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Master's Degrees Completed 2015

Nicole Coetzee, 2015. "Heat Transfer Coefficients of Smooth Tubes in the Turbulent Flow Regime"

Several well-known equations are available in literature that can be used to determine the heat transfer coefficients of smooth tubes in the turbulent flow regime. When comparing the results of these equations they vary over a considerable range. Although in many cases it is assumed that the Gnielinski equation is one of the most accurate as it is the most recently developed correlation, the uncertainty of this equation has not be quantified as yet. Many of the equations have been developed using the data that was obtained from experimental testing as far back as 80 years ago. As more accurate instrumentation is available, it should be possible to collect more accurate data during experimental testing. The purpose of this study was threefold: to take accurate heat transfer measurements and to quantify the uncertainties of the Nusselt numbers as a function of Reynolds number; to compare the measured data with existing correlations and  to develop an accurate Nusselt number correlation from the data. Experiments were conducted on two smooth circular tubes with a heat transfer length 3.75 m with inner tube diameters of 8.3 mm and 14.2 mm using water over a Reynolds number range of 10 000 to 220 000 and a Prandtl number range of 3.2 to 4. Surface roughness analysis was also performed to ensure the tubes can be considered as smooth tubes. Pressure drop measurements were also taken over tube lengths of 4.1 m as a function of Reynolds number. The experiments were conducted using a tube-in-tube heat exchanger with the hot water in the inner tube and the cold water in the annulus operating in a counter flow configuration. The friction factor values were determined from pressure drop measurements and the heat transfer coefficients were determined from the Wilson Plot method using temperature and mass flow rate measurements. The average uncertainties of the friction factors and Nusselt numbers were both less than 3%. The results were compared to the existing literature and it was found that at lower Reynolds numbers the Nusselt numbers deviated slightly to those of Gnielinski with the results comparing more closely with increasing Reynolds number. The friction factor results correlated well with that of Blasius. A new Nusselt number correlation, which is a function of Reynolds number, Prandtl number and friction factor, was developed. The equation predicted all the measured values within 3%. The equation is, however, limited in not taking into consideration large variations in fluid properties. 

Supervisor:         Prof J.P. Meyer



Torsten Peter Löwe,2015. "Modelling of a Passive Hydraulic Steering for Locomotives"


During the past few decades, substantial improvements were made to rail infrastructure worldwide. This was necessary to accommodate the ever increasing transportation demand and requirements. Nowadays, trains are required to transport heavier loads and to travel at higher speeds.
One of the major improvements was achieved by the development of the off-flange curving bogie designs to reduce wheel and rail wear. Off-flange designs include passive steering and actively controlled steering. The development and implementation of self-steering bogies on locomotives was promoted in the early 1980’s by two major locomotive manufacturers. Up to date, thousands of these locomotives, with built-in self-steering bogies, have been manufactured and taken into service (Swenson, 1999).
Most self-steering bogies have mechanical linkage systems to steer the wheel sets. As an alternative to the mechanical linkage system, the DCD Group (a South African manufacturer of rail and mining equipment) initiated the development of a Passive Hydraulic Steering (PHS) system.
First PHS prototype systems, developed by DCD, have proven that huge wear reduction possibilities exist on both, rails and wheels. In addition the prototype systems also significantly decreased noise and vibration levels when negotiating tight corners (Swenson & Scott, 1996 and DCD Rolling Stock, 2012). However, existing prototype solutions require further improvements and development for optimisation. To be able to identify and implement improvements, the need exists to perform modelling and testing of the systems to obtain a better understanding of the operation and suitability of a complete unit.
The aim of this research project is thus, to mathematically model an existing prototype PHS system and validate the model with data from experiments and tests. This model can then be used in order to improve and optimise performance, cost and reliability of the system, before mass production is considered.
A literature survey was conducted, focusing on general wheel and rail wear mechanisms, techniques to improve wheel and rail life and on existing techniques for modelling the hydraulics and multi-body dynamics of locomotive systems.
The literature survey was followed by extensive laboratory tests on component basis, a quarter PHS system and on the full PHS system. From these tests all parameters needed for the characterisation of the PHS and the mathematical model were determined. These tests also provided data for the validation of the PHS model.
Finally, a mathematical model of the PHS system was successfully generated and validated. This model can now be used in a multi-body dynamic locomotive simulation to evaluate its effectiveness.
The results and findings of the literature survey, experiments and modelling are reported on and discussed in this report.

