Pool boiling heat transfer enhancement over cylindrical tubes with water at atmospheric pressure, Part II: Experimental results and bubble dynamics for circumferential V-groove and axial rectangular open microchannels

A two-part experimental study is conducted on pool boiling heat transfer over enhanced cylindrical microchannel test surfaces with water at atmospheric pressure. In this Part II of the study, the effects of circumferential V-groove microchannels and axial rectangular microchannels are reported. These experiments were performed in the horizontal as well as vertical orientations. The heat transfer performances of the modified surfaces are compared with that of a plain surface. At a heat flux of 1070 kW/m2 a maximum heat transfer coefficient of 96 kW/m2 K was achieved with an axial rectangular microchannel test section in the vertical orientation. Videos captured using a high speed camera were analyzed and the boiling heat transfer mechanisms seen at low and medium heat flux conditions were discussed. The enhancement in the heat transfer performance, and the improvement of the critical heat flux observed with these modified test sections have been attributed to the liquid rewetting phenomenon of the heated surfaces through the microchannels.