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Thermodynamic simulation and experimental validation of a cascaded two-stage organic Rankine cycle
(2015)
A 3-stage model of a low pressure steam turbine has been equipped with extensive instrumentation to assess aerodynamic performance and for validation of numerical methods. Emphasis was placed on detailed measurements at the inlet and exit of the last stage guide vane. Two configurations were examined – the first used a guide vane with lean in the last stage and a the second used a guide vane combining both lean and sweep. The experimental results from the two configurations have been compared with data from various design methods, ranging from through-flow to 3D multistage viscous simulations with mixing planes. Despite the challenges in modelling complex rotating blades in a wet steam environment, good agreement between the numerical and experimental results was obtained, and the new stator vane combining lean and sweep achieved the desired increase in reaction at the hub section.
This article is on a university course called Engineering Conferences. It has been developed by the authors and installed as a mandatory part of the curriculum in three master’s degree programs for engineering students. The participants of the course are postgraduates with different nationalities, mostly German, and different technical backgrounds. They study Mechanical Engineering, Simulation and Experimental Technology, or International Business Engineering. The basic idea of the course concept goes far beyond simply teaching the standards of academic writing and skills for using scientific publications. By using a learnercentered approach, the students get engaged in typical activities around an active attendance of a real conference. Students learn to locate the field of their bachelor thesis or project report in the world of research communities, scientific journals and engineering conferences. They learn about conferences matching their bachelor/project topic. They write a paper complying with common academic standards, submit it to a mock-up conference, and review submissions of their fellow students. Students also produce a poster and have to defend it in a poster session held publicly on the campus. Engineering Conferences is a course on scientific communication and presentation that also aims at the development of other skills and competences needed in the world of modern engineering.
The organic Rankine cycle (ORC) is an established thermodynamic process that converts waste heat to electric energy. Due to the wide range of organic working fluids available the fluid selection adds an additional degree of freedom to the early design phase of an ORC process. Despite thermodynamic aspects such as the temperature level of the heat source, other technical, economic and safety aspects have to be considered. For the fluid selection process in this paper, 22 criteria were identified in six main categories while distinguishing between elimination and tolerance criteria. For an ORC design, the suggested method follows a practical engineering approach and can be used as a structured way to limit the number of interesting working fluids before starting a detailed performance analysis of the most promising candidates. For the first time the selection process is applied to a two-stage reference cycle which uses the waste heat of a large reciprocating engine for cogeneration power plants. It consists of a high temperature and a low temperature cycle in which the condensation heat of the high temperature (HT) cycle provides the heat input of the low temperature (LT) cycle. After the fluid selection process the detailed thermodynamic cycle design is carried out with a thermodynamic design tool that also includes a database for organic working fluids. The investigated ORC cycle shows a net thermal efficiency of about 17,4% in the high temperature cycle with Toluene as the working fluid and 6,2% in low temperature cycle with iso-Butane as the working fluid. The electric efficiency of the cogeneration plant increases from 40,4% to 46,97% with the both stages of the two-stage ORC in operation.
Undergraduate students in engineering often have little exposure to the world of scientific publishing and the culture of sharing research work. While the beneficial exchange between research and teaching is well promoted, while the benefits of conference participation for students have already been surveyed and while courses on scientific writing are readily available, the concept of mini-conferences as part of the curriculum combines all three aspects into one. Therefore, the course "Engineering Conferences" was developed and installed as a mandatory part of a master program for engineering students. The idea is to go beyond simply teaching the standards of academic writing and skills for using scientific publications. By using a learner-centered approach, the students are engaged in typical activities around an active attendance of a real conference. Based on their bachelor thesis, they write a paper complying with common academic standards, submit the paper and review submissions of their fellow students. They also produce a poster and defend their work in a poster session held publicly on campus. This contribution is based on the experience from the first four terms teaching the course. It explains the didactic rationale behind the concept and individual teaching modules, it comprises the collection of useful resources for teaching and organizing scientific publishing and it includes the consequences drawn from course evaluation results.
This paper deals with the investigation in the technical and economic feasibility of a low-pressure steam cycle for the waste heat utilization of a combined heat and power (CHP) plant. The proposed steam cycle uses a pressure level below atmospheric pressure which allows to use the heat from the motor’s coolant for evaporation. The cycle is designed to increase the maximum electrical efficiency of a gas fired reciprocating engine and to increase operational flexibility for varying heat demand. Since organic Rankine cycle (ORC) plants are already being successfully used for waste heat recovery, advantages of the low-pressure steam Rankine cycle (SRC) over an ORC are highlighted. Many ORC fluids are toxic or flammable and thus have an increased risk potential compared to water. An additional advantage of water compared to ORC fluids is the possibility of superheating above 500 °C. The technical feasibility of the proposed cycle is studied by the detailed design of the individual system components such as turbine, heat exchangers and auxiliary parts. This includes the detailed thermodynamic design of the full cycle for the calculation of plant efficiencies. As a basis for future experimental investigations, a cogeneration plant with an electrical output of 50 kWel was selected as a heat source for the steam cycle. The design geometries of the turbine and the heat exchanger are presented along with efficiency and cost predictions. A subsonic radial turbine with a rotational speed of 155,000 rpm was selected. The maximum turbine output is about 7.5 kW. The use of a flash evaporator is investigated to reduce the size and the costs for the evaporator. The suggested steam cycle helps to increase the electrical efficiency of the CHP plant by 4.5% points from an original 36.5% to 41.0%. The steam cycle itself has an electrical efficiency of approximately 9%. Additionally, the condensation temperature in this steam cycle is high enough to be extracted as useful heat output to generate domestic water with a temperature of 50°C.
