M.Ing. / Heat transfer and pressure drop characteristics during in-tube condensation of nonazeotropic mixtures of R22/R142b in a smooth helically coiled copper tube with an inside diameter of 8.11 mm are investigated. The experimental results are compared with prediction from correlation. The coefficient of performance of.the heat pump built and used for experiments has been studied. The mass flux of the refrigerant was varied during the course of the experiments. At similar mass flow rate of fluids, the average heat transfer coefficients for mixtures were lower than those for pure refrigerant R22 used as reference for comparison. Also, the heat transfer coefficients of all the refrigerants increased with increasing mass flux.

M.Ing. / Heat pump water heaters (HPWHs) have been identified as a viable replacement for electrical resistance heaters due to their high efficiency and reliability. Heat exchangers are a crucial part of HPWHs, and play a vital role in improving the system's coefficient of performance (COP). Experimentally analysing a heat exchanger is usually a slow and highly labour intensive practice since vast amounts of data have to be logged and mathematically manipulated to obtain results. A lot of time and money could be saved, if this process were to be automated. The first objective of this study was to develop a software program capable of calculating the heat transfer correlation constants of a tube-in-tube heat exchanger using the modified Wilson plot technique from data obtained through water-to-water experimentation at different flow rates and temperatures. The data was to be captured by using data acquisition equipment capable of measuring temperature from several Ptl 00-type temperature sensors, pressure transducers as well as Coriolis flow meters, all within a few seconds, thus giving virtually steady-state measurements. The second objective of this study was to develop a software package, capable of capturing and manipulating data from a HPWH system using the same tube-in-tube heat exchanger and using R-22 as refrigerant. The software package had to be capable of capturing all the required experimental data from the system and calculate the local and average heat transfer coefficients on-line and display it to the user. It also had to capture it in the form of a spreadsheet data file for further manipulation. The success of the software package would depend on the results achieved, as well as the time saved with its implementation. To verify the results, the output of the program was compared with the findings of various other researchers. It was found that the output of the program compared well with the results obtained by other researchers, both for the average heat transfer coefficient as well as the local heat transfer coefficients. The time taken for a full set of data was as little as 30 minutes, compared to many hours previously needed to achieve stable results. The software package has thus succeeded in fulfilling its objective to reduce the time taken to achieve accurate results during heat transfer experimentation.

M.Tech. / Due to electric (E) and magnetic (H) fields that vary with space (r) and time (t) in the microwave cavity, and due to the inhomogeneous nature of the minerals, heating a mineral in a microwave oven gives an inherently non-uniform temperature distribution. The objective of the project is to introduce a mathematical model that will demonstrate the thermal interaction between ilmenite mineral (FeTiO3) and microwaves. The simulation presents the temperature distribution in the sample based on the conditions imposed on its boundaries. The field distribution in the cavity is simulated, and then the thermal analysis is performed using the lumped thermal capacity model. The temperature distribution in the sample is also simulated using the general heat conduction equation. Finite difference method is used two solve the two-dimensional unsteady heat conduction equation. The simulation of the field distribution in the cavity reveals that there are position of intense electric and magnetic field in the oven. This is demonstrated by experiment 6, where samples are heated at different positions in the oven for the same duration and different temperatures in the samples were measured. Electromagnetic wave propagation was also studied. It became apparent that the electric and magnetic field can not be treated independently from each other, because the changing electric field produces a changing magnetic field and the newly produced changing magnetic field produces a changing electric field, which is an electromagnetic wave. It is also proved that, considering the relationship given by Maxwell's equations, the electric and magnetic fields are not only space out of phase but they are also time out of phase, meaning that the one quantity is leading while the other is lagging. Based on the available mathematical evidence it was suggested to fit the conventional representation of the electromagnetic field, which show the electric field and the magnetic field at right angle to each other and in time phase, to the new representation which would highlight the fact that the electric and magnetic fields are time out of phase. The study of electromagnetic wave propagation has proved that the one-dimensional conventional representation of electromagnetic waves is inadequate. It does not support the fact that there are a number of resonant modes that exists in the cavity which has long been proved and accepted by authors in the field of electromagnetism. This is very much clear when dealing with electromagnetic waves in three dimensional space.

