Development and Commissioning of a Blowdown Facility for Dense Gas Vapours

Abstract

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.

Direct velocity measurements in high-temperature non-ideal vapor flows

“…These analyses are currently underway, but, however, beyond the scope of this paper. However, similar results were observed by Robertson et al (2019): deviations of P∕P T at the last pressure tap between CFD and experiments were found to be 9.12-23.36% and 6.63-20.65% for Peng-Robinson and Helmholtz energy models, respectively, for a supersonic flow ( M ≈ 2 ) of R1233zd(E).…”

Nitrogen Experiments on a Supersonic Linear Cascade For ORC Applications

“…In this context, experiments are further complicated by the need of instrumentation requiring fluid-specific and condition-specific calibration. Consequently, experiments conceived and performed up to date, with the aim of validating CFD tools [1], typically employed simplified flow configurations, such as either isentropic expansions within planar converging-diverging nozzles [2,3,4] or supersonic flows over aerodynamic bodies [5]. Such experiments entail the indubitable advantage of providing nearly isentropic flows in large portions of the domain (typically upstream/downstream of shock structures); this greatly simplifies the measurement system, since it allows resorting to measurement techniques that do not require aerodynamic calibration.…”

Pneumatic System for Pressure Probe Measurements in Transient Flows of Non-Ideal Vapors Subject to Line Condensation

“…Amongst active nozzle-fitted facilities (those with test sections featuring simple geometries such as nozzles) suitable for pressure probes testing, is the Test Rig for Organic VApors (TROVA) [10] at the Laboratory of Compressible Fluid-dynamics for Renewable Energy Applications (CREA Lab) of Politecnico di Milano, where all the experimental campaigns concerning the present work were carried out. Other plants of this kind are the ORCHID at TU Delft [11], the CLOWT at Muenster University of Applied Sciences [12] and the dense-gas blowdown facility at Imperial College London [13]. Several turbine-fitted facilities such as the LUT micro-ORC test rig at Lappeenranta -Lahti University of Technology [14] exist too.…”