The study of thermoacoustic effects is a relatively new area, particularly in application to thermoacoustic engines. For thermoacoustic engines to be commercially viable, there are still many aspects to be investigated, not only practical aspects but also at the fundamental level of physics. Particularly lacking is research on heat exchangers in thermoacoustic engines, despite the fact that this is one of the most important components, for which a design methodology does not yet exist. The primary aim of this work was to investigate the design methodology for heat exchangers in thermoacoustic devices to improve their efficiency. In this work, second law analysis was chosen as the design methodology and was applied to a simplified model of heat exchangers in thermoacoustic engines and its validity was examined. However, for the analysis to be useful to design practical devices, further knowledge of the heat transfer mechanism in oscillatory, compressible flow, and on the development of boundary layers under such conditions are required. This is not currently available for thermoacoustic devices. The commercial software PHOENICS was used to investigate this oscillatory heat transfer problem numerically. To test the capability of the software for simulating thermoacoustic phoenomena, two dimensional standing waves and thermoacoustic couples were simulated at various operating conditions and geometries, including conditions very close to those at heat exchangers in thermoacoustic engines. The results were compared with existing analytical solutions and the results of numerical simulations from others and showed that PHOENICS is capable of simulating thermoacoustic effects. However, the accuracy of second order effects, such as heat flux induced by thermoacoustic effects, was limited by the capability of PHOENICS and the results should be interpreted with this in mind. Energy and flow fields from thermoacoustic couple simulations were investigated from plots of energy vectors, energy lines, instantaneous velocity fields, particle traces and energy dissipation.The dependence of such quantities on plate spacing, plate length and Mach numbers are presented. One important result from these test which is relevant to the design of regenerators or heat exchangers in thermoacoustic engines was that a net heat pumping effect appears only near the edges of thermoacoustic couple plates, within about a particle displacement distance from the edges. Also it was observed that the energy dissipation near the plate is proportional to the plate surface area but increases quadratically as the plate spacing is reduced. The results also indicated the presence of larger scale vortical motion outside the plates which disappeared as the plate spacing was reduced. The presence of such vortical motion did not seem to influence the heat transfer to the plates. In order to simulate heat exchangers in thermoacoustic engines without simulating the whole device, boundary conditions representative of those near the ends of the regenerator plate were considered and tested. Although in some test cases, the simulation converged to a solution with minimal energy imbalances, there was a major discontinuity in the energy flux vectors near the boundary. Further investigations (both numerical and experimental) are required to provide further insight into the boundary conditions which need to be specified for future simulations of heat exchangers in thermoacoustic engines.

A detailed second law analysis of dual fuel LTC is not yet available in the open literature even though dual fuel low temperature combustion (LTC) has been studied before. To address this gap, a previously validated, closed-cycle, multi-zone, simulation of diesel-natural gas dual fuel LTC was used to perform a second law analysis. In the current study, a 2.4-liter single-cylinder research engine operating at a nominal load of 6 bar BMEP and 1700 rpm was used. Zone-wise thermodynamic irreversibilities as well as total cumulative entropy generated and lost available work over the closed cycle were quantified. Subsequently, two convenient second-law parameters were defined: (1) the “lost available indicated mean effective pressure” (LAIMEP), which can be interpreted as an engine-size-normalized measure of available work that is lost due to thermodynamic irreversibilities (analogous to the relationship between indicated mean effective pressure and indicated work); (2) fuel conversion irreversibility (FCI), which is defined as the ratio of lost available work to total fuel chemical energy input. Finally, parametric studies were performed to quantify the effects of diesel start of injection, intake manifold temperature, and intake boost pressure on LAIMEP and FCI. The results show that significant entropy generation occurred in the flame zone (52-61 percent) and the burned zone (31-39 percent) while packets account for less than 6 percent of the overall irreversibilities. Parametric studies showed LAIMEPs in the range of 645-768 kPa and FCIs in the range of 32.8-39.2 percent at different engine operating conditions. Although the present study focused on dual fuel LTC, the conceptual definitions of LAIMEP and FCI are generally applicable for comparing the thermodynamic irreversibilities of IC engines of any size and operating on any combustion strategy.

