Organic Rankine Cycle Applications Research Articles

Experimental observation and thermodynamic modeling of non-ideal expanding flows of siloxane MDM vapor for ORC applications

This paper reports extensive experimental results characterizing the supersonic expansion of an organic vapor in non-ideal thermodynamic conditions typical of organic Rankine cycle (ORC) turbines. Data are also employed to assess the accuracy of different thermodynamic models used to describe non-ideal expansions.Experiments were carried out on a converging-diverging nozzle test section, where siloxane vapor MDM expanded in the proximity of the saturation curve, the typical operating region of ORC expanders, thus proving the importance of the present investigation for ORC technology. Indeed, detailed experimental data representative of ORC expansions, useful for design tool assessment, were lacking in the open literature up to date.Two nozzles, featuring exit Mach number of 2.0 and 1.5, were tested from highly non-ideal states to dilute gas conditions. The nozzle flow was characterized by measuring total pressure, total temperature and static pressure along the axis. The Mach number was measured at the centerline through schlieren imaging. Vapor expansion was found to be dependent on inlet conditions, thus proving the flow non-ideality.State-of-the-art thermodynamic models proved their capability of fully describing the flow non-ideality, while simpler and easier to implement equations of state, such as van der Waals, can be acceptable for preliminary expander calculations.

Variable volume ratio free-piston expander: Prototyping and experimental campaign

This paper describes a dynamic model developed for control purposes of a novel free piston expander with a variable built-in volume ratio (BVR) for Organic Rankine Cycle (ORC) applications. In order to demonstrate the feasibility of the proposed concept and to validate the theoretical model, a test setup has been designed and built. An integrated two degree of freedom motor/alternator is substituted by a separate linear motor/alternator (LMA) and a rotary servo motor designed and made from standard industrial components. This approach gives a possibility to develop and to test the integrated alternator and the expander itself independently. The model has been validated by running the setup in compressor mode. The cogging force and the frictional force, which are difficult to obtain analytically, are identified and mapped. An estimation of the geometry and the mass of the piston of an integrated design has also been made. The obtained results are sufficient for the control algorithm development since all acting forces are calculated or estimated with a good accuracy.

Error analysis of ORC performance calculation based on the Helmholtz equation with different binary interaction parameters of mixture

Mixtures have been widely researched for organic Rankine cycle (ORC) application in various heat sources. In this contribution, the calculation errors of cycle performance are analyzed by investigating the property uncertainty of mixtures, based on the Helmholtz equation with three groups of binary interaction parameters (BIPs), namely newly fitted BIPs, estimated BIPs and BIPs = 1. Nine mixtures are selected for the ORC and six cycle parameters are determined under different molar fractions of mixtures and operating temperatures. When the results of fitted BIPs are considered as the basis of comparison, the relative errors of estimated BIPs and BIPs = 1 vary greatly with the molar fraction of mixture, but change little with the cycle temperature. Deviations of pumping work are always the largest among the considered parameters. For HFC/HFC mixtures, cycle parameters except pumping work have average absolute relative deviations (AARDs) less than 5%. However, for HFC/HC and HFO/HC mixtures, BIPs = 1 has an AARD of pumping work larger than 20%. The AARD of pumping work for the estimated BIPs is around 10%. In the future performance evaluation of alternative mixture with few or no experimental data, the above analysis is helpful for choosing appropriate BIPs and estimating the errors of cycle parameters.

Experimental evaluation of nucleate pool boiling heat transfer correlations for R245fa and R1233zd(E) in ORC applications

