PhD's completed

The following is a list of PhD's dissertations that were recently completed in this Department. 

All these dissertations can be found at UPSpace, the University of Pretoria Institutional Repository. The abstracts of older PhD from the department are summarized in the following archive pages:

[2021-2020][2019] [2018] [2017] [2016] [2015] [2014-2011] [2010-2004

2022

B Barbieri Huyssen, 2022, "FLOW-FIELD IN A LINEAR CASCADE OF FILLETED VANES WITH ENDWALL FILM COOLING" 

The thermal performance of the axial-flow gas turbine, while adhering to certain design limitations, can be improved by reducing the total pressure losses and increasing the film-cooling coverage on the endwall of the blade and vane passages. The secondary flows near the passage endwall and film-cooling hole configurations in the endwall primarily increase the passage aerodynamic losses. According to previous studies, a leading-edge fillet, which fills the intersection of the nozzle guide vane (NGV) and the endwall, has the potential to mitigate the secondary flows. Past investigations also indicate that the interactions between the endwall secondary flows and flows from the film-cooling holes drag the cooling flow towards the passage suction-side and away from the endwall region leaving the pressure-side endwall unprotected from the hot combustion gas. The interactions of secondary flows with the film-cooling flows often add to the total pressure losses. This thesis experimentally investigated the detailed flow field in a filleted vane cascade, employing the endwall film-cooling flow. The objectives were: (i) to reduce the endwall secondary flows in the vane passage through the benefit of a leading-edge fillet and (ii) to combine the effects of fillet with the effects of some new configurations of the leading-edge coolant holes in the endwall in order to cover the pressure-side of the passage endwall with the coolant flow. The experiments were performed in a low-speed linear vane cascade employing the GE-E3 first-stage nozzle guide vane profile with a new leading-edge fillet. The film-cooling flow was delivered through the holes located in the endwall upstream of the cascade as well as inside the cascade passage. The main observations from the measurements were as follows:

• The fillet reduced the total pressure losses in the vane passage because it weakened the passage vortex in the endwall region.

• The distribution of flow temperature showed that the distributions or coverage of film-cooling flows in the endwall region were better with the leading-edge cylindrical holes and leading-edge slots compared to other cases.

• The leading-edge diffused cylindrical-hole configuration provided the lowest pressure losses and good cooling film coverage of the endwall only without the filleted vanes.

Keywords: LE fillet, film-cooling, secondary flows, nozzle guide vane, passage loss.

 

Supervisor:Dr. Gazi Mahmood 

Co-Supervisor: Prof J.P. Meyer


W.J. van den Bergh, 2022, "Experimental investigation into the influence of low mass and heat fluxes and transient perturbances in vapour quality and heat flux on the behaviour of boiling R-245fa in horizontal pipes."

The use of solar power to generate electricity has gained increasing traction over the last few decades, particularly to offset the excessive reliance on fossil fuels and the anthropogenic effect this has on the climate. While photovoltaic electricity production plays a significant role, concentrated solar power, coupled with thermal energy storage, is more suited for providing baseline grid power. One of the prime areas still needing research, however, is the transient nature of solar irradiation and its influence on the operation of the power conversion process.

When a fluid undergoes flow boiling for use in thermodynamic power cycles within the solar field, it enables the implementation of direct steam generation cycles. These offer several thermodynamic, environmental and cost advantages. However, the thermal operating state of these flow-boiling scenarios is more susceptible to short term transient changes in the fluid inlet condition and thermal heat flux boundary condition caused by, for instance, temporal cloud coverage of the collector field. Little documented research into such conditions is available.

In order to help address the current lack of data, this experimental investigation was conducted, in which, particularly, the heat transfer coefficient and pressure drop behaviour was studied for cases where little or no data is available. An experimental test installation was designed and constructed to quantify various effects on the heat transfer coefficient of the flow and pressure drop within the fluid. A horizontal test section was considered which consisted of a uniformly heated pipe with a length of 800 mm and an inner diameter of 8.31 mm. R‑245fa was selected as testing fluid since it has been proposed for possible use in organic Rankine cycles which can also be operated as direct steam generation cycles. For practical reasons a saturation temperature of 35 °C was used and focus was placed on aspects that have not yet received significant attention in literature.

