NUMERICAL-SIMULATION OF CAPILLARY-TUBE EXPANSION DEVICES

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