The preliminary fluid dynamic design of turbomachinery operating with non-standard working fluids and unusual operating conditions and specifications can be very challenging because of the lack of know-how and guidelines. Examples are the design of turbomachinery for small-capacity organic Rankine cycle and supercritical CO₂ cycle power plants, whereby the efficiency of turbomachinery components has also a strong influence on the net conversion efficiency of the system. These machines operate with the fluid in thermodynamic states which, for part of the process, largely deviate from those obeying to the ideal gas law. This in turn implies the presence of so-called non-ideal compressible fluid dynamics effects. Active subspaces, a model reduction technique, is at the basis of the methodology presented here, which is aimed at the optimal meanline design of unconventional turbomachinery. The resulting surrogate model depends on a very small set of non-physical variables, called active variables. The procedure integrates into a single constrained optimization framework the selection of the working fluid, the thermodynamic cycle calculation and the preliminary sizing of the turbomachinery component. As a demonstration of the advantages of the proposed approach, the design of a 10kW mini organic Rankine cycle turbine with a turbine inlet temperature of 240°C is illustrated. In this case, approximately the same maximum efficiency is estimated for three dissimilar turbines operating with different working fluids and rather different thermodynamic cycles. The use of active subspaces allows the seamless evaluation of the sensitivity of results to input parameters, both those related to the machine and the working fluid. The novel design procedure is compared in terms of computational efficiency to a conventional approach based on the coupling of a genetic algorithm directly with a meanline code. Results show that the calculation based on the use of surrogate models is more than two orders of magnitude faster. The surrogate can be used to solve any design problem within the specified boundaries of the design envelope. Results are affected by uncertainty on the estimation of losses and of non-ideal compressible fluid dynamics effects, which, in turn, do not affect the applicability of the method, which will become quantitatively accurate once this information will be available. Work to this end is underway in various laboratories.

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There is an increasing demand on small satellites to provide greater mission flexibility and capability while reducing their associated launch costs. Solar thermal propulsion (STP) is an attractive candidate to meet this requirement. STP provides specific impulses between 200 and 1000 s and thrust-to-weight ratios of 10-4 to 10-1 which fall in-between that of conventional chemical and electric propulsion systems. The unique coupling of an STP system with a thermal-to-electric energy conversion system has the potential to offer mass and volume savings by sharing on-board resources. The feasibility of using a micro-Organic Rankine Cycle (ORC) to generate the low on-board electrical power required by a small satellite and its associative design challenges are also discussed. The expected benefits of including a micro-ORC system are to increase the electric conversion efficiency, life expectancy, and cost-saving compared to traditional photovoltaic (PV) systems. The design consideration of a 100 W micro-ORC power system is also detailed focusing on both cycle thermodynamic performance and turbine characteristics. Results highlight the importance of considering the minimum blade height that can be manufactured with current state-of-the-art technology so that realistic operating conditions are selected. The cyclic siloxane D5 working fluid was selected for the micro-ORC based on a compromise between feasible turbine characteristics and cycle efficiency. The proposed system employs the unique coupling of using a single receiver to heat both the propellant and working fluid of the propulsion and electrical power system respectively. This system is shown to be a potential candidate for deorbit missions providing a delta-V of 540 m/s to microsatellites. Ammonia and water are promising propellant candidates for this system. The report highlights the benefits of an integrated solar thermal system capable of co-generating electrical power and propulsion for high delta-V missions to increase the capability of microsatellites. @en

