Enhanced pool boiling heat transfer, with simultaneous increase in critical heat flux (CHF) and heat transfer coefficient (HTC), is desired to improve overall system efficiency and reduce equipment size and cost. This paper focuses on combining graphene oxide (GO) and porous copper particles to generate microstructures based on their ability to enhance HTC, CHF, or both. Three pool boiling performance characteristics based on CHF improvements and wall superheat reductions are identified: Type I—reduction in wall superheat only, type II—increase in CHF only, and type III—increase in CHF with reduction in wall superheat at higher heat fluxes. Specific microscale morphologies were generated using (a) screen-printing and (b) electrodeposition techniques. In type-I, rapid bubble activity due to increased availability of nucleation cavities was seen to influence the reduction in the wall superheats, while no increase in CHF was noted. Roughness-augmented wettability was found to be the driving mechanism in type-II enhancement, while wicking and increased nucleation site density were responsible for the enhancement in type-III. An HTC enhancement of ∼216% in type-I and a CHF improvement of ∼70% in type-II were achieved when compared to a plain copper surface with water. In type-III enhancement, a CHF of 2.2 MW/m2 (1.8× over a plain surface) with a HTC of 155 kW/m2 °C (∼2.4× over a plain surface) was obtained. Furthermore, close correlation between the boiling performance and the microscale surface morphology in these three categories has been identified.

We demonstrate a novel technique to achieve highly surface active, functional, and tunable hierarchical porous coated surfaces with high wickability using a combination of ball milling, salt-templating, and sintering techniques. Specifically, using ball-milling to obtain graphene nanoplatelets (GNP) draped copper particles followed by salt templated sintering to induce the strength and cohesiveness to the particles. The salt-templating method was specifically used to promote porosity on the coatings. A systematic study was conducted by varying size of the copper particles, ratio of GNP to copper particles, and process parameters to generate a variety of microporous coatings possessing interconnected pores and tunnels that were observed using electron microscopy. Pool boiling tests exhibited a very high critical heat flux of 289 W/cm² at a wall superheat of just 2.2 °C for the salt templated 3 wt% GNP draped 20 µm diameter copper particles with exceedingly high wicking rates compared to non-salt-templated sintered coatings. The dramatic improvement in the pool boiling performance occurring at a very low surface temperature due to tunable surface properties is highly desirable in heat transfer and many other engineering applications.

Pool boiling heat transfer has proven to be the most effective ways to dissipate the large amount of heat fluxes and achieve the efficient cooling in many industrial applications including high-power electronics cooling, data center cooling, heat exchangers, batteries, refrigeration, and air conditioning. With the aggressive net-zero carbon footprint goals set up by the numerous industries across the globe, the need for development of innovative two-phase cooling solutions is of utmost importance. Graphene, being the highest thermal conductivity material, has been implemented in numerous studies for improving both the critical heat flux (maximum possible heat removed before thermal runaway of the heater surface) and a heat transfer coefficient (determines how efficiently the heat is removed) in pool boiling heat transfer. Initially, this chapter introduces various graphene-based nanomaterials and basics related to structure and characterization of graphene. Later, the highlights of some of the notable research work related to the graphene-based coatings for pool boiling enhancements are discussed. The responsible mechanism for such higher performance is summarized. Concluding remarks and industrial applicability of these techniques are also discussed in this section.

Pool boiling heat transfer is extensively used in various industrial applications such as cooling of reactor cores in nuclear power plants, steam generation in industrial boilers, and cooling of high heat flux generating electronic equipment to efficiently dissipate a large amount of heat by maintaining lower surface temperatures. The maximum heat flux dissipated during pool