Fig. 9

Overview of transient vapor bubble ebullition data for subcooled cross-flow in a microchannel. a Transient micro-to-nanoscale HTC (h _ω , left axis) and transient macroscale HTC (h_ω→0, right axis) during bubble ebullition cycle in subcooled flow boiling. The dashed-line (- -) is the HTC expected for stagnant subcooled water at 100 °C; b–d Selected images from the recorded video. Time stamp: elapsed time after subcooled flow was introduced in the microchannel. Subcooled flow: left-to-right with \(T_{\mathrm{f}}^\infty \approx\)22 °C, D_h ≈ 480 μm, \(\overline \nu _{\mathrm{flow}}\)≅3.5 m s⁻¹ (t < 6 s) and \(\overline \nu _{\mathrm{flow}}\)≅3.9 m s⁻¹ (t > 6 s). White scale bar=100 μm. Red-border circle overlay: representative size and location of laser-induced hot-spot; b Image acquired before vapor bubble release; c, d Images acquired after nucleation of a new vapor bubble; e Relative size of the vapor bubble during the ebullition cycle with respect to the 1/e² diameter of the hot-spot (D_VB/2\(\overline w\)); f Other key dimensionless thermal transport data acquired during the transient subcooled flow boiling experiment: \(h_\omega {\mathrm{/}}h_0^{{\mathrm{scw}}}\), HTC ratio between micro-to-nanoscale HTC and the stagnant subcooled water HTC at 100 °C; ΔT_w/ΔT_w,ONB, wall superheat ratio between that measured (over 2\(\overline w\)) and that predicted at incipience;⁴⁶ and \(q''_{\mathrm{f}} {\mathrm{/}}q''_{\mathrm{net}}\), ratio of the hot-spot heat flux into the coolant (wall-to-fluid) relative to total hot-spot heat flux. Laser heating power: \(\widetilde P\) ≈ 7.5 mW (corresponding to \(q''_{{\mathrm{net}}} \approx\)2.9 kW cm⁻² with 2\(\overline w\) ≈ 19 μm, ΔT_w,ONB ≈ 43 °C, and \(h_0^{{\mathrm{scw}}} \approx\)4100 kW m⁻² K⁻¹)