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.