This preliminary study considers a combined cycle configu­ration for aeroengines, whereby thermal energy from the exhaust of the gas turbine is partly recovered in order to obtain additional mechanical power. The waste heat recovery system is based on a closed thermodynamic bottoming cycle with supercritical car­ bon dioxide (scCO2) as working fluid, allowing to achieve a very high power density. As first step of the investigation a thermo­dynamic cycle analysis of the combined-cycle engine (CCE) is carried out. Results are compared to those of the intercooled­recuperative engine (IRE) configuration for the same operating conditions and calculated under the same modeling assumptions. The estimated nominal SFC of the proposed CCE configuration is approximately 20% lower compared to that of a conventional turbofan, and 6% lower than that of the IRE, if pressure drops in the heat exchangers are neglected. Such large gain justified further analysis, by including the preliminary sizing of main components. Once the sizing of heat exchangers is factored in, the thermodynamic benefit of the CCE is offset by the penalty due to the weight of the additional equip­ment. This is mainly caused by i) the space constraints of the turbofan nacelle, which strongly limit the recoverable thermal power, and ii) the lack of proper het exchanger technology for such a highly unconventional application. These issues, and the many other that need consideration, will be addressed in an upcoming research project encompassing a much wider scope involving new aircraft and propulsion system @en

The article presents a uew procedure to calculate the wall temperature of film-cooled turbine blades. The metlwdology utilises semi-empirical relations for film cooling effectiveness developed for a fiat plate. The correlations are corrected a-posteriori with parameters (Mach number, pressure, etc) resulting from aerodynamic analysis, performed by means of the Multiple Blade Interacting Streamtube Euler Solver (MISES). With the proposed approach, the disadvantages ofthe classical 2D approaches, that deal with slots instead of fihn cooling holes, are circumvented. The procedure is validated against experimental data and 3D CFD results. It is shown that the method, despite its simplicity', is relatively accurate and computationally efficient, the computational cost being an order of magnitude lower than that ofthe conventional approaches. Thanks to this, the methodology is suited for Multidisciplinary Design Optimisation (MDO) problems involving the conceptual aerothermo-structural design of turbine blades.@en

High temperature Organic Rankine Cycles power systems of low power capacity, i.e. 3-50 kW_e, are receiving recognition for distributed and mobile energy generation applications. For this type of power plants, it is customary to adopt a radial-turbine as prime mover, essentially for their ability to cope with very large volumetric flow ratio with limited fluid-dynamic penalty. To date, the design of such turbines is based on design guidelines and loss models developed mainly for turbo-chargers, subsequently adapted by means of non-validated computational fluid-dynamic calculations. In the attempt to provide data sets for CFD validation and calibration of loss models, a mini-ORC radial inflow turbine delivering 10kW of mechanical power will be realized and tested in the Propulsion and Power Laboratory of TU-Delft. The fluid dynamic design and characterization of the machine is detailed in this paper. According to available models, the results indicate that the optimal layout of mini-ORC turbines can substantially differ from that of radial-inflow turbines utilized in more traditional applications, strengthening the need of experimental campaigns to support the conception of new design practices.

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Mini ORC power systems with the capability to deliver 3-50 kWe are receiving increased recognition for applications such as heat recovery from automotive engines, or distributed power generation from geothermal reservoirs and solar irradiation. Efficient and reliable expanders are the enabling components of such power systems, and all the related developments are currently at the research stage [1]. In the open literature experimental gas dynamic data is limited concerning the fluids and the flow conditions of interest for ORC expanders [2]. Therefore, CFD tools used for the fluid dynamic design of these components cannot be validated against reliable test cases. To bridge this gap, new experimental facilities are currently being built, such as the ORCHID setup [3]. The availability of proper experimental datasets is not, however, the sole requirement for validating a CFD code. Another precondition, equivalently important, is to define an appropriate validation methodology. This paper introduces the first steps towards the validation of a CFD solver for non-ideal compressible flows. Notably, a numerical procedure based on uncertainty quantification analysis has been conceived to assess the accuracy of the thermophysical sub-model of the code, i.e. the equation of state (EoS). Due to the lack of suitable experimental data, a synthetic dataset is generated and used to investigate the validity of the procedure. The associated validation exercise confirms the applicability of the proposed procedure, but also points out that the adopted validation metrics should be complemented with additional statistical indicators.

