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The organic Rankine cycle is low grade heat recovery technology, for sources as diverse as geothermal, industrial and vehicle waste heat. The working fluids used within these systems often display significant real-gas effects, especially in proximity of the thermodynamic critical point. 3D Computational Fluid Dynamics (CFD) is commonly used for performance prediction and flow field analysis within expanders, but experimental validation with real gases is scarce within the literature. This paper therefore presents a dense-gas blowdown facility constructed at Imperial College London, for experimentally validating numerical simulations of these fluids. The system-level design process for the blowdown rig is described, including the sizing and specification of major components. Tests with refrigerant R1233zd(E) are run for multiple inlet pressures, against a nitrogen baseline case. CFD simulations are performed, with the refrigerant modeled by Ideal Gas, Peng-Robinson, and Helmholtz Energy Equations of State. It is shown that increases in fluid model fidelity lead to reduced deviation between simulation and experiment. Maximum and mean discrepancies of 9.59% and 8.12% in nozzle pressure ratio with the Helmholtz Energy EoS are reported. This work demonstrates an over-prediction of pressure ratio and power output within commercial CFD packages, for turbomachines operating in non-ideal fluid environments. This suggests a need for further development and experimental validation of CFD simulations for highly non-ideal flows. The data contained within this paper is therefore of vital importance for the future validation and development of CFD methods for dense-gas turbomachinery

Real-gas effects have a significant impact on compressible turbulent flows of dense gases, especially when flow properties are in proximity of the saturation line and/or the thermodynamic critical point. Understanding of these effects is key for the analysis and improvement of performance for many industrial components, including expanders and heat exchangers in organic Rankine cycle systems. This work analyzes the real-gas effect on the turbulent boundary layer of fully developed channel flow of two organic gases, R1233zd(E) and MDM - two candidate working fluids for ORC systems. Compressible direct numerical simulations (DNS) with real-gas equations of state are used in this research. Three cases are set up for each organic vapour, representing thermodynamic states far from, close to and inside the supercritical region, and these cases refer to weak, normal and strong real-gas effect in each fluid. The results within this work show that the real-gas effect can significantly influence the profile of averaged thermodynamic properties, relative to an air baseline case. This effect has a reverse impact on the distribution of averaged temperature and density. As the real-gas effect gets stronger, the averaged centre-to-wall temperature ratio decreases but the density drop increases. In a strong real-gas effect case, the dynamic viscosity at the channel center point can be lower than at channel wall. This phenomenon can not be found in a perfect gas flow. The real-gas effect increases the normal Reynolds stress in the wall-normal direction by 7–20% and in the spanwise direction by 10–21%, which is caused by its impact on the viscosity profile. It also increases the Reynolds shear stress by 5–8%. The real-gas effect increases the turbulence kinetic energy dissipation in the viscous sublayer and buffer sublayer <inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mo stretchy="false">(</mml:mo><mml:msup><mml:mi>y</mml:mi><mml:mo>∗</mml:mo></mml:msup><mml:mo><</mml:mo><mml:mn>30</mml:mn><mml:mo stretchy="false">)</mml:mo></mml:math></inline-formula> but not in the outer layer. The turbulent viscosity hypthesis is checked in these two fluids, and the result shows that the standard two-function RANS model (<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mi>k</mml:mi><mml:mo>−</mml:mo><mml:mi>ϵ</mml:mi></mml:math></inline-formula> and <inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mi>k</mml:mi><mml:mo>−</mml:mo><mml:mi>ω</mml:mi></mml:math></inline-formula>) with a constant <inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mi>C</mml:mi><mml:mi>μ</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:mn>0.09</mml:mn></mml:math></inline-formula> is still suitable in the outer layer <inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mo stretchy="false">(</mml:mo><mml:msup><mml:mi>y</mml:mi><mml:mo>∗</mml:mo></mml:msup><mml:mo>></mml:mo><mml:mn>70</mml:mn><mml:mo stretchy="false">)</mml:mo></mml:math></inline-formula>, with an error in ±10%.

