A systematic study of pool boiling heat transfer on structured porous surfaces: From nanoscale through microscale to macroscale

Abstract

An experimental study has been conducted to examine the effects of macroscale, microscale, and nanoscale surface modifications in water pool boiling heat transfer and to determine the different heat transfer enhancing mechanisms at different scales. Nanostructured surfaces are created by acid etching, while microscale and macroscale structured surfaces are synthesized through a sintering process. Six structures are studied as individual and collectively integrated surfaces from nanoscale through microscale to macroscale: polished plain, flat nanostructured, flat porous, modulated porous, nanostructured flat porous, and nanostructured modulated porous. Boiling performance is measured in terms of critical heat flux (CHF) and heat transfer coefficient (HTC). Both HTC and CHF have been greatly improved on all modified surfaces compared to the polished baseline. Hierarchical multiscale surfaces of integrated nanoscale, microscale, and macroscale structures have been proven to have the most significant improvements on HTC and CHF. The CHF and HTC of the hierarchical multiscale modulated porous surface have achieved the most significant improvements of 350% and 200% over the polished plain surface, respectively. Experimental results are compared to the predictions of a variety of theoretical models with an attempt to reveal the different heat transfer enhancing mechanisms at different scales. It is concluded that models for the structured surfaces at all scales need to be further developed to be able to have good quantitative predictions of CHFs on structured surfaces.

Independent and collective roles of surface structures at different length scales on pool boiling heat transfer

“…1) to create different contact angles from that on a plain Cu surface, which enables to examine the magnitude of CHF enhancements due to the hydrodynamic instability wavelengths changed by contact angles. Equilibrium contact angles (measured when a droplet is not about to move), advancing contact angles (measured when a droplet is about to replace vapor/gas phase in front of it), and receding contact angles (measured when a droplet is about to give way to vapor/gas phase in front of it), are measured by a standard contact angle goniometer system2630 on a plain Cu surface, 200 nm, and 500 nm diameter spherical Cu nanocavity surfaces. The smooth plain Cu surface has an average equilibrium contact angle of 78.35° with a standard deviation of 1.13°.…”

“…In order to further examine the surface structure effects on hydrodynamic instability at CHF, solid and porous Cu pillar surfaces have been created by the same sintering method reported in literature2635, as shown in Fig. 6.…”

“…The Cu powder used to synthesize the porous pillars has a size range of 550–600 μm, and the resulted pore sizes in the porous pillars are found to be in the range of 50 to 220 μm. The details of the Cu pillars synthesis process could be found in previous work2635. The full length Cu pillars should be able to penetrate through the vapor blanket covering the bottom heating surface once CHF has been reached, but the ¼ length Cu pillars should be just comparable to the thickness of the vapor blanket, which can not effectively prevent neighboring vapor columns from collapsing together at the onset of CHF.…”

“…The weakness of this model is the failure to take into consideration of heating surface effects on hydrodynamic instability. Recent pool boiling studies on micro/nanoscale surface structures2324252627282930 have led to the latest modifications of the hydrodynamic instability model with heating surface effects, including the contact angles1415, surface wettability, and capillary wicking1617. On the other hand, the study of hydrodynamic instability models on porous structures has been pursued, too, which focuses on controlling hydrodynamic instability wavelength by the characteristic length of porous structures of micron and millimeter scales263132.…”

“…Recent pool boiling studies on micro/nanoscale surface structures2324252627282930 have led to the latest modifications of the hydrodynamic instability model with heating surface effects, including the contact angles1415, surface wettability, and capillary wicking1617. On the other hand, the study of hydrodynamic instability models on porous structures has been pursued, too, which focuses on controlling hydrodynamic instability wavelength by the characteristic length of porous structures of micron and millimeter scales263132. It has been proposed that the hydrodynamic instability wavelength can be controlled by pore size, d , and surface pore distribution through porosity, ε , as on a flat porous structure31, and the pitch of porous cones as 2632, where σ is surface tension, g is gravitational acceleration, ρ l and ρ v are densities of liquid and vapor.…”