Jürg Alexander Schiffmann

Related publications (122)

Design and experimental investigation of a herringbone grooved gas bearing supported turbocharger

Wanhui Liu, Jürg Alexander Schiffmann

The design of an oil free turbocharger supported on herringbone grooved gas bearing was formulated as a multi-objective problem, which was solved by coupling a reduced order parametric model for gas bearing supported rotors with an evolutionary algorithm. The resulting Pareto curve offers the best possible tradeoff between the two competing objectives of maximizing the rotordynamic stability and minimizing the power losses. After manufacturing, the turbocharger was successfully operated up to 170 krpm, beyond a hybrid mode composed of a rigid body cylindrical and a flexural mode. Supercritical operation above a flexural bending mode on rigid gas lubricated bearings was possible due to the smart selection of the location of two balancing planes, to avoid the excitation of the flexural mode. A coast down test suggests minimum friction torque and therefore lift-off at 3 krpm. Bearing power loss tests performed at 40 ℃ and 100 ℃ showed that the herringbone grooved gas bearing supported oil-free turbocharger yields only 50% to 70% of the traditional semi-floating oil bearing supported turbochargers, which is a distinct advantage in terms of efficiency and cut in emissions. The theoretical and experimental results presented in this paper clearly demonstrate the rotordynamic feasibility of oil-free turbochargers supported on herringbone grooved gas bearings.

2023

Design, computational and experimental investigation of a small-scale turbopump for organic Rankine cycle systems

Jürg Alexander Schiffmann, Sajjad Zakeralhoseini

The hydraulic design, computational analysis, and experimental investigations of a high-speed small-scale turbopump for mobile waste heat recovery applications based on organic Rankine cycle systems are presented in this paper. Such applications demand high-pressure rise, lightweight, and compact pumping systems with simple construction. The investigated turbopump features an unshrouded 37.75 mm tip diameter single-stage centrifugal pump equipped with eight radial blades, eight splitters blades, and a rectangular axisymmetric volute. The pump should deliver 0.28 kg/s of mass flow rate with a pressure rise of 20 bar at a designed rotational speed of 25,000 rpm. Due to uncertainties observed in employing state-of-the-art 1-d methods that are valid for much larger machines, computational fluid dynamics is utilized to obtain a design meeting the specifications. The pump’s performance is evaluated experimentally at different rotational speeds, mass flow rates, and impeller tip clearances. The excellent agreement between experimental data and predictions from computational fluid dynamics validates the design methodology and computational results. The turbopump’s characteristics are then utilized to estimate the possible performance improvement of a target organic Rankine cycle using the novel turbopump instead of a commercial multi-stage centrifugal pump. The comparison suggests that the novel turbopump increases the efficiency of the target organic Rankine cycle by 0.3 % points and decreases its back-work ratio by nearly 50 %. The novel turbopump is approximately ten times more compact compared to commercial systems.

2023

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Increasing the capability of automated driving vehicles is motivated by environmental, productivity, and traffic safety benefits. But over-reliance on the automation system is known to cause accidents. The role of the driver cannot be underestimated as it will ultimately be the most relevant aspect for trust building and social acceptance of this technology. Here we introduce a driver-oriented automation strategy to achieve collaborative steering. Our approach relies on three major functionalities: interaction, arbitration, and inclusion. The proposed control strategy is grounded in the concept of shared control enabling driver intervention over the automation without deactivation. Well-defined physical human-robot interaction types are made available with the arbitration strategy. The automated driving trajectory is adapted to include the driver intent into the tactical level of trajectory planning. This enables driver initiated rerouting and consistent coordination of all vehicle actuators. In this way, automated vehicles, which rely on sight only, are augmented with the incorporation of the driver intent. The driver is neither replaced by nor excluded from the automation, rather their role remains active to the benefit of trust building and driving safety.

