Investigation on the effect of rigid rod-like particles on a turbulent pipe jet by means of High-Speed Particle Image Velocimetry

The understanding of turbulent two-phase flows in which a solid phase is dispersed into a fluid is of great importance in many industrial applications such as cyclone separators, post-combustor devices and chemical reactors. Many efforts have been given in this direction and the majority of investigations can be divided into two main frameworks: effect of the solid particle dispersion and modification of the fluid phase due to the presence of a solid phase. The former is of particular interest for industrial applications where particle concentration within the flow represents a key factor, such as combustion. The latter, specifically the turbulent modulation induced by the dispersed phase in the flow, has been the subject of several works and a comprehensive review is given in Balachandar and Eaton (2010). In particular, Gore and Crow (1989) proposed a single parameter, Reynolds number independent, to characterize particle effects on the turbulence intensity of the fluid phase. This parameter is the ratio d/L among solid-phase diameter, d, to the characteristic eddy length scale, L ,of the fluid phase. For d/L >0.1, the turbulence intensity will be enhanced by solid phase addition, whereas for d/L<0.1 a decrease occurs. A historically important class of turbulent two-phase flows, is represented by jets, which can be also found in several areas of engineering involving mixing, combustion and exhausting devices. Consequently, many works have been developed which focus on turbulence modification induced by solid particles in various jet configurations (Fleckhaus et al. (1987), Prevost et al. (1996) among others). As a matter of fact, in all these investigations, due to experimental constraints, spherical particles are used as dispersed phase. On the other hand, in many applications, the dispersed phase cannot deemed as spherical (pulp production, paper manufacturing, cloud formation), whereas rodlike particles, characterized by high (>5) geometrical aspect ratio, provide a much better approximation of the effective dispersed phase (i.e. wood fibres or ice crystals). Although there are works on the effect of fiber suspensions in turbulent flows (Krochak et al (2008), Parsa et al (2011) and Parsheh et al (2005)) yet there is a lack of studies on the effects of rodlike particles on turbulence in jet flows. In this work a fiber-laden fully developed turbulent water pipe jet flow is investigated by means of a High Speed Particle Image Velocimetry (PIV) system. Experiments were carried out with two different fibers mass fraction concentration (C1=0.002% and C2=0.006% ) in the Reynolds number range 5000-9000. Optical experimental observation of turbulent two-phase flows requires a non-trivial image processing phase in order to discriminate the dispersed phase from the carrier flow seeded with particle tracers. An overview on various phase discrimination methods may be found in Khalitov and Longmire (2002) and Kiger and Pan (1999). The phase discrimination method proposed in this work is a simple extension of that proposed by Kiger and Pan (1999) and validation by means of artificial images is provided. Discrimination of phases allowed simultaneous velocity calculation of both fluid and dispersed phases. Flow features modifications induced by the dispersed phase were investigated by comparing data to single-phase flow in the near-field region of the jet (x/D < 5, where D is the pipe/hole diameter). Interactions between fluid and fibers were also investigated analyzing particles velocity data.