Browsing by Author "Govender, Saneshan."

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Coriolis effect on the stability of convection in mushy layers during the solidification of binary alloys.
(2000) Govender, Saneshan.; Vadasz, Peter.;
We consider the solidification of a binary alloy in a mushy layer subject to Coriolis effects. A near-eutectic approximation and large far-field temperature is employed in order to study the dynamics of the mushy layer in the form of small deviations from the classical case of convection in a horizontal porous layer of homogenous permeability. The linear stability theory is used to investigate analytically the Corio lis effect in a rotating mushy layer for, a diffusion time scale used by Amberg & Homsey (1993) and Anderson & Worster (1996), and for a new diffusion time scale proposed in the current study. As such, it is found that in contrast to the problem of a stationary mushy layer, rotating the mushy layer has a stabilising effect on convection. For the case of the new diffusion time scale proposed by the author, it is established that the viscosity at high rotation rates has a destabilising effect on the onset of stationary convection, ie. the higher the viscosity, the less stable the liquid. Finite amplitude results obtained by using a weak non-linear analysis provide differential equations for the amplitude, corresponding to both stationary and overstable convection. These amplitude equations permit one to identify from the post-transient conditions that the fluid is subject to a pitchfork bifurcation in the stationary case and to a Hopf bifurcation associated with the overstable convection. Heat transfer results were evaluated from the amplitude solution and are presented in terms of the Nusselt number for both stationary and overstable convection. They show that rotation enhances the convective heat transfer in the case of stationary convection and retards convective heat transfer in the oscillatory case, but only for low values of the parameter X I = 8 Pr ~ 0 So· The parameter 1/ X I represents the coefficient of the time derivative term in the Darcy equation. For high X I values, the contribution from the time derivative term is small (and may be neglected), whilst for small X I values the time derivative term may be retained.
The development of a guideline to assist with compiling asset management plans for transmission lines.
(2010) Mansingh, Sharan.; Ijumba, Nelson Mutatina.; Govender, Saneshan.
The overhead transmission line is a fundamental component in the power supply system as it links electricity supply to the various points on the electrical network. Failure of the transmission overhead line will result in interruption of supply and depending on the network configuration may result in long term outages. It is therefore essential that the overhead transmission line asset is inspected and maintained regularly to prevent premature failure. Newer approaches to maintenance management are required to improve the overhead transmission lines performance and reduce the cost and risk associated with the asset. Asset management is seen as the process that can be adopted to enhance overall management of the overhead transmission line. The review of maintenance practices of various Utility’s and that of a pilot site made up of selected lines within Eskom’s North East Transmission Grid revealed numerous shortcomings in the current practices largely due to the application of traditional (non-holistic) methods. This situation supports the development of asset management plans which will cater for improvement in performance, reduction in the risk and cost and achieving service level targets. This research has used asset management principles to design a guideline in the form of a flowchart for effective maintenance management for overhead transmission lines. The key benefits/advantages of the maintenance management guideline are as follows: It is closed loop and process driven. Decision making is more scientific because it requires the use of historical performance data, detailed asset condition information and encourages quantitative analysis. Promotes defect and condition assessment tracking via the condition database. Rather than focusing mainly on defect management, the asset manager will be directed towards the performance specifications and the condition database to establish individual action plans which can be prioritized against short, medium and long term improvement plans per specific asset.
Economic evaluation of a district cooling system incorporating thermal storage.
(2003) Bannerman, Andrew.; Govender, Saneshan.
The following report investigates district cooling systems. This form of technology provides an alternative means of providing cooling. In a traditional cooling system each building would include cooling equipment to serve only that building. District cooling differs in that water is chilled at one location and pumped to two or more buildings. District cooling has many benefits over traditional cooling systems. This report, however, aims to determine the economic benefits (if any) of district cooling systems. The location chosen as a model for this study was the University of Natal (Durban) campus. This campus currently operates a district cooling system serving six buildings. This study is hypothetical in nature, as the cooling system is already finalized and operational. The aim of this dissertation is to answer the question of which would be the more attractive alternative if the University were in a position of having to install a completely cooling system. One of the most important steps in this process is the calculation of cooling loads. The cooling load was estimated for each of the buildings associated with the district cooling system. The LOADEST software package was used to derive these cooling loads. The accuracy of LOADEST software was also validated in this study. The bulk of this report is composed of the preliminary work required to obtain capital and operating costs for cooling systems, including validation of cooling load calculation software. It was felt that this prelimiinary work justified inclusion in the final report to provide accurate representation of the steps taken before any economic evaluation could be reached. The capital and operating costs of the district cooling system and a more traditional system were compared. It was found that the district cooling system reduces operating costs significantly, although it's capital cost is higher than the traditional system against which it was compared.