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Currently, turbomachinery design optimization methodologies are mainly restricted to steady state approaches, due to the high computational cost associated with time-accurate shape optimization algorithms. However, the possibility to include unsteady effects in turbomachinery optimization can significantly increase the level of accuracy of the design predictions, leading to a more realistic representation of the actual performance and ultimately to a substantial increase in operating efficiency. Unsteady effects are particularly relevant in Organic Rankine Cycle turbines. A trade-off between high-fidelity time-accurate unsteady simulations of the flow solution and computational cost is therefore needed at design level. In this paper, a first application of the harmonic balance method to non-ideal compressible flows is presented. The methodology allows to solve the unsteady flow equations for a set of specified frequencies only, with significant computational time savings. An algorithm is proposed for non uniform time sampling in order to resolve frequencies that do not need to be integral multiple of one fundamental harmonic. This enables the solution of quasi-periodically forced non-linear flow problems, in combination with complex fluid models based on accurate equations of state. The method is applied to the unsteady analysis of a supersonic Organic Rankine Cycle stator with quasi-periodic inlet operating conditions, showing about one order magnitude lower computational cost compared to time-accurate simulations.

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The fluid dynamic preliminary design of unconventional turbomachinery is customary done with meanline design procedures coupled with gradient-free optimizers. This method features various drawbacks, since it might become computationally expensive, and it does not provide design insights or guidelines to the designer. This work proposes a strategy to abate this disadvantages, namely, the construction of a reduced-order model by means of active sub-spaces, and the use of the surrogate combined with a gradient-based optimizer. The case study is the design optimization of a Organic Rankine Cycle radial inflow turbine. The results show that active subspaces exist for this application, and that it is possible to construct a surrogate with an approximate error of ±1% for the total-to-static efficiency. Additionally, the optimization using the surrogate leads to accurate results and a computational cost at least four times faster. Furthermore, the results reveal that the models for unconventional turbomachinery feature multiple regions containing constrained optima. Active subspace methods thus prove to be a promising alternative for optimization of unconventional turbomachinery.@en

Organic Rankine Cycle (ORC) power systems are receiving increased recognition for the conversion of thermal energy when the source potential and/or its temperature are comparatively low. Mini-ORC units in the power output range of 350 kWe are actively studied for applications involving heat recovery from automotive engines and the exploitation of solar energy. Efficient expanders are the enabling components of such systems, and all the related developments are at the early research stage. Notably, no experimental gasdynamic data are available in the open literature concerning the fluids and flow conditions of interest for mini-ORC expanders. Therefore, all the performance estimation and the fluid dynamic design methodologies adopted in the field rely on non-validated tools. In order to bridge this gap, a new experimental facility capable of continuous operation is being designed and built at Delft University of Technology, the Netherlands. The Organic Rankine Cycle Hybrid Integrated Device (ORCHID) is a research facility resembling a state-of-Theart high-Temperature ORC system. It is flexible enough to treat different working fluids and operating conditions with the added benefit of two interchangeable Test Sections (TS's). The first TS is a supersonic nozzle with optical access whose purpose is to perform gas dynamic experiments on dense organic flows in order to validate numerical codes. The second TS is a test-bench for mini-ORC expanders of any configuration up to a power output of 100 kWe. This paper presents the preliminary design of the ORCHID setup, discussing how the required operational flexibility was attained. The envisaged experiments of the two TS's are also described.

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This work proposes and assesses two numerical models for solving high-speed condensing flows in metastable conditions. Each model involves a set of governing equations (mass, momentum, and energy) for the mixture or the continuum phase, i.e. the vapor, and two additional transport equations to characterize the dispersed phase. Such relations are formulated through the so-called method of moments that allows to represent the wetness fraction and the number of droplets of the liquid. The transport relations are discretized in space by means of a new coupled up-wind scheme. A segregated implicit time integration strategy is exploited to hasten the convergence of the full system to steady-state. The performance and accuracy of both models are thoroughly investigated on a reference quasi-1D problem and confronted against experimental data and more advanced two-phase flow models. Results show that experimental observations are adequately predicted, especially concerning the droplets dimension. It is additionally inferred that the new upwind flux is beneficial to improve robustness of the underlying numerical methods. Finally, it is demonstrated that the continuum phase model outperforms the mixture one in terms of numerical stability and computational cost, thereby making it very promising for the extension to multi-dimensional problems.@en