Evaluation of bubble/drop velocity and slip velocity by a single normal hot-film probe placed in a two-phase flow

Abstract

This paper describes a new method of evaluating the bubble/drop velocity using the signal from a hot-film probe placed in either a liquid-gas two-phase flow or a mixture flow of two immiscible liquids. In previous works the dispersed phase velocity was measured by dual optical or resistance probes, while hot-film probes were used to measure the volume fraction and statistical velocity parameters of the continuous phase. This paper describes the extension of the hot-film technique to velocity measurements of both the continuous and dispersed phase with a single hot-film sensor. This technique was found to be beneficial in terms of direct slip velocity measurements. Experimental tests were mainly carried out using kerosene-water flow and additional tests were performed for water-air flows. These were interface tests simulating the arrival of a bubble/drop front at the sensor location, and drop tests in still water and in a large-scale recirculating vertical kerosene-water pipe flow. Three features of the `U'-shaped signal corresponding to the passage of a bubble/drop were investigated: (i) the voltage overshoot at the bubble/drop arrival, (ii) the corresponding maximum (negative) slope and (iii) the `kerosene' voltage value at the bottom of the `U'-shaped signal. This paper presents the applicability/limitations of these methods for bubble/drop velocity evaluations.