Optical Diagnostics for Non-Ideal Compressible Fluid Dynamics

Optical based techniques are commonly employed in many technical and scientific fields, and those able to give qualitative and/or quantitative results about density and velocity fields are valuable tools to investigate compressible flows. In recent years a strong interest has emerged about non-ideal compressible flows also due to their relevance for variety of applications where such flows are encountered, including for example wind tunnels for aerodynamic testing and turbomachinery. Moreover, such flows theoretically show peculiar behaviour with respect to their ideal counterpart giving rise for example the possibility of rarefaction shocks to occur. Actually very few examples of application of optical diagnostic techniques along with measured data are reported in the literature about non-ideal compressible flows. This work aim to present a brief summary of available optical techniques that are applied within the TROVA (Test Rig for Organic VApors) facility at the CREA laboratory of Politecnico di Milano. The TROVA test rig is designed to operate with siloxane, a family of silicon oil of particular interest for high temperature Organic Rankine Cycle (ORC) applications. MDM (Octamethyltrisiloxane-C₈H₂₄O₂Si₃) or either MM (Hexamethyldisiloxane-C₆H₁₈OSi₂) are currently used as a working fluid. To investigate such kind of flows innovative Schlieren and LDV techniques have been developed, tested and applied on the TROVA blow-down wind tunnel. The paper aims to provide a description of the measurement set-up used at CREA laboratory by considering also constraints and issues in their application and design. To support the conclusions, exemplary results achieved so far are reported. 1 Introduction The interest towards non-ideal fluid dynamics has grown in recent years due to their relevance to industrial applications. Here, the term non-ideal is used to indicate the occurrence of peculiar flow behaviour because of departure from dilute, ideal-gas thermodynamics. It is interesting to observe that such kind of flows can theoretically show unusual phenomena; for example, in the so called non-classical regimes rarefaction shocks are physically admissible [1] [2]. More details about the theoretical analysis of so called non-classical regime can be found for example in [2, 1, 3, 4]. Optical techniques such as Schlieren and Laser Doppler Velocimetry (LDV) are a valuable tools to investigate compressible-fluid flows thanks to their ability to provide information about density gradients and flow velocities. Nevertheless, to the authors knowledge, example of their application to fluid flows featuring non-ideal behaviour close to liquid-vapour saturation and critical point, or within the supercritical region are still lacking. The TROVA facility is a blow-down wind tunnel purposely designed to investigated non-ideal compressiblefluid flows by using, but not limited to, Schlieren and Laser Doppler Velocimetry (LDV) techniques. This paper offers a brief description on how those techniques have been successfully implemented, and it briefly analyses some constrains and issues for their application. Results achieved so far are reported.