Supervisor: Prof P.S. Els
Co-Supervisor: Dr C. Kat

 


 

 Mangolo Masenya, 2015. "Enhancing the maintenance system for critical pumps in a power station"

Maintenance is a significant aspect of asset management. Recent findings on Eskom’s asset management system in 2013 and the 2014 publication of the new asset management standard (ISO 55000) has highlighted the need to evaluate maintenance within the power utility to ascertain the degree of alignment to the minimum requirements stipulated in the standard. This research project derives requirements for a maintenance system, evaluates current maintenance processes and activities at a power station, and offers recommendations for improvement where gaps are identified.

The principal objective of this research project is to enhance maintenance of critical assets in the Eskom power production process, using the Eskom Matimba power station as the basis for the investigation. This objective is achieved by first establishing the current maintenance system and practices at the power station through collection and review of documented maintenance-related information, policies and procedures from the station. Additional maintenance-related information is also sourced through interaction and interviews with staff employed by the power station.

The research then gathers the requirements for an effective and efficient maintenance organization from literature, as well as asset management standards, and systematically evaluates the existing maintenance system for selected assets (critical pumps in electricity generation process) against these requirements with the aim of identifying improvement opportunities to the current maintenance system.

In addition to frameworks for effective maintenance, the research also investigates quantitative tools for optimizing maintenance and develops a reliability engineering model to optimise asset maintenance costs for selected critical pumps. The research also highlights factors that influence the success of improvement initiatives such as adoption of ISO standards. 

Findings from evaluating the maintenance system for critical pumps at the power station against asset management requirements are that fundamental requirements for a maintenance system such as maintenance resource allocation, documented maintenance policies, computerised maintenance management systems and commitment to continuous improvement are met and in place. Areas of improvement identified and recommendations include increasing proactive maintenance; enhancing comprehensive asset information management, increasing asset management awareness and increasing functional or departmental integration.

The findings from the research are applicable to the power station studied and offer a starting point for a maintenance improvement initiative for this station. However, the findings and recommendations are seen to be applicable to maintenance of other critical assets in similar power plants and are potentially beneficial in establishing a maintenance system for new power plants.

supervisor: Prof PS Heyns


 

Benjamin Gwashavanhu, 2015. "Evaluation of optical techniques applied to online turbomachinery blade vibration measurements".

 

Understanding the dynamic characteristics of blades is important in the online condition monitoring of turbomachinery.  Conventionally contact methods are used for this purpose.  However improvements in technology now allow for the use of non-contact methods.  Contact measurement techniques for turbomachinery blade vibration analysis typically involve the use of strain gauges and accelerometers.  These present some complications when analyzing rotating machinery.  Being contact in nature, mass loading can affect the integrity of measurements captured.  Turbomachines typically operate under the adverse conditions of high temperatures, high flow rates and sometimes wet environments.  This significantly reduces the life of contact transducers installed to capture the blade dynamics.  Installation of telemetry systems for signal transmission is also necessary.  In addition to being invasive and expensive, telemetry systems can introduce electrical noise.

These factors make it desirable to explore the applicability of various optical non-contact methods for analyzing turbomachine blade vibrations, such as Laser Doppler Vibrometry (LDV) and photogrammetry.  Both techniques have been successfully used to analyze vibrations of structures.  Photogrammetry is a full-field measurement technique which allows for non-intrusive simultaneous measurement of vibrations at different locations on a blade.  This is particularly important for the updating of numerically developed models of structures, investigation of structural global dynamics, and more effective localization of damage.  Accelerometers have been used to validate a variation of photogrammetry, three dimensional point tracking (3DPT), for rotational applications and discrepancies attributed to the contact nature of accelerometers were observed.  To build confidence in the use of 3DPT as a non-contact method for analyzing rotating machines, it is necessary to investigate how well it correlates with various non-contact methods.  Through such an investigation aspects that need to be addressed when using 3DPT to analyse turbomachines can be identified.  If reliable measurements can be obtained using this technique, further investigations such as online damage detection and characterization in rotating structures can be conducted.

In this study Tracking Laser Doppler Vibrometry (TLDV) and 3DPT are used as non-contact methods to investigate the online vibrations of a turbomachine test rotor.  To employ TLDV on the test rotor, the dynamics of the scanning mirrors of a Polytec Scanning Vibrometer (PSV) are characterized using a frequency response approach.  Look-up tables are constructed to provide the necessary phase angle compensation for the two signals supplied to the mirrors, to obtain a circular scanning path.  Photogrammetric 3DPT is then validated by tracking the TLDV laser spot focused on one of the test rotor blade using high speed cameras, and comparing the 3DPT measurements to TLDV blade out-of-plane vibration measurements.  The correlation between the two non-contact measurement techniques is presented. This establishes the validity of the employed scanning system, and also serves to show how well the two non-contact methods correlate with each other, when investigating dynamics of turbomachinery blades.  3DPT is then used to analyze the responses of the test rotor blades under excitation. Various Operational Deflection Shapes (ODSs) of the blades are identified and the results obtained are presented.

