Performance analysis of a travelling-wave looped tube thermo-acoustic engine
- Authors: Balonji, Serge
- Date: 2018
- Subjects: Heat engineering , Heat-engines - Thermodynamics , Refrigerators , Heat - Transmission , Low temperature engineering
- Language: English
- Type: Masters (Thesis)
- Identifier: http://hdl.handle.net/10210/280505 , uj:30143
- Description: M.Tech. (Mechanical Engineering) , Abstract: The present study focuses on analysing the performance of a travelling-wave looped-tube thermo-acoustic engine. The introduction of thermo-acoustic theories has provided a path for the technology to be developed further, in an attempt to replace the conventional refrigeration systems, power production and other pollutant energy production systems in the future. The thermo-acoustic field is becoming one of the emerging technologies dealing with conversion of energies. The earth population is growing faster for the planet to sustain its consumption and for the environment to control the pollution. Our home refrigerators are ones of appliances contributing to the pollution of the environment by their toxic coolants and their lubricants. Burning of fossils to produce electricity is another way of contributing to the pollution. Thermo-acoustic device show potential alternative on reducing air pollution. It can be driven by wasted heat, has no toxic coolant and does not require the use of lubricant. Because of these advantages and many others, thermo-acoustic devices are attracting more attention from researchers. However the technology is still in its developing stage because of the currents limitations related to the performance of existing models. Thermo-acoustic engine convert heat into a sound wave. Previous studies have searched ways and means to improve thermo-acoustic devices performance. Some have chosen to optimise the length and the configuration of the resonator; others have focussed on the gases and their mixtures; while number of researchers has focussed on the materials and configuration of the stack or regenerator, described as the heart of the devices. In the present, study we have pointed out the effect of the variation of some crucial parameters on the performance of thermo-acoustic devices. A travelling-wave thermo-acoustic engine was designed using numerical approximation provided by a modelling code DELTAEC (Design Environment for Low-amplitude Thermo-Acoustic Energy Conversion). The design developed focuses, in particular, on the effects of different ceramic substrate configurations (diameter, length, porosity and position) on the performance of the device. Meaningful comparison on the effect of the ceramic substrates configuration is provided in order to predict the performance of the device. In addition, guidance on the identification and the selection of the best geometrical configurations of ceramic substrates have been provided. The results obtained from the DELTAEC simulation were used to design and build a hermetically sealed travelling-wave thermo-acoustic engine using low cost materials. The device was able to...
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Aspekte van vibrasies in romp- en buis hitteruilers
- Authors: Jordaan, Christiaan Cornelius
- Date: 2015-02-12
- Subjects: Heat - Transmission , Turbulence , Eddies
- Type: Thesis
- Identifier: uj:13303 , http://hdl.handle.net/10210/13320
- Description: M.Ing. (Mechanical Engineering) , Shell and tube heat exchangers are commonly used in industrial processes. In the heat exchangers the tubes incur flow induced vibrations. These vibrations lead to fretting on the perimeter of the tube at the baffle plates and to fatigue failure at the tube sheets. Various studies were and are done to qualify and quantify these vibrations. This is then used to develop design criteria and methods for lowering the vibration amplitudes of the tubes. In the thesis the response of a tube bundle in cross flow is shown. The effect of increasing cross flow velocity on the vibration amplitude of the tubes is determined in a low speed wind tunnel. The results show similarity to the work done by Gorman [27]. By raising the natural frequency of the tubes in the first row upstream it is shown that the vibration peaks exhibited by the unmodified model.at cross flow velocities lower than the critical flow speed do not materialize in the modified tube bundle. This modification can be implemented by decreasing the unsupported length of the tubes in the first row. The inclusion of fluid damping when calculating the total damping of the tubes according to Blevins [24] is shown in the experiment. From experimental results it is shown that the prediction of the critical velocity according to Blevins [24] results in a better prediction of this velocity. Numerical solutions of two dimensional flow over a single cylinder and a pair of cylinders are obtained by simulating the flow on the STAR CD flow simulation package. The effect of neighbouring tubes on one another is determined by qualifying and quantifying the flow over and the forces on the tubes. The vortex shedding frequency determined from the simulation coincides with experimental results. This shows great potential in future simulation of flow over a tube bundle. A computerised design model is also developed. Themodel determines the vibration potential of the tube bundle in a shell and tube heat exchanger. If needed, modifications to new designs and built heat exchangers can be done and the vibration potential is easily determined with the model. The results of the experimental work are used to further develop the design criteria preventing detrimental vibrations in heat exchangers. Promising results to decrease vibration amplitudes of tubes in tube bundles are obtained in the experiments done. This and the results obtained in the simulation of the vortex shedding frequencies that coincide with the results in the experimental work, show great potential for flow simulation and development of methods to decrease flow induced vibrations of tubes in shell and tube heat exchangers.