In binary geothermal power plants based on the Organic Rankine Cycle (ORC) typically shell-and-tube heat exchangers are used as evaporators. In the shell-side, nucleate boiling of the working fluid takes place on the external surfaces of tubes. For the replacement of fluids with high global warming potential (GWP) or selection of efficient working fluids, a comprehensive evaluation has to be performed. Therefore, the knowledge about the nucleate pool boiling heat transfer coefficient (HTC) in combination with the electrical power output is necessary. In this study, the focus is led on the experimental evaluation of nucleate pool boiling heat transfer correlations for R245fa and its possible replacement R1233zd(E) in ORC applications. The nucleate boiling HTC on a horizontal tube and the electrical power generation of a 1 kW scroll expander are simultaneously measured with an ORC test rig for both fluids. The thermal input is provided by an electrically heated preheater and evaporator. Nucleate boiling takes place on a plain copper tube with an outer diameter of 32 mm and a heated length of 822 mm. The surface temperature of the copper tube is determined by thermocouples within the tube in consideration of thermal conduction. The obtained measurement results, regarding heat transfer characteristics as well as power output, show that the working fluid R245fa performs better at equal saturation temperatures due to the higher density and saturation pressure, and the lower viscosity. The HTC for R245fa is up to 43.2% higher in comparison to R1233zd(E). The experimental HTC are compared to selected nucleate pool boiling HTC correlations. The evaluation reveals that correlations according to Cooper and Gorenflo et al. show the slightest mean absolute deviations between 4.75% and 15.65% for both working fluids.

Residual steam recovery in oil refineries: technical and economic analyses

The concern about environmental issues and economic competitiveness has brought discussions on how to make processes more efficient. In Brazil, most oil refineries were introduced between the 1950s and 1980s. Since then, structural changes were carried out which resulted on differences between the quantity of steam demanded for the modified processes and the quantity of steam produced by utility plants. These differences were created by the addition of new processes and modification on the composition of the oil processed over time. This work proposes a methodology to analyze the technical and economic feasibility of the exploitation of the steam wasted in oil refineries. For these analyses, three different technologies are proposed: organic Rankine cycles (ORCs) to generate power, absorption chillers to increase a gas turbine power production, and the pre-heating boiler’s feedwater to reduce fuel consumption. The economic analysis takes into account three different scenarios. The methodology was applied to a refining unit in Brazilian Northeast. Results show that all solutions are feasible technically and economically, except for one solution in the worst-case scenario. The application of ORCs to a 30-t/h stream of steam at 3.5 bar can generate 3364.9 kW of electricity (54.15% of exergy efficiency) with a return of the investment between 4.9 and 7.6 years. For a 10-t/h stream of steam at 1.4 bar, the application of ORCs can generate 878.6 kW of electricity (50.55% of exergy efficiency) with a return of the investment between 13.9 and 22.7 years. This application was not economically feasible in the worst-case scenario. The application of absorption chillers to reduce a gas turbine inlet air temperature, using 3.68 t/h of steam at 1.4 bar, increases in 2650.0 kW (12.5%) the power generated by the turbine (43.7% of exergy efficiency for the extra power). This investment returns in between 2.6 and 4.4 years. The use of a 30-t/h stream of steam at 3.5 bar can elevate boiler’s feedwater temperature from 147.8 to 150.7 °C, which results in a 0.3% reduction in boiler’s fuel consumption (7.7% of the exergy available), which returns the investment in between 3.2 and 5.2 years.

Impact of shape-optimization on the unsteady aerodynamics and performance of a centrifugal turbine for ORC applications

This paper presents the results of the application of a shape-optimization technique to the design of the stator and the rotor of a centrifugal turbine conceived for Organic Rankine Cycle (ORC) applications. Centrifugal turbines have the potential to compete with axial or radial-inflow turbines in a relevant range of applications, and are now receiving scientific as well as industrial recognition. However, the non-conventional character of the centrifugal turbine layout, combined with the typical effects induced by the use of organic fluids, leads to challenging design difficulties. For this reason, the design of optimal blades for centrifugal ORC turbines demands the application of high-fidelity computational tools. In this work, the optimal aerodynamic design is achieved by applying a non-intrusive, gradient-free, CFD-based method implemented in the in-house software FORMA (Fluid-dynamic OptimizeR for turboMachinery Aerofoils), specifically developed for the shape optimization of turbomachinery profiles. FORMA was applied to optimize the shape of the stator and the rotor of a transonic centrifugal turbine stage, which exhibits a significant radial effect, high aerodynamic loading, and severe non-ideal gas effects. The optimization of the single blade rows allows improving considerably the stage performance, with respect to a baseline geometric configuration constructed with classical aerodynamic methods. Furthermore, time-resolved simulations of the coupled stator-rotor configuration shows that the optimization allows to reduce considerably the unsteady stator-rotor interaction and, thus, the aerodynamic forcing acting on the blades.

