Two-wave structure of liquid film and wave interrelation in annular gas-liquid flow with and without entrainment

Abstract

The wavy structure of liquid film in downward annular gas-liquid flow with and without liquid entrainment is examined for a wide range of gas velocities using high-speed laser-induced fluorescence technique. It is shown that the wavy structure is always represented by two types of waves, long-lived and short-lived waves with the latter always generated at the back slopes of the long-lived waves. Main regularities of short-lived wave generation and evolution are also described.Annular flow represents the combined flow of liquid film along the channel wall and high-velocity gas stream along the center of the channel. At high gas and liquid flow rates liquid droplets are entrained from the film surface into the core of the gas stream. Entrainment contributes significantly to integral heat and mass transfer, and its modeling entails high practical interest. It is now acknowledged that two types of waves coexist on the film surface in case of entrainment: long-length disturbance waves with amplitude several times higher than the average film thickness and small-scaled ripple waves. According to high-speed video observations of Woodmansee and Hanratty, 1 entrainment occurs due to "acceleration, lifting and subsequent scattering of ripples on disturbance waves crests."For physical modeling of entrainment phenomenon, detailed information on the origin of the two types of waves is required. At present ripples are considered to be omnipresent on the film surface in the presence of turbulent gas stream, even for very low film Reynolds numbers. With liquid flow rate growth disturbance waves appear and entrainment occurs. 2,3 Existence of critical Reynolds number is the characteristic feature of entrainment phenomenon. At Reynolds numbers lower than this value entrainment does not occur irrespective of gas stream velocity. 2,3 This feature is usually explained by the absence of disturbance waves in regimes without entrainment ͑hereinafter such regimes will be referred to as "no-entrainment regimes"͒. So, the contemporary notion of the wavy structure of annular flow is as follows: when entrainment occurs, there exist two independent types of waves on liquid film surface-disturbance and ripple waves-and when entrainment is absent, only ripple waves exist.Some attempts have been made to obtain a more detailed representation of the wavy structure in annular flows. Sekoguchi et al., 4 using a complicated set of conductivity sensors for spatiotemporal investigation of liquid film thickness, found "ephemeral waves," which sometimes appear on the back front of disturbance waves and move slower than the latter. Ohba and Nagae 5 observed one more type of waves in the narrow region near the transition to entrainment in upward annular flow. Those waves, which they called "ring waves," were shorter than disturbance waves and wider than ripples. But common practice is to divide waves into disturbance waves and ripples and to study them separately. 6,7 The ripples are known to be short lived and coherent only for short ͑about several wavele...

Disturbance wave development in two-phase gas–liquid upwards vertical annular flow

“…gas-liquid oil wells), evaporators and condensers (Hewitt and Hall-Taylor 1970;Andreussi et al 1985;Schadel et al 1990;Alekseenko et al 1994Alekseenko et al , 2009Pan and Hanratty 2002;Park et al 2004). This flow regime corresponds to a situation in which the liquid phase flows in the form of a film on the inside of the conduit wall, in addition to droplets entrained within the gas phase.…”

“…Achieving a fundamental understanding of the mechanisms underlying the onset and evolution of disturbance waves is important for a number of reasons. For instance, it seems likely that (at least at low liquid viscosities) disturbance waves are a necessary condition for the entrainment of droplets from wavy interfaces (Cousins et al 1965;Cousins andHewitt 1968a, 1968b;Hewitt and Roberts 1969;Azzopardi 1986Azzopardi , 1997Owen andHewitt 1986, Pan andHaratty 2002;Sawant et al 2008a;Alekseenko et al 2009). Disturbance waves also have a dominant role in the shear stress at the interface in annular flow and measurements of wall-shear stress underneath such waves are reported by Martin (1983) who made simultaneous measurements at the same locality of shear stress (using a hot film probe device) and film thickness (using a conductance probe device).…”

“…One of the most characteristic and important interfacial wave phenomena in annular flow is the formation of large waves (known as disturbance waves) at the interface (Taylor 1963;Hewitt and Hall-Taylor 1970;Azzopardi 1986Azzopardi , 1997Alekseenko et al 1994Alekseenko et al , 2009). Disturbance waves have been observed above a threshold liquid flow-rate value, and remain correlated over considerable distances.…”

Stability of a moving radial liquid sheet: experiments

“…In this paper, we report a simple planar laser induced fluorescence technique similar to that of Alekseenko et al (2009) to measure the instantaneous local thickness of a thin moving liquid sheet. This technique captures not only the thickness profiles of an undulating liquid sheet but also that of the centerline displacement of the wavy liquid sheet in response to external disturbances.…”

“…A calibration was performed so that the distance between the upper and the lower boundaries of the fluorescent image yielded the true film thickness. A different approach to extracting thickness information using laser induced fluorescence method has been applied to liquid films flowing over a solid substrate or confined between transparent solid substrates (Alekseenko et al 2009). Here, a non-linear Beer-Lambert law relating the incident light intensity, emission light intensity, film thickness, fluorescent dye concentration and extinction coefficient was used to determine the local variations in the film thickness.…”