In current design methods for gas turbines there are important features of the flow which are not yet within the scope of the available prediction methods for both the calculation of surface pressures and heat transfer rates. Such features include the prediction of three-dimensional viscous flowfields, the accurate location and strengths of the secondary flow regimes in a turbine passage, and allowance for time-dependent variations. It is the understanding of the time-varying phenomena which is the subject of this study. Such phenomena occur due to the periodic interaction between stages in a turbine, either that of a nozzle guide vane on a rotor downstream or vice-versa. In most contemporary designs of turbines the effects are due primarily to the wakes from the trailing-edge of the upstream airfoil, and to any associated shock structures resulting from transonic exit flow Mach numbers. The present investigation is concerned with furthering knowedge of these wake and shock interactions, using a method of simulation established in the Isentropic Light Piston Tunnel and Oxford. Measurements of heat transfer rates and pressures are presented, supported by flow visualisation methods such as surface oil-dots and schlieren photography, for two examples of high-pressure turbine rotor blades. The majority of analysis deals with the first of these (a highty-loaded transonic profile) whilst the second blade (designed for use in a large civil engine) is included for investigation of the effects of flow unsteadiness on the film cooling process The transition process is examined in detail by use of wide bandwith heat transfer measurements, and a new method derived for modelling this process. It has been possible to observe the effect of the enhanced turbulence in the simulated nozzle guide vane wake and effects due a shock-boundary layer interaction. The reaction of the blade boundary layers to these disturbances is identified, and trajectories of disturbed events tracked along the blade surfaces. The measurements which have been taken allow for some aspects of wake and shock interactions to be included in the design process for turbine blading. A better understanding has been obtained of how these types of transient flow regimes affect the boundary layers on the blade surfaces.

M.Tech. (Mechanical Engineering) / Eskom fossil fuel power stations operate under various conditions and the ageing fleet of fossil fuel power stations are under strain due to the rising demand in electricity. The quest for cheaper electricity in growing businesses and bringing foreign investment into South Africa is significant for economic growth. Therefore, minimizing operation costs and improving availability, reliability and thermal heat rate are key objectives for the operation of the business. Thermal heat rate calculation is a method used to assess the performance of fossil fuel power stations. It provides an indication of the thermal performance which reflects the condition and operation of the plant. This dissertation refers to the tools used to measure the condenser performance and to identify the reason for unhealthy condenser performance and an increase in thermal heat rate. Problem: Poor condenser performance causes significant losses in generation and thermal heat rate in fossil fuel plants. Loss in generation and thermal heat rate is caused by condenser backpressure. This has an adverse effect on turbine thermal heat rate and henceturbine efficiency. The performance of the condenser affects the turbine performance to a large extent. Decreased condenser performance will increase the turbine thermal heat rate, i.e. kJ/kWh, because of the increased steam consumption per kWh of energy generated and therefore lowering the overall turbine efficiency. This will result in poor thermal heat rate efficiency resulting in an increased coal burn rate, i.e. kg/kWh, and an increased fuel bill and carbon foot print for the fossil fuel power station. The aim of this dissertation is to determine the effects of condenser performance on thermal heat rate and to provide the engineering and operating personnel with a guide for monitoring condenser back pressure in order to improve turbine thermal heat rate and turbine efficiency. Method and Procedure: Chapter 4 describes the method and procedure used for the pre and post outage evaluation to cost-effectively determine the current condition of the condenser and to quantify the increase in thermal heat rate due to condenser degradation. The pre and post outage evaluation offered an opportunity to evaluate and quantify the effectiveness of the maintenance program and the value of the capital layout undertaken during the scheduled Unit outage. Findings: Chapter 5 discusses the findings of the pre and post outage data analysis of the condenser using the EtaPRO system as the tool. Re-tubing 50% of the condenser inner loop tubes and cleaning the other 50% of the outer loop tubes demonstrated a decrease in condenser backpressure and therefore an improvement in thermal heat rate. This will also result in an improvement in turbine cycle efficiency and reduce coal cost.

No description available.

The aim of this study was to determine the influence of alumina increase on the functioning of casting powder during continuous casting. Two aspects of the powder's performance were experimentally measured, namely the influence of alumina on the viscosity and the crystallisation behaviour. These two aspects were then related to the in-mould functioning of the casting powder with the aid of literature references. Casting slag must provide lubrication between the mould surface and the thin steel shell. Alumina increase will cause viscosity increase in casting slags and will hinder lubrication when the inflow of casting slag into the mould/strand gap deteriorates. Heat transfer across the gap is determined by the amount of solid casting slag and the form (glass or crystalline) thereof. Heat transfer across a crystalline material may be up to seven times lower than that across a glass phase. The increased alumina will serve to increase the ratio of glassy phase to crystalline phase in the gap, so increasing the heat transfer. To determine the true extent of alumina increase with modern clean steel practice, samples were taken from the moulds of both the VI and V2 continuous casters at ISCOR Vanderbijlpark. The influence of this alumina increase on the viscosity and the crystallisation behaviour of the casting slag were experimentally measured. Viscosity measurements were done with a rotating bob viscometer (in a vertical tube furnace) on two commercial casting powders (with increasing AI2O3 content). Viscosity prediction models were evaluated with the data from the measured viscosity values. Crystallisation measurements were done by quench experiments with the aid of the hot thermocouple technique. Crystallisation behaviour after a specified heat cycle was presented as the percentage opaque material (crystalline) to vitreous material (glassy) measured with an optical microscope. For the seven sequence casts during which samples were collected from the mould, it was found that the alumina content of the casting slag reached a steady-state value within the first ladle (first 40 minutes) with an increase of 3 to 4 mass %. The influence of a 4 mass % alumina increase on the measured viscosity is small enough that proper lubrication function of the slag will not be negatively affected. Viscosity prediction models vary in their accuracy and are limited with respect to the composition range of the casting powder and the temperature range for which they are valid. The increased alumina content was found to have a strong influence on the percentage crystalline material present. After the sample was heated to 1300°C, kept there for 10s, and then quenched, the crystalline material will decrease from 60 % to 30 % for 5 mass % alumina added. This decreased crystalline material present may notably increase heat transfer. Full crystallisation does not occur at a single temperature, and crystallisation occurs over a temperature interval (of up to 200°C). This means that crystallisation temperature values quoted by casting powder manufacturers depend on the heat cycle and the technique used during crystallisation measurements. Several crystalline phases are usually present in solidified casting slag and these crystalline phases are strongly influenced by the alumina content: the stable crystalline phases may change as alumina content increases. For the current alumina increase in casting powders the effect on viscosity is small, while crystallisation behaviour may be influenced to a greater extent. / Dissertation (M Eng (Metallurgical Engineering))--University of Pretoria, 2006. / Materials Science and Metallurgical Engineering / unrestricted

