Characterization of plug and slug multiphase flows by means of image analysis

Multiphase flow is involved in a wide range of applications, and among the flow patterns that a multiphase mixture may develop in its flow, the intermittent one is particularly complex both in behaviour and for analysis. Experimental analysis about the characteristics of the flow structures (plugs and slugs) is therefore still mandatory for a detailed description of the phenomenon. In this work an image-based technique for the determination of the plug/slug characteristics was applied to air-water, oil-air and three-phase oil-water-air flows in horizontal ducts with different diameters, with superficial velocities of the phases in the range 0.2-2.1 m/s. The technique is based on the acquisition of a video of the flow and the conversion of each frame (or part of it) into a Boolean signal, in which the non-zero part represents the structure of interest. Concatenation of such signals along the singleton dimension creates a space-time representation of the flow, from which information about the flow velocities, the structure lengths and frequencies and the void fraction can be extracted. Focus here is particularly on the performances of the technique when using high-speed videos. The results were also compared with the predictions of the drift-flux model

Water holdup estimation from pressure drop measurements in oil-water two-phase flows by means of the two-fluid model

The Two-Fluid Model (TFM) has been applied to determine water holdup from pressure drop measurements for core-annular flows in horizontal pipes. The fluids are Milpar 220 oil (Ïo=890 kg/m3, μo=0.832 Paâ¢s at 20 °C) and tap water (μw=1.026Ã10-3 Paâ¢s at 20°C). The investigated volume flow rates range from 2 to 6 m3/h, for water, and from 1 to 3.5 m3/h, for oil, respectively. The results are in very good agreement with available experimental data from the literature and a simple correlation between water holdup and water input fraction has been benchmarked to the overall data set. Eventually, the TFM endowed with the holdup correlation has been adopted to predict the pressure drop with quite satisfactory results: 98% of data fall within a percentage error of ±10%, 99% of the data fall within ±15%, and all the data are predicted within ±20%. On the other hand, the mean absolute relative error for the pressure drop reduction factor is 5.5%

A mechanistic model to predict pressure drop and holdup pertinent to horizontal gas-liquid-liquid intermittent flow

In this work a mechanistic model is proposed to predict pressure drop and phase holdup for viscous oil-water-gas flows within horizontal duct. The model is suitable for three phase intermittent (slug) flows where oil and water phases are fully mixed. However, validation is also made for slug flow with core-annular oil and water flow. This approach makes use of a correlation for total slug unit length developed by Babakhani Dehkordi (2017) as an input in the model so that the continuity equation is explicitly solved, reducing complexity of the present models in the literature survey. Furthermore, oil and water are assumed to have a homogeneous behavior to simplify three-phase flow equations. The model predictions are compared with experimental data of viscous oil-water-gas slug flows. Results revealed that inclusion of slug unit data together with assumption of homogeneous flow for oil and water in mechanistic model improved prediction of pressure drop over the range of investigated flow conditions

Study of viscous oil-water-gas slug flow in a horizontal pipe

Slug flow characteristics for viscous oil-water-air are experimentally conducted in a horizontal pipe with a 40 mm ID and 12 m long. Results of experimental pressure gradient are presented, taking into account μo = 0.83 Pa s. Water and air superficial velocities ranged 1.02–2.1 m s−1 and 0.22–1.9 ms−1, respectively. Flow patterns were observed and images captured by using a video camera. New data-sets on such flow that include translational bubble velocity, slug and elongated bubble length, total slug unit length and frequency are experimentally provided. Translational velocity of slug units is measured by means of optical probes and video camera, and compared with modified Nicklin (1962) correlation. Moreover, slug body, elongated bubble and slug unit lengths are measured by optical probe. Statistical analysis was adopted based on measured data of slug length to develop probability density function (PDFs). As a result, a new expression to compute slug unit length in terms of liquid, gas superficial velocities, and pipe diameter was proposed