FLOW PATTERNS DURING FLOW BOILING AND CONVECTIVE CONDENSATION OF R1234ze(E) INSIDE A MICROFIN TUBE
Due to increasing environmental concerns, the phase-out of refrigerants commonly used in HVAC devices has significantly progressed in the last decade. Hence, it became imperative to find suitable substitutes for tomorrow's HVAC systems. With the announced ban of the widely used R134a, the industrial interest shifted toward new environmentally-friendly replacements, among which the hydro-fluoro-olefin (HFO) R1234ze(E), characterized
by a smaller global warming potential and a much shorter atmospheric lifetime than R134a, is particularly promising.
In this regard, the article reports experimental investigations on flow patterns during flow boiling and convective condensation inside a microfin tube (9.52 mm o.d. and 60 triangular fins), with the ultimate objective of comparing
the performances of these two refrigerants.
Experiments were performed to test a wide range of operating conditions and to build the flow pattern maps for the microfin tube using both refrigerants. The data were overlapped to some of the flow pattern maps developed for the smooth tube to highlight the main differences.
The study of the flow visualization led to the following conclusions: the swirled microfins promote the annular flow, which onsets at a lower mean quality and mass fluxes in both evaporation and condensation. Moreover, during flow boiling, the dryout is shifted towards larger mean qualities than the ones observed for the smooth tube. Eventually, R1234ze(E) behaves very similarly to R134a.