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A test section consisting of a circumferential array of conductance probes has been developed to measure the thickness distribution around the pipe wall of a liquid layer flowing in near horizontal pipes. When the film thickness is known, the array can be employed to measure the local film flow rate by injecting a high conductivity tracer into the liquid flowing at pipe wall. The test section consists of a short pipe made of a non-conducting material installed in a flow rig designed to operate at an appreciable pressure (40 bar). The flow loop is made of metallic pipes connected to the electrical earth. The conductance probes are made of three parallel, rigid wires spaced along the flow direction and are used to measure the height or the electrical conductivity of the liquid layer. The three-electrode geometry is aimed at minimizing current losses toward earth. The simultaneous operation of all the probes of the array, without multiplexing, allows a substantial reduction of current dispersion and a good circumferential resolution of film thickness or conductivity measurements. The probe geometry may generate an appreciable disturbance to the gas–liquid interface. This aspect of the proposed method has been studied with an experimental and numerical investigation relative to free falling liquid layers.

There is a high industrial interest today in the development of accurate measurement techniques to support the modelling of gas-liquid flow phenomena. When dealing with stratified flow in horizontal or inclined pipes, the main problem is that in the range of medium to large gas velocities there are very few data available for the development or validation of flow models. These data should include the measurement of the pressure gradient, the circumferential liquid film distribution, the liquid hold-up and the fraction of entrained droplets. This work describes the development and experimentation of a measurement set-up for generating these data at flow conditions as close as possible to those of industrial interest, in terms of pipe diameter and physical properties of the fluid. This paper focuses on the measurement technique, which is based on conductivity measurements by arrays of needle-shaped electrodes, and illustrates one practical implementation and its validation.

This paper shows the experimental results that have been performed to identify transitions from Slug flow in horizontal pipes. The flow patterns were identified by the analysis of local and cross section averaged signals of liquid hold-up collected by conductivity probes. New mechanisms were suggested to identify the boundaries of the Slug flow regimes. A new approach to identify the range of existence of Slug flow in long pipelines is also suggested.

In stratified gas–liquid horizontal pipe flow, growing long wavelength waves may reach the top of the pipe and form a slug flow, or evolve into roll-waves. At certain flow conditions, slugs may grow to become extremely long, e.g. 500 pipe diameter. The existence of long slugs may cause operational upsets and a reduction in the flow efficiency. Therefore, predicting the flow conditions at which the long slugs appear contributes to a better design and management of the flow to maximize the flow efficiency. In this paper, we introduce a wave transition model from stratified flow to slug flow or roll-wave regimes. The model tracks the wave crest along the pipe. If the crest overtakes the downstream wave end before hitting the top of the pipe, a roll-wave is formed, otherwise a slug. For model validation we performed measurements in air–water horizontal pipe flow facilities with internal diameters of 0.052 and 0.06 m. Furthermore, we made numerical calculations using a transient one-dimensional multiphase flow simulator (MAST) which adopts a four-field model. The model presented in this paper successfully predicts the evolution of waves and their transition into either slugs or roll-waves. It also predicts the formation time of slugs and roll-waves with a satisfactory agreement.

A set of experiments has been performed to analyse gas-phase distribution in horizontal bubble flow. The experimental data have been obtained by means of conductance probes and include measurements of the local void fraction and the bubble diameter and velocity. Present observations show large variations of the local void fraction and the bubble size moving from the top to the bottom of the pipe, with local maxima located close to the upper pipe wall. Measurements also show that, at the boundary of the dispersed bubble flow regime, the bubble size, and therefore the surface available for heat and mass transfer, changes abruptly for small variations of the liquid or gas flowrate. This change allows the transition between the dispersed and the elongated bubble regimes to be identified with good precision. Present data make it possible to develop a new correlation for the maximum bubble size in dispersed bubble flow.

This study investigates the use of low-temperature catalytic oxidation for the removal of H 2 S from tail gases originating from geothermal plants, with special focus on the effect of water on the overall performance of the activated carbon catalyst. It is shown that water strongly influences the reaction rate and the total amount of sulfur that can be adsorbed on the catalyst prior to regeneration. It is suggested that the reaction takes place in a thin water layer, inside the carbon pores, from the reaction of dissolved H 2 S with chemisorbed oxygen.

In vertical annular flow, the motion of droplets in the gas core is dominated either by a diffusion mechanism, when the droplet size is small, or by inertial effects when droplets are large. These two mechanisms have to be considered when predicting deposition rates. Furthermore, since droplet-droplet interactions influence droplet motion, a deposition model should also account for collisions and coalescence among droplets. After reviewing the available deposition models, the effect of coalescence on droplet motion is theoretically analyzed. The results demonstrate that coalescence extends droplets residence time in the gas core thus decreasing the deposition coefficient. On the basis of these results, a new deposition model which accounts for the two deposition mechanisms and includes the effect of coalescence is proposed and compared against existing experimental data.

The dispersion of particles in turbulent duct flow under the influence of electrostatic fields is studied using direct numerical simulation. In this new approach, particles are moved in the temporally and spatially varying turbulent flow field under the influence of electrostatic and gravitational body forces, as well as fluid dynamic drag. The simulations agree well with previously performed experiments (done in geometries typical of wire-plate and plate-plate electrostatic precipitators) not only in the overall collection efficiency of particles, but in particle concentration profiles at various axial locations in the flow direction. This gives confidence in the technique that may be used to study different precipitator geometries and flow field configurations, supplementing costly and difficult experiments. Furthermore, information is obtained at a much more detailed level than is possible via experiments, allowing insights into the mechanisms dominating particle collection.

A theoretical model has been developed to simulate the reacting boundary layer flow over a horizontal fuel surface. This model is based on the assumption that molecular diffusivities are equal for all species and on an approximate formulation of the heat balance equation. Fuel oxidation has been assumed to be infinitely fast.

Resultats de l'analyse de films cinematographiques obtenus avec la methode de la lumiere parallele de Hewitt et Whalley. Correlation des resultats par une equation simple qui peut etre reliee a l'analyse de la rupture d'onde de Taylor

The tracer method, adopted by a number of experiments for the measurement of the rate of liquid interchange between the wall layer and the dispersed phase in a fully developed annular flow, has been critically examined on the basis of a more accurate description of the flow behavior of entrained droplets. The results obtained indicate that this experimental technique can give reliable results in a wide range of flow conditions when the experiments are properly designed. Present measurements of the interchange show that the dimensionless deposition coefficient is a function of the concentration of entrained droplets. This effect appears to be due to the dependence on concentration of droplet size. The rate of entrainment is proportional to the liquid film flowrate and to the gas velocity squared.

The structure of the diffusion flame formed on a liquid fuel surface with a parallel oxidizer flow was studied both analytically and experimentally. Only the case of ethyl alcohol burning in air has been considered in this paper. The temperature and species concentration profiles across the boundary layer were determined by means of fine, carefully calibrated thermocouples and of gas chromatography. The gas analysis yielded the concentrations for the following species: N 2 , C 2 H 5 OH, O 2 , CO 2 , H 2 O, CO, CH 4 , C 2 H 2 +C 2 H 4 , C 2 H 6 , CH 3 CHO, H 2 . The mathematical model is based on the Shvab-Zeldovic formulation of conservation equations with physical properties considered as functions of temperature and species concentration. The agreement obtained between the experimental and theoretical profiles of temperature and of normalized elemental mass fractions of the atomic species N, O, C, H is satisfactory when the fuel diffusivity in nitrogen replaces the actual species and thermal energy diffusivities in the conservation equations.