Experimental and numerical analyses of a 5 kWe oil-free open-drive scroll expander for small-scale organic Rankine cycle (ORC) applications

Organic Rankine cycles (ORCs) are thermodynamic power cycles designed to generate work from a wide range of heat source conditions. In particular, low-grade waste heat recovery (WHR) (<150 °C) can be effectively exploited with such systems. The efficiency of an ORC is highly dependent on its expander performance. In the low power output range (<10 kW), scroll expanders are cost-effective. Although a number of researchers have investigated the use of scroll compressors as expanders, very little work has been carried out in modeling and investigating the performance of oil-free expanders. In this work, an experimental evaluation of a newly designed open-drive oil-free scroll expander was performed. The expander had a nominal capacity of 5 kW, built-in volume ratio of 3.5, and was integrated into an ORC test-rig with R245fa as working fluid. The experimental data consisted of 75 data points that were used to map the oil-free operation of the scroll expander over five expander rotational speeds (from 800 rpm to 3000 rpm). Two heat sources inlet temperature, i.e. 85 °C and 110 °C, were investigated. The scroll expander achieved a maximum overall isentropic efficiency of 0.58 for the temperature source of 110 °C, for the imposed specific volume ratio of 6.12 at rotational speed of 1600 rpm. For the same heat source, the maximum expander power output was 3.75 kW for an imposed specific volume ratio of 6.55 and rotational speed of 2500 rpm. Besides the experimental work, the performance of the expander was characterized by means of a semi-empirical model to break-down the different loss terms. A well known model available in the literature was extended to account for the major frictional losses in a scroll machine, i.e. bearings, tip-seals and other sources of friction. Additionally, an Artificial Neural Network (ANN) modeling approach was also proposed to achieve higher accuracy in mapping expander performance for use in system simulation. The experimental data and model source codes are provided as supplementary materials.

A Review of Working Fluids for Organic Rankine Cycle (ORC) Applications

The organic Rankine cycle (ORC) systems are commercially employed for small scale and large scale thermal conversion to electricity of a large variety of abundant heat sources such as exhaust waste heat and some renewable energy sources where conventional steam Rankine and open-gas cycle turbine cycles cannot provide any viable, sustainable, techno-economic solution for power generation. The ORC operates the conventional steam Rankine cycle in subcritical level but can as well operate trans critical cycle while employing heavy molecular organic fluids in place of steam which makes the ORC suitable for medium and low grade heat conversion into electricity. No singular ideal fluid exists for any application and hence over 600 pure and zeotropic mixtures have been investigated by various researches for their best suitability for different applications and operating conditions based on performance characteristics such as efficiency, cost and environmental impacts. In this study a review of working fluids selection for different applications has been conducted. This study helps in identifying the possible most suitable organic fluids for various ORC applications depending on the operating conditions.

Multi-objective optimization and exergetic analysis of a low-grade waste heat recovery ORC application on a Brazilian FPSO

This paper presents an analysis of the application of an Organic Rankine cycle (ORC) for power generation of a Brazilian FPSO (Floating Product Storage Offload). The peculiarity of this analysis is the investigation of power generation using low-temperature waste heat sources. Low-grade sources represent a significant amount of heat rejected on the platform. The primary production in the platform processes was evaluated using ASPEN-HYSYS® software v.8.6. The main sources of low-temperature residual heat were preliminarily identified, and the highest potential for energy recovery was: the heat rejected in the intercoolers and aftercoolers of the compression processes in the Main Compression Unit and the CO2 Compression Unit. For the development of this study, a computational tool was elaborated in MATLAB® to evaluate the thermodynamic performance and to predict the design of the heat exchanger of the ORC. A multi-objective optimization was conducted to verify the ORC application at the established sources. The higher net power was obtained at Main Compression Unit heat recovery, operating with R245CB2 as working fluid. This application allows to generate up to 2063 kW with a heat transfer area of 2997 m2, providing a 23.6% increase in exergy efficiency of the system. The results of this study suggest that the application of ORC cycles on FPSO platforms for heat recovery from low-temperature sources allows an essential increase in the energy and exergetic efficiency of the production processes of the platform. Although the ORC doesn’t give a substantial increase in the supply of electricity, they contribute to less gas consumption in gas turbines. In this way, contributing to a significant reduction of GHG emissions.

