Capillary tube, non-adiabatic, numerical model
In this work a numerical model to simulate the thermal and fluid-dynamic phenomena inside non-adiabatic capillary tubes is presented. The model presented herein is an improved version of the distributed model detailed in . It is based on a pseudo-homogeneous two-phase flow model where the governing equations (continuity, momentum, energy and entropy) are integrated over the discretized fluid domain and solved by means of a step-by-step scheme. The main novelty of the improved algorithm is its enhanced capability to address many of the convergence issues typically found in distributed models as stated by  and . In addition, the newest version of the model allows the simulation of both concentric and lateral configurations whereas only the concentric configuration was available in the previous version. The initial section of this work is focused on explaining the new resolution procedures and features included in the enhanced algorithm that allow a better convergence performance. The predictions of the new model are compared against simulations carried out with other distributed models proposed in the open literature. The comparison shows that the new model succeeds (convergence is attained) in cases where high convergence difficulties have been reported. The subsequent section is devoted to the implementation and validation of the lateral configuration. On one hand, both the resolution approach and the hypotheses considered are briefly described, and on the other, the model predictions are compared against experimental data found in the open literature where good agreement is observed. And finally, the last section presents parametric studies on capillary tubes used in household refrigerators working with isobutane. The influence of the heat exchanger length and its relative position over the whole capillary tube are analysed for both configurations: concentric and parallel. In addition, the influence of the capillary tube inlet condition at a constant pressure (subcooling degree for single-phase flow and vapour quality for two-phase flow) is analysed. REFERENCES  N. Ablanque, J. Rigola, C.D. Pérez-Segarra, A. Oliva, “Numerical simulation of capillary tubes. Application to domestic refrigeration with isobutane”, International Refrigeration and Air Conditioning Conference at Purdue, 2377, Purdue, IN, USA, 2010.  C.J.L. Hermes, C. Melo, J.M. Gonçalves, “Modeling of non-adiabatic capillary tube flows: A simplified approach and comprehensive experimental validation”, International Journal of Refrigeration, vol. 37, pp. 1358-1367, 2008.  P.K. Bansal, C. Yang, “Reverse heat transfer and re-condensation phenomena in non-adiabatic capillary tubes”, applied Thermal Engineering, vol. 25, pp. 3187-3702, 2005.