Boiling with ultralow superheat using confined liquid film

Abstract

Abstract High-performance thermal management devices require both high heat flux and high heat transfer coefficient (HTC). Boiling heat transfer provides an effective way toward these goals by utilizing the large latent heat of vaporization. However, HTC of typical pool boiling is hindered by thick liquid layer with relatively slow bubble dynamics. We recently demonstrated a “thin film boiling” mechanism that significantly increases both the critical heat flux and HTC of boiling, when the liquid film is thinner than the thermal boundary layer. In this work, we further explore the “thin film boiling” regime by controllably confining the liquid water film within a narrow gap between the heater wall and a hydrophobic vapor-permeable membrane. We systematically vary the gap thickness from 1.7 mm to 190 μm and demonstrate an obvious reduction in the liquid-vapor phase change resistance by reducing the liquid layer thickness. With smaller gaps, we find a decreasing trend in the effective superheat of the thin film boiling, reaching as low as 3.5±0.3 K at heat flux of 133.8±7.7 W cm-2, where the effective superheat is the difference between the heater wall temperature and the saturation temperature of the liquid in the gap and represents the liquid-vapor phase change resistance only, excluding the vapor flow resistance from the liquid-vapor interface to the far field. The ultralow effective superheat leads to a high HTC associated with the phase change process of 38.4±1.0 W cm-2 K-1 in the 190 μm gap. We further show that there is a transition from the thin film boiling to evaporation regime when liquid water recedes into the nanopores of the porous heating membrane. The HTC at these transition points can be well captured by the Hertz-Knudsen (H-K) relationship with an accommodation coefficient of ∼0.16, which also demonstrate that the phase change process at these transition points are close to the kinetic limit.