Performance and operation of micro-ORC energy system using geothermal heat source

In the electricity production sector, geothermal energy is considered a reliable energy source because of its independence of seasonal, climatic and geographical conditions. Low-temperature geothermal wells present a huge potential of exploitation, as the development of binary cycles and the technological improvement in drilling make this heat source a competitive solution for electricity generated distribution and self-consumption. The Organic Rankine Cycle (ORC) is currently the best solution to convert heat into electricity using low enthalpy heat sources. The ORC technology is already mature and widespread for medium and large-scale power plants, applying for geothermal, solar, biomass or waste heat recovery exploitation. Micro-scale ORC applications are still not diffused in the market: the system layout, the working fluid selection and the expander architecture can significantly vary depending on the specific realization requirements, thus a standard configuration has not established yet.In this paper, a particular case study of a micro-ORC power system using a geothermal well is presented. The application in analysis is a plug-and-play ORC facility, currently installed and operating in a pool centre. The system layout and the main components are described. The heat source is a geothermal well, which continuously supplies (by pressure difference) liquid water at a temperature lower than 60 °C to a binary Rankine cycle working with R134a. The ORC system is driven by a prototypal radial-piston expander and adopts an external-gear feed pump and a recuperative cycle. It is developed for working continuously, delivering the generated electricity directly into the grid. The facility is provided with temperature, pressure and electric power sensors for monitoring the operation and for a preliminary evaluation of the performance. The global efficiency of expander and feed pump and the ORC net efficiency have been evaluated at the regular working conditions of the geothermal well, showing values equal to, respectively, 53 %, 41 % and 4.4 %.

Modeling and optimization criteria of scroll expander integrated into organic Rankine cycle for comparison of R1233zd(E) as an alternative to R245fa

HFCs are suggested to be banned in 2020 because of high GWP (Global Warming Potential). New type of HFO refrigerant R1233zd(E) is proposed as a drop-in replacement to R245fa for organic Rankine cycle application considering the similar thermo-physical properties. In this paper, a description of previous experimental comparison between two refrigerants is presented in the first section. In the second section, further investigation in expansion procedure is implemented with a semi-empirical expander model, which is validated with experimental data based on ‘Genetic Algorithm’. Internal leakage, mechanical friction and heat transfer are presented as main irreversible losses. Input parameters are assigned to mass flow rate, expander rotational speed, supply temperature and exhaust pressure. Supply pressure, exhaust temperature and net power are computed as output results. The maximum deviation between the measured and predicted results are 3.35%, 2.24 K and 6.09% respectively. In the last section, polynomial curve-fittings of dimensionless expander efficiency are conducted for wider prediction of operating range of the expander. Values of filling factor and expander isentropic efficiency are predicted with R2=99.517% and R2=97.997%. Curve-fittings of expander efficiency can be integrated into systematic simulation, which is aimed for further optimization of cycle performance to better take advantage of new refrigerants.

Experimental comparison and optimization guidance of R1233zd(E) as a drop-in replacement to R245fa for organic Rankine cycle application

