Siloxanes: A new class of candidate Bethe-Zel’dovich-Thompson fluids

Abstract

This paper presents a new class of Bethe-Zel'dovich-Thompson fluids, which are expected to exhibit nonclassical gasdynamic behavior in the single-phase vapor region. These are the linear and cyclic siloxanes, light silicon oils currently employed as working fluids in organic Rankine cycle turbines. State-of-the-art multiparameter equations of state are used to describe the thermodynamic properties of siloxanes and to compute the value of the fundamental derivative of gasdynamics ⌫, whose negative sign is the herald of nonclassical gasdynamics. Siloxane fluids starting from D 6 and cyclic siloxanes of greater complexity, and MD 3 M and linear siloxanes of greater complexity are predicted to exhibit a thermodynamic region in which ⌫ is negative and hence nonclassical wavefields are admissible. As an exemplary case, a nonclassical rarefaction shock wave propagating in fluid D 6 is studied to demonstrate the possibility of using siloxane fluids in nonclassical gasdynamic applications and to experimentally verify the existence of nonclassical wavefields in the vapor phase. The sensitivity of the present results to the considered thermodynamic model of the fluid is also briefly discussed.

Exact solutions to non-classical steady nozzle flows of Bethe–Zel’dovich–Thompson fluids

“…Note, however, that the extension of the negative-Γ region and, in turn, of the thermodynamic region of reservoir states related to non-classical regimes is overestimated by the van der Waals thermodynamic model employed in the present work, if compared to more sophisticated equations of state (see e.g. Thompson & Lambrakis 1973 The computed reservoir conditions leading to non-classical regimes are expected to lie in a range of temperatures above the thermal stability limit of the working fluid (see Calderazzi & Colonna 1997;Colonna & Silva 2003;Colonna et al 2007;Pasetti, Invernizzi & Iora 2014). Therefore, in a future attempt to observe non-classical nozzle flows, an experimental set-up consisting of a nozzle directly connected to a reservoir appears to be impracticable.…”

“…On the other hand, in order to avoid thermal decomposition, a viable experiment could be devised in which the stagnation enthalpy of the fluid is increased via heating and/or compression by external means before entering the nozzle, using for example a compressor, while keeping the static temperature below the thermal decomposition limit. In this respect, among candidate BZT fluids, siloxanes are possibly the best class of fluids for experiments and applications involving non-classical gasdynamic phenomena (see Colonna et al 2007). Experimental investigations (see Angelino & Invernizzi 1993;Colonna 1996) indicate that siloxanes in stainless steel vessels are thermally stable at the temperatures characterizing states that are expected to support negative nonlinearities, provided that the fluid is sufficiently purified (see also Pasetti et al 2014).…”

Admissibility region for rarefaction shock waves in dense gases

“…This modern equation of state allows accurate computations of all relevant thermodynamic properties over the entire thermodynamic range (even close to the critical point) with the accuracy required for design and analysis of advanced technical applications. Recently, 12-parameter equations of state of the Span-Wagner type for selected linear and cyclic siloxanes have become available, see and Colonna, Nannan & Guardone (2008b), and siloxanes were proposed as candidate BZT fluids, see Colonna & Silva (2003) and Colonna, Guardone & Nannan (2007). These fluids are light silicon oils currently used in Organic Rankine Cycle turbomachinery applications and considered as potential candidates for the experimental verification of the existence of nonclassical phenomena, see Zamfirescu et al (2006a, b) and Colonna et al (2008a).…”

“…Note that, even for the most complex siloxane fluids, the conditions depicted in figure 12 are never encountered and all plots qualitatively resemble the results shown in figure 10. To conclude, as first noticed by Thompson & Lambrakis (1973), it should be recalled that the value of Γ and therefore the shape and size of the RSR for a given fluid strongly depends on the thermodynamic model used to describe the fluid properties, whose accuracy is to be validated against experimental results, see Colonna et al (2007).…”

Quasi-1D Compressible Flow of Hydrocarbon Fuel

“…They developed numerical methods to solve real fluid's compressible flow, 14 and built the flexible asymmetric shock tube (FAST) for dense gas experiments. 15 They are interested in organic Rankine cycle utilizing a family of siloxane as working fluid, 16 which has a large area of negative  near the critical point. However, hydrocarbon fuels do not have large 0  region but a large 0 1   region near the critical point.…”