Combined effects of liquid wicking and hydrodynamic instability on pool boiling critical heat flux by two-tier copper structures of nanowires and microgrooves

Abstract

Two-tier structures of Copper Nanowires (CuNWs) and microgrooves have been fabricated on copper substrates. Pool boiling heat transfer experiments have been conducted to study the combined effects of wicking by CuNWs and modulated wavelength by microgroove pitches on pool boiling Critical Heat Flux (CHF). First, 70 nm diameter CuNWs with three different heights of 5 µm, 15 µm, and 25 µm have been synthesized on plain copper surfaces via in-house made Anodic Aluminum Oxide (AAO) templates. The wicking effect of CuNWs at different heights on pool boiling CHF has been measured. Second, microgrooves of 262 µm wide and 518 µm deep at three different mesh pitches of 0.5 mm, 1.0 mm, and 4.0 mm have been fabricated to study the modulated wavelength effect on pool boiling CHF. Last, 25 µm height CuNWs have been integrated with various microgroove meshes to create two-tier structures. The highest pool boiling CHFs of each type of structures are achieved on CuNWs at 25 µm height, microgrooves at 0.5 mm mesh pitch, and two-tier structures of CuNWs at 25 µm height and microgrooves at 0.5 mm mesh pitch are 189.3 W/cm2, 236.4 W/cm2, and 246.3 W/cm2, respectively. The CHF enhancements of these structures over that on a plain surface are 68.6%, 110.5%, and 119.3%, respectively. The experimental results have demonstrated that both wicking effect and modulated wavelength effect contribute to pool boiling CHF enhancement on the two-tier structures, and the collective effect depends on the combination between CuNWs height and microgroove mesh pitch. An existing theoretical model of pool boiling CHF has been extended to include liquid wicking effect and contact angle effect on CuNWs surfaces, and a new correlation for pool boiling CHF on the two-tier structures has been developed to include contributions from both liquid wicking by CuNWs and modulated wavelength by microgrooves.