Applied Mechanical Design Laboratory

Laboratory

Experimental and numerical investigation of the unbalance behavior of rigid rotors supported by spiral-grooved gas journal bearings

Elia Iseli, Jürg Alexander Schiffmann

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

Robust design using multiobjective optimisation and artificial neural networks with application to a heat pump radial compressor

Soheyl Massoudi, Cyril Picard, Jürg Alexander Schiffmann

Although robustness is an important consideration to guarantee the performance of designs under deviation, systems are often engineered by evaluating their performance exclusively at nominal conditions. Robustness is sometimes evaluated a posteriori through a sensitivity analysis, which does not guarantee optimality in terms of robustness. This article introduces an automated design framework based on multiobjective optimisation to evaluate robustness as an additional competing objective. Robustness is computed as a sampled hypervolume of imposed geometrical and operational deviations from the nominal point. In order to address the high number of additional evaluations needed to compute robustness, artificial neutral networks are used to generate fast and accurate surrogates of high-fidelity models. The identification of their hyperparameters is formulated as an optimisation problem. In the frame of a case study, the developed methodology was applied to the design of a small-scale turbocompressor. Robustness was included as an objective to be maximised alongside nominal efficiency and mass-flow range between surge and choke. An experimentally validated 1D radial turbocompressor meanline model was used to generate the training data. The optimisation results suggest a clear competition between efficiency, range and robustness, while the use of neural networks led to a speed-up by four orders of magnitude compared to the 1D code.

CAMBRIDGE UNIV PRESS2022

A reduced order model to predict the machining time and cost of small-scale radial-inflow turbines

Matthias Geiselhart, Jürg Alexander Schiffmann

This paper proposes a novel approach to predict the machining time of small-scale radial-inflow turbine impellers for organic Rankine cycle power systems based on preliminary design parameters. The time prediction method uses machining databases of different milling tools for the estimation of the cutting parameters, on which basis the flow channel volume and surfaces are discretized for the generation of preliminary milling toolpaths. The parameter selection comprises the material selection, the volume removed, the demanded surface quality and the tool selection governed by the accessibility due to geometrical limitations induced by the blades. The model is verified using computer-aided manufacturing software for two radial-inflow turbines cases: a state-of-the-art turbine using air and a turbine using the working fluid Novec 649 for a heat recovery application. The results highlight the governing role of the tool slenderness ratio on the actual manufacturing time. The relative time distribution among roughing, semi-finishing and finishing operations remain similar when considering the same production chain. The tool roughing strategy holds potential for optimization by contributing to approximately 60 % of the total milling time. Furthermore, the results indicate that the turbine manufacturing time can be improved by up to approximately 44 % when the maximum permissible load limits of the tools are exploited by customized cutting data. The presented machining time model can be applied in future works to predict the manufacturing costs of impellers and to quantify the influence of individual design features on the total cost by performing a sensitivity analysis. Moreover, the design space for optimal designs of small-scale radial-inflow turbines for organic Rankine power applications considering both the manufacturing time and the performance of the expander can be explored.

Technical University of Munich2021

The effects of the tip clearance on the performance of small-scale turbopumps for ORC applications; analysis and modeling

Jürg Alexander Schiffmann, Sajjad Zakeralhoseini

In this paper, the influence of tip clearance on the performance of small-scale turbopumps is studied numerically on extensive parameter ranges suitable for organic Rankine cycle applications. A novel and fully parameterized design model is developed and used to generate a wide range of turbopumps and their fluid domains to carry out three-dimensional computations across the impeller stage. Impellers are investigated at different operating conditions and the accomplished results are analyzed to characterize the performance at design and off-design conditions. The CFD calculations demonstrate that the slip factor is dependent not only on its geometrical parameters as considered by most correlations but also on its operating conditions. The slip factor decreased almost linearly as the flow rate increased. In addition, the tip clearance ratio influences the slip factor and its influence is non-linear. The head rise decreases as the tip clearance ratios increased, however, for radial impellers, the higher head rise was observed for small tip clearance ratios.

Technical University of Munich2021

Experimental investigation and analysis of sequential jet impingement concepts for enhanced turbine cooling

