An air-water experimental system consisting of two inlets and one outlet is constructed and characterised. it reaches the state of sliding mode, or equivalently, two phase slug flow. The linear hydraulic model proposed in the literature is adequate to describe it. Experimental data are used to tune this model. The resistance to the flow of air through the outlet valve during the two phase flow is much larger than that when air alone flows out. At the operating range, the resistance to water flow is not affected by the presence of air. (C) 2003 Elsevier Ltd. All rights reserved.