An experimental and numerical convective heat transfer analysis over a transonic gas turbine rotor blade.
(2006) Cassie, Keith Baharath.; Govender, Saneshan.
An experimental and numerical investigation of the flow and convective heat transfer distribution around a high turning angle gas turbine rotor blade has been carried out at the University of Kwa-Zulu, Durban campus. This study in gas turbine blade aerothermodynamics was done to meet the research and development requirements of the CSIR and ARMSCOR. The experimental results were generated using an existing continuously running supersonic cascade facility which offers realistic engine conditions at low operating costs. These results were then used to develop and validate a 2-D model created using the commercially available Computational Fluid Dynamics (CFD) software package, FLUENT. An initial phase of the study entailed a restoration of what was an unoperational experimental facility to a state capable of producing test simulation conditions. In the analysis, a 4-blade cascade system with provisions for an interchangeable, test blade was subjected to the steady state conditions set up by the facility. Firstly, the flow was characterised by evaluating the static pressures around the midspan of a pressure measurement test blade. This was done using two pressure transducers, a scanivalve, an upgraded data acquisition system and LABview software. The method for measuring the heat transfer distributions made use of a transient measuring technique, whereby a pre-chilled Macor test blade, instrumented with thin film heat flux gauges was rapidly introduced into the hot cascade flow conditions by displacing an aluminum dummy blade while still maintaining the flow conditions. Measurement of the heat flux and generation of the isothermal heat transfer co-efficient distributions entailed re-instrumentation of the test blade section with gauges of increased temperature sensitivity along with modifications of the associated electrical circuitry to improve on the quality of experimental data. Both the experimental flow and heat transfer data were used to validate the CFD model developed in FLUENT. An investigation into different meshing strategies and turbulence models placed emphasis on the choice of model upon correlation. The outcome of which showed the k -co model's superiority in predicting the flow at transonic conditions. A feasibility study regarding a new means of implementing a film cooled turbine test blade at the supersonic cascade facility was also successfully investigated. The study comprised of experimental facility modifications as well as cascade and blade redesigns, all of which were to account for the requirements of film cooling. The implementation of this project, however, demanded the resources of both time and money of which neither commodity was available.
Experimental and theoretical analysis of heat transfer on a transonic blade.
(2008) Essa, Zaahir Ahmed.; Inambao, Freddie Liswaniso.; Govender, Saneshan.
This dissertation involves the experimental and numerical analysis of heat transfer on a transonic high turning angle gas turbine blade, which has been performed on the supersonic cascade experimental facility at the University of KwaZulu-Natal (UKZN), as part of the continuous research and development project run under ARMSCOR. Efforts have been made to keep constant maintenance on the experimental rig. This now functional rig was used to generate experimental results, which were used to validate numerical models created using the commercially available computational fluid dynamics (CFD) package of FLUENT. The facility at UKZN is a continuously running cascade system, which consists of a plenum run under vacuum pressure and houses a four-blade cascade. One of these SMR-95 turbine blades is instrumented with thin-film gauges, which allow heat transfer measurement via a heat transfer analogy through electrical circuit boards. This blade is interchangeable with an instrumented blade with pressure tappings along its span for pressure distribution tests. This facility was used to validate flow measurements, and results compared to previous test data conducted on the rig, using two pressure transducers, a scanivalve and a data acquisition system with LabView software. The method of generating heat transfer measurement results involved pre-chilling the test blade in a cooling box, before rapidly plunging it directly into a hot-air stream. Re-instrumented and more sensitive thin-film gauges would react resistively according to the temperature change. The heat transfer coefficient distribution was calculated using LabView. The turbulence intensity at the inlet of the cascade was varied using a grid of rods of varying diameter. For 15% turbulence intensity, there was a 16% overall increase in heat transfer on the pressure side, and 25% increase on the suction side. For 25.5% turbulence intensity, there was an overall increase of 23% on the pressure side and 40% on the suction side. The results compared favourably to that of previous results generated by Stieger (1998). The experimental results were used to validate and compare to the CFD model developed in FLUENT. Improvements were made with the meshes developed previously, and results obtained showed that the general trend of distribution was similar, although certain models varied in the correct prediction of magnitude. This research includes a comprehensive study of various methods of numerical heat transfer measurement techniques, which would be used to replace the current ageing electrical heat transfer analogue method used at UKZN.
