Expansion device, Capillary tube, Meso-cooling
Nowadays there is a growing interest for compact cooling systems, the so-called meso-refrigerating systems, and thus there is also a need for innovative components which could be applied in these systems. The expansion device of meso-refrigerating systems must be properly sized not only to improve the system performance but also to work properly under varying load conditions. The aim of this work is to study the expansion process of HFC-134a through two distinct expansion devices in order to get a better understanding of the fluid flow phenomena and to support the development of mathematical models. To this end an experimental apparatus was designed and constructed in order to reproduce and control typical operating conditions of meso-cooling systems. The experiments were designed according to the factorial design technique. The first part of this work focused on small diameter adiabatic capillary tubes, when it was found that the capillary tube I.D. has the greatest impact on the refrigerant mass flow rate. It was also found that the developed model predicted 90% of the experimental data points within a band of error of Â± 10%. Further experiments were carried out to study the effect of the ingestion of vapor bubbles on the mass flow rate. The second part of this work focused on pulsating capillary tubes â€“ series association of a PWM-controlled metering valve, an intermediate chamber and an adiabatic capillary tube â€“ when it was found that the refrigerant mass flow rate is greatly affected by the valve duty cycle. The fluid flow phenomenon for this particular expansion device was not well captured by the model which provided mass flow rate deviations of Â±30 % and overestimated the pressure in the intermediate chamber.