The use of ODSs obtained from 3DPT to investigate irregularities along turbomachinery blades is also presented.  This information is used to show that ODSs captured using 3DPT can be used to online detect and localize blade damage in turbomachines.

supervisor: Prof PS Heyns


                          

Jan-Sjoerd van den Bergh, 2015. "Effects of Friction and Gas Modelling on Vehicle Dynamics Simulation".

 

 

Validated simulation models have become ever more important in the current technological and economic environment, where simulation is an integral part of the design process. In the field of vehicle dynamics, it is no different, where vehicle manufacturers and researchers are relying more heavily on simulation than ever before. In the competitive field of research and development, the phrase “as accurate as possible, as complex as is necessary” rings true for vehicle models. Due to the “as complex as necessary” approach, many complex phenomena such as suspension kinematics and suspension friction remain un-modelled, as the assumption is made that the effects are negligible. The seemingly negligible effects negatively affect the validity of simulation models, especially when deviating from the specific manoeuvre for which the model was originally created.

In this study, focussed on a vehicle with a hydropneumatic suspension system, the effect of gas modelling methodology, friction, and friction modelling strategy on the validity the suspension unit characteristics, and a full non-linear vehicle dynamics model is presented. The approach to gas modelling included three permutations of the ideal gas formulation, namely isothermal, adiabatic, and a heat transfer dependent thermal time-constant approach. The effects of friction were accounted for using a rudimentary lookup table approach, a LuGre, and a Modified LuGre friction model, while using the case where friction is neglected as reference.

The results showed that the gas modelling approach, and the effects of friction, each have a significant effect on model accuracy and validity when compared to physical test results. The improvement is witnessed on both the single suspension unit characteristic as well as on the full non-linear simulation model. This effectively proves that seemingly negligible effects may have a significant effect on model validity.

Study Leader:            Prof. N.J. Theron

Co-Study Leader:       Prof. P.S. Els

 


 

Henriette Christine Nolte, 2015. "Analysis and Optimisation of a Receiver Tube for Direct Steam Generation in a Solar Parabolic Trough Collector"

This study focused on a numerical second law analysis and optimisation of a receiver tube operating
in a parabolic trough solar collector for small-scale application. The receiver functioned in a Rankine cycle. The focus was on entropy generation minimisation in the receiver due to the high quality exergy losses in this component. Water functioned as the working fluid and was heated from ambient conditions (liquid) to a superheated state (vapour), consequently, the receiver tube was subject to both single phase as well as two-phase flow.
Entropy generation in the receiver tube was mainly due to finite temperature differences as well as fluid friction. The contribution of each of these components was investigated. Geometrical as well as operating conditions were investigated to obtain good guidelines for receiver tube and plant design. An operating pressure in the range of 1 MPa (Tsat = 180◦C) to 10 MPa (Tsat = 311◦C) was considered. Furthermore a mass flow range of 0.15 kg/s to 0.4 kg/s was investigated.

Results showed that beyond a diameter of 20 mm, the main contributor to the entropy generation was the finite temperature differences for most conditions. Generally, operating pressures below 3 MPa showed bad performance since the fluid friction component was too large for small operating pressures. This phenomenon was due to long two-phase lengths and high pressure drops in this region. The finite temperature difference component increased linearly when the tube diameter was increased (due to the increase in exposed area) if the focused heat flux was kept constant. However, the fluid friction component increased quadratically when the diameter was reduced.
In general when the concentration ratio was increased, the entropy generation was decreased.
This was due to more focused heat on each section of the receiver pipe and, in general, resulted in shorter receiver lengths. Unfortunately, there is a limit to the highest concentration ratio that can be achieved and in this study, it was assumed to be 45 for two-dimensional trough technology.
A Simulated Annealing (SA) optimisation algorithm was implemented to obtain certain optimum parameters. The optimisation showed that increasing the diameter could result in a decrease in entropy generation, provided that the concentration ratio is kept constant. However, beyond a certain point gains in minimising the entropy generation became negligible. Optimal operating pressure would generally increase if the mass flow rate was increased. Finally, it was seen that the highest operating pressure under consideration (10 MPa) showed the best performance when considering the minimisation of entropy in conjunction with the maximisation of the thermodynamic work output.

