Thermal stability of linear siloxanes and their mixtures

The working fluid thermal stability is one of the crucial features of an effective organic Rankine cycle. Hexamethyldisiloxane (MM -C6H18OSi2) and octamethyltrisiloxane (MDM -C8H24O2Si3) are siloxane fluids currently exploited in high temperature organic Rankine cycles. However, data about their thermal stability are scarce or absent in literature. This manuscript presents a study of their behavior and decomposition at operating temperatures in the range 270 - 420 degrees C. The assessment of thermal stability can be performed with several methods, which are either based on pressure anomalous variation in isothermal stresses or on the deviation of the saturation curves experimentally obtained before and after the fluid is thermally stressed. An enhanced method is proposed here, based on chemical analysis of both vapor and liquid phases of the sample before and after it is subjected to thermal stress. A comparison of the pre-and post-stress vapor-liquid equilibrium curve complements the analysis. Results proved a higher stability for MM than for MDM. Moreover, due to the current interest in applying mixtures in organic Rankine cycles, an equimolar mixture of MM and MDM was also tested, which exhibit a behavior that appears to be different from the simple superimposition of pure fluid ones

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

Abstract 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 T T ≃ 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

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

Direct velocity measurements in a non-ideal expanding flow of a high temperature organic vapor were performed for the first time using the laser Doppler velocimetry technique. To this purpose, a novel seeding system for insemination of high-temperature vapors was specifically conceived, designed, and implemented. Comparisons with indirectly measured velocity, namely inferred from pressure and temperature measurements, are also provided. Nozzle flows of hexamethyldisiloxane (MM, C6H18OSi2) at temperature up to 220∘C and pressure up to 10 bar were taken as representative of non-ideal compressible-fluid flows. The relative high temperature, high pressure and the need of avoiding contamination pose strong constraints on the choice of both seeding system design and tracer particle, which is solid. A liquid suspension of tracer particles in hexamethyldisiloxane is injected through an atomizing nozzle in a high-temperature settling chamber ahead of the test section. The spray droplets evaporate, while the particles are entrained in the flow to be traced. Three different test cases are presented: a subsonic compressible nozzle flow with a large uniform region at Mach number 0.7, a high velocity gradient supersonic flow at Mach number 1.4 and a near-zero velocity gradient flow at Mach number 1.7. Temperature, pressure and direct velocity measurements are performed to characterize the flow. Measured velocity is compared with both computational fluid dynamics (CFD) calculations and velocity computed from pressure and temperature measurements. In both cases, the thermodynamic model applied was a state-of-the-art Helmoltz energy equation of state. A maximum velocity deviation of 6.6% was found for both CFD simulations and computed velocity. Graphical abstract: [Figure not available: see fulltext.]

Design and commissioning of a laser doppler velocimetry seeding system for non-ideal fluid flows

The design, the construction and the commissioning of a seeding system for Laser Doppler Velocimetry operating in non-ideal conditions, namely in the close proximity of the liquid-vapor saturation curve and critical point, is presented. The system is implemented in the Test Rig for Organic VApors (TROVA), a facility built at CREALab (Politecnico di Milano) with the aim of characterizing non-ideal gas flows representative of those occurring in Organic Rankine Cycle turbine passages. The tested fluid is the siloxane MDM (Octamethyltrisiloxane – C8H24O2Si3), a silicon oil of particular interest for high temperature ORC applications. Depending on the test operating conditions, the fluid under scrutiny expands in a convergent-divergent nozzle from total pressure and total temperature ranging from 4 bar to 25 bar and from 253:2 C to 310:3 C respectively, therefore the seeding has to be injected in a high temperature and high pressure environment, without altering the thermo-fluid dynamic behavior of the fluid. A suspension of the tracer particles (titanium dioxide, TiO2 or silicon dioxide, SiO2) in the working fluid is atomized into the flow, in a plenum ahead of the nozzle inlet. Since the surrounding fluid is in superheated vapor (or supercritical) conditions, the spray then evaporates leaving the solid particles free to follow the flow. The designed system consists of a tank, pressurized with nitrogen and containing the MDM-seeding suspension, of a jet mixing system, to maintain the suspension stirred, and of a drawing line ending with the atomizing nozzle. During normal operation, the tank is pressurized at a pressure higher than the plenum one and the fluid flows naturally through the atomizer. The system has been commissioned and validated through the verification of its operation. The system is suitable for all cases where optical measurements (LDV, PIV, etc.) have to be applied in high temperature, high pressure conditions similar to those occurring in the TROVA and whenever the use of auxiliary fluids different from the working one is not feasible. The reported test proves the suitability of the system in properly seeding the flow.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

Thermal stability of hexamethyldisiloxane and octamethyltrisiloxane

A thermal stability test-rig for organic Rankine cycles working fluids was designed and commissioned at the Laboratory of Compressible-fluid dynamics for Renewable Energy Applications (CREA Lab) of Poli- tecnico di Milano, in collaboration with the University of Brescia. The set-up is composed by a vessel containing the fluid, heated for about 80 h at a constant stress temperature. During the test, the pressure is monitored to detect thermal decomposition of the fluid. After the test, the vessel is placed in a thermal bath, where the vapor pressure is measured at different values of temperature lower than the stress temperature and critical temperature and is compared to that obtained before the fluid underwent thermal stress. If departure from the initial fluid behavior is observed, thermal decomposition occurred and a chemical analysis of the sample is carried out on both liquid and vapor phase using gas chro- matography and mass spectrometry. Experimental results are reported for the pure siloxane fluids MM (Hexamethyldisiloxane, C6 H18 OSi2 ) and MDM (Octamethyltrisiloxane, C8 H24 O2 Si3 ), showing that limited but appreciable decomposition is occurring at 240C and 260C respectively