The organic Rankine cycle is capable of converting the low-temperature waste heat into electricity. The commonly used working fluid R245fa will be phased out in the near future because of the significant impact to climate change. In that case, a new refrigerant R1233zd(E) with extremely low GWP is proposed as an environmental friendly substitute in this paper. The investigation is processed with four steps: firstly, a thermodynamic analysis was carried out for prior prediction of the applicability of R1233zd(E) as an alternative to R245fa; secondly, an experimental comparison between two refrigerants was implemented under a design of extensive operating conditions; Afterwards, experimental results were presented. Differences in expansion and evaporation procedure based on three non-dimensionless indicators were analyzed; Eventually, a multi-objective optimization guidance involved with aforementioned indicators was proposed. Comparing the maximum cycle thermal efficiency, R1233zd(E) leads to approximately 3.8% higher than R245fa. Comparing the maximum output electrical power, R1233zd(E) leads to 4.5% better than R245fa. R1233zd(E) is proven as an appropriate alternative to R245fa based on current study. Prediction precision of the volume ratio dependent curves of filling factor and isentropic effectiveness are within 1.2% and 2.7%, which can be used to model a certain expander in optimization procedure.

Experimental study of R134a flow boiling in a horizontal tube for evaporator design under typical Organic Rankine Cycle pressures

The heat transfer characteristics of organic fluids at high pressures for Organic Rankine Cycle (ORC) application has not been well studied and relevant experimental results are scarce. R134a flow boiling was investigated experimentally in a 10.3 mm horizontal tube at high pressures of 2.5–3.3 MPa (0.62–0.81Pc) with mass fluxes of 300–600 kg/m2s, heat fluxes of 20–50 kW/m2. The results showed that dryout more easily occurred at the top surface at higher pressures indicating the predominant flow pattern was stratified flow with partial dryout. With the expansion of dryout, the heat transfer coefficient decreased with increasing vapor quality. The heat flux and pressure affected the heat transfer coefficient at vapor qualities less than around 0.3, while higher mass fluxes improved the heat transfer for the whole range of vapor qualities. An improved correlation was developed for stratified flow conditions by combining the Gungor and Winterton correlation and the Wojtan correlation to overcome their respective defects in predicting cases in this study. The combined correlation has a better prediction accuracy with a mean absolute prediction error of 12.5% for 270 data points for various experimental stratified conditions and the prediction accuracy is less sensitive to the experimental parameters.

Parametric optimization and thermodynamic performance comparison of single-pressure and dual-pressure evaporation organic Rankine cycles

Dual-pressure evaporation organic Rankine cycle (ORC) involves two evaporation processes with different pressures, and can significantly reduce the exergy loss in the heat absorption process compared with conventional single-pressure evaporation ORCs. However, the applicable heat source temperatures of dual-pressure evaporation ORCs and the effects of the working fluid thermophysical properties on the applicable conditions remain indeterminate. Optimal cycle parameters for various heat source temperatures also need to be studied. Solving these questions is crucial for the application and promotion of dual-pressure evaporation ORCs. This study focuses on a typical dual-pressure evaporation ORC driven by the 100–200 °C heat sources without a limit on the outlet temperature. Nine pure organic fluids were selected as working fluids. Evaporation pressures and evaporator outlet temperatures of the single-pressure and dual-pressure evaporation ORCs were optimized, and their optimized system thermodynamic performance was compared. Results show that the applicable heat source temperature range of the dual-pressure evaporation ORC (Wnet,dual>Wnet,single) generally increases as the working fluid critical temperature increases. The upper limit of the applicable heat source temperatures (THS,in_TP), working fluid critical temperature and pinch point temperature difference generally conform to a linear relation. For the heat source temperature below THS,in_TP, the maximized net power output of the dual-pressure evaporation ORC is larger than that of the single-pressure evaporation ORC. Furthermore, the increment generally increases as the heat source temperature decreases, and the maximum increments are 21.4–26.7% for nine working fluids. For the heat source temperature above THS,in_TP, the dual-pressure evaporation ORC is unbefitting.