Michele Gaffuri

With the future decarbonization of the electricity production, gas turbines will be an important tool for the stabilization of the electricity grid by meeting the peak of energy demand using zero-carbon fuels like synthetic gas and hydrogen.The combustion temperature in gas turbines has increased steadily over the years in a quest to increase the thermal efficiency of the machine, so that in modern gas turbines active cooling of blades and vanes is required to ensure safe operation.The need for cooling is exacerbated by the prospect of using hydrogen as fuel, since it has higher stoichiometric combustion temperature, and the combustion products include water vapor, which increases the load on parts exposed to the hot flow.For future turbines, therefore, the thermal management is of the utmost importance; this includes both the cooling system itself - internal cooling and film cooling - as well as protective coatings on the exposed surface of the parts.Additive manufacturing of metallic parts is already a reality for production of static turbine parts, and in the future rotating blades could also be produced by this method.This paradigm shift opens the design space of turbine cooling systems by reducing the manufacturing constraints of the parts, so that radically new cooling concepts can be implemented.For internal cooling, jet impingement achieves the highest local heat transfer and is therefore used in the most highly stressed regions of the turbine parts.The crossflow caused by the spent air of the jets, however, limits the number of jets that can be used in a channel while maintaining high cooling performances, especially in narrow channels with maximum crossflow configuration.The main objective of this work is the experimental investigation of a new impingement channel design that reduces the crossflow by stacking two consecutive shorter channels connected via a plenum.In total, 34 large scale models have been investigated at engine-relevant Reynolds numbers using a new transient liquid crystal technique. Experimental data include high resolution heat transfer coefficients on the target plates as well as pressure data at several locations, which allows to determine the pressure drop of each part of the channel.In a first, exploratory phase, the effect of the variation of the number of jets and of the length of the transition zone has been studied.Moreover, bypass holes that connect directly the two channels and allow for an initial crossflow in the second channel have been investigated.A low heat transfer region has been identified in the transition zone; therefore, several heat transfer enhancement devices have been installed in this zone. Ribs, pins, and cross-section reductions allow to increase the heat transfer coefficient levels.Combinations of these devices provide higher heat transfer enhancement with a marginal increase of the pressure drop.An analytical approach has been used to estimate the metal temperatures of a part cooled by the sequential channels in comparison to a conventional narrow impingement channel at the same massflow, with the new concept achieving lower temperature when using high performance heat transfer enhancement designs. With the coolant mass flow reduction allowed by the new design, an improvement of the thermal efficiency of 0.7% has been estimated when considering the thermodynamic cycle of a modern gas turbine.

EPFL2021

Stability and Unbalance Analysis of Rigid Rotors Supported by Spiral Groove Bearings: Comparison of Different Approaches

Elia Iseli, Jürg Alexander Schiffmann

The dynamic behavior of spiral-grooved gas bearing supported four degrees-of-freedom (DOF) rotors is investigated by means of linearized bearing force coefficients and full time-integrated transient analysis. The transient method consists of a state-space representation, which couples the equations of motion with the compressible thin-film fluid equation. The linearized method is based on the perturbation analysis around a given eccentric shaft position e, allowing to compute the static and linear dynamic bearing force coefficients at different excitation frequencies. The two methods are compared for a variation of test rotors and bearing geometries in a given compressibility number interval of K = [0,40]. The limitations and weaknesses of the linearized model are presented. It is shown that shafts with two symmetric herringbone-grooved journal bearings (HGJBs) have their maximum stability and load capacity if the center of gravity lays in the middle of the two bearings. For symmetric rotors (la/lb = 1), the two rigid modes, cylindrical and conical, are present and are influenced by the mass and transverse moment of inertia independently. For asymmetric rotors (la/lb < 1), the stability region decreases, and the modes have a mixed shape. It is no longer possible to clearly distinguish between pure cylindrical and pure conical mode shapes. The two methods predict the critical mass and critical transverse moment of inertias within a difference of

2021

Thermal management of components for high energy physics experiments and space applications

Timothée Robert Sophus Frei

High-energy physics (HEP) experiments and space missions share a common challenge: thermal management in harsh environments. The severe constraints in both fields make cooling of electronic components a major design concern. This thesis aims to develop high thermal conductivity, radiation hard devices for thermal management in HEP and space application with the help of micro-technologies.Continuous advances in micro-engineering have opened the door to the development of smaller and more efficient cooling devices capable of handling increasing power densities with minimum mass and volume penalties. In this respect, previous works have focused on the use of microchannels etched in single crystal silicon (ScSi) wafers to circulate a cooling fluid. However, whilst this technique represents the state-of-the-art for thermal management of silicon detectors, it poses several challenges, particularly those associated with the fluidic interconnections, compatibility with high operating pressures and coverage of large areas. A cooling scheme relying on two independent fluidic loops is proposed to overcome these limitations. The two cooling loops, referred to as the primary and the secondary cooling loops (i.e. PCL and SCL respectively), are connected through thermal and mechanical interfaces but do not share any fluidic interface. Both circuits work together to remove heat and control the temperature of multiple electronic components. The SCL transfers the heat generated at the source to the PCL, which transports it further away and dissipates it to the environment.The present thesis investigates the use of micro-oscillating heat pipes (uOHPs) to implement the secondary cooling loops. These devices offer great potential, particularly if embedded in silicon substrates. However, a better understanding of the mechanisms responsible for their self-actuated, two-phase flow is essential to produce high performance devices which meet the stringent requirements of particle detectors and spacecraft. Two types of dual-diameter uOHPs were microfabricated: the first version used a glass wafer to close trenches etched in a silicon substrate, while an all-silicon construction was adopted in the second type. The thermocompressed interface used to bond the two silicon wafers yielded excellent pressure resistance. The thermal performance of the of uOHPs was studied for different working fluids, charging ratios, orientations, and heat inputs, taking advantage of the transparent cover in the glass devices to monitor the two-phase flow for the different conditions. The results revealed that the performance of the proposed uOHPs was orientation-independent. Furthermore, under certain conditions the equivalent thermal conductivity of acetone-charged devices exceeded that of an equivalent copper strip.Finally, an innovative technique for charging and sealing the micro-cooling heat pipes was developed. It offers a low-volume, small-footprint solution to enclose working fluids in the devices. The proposed technique, which relies on a compressed Indium preform to seal the inlet charging port, yield very good leak-tightness results.

