Operational regimes in a closed loop pulsating heat pipe

• The systematic analysis of the flow patterns, their transitions and thermal performance of a CLPHP was carried out.
• The effect of the flow dynamics on the thermal performance of a CLPHP was assessed and presented as ‘operational maps’.
• Local heat transfer coefficients were extracted by applying a mechanistic model for the evaporation in the CLPHP microchannels.
• Heat transfer coefficients were used to qualitatively account for the heat transfer characteristics in the CLPHP evaporator U-turn.
• Thin film evaporation is the dominant thermal mechanism, single-phase heat transfer and localized nucleate boiling are of secondary importance.

Synchronized thermal and visual investigation was carried out on a single-turn channel CLPHP using R245fa as the working fluid. The tests were carried out at filling ratios from 10 to 90% and heat inputs from 2 to 60 W for vertical and inclined orientations. A systematic analysis of the flow patterns, their transitions and thermal resistance measurements suggests a strong coupling between the two-phase flow pattern and the system thermal behavior. The effect of the flow dynamics on the system thermal performance was also qualitatively and quantitatively assessed and presented as ‘operational maps’. Local time-averaged heat transfer coefficients were extracted by applying a-state-of-the-art mechanistic model for the evaporation of elongated bubbles in the CLPHP microchannels using the flow measurements. The obtained local and averaged results were then used to qualitatively assess and account for the heat transfer characteristics in the CLPHP evaporator U-turn for the different flow patterns. Based on this analysis, thin film evaporation was found to be the dominant thermal mechanism, while heat transfer into the oscillating liquid slug and localized nucleate boiling were of secondary importance.