Search

This paper describes a method for computing: (1) the chemical composition of the first drop of condensate which forms at dew-point temperature through expansion of superheated steam, and (2) the saturation index of the drop with respect to relevant solid phases, such as halite, amorphous silica, boric acid, borax and sal ammoniac. Boiling-point elevation is taken into account in these calculations. Preliminary application to some wells in the Larderello geothermal field indicate that: (1) the high concentration of HCl in the steam causes both the low pH and very high TDS of the first drop; (2) the lower the dew-point temperature, the higher the TDS of the first drop; (3) for a given chemical composition, the lower the steam pressure, the higher the risk of corrosion and scaling in the steam path.

Air-water slug flow at atmospheric conditions in horizontal pipes of 31 and 53 mm i.d. has been characterized experimentally with local (optical) and cross-sectional (conductance) probes. The objective of the work is to improve the description of slug flow and the closure relations required in mean kinematic slug flow models. Local measurements include the radial void fraction distribution in the slugs, the size of the dispersed bubbles in the slug and the aeration of the liquid layer under the slug bubble. From cross-sectional holdup measurements, slug lengths and frequencies are determined, as well as the length of the highly aerated mixing zone in the front of the slugs. Measured values of film holdup at the tail of the slug bubbles and of slug frequency are compared with a physical model of slug flow derived from the work of Dukler & Hubbard.

Air/water slug flow in 53 and 90 mm i.d. horizontal pipes has been investigated for a large range of gas and liquid velocities. For this purpose a special instrumentation with simultaneous data acquisition and analysis has been developed. This has been used to determine the mean slug characteristics (length, holdup and velocity) and their statistical distributions.

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 aeration of a liquid body advancing over a slow moving liquid layer in a pipe has been experimentally investigated with a conductance method. The 50 mm i.d. pipe section was close to horizontal and the fluids were air and water at atmospheric conditions. It has been found that net gas entrainment only occurs when the relative velocity between the advancing front and the liquid layer is greater than a limiting value. The rate of gas entrainment is proportional to the relative velocity between the advancing slug and the liquid layer, and to the interfacial width of the layer. Measured pressure drops, film heights and gas volume fractions in the advancing slug agree well with available correlations or with similar measurements taken under conditions of fully developed slug flow.

The void fraction in liquid slugs has been determined for air—water fiow in horizontal and near-horizontal pipes by a newly-developed conductance probe technique. A semi-empirical correlation has been developed and compared with the present measurements and available data. This correlation predicts reasonably well the observed effects of diameter, inclination and physical properties.

Measurements of the effect of liquid viscosity on the initiation of roll waves in a horizontal gas-liquid flow are presented. These results are interpreted by an analysis based on the calculation of the growth of long wavelength disturbances.

The point of onset of large disturbance waves and droplet entrapment has been determined for the annular downward flow of air-water mixtures in a 24 mm ID tube. Experimental results are in a good qualitative agreement with the previous work by Webb and Hewitt1. These results indicate that 1the critical gas velocity at the onset of entrainment is lower than the value predicted by available correlations 2the critical gas velocity continuously decreases at increasing the liquid Reynolds number 3below a liquid Reynolds number of 270 no entrainment occurs even at gas velocities as high as 80 m/sec A correlation for the critical gas velocity which is based on a simple physical model of atomization, is also proposed.

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.

Measurements of liquid hold-up and pressure drop are reported for stratified flow in a slightly inclined (0.65° and 2.1°), 5 cm pipeline. Velocity profiles in the gas phase have been determined for a limited number of flow conditions. Semi-empirical correlations are proposed for the transition to slug flow, the interfacial friction factor and the liquid hold-up.

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.

A mechanistic model of slug flow based on a set of mass and momentum conservation equations and a number of empirical closure relations is presented. It is then shown that a simplified version of the model, which physically corresponds to the flow of long liquid slugs (long slug model, LSM), can be immediately derived from the general model. The LSM is much simpler than the general model; at the same time, the predictions obtained in the two cases are very similar, when the comparison is limited to the computation of the pressure gradient and the mean liquid holdup. The LSM successfully correlates a large set of experimental measurements relative to three pipe diameters (18, 50, and 90 mm), four inclinations (0[degree], [+-]3[degree], 0.3[degree]), and two pipe lengths (17 and 34 m).