Firstly, the behaviour of the steady-state heat transfer coefficient for relatively low mass fluxes (less than 100 kg/m2s) and relatively low heat fluxes (less than 7.5 kW/m2) was investigated over the entire vapour quality range from 0.05 to 0.90. It was found that a higher mass flux always resulted in a higher heat transfer coefficient for a given heat flux throughout the quality range. However, at the lowest heat flux considered, the heat transfer coefficient exhibited anomalous behaviour in that it was up to 70% higher at vapour qualities of 0.15 to 0.30 than higher heat fluxes.

Secondly, the effects of an abrupt change in vapour quality at the inlet to the test section on the heat transfer coefficient was studied. Mass fluxes of 200 and 300 kg/m2s were considered at vapour qualities of 0.15 to 0.40, such that several flow pattern conditions were covered. It was found that for a step size magnitude of 0.13 in the vapour quality, the actual heat transfer coefficient differed from the expected quasi-steady-state heat transfer coefficient during the transient. It was higher than the expected in the downward step, and lower during an upward step.

Lastly, the impact of a changing heat flux on the flow boiling heat transfer coefficient was considered. These tests were performed at a mass flux of 200 kg/m2s over a vapour quality range of 0.10 to 0.85. Based on actual DNI data, a reduction of 75% in heat flux over a period of 30 s with a step, triangular and sinusoidal waveform pulse was applied in a controlled fashion. It was found that for an abrupt step reduction in the heat flux, that the average heat transfer coefficient during the perturbance was 30% lower than the unperturbed value. For triangular and sinusoidal pulses, the heat transfer coefficient was 8% lower. During all tests, the pressure gradient through the test section was unaffected by any form of perturbance.

Supervisor: Professor J. Dirker 

Co-Supervisors: Professor J.P. Meyer , Professor C.N. Markides 

 


Modaser Hamid Morahed Momin, 2022, "Experimental investigation into natural convection heat transfer enhancement by convective hybrid nanofluids in a square cavity "

In the past decade, nanotechnology research has been increased rapidly. Reports reveal that nanofluids are beneficial heat transfer fluids for engineering applications as superior heat transfer fluids to traditional conventional heat transfer fluids such as water, ethylene glycol, engine oil, mixture EG/W, etc. In addition, the nanofluids have been good to occupy as operational fluids to transfer heat in numerous industrial equipment such as heat exchangers, cooling devices and solar collectors. Researchers continuously have been doing research and investigations to attain the distinctive features of nanofluids and thermophysical properties; the first significant feature, thermal conductivity, has more influence on heat transfer. The idea of hybridization of different kinds of nanomaterial to prepare “hybrid” nanofluids have developed as progressive heat transfer fluids through better thermophysical properties and convective heat comparative to single nanofluids. The investigation supports that the hybrid nanofluids have the capability to increase the effective thermal conductivity of traditional conventional base fluids and are beneficial for various engineering applications.

The current study is interested in applying hybrid nanofluids synthesized by ZnO-MWCNT/DIW, ZnO-GPN/DIW, and ZnO-MWCNT/W: EG on natural convection heat transfer inside a square cavity. Hybrid nanofluids were synthesized by using the two-step process. Various techniques achieved NFs stability and surface characterization. The HNFs were stable for a long period, and no sedimentation was detected. The effective thermal conductivity and viscosity experimentally were measured by standard devices under the influence of PWR, base fluids, temperature difference, and volume concentration. Base fluid κ and superior κ of nanopowder, rising temperature and phi were observed to augment κ of HNF. The κ augmentation by 23.11% for (ZnO: MWCNT_20:80/DIW) at (0.1 vol. % and 55oC), 34.72% for (40:60 – ZnO: MWCNT/W: EG (80:20)) at (0.2 vol. % and 55oC), 27.47% (ZnO: GPN_25:75/DIW) at (0.1 vol. % and 55oC). Temperature increase diminished μ and base fluids μ, higher density of nanopowder, phi improved it for AZM, BAZM, and AZG by an increase of 33.33%, 54.92%, and 12.28%, respectively. Furthermore, models were established for calculating the κ and μ of AZM, BAZM, and AZG as a function of PWR, temperature, and  phi at the considered range from the found experimental data. Heat transfer performance of AZM, BAZM, and AZG was studied by occupying it into a square cavity by measuring natural convection parameters. The observation of natural convection parameters showed that nanocomposites to be superior to those of the single nanofluids of ZnO, which showed the benefit of NP hybridization for industrial application. Furthermore, the experimental data were established to predict the κ, μ and Nu of the formulated AZM,BAZM  and AZG.

Supervisor: Professor M. Sharifpur 

Co-Supervisor: Professor J.P. Meyer

Published by Barbara Huyssen

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