The Organic Rankine Cycle is a candidate technology for low grade heat recovery, from sources as diverse as geothermal, solar and industrial/vehicle waste heat. The organic working fluids used within these systems often display significant real-gas effects, especially in proximity of the thermodynamic critical point. Significant research has therefore been performed on the design of real-gas expansion devices, including both positive displacement and rotordynamic machinery. 3D Computational Fluid Dynamics (CFD) is commonly used for performance prediction and flow field analysis within expanders, and experimental validation of these simulations within a real-gas environment are scarce within the literature. This paper therefore presents a dense-gas blowdown facility constructed at Imperial College London, for the purpose of experimentally validating numerical simulations of these fluids. The system-level design process for the blowdown rig is detailed within this paper, including the sizing and specification of major components. A hemispherically-ended 3.785 L cylinder was selected as the main blowdown vessel, allowing a designpoint pressure and temperature of 3751 kPa and 477 K, respectively. Regulating valves were placed either side of the test section, allowing a Pressure Ratio to be fixed across the measurement section. The primary design focus of this paper is that of the test section — a converging-diverging nozzle producing an expansion of Mach 2 at the nozzle exit plane. The nozzle profile is generated by Method of Characteristics (MoC) modified to account for real-gas effects. Both mechanical and fluid dynamic design are discussed, along with location and thermal management of the nine pressure transducers, located along the nozzle centreline. A series of blowdown tests are conducted, firstly for a fluid conforming closely to the ideal gas Equation of State - Nitrogen (N2) at room temperature. A comparison between the experimental measurements and a CFD analysis of these results is taken as a benchmarking example. A second set of tests with refrigerant R1233zd(E) are run, across multiple inlet pressures - CFD simulations are subsequently performed, with the refrigerant modeled by Ideal Gas, Peng-Robinson, and Helmholtz energy (via REF-PROP) Equations of State. An error analysis is conducted for each, identifying that an increase in fluid model fidelity leads to reduced deviation between simulation and experiment. An average discrepancy of 11.1% in nozzle Pressure Ratio with the Helmholtz energy EoS indicates an over-prediction of expander power output within the CFD simulation.

Mobile organic Rankine cycle (MORC) systems represent a candidate technology for the reduction of fuel consumption and CO2 emissions from heavy-duty vehicles. Through the recovery of internal combustion engine waste heat, energy can be either compounded or used to power vehicle ancillary systems. Waste heat recovery systems have been shown to deliver fuel economy improvements of up to 13% in large diesel engines [1]. Whilst the majority of studies focus on individual component performance under specific thermodynamic conditions, there has been little investigation into the effects of expander specification across transient test cycles used for heavy-duty engine emission certification. It is this holistic approach which will allow prediction of the validity of MORC systems for different classes of heavy-duty vehicle, in addition to providing an indication of system performance. This paper first describes a meanline (one-dimensional simulation along a mean streamline within a flow passage) model for radial ORC turbines, divided into two main subroutines. An on-design code takes a thermodynamic input, before generating a candidate geometry for a chosen operating point. The efficacy of this design is then evaluated by an off-design code, which applies loss correlations to the proposed geometry to give a prediction of turbine performance. The meanline code is then executed inside a quasi-steady-state ORC cycle model, using reference emission test cycles to generate exhaust (heat source) boundary conditions, generated by a simulated 11.7L heavy-duty diesel engine. A detailed evaporator model, developed using the NTU-effectiveness method and single/two-phase flow correlations, provides accurate treatment of heat flow within the system. Together, these elements allow estimation of ORC system performance across entire reference emission test cycles. In order to investigate the limits of MORC performance, a Genetic Algorithm is applied to the ORC expander, aiming to optimize the geometry specification (radii, areas, blade heights, angles) to provide maximal time-averaged power output. This process is applied across the reference duty cycles, with the implications on power output and turbine geometry discussed for each. Due to the large possible variation in thermodynamic conditions within the turbine operating range a typical ideal-gas methodology (generating a single operating map for interpolation across all operating points) is no longer accurate — a complete off-design calculation must therefore be performed for all operating points. To reduce computational effort, discretization of the ORC thermodynamic inputs (temperature, mass flow rate) is investigated with several strategies proposed for reduced-order simulation. The paper concludes by predicting which heavy-duty emission test cycles stand to benefit the most from this optimization procedure, along with a comparison to existing transient results. Duty cycles containing narrow bands of operation were found to provide optimal performance, with a Constant-Speed, Variable-Load cycle achieving an average power output of 4.60 kW. Consideration is also given to the effectiveness of the methodology contained within the paper, the challenges of making ORC systems viable for mobile applications, along with suggestions for future research developments.

Real-gas effects have a significant impact on compressible turbulent flows of dense gases, especially when flow properties are in proximity of the saturation line and/or the thermodynamic critical point. Understanding of these effects is key for the analysis and improvement of performance for many industrial components, including expanders and heat exchangers in organic Rankine cycle systems. This work analyzes the real-gas effect on the turbulent boundary layer of fully developed channel flow of two organic gases, R1233zd(E) and MDM-two candidate working fluids for ORC systems. Compressible direct numerical simulations (DNS) with real-gas equations of state are used in this research. Three cases are set up for each organic vapour, representing thermodynamic states far from, close to and inside the supercritical region, and these cases refer to weak, normal and strong real-gas effect in each fluid.