Flow visualization and evaporation heat transfer of R32/R1234ze(E) in aluminum cross- and straight-grooved tubes.

Flow regime and heat transfer performance have been extensively characterized in various grooved tubes using refrigerants. However, the effect of secondary grooves in cross-grooved tubes on flow regime transition in zeotropic refrigerant mixtures, such as R32/R1234ze(E), is limited. This study experimentally investigates the flow regime, heat transfer, and pressure drop of the zeotropic refrigerant mixture R32/R1234ze(E) in horizontal aluminum tubes with cross and straight grooves. The cross-grooved tube has primary straight grooves and secondary spiral grooves, with a maximum inner diameter of 6.88 mm. The local heat transfer coefficients (HTCs) of R32/R1234ze(E) and R32 were measured for both tube types. Typical flow regimes in both tube types were visualized: stratified, wavy, transition, and annular flows. At lower mass fluxes, the HTCs in the crossgrooved tube exceeded those in the straight-grooved tube by 1.2–3.4 and 1.2–1.7 times for R32 and R32/ R1234ze(E), respectively. This was because the secondary grooves lifted the liquid film to the upper side of the tube. Conversely, at higher mass fluxes, the average HTCs in the cross-grooved tube were lower than those in the straight-grooved tube because the liquid was easily trapped in the cross-grooves. The frictional pressure drops in the cross-grooved tube exceeded those in the straight-grooved tube by up to 40 %, whereas this difference was small at a lower mass flux of approximately 50 kgm^{− 2} s^{− 1} . These findings can potentially improve heat transfer in refrigeration and related applications.