Ingo Jahn

Development of a Computational Tool to Simulate Foil Bearings for Supercritical CO2 Cycles

The foil bearing is an enabling technology for turbomachinery systems, which has the potential to enable cost efficient supercritical CO cycles. The direct use of the cycles working fluid within the bearings results in an oil-free and compact turbomachinery system; however, these bearings will significantly influence the performance of the whole cycle and must be carefully studied. Moreover, using CO as the operating fluid for a foil bearing creates new modeling challenges. These include highly turbulent flow within the film, non-negligible inertia forces, high windage losses, and nonideal gas behavior. Since the flow phenomena within foil bearings is complex, involving coupled fluid flow and structural deformation, use of the conventional Reynolds equation to predict the performance of foil bearings might not be adequate. To address these modeling issues, a threedimensional flow and structure simulation tool has been developed to better predict the performance of foil bearings for the supercritical CO cycle. In this study, the gas dynamics code, EILMER, has been extended for multiphysics simulation by implementing a moving grid framework, in order to study the elastohydrodynamic performance of foil bearings. The code was then validated for representative laminar and turbulent flow cases, and good agreement was found between the new code and analytical solutions or experiment results. A separate finite difference code based on the Kirchoff plate equation for the circular thin plate was developed in Python to solve the structural deformation within foil thrust bearings, and verified with the finite element analysis from ANSYS. The fluidstructure coupling algorithm was then proposed and validated against experimental results of a foil thrust bearing that used air as operating fluid. Finally, the new computational tool set is applied to the modeling of foil thrust bearings with CO as the operating fluid.

Effect of operating conditions on the elasto-hydrodynamic performance of foil thrust bearings for supercritical CO2 cycles

In order to efficiently utilize the abundant solar resources in Australia, the supercritical CO cycle is proposed as an alternative to conventional steam power cycles due to high thermal efficiency and compact system layout. To mature the technology readiness of the supercritical CO cycle, each part, including turbine, compressor, seals and bearings, needs to be evaluated and possibly re-designed under consideration of the high density working fluid. One key technology is the foil thrust bearing, which is an enabler for high speed operation and oil-free systems. Bearings are at the core of the turbomachinery system and have a significant influence on the performance of the whole system. In this paper, a quasi three-dimensional fluid-structure model, using computational fluid dynamics for the fluid phase is presented to study the elasto-hydrodynamic performance of foil thrust bearings. For the simulation of the gas flows within the thin gap, the computational fluid dynamics solver Eilmer is extended and a new solver is developed to simulate the bump and top foil within foil thrust bearings. These two solvers are linked using a coupling algorithm that maps pressure and deflection at the fluid structure interface. Results are presented for ambient CO conditions varying between 0.1 to 4.0MPa and 300 to 400K. It is found that the centrifugal inertia force can play a significant impact on the performance of foil thrust bearings with the highly dense CO and that the centrifugal inertia forces create unusual radial velocity profiles. In the ramp region of the foil thrust bearings, they generate an additional inflow close to the rotor inner edge, resulting in a higher peak pressure. Contrary in the flat region, the inertia force creates a rapid mass loss through the bearing outer edge, which reduces pressure in this region. This different flow field alters bearing performance compared to conventional air foil bearings. In addition, the effect of turbulence in load capacity and bearing torque is investigated. This study provides new insight into the flow physics within foil bearings operating with dense gases and for the selection of optimal operating condition to suit foil thrust bearings in supercritical CO cycles.

Development of a Fluid-Structure Model for Gas-Lubricated Bump-Type Foil Thrust Bearings

In the present study, a computational model for the coupled fluid-structure simulation of bump-type foil thrust bearings is developed. A three-dimensional compressible Navier-Stokes solver is extended to model the fluid flow within the thin gap. In addition, a new solver is developed to model the bump and top foils within the foil thrust bearings. These two solvers are linked with a coupling algorithm that maps pressure and deflection at the fluid-structure interface. The theory and verification of this coupling algorithm are detailed as the focus of this paper. Finally, this coupled fluid-structure simulation for the foil thrust bearings is validated with experiment results from the literature. The resulting fluid-structure model can be used to assist the design of bump-type foil thrust bearings for various applications.

