP) due to the presence of both continuous and integer variables, as well as non-linear constraints governing the problem. Two illustrative examples are applied to the formulation in which the optimal number of multiple heat storage vessels is not known beforehand. The results rendered from the model show a decrease in the external utilities, in the form of cooling water and steam, compared to the base case where no integration is considered and the case where only one heat storage vessel is used. / MT 2018

A Dissertation for the Master’s Degree in Mechanical Engineering School of Mechanical, Industrial & Aeronautical Engineering Date: 02/02/2017 / The Friction Stir Welding process is a rotating tool, that consists of a specialy designed shoulder and pin, that is plunged into the joining line of the required material and traverses along this line. The friction is induced by the rotating tool causes the workpiece material to rise to an operating temperature of 70% to 90% of the workpiece material's melting temperature and resulting in, no phase change, nor any defects associated with phase change, occurs in the workpiece. The increased temperature of the material causes the shear yield strength to drastically decrease thus allowing the two pieces to plasticise, easily stir around the tool and subsequently join. As the tool traverses along the workpiece, the softened material cools in the wake of the rotating tool and recrystallises, forming a ne grained microstructure. Attempts to develop an innovative tool to correlate the resulting of thermal models with process parameters are scarce. In this work, 6056-T4 and 6082-T6 Aluminum alloy sheets are friction stir welding at different rotational and translational speeds during the experimental aspect and material 2024-T3 for the analytical calculations. The effects of process parameters on the resulting thermal and mechanical properties are investigated. The results show that the use of coolant during the friction stir weld decrease heat generation substantially, this can also affect the force of the weld. It is also observed that the shear strenght of the processed sheet depends strongly on the rotational and translational speeds as weld as the thermal aspect and varies widely within the processed region, this was shown in this study by evaluating the thermal aspects of different weld types namely the Standard tool, Bobbin tool and the innovative tool. In addition. The proposed approach involves determination of the use of the friction stir welding in different thermal conditions and championing the use of an innovative tool. / MT2017