Investigation of a scroll expander driven by compressed air and its potential applications to ORC

Micro combined heat and power systems using an organic Rankine cycle (ORC) as a prime mover are under development. The ORC is capable of utilizing various heat sources. The key component in the ORC is an expander to produce the electric power, yet such an expander for residential applications has, to date, not been commercially available. The present study aims to investigate the performance of a scroll expander that has been converted from a commercial refrigeration scroll compressor, as these scroll machines are generally mass-produced as compressors in refrigeration applications. The viability of converting a scroll compressor into an expander for use in an ORC was examined in this paper. A test rig was built where the expander was driven by compressed air and the scroll expander was evaluated under various operating conditions, providing fundamental data that could form the basis for ORC applications. It has been found that conversion of a scroll compressor into an expander is feasible without major modifications. Experiments showed that the power output of the expander increased significantly with increasing rotating speed. The mechanical efficiency of the expander ranged from 62.7% to 60.9% at the rotating speeds of 1600–3995 rpm and the adiabatic efficiency increased from 55.5% to 58.5% at the tested rotating speeds, whereas the total efficiency varied between 34.8% and 35.6%. Based on the expander tested, an ORC system was specified and the resulting efficiency of the ORC, through modelling, was determined to be 6.9%. It was noted that the built-in volume ratio of the compressor may need to be optimized, when operating in reverse, as an expander, in order to achieve a higher ORC efficiency.

Design and process integration of organic Rankine cycle utilizing biomass for power generation

Indonesia has high potential biomass energy sources from palm oil mill industry activities. The growing interest on Organic Rankine Cycle (ORC) application to produce electricity by utilizing biomass energy sources are increasingly due to its successfully used for generating electricity from rejected waste heat to the environment in industrial processes. In this study, the potential of the palm oil empty fruit bunch, and wood chip have been used as fuel for biomass to generate electricity based ORC with combustion processes. The heat from combustion burner was transfer by thermal oil heater to evaporate ORC working fluid in the evaporator unit. The Syltherm-XLT thermal oil was used as the heat carrier from combustion burner, while R245fa was used as the working fluid for ORC unit. Appropriate designs integration from biomass combustion unit to ORC unit have been analyzed and proposed to generate expander shaft-work. Moreover, the effect of recuperator on the total system efficiency has also been investigated. It was observed that the fuel consumption was increased when the ORC unit equipped recuperator operated until certain pressure and decreased when operated at high pressure.

Integrated biomass pyrolysis with organic Rankine cycle for power generation

The growing interest on Organic Rankine Cycle (ORC) application to produce electricity by utilizing biomass energy sources are increasingly due to its successfully used to generate power from waste heat available in industrial processes. Biomass pyrolysis is one of the thermochemical technologies for converting biomass into energy and chemical products consisting of liquid bio-oil, solid biochar, and pyrolytic gas. In the application, biomass pyrolysis can be divided into three main categories; slow, fast and flash pyrolysis mainly aiming at maximizing the products of bio-oil or biochar. The temperature of synthesis gas generated during processes can be used for Organic Rankine Cycle to generate power. The heat from synthesis gas during pyrolysis processes was transfer by thermal oil heater to evaporate ORC working fluid in the evaporator unit. In this study, the potential of the palm oil empty fruit bunch, palm oil shell, and tree bark have been used as fuel from biomass to generate electricity by integrated with ORC. The Syltherm-XLT thermal oil was used as the heat carrier from combustion burner, while R245fa was used as the working fluid for ORC system. Through Aspen Plus, this study analyses the influences on performance of main thermodynamic parameters, showing the possibilities of reaching an optimum performance for different working conditions that are characteristics of different design parameters.

Life cycle greenhouse gas evaluation of organic rankine cycle using refuse-derived fuel from municipal solid waste

In Thailand, the rise in population and economic growth have caused waste generation to increase rapidly, leading to increases in greenhouse gases (GHGs) being released to the atmosphere from waste landfills. Application of waste-to-energy technology to produce electricity can reduce waste accumulation in landfills in addition to lessening the GHG emissions. This study aimed to evaluate life cycle GHG emissions from application of organic Rankine cycle (ORC) as waste-to-energy technology using refuse-derived fuel (RDF), which is produced from municipal solid waste, according to the life cycle assessment approach. The functional unit is defined as 1 kWh. The results indicate that the GHG emissions of 1 kWh of power generated from a RDF hybrid with an ORC power generation system are mostly contributed from RDF combustion in the operation stage, which accounts for 77% of the total. The RDF hybrid ORC shows fewer GHG emissions than the existing combustion-based power generation in Thailand, except for natural gas with the combined-cycle technology. The RDF hybrid ORC power generation can reduce 51.47% of GHG emissions when compared with an open dump, and 34.31% when compared with a landfill. The RDF hybrid ORC was also demonstrated as an environmentally friendly system that can be considered as a source of power generation in power planning in the future.

