Int. J. Therm. Sci. 41 (2002) 17–28 Evolution of the two-phase ﬂow in a vertical tube—decomposition of gas fraction proﬁles according to bubble size classes using wire-mesh sensors Horst-Michael Prasser ∗ , Eckhard Krepper, Dirk Lucas Forschungszentrum Rossendorf e.V., P.O. Box 510119, 01314 Dresden, Germany Received 24 November 2000; accepted 12 February 2001 Abstract The wire-mesh sensor developed by the Forschungszentrum Rossendorf produces sequences of instantaneous gas fraction distributions in a cross section with a time resolution of 1200 frames per second and a spatial resolution of about 2–3 mm. At moderate ﬂow velocities (up to 1–2 m·s -1 ), bubble size distributions can be obtained, since each individual bubble is mapped in several successive distributions. The method was used to study the evolution of the bubble size distribution in a vertical two-phase ﬂow. For this purpose, the sensor was placed downstream of an air injector, the distance between air injection and sensor was varied. The bubble identiﬁcation algorithm allows to select bubbles of a given range of the effective diameter and to calculate partial gas fraction proﬁles for this diameter range. In this way, the different behaviour of small and large bubbles in respect to the action of the lift force was observed in a mixture of small and large bubbles.  2002 Éditions scientiﬁques et médicales Elsevier SAS. All rights reserved. Keywords: Two-phase ﬂow; Gas-liquid ﬂow; Bubble size measurement; Gas fraction measurement; Flow pattern; Wire-mesh sensors 1. Introduction Forces acting on bubbles in a gas-liquid ﬂow strongly depend on the bubble diameter. This is well known for the drag force, which was investigated by many authors. In the ﬁeld of the so-called non-drag forces, which act perpendicular to the ﬂow direction, there are still open questions. Recently, it was found, that the lift force changes its sign at some critical bubble diameter [1], e.g., in a vertical upwards pipe ﬂow, small bubbles are moved towards the wall, while bubbles with a diameter greater than the critical migrate towards the centre of the tube. Concerning the other non-drag forces, i.e., the lubrication force and the turbulent dispersion force, there are also still needs to complete the models. The experimental input for the study of these forces comes mainly from the observation of single bubbles. In this paper, we would like to introduce a method that allows to effectively analyse the motion of bubbles depending on their size in a multi-disperse ﬂow. For this purpose, measurements * Correspondence and reprints. E-mail address: prasser@fz-rossendorf.de (H.-M. Prasser). with our wire-mesh sensor [2] in a vertical upwards ﬂow were carried out. In [3] we presented a method to measure bubble size distributions by evaluating wire-mesh sensor data. Now, we will show, that the bubble identiﬁcation algorithm can be used to decompose radial gas fraction proﬁles according to bubble size classes. Measurements taken at different distances from a gas injection show the evolution of the gas fraction proﬁles for different bubble size classes separately. This makes it possible to study the net motion of the bubbles in the direction perpendicular to the ﬂow, and to check theoretical models of the non-drag forces in a realistic multi-disperse ﬂow. 2. Experimental test facility The methods described below will be demonstrated by means of experimental data obtained in a vertical pipe ﬂow. Measurements were performed at the MTLOOP test facility [4], a two-phase ﬂow test loop of the Institute of Safety Research. The loop was operated with an air– water mixture at atmospheric pressure and a temperature of 30 ◦ C. A vertical test section (Fig. 1) with an inner diameter of 51.2 mm was used. Air was injected in the lower 1290-0729/02/$ – see front matter  2002 Éditions scientiﬁques et médicales Elsevier SAS. All rights reserved. PII:S1290-0729(01)01300-X