Enhanced pool boiling heat transfer rates on graphene nanoplatelets coatings

The present work explores the potential of a simple drop-casting method for the preparation of expanded graphite-based graphene nanoplatelets (GNPs) coatings to enhance pool boiling heat transfer performance with a dielectric fluid. Three coatings (1 mL, 3 mL, and 5 mL surfaces) of varying thicknesses (≈33 μm, 54 μm, 138 μm) are prepared onto aluminium substrates via low-temperature drop casting method using in-house prepared GNPs suspension with water. All surfaces are characterized for their morphological features and wettability behaviour. Subsequently, an uncoated and three coated surfaces, are subjected to pool boiling tests using Promosolv-DR3 as the working fluid. Experiments are conducted on each surface by gradually increasing the heat flux from the onset of bubble nucleation to the Critical Heat Flux (CHF). For each heat flux, surface superheat is measured, and the boiling dynamics are recorded using high speed camera at 4000 fps. It is observed that all coated surfaces substantially improve overall boiling heat transfer coefficient (BHTC) as compared to the baseline uncoated surface. Notably, 1 mL coated surface exhibits the highest enhancement ranging from 660% (at q'' ≈ 2.7 W/cm2) to 180% (at heat flux prior to CHF) relative to the uncoated surface. The improvement in BHTC is specifically attributed to the increased bubble nucleation site density (NSD) due to the porous nature of the coatings coupled with their increased surface roughness. In the context of CHF, the uncoated surface outperforms all coated surfaces with 5 mL coated surface realizing maximum CHF reduction of 18%. The CHF deterioration is attributed to conductive thermal resistance and NSD behaviours of the coatings. The aging test comprising of boiling tests for a duration of one week (8hrs/day) revealed minor changes in the boiling performance as well as coating appearance, suggesting robustness of the coatings against boiling processes.