Void fraction measurement using an imaging and phase isolation method in horizontal annular flow

Abstract

A new imaging method was proposed to measure the void fraction of annular flow based on phase isolation technology in a horizontal circular tube. As the gas–liquid mixture passes through the phase isolation device, which is arranged upstream, a strong swirl flow is created due to centrifugal effect. The liquid phase is pushed to the tube wall and forms a uniform liquid film, while the gas phase is concentrated to the tube center and forms a gas core. This rectified core-annular flow has a more smooth and clear phase interface than that of natural annular flow, which makes the accurate measurement of some inherent flow parameters of gas–liquid two-phase flow possible and much easier to perform. A backlight-collimated illumination and high-resolution CCD camera were employed to capture the gas core and liquid film. A calibration experiment was conducted to acquire an accurate edge detection criterion for recognition of the phase interface. The morphological image characteristics of the core-annular flow and the beam path diagram of imaging procedure were analyzed in detail and a corresponding image processing algorithm was developed. The working fluids were air and water and the ranges of void fraction covered in the sexperiment were 0.736–0.978(Usg = 4.35 m s−1–39.12 m s−1, Usl = 0.016 m s−1–0.504 m s−1). For each experiment condition, about 800 raw images were processed to obtain an average result. Comparisons to a representative model of void fraction of natural annular flow showed that the void fraction of the core-annular flow rectified by the phase isolation device remains well consistent with that of natural annular flow in the range of low-gas volume fraction, while the void fraction of core-annular flow becomes a little lower than that of natural annular flow as the gas volume becomes very high.

Liquid flow measurement using phase isolation and an imaging method in horizontal gas–liquid two-phase flow

“…The uniform shape of swirl core-annular flow enables the flowrate measurement of liquid phase in gas-liquid two-phase flow by electromagnetic flowmeter to be more accurate. The authors also successfully measured the void fraction and liquid film thickness in a previous study [48] using phase isolation and an imaging method, in which we also found that phase isolation not only provides a uniform phase interface but also generates many microbubbles in the liquid film as well. Therefore, it is rational to suppose that these phase-isolation-induced microbubbles may be able to act as tracers to measure the flow of liquid film.…”

“…Under the action of centrifugal force, the original flow pattern is redistributed into a swirl core-annular flow. With this flow pattern, the majority of the gas phase gathers into the pipe's center and forms a gas core while the liquid phase diffuses to the pipe's inner surface and forms a liquid film [48]. The interface of the two phases becomes smoother and more uniform.…”

“…In contrast to natural annular flow, the centrifugal effect caused by phase isolation introduces the vertical velocity in swirl core-annular flow. We have investigated the difference of void fraction between swirl core-annular flow and natural annular flow in our previous study [48]. The results showed that the void fraction is only slightly changed after phase isolation due to the fact that the phase isolation decreases the droplet entrainment.…”

“…A calibration experiment is conducted to obtain the velocity coefficient and the effect of gas and liquid velocities are discussed in detail. The measurement method of void fraction which was proposed in our previous study [48] is presented briefly and employed directly. Finally, the flowrate of liquid film is determined by mean axial velocity and flow area of liquid film, and the uncertainty analysis is discussed.…”

Liquid film characteristics measurement based on NIR in gas–liquid vertical annular upward flow

“…Numerous studies had been conducted on liquid film thickness employing experimental methods. The main measurement techniques can be mainly classified into acoustic [13][14][15][16], radiometric [17], electrical [18][19][20][21], and optical [22,[23][24][25][26] methods, of which electrical method [18][19][20][21] and optical method [22,[23][24][25][26] are the most widely used.…”