EPFL2021

Automated Design: A Journey Across Modelling, Optimization, and Education

Cyril Picard

Machine intelligence greatly impacts almost all domains of our societies. It is profoundly changing the field of mechanical engineering with new technical possibilities and processes. The education of future engineers also needs to adapt in terms of techniques and even skills. Using the design of electro-mechanical actuators as a common thread, this work explores the many-facets of automated design: modeling, optimization, and education, and looks for the prerequisites essential to its successful application. The journey starts by building a modular and integrated model. It focuses on the prediction of system-level specifications that yield high added-value for decision-makers and shorten the path from the model to the final product. Combined with multi-objective evolutionary algorithms (MOEAs) and visualization tools, the model forms an automated design tool that helps engineers and decision-makers to rapidly get important insights into their design task. Its potential and benefits are validated through two specific applications. The results, however, also highlight a gap between the reported performance of optimizers on common benchmark problems and the actual performance on these problems. To further develop optimizers, appropriate and realistic benchmark problems are needed. A subset of the integrated design model is used to formulate a new test suite called MODAct, composed of 20 constrained multi-objective optimization problems (CMOPs) with variable levels of complexity. In addition, numerical approaches to evaluate the constraint landscape of CMOPs are introduced and applied to identify the differences in features of MODAct against 45 benchmark problems from literature. Further, the convergence performance of three algorithms on the same problems highlights the key role of constraints and, in particular, the number of simultaneously violated constraints in MODAct problems. In a next step, existing constraint handling strategies suitable for MOEAs along with a newly proposed technique for many-constraint problems are evaluated. Their parameters are tuned for different problems. The performance of the various configurations further highlights the difference between MODAct and other benchmark problems and show the highly competitive results of the proposed constraint handling technique on realistic design problems. As the technical limits are removed, the impact of automated design on the work of future engineers should be considered. On the one hand, the development of professional skills by students working on team project in different settings has been evaluated thanks to 205 students from three classes. Explicitly addressing these skills within the project seems key to support stronger and broader learning, suggesting changes that do not require a full curriculum redesign. On the other hand, nine groups (33 students) have been asked to design an actuator using a conventional approach followed by an automated design approach. The actuators suggested by students using the automated tool outperform the designs obtained through the traditional approach. Six groups even suggest solutions cheaper, three of which are also smaller, than the product of experienced industry engineers. Students proved thus capable of leveraging the tool within a short time. The analysis of their mistakes suggests possible improvements for future tools. As these students leave university, they carry the hope to see such methods spread in industry.

EPFL2021

Techno-Economic Optimization of an Integrated Biomass Waste Gasifier–Solid Oxide Fuel Cell Plant

Victoria Xu Hong He, François Maréchal, Arata Nakajo, Maria del Mar Perez Fortes, Jürg Alexander Schiffmann, Jan Van Herle

With a growing energy demand in a carbon-constrained society, fuels cells powered by renewable fuels, and specifically solid waste, are seen as interesting contributors to the energy portfolio. The alternative energy industry needs to reduce costs, enhance efficiency, and demonstrate durability and reliability to be economically feasible and attractive. This paper addresses biomass waste gasification in distributed energy systems, using a solid oxide fuel cell (SOFC) to produce electricity and heat. The potential and optimal plant efficiency and layout (i.e., anode off-gas (AOG) recirculation point via small-scale turbomachinery and heat exchanger network) are analyzed through a multi-stage approach that includes scenario evaluation and multi-objective optimization via a hybrid optimization strategy with heuristics and mathematical programming. The results in this paper summarize the most convenient operating conditions and provide an optimized heat exchanger network (HEN). The AOG recirculation toward the gasifier combustor is the preferred option; the electrical and thermal efficiencies can separately go up to 49 and 47%, respectively. The combined total efficiency ranges between 76 and 82%, and the area of heat exchange, which corresponds to an amount of heat exchanged between 91 and 117 kW, is within 6–14m2.

2021