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

Abstract

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$$_6$$ 6 H$$_{18}$$ 18 OSi$$_2$$ 2 ) at temperature up to $$220\,^\circ \mathrm {C}$$ 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

Nonideal Compressible Fluid Dynamics of Dense Vapors and Supercritical Fluids

“…Experimental data confirm the nonideal dependence of the pressure ratio across the shock on stagnation conditions. A laser Doppler velocimetry (LDV) system, including a novel seeding device, was employed for measurements of subsonic and supersonic flows (Gallarini et al 2021). The velocity field was compared with the result of flow simulations and with velocities computed from pressure and temperature measurements.…”

“…The successful use of directional multi-hole probes in NICFD flows is not documented in the literature yet, and first attempts resulted in quite inaccurate measurements (see Schaffer et al 2022). Two-dimensional LDV has been recently employed to measure the flow velocity of MM vapor (Gallarini et al 2021) by solving the challenge of seeding such a high-temperature flow. The seeding method is also applicable to PIV.…”

Nitrogen Experiments on a Supersonic Linear Cascade For ORC Applications

“…In such experiments, upstream total conditions, static pressure measurements along the expansion and schlieren visualization techniques are normally sufficient to characterize the flow. Also optical velocimetry techniques can be applied, though considerably challenging in such flow environment (see [6]). Despite being relevant for canonical flow characterization and for the development of measurement techniques capable of operating at high temperature, pressure, and with possibly condensing vapors, their relevance for turbine applications is only limited since, without blade rows, wakes, semi-bladed regions, trailing edge structures, and crucial loss mechanisms are not reproduced.…”