A study is made of the simultaneous heat and mass transfer processes which occur within a finite wood cylinder of circular cross-section when it is convectively dried. Governing transport equations are developed allowing for the functional dependence of both thermal and moisture diffusivities on moisture content and temperature. Derivative type boundary conditions are included in the analysis. The equations are formulated using a cylindrical coordinate system because it is well suited to modelling wood's anisotropy. In the case considered, the axial coordinate direction is aligned with the wood-grain. Due to the coupling and non-linearity present in the transport and boundary equations, an implicit finite-difference solution scheme is formulated. The three-time-level scheme uses an equation splitting technique to simplify its solution on the computer. A mathematical wood-model, as available in the literature, is refined and used to determine moisture and thermal diffusivities, and mass transfer boundary conditions. Results from this wood-model apply to softwoods below the fiber-saturation moisture content. Combined diffusivities of liquid and vapour are calculated for the radial and axial directions and results for the radial direction are compared to those found from diffusion experiments. Desorptional isotherms are used in the mass transfer boundary condition equations to relate surface humidity and moisture content. The wood-model is used in the mass transfer equation to determine wood drying behaviour under isothermal conditions. Initial moisture contents are uniform and equal to the fiber-saturation value. Solutions are presented to give local and average moisture content as a function of time as well as moisture content profiles. The effect of drying temperature was investigated for one case of wood density and shrinkage. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

M.Ing. (Mechanical Engineering) / In snow shooting, pressurised liquid carbon dioxide is injected via a lance into a permeable snow bag mounted near the ceiling of an insulated transport container. The decrease in pressure causes the liquid carbon dioxide to convert to "snow" and vapour inside the snow bag. The snow bag acts as a phase separator, allowing the sublimated snow to cool down the products inside a container. In this thesis the heat transfer coefficients of such a snow bag were determined experimentally and theoretically. It was found, that on average the measured heat transfer coefficient was 31% lower than the theoretical prediction. The theoretical model was used to correlate the experimental heat transfer coefficient as function of snow height. With this correlation it was possible to predict the time of the snow life to within 24%.

M.Ing. / Gold ore deposits are depleted near the surface of the earth and the remaining deposits are located at great depths of over 3000m. Geothermal energy will cause the ventilation air temperature to increase above 50°C, and therefore it must be cooled to an effective working temperature. The cost of providing acceptable air temperatures to the workers in the mine plays an important role in the feasibility of whether to proceed with a mining project. At present the air in South African gold mines is cooled by a number of processes including bulk air coolers and direct contact spray chambers. In all of the above mentioned processes large quantities of water need to be pumped into the mine where it is used as the cooling fluid to reduce the air temperature. One of the recent developments for cooling the ventilation air uses ice, produced on surface in large quantities and transported underground in conventional plastic piping. It is regarded as the most effective system for creating chilled cooling water. To create an even more effective process, the properties of a new coolant will be investigated. In the new process, ice is transported into the mine and then mixed with a brine solution. The presence of the salt in the brine creates a freezing point depression and an endothermic chemical reaction causing the temperature of the mixture to decrease well below zero degrees Celsius. This chilled brine solution will be used instead of water to decrease the temperature of the ventilation air. The aim of the project is to conduct an analytical and experimental evaluation by using ice and brine as the coolant. In the experimental investigation the coolant is pumped over a flat, multi-plate heat transfer surface where the air flow is maintained at a constant rate. This simplifies the mathematical modeling and gives useful relationships between the experimental results and the theoretical results.