The biorefinery concept is attractive. Increasing international concerns over issuessuch as climate change have led to political as well as social pressures for a shift fromfossil fuels to renewable resources and biomass is one abundant renewable resource.Biomass has the potential of supplying many of the fuels and chemicals which arecurrently dependent on petroleum. Much development is still needed in the field ofbiorefineries and a systematic approach to evaluate and compare process technologiesand to suggest optimizations seems necessary.The objective of this thesis is to develop entropy analysis as a possible evaluation toolfor optimization of biorefinery processes. This is a new application of entropyanalysis which is rarely discussed in the literature. The scientific basis of the entropyanalysis is described and the proposed methodology is explained. The position ofentropy analysis among other system analysis tools such as exergy analysis and lifecycle assessment is discussed along with entropy analysis earlier applications.A case study is introduced which is the IBUS (Integrated Biomass Utilization System)project in Denmark. The idea in IBUS is to integrate the biomass plant with a powerplant to utilize the surplus steam from the power plant for the internal use of thebiorefinery. The suggested method of entropy analysis is applied to this case study tocompare different processes for production of ethanol along with solid biofuel andanimal feed from Danish wheat straw. The evaluation is a gate to gate analysis inwhich production of energy carriers are also included in addition to biorefining ofwheat straw. A parallel life cycle assessment study with equivalent system boundariesis also carried out to compare the results with a conventional environmental systemsanalysis method.The results from the entropy analysis of the IBUS case study show that fermentationof C5 and C6 sugars by yeast is the most efficient process thermodynamically whilefermentation of only C6 sugars by yeast is the least efficient among the three casesstudied. Integration of the biorefinery with a coal fired CHP plant is identified as awise choice by the results of the entropy analysis method.For the IBUS process alternatives investigated in this study, the entropy results andthe LCA results (aggregated environmental load) are in correlation; entropy results areconsistent with weighting results based on two different weighting methods namelyEco indicator 99 and EPS 2000. Entropy generation is also in correlation withproduction cost for the processes analyzed in this evaluation. Another observation isthat cooling in the biorefining process contributes highly in the generation of entropy.This potential improvement option is not surfaced by the LCA conducted.The potential for further investigation and development of the tool is recognizedreflecting on some interesting observations in the results. Improvement of the tool ishighly possible for example by supplementing other implications of entropy inprocess design such as "waste potential entropy" concept which is developed as aneco-toxicity measure. / Uppsatsnivå: D

This study focused on a numerical second law analysis and optimisation of a receiver tube op- erating 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 uid 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 ow. Entropy generation in the receiver tube was mainly due to nite temperature di erences as well as uid 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 ow 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 nite temperature di erences for most conditions. Generally, operating pressures below 3 MPa showed bad performance since the uid 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 nite temperature di erence component increased linearly when the tube diameter was increased (due to the increase in exposed area) if the focused heat ux was kept constant. However, the uid 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 ow 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. / Dissertation (MEng)--University of Pretoria, 2014. / tm2015 / Mechanical and Aeronautical Engineering / MEng / Unrestricted

Many industrial processes have low-temperature waste heat sources that cannot be efficiently recovered. Low grade waste heat has generally been discarded by industry and has become an environmental concern because of thermal pollution. This has led to the lookout for technologies which not only reduce the burden on the non-renewable sources of energy but also take steps toward a cleaner environment. One approach which is found to be highly effective in addressing the above mentioned issues is the Organic Rankine Cycle (ORC), which can make use of low- temperature waste heat to generate electric power. Similar in principle to the conventional cycle, ORC is found to be superior performance-wise because of the organic working fluids used in the cycle. The focus of this study is to examine the ORC using different types of organic fluids and cycle configurations. These organic working fluids were selected to evaluate the effect of the fluid boiling point temperature and the fluid classification on the performance of ORCs. The results are compared with those of water under similar conditions. In order to improve the cycle performance, modified ORCs are also investigated. Regenerative ORCs are analyzed and compared with the basic ORC in order to determine the configuration that presents the best thermal efficiency with minimum irreversibility. The evaluation for both configurations is performed using a combined first and second law analysis by varying certain system operating parameters at various reference temperatures and pressures. A unique approach known as topological method is also used to analyze the system from the exergy point of view. Effects of various components are studied using the exergy-wheel diagram. The results show that ORCs using R113 as working fluid have the best thermal efficiency, while those using Propane demonstrate the worse efficiency. In addition, results from these analyses demonstrate that regenerative ORCs produce higher efficiencies compared to the basic ORC. Furthermore, the regenerative ORC requires less waste heat to produce the same electric power with a lower irreversibility.