Distinguishing evaporation-like and boiling-like modes of pseudo-boiling in supercritical pressures

Abstract

In this study, we identify whether evaporation-like (EL) mode and boiling-like (BL) mode occur under supercritical pressures by investigating the heat transfer of platinum (Pt) and nickel-chromium (NiCr) wires, with a length of 22.0 mm and diameter of 70 μm, in carbon dioxide liquid at 8 and 10 MPa. Using a direct photographic technique based on the light refractive principle, we analyse the brighter/darker pattern in a varied-density field. The vapor–liquid interface is quantified using the grey extraction method. Our results show that beyond the limit of natural convection (NC), only the EL mode is identified for the Pt wire. However, for the NiCr wire, the EL mode, the transition from EL to BL (TEB) mode, and the BL mode are observed. The opposing gravity and drag forces cause downward bending of the Pt wire and upward bending of the NiCr wire. In the TEB region, the varying importance of interfacial expansion velocity over the buoyancy velocity accounts for the piston motion of the NiCr wire. During falling of the wire, the lighter fluid penetrating the heavier fluid causes interfacial instability to trigger the intermittent BL heat transfer. Beyond the TEB region, continuous BL heat transfer is observed. We introduce the small-perturbation theory to achieve a new non-dimensional parameter, H3. For the Pt wire, a smaller (either negative or positive) H3 explains why only the EL mode appears. For the NiCr wire, the negative H3 in the TEB region proves that the intermittent BL heat transfer is caused by the piston motion of the wire, but the positive and ∼30 times larger H3 indicates that the continuous BL heat transfer is caused by the temporal-spatial temperature variations. Furthermore, we observe that the BL mode occurs in a vapor-like layer instead of wall cavities, which is different from subcritical boiling. The BL mode displays random and disordered features, like those of subcritical boiling. This work elaborately explores common and uncommon features between supercritical pseudo-boiling and subcritical boiling, and uncovers the secret of two modes of pseudo-boiling.