**1. Introduction**

Pool boiling heat transfer has been widely used in numerous engineering systems, such as aircraft and spacecraft thermal management, high power electronics cooling, heat exchangers, nuclear reactors, air conditioning, thermal power generation, etc. [1–6]. Compared to natural and forced convection without phase change, the advantage of pool boiling is a higher heat removal rate from a surface while maintaining a low superheat (i.e., the temperature difference between the surface and the boiling liquid), which is an important advantage for its use in dissipating highly concentrated thermal loads.

Pool boiling is the process of vaporization at the solid–liquid interface. and it occurs when the temperature of the surface exceeds the saturation temperature of the liquid at the given pressure. The characteristics of the pool boiling heat transfer process can be described by the boiling curve, which was first reported by Nukiyama [7]. In the first phase of boiling heat transfer, all of the heat is dissipated through single-phase natural convection. When the surface superheat is high enough, vapor bubbles begin to form in the cavities on the surface (i.e., heterogeneous boiling takes place), which represents the onset of nucleate boiling (ONB) and the inception of the nucleate boiling phase. The nucleate boiling heat transfer regime is the most effective heat transfer region of the pool boiling process due

**Citation:** Hadži´c, A.; Može, M.; Arhar, K.; Zupanˇciˇc, M.; Golobiˇc, I. Effect of Nanoparticle Size and Concentration on Pool Boiling Heat Transfer with TiO<sup>2</sup> Nanofluids on Laser-Textured Copper Surfaces. *Nanomaterials* **2022**, *12*, 2611. https:// doi.org/10.3390/nano12152611

Academic Editor: S. M. Sohel Murshed

Received: 18 July 2022 Accepted: 27 July 2022 Published: 29 July 2022

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to very high heat transfer coefficients at low surface superheat values, making it the most suitable method for various engineering applications. The heat transfer coefficient (HTC), representing the ratio between dissipated heat flux and the corresponding wall superheat, is the most common metric to describe the heat transfer intensity. With increasing heat flux, the number of active nucleation sites on the surface increases. When the population of bubbles becomes too high (at a high heat flux), neighboring bubbles coalesce extensively (i.e., merge on or above the surface), forming an insulating blanket of vapor covering the heating surface and thereby significantly decreasing the heat transfer intensity. This phenomenon is known as the boiling crisis, and the maximum heat flux associated with its incipience is the critical heat flux (CHF). After CHF onset, the boiling process transitions towards film boiling, which is characterized by a high increase in wall temperature and a large decrease in the HTC [8,9].

While boiling heat transfer represents an efficient cooling method, the HTC and CHF need to be enhanced to meet the specifications of certain applications, allowing for the safe and efficient operation of such systems. The enhancement of heat transfer in the nucleate boiling regime can be achieved in various ways, including by (i) changing the characteristics of the boiling surface, (ii) modifying the surface-fluid interaction, (iii) modifying the working fluid, or (iv) changing the operating conditions [10,11]. The aim of these methods is generally to lower the ONB and increase both the CHF and the HTC [11–13]. Most approaches to boiling enhancement only consider one technique, meaning that possible combinations and synergies between more techniques are largely unexplored. This is addressed in the present study, where surface modification via laser texturing is coupled with fluid modification through the addition of nanoparticles, with the aim of obtaining superior boiling performance to that using a single enhancement approach.
