Pool boiling enhancement by nanotextured surface of hierarchically structured electroplated Ni nanocones

Electroplated Ni nanocones (NiNCs) with a hierarchical structure were used to enhance the pool boiling properties of a surface. The electroplating time was varied to produce NiNCs with varying texturing parameters, which affected the morphology, roughness, and surface area of the boiling surface. These parameters affected the number of boiling nucleation sites and the surface wettability. The resulting values of the critical heat flux (CHF) and effective heat-transfer coefficient (h_eff) were derived. The heat flux (q″) sharply increased when the superheating temperature (ΔT_sat) exceeded the temperature at the onset of nucleate boiling, which distinguished the natural convection regime from the boiling regime. Optimal texturing was obtained at an electroplating time of t_ep = 5 min and increased the CHF by 36% compared to that of the bare Cu substrate: q″_opt = 214 and q″_bare = 157 kW⋅m⁻². The value of h_eff increased correspondingly from 5.7 to 10.5 kW⋅m⁻²⋅°C⁻¹. The Rayleigh and Nusselt (Nu) numbers in the natural convection regime were also the highest for the optimal texturing case. The total surface area was the largest when the surface roughness reached a maximum. A dimensionless parameter, the relative roughness (r*), was introduced to represent the ratio of the surface roughness (R) to the total surface area (A). The CHF, ΔT_sat, bubble diameter (D_b), boiling Nusselt number (Nu_b), and experimental factors (C_CHF and C_heff) can be linearly expressed in terms of r*, confirming the importance of textured surfaces for the enhancement of the pool boiling performance.