Vibration-assisted laser surface texturing of metals as a passive method for heat transfer enhancement

• Published in: Experimental thermal and fluid science 2015-11, Vol.68, p.499-508
• Main Author: Grabas, B.
• Format: Article
• Language: English
• Subjects: Austenitic stainless steels; Fluid dynamics; Heat transfer; Laser beams; Laser melting; Laser treatment; Lasers; Melts; Metal; Passive method; Pool boiling; Scanning; Surface roughness; Texturing; Vibration
• ISSN: 0894-1777; 1879-2286
• DOI: 10.1016/j.expthermflusci.2015.06.006

•Metal samples during laser melting are subjected to circular vibration.•The effect of circular vibration on the laser melts is analysed.•The presence of vibration affects the shape of laser melts.•The roughness of laser-vibration melts is considerably increased.•Impact of vibration-assisted laser melting on heat transfer enhancement is considerable. A novel passive method of heat transfer enhancement is presented. The method consists of surface texturing using a scanning laser beam that melts the surface of the metal element subjected to vibration. A generator produces circular vibration with amplitude of 3mm, and the vibration plane is perpendicular to the axis of the moving beam. The tests were performed at a steady circular vibration frequency on flat steel workpieces (AISI 304). The melts were produced at various laser power and laser beam scanning velocities but with the set vibration frequency f=105Hz in all cases. These melts were then subjected to microscopic observations and the measurements of profilometric parameters Ra and Sa, the pool boiling heat transfer coefficient and the heat flux for distilled water. Laser melting of technically smooth metal specimen surfaces produced structures with varied roughness, for which Ra (ISO 4287/1:1984) and Sa (ISO 25178-2:2012) had values considerably higher than those measured for technically smooth surfaces. Also thermal measurements of the laser-vibration textured surfaces indicated a significant, more than fourfold increase in the heat transfer coefficient and increased ability to transfer heat fluxes. A substantial rise in the value of the critical heat flux qCHF was thus observed, along with higher roughness values of the test samples. The sample with the highest Ra=25.709μm, and the samples with Ra=9.637μm are capable of transferring qCHF∼317kW/m2 and qCHF∼220kW/m2, respectively. For the sample with a technically smooth surface, Ra=0.528μm and qCHF∼176kW/m2. The tests aimed at finding potential applications of this technology to passive heat transfer enhancement on the surfaces of heating plates.