The ability to understand and predict the pressure losses of orifices is important in order to improve the air flow within the secondary air system. This experimental study investigates the behavior of the discharge coefficient for circular orifices with inlet cross flow which is a common flow case in gas turbines. Examples of this are at the inlet of a film cooling hole or the feeding of air to a blade through an orifice in a rotor disk. Measurements were conducted for a total number of 38 orifices, covering a wide range of length-to-diameter ratios, including short and long orifices with varying inlet geometries. Up to five different chamfer-to-diameter and radius-to-diameter ratios were tested per orifice length. Furthermore, the static pressure ratio across the orifice was varied between 1.05 and 1.6 for all examined orifices. The results of this comprehensive investigation demonstrate the beneficial influence of rounded inlet geometries and the ability to decrease pressure losses, which is especially true for higher cross flow ratios where the reduction of the pressure loss in comparison to sharp-edged holes can be as high as 54%. With some exceptions, the chamfered orifices show a similar behavior as the rounded ones but with generally lower discharge coefficients. Nevertheless, a chamfered inlet yields lower pressure losses than a sharp-edged inlet. The obtained experimental data were used to develop two correlations for the discharge coefficient as a function of geometrical as well as flow properties.
High heat losses via exhaust gas and coolant in internal combustion engines (ICE) are the basis for numerous investigations regarding downstream processes for power generation. The most promising concepts are Organic Rankine Cycles (ORC) and Steam Rankine Cycles (SRC). In previous work of the Center of Innovative Energy Systems, Düsseldorf (Germany), the technical and economic feasibility of a low pressure SRC has been investigated and the advantages in comparison to organic Rankine cycles are highlighted. A distinctive feature of the cycle is the use of the ICE coolant heat for evaporation, which limits the cycles maximum steam pressure to values below atmospheric pressure. This work presents first results of a test rig with data reconciliation according to DIN 2048 to validate the simulation results and design calculations. As a basis for the experimental investigations, a gas fired CHP plant was selected. The design of the test rig is optimized for the operation in the laboratory, where the coolant heat is emulated by a tempering device and the exhaust heat of the CHP plant is emulated by a gas burner. With the designed test rig, it is possible to control the volume flow and the temperatures of the coolant and exhaust gas, so that different load conditions of the gas fired CHP plant can be investigated. For initial tests, the turbine in the SRC is replaced by a throttle to achieve the pressure drop of the turbine. The experimental results show, that the exhaust and coolant heat of a 36 kWel CHP plant can be emulated and the performance expectations of the cycle can be met in stable steady-state conditions. Based on the measurement results and the turbine design calculations an electric power output of the cycle of 3.8 kW will be possible, which results in a cycle efficiency of about 6.5 % and an increase of the electrical power output of the CHP plant of about 10 %. The results show that the plant concept is technically feasible and, with further optimization, also represents an alternative to ORC plants in terms of increasing the efficiency of a cogeneration plant.
Adaptive and contacting seals such as brush seals have been successfully applied to turbomachinery for several years. In large steam turbine applications, however, various challenges still persist. Special focus is directed at the long-term performance and longevity of brushes on conventional spring-backed seal segments in steam turbines. This issue is particularly related to wear during startup conditions. This paper discusses the results of wear tests, derived from simulated transient turbine behavior, where the resultant seal leakage under steady state conditions is monitored. It is shown that the brush seal is significantly capable of adapting to varying operating conditions, but exhibits a degree of performance degradation during the initial startups. Together with previously reported mid-term wear data and an experience based long-term phenomenological approach a general model for brush seal performance degradation is developed. This model can be used for performance prediction and exerts influence on brush seal design.
Striving for smaller losses in turbomachinery has led to many advancements in the design of seals. Modern sealing concepts such as brush seals hold a great potential to increase the efficiency of both flight engines and stationary turbines. At the same time, in order to maintain stable operating conditions of the rotor, swirl-induced forces must be kept at a minimum in the sealing channels. Therefore, the influence of the permeable and flexible bristle pack of brush seals on the flow around the rotor surface must be known. In this paper the swirl flow in the cavities of two different seal geometries is studied experimentally and numerically. A conventional three-tooth labyrinth serves as a reference. A second seal arrangement with a bristle pack upstream of two teeth is compared with the reference labyrinth. The swirl is evaluated experimentally from total and static pressure measurements in various axial and circumferential positions. Additionally, the axial swirl distribution is calculated using computational fluid dynamics (CFD). Here, the numerical model of the brush seal is based on the porous medium approach and is calibrated using the experimental values of the leakage and the bristle clearance by adjusting the thickness of the bristle pack. The calibrated CFD model is then used to study the impact of the brush seal on the swirl component of the sealing flow. The observed significant decrease of the swirl by the brush seal shows good agreement with the experimental data. The impact of changes in bristle pack clearance on the swirl is also investigated and compared with experimental evidence. The aim is to show that the brush seals have a natural tendency to interrupt seal swirl. They can therefore be used for swirl control in order to create a beneficial impact on the dynamic stability of turbomachines.