Supervisors: Prof. T. Bello-Ochende, Prof. J.P. Meyer


M.Everts, 2015. "Heat Transfer and Pressure Drop of Developing Flow in Smooth Tubes in the Transitional Flow Regime"

Heat exchangers have a wide range of applications and engineers need accurate correlations to optimise the design of these heat exchangers.  During the design process, the best compromise between high heat transfer coefficients and relatively low pressure drops is usually in the transitional flow regime.  Limited research has been done on tube flow in the transitional flow regime. These studies considered either fully developed flow, or average measurements of developing flow across a tube length.  No research has been done with the focus on developing flow in smooth tubes in the transitional flow regime.  Therefore, the purpose of this study was to experimentally investigate the heat transfer and pressure drop characteristics of developing flow in the transitional flow regime.  An experimental set-up was designed, built and validated against literature.  Heat transfer and pressure drop measurements were taken at Reynolds numbers between 500 and 10 000 at three different heat fluxes (6.5, 8.0 and 9.5 kW/m2).  A total of 398 mass flow rate measurements, 19 158 temperature measurements and 370 pressure drop measurements were taken.  Water was used as the test fluid and the Prandtl number ranged between 3 and 7.  The test section was a smooth circular tube and had an inner diameter and length of 11.52 mm and 2.03 m, respectively.  An uncertainty analysis showed that the uncertainties of the Nusselt numbers and Colburn j-factors varied between 4% and 5% while the friction factor uncertainties varied between 1% and 17%.  Five different flow regimes (laminar, developing laminar, transitional, low-Reynolds-number-end and turbulent) were identified in the first part of the tube during the experiments and nomenclature was developed to more clearly identify the boundaries of the different flow regimes.  The developing laminar regime was unique to developing flow and decreased along the tube length.  Both the start and end of transition were delayed along the tube length and the width of the transition region decreased slightly.  This is in contrast with the results obtained in literature where the effect of the non-dimensional distance from the inlet on fully developed flow in the transition region was investigated.  Transition was also slightly delayed with increasing heat flux, but secondary flow effects had no significant influence on the width of the transition region.  The relationship between heat transfer and pressure drop was investigated and correlations were developed to predict the Nusselt number as a function of friction factor, Reynolds number and Prandtl number in the laminar, transitional, low-Reynolds-number-end and turbulent flow regimes.  Overall, it can be concluded that the heat transfer characteristics of developing and fully developed flow differ significantly and more work needs to be done to fully understand the fundamentals before the heat transfer and pressure drop characteristics are fully understood.

Supervisor:         Prof J.P. Meyer

 


Johannes Marthinus Koorts, 2015. "Entropy Minimisation and Structural Design for Industrial Heat Exchanger Optimisation"

 

The mass flow rate for shell-and-tube, tube-fin and tube-in-tube industrial-type heat exchangers can be optimised by minimising entropy generation over a finite temperature difference. The purpose of the work was to apply the principles of entropy generation minimisation based on the second law of thermodynamics to determine whether the intercept between entropy generation due to heat transfer and fluid friction is a good approximation for the global minimum entropy generation, as well as to optimise a number of variables. Optimisation was achieved by applying numerical methods.

In order to yield meaningful results, the optimisation was done by setting a number of boundary conditions such as the maximum inlet steam temperature. The heat exchanger optimisation was based on a case study of the heat exchangers used at Columbus Stainless’ cold products division.

The mentioned case study consisted of 27 industrial-types of heat exchangers with power ratings ranging between 100 and 800 kW. The original specification included lengths varying between 1 and 2.42 m, shell-side diameters from 0.4 to 2 m and number of tubes varying from 1 to 20. The medium mass flow rates ranged between 0.048 and 0.855 kg/s while the steam mass flow rates ranged between 0.029 and 0.236 kg/s. The medium output temperatures were between 303 and 403K

The effect of various conditions was taken into account. Apart from the conditions already mentioned, the specific material properties at their specific pressures and temperatures (such as density, viscosity, conductivity, heat transfer coefficient) were also taken into account in the calculations.

Through numeric optimisation it was possible to conclude that the main variables that affected entropy generation were the steam inlet temperature, followed by the tube-side diameter for the given sample set. These variables were thus used in all further graphs as the main variables to be changed.

