Czech Academy of Sciences, Institute of Thermomechanics

Abstract Hydrofluoroethers (HFEs) are considered promising replacements for the high global warming potential (GWP) per- and polyfluoroalkyl substances (PFAS) in refrigeration, heat transfer, and electronic cooling applications. However, the scarcity and inconsistency of available thermophysical property data have hindered their reliable implementation and modeling. This study reports a comprehensive experimental investigation of HFE-7300 (1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-(trifluoromethyl)pentane), including measurements of critical properties, density, vapor pressure, isobaric heat capacity, and speed of sound over wide temperature and pressure ranges. The experiments were complemented by ab initio calculations of ideal-gas thermodynamic properties, resulting in a consistent dataset suitable for model development. Correlations for vapor pressure, density, and heat capacities were derived and mutually validated for internal thermodynamic consistency. These results provide a robust foundation for developing a multiparameter equation of state for HFE-7300, while the performance of cubic, PC-SAFT, and LKP equations of state was also evaluated as an interim approach.

Previous studies on particle-laden flows have focused mainly on either small, heavy particles or large, and weakly buoyant particles. We present measurements in the regime of large and heavy particles, investigating the particle-turbulence dynamics over a parameter space covering a wide range of particle volume fractions, Stokes and Froude numbers at low to moderate Reynolds numbers. We consider the flow in a vertical pipe, thus allowing for the investigation of the two-way coupling between particles and the carrier phase turbulence while excluding the effect of gravity-induced settling on the wall-normal migration of particles. Pressure drop measurements indicate a linear increase in the skin friction coefficient with the volume fraction of particles. The growth rate of the skin friction coefficient was found to scale inversely with the Froude number. Using time-resolved particle image velocimetry measurements, we show that the carrier phase turbulence is modulated even at particle concentrations as low as 0.3%, indicating that particle-induced stresses due to the distortion of fluid streamlines cannot be neglected. Additionally, using Voronoi tessellations to quantify the particle concentration field, we observe a strong deviation from the Poisson behavior for cases both with and without turbophoresis. The results of particle tracking show that the particles move in roughly straight-line trajectories, which deviate from the flow streamlines with a mean angle of about 2° and a maximum angle of about 9°. The particles were also observed to be situated away from vortex cores indicating the occurrence of the centrifuge mechanism.

A complex adjustable car spoiler opens or closes in dependence on ride properties: the closed state increases negative lift (better for curves), while the open state ensures lower drag (better for straight segments of the road). Both these configurations are studied by using Particle Image Velocimetry method in a wind tunnel at model scale. The three measured planes are perpendicular to the main flow. The wake is characterised by upwind (stronger past the closed variant) and by a pair of wing-tip vortices, although the spoiler wing has no end, it is rounded downwards instead. These vortices are stronger past the closed variant, they migrate towards each other, while their positions correlate by about 30 %. The circulations of both vortices are strongly correlated by about 90 % for the closed case, while this correlations is much smaller (∼25 %) for the open case.

Contactless, laser-based resonant ultrasound spectroscopy was utilized to monitor changes in elastic properties in single-crystalline NiTi shape memory alloy. It was observed that the elastic behavior of the temperature-induced B19$^\prime$ martensite, which is formed by a fine mixture of variants, adopts the symmetry elements of the parent austenite phase, and thus, the changes over the transformation temperature can be represented by the temperature evolution of three cubic elastic coefficients. The experiments confirm that the transition during the cooling run is preceded by pronounced softening of the $c_{44}$ elastic coefficient, which leads to nearly complete vanishing of elastic anisotropy prior to the transition. Below the transition, this coefficient remains soft, and the character of anisotropy switches from $c_{44}/c^\prime>1$ to $c_{44}/c^\prime<1$. We rationalize this behavior from the mechanical instability of the B19$^\prime$ lattice with respect to shears along the (001)$_{B19^\prime}$ plane, which is known from first-principles calculations.