Assessment on R-410A saturated flow boiling heat transfer subject to periodic oscillating heat flux inside an annular duct

The objective of this study is concerned with assessing experimentally the heat transfer of time-dependent saturated flow boiling (TDSFB) for refrigerant R-410 A stemming from oscillations in the heat flux inside an annular duct with the gap of delta = 2 mm in order to improve the energy efficiency of heat exchanger system. The influences of operating parameters, such as mean applied heat flux q, heat flux oscillation amplitude Delta q/q, mass flux of refrigerant G, oscillation period tp, and saturation temperature Tsat, on heat transfer characteristics of TDSFB are systematically explored. From the present experimental results, it is found that increasing mean applied heat flux enhances the average wall superheat and boiling heat transfer coefficient (BHTC, hr); while an increase in either Delta q/q or tp exerts very minor influence on the boiling curve and hr. As the q oscillates in the triangular waveform, the wall temperature (Tw) varies in sinusoidal waves at the same frequency of q. At low, intermediate, and high q, three different flow patterns versus time are observed and named single-phase flow (SPF), intermittent boiling flow (IBF), and persistent boiling flow (PBF). The oscillation amplitudes of Tw and hr are increased with the increases of Delta q/q. Enhancing either G or Tsat tends to result in a slight decrease in Tw, but an increase in hr. Moreover, the Tw does not fluctuate with the oscillation of lower q under a shorter tp due to the tube thermal inertia, but oscillates significantly as the tp is raised. Finally, a correlation equation is presented to identify the bounds, distinguishing the three different flow patterns of R-410 A TDSFB subject to oscillatory heat flux inside horizontal annuli.