These variables were manipulated and the entropy generation due to fluid friction and heat transfer were independently plotted to determine if the intercept of these two lines could be used as a good approximation of the global minimum entropy generation. It was found that the approximation is not that good, unless one uses the entropy generation due to operational heat transfer, which yielded deviations between the intercept values and the global minimum ranged between 0.21 % and 21.88 % with an average deviation of 6.52 %, which is considered to be a good approximation. The constructal design theory does thus hold well under the current operating conditions. 

It was clear that the main mechanism contributing to entropy generation was the effect of fluid friction, although this was only the case at smaller tube diameters. Of the entropy generated due to fluid friction the majority was contributed due to the tube side (steam), with almost no entropy generated due to fluid friction in the shell-side (medium) of the heat exchanger.

The effect of a number of variables on entropy generation were discussed and plotted. It was seen that an increased inlet temperature resulted in most cases in less entropy generated and that larger tube diameters have a similar effect due to less entropy generated due to fluid friction.

By using the principles of entropy generation minimization the entropy generated of each heat exchanger could be reduced by between 2% and 64%, with the tube-fin heat exchangers having the largest scope for improvement in the sample set used. 

By using the principals of construal design, the intercepts of the entropy generation due to heat transfer and due to fluid friction was used to determine the optimal diameter. This correlation yielded very good results (Within 1% of the global minimum entropy generation) to predict the global minimum entropy generation.

Supervisors:Tunde Bello-Ochende, Josua P. Meyer

Keywords: Entropy, shell-and-tube, tube-fin, tube-in-tube, heat exchanger, heat transfer, irreversibility, model, thermodynamics, numerical, optimisation, geometric, constructal design

 


Jacob E. van der Westhuizen, 2015. "INVESTIGATION INTO USING LIQUID CRYSTAL THERMOGRAPHY AS PRIMARY TEMPERATURE MEASUREMENT TECHNIQUE FOR OBTAINING LOCAL WALL TEMPERATURES AND HEAT TRANSFER COEFFICIENTS IN TUBE-IN-TUBE HEAT EXCHANGERS." 

 

The feasibility of obtaining local wall temperatures by using liquid crystal thermography (LCT) in a counter‑flow tube-in-tube heat exchanger was investigated. Local annulus-side heat transfer coefficients at the inlet and thermodynamically and hydrodynamically underdeveloped regions were also obtained while operating at steady conditions.

The heat transfer coefficients of the tube‑in‑tube heat exchanger are, however, disputed in the literature, as conflicting sources are easily found. In most literature sources the problem is simplified by assuming constant heat transfer coefficients throughout the length of the heat exchanger and the boundary layer growth is generally ignored at inlet regions.

Thermocouples pose practical problems when measuring temperatures in heat exchangers. LCT is investigated as alternative surface temperature measurement technique. This study aims to develop a methodology for directly measuring wall temperatures inside a tube‑in‑tube heat exchanger. These temperatures were further used to calculate local heat transfer coefficients.

In this study, a 1m long tube-in-tube test section with an annulus diameter ratio of 0.54 (ratio of the inner wall of the annulus to its outer wall) was constructed, in which liquid crystal thermography was employed as an alternative wall temperature measurement technique to thermocouples. Temperature-sensitive paint was applied to the inner wall of the annulus in order to measure the wall temperatures non-intrusively. Complete temperature maps could be constructed for different thermal conditions which indicated differences of up to 10 °C in wall temperature at the inlet regions, which would have been difficult to capture with thermocouples. This study covered a total of nine different annular flow and thermal conditions for cooled and heated cases. The annular flow conditions ranged from laminar (Re = 1000) flow to fully turbulent flow (Re = 13 800).

In general, the heat transfer coefficients were found not to be constant along the length of the heat exchanger. The averaged heat transfer coefficients at the inlet were compared with existing correlations in the literature for full-length heat exchangers and were found to be higher by an average of 44 % over the data presented. Uncertainties on the local heat transfer coefficient were found to be approximately 80% for the cooled annulus cases and 45% for a heated annulus. This was mostly due to the practical laboratory restrictions imposed by fluid temperature limits.

It was found that liquid crystal thermography could be used successfully for directly measuring the wall temperatures of tube-in-tube heat exchangers with very low surface temperature uncertainties (0.03 °C). With the approach developed in this study, a method was found for determining local heat transfer coefficients without introducing wall thermocouples or any other disturbances in the passage of the annular fluid.

Supervisors: Dr Jaco Dirker and Prof Josua P. Meyer

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Last edited by Bradley BockEdit