Effects of Dynamic Surface Wettability on Pool Boiling Behavior

Taylor P Allred, Purdue University

Abstract

Wettability has been shown to play a critical role in the pool boiling behavior of a surface. In this thesis, the effects of surface wettability on boiling behavior are further examined, with a particular focus on understanding the role of dynamic surface wettability (i.e. receding contact angle, advancing contact angle, and contact angle hysteresis). Hydrophobic and superhydrophobic surfaces are shown to have favorable boiling performance if the receding contact angle of the surface is sufficiently low, contrary to previous reports that found them to be ineffective boiling surfaces when considering only the static contact angle. To explain this behavior, the roles of both the receding and advancing contact angles during boiling are clarified. Additionally, the effect of different dynamic wetting behaviors on heat transfer mechanisms during single bubble growth are established in order to develop a comprehensive, mechanistic understanding of the role of wettability during boiling. The critical importance of dynamic surface wettability is first demonstrated through studying boiling from superhydrophobic surfaces. These surfaces have stark differences in boiling behavior depending on the initial wetting state of the surface, which determines the effective dynamic wettability. Superhydrophobic surfaces are fabricated on metal test blocks and evaluated in a controlled pool boiling environment. Two degassing procedures are utilized to achieve one of two different initial wetting states prior to the start of a pool boiling experiment. The lowhysteresis Cassie-Baxter state leads to film boiling as a result of vapor spreading readily over the surface. The high-hysteresis Wenzel state leads to efficient nucleate boiling, never before seen on a superhydrophobic surface. Biphilic surfaces, with alternating superhydrophobic