An experimental study of flow boiling from downward-facing heated wall in inclined channels

Abstract

On downward-facing heated wall in an inclined channel the bubble nucleation and departure behavior are affected by large bubbles near the heated wall as the vapor bubbles tend to slide along the upper heated wall. In order to investigate the boiling behavior in this geometry, an experiment was carried out with 1.2 m long rectangular channel, 0.06 m × 0.1 m of channel cross section. The channel was inclined by 10°, 20°, or 30° from the horizontal plane. The size of the heated wall was 0.06 m wide, 0.75 m long. The heat flux on the heated wall was provided by the Joule heating of the thin stainless steel plate bonded to the Teflon wall by DC electric current. The tests were conducted with near-saturated water at atmospheric pressure. The heat flux was varied in the range of 40–240 kW/m2 and the mass flux was in the range of 100–300 kg/m2 s. The temperatures of the heated surface were measured at five points and were used for calculating heat transfer coefficients. High speed video images showed that the bubbles were sliding along the heated wall, continuing to grow and combining with the bubbles growing at the nucleation sites in the downstream. These large bubbles continued to slide along the heated wall and formed elongated slug bubbles. The measured flow boiling heat transfer coefficients for the range of the test parameters were 5000–12,000 W/m2 K. By performing video image analyses, the length, velocity and frequency of slug bubbles sliding on the upper heated wall were measured as a function of wall heat flux, mass flux, and the inclination angle. These sliding bubble parameters were reduced to so-called Reduction Factor and an empirical correlation of the reduction factor is proposed. These sliding slug bubble parameters can be used to model wall boiling on inclined, downward-facing heated wall.