Experimental characterisation of the stiffness and leakage of a prototype leaf seal for turbine applications

The leaf seal, a seal comprising multiple flexible elements, offers similar leakage to a brush seal, but may have other benefits that make it a more attractive option in some applications. This paper details an experimental investigation of the leakage and stiffness of a pair of prototype leaf seals carried out in a slow speed rotating rig at differential pressures of up to 0, 4 MPa. The stiffness is characterised by measuring the force required to create excursions of the housing relative to the seal. The results are presented in terms of the effective annular clearance of an idealised labyrinth seal. For a single seal design, in which contact between the leaf tips causes inherent damping in the seal, the effect of changes in seal housing geometry are reported. The results observed are explained with reference to the flow field in the leaf pack.

The University of Queensland refrigerant and supercritical CO2 test loop

The use of organic refrigerants or supercritical CO (sCO ) as a working fluid in closed loop power cycles has the potential to revolutionise power generation. Thermodynamic cycle efficiency can be improved by selecting bespoke working fluids that best suit a given combination of heat source and heat sink temperatures, but thermal efficiency can be maximised by pairing this with a custom made turbine. This work describes the development and design of a new 100kW thermal laboratory-scale test loop at the University of Queensland. The loop has capabil-ities for characterising both simple and recuperated refrigerant and sCO organic Rankine cycles in relation to overall cycle performance and for the experimental characterisation of radial inflow turbines. The aim of this facility is to generate high quality validation data and to gain new insight into overall loop performance, control operation, and loss mechanisms that prevail in all loop components, including radial turbines when operating with supercritical fluids. The paper describes the current test loop and provides details on the available test modes: An organic Rankine cycle mode, a closed loop Brayton cycle mode, and heat exchanger test mode and their respective operating ranges. The bespoke control and data acquisition system has been designed to ensure safe loop operation and shut down and to provide high quality measurement of signals from more than 60 sensors within the loop and test turbine. For each measurement, details of the uncertainty quantification in accordance with ASME standards are provided, ensuring data quality. Data from the commissioning of the facility is provided in this paper. This data confirms controlled operation of the loop and the ability to conduct both cycle characterisation tests and turbomachinery tests.

[Retracted publication] Design and Testing Process for a 7Kw Radial Inflow Refrigerant Turbine At the University of Queensland

The Queensland Geothermal Energy Centre of Excellence (QGECE) has been developing a small 7 kW refrigerant radialinflow turbine assembly. Such turbines, when used with organic fluids (e.g. refrigerants), result in power cycles that can have a superior thermodynamic efficiency compared to traditional power cycles and turbines in the low to medium temperature range (100-250°C). The intended use for the UQ 7kW turbine unit is validation of CFD simulations, characterisation of turbomachinery loss mechanisms, and validation of 1-D design methodologies. This paper describes the structural and aerodynamic design process that has led to completion of the turbine unit. The first generation aerodynamic design (rotor and stator) and operating points were selected using the QGECEs 1-D mean line design software TOPGEN, to obtain a simple and robust turbine. Results from preliminary CFD simulations to verify the volute and stator operation and stage simulations to provide design and off-design performance characteristics and structural loads are presented. The turbine assembly was designed with modularity in mind to allow future turbine design iteration. Design information is provided for the overall turbine concept and the modular sub-components, including volute, magnetic coupling, bearing chamber design, shaft rotordynamics, FEA analysis and the instrumentation scheme. The paper concludes with a summary of the planned tests.

