B.J. Azzopardi

Drops in annular two-phase flow

Abstract Drops, one of the forms in which liquid is present in annular gas-liquid flow, are formed from the wall film, carried by the gas or vapour and redeposited. During this time they exert a strong influence on many important parameters of both flow and heat transfer. The available information on the creation, size and velocity, and removal of drops is identified and reviewed. This review shows that there is an extensive literature on drops and the associated topic of waves in annular gas-liquid flows. In spite of the large number of papers that have been published, there are still some fundamental questions which remain unanswered and there are large gaps in the parameter ranges to be considered.

Drop size distributions in dispersed liquid–liquid pipe flow

Abstract This paper examines drop size distributions in a 0.063 m pipe for a two-phase mixture of kerosene and aqueous potassium carbonate solution. Measurements have been made for both vertical upflow and horizontal geometries, for mixture velocities ranging from 0.8 to 3.1 m/s. Two optical measurement techniques, a backscatter technique using a Par-Tec 300C and a diffraction technique using a Malvern 2600, have been used to obtain the drop size distributions of the dispersions created. Both measurement techniques have been found to be limited to different concentration ranges. Stratification of drop size was observed for low mixture velocities in a horizontal geometry. This did not occur for the vertical geometry. The drop distributions obtained were found to fit an upper limit log-normal distribution (ULLN). The theory of Hinze has been found to agree well with experimentally determined values of maximum drop diameter at low dispersed phase concentrations. At high concentrations, neither Hinze theory, nor a modified version proposed previously, adequately describe the data obtained.

A physically based correlation for drop size in annular flow

Abstract The derivation of a correlation for drop size in annular flow based on a mechanistic model is presented. Optimum values of four constants were obtained by a fit to measured data over a wide range of gas and liquid flow rates, physical properties and pipe diameter.

Plant Application of a T-Junction as a Partial Phase Separator

It is considered good practice for a distillation column with a two-phase (vapour/liquid) feed to separate the phases and feed them into the column at different points. The phase separation is normally effected in a cylindrical vessel. The cost of design, construction and installation of such a vessel is not trivial. This paper presents an alternative, more economical approach to tackle the task. It utilizes the maldistribution of the phases that occur when a gas/liquid stream is divided at a T-junction. The design approach adopted to specify the geometrical parameters of the junction and surrounding pipework is described. Post installation tests, which were carried on the equipment to verify its correct operation, are also reported. The plant management are delighted with the operation of the equipment.

The split of annular two-phase flow at a small diameter T-junction

Abstract This paper extends the range of diameters (0.009–0.127 m) for which information on the maldistribution of the phases at a T-junction is available. Data are presented for a smaller T-junction whose diameter is 0.005 m in all branches. The flow rates studied result in annular flow approaching the junction. Gas superficial velocities of 46–60 m/s and liquid superficial velocities of 15–20 m/s were studied. When the present data are compared to those from larger diameter junctions for similar superficial inlet velocities, it is seen that decreasing the pipe diameter increases the fraction of liquid taken off. It is suggested that this trend is due to the lower entrained fraction and more uniform film around of the pipe circumference for the smaller pipes. The measured split data were compared with the predictions of published models.

The split of horizontal annular flow at a T-junction

Abstract A model is presented to predict the flow split of horizontal air-water annular flow at a T-junction with a side arm at an arbitrary inclination to the horizontal. The distribution of the film thickness and film flow rate around the circumference of the inlet pipe is predicted and the results implemented in a model to determine the flow split at a T-junction. Under certain conditions, an abrupt increase in the amount of liquid extracted into the side arm is observed due to liquid coming to rest in the main pipe. A method is presented to include this phenomenon taking into account the distribution of film flow rate. Predictions are shown to be in good agreement with available flow split data taken at normal and reduced T-junctions with main pipe diameters of 0.032 and 0.038 m with horizontal, vertically upward and vertically downward side arms.

Drop size measurements in Venturi scrubbers

Venturi scrubbers are high efficiency gas cleaners in which suspended particles are removed from gas streams by drops formed by liquid atomisation, usually in the Venturi throat. The size of the drops formed are of fundamental importance to the performance of the equipment, both in terms of pressure drop and dust removal efficiency. In this study, drop sizes in a cylindrical laboratory-scale Venturi scrubber were measured using a laser diffraction technique. Gas velocity and liquid to gas ratios varied from 50 to 90m/s and 0.5 to 2.0l/m3, respectively. Water was injected using two different arrangements: either as jets in the throat or as a flim just upstream of the convergence. Drop size measurements were performed at three positions in the case of jet injection: two located along the throat, and the last one at the end of the diffuser. The present data shows that the Sauter mean diameter of the spray can be well correlated by the equation of Boll et al. (J. Air Pollut. Control Assoc. 24 (1974) 932). Drop size distributions are satisfactorily represented by a Rosin–Rammler function. This paper also provides a simple method for calculating the parameters of the Rosin–Rammler function. As a result of this work, drop sizes in Venturi scrubbers can be estimated with much higher accuracy.

Comparison between wire mesh sensor and gamma densitometry void measurements in two-phase flows

Wire mesh sensors (WMS) are fast imaging instruments that are used for gas–liquid and liquid–liquid two-phase flow measurements and experimental investigations. Experimental tests were conducted at Helmholtz-Zentrum Dresden-Rossendorf to test both the capacitance and conductance WMS against a gamma densitometer (GD). A small gas–liquid test facility was utilized. This consisted of a vertical round pipe approximately 1 m in length, and 50 mm internal diameter. A 16 × 16 WMS was used with high spatial and temporal resolutions. Air–deionized water was the two-phase mixture. The gas superficial velocity was varied between 0.05 m s−1 and 1.4 m s−1 at two liquid velocities of 0.2 and 0.7 m s−1. The GD consisted of a collimated source and a collimated detector. The GD was placed on a moving platform close to the plane of wires of the sensor, in order to align it accurately using a counter mechanism, with each of the wires of the WMS, and the platform could scan the full section of the pipe. The WMS was operated as a conductivity WMS for a half-plane with eight wires and as a capacitance WMS for the other half. For the cross-sectional void (time and space averaged), along each wire, there was good agreement between WMS and the GD chordal void fraction near the centre of the pipe.

Effects of initial bubble size on flow pattern transition in a 28.9 mm diameter column

Abstract The experimental results described in this paper were carried out in a 28.9 mm diameter column at a constant water velocity of 0.356 m/s for four different bubble sizes. The void fraction waves were measured with impedance void fraction meters. It has been found that the initial bubble size has strong effects on the flow pattern transition and the instabilities of void fraction waves. The critical void fraction at which the flow pattern transition happens decreases with increasing bubble size. At a constant liquid velocity with increasing gas flow rate, the point at which the system gain factor becomes larger than 1, and the point where the wave velocity gradient first becomes negative, also decrease with increasing bubble size. This study has confirmed that the instability of the void fraction wave is not the factor that causes the bubble-to-slug flow pattern transition in gas–liquid verticle flow.

Detailed Measurements of Vertical Annular Two-Phase Flow—Part I: Drop Velocities and Sizes

Phase anemometry and laser diffraction techniques have been employed to measure drop sizes in annular two-phase flow. The former technique also provides drop velocities. When converted to the same basis, the drop size distributions from the two techniques are in agreement. Drop velocities were 20 percent below the corresponding local velocities for the gas. Standard deviations of the drop velocities were 10 to 65 percent higher than those for the gas.