*Article* **Effect of Nanoparticle Size and Concentration on Pool Boiling Heat Transfer with TiO<sup>2</sup> Nanofluids on Laser-Textured Copper Surfaces**

**Armin Hadži´c, Matic Može \* , Klara Arhar, Matevž Zupanˇciˇc and Iztok Golobiˇc**

Faculty of Mechanical Engineering, University of Ljubljana, Aškerˇceva 6, 1000 Ljubljana, Slovenia; armin.hadzic@fs.uni-lj.si (A.H.); klara.arhar@fs.uni-lj.si (K.A.); matevz.zupancic@fs.uni-lj.si (M.Z.); iztok.golobic@fs.uni-lj.si (I.G.)

**\*** Correspondence: matic.moze@fs.uni-lj.si; Tel.: +386-1-4771-309

**Abstract:** The enhancement of boiling heat transfer has been extensively shown to be achievable through surface texturing or fluid property modification, yet few studies have investigated the possibility of coupling both enhancement approaches. The present work focuses on exploring the possibility of concomitant enhancement of pool boiling heat transfer by using TiO<sup>2</sup> -water nanofluid in combination with laser-textured copper surfaces. Two mass concentrations of 0.001 wt.% and 0.1 wt.% are used, along with two nanoparticle sizes of 4–8 nm and 490 nm. Nanofluids are prepared using sonification and degassed distilled water, while the boiling experiments are performed at atmospheric pressure. The results demonstrate that the heat transfer coefficient (HTC) using nanofluids is deteriorated compared to using pure water on the reference and laser-textured surface. However, the critical heat flux (CHF) is significantly improved at 0.1 wt.% nanoparticle concentration. The buildup of a highly wettable TiO<sup>2</sup> layer on the surface is identified as the main reason for the observed performance. Multiple subsequent boiling experiments using nanofluids on the same surface exhibited a notable shift in boiling curves and their instability at higher concentrations, which is attributable to growth of the nanoparticle layer on the surface. Overall, the combination of nanofluids boiling on a laser-textured surface proved to enhance the CHF after prolonged exposure to highly concentrated nanofluid, while the HTC was universally and significantly decreased in all cases.

**Keywords:** nanoparticles; nanofluids; boiling; heat transfer; heat transfer enhancement; nanoparticle deposition