High-Speed Characterization of a Prototype Leaf Seal on an Advanced Seal Test Facility

Advanced contacting seals, such as leaf seals or brush seals, can offer reduced leakage during engine operation when compared to conventional labyrinth seals. The flexible elements of these seals provide better compliance with the rotor during flight maneuvers. The functionality and performance retention attributes of an engine-scale prototype leaf seal have been investigated on a seal test facility at Rolls-Royce that achieves engine-representative pressures and speeds and allows dynamic control of the seal position relative to the rotor, both concentric and eccentric. In this paper, the experimental setup and the test method are described in detail, including the quantification of the measurement uncertainty developed to ASME standard PTC 19.1. Experimental data are presented that show the variations in leakage and torque over typical variations of the test parameters. Insight is gained into the interactions between the operating pressure and speed and the concentric and eccentric movements imposed on the seal.

Improved Understanding of Stiffness in Leaf-Type Filament Seals

Filament seals, such as brush seals and leaf seals, are investigated as a potential improved seal for gas turbine applications. As these seals operate in contact with the rotor, a good understanding of their stiffness is required in order to minimize seal wear and degradation. This paper demonstrates that the filament and complete seal stiffness is affected in comparable magnitudes by mechanical and aerodynamic forces. In certain cases, the aerodynamic forces can also lead to an overall negative seal stiffness which may affect stable seal operation. In negative stiffness, the displacement of the seal or rotor into an eccentric position causes a resultant force, which, rather than restoring the rotor to a central position, acts to amplify its displacement. Insight into the forces acting on the seal filaments is gained by investigating a leaf seal, which consists of a pack of thin planar leaves arranged around the rotor, with coverplates on either side of the leaf pack, offset from the pack surfaces. The leaf seal is chosen due to its geometry being more suitable for analysis compared to alternative filament seals such as the brush seal. Data from two experimental campaigns are presented which show a seal exhibiting negative stiffness and a seal exhibiting a stiffness reduction due to aerodynamic effects. An empirical model for the forces acting on leaf filaments is developed based on the experimental data, which allows separation of mechanical and aerodynamic forces. In addition a numerical model is developed to analyze the flow approaching the leaf pack and the interleaf flow, which gives an insight into the causes of the aerodynamic forces. Using the empirical and numerical models together, a full picture of the forces affecting leaf stiffness is created. Validation of the models is achieved by successfully predicting seal stiffness for a further data set across the full range of operating conditions. The understanding of aerodynamic forces and their impact on filament and seal stiffness allows for their consideration in leaf seal design. A qualitative assessment of how they may be used to improve seal operation in filament seals is given.

Experimental Investigation of a Leaf Seal Prototype at Engine-Representative Speeds and Pressures

The application of compliant filament seals to jet engine secondary air systems has been shown to yield significant improvements in specific fuel consumption and improved emissions. One such technology, the leaf seal, provides comparable leakage performance to the brush seal but offers higher axial rigidity, significantly reduced radial stiffness, and improved compliance with the rotor. Investigations were carried out on the Engine Seal Test Facility at the University of Oxford into the behavior of a leaf seal prototype at high running speeds. The effects of pressure, speed, and cover plate geometry on leakage and torque are quantified. Earlier publications on leaf seals showed that air-riding at the contact interface might be achieved. Results are presented which appear to confirm that air-riding is taking place. Consideration is given to a possible mechanism for torque reduction at high rotational speeds.

Heat Transfer in Saline Water Evaporative Cooling

ABSTRACT The present paper provides an overview and review on heat and mass transfer involved in evaporation of single saline water droplets as well as pure and saline water sprays for cooling purposes. The aim is to demonstrate the advances that have been made toward the deployment of Natural Draft Dry Cooling Tower operating with saline water. A new theoretical model for evaporation of single solid containing droplet is reviewed, and the corresponding advantages are discussed. Moreover, a comparison between pure and saline water sprays is reported and performance correlations are compared. The new approaches to implement saline water injections in numerical simulation and nozzle arrangement in a cooling tower reported in this work provides valuable tools for designing an efficient cooling tower. This work shows that in addition to fresh water preservation and cost savings due to water purification, using saline water can also lead to enhance cooling efficiency by up to 8% and build more compact cooling towers. Overall it has been shown that the use of saline water is a viable and promising concept that requires further exploration.