Manfred Wirsum

Experimental Investigation of Steady State and Transient Heat Transfer in a Radial Turbine Wheel of a Turbocharger

Turbochargers make an essential contribution to the development of efficient combustion engines by increasing the boost pressure. In recent years, there has been a trend towards enhanced turbine inlet temperatures, which cause heat fluxes within the turbocharger. Due to the high rotational speed, the centrifugal force and thermal stress of the turbine components rise inevitably. In addition to the enhanced temperature level, due to the variation of the load and speed of the engine in cold start, acceleration and deceleration periods, the turbine inlet temperature is changing permanently, which leads to higher thermal loads. The flow state and thus the heat transfer in the turbocharger are constantly changing within a single cycle. This induces an unsteady temperature profile, which is essential for the thermal stress and thus the prediction of the component life cycle.The present study reports about the results of the experimental steady state and transient heat transfer investigations of a turbocharger which are conducted at a hot gas test rig. The investigations are performed transiently between different steady state operating points. In order to simulate the real driving conditions, the turbine inlet temperature is changed between a high and low temperature level abruptly (thermal shock) or cyclically at an approximately constant mass flow. The flow parameters at the inlet and outlet of the turbine as well as material and surface temperatures of the turbine wheel and casing are recorded. Additionally the compressor as well as the bearing housing inlet and outlet conditions are measured. The heat flux between the components is analyzed by means of the measured data.Copyright

Numerical and Experimental Investigation of Tangential Endwall Contoured Blades in a 2-Stage Turbine

This paper deals with the results of numerical and experimental investigations of tangential endwall contoured (TEWC) blades. The contouring is restricted to the blade passage and is implemented on the root and the shrouds for all blade rows. The investigated results of the new blade design have been compared to a reference blade design, which differs only in terms of the contouring. For this reason, the differences of the flow conditions can solely be attributed to the endwall contouring. All investigations have been carried out at different blade loadings in order to determine the potential improvement of the blading with respect to the efficiency.The analysis has emphasized an efficiency improvement for the design point and an efficiency decrease for high blade loading. The under- and overturning are considered by means of the angle distributions behind the stages and serve as an indicator for the losses caused by the passage vortex. An attenuation of the passage vortex in the casing area resulting from the load relief in the casing-sided area can be observed. The contouring in the hub-area however leads to a higher development of the horseshoe vortex at high blade loading.Copyright

Advanced Bottoming Cycle Optimisation for Large Alstom CCPP

This paper presents the fundamentals of an evolutionary, thermo-economic plant design methodology, which enables an improved and customer-focused optimization of the bottoming cycle of a large Combined Cycle Power Plant. The new methodology focuses on the conceptual design of the CCPP applicable to the product development and the pre-acquisition phase. After the definition of the overall plant configuration such as the number of gas turbines used, the type of main cooling system and the related fix investment cost, the CCPP is optimized towards any criteria available in the process model (e.g. lowest COE, maximum NPV/IRR, highest net efficiency). In view of the fact that the optimization is performed on a global plant level with a simultaneous hot- and cold- end optimization, the results clearly show the dependency of the HRSG steam parameters and the related steam turbine configuration on the definition of the cold end (Air Cooled Condenser instead of Direct Cooling). Furthermore, competing methods for feedwater preheating (HRSG recirculation, condensate preheating or pegging steam), different HRSG heat exchanger arrangements as well as applicable portfolio components are automatically evaluated and finally selected. The developed process model is based on a fixed superstructure and copes with the full complexity of today’s bottoming cycle configurations as well with any constraints and design rules existing in practice. It includes a variety of component modules that are prescribed with their performance characteristics, design limitations and individual cost. More than 100 parameters are used to directly calculate the overall plant performance and related investment cost. Further definitions on payment schedule, construction time, operation regime and consumable cost results in a full economic life cycle calculation of the CCPP. For the overall optimization the process model is coupled to an evolutionary optimizer, whereas around 60 design parameters are used within predefined bounds. Within a single optimization run more than 100’000 bottoming cycle configurations are calculated in order to find the targeted optimum and thanks to today’s massive parallel computing resources, the solution can be found over night. Due to the direct formulation of the process model, the best cycle configuration is a result provided by the optimizer and can be based on a single-, dual or triple pressure system using non-reheat, reheat or double reheat configuration. This methodology enables to analyze also existing limitations and characteristics of the key components in the process model and assists to initiate new developments in order to constantly increase the value for power plant customers.Copyright

