Viscous fingering and heat transfer during boiling in a Hele–Shaw cell

• Viscous fingering during boiling with water in a Hele–Shaw cell was generated.
• Explosive evaporation process with high interface velocities up to 15 m/s.
• An optimum heat transfer occurred for special values of the spacing.
• The measured high values were attributed to substantial micro-layer evaporation.

Nucleate boiling in a Hele–Shaw cell was experimentally investigated considering the effects of plate gap, heat flux level, and heating area size. The fluid was pure and de-mineralized water at atmospheric pressure. The highly confined bubble dynamics were visualized by means of a high speed camera mounted perpendicular to the surface. Viscous fingering was observed in case of special combinations of plate spacing, heat flux, and sub cooling. That viscous fingering led to a small-scale roughening of the fast evaporating front. The interface velocity and the gap size were identified as major parameters for the onset of that instability, and the observed dynamics was in a reasonable agreement with theoretical considerations. The boiling heat transfer coefficients were determined, too. It was found that the spacing was of major importance for the heat transfer level that was rather high for special combinations of heat flux level and plate spacing. This behavior was explained as result of the micro-layer evaporation and the refilling of the Hele–Shaw cell after a computer-based image processing of the high speed cinematography.