Mechanical-capillary-driven two-phase loop: Feedback control for thin-film evaporation and capillary limit enhancement

Capillary-driven thin-film evaporation in the evaporator was modulated using feedback control of the mechanical pump flow rate and capillary pumping head in a mechanical-capillary-driven two-phase loop. The shift from cold-start (forced-convection dominant condition) to post cold-start (boiling dominant condition) was signified by a sudden drop in the evaporator thermal resistance at relatively low heat inputs. In the post cold-start, positive capillary pressure head resulted in another important transition of boiling conditions in the evaporator transitioning from mechanically-driven flooded boiling (flooded mode) to the capillary-driven thin-film evaporation (capillary mode) showing a small but continuous decrease in the thermal resistance. The feedback control significantly expanded the range of the capillary mode by 285% and reduced the mechanical pumping power consumption by 48% compared to a constant flow case. In this study using a monolayer wick in the evaporator, the highest capillary limit was measured to be 227.2 W/cm(2) with a thermal resistance of 0.190 K-cm(2)/VV for the highest pump flow rate of 7.5 g/s and the highest flow restriction in the liquid return line from the evaporator.