Refrigerant distribution in horizontal headers with downward minichannel-branching conduits: Experiment, empirical correlation and two-phase flow pattern map

The system energy efficiency will decline with the decrease of evaporator capacity because the use of more branches in evaporators increases maldistribution. This paper presents experimental study of several design options for downward flow to reduce the maldistribution. Two-phase flow distribution in a header type evaporator applied for CO₂ have been experimentally investigated because the refrigeration system based on carbon dioxide (CO₂) as a refrigerant is near ideal. Since the CO₂ system works around critical pressure, an alternative method to predict the flow distribution of CO₂ header type evaporator is inevitable. R134a is used as the alternative working fluid in the present study. Similarity hypothesis between CO₂ and R134a is applied (refer to Wijayanta et al., 2016). R134a headers are tested and the experimental work is applicable to CO₂. The R134a experiments were conducted at saturation temperature of about 21 °C, refrigerant mass flow range between 10 and 30 kg/h which corresponds to about 44 and 130 kg/m² s in the 9 mm i.d. header, and average vapor quality in the test section inlet of about 0.1–0.4. The test section consists of a horizontal header with 3 and 6 vertically downward replaceable branching conduits. The vapor-liquid phase mass flows enter into the branches are measured to propose the configuration of the header that has the minimum maldistribution. Empirical correlation of the vapor-liquid distribution is developed. Estimated CO₂ distribution for the proper header is determined based on R134a experimental data using the similarity hypothesis. The developed flow pattern map for R134a and CO₂ is also proposed.