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A numerical evaluation of multi-dimensional heat transfer effects in greenhouses
- Authors: Kruger, Sunita
- Date: 2015
- Subjects: Greenhouses - Climate , Heat - Transmission
- Language: English
- Type: Doctoral (Thesis)
- Identifier: http://hdl.handle.net/10210/58126 , uj:16421
- Description: Abstract: Greenhouses are used extensively in both agricultural and horticultural industries. Cultivating crops in greenhouses is an energy-intensive process, especially as far as ventilation is concerned. The use of natural ventilation as an alternative to control the indoor climate of greenhouses has become increasingly popular. Natural ventilation in greenhouses is due to the air exchange process between the internal and external environment of the greenhouse mainly due to thermal buoyancy and wind-induced forces. The main objective of this thesis is to determine the effect of dimensional and geometrical influences on the heat transfer characteristics of greenhouses using Computational Fluid Dynamics. This entails an investigation into the effect of geometric design changes on the thermal and flow performance of greenhouses due to buoyancy driven flow. The study also includes a numerical investigation into the effect of plant benches on the indoor climate of greenhouses. This thesis commences with a review of the available literature, and identified areas in the research that require more attention. A background on the numerical tool used (Computational Fluid Dynamics) was given, as well as a background on the experimental methods used in this thesis. In this research study, the investigation was initiated with analysis of a simple, zero-degree roof angle cavity, which represents a square greenhouse. Results were compared to experimental results found in the literature and a good comparison was found. Confidence was established in the two-dimensional CFD model. A three-dimensional zero-degree roof angle cavity was also created and verified against data found in the literature. A good comparison between Nusselt numbers was found, and therefore confidence in the three-dimensional CFD model was also established. The three-dimensional CFD model approach was used to investigate the multi-dimensional heat transfer effects of the zero-degree greenhouse cavity. A significant difference between the two- and three-dimensional cases was observed. The next step was to modify the zero-degree roof angle cavity, by adding an angled roof. This represented a simple, single-span greenhouse. The heat transfer in the cavity subject to various roof angles was investigated, and Nusselt-Rayleigh number relationships were established. A three-dimensional cavity of similar dimensions was investigated and compared to the two-dimensional cavity. Temperatures were found to be higher, and velocities much lower compared to the two-dimensional case. A roof ventilator was added to the one side of the greenhouse, and it was found that adding a ventilator does have a noticeable effect on the heat transfer inside the cavity. Nusselt-Rayleigh number relationships were also established for the... , D.Ing.