Heat transfer enhancement in nano-fluids suspensions : thermal wave effects and hyperbolic heat conduction.
(2005) Vadasz, Johnathan John.; Govender, Saneshan.
The spectacular heat transfer enhancement revealed experimentally in nanofluids suspensions is being investigated theoretically at the macro-scale level aiming at explaining the possible mechanisms that lead to such impressive experimental results. In particular, the possibility that thermal wave effects via hyperbolic heat conduction could have been the source of the excessively improved effective thermal conductivity of the suspension is shown to provide a viable explanation although the investigation of alternative possibilities is needed prior to reaching an ultimate conclusion.
Reliability analysis of power transformers : case : Eskom Distribution Eastern Region, 1MVA to 80MVA power transformers.
(2007) Chetty, Manogaran.; Ijumba, Nelson Mutatina.; Govender, Saneshan.
This dissertation analyses the reliability of power transformers and its impact of failure on system performance. Eskom Distribution, Eastern Region is used as a practical case study, which has an installed transformer base of 6066MVA comprising of 428 transformers ranging from 1 MVA to 8OMVA with voltage levels of 6.6kV to I 32kV. The literature review illustrates the theory and principles of transformers, evolution and changes in design criteria, the function of cellulose and insulating oil, failure modes. operations and maintenance practices and factors affecting the distribution systems performance. This study included a conditional assessment and an oil analysis review of transformers at Eskom. A method to trend multiple oil samples was developed and illustrated. The research further investigates the reliability of series and parallel systems using actual component reliability values. A study was conducted to establish the degree of network firmness. Transformer failure data was analysed and were shown to be characteristic of a bathtub curve. Defects from on site inspections were analysed and identified oil leaks as a maintenance focus area. The Distribution Supply Loss Index was determined to be the major impact Key Performance Index due to transformer failures. Transformer failures using statistical methods, showed HV/LV winding to be the main component to fail. The cost of a transformer failure to Eskom and the customer was determined. International Benchmarking was investigated to establish the criteria for network reliability indices and to compare the network infrastructure and performance of international utilities and Eskom. The later part of the study involved the analysis of a risk ranking methodology to establish a risk ranking matrix. The transformers were ranked according to the matrix, identifying the high risk focus areas. Projects were raised within Eskom to replace the identified high risk transformers. This study has concluded that the reliability of transformers is impacted by the changes in transformer design, increased maintenance defects and inadequate transformer protection at substations. The reduced oil volume per kVA, increased hot spot and ambient temperature, and compact tank designs have resulted in the cellulose being overheated and fault gases being produced during normal operating conditions. The increase in load demand from the existing transformer fleet and a reduction in capital expenditure to maintain and build additional substations have also contributed to accelerated aging, since the transformers are forced to operate at 100 percent loading. There is an increase in transformer maintenance defects due to in sufficient operational staff, high staff turnover, reduced skills transfer. and insufficient network contingencies to allow for planned outages to clear the defects identified. The failure analysis showed that the main component to fail is HV/LV windings. The winding failures were traced to there being no or inadequate transformer protection at -20% of substations.
Thermal shock and CFD stress simulations for a turbine blade.
(2002) Ganga, Deepak Preabruth.; Govender, Saneshan.
