The gas-liquid flow pattern with an air core
Fig.1 The structure, principle and flow pattern of the gas-liquid separator cited from Ref.[3]
In our previous study [5], we used stereo particle image velocimetry to obtain the time averaged velocity distributions across the swirl chamber of the separator. However, the information of velocity profile is not sufficient to model the interacting terms such as the turbulent dispersion force [6]. Also important for modeling the gas-liquid two-phase flow is the choice of turbulence model [7]. Due to the lack of elaborated turbulence data, validating the turbulence model for the liquid phase is not realistic [8,9]. Therefore, an experimental work aiming to obtain the turbulent field of the liquid phase such as the turbulent kinetic energy and the Reynolds shear stresses is in demand.
The earliest work to measure the turbulent quantities of swirl flow to date is the study by Kitoh [10], in which hot wire anemometer was adopted to measure the instantaneous velocities of the swirl flow in a straight pipe. Based on the tangential velocity profile, the swirl flow regime is classified into three regions: wall, annular and core. The maximal Reynolds stress appears in the boundary area between the annular region and the core region. It was pointed out that the turbulent data in the central region and regions close to the wall in the swirl flow is not reliable due to the limitation of hot wire probe. With the development of fluid measurement technology, particle image velocimetry (PIV) proves to be an advanced tool for turbulence study. Many studies focusing on the turbulent characteristics of swirl flow were conducted [11-14]. Pruvost [15] was the first to perform a 2D PIV measurement of turbulence intensities for a swirl flow in a straight pipe where the swirl flow was generated by tangential inlets. The experimental results indicate that the magnitude of each turbulence intensity differs significantly. The anisotropic is represented by the fact that the magnitude of turbulence intensity for tangential velocity component is two times of other two components. One limitation to measure the three velocity components by 2D PIV is three velocity components for a point was not measured simultaneously [16]. A preferable solution to get around the shortcomings of 2D PIV is using stereoscopic PIV. Liu [17] successfully implemented the 2D-3C PIV to measure the turbulence quantities including the turbulent kinetic energy and Reynolds stresses in a microscale multi-inlet vortex reactor, in which a confined swirl flow generated by tangential inlets dominates. Abundant turbulence data was extracted, from which it can be seen the turbulent kinetic energy and Reynolds stresses are mainly concentrated in the core region and the wandering of vortex center influences the turbulence distribution. Thus, aiming to acquire the turbulence data of the gas-liquid separator, we used the stereo PIV of Dantec to measure the velocity fluctuations of the swirl flow concerned.
The paper is organized as follows. The Section 2 presents the experimental set-up and instrumentation followed by a brief description of stereoscopic PIV measurement. Analysis of turbulence quantities is performed in Section 4 and conclusions are given in the last Section.

2 Experimental set-up

A three dimensional schematic diagram of the experimental apparatus is depicted in Fig. 2. The loop mainly consists of one venturi bubble generator, one gas-liquid separator and other components like pump and valves. Auxiliary components including an air compressor, an air storage tank, a reducing valve and a Venturi-type bubble generator. Tap water was taken as the liquid phase, of which the volumetric flow rates were measured by electromagnetic flow meter.