Numerical simulation of capillary-tube expansion devices

Abstract

A numerical method has been developed to simulate the thermal and fluid-dynamic behaviour of capillary-tube expansion devices. The governing equations of the flow (continuity, momentum and energy), written in one-dimensional and transient form over finite control volumes, have been solved using an implicit step-by-step numerical technique. This formulation requires the use of additional equations for the evaluation of convective heat transfer, shear stress and void fraction. A special treatment of the point of transition between single-phase and two-phase fluid flow has been implemented together with a Newton-Raphson algorithm for the evaluation of the inlet mass flow rate corresponding to the given boundary conditions. The numerical model allows analysis of aspects such as geometry, type of fluid, critical and non-critical flow conditions, boundary conditions (heat transfer, inlet and discharge pressures etc.) and transient aspects. Good agreement between numerical and experimental data from three different authors (Bolstand and Jordan, Whitesel, Mikol) has been obtained. Some illustrative results are presented to indicate the main characteristics of the numerical model.