Presented here are the results from the steady state flow boiling of R245FA in a laboratory scale horizontal stainless-steel test tube with an inner diameter of 8.5 mm and a length of 900 mm at a saturation temperature of 35 °C and 40 °C. Experiments were conducted at mass fluxes ranging between 200 and 300 kg/m²s at inlet vapour qualities from 0.2 to 0.7 under uniform, and non-uniform imposed heat flux cases that are expected to exist in horizontal parabolic trough solar collectors. Nine (9) different heat flux distributions were investigated. Local and average heat transfer coefficients (HTC’s) were determined based on wall temperature measurements taken along the length and around the circumference of the test section. Through the choice of the fluid being linked to the possible usage of DSG technology in organic Rankine cycles, the qualitative trends and observed performance variations can be used to predict the same for a working fluid such as water. It was found that the non-uniformity of the heat flux greatly alters the HTC’s of the fluid undergoing boiling but has no effect on the pressure drop characteristics of the fluid undergoing boiling. Heating only on the sides of the tube yielded HTC’s that were 46 % lower than achieved under uniform heating. Heating only from the top proved to be more effective in heat transmission to the fluid than heating only from the bottom (as is the case on PTC solar fields), by only a slight margin, and both these cases yielded HTC’s that were 30 % lower than the uniform heating case. Applying a bell curve heat flux distribution over the tube walls yielded overall HTC’s that differed from the uniform case by a maximum of 5 %, even as the peak heat flux position changes around the circumference of the tube. A further study may be done to quantify the degree to which the non-uniformity of the heat flux influences the local HTC’s, and to develop correlations that may aid in predicting these cases. An integration with flow pattern mapping may also be done to solidify the understanding of the phenomenon governing these observations. / Dissertation (MEng)--University of Pretoria, 2021. / Department for International Development (DFID) through Royal Society-DFID Africa Capacity Building Initiative. / The UK Engineering and Physical Sciences Research Council (EPSRC) [grant numbers EP/T03338X/I and EP/P004709/1]. / Russian Government "Megagrant" project 075-15-2019-1888. / Mechanical and Aeronautical Engineering / MEng / Unrestricted

This thesis presents numerical modelling and analysis of a solar Organic Rankine Cycle (ORC) for electricity generation. A regression based approach is used for the working fluid property calculations. Models of the unit&rsquo / s sub-components (pump, evaporator, expander and condenser) are also established. Steady and transient models are developed and analyzed because the unit is considered to work with stable (i.e. solar + boiler) or variable (i.e. solar only) heat input. The unit&rsquo / s heat exchangers (evaporator and condenser) have been identified as critical for the applicable method of analysis (steady or transient). The considered heat resource into the ORC is in the form of solar heated water, which varies between 80-95 0C at a range of mass flow rates between 2-12 kg/s. Simulation results of steady state operation using the developed model shows a maximum power output of around 40 kW. In the defined operation range / refrigerant mass flow rate, hot water mass flow rate and hot water temperature in the system are identified as critical parameters to optimize the power production and the cycle efficiency. The potential benefit of controlling these critical parameters is demonstrated for reliable ORC operation and optimum power production. It is also seen that simulation of the unit&rsquo / s dynamics using the transient model is imperative when variable heat input is involved, due to the fact that maximum energy recovery is the aim with any given level of heat input.

A turbine was investigated by various methods of calculating its efficiency. The project was based on an existing impulse turbine, a one-stage turbine set in an organic Rankine cycle with the working fluid being R245fa. Various methods of loss calculation were explored in the search for a method sufficiently accurate to make valid assumptions regarding the turbine performance, while simple enough to be time efficient for use in industrial research and development.  The calculations were primarily made in an isentropic manner, only taking into account losses due to the residual velocity present in the exit flow. Later, an incidence loss was incorporated in the isentropic calculations, resulting in additional losses at off-design conditions. Leaving the isentropic calculations, the work by Tournier, “Axial flow, multi-stage turbine and compressor models” was used. The work presents a method of calculating turbine losses separated into four components: profile, trailing edge, tip clearance and secondary losses. The losses applicable to the case were implemented into the model. Since the flow conditions of the present turbine are extreme, the results were not expected to coincide with the results of Tournier. In order to remedy this problem, the results were compared to results obtained through computational fluid dynamics (CFD) of the turbine. The equations purposed by Tournier were correlated in order to better match the present case. Despite that the equations by Tournier were correlated in order to adjust to the current conditions, the results of the losses calculated through the equations did not obtain results comparable to the ones of the available CFD simulations. More research within the subject is necessary, preferably using other software tools.

An experimental investigation of condensation of hydrocarbons and hydrocarbon/refrigerant mixtures in horizontal tubes was conducted. Heat transfer coefficients and frictional pressure drops during condensation of a zeotropic binary mixture of R245fa and n-pentane in a 7.75 mm internal diameter round tube were measured across the entire vapor-liquid dome, for mass fluxes ranging from 150 to 600 kg m-2 s-1, and reduced pressures ranging from 0.06 to 0.23. Condensation experiments were conducted for the mixture, as well as its pure constituents over a similar range of conditions. In addition, condensing flow of the hydrocarbon propane was documented visually using high-speed video recordings. Results from these experiments were used to establish the two-phase flow regimes, void fractions, and liquid film thicknesses during condensation of propane flowing through horizontal tubes with internal diameters of 7 and 15 mm. These measurements were made over mass fluxes ranging from 75 to 450 kg m-2 s-1, operating pressures ranging from 952 to 1218 kPa, and vapor qualities ranging from 0.05 to 0.95. Liquid film thickness and void fraction data were subsequently be used to assist the development of heat transfer and pressure drop models. In particular, the heat transfer coefficients and pressure drops observed in the mixture were compared with the corresponding values for the pure constituents. Models for heat transfer and pressure drop in the pure components as well as the mixtures were developed based on the data from the present study. This work extends the available literature on two-phase flow regimes for air-water mixtures, steam, and refrigerants to include hydrocarbons. Additionally, the limited information on condensation in multi-constituent hydrocarbon-hydrocarbon and refrigerant-refrigerant mixtures was extended to include hydrocarbon-refrigerant mixtures. The findings of this study are expected to benefit applications such as refrigeration, low-grade heat-driven power generation, and the development of heat exchangers for the chemical and process industries.