2023

Readiness evaluation of SOFC-MGT hybrid systems with carbon capture for distributed combined heat and power

Michele Gaffuri, Victoria Xu Hong He, Jürg Alexander Schiffmann, Jan Van Herle

The aggravation of climate change calls for the timely development and implementation of highly efficient, renewable, zero- or negative-emission energy solutions with high technological readiness and feasibility levels. Solid oxide fuel cells (SOFC) constitute an appealing solution since they are close to commercial viability and can generate electricity and heat from a wide variety of fuels, for instance gas from renewable sources. In this paper, we consider the coupling of SOFC with Brayton cycles via micro gas turbines (MGT) due to their suitability for a compact small-scale energy system (10–100 kWe) and their high readiness level as stand-alone systems and in combination with suitable carbon capture and storage (CCS) technologies. An integrative literature review approach is used to qualitatively evaluate different SOFC-MGT integration strategies and CCS to devise a suitable small-scale SOFC/MGT-CCS system. A promising layout of a hybrid system based on atmospheric pressure SOFC and MGT has been identified, which uses an inverted Brayton cycle to expand the SOFC outlet gases. This system has the potential to achieve similar efficiency as the widely researched pressurized system with less complexity. Further, implementing an oxy-combustion process between the SOFC and the MGT inlet, a pure stream of CO2 is obtained, easing its capture.

2023

The influence of splitter blades and meridional profiles on the performance of small-scale turbopumps for ORC applications; analysis, neural network modeling and optimization

Jürg Alexander Schiffmann, Sajjad Zakeralhoseini

This work presents the influence of splitters on the performance of small-scale turbopumps for organic Rankine cycle applications numerically. Different design parameters, such as impeller diameter at the outlet, number of blades, blade angle at the trailing edge, and the length and location of the splitter blades, are studied for both shrouded and unshrouded impellers at tip clearance ratios of 0.05 and 0.10 (relative to blade height). The results suggest that splitter blades can increase impeller head coefficient and slip factors up to 10%–24% depending on the blade outlet angle. At the same time, the total efficiency is not influenced significantly. The CFD calculations predict that the placement of splitters in the middle of the blade channel leads to the maximum head coefficient for shrouded impellers. In contrast, for the tip clearance ratio of 0.10, a higher head coefficient is observed for placement closer to the suction side of the main blade. After that, different meridional profiles are studied to investigate their influence on the performance characteristics and mitigation of tip clearance effects. Profiles with a sharper change of cross-section area at the inlet rather than a linear change are found more advantageous, as higher head rise, total efficiency, and static efficiency are observed. In the end, the splitter blade profiles are optimized through techniques of design of experiments, leading to a 2.43% improvement in total efficiency. In contrast, the head rise remains constant, and static efficiency decreases by 0.49% for an impeller with a tip clearance ratio of 0.05.

2023

Thermal management for gas lubricated, high-speed turbomachinery

Wei Liu, Luis Eric Olmedo Ocampo, Jürg Alexander Schiffmann

High-speed turbomachinery is commonly designed to achieve high power densities. Limited space for active cooling results in a challenging thermal management. A thermal modeling approach leveraging modern declarative programming capabilities is presented, yielding an efficient dynamic model capable of real-time simulation while achieving accurate results. These properties enable the inclusion of thermal management strategies in an early stage of the design process. Further, the effect of varying thermal and transport properties of materials and fluids during transient conditions is included and is suggested to yield a high impact on thermal loads and heat evacuation capabilities. The often neglected fluid advection within the system is modeled by integrating a 1D fluid network from MatlabTM SimscapeTM to the thermal model, displaying a significant impact on the temperature estimation for critical parts. The accuracy of the presented model is verified against three gas-bearing supported high-speed turbomachinery experiments for stationary and transient operation.

2023

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Adiabatic pressure drop measurements of two-phase Carbon Dioxide (CO2) have been carried out in horizontal, vertical upward and downward direction with a dedicated test facility at the European Organization for Nuclear Research (CERN). A database of more than 1100 measurements, consisting of 512 data points in horizontal and 295 data points for each vertical up- and downflow respectively, has been compiled within this study. (...) The analysis of the pressure drop signals reveals that pressure fluctuations are increasing around the slug flow regime and the oscillation effects get magnified in vertical direction. The measurements in horizontal direction are compared to 18 frictional pressure drop models. For the comparison of the vertical data, 21 void fraction correlations accounting for the static head have been combined with the frictional models. The data sets of the horizontal and vertical upflow measurements are well predicted by several pressure drop models with an acceptable statistical significance. However, the prediction models perform less accurately for vertical downflow and it has been found that the static head is not well described by the void fraction correlations. In particular at low mass velocities, the effects of phase separation due to dominant buoyancy forces are not well incorporated by the void fraction models considered. In addition to the comparison of the entire data sets, subdivided analyses accounting for the flow patterns are provided within the present work.

