Bubble-Point Measurements and Modeling of Binary Mixtures of Linear Siloxanes.

The bubble-point pressures of three binary mixtures of linear siloxanes have been measured. The binary mixtures consist of hexamethyldisiloxane (MM) which is mixed with either octamethyltrisiloxane (MDM), decamethyltetrasiloxane (MD2M), and dodecamethylpentasiloxane (MD3M). For each mixture, three compositions were measured where MM was present in approximately 25 mol%, 50 mol%, and 75 mol%. The bubble-point pressures were measured over a temperature range of 270 K to 380 K for all mixtures. Large uncertainties are observed for the lower temperatures (below 320 K) due to non-condensable impurities. A detailed analysis is performed to determine the effect of non-condensable gases on the measured bubble-point pressure data. The newly obtained bubble-point pressure data is used to determine new binary interaction parameters for the multicomponent Helmholtz energy model. The data used for the fitting of the binary interaction parameters are weighted by the relative uncertainty, this ensures that data points with high uncertainty contribute less to the final binary interaction parameter. In this work, a description of the experimental apparatus and measurement procedure is given, as well as the measured bubble-point pressure data and newly obtained binary interaction parameters.

Shock loss measurements in non-ideal supersonic flows of organic vapors.

This paper presents the first ever direct measurements of total pressure losses across shocks in supersonic flows of organic vapors in non-ideal conditions, so in the thermodynamic region close to the liquid-vapor saturation curve and the critical point where the ideal gas law is not applicable. Experiments were carried out with fluid siloxane MM (hexamethyldisiloxane, C 6 H 18 OSi 2 ), commonly employed in medium-/high-temperature organic Rankine cycles (ORCs), in the Test Rig for Organic VApors (TROVA), a blowdown wind tunnel at the Laboratory of Compressible fluid dynamics for Renewable Energy Applications (CREA lab) of Politecnico di Milano. A total pressure probe was inserted in superheated MM vapor flow at Mach number ∼ 1.5 with total conditions in the range 215 - 230 ∘ C and 2 - 12 bar at varying levels of non-ideality, with a compressibility factor evaluated at total conditions between Z T = 0.68 - 0.98 . These operating conditions are representative of the first-stage stator of ORC turbines. Measured shock losses were compared with those calculated from pre-shock quantities by solving conservation equations across a normal shock, with differences always below 2 % attesting a satisfactory reliability of the implemented experimental procedure. An in-depth analysis was then carried out, highlighting the direct effects of non-ideality on shock intensity. Even at the mildly non-ideal conditions with Z T ≳ 0.70 considered here, non-ideality was responsible for a significantly stronger shock compared to the ideal gas limit at same pre-shock Mach number, with differences as large as 6 % .

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, C 6 H 18 OSi 2 ) 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.