The Potential of Recuperated Semiclosed CO2 Cycles

Semi-closed cycles are characterized by firing a fuel with technically pure oxygen. The combustion gases mainly consist of CO2 and H2O. The use of the exhaust heat of the turbine in a steam bottoming cycle is the most common approach to achieve a sufficient thermal efficiency. In this paper it is shown that a semi-closed CO2 cycle with thermal recuperation avoids the complexity of a bottoming steam cycle and at the same time adds the potential of supercharging the cycle to high pressure. This allows both a very high power density and a discharge of fairly clean CO2 under sufficiently high pressure, which may be used directly or after separation of parasitic gases for enhanced oil recovery or other kind of disposal methods. We show in this paper preliminary cycle calculations, the optimization of pressure ratio, pressure level, turbine inlet temperature and recuperator assumptions. The real gas properties influence the cycle optimization and especially the temperature match within the recuperator.© 2006 ASME

UNSTEADY CONJUGATE HEAT TRANSFER INVESTIGATION OF A MULTISTAGE STEAM TURBINE IN WARM-KEEPING OPERATION WITH HOT AIR

In pursuit of flexibility improvements, General Electric has developed a product to warm-keep high/intermediate pressure steam turbines using hot air. In order to optimize the warm-keeping operation and to gain knowledge about the dominant heat transfer phenomena and flow structures, detailed numerical investigations are required. For the sake of the investigation of the warm-keeping process as found in the presented research, single and multistage numerical turbine models were developed. Furthermore, an innovative calculation approach called the Equalized Timescales Method (ET) was applied for the modeling of unsteady conjugate heat transfer (CHT). In the course of the research, the setup of the ET approach has been additionally investigated. Using the ET method, the mass flow rate and the rotational speed were varied to generate a database of warm-keeping operating points. The main goal of this work is to provide a comprehensive knowledge of the flow field and heat transfer in a wide range of turbine warm-keeping operations and to characterize the flow patterns observed at these operating points. For varying values of flow coefficient and angle of incidence, the secondary flow phenomena change from well-known vortex systems occurring in design operation to effects typical for windage, like patterns of alternating vortices and strong backflows. Furthermore, the identified flow patterns have been compared to vortex systems described in cited literature and summarized in the so-called blade vortex diagram. The anylysis of heat transfer in turbine warm-keeping operation is additionally provided.

Using Scenarios for Interdisciplinary Energy Research. A Process Model

The transition towards renewable energies is not only a technical, but also an economic and social challenge. Without an economic perspective that takes into account risk and uncertainty, a technically feasible scenario can easily lead to financial losses. Likewise, a technically and economically feasible scenario which is not in line with public acceptance is difficult to implement and the diffusion of new technologies is hindered. It is therefore apparent that, for a holistic evaluation, new energy scenarios need to be considered from more than one perspective. The challenge in an interdisciplinary approach is to find a common analytical framework, which is a prerequisite to be able to integrate data and combine approaches from different disciplines into one holistic model. This paper suggests a process model for interdisciplinary collaboration and argues how within these, scenarios can be used as common frames of reference by taking a current interdisciplinary energy project as example. Finally, challenges and opportunities of the process model are discussed.

Betriebsverhalten und Aerodynamik einer kompakten Radialverdichterstufe mit Pipe-Diffusor

Performance and Aerodynamics of a Compact Centrifugal Compressor Stage with Pipe-Diffuser This thesis represents an experimental investigation of a compact aero engine centrifugal compressor stage with pipe-diffuser and tandem deswirler in which the vanes of the first deswirl row are immerged into the diffuser channel. This geometrical arrangement eliminates a vaneless space between diffuser and deswirler, which is present in current conventional centrifugal compressor stages and can result into much lower outer stage diameters. In addition to the nominal centered alignment of the first deswirl row vanes within the diffuser channel, a variation of the circumferential positioning of the deswirler towards diffuser suction and pressure side was conducted. Furthermore, a bleed extraction between impeller and diffuser was varied. For a comparison of the compact compressor stage to conventional stage designs, additionally three compressor stages with vaneless space between diffuser and deswirler were experimentally investigated, whereas each stage contained a different deswirl system. Impeller and diffuser inlet geometry of all four stages were the same. The study demonstrates that the compact centrifugal compressor stage reaches similar performance values in terms of pressure build-up and efficiency as the stages with decoupled deswirlers. In comparison to a conventional stage with simple one row prismatic deswirl vanes, the compact stage even shows a significant increase in performance. However, the compact stage looses surge margin compared to all three reference stages. Also, a shift of the characteristics of the different stage components was detected. The impeller of the compact stage shows a slightly higher pressure build-up and slightly lower work input. Furthermore, the diffuser inlet shows a significant increase in pressure build-up which is linked to the loss in surge margin. Within the diffuser channel the pressure build-up is lower due to the lower dynamic head at the channel inlet. This results from the increased diffusion at the diffuser inlet. The three-dimensionally designed deswirlers of the compact stage and of two reference stages are showing equal characteristics, whereas the pressure build-up of the deswirler of the last reference stage with prismatic vanes is significantly decreased and the total pressure loss increased. With a total-to-static stage balancing, a variation of the circumferential positioning of the deswirler to the diffuser of the compact compressor stage shows very small changes in pressure build-up and efficiency. However, the surge margin increases from suction to pressure sided positioning of the deswirler relative to the diffuser. With a total-to-total balancing of the stage, the pressure sided positioning of the deswirler relative to the diffuser shows the smallest losses but also the highest Mach-numbers at the stage exit.