2022

The forced response of rigid rotors, supported by herringbone-grooved gas journal bearings are investigated numerically and experimentally. The rotor configurations are varied by using end cylinders, attached at both sides of the shaft. Three rotors with different center of mass positions relative to the bearing locations are investigated for a variation of unbalance conditions. The numerical predictions by a quasi-linear unbalance approach and a full transient analysis are compared with experimentally obtained data. Different unbalance configurations are tested for validating the numerical models at orbital motions larger than 50% of the clearance and for bearing compressibility numbers of up to lambda = 42. The investigation shows a maximum difference of 19.5 % between nominal model predictions and experimental data at maximum orbital motions. The influence of the center of mass position on the rotor motion is proven to be significant and agrees with the numerical predictions. Further, the test unit is operated at different tilt angles, allowing to investigate the influence of static loading due to gravitational force on the forced response. It is found, that under different static loading, the rotor orbital motion remains unaltered and its influence can be neglected in the unbalance analysis.

ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD2022

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This paper presents the results of a theoretical and experimental investigation of the potential of enhanced groove geometries to increase the bearing clearance of a Herringbone Grooved Journal Bearing (HGJB) supported rotor. The theoretical study investigates various groove geometries of different complexities and their effect on the stability threshold of a particular rotor geometry. The theoretical results obtained from a rigid-body rotordynamic model suggest an increase of more than 300% in instability onset speed when enhanced groove geometries are used compared to a classical, helically grooved rotor featuring the same radial bearing clearance. As part of the experimental investigation, one rotor shaft with classical grooves, representing the baseline rotor, and four rotors of identical diameter and clearance, but featuring enhanced grooves of varying degrees of complexity, were manufactured and experimentally tested. Good agreement between the experimentally determined speed of instability onset and the prediction was found for the investigated enhanced groove patterns. Experimental results of the classical rotor suggest the onset of instability to occur at a rotational speed of 56 krpm, whereas a speed of 180 krpm was achieved when enhanced groove geometries were applied to the rotor, which agrees very well with the theoretically predicted results and confirms the potential of enhanced groove geometries to stabilize HGJBs. Furthermore, the rotor featuring only a varying groove angle along the rotor axis was found to perform similarly to fully enhanced grooves of varying groove width, depth, and angle, hence representing a good trade-off between performance increase and design cost.

2022

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A test facility to investigate flow pattern transitions of vertical two-phase flow of CO2 has been built within the scope of the high-luminosity detector upgrades at the European Organization for Nuclear Research (CERN). Adiabatic flow pattern observations for both vertical up- and downflow are recorded with high-speed imaging in tubes of 8 mm inner diameter, with saturation temperatures in the range of −25◦C to +5◦C and mass velocities ranging from 100 kgm−2 s−1 to 450 kgm−2 s−1. A database of 431 flow pattern observations in upward and 123 in downward direction has been compiled. The recorded data have been analysed with machine learning techniques and a previously trained Frame- and Flow-Regime-Classifier is used for the flow regime classification. The observed two-phase flow pattern transitions did not match the transition lines of existing flow pattern maps. As a consequence, new transition lines for the bubbly-to-slug, slug-to-churn and churn-toannular transitions have been developed for both vertical upflow and downflow respectively and condensed into new flow pattern maps. It is concluded, that the flow regime transitions are strongly depended on vapour quality, mass velocity, the flow direction and the fluid properties. Compared to horizontal flow, a dryout region is not observed and the liquid film of the annular flow regime dries out symmetrically at vapour qualities close to 𝑥 = 1.

2022