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Long term overheating and its effect on plant availability
- Authors: Madjoe, Sean Michael
- Date: 2015
- Subjects: Boilers - Efficiency , Coal-fired power plants , Heat - Transmission , Steam-boilers - Creep , Boilers - Testing
- Language: English
- Type: Masters (Thesis)
- Identifier: http://hdl.handle.net/10210/56161 , uj:16338
- Description: Abstract: This mini-dissertation investigates the impact that Long Term Overheating and Creep (LTOC) has on coal fired power stations and the availability implications thereof. The need for the investigation arose from a single coal fired power station, with multiple boilers with a total generating capacity of greater than 400MW, of which LTOC is the dominant boiler tube failure mechanism. For the purposes of the mini-dissertation, the power station was simply titled Power Station X. A summary is provided of the various boiler tube failure mechanisms as well as a brief description of availability. The mini-dissertation makes use of case studies in researching the matter of LTOC. It was seen that LTOC is a global matter, which is typically seen in ageing power stations. The major discussion points arising from the mini-dissertation is that of a boiler tube failure management program as well as correctly understanding the causes of boiler tube failures. With an adequate and correctly functioning boiler tube failure management program one can reduce and manage the occurrence of boiler tube failures. The second major discussion point arising from the mini-dissertation is that of correctly understanding the contributing factors to boiler tube failures. A lot of emphasis has been placed on the actual mechanisms and identification of failures, however the contributing factors and root cause has not been thoroughly investigated. Additionally one also has to identify the various areas prone to creep and the necessary remedial and mitigating actions for the removal and or replacement of such components, while keeping in mind the financial implication thereof. The mini-dissertation concludes with possible areas of future research as well as possible remedial actions that can be taken. , M.Ing. (Engineering Management)
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Heat transfer coefficient of a snow bag
- Authors: Da Veiga, Willem Richter
- Date: 2014-09-10
- Subjects: Heat - Transmission , Dry ice , Refrigerated transport , Snow bag
- Type: Thesis
- Identifier: uj:12226 , http://hdl.handle.net/10210/11993
- Description: 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%.
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Effects of condenser performance on thermal heat rate
- Authors: Narainsingh, Soomesh
- Date: 2014-03-17
- Subjects: Coal-fired power plants , Heat - Transmission , Condensers (Steam)
- Type: Thesis
- Identifier: uj:4346 , http://hdl.handle.net/10210/9695
- Description: 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.
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Condensation heat transfer and pressure drop coefficients of R22/R142b in a water cooled helicaly coiled tube-in-tube heat exchanger.
- Authors: Kebonte, Shiko A.
- Date: 2012-08-20
- Subjects: Heat exchangers - Fluid dynamics , Heat - Transmission , Refrigerants , Condensation
- Type: Thesis
- Identifier: uj:2741 , http://hdl.handle.net/10210/6181
- Description: 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.
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Roasting of sulphide concentrates and acid leaching of calcines emanating from conventional and microwave assisted roasting
- Authors: Phiri, Thulane Raymond
- Date: 2012-06-05
- Subjects: Leaching , Gold - Metallurgy , Heat - Transmission , Microwave heating
- Type: Thesis
- Identifier: http://ujcontent.uj.ac.za8080/10210/380427 , uj:2395 , http://hdl.handle.net/10210/4849
- Description: M.Tech. , The study presented, investigated the acid leaching behavior of calcines emanating from conventional roasting and microwave assisted roasting of sulphide ores. The aim was to investigate a possible change in the mineral composition of the calcine produced and to investigate the leaching behavior of the calcine produced by conventional and microwave assisted roasting. Two sulphide concentrates were received, pyrite concentrate from OTJIHASE concentrator and sphalerite concentrate from ZINCOR. The mineralogical study was done using the XRD (PANalytical X-PertPro X-Ray diffractometer) to determine the mineral composition of the concentrate and the head grade of each concentrate was determined with an Atomic Absorption Spectrometry (AAS) machine for copper, iron and zinc. The concentrates were wet screened at 500, 355, 212, 150, 106 and 75 μm screens for size distribution and for the purpose of the study it was required that 80% of concentrate should pass the 75 μm screen since after roasting, the calcine obtained were to be acid leached. Conventional roasting was carried out at temperatures of 500oC, 700oC, 850oC and 1000oC at durations of 5min, 10min, 20min, 30min, 1hour and 2hours, while microwave assisted roasting carried out at powers of 200W, 400W, 600W, 800W and time intervals of 3s, 10s, 30s, 30min and 1hour
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