Optimal Aerodynamic Design of a Transonic Centrifugal Turbine Stage for Organic Rankine Cycle Applications

This paper presents the results of the application of a shape-optimization technique to the design of the stator and the rotor of a centrifugal turbine conceived for Organic Rankine Cycle (ORC) applications. Centrifugal turbines have the potential to compete with axial or radial-inflow turbines in a relevant range of applications, and are now receiving scientific as well as industrial recognition. However, the non-conventional character of the centrifugal turbine layout, combined with the typical effects induced by the use of organic fluids, leads to challenging design difficulties. For this reason, the design of optimal blades for centrifugal ORC turbines demands the application of high-fidelity computational tools. In this work, the optimal aerodynamic design is achieved by applying a non-intrusive, gradient-free, CFD-based method implemented in the in-house software FORMA (Fluid-dynamic Opti-mizeR for turboMachinery Aerofoils), specifically developed for the shape optimization of turbomachinery profiles. FORMA was applied to optimize the shape of the stator and the rotor of a transonic centrifugal turbine stage, which exhibits a significant radial effect, high aerodynamic loading, and severe non-ideal gas effects. The optimization of the single blade rows allows improving considerably the stage performance, with respect to a baseline geometric configuration constructed with classical aerodynamic methods. Furthermore, time-resolved simulations of the coupled stator-rotor configuration shows that the optimization allows to reduce considerably the unsteady stator-rotor interaction and, thus, the aerodynamic forcing acting on the blades.

Experimental observation of non-ideal expanding flows of Siloxane MDM vapor for ORC applications

Extensive experimental results characterizing the supersonic expansion of an organic vapor in non-ideal conditions are reported in this paper for the first time. The collected data also allowed the assessment of the accuracy of Computational Fluid Dynamic (CFD) tools employed to predict the non-ideal behavior of such flows, including the consistency of thermodynamic models adopted.The investigation has been carried out on the converging-diverging nozzle test section of the Test Rig for Organic VApors (TROVA), at the Laboratory of Compressible fluid-dynamics for Renewable Energy Application (CREA) of Politecnico di Milano. Supersonic nozzle flow was chosen as the simplest one of significance for organic Rankine cycle (ORC) turbine channels.The working fluid under scrutiny is Siloxane MDM, a widely employed compound for high temperature ORCs. MDM vapor expands through the TROVA nozzle at moderate non-ideal conditions in the close proximity of the vapor saturation curve. This is the region where ORC expanders typically operate, thus proving the relevance of the investigation for the ORC community. Indeed, detailed experimental data representative of typical ORC expansions were lacking in the open literature up to date.Two different nozzle geometries, featuring exit Mach number of 2.0 and 1.5 respectively, were tested, exploring a wide range of thermodynamic inlet conditions and diverse levels of non-ideality, from moderate non-ideal state, indicated by a compressibility factor Z = Pv/RT ≃ 0.80, to dilute gas conditions, Z ≥ 0.97. Maximum operating total pressure and temperature are Pt ≃ 5 bar and TT ≃ 250 °C. The nozzle flow is characterized in terms of total pressure, total temperature, static pressure at discrete locations along the nozzle axis, and schlieren imaging. In contrast to the well known case of polytropic ideal gas, the vapor expansion through the nozzle is found to be dependent on the inlet conditions, thus proving the non-ideal character of the flow. This influence is found to be consistent with the one predicted by the quasi-1D theory coupled with simple non-ideal gas models.Experimental data at the nozzle centerline are compared with those resulting from a two-dimensional viscous CFD calculation carried out using the SU2 software suite and the improved Peng Robinson Stryjek Vera (iPRSV) thermodynamic model. A very good accordance is found, demonstrating the high accuracy of the applied tools.