Oscillate boiling from microheaters

Abstract

We report about an intriguing boiling regime occurring for small heaters embedded on the boundary in subcooled water. The microheater is realized by focusing a continuous wave laser beam to about 10 µm in diameter onto a 165 nm-thick layer of gold, which is submerged in water. After an initial vaporous explosion a single bubble oscillates continuously and repeatably at several 100 kHz. The microbubble's oscillations are accompanied with bubble pinch-off leading to a stream of gaseous bubbles into the subcooled water. The self-driven bubble oscillation is explained with a thermally kicked oscillator caused by the non-spherical collapses and by surface pinning. Additionally, Marangoni stresses induce a recirculating streaming flow which transports cold liquid towards the microheater reducing diffusion of heat along the substrate and therefore stabilizing the phenomenon to many million cycles. We speculate that this oscillate boiling regime may allow to overcome the heat transfer thresholds observed during the nucleate boiling crisis and offers a new pathway for heat transfer under microgravity conditions.Introduction.-The thermal energy transfer from a heater into a liquid is greatly increased once the boiling temperature is reached and vapor bubbles are formed. Then the previous convective driven flow becomes advected by the growing and detaching bubbles rising into the bulk under the action of gravity. By increasing the temperature of the heater further more bubbles are nucleated until a continuous vapor layer is formed [1]. In this so-called film boiling regime heat transfer is strongly reduced; this regime limits the design and efficiency of common heaters. To overcome this boiling crisis, research is focused on the enhancement of heat transfer while avoiding the transition to film boiling. Current promising approaches are the texturing of the heater surface [2] or the use of thin electrically heated wires [3]. Here we report a nucleate boiling regime on a flat substrate where the vapor bubble does not detach from the surface yet transports heat through a flow caused by bubble oscillations and thermocapillary stresses. This letter starts with a description of this unexpected oscillate boiling regime and relates the phenomenon with reports from literature having some similarity before we provide two simple models. The first model, based on a set of ordinary differential equations (ODEs), explains the orgin of the bubble oscillation and the dependency of the oscillation frequency on the heater power. The second model describes the thermal gradients and the resulting thermocapillary flow using a Navier Stokes solver which is compared to the experimental measurements. This novel oscillate boiling regime may allow to overcome the film boiling regime through an auto-oscillatory flow from a constant heat input.

Single Oscillating Bubble from Radiofrequency Fiber Electrode

“…2b) then rapidly collapse during the next 28 μs before the next oscillation cycle starts. Small gas bubbles are extruded from the oscillating cavitation bubble to compensate rectified diffusion, as reported previously (4). Similar bubble dynamics are also observed for the driven frequency of 0.3 and 5 MHz ( Fig.…”

In-phase synchronization between two auto-oscillating bubbles

“…In contrast, the unstable mode is driven by a more gentle bubble's inertia and can be affected by the acoustic emission. To support this hypothesis, we extend our model [23] with the interaction term p i . For the unknowns R i and temperature T i with i = 1, 2 numbering the bubble, we solve the coupled Keller-Miksis equation [27] and the energy equation for the gas or vapor inside the bubbles.…”

“…where R is the bubble radius, d the distance, and ρ the density of the liquid. In stable oscillate boiling the liquid jet impact prematurely disrupt the bubble's natural collapse, causing rapid vaporization and rapid reexpansion of the bubble driven by the expanding vapor [22,23]. The pressure interaction from the neighboring bubble is therefore not sufficient to overcome this violent mechanism.…”

“…Oscillate boiling shows improved heat transfer efficiency compared to nucleate boiling [22,23]. Towards application, this phenomenon needs to be scaled up or numbered up to multiple oscillateboiling bubbles covering the whole heating floor.…”

“…Once a bubble is nucleated on the surface of the submerged microheater, it exhibits volume oscillations. We have recently demonstrated an electric version of the microheater [22], which improves the control over size and input energy as compared to an earlier laser-based heater [23]. The cyclic vaporization of liquid for a brief time interval (approximately 30 ns) at minimum bubble volume supplies the necessary heat input to overcome the energy dissipation of the system caused mainly by viscosity and acoustic radiation.…”