Pool boiling refers to a heat transfer phenomenon that occurs when a horizontal surface immersed in a quiescent fluid is heated, where heat is transferred through natural convection driven by buoyancy-induced density differences as well as through the nucleation, growth, and detachment of vapor bubbles. It denotes a boiling process in which fluid motion is generated solely by natural convection, in the absence of any externally imposed forced flow. Boiling is generally classified into four distinct regimes: natural convection boiling, nucleate boiling, transition boiling, and film boiling. Among these, the Onset of Nucleate Boiling (ONB), Heat Transfer Coefficient (HTC), and Critical Heat Flux (CHF) are regarded as the key governing parameters.

The present pool boiling experimental apparatus is designed to enable the observation of boiling phenomena under well-controlled and rigorously defined conditions. The pool boiling chamber is designed and fabricated to minimize heat losses to the surroundings while maintaining stable pool boiling conditions. A comprehensive performance analysis of the pool boiling behavior is conducted using high-precision instrumentation, including a high-speed camera, an accurate temperature control system, and dedicated power supply equipment.

The present flow boiling experimental facility is characterized by the following features. Refrigerant circulation is driven by a gear pump, while impurities are removed via an in-line filter, and the mass flow rate is accurately measured using a Coriolis mass flow meter. The inlet temperature is precisely controlled by a subcooler, and after passing through the test section, the working fluid is re-condensed in a plate-type heat exchanger, thereby completing the circulation loop. The system is configured as a fully sealed closed-loop circuit, in which the saturation pressure is regulated through a cooling coil installed inside the reservoir tank. Even when the test section is isolated, a bypass loop ensures the continuous maintenance of a closed-loop operation. Prior to refrigerant charging, it is essential to evacuate the loop under vacuum to completely remove any non-condensable gases from the system, as their presence can significantly affect flow boiling performance and measurement accuracy.

In the present study, sandblasting was employed to tailor the surface roughness, wherein surface textures were generated through high-pressure abrasive particle impingement. Stainless steel (SUS) wire-cut abrasive media with particle sizes of 0.2, 0.4, and 0.6 mm were used to establish distinct roughness conditions. As the particle size increased, both the depth and lateral scale of surface asperities became more pronounced, resulting in an overall increase in surface roughness. The flow boiling experiments revealed that surfaces treated with larger abrasive particles exhibited superior heat transfer performance. This enhancement is attributed to an increased density of active nucleation sites and a rise in surface energy, which promote the formation of smaller and more uniformly distributed vapor bubbles, ultimately leading to a significant enhancement of flow boiling heat transfer.

The microporous surfaces employed in this study are capable of enhancing nucleate boiling heat transfer by suppressing vapor film formation and inducing capillary-driven liquid transport, thereby significantly improving pool boiling heat transfer performance. Micro-thick metallic foam (MMF), characterized by its high porosity and interconnected ligament network, effectively promotes both nucleate boiling activity and continuous liquid replenishment to the heated surface. The dual-layer microporous structure, formed by stacking MMF onto a baseline substrate, facilitates efficient vapor evacuation while simultaneously strengthening capillary flow, leading to improved boiling stability and heat transfer. The mixed-size sintered copper powder surface, fabricated by combining copper powders of varying diameters, generates a dual-scale pore architecture, which enables the simultaneous enhancement of the heat transfer coefficient (HTC) and the critical heat flux (CHF).

Cross-sectional scanning electron microscopy (SEM) imaging enables a detailed visual analysis of the pore size distribution, layer thickness, and interfacial bonding quality of the microporous structures. To quantitatively characterize the surface properties, time-resolved contact angle measurements are performed, allowing for the evaluation of the wettability and surface energy of the working fluid on the engineered surfaces. The wicking performance is assessed based on the droplet spreading dynamics, through which the surface capillary transport behavior and liquid replenishment capability can be quantitatively evaluated.

Because aluminum readily reacts with water, which is otherwise an effective working fluid, acetone, possessing a high latent heat of vaporization, is predominantly employed as the working fluid for phase-change heat transfer applications involving aluminum surfaces. In this study, a microporous structure was fabricated via aluminum particle brazing, and the resulting surface was utilized to enhance boiling heat transfer performance significantly. Compared with previous studies employing either aluminum surfaces or acetone as the working fluid, the present results demonstrate substantially lower wall superheat and a markedly higher critical heat flux (CHF), thereby confirming the superior thermal performance of the aluminum–acetone combination. Furthermore, based on the experimental data, an empirical correlation was developed to predict the pool boiling CHF as a function of the coating parameters of the microporous surface.