A 2-D CFD / FEM model to simulate thermal stresses in a turbine blade has been set up using the software FLUENT and FIDAP. The model was validated against the data of Bohn et. al. (1995) and was used to simulate 5 test cases. The numerical model was set up for a single Mark II nozzle guide vane (NGV) and utilised the appropriate boundary conditions for the surrounding flow field. A commercially available software code, FLUENT, was used to resolve the flow field, and heat transfer to the blade. The resulting surface temperature profile was then plotted and used as the boundary conditions in FIDAP (a commercial FEM code) to resolve the temperature and stress profile in the blade. An additional solver within FLUENT essentially superimposes an additional flow field as a result of the NGV vibration in the flow field. The pressure, temperature and heat transfer coefficient distribution, from FLUENT, were compared to those from Bohn et. al. (1995). The model predicted the distributions trends correctly, with an average over-prediction for temperature, of 10 % on the suction side and 6 % on the pressure side. This was restricted to the region from leading edge to 40 % chord on both sides of the blade. The blade temperature and equivalent stress contour trends were also correctly predicted by FIDAP. The blade temperature was over-predicted by and average of 1.7 %, while the equivalent stress magnitude was under-predicted by a worst case of 43 %, but the locations of maximum stress were correctly predicted. The reason for the differences between the stresses predicted by FLUENT / FIDAP and the data given in Bohn et. al. (1995), is believed to be the results of the temperature dependence of the material properties for the blade (ASTM 310 stainless steel), used in the two studies, not being identical. The reasoning behind this argument is because the distribution trends and contour variation, predicted by the model, compared favourably with the data of Bohn et. aI., and only the equivalent stress magnitude differed significantly. This completed the validation of the FLUENT / FIDAP model. The model was used to simulate test cases where temperature (i.e. turbine inlet temperature or TIT), at the model inlet (Le. the pressure inlet boundary in FLUENT), was set up to be time varying. Four simplified cases, viz single shock, multiple shocks, simplified cycle and multiple cycles, and a complex cycle (a mission profile) were simulated. The mission profile represented typical gas turbine operational data. The simulation results showed that stress was proportional to TIT. Changes in TIT were seen at a later time in the stress curve, due to conduction through the blade. Steep TIT changes, such as the shock loads, affected stress later than gentler TIT changes - the simplified and multiple cycles. These trends were consistently seen in the complex cycle. The maximum equivalent stress was plotted against TIT to try and develop a loose law that gives maximum equivalent stress as a function of TIT. A 4th order polynomial was fitted through the maxima and minima of the maximum equivalent stress plot, which gave the maximum and minimum stress as a function of TIT. This function was used calculate the maximum and minimum and mean equivalent stress using the TIT data for the mission profile. Thus, the FLUENT I FIDAP model was successfully validated, used to simulated the test cases and a law relating the equivalent stress as a function of TIT was developed.
Thermal shock and thermal stress prediction on a highly loaded turbine nozzle guide vane based on an aerodynamic and thermal analysis.
(2005) Kulik, Krzysztof.; Govender, Saneshan.
A 2-D plain strain CFD/FEM model to simulate thermal shocks and stresses in a turbine blade has been set up using the commercially available software FLUENT and NASTRAN. The model was validated against the experimental data of Bohn et. al. and used to simulate real test cases. The steady state numerical model was set up for a single Mark II nozzle guide vane using the correct boundary conditions to resolve the flow field. A combined laminar and turbulent model was developed in FLUENT that was used to highly accurately predict the pressure, temperature and heat transfer coefficient distribution on the blade surface as well as the temperature distribution on the cooling holes inside the blade. The resulting temperature profiles on the blade and cooling holes were used as boundary conditions for the FEM analysis to resolve the internal temperature and stress profiles. The pressure, temperature and heat transfer distribution on the blade, from FLUENT, were compared to those from Bohn et. al. The predicted pressure distribution was exact with the experimental results and the predicted temperature distribution had an average overprediction of 1.4 % on both the pressure and suction side. The internal temperature profile predicted by NASTRAN was correctly predicted with an average over-prediction of 2 %. The stress contours were accurately predicted with the stress magnitude varying by 17 % to that of Bohn et. al. The reason for the difference between the MSC.NASTRAN and Bohn et. al. stress results is believed to be purely solver related. Bohn et al. used a FEM package called MSC.MARClMentat. With the steady state model validated, transient test cases were simulated that represent typical operational data. The mission profile was obtained for the T-56 engine found on the C130 cargo plane. The model was used to simulate the test case where the turbine inlet temperature (TIT) varied with time. The simulation results showed that stress was proportional to TIT, where changes in the TIT were seen later in the stress curve, due to conduction in the blade. Steep TIT changes, such as shock loads affected stress later than gentler TIT changes. Thus, the FLUENT / NASTRAN model was successfully validated, and used to simulate a flight mission profile. The goal to calculate quality unsteady stress profiles was achieved and forms the boundary conditions for thermal fatigue calculations.