Studies of gas-liquid flow in bends

de Sá Ribeiro, Albina Maria (1994). Studies of gas-liquid flow in bends. University of Birmingham. Ph.D.

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Abstract

The present investigation is concerned primarily with air-water flow in a horizontal 0.032 m ID tube, and the influence of a 90° horizontal bend on the flow characteristics. Visualisation studies using high speed still photography and cine film, and entrainment and drop size measurements were conducted before and after the bend. Entrained mass fluxes were determined from film flow measurements carried out using the film removal technique, while the drop size distributions were measured with a laser diffraction technique. During these measurements the pressure in the test section was held between 1.0-1.4 bar, and at ambient temperature.

Prior to the horizontal flow study, drop size and film flow rates were measured for vertical air-water flow in a 0.01026 m ID tube. This extended the work of Jepson (1992) who reported the effect of gas density and surface tension on film flow rate, drop size and deposition mass transfer coefficient. Modifications to the equipment described by Jepson (1992) allowed an extension of the measurements to higher flow conditions.

The visualisation study was taken across flow conditions that include stratified, annular and pseudo-slug flow regimes. Still photographs show the presence of air bubbles entrained in the liquid film, and the creation of liquid drops at the crest of roll waves. Drops were seen to be entrained from the liquid film by both bag break-up and ligament break-up mechanisms. At the bend, the phenomenon of film inversion was seen to occur. Also, a secondary flow existing in the gas phase at the bend can be responsible for a swirl movement observed in the liquid film, in which at the upper part of the tube the liquid was pulled from the outer wall of the bend to the top of the tube in an anti-clockwise, cork screwing fashion. In the lower half, the liquid film was drawn from the outer wall towards the bottom of the tube in a clockwise motion. From the cine films, information on drop velocity was also extracted. This showed the axial drop velocity to be constant over the time frame of analysis. No significant correlation was found between the drop size and the axial drop velocity.

The entrainment results in the horizontal tube showed that for stratified/annular conditions the entrained liquid mass flux increases with liquid flow rate (for a fixed gas flow rate), and in some instances plateau conditions were reached. However, for the pseudo-slug regime the level of entrainment falls considerably. For the whole range of flow conditions studied, the entrained liquid mass flux increases with superficial gas velocity, except for GL = 10 kg/m 2 s where the amount of entrainment is constant. The reduction in entrained liquid mass flux after the bend above certain flow conditions, is caused by drops depositing on the outside wall of the bend.

For the flow conditions under study, the Sauter mean diameter varies between 60-110 um. Gas velocity has a strong influence on drop size, ie, the higher the gas velocity the smaller the drop size. The effect of liquid flow rate is somewhat more complex. At the lower liquid flow rates, drop size seems to be controlled by the entrainment mechanisms, while at the higher liquid flows drop coalescence has a dominant effect. The influence of the 90° bend on the drop size distribution was to increase the diameter of the drops. Both the entrained liquid mass flux and drop size were found to be lower for horizontal annular flow than in vertical flow, for the same flow conditions and tube diameter.

The measurements carried out for air-water flow in the vertical 0.01026 m ID tube, showed the entrained mass flux to increase with both gas and liquid flow rates. For the flow conditions analysed, the Sauter mean diameter varies between 26-45 um. Drop size was seen to be influenced by gas and liquid flow rates, following similar trends to those observed during the horizontal study.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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