Influence Of Secondary Flow Phenomena On Boundary Layer Thickness And Wall Heat Flux In Scalloped Radial Turbines

This paper deals with investigations of the boundary layer and the influencing flow phenomena in radial turbine wheels. Therefore the interrelationship between secondary flow phenomena, boundary layer thickness, thermodynamic state at the edge of the boundary layer and the wall heat flux is investigated. The results based on experimental and numerical heat transfer investigations of a scalloped turbocharger turbine wheel for commercial application. The investigations are performed and calculated for steady state operating points with a total turbine inlet temperature of 600°C. The numerical investigations aim to model the flow field and the heat transfer between the fluid and solid state and provide the basis for the following considerations. For the determination of the boundary layer thickness six criteria based on the flow velocity and the velocity gradient were defined. The evaluation of the thermodynamic state at the boundary layer edge show a relation between this thermodynamic state and the wall heat flux and enable a better understanding of the characteristic of the wall heat flux distribution and the specific conditions if there is a local heat input to or heat output from the radial turbine wheel.

Speed-Up Methods for The Modeling of Transient Temperatures With Regard to Thermal and Thermomechanical Fatigue

The accurate prediction of the life cycle in turbomachinery design is one of the most challenging issues. Traditionally, life cycle calculations for radial turbine wheels of turbochargers focus on mechanical loads such as centrifugal and vibrational forces. Due to steadily increasing exhaust gas temperatures of automotive and commercial engines in the last years, thermo-mechanical fatigue in the turbine wheel is a major topic of current investigations. In order to account for the thermally induced stresses in the turbine wheel and the turbine housing as a part of the standard design process, a fast method is required for predicting metal temperatures. In the present paper, a fast method to calculate the transient temperatures in a radial turbine is presented. In this method the specific heat capacity of the solid state is reduced by a “speed up factor” in order to shorten the duration of a transient heating or cooling process. With the shortened processes, the computing times can be reduced significantly. After the calculations, the resulting times are transferred into realistic heating or cooling times by multiplying them with the speed up factor. The method is evaluated against experimental data and against the results of a numerical method known from literature. The method shows a good agreement with those data.

Analysis of Long-Term Gas Turbine Operation With a Model-Based Data Reconciliation Technique

The continuous monitoring of gas turbines in commercial power plant operation provides long-term engine data of field units. Evaluation of the engine performance is challenging as, apart from variations of operating points and environmental conditions, the state of the engine is subject to changes due to the ageing of engine components. The measurement devices applied to the unit influence the analysis by means of their accuracy, which may itself alter with time. Furthermore, the available measurements do usually not cover all necessary information for the evaluation of the engine performance. To overcome these issues, this paper describes a method to systematically evaluate long term operation data without the incorporation of engine design models since the latter do not cover performance changes when components are ageing. Key focus of the methodology thereby is to assess long-term emission performance in the most reliable manner.The analysis applies a data reconciliation method to long-term operating data in order to model the engine performance including non-measured variables and to account for measurement inaccuracies. This procedure relies on redundancies in the data set due to available measurements and the identification of suitable additional constituting equations that are independent of component ageing. The resulting over-determined set of equations allows for performing a data set optimization with respect to a minimal cumulated deviation to the measurement values, which represents the most probable, real state of the engine. The paper illustrates the development and application of the method to analyse the gas path of a commercial gas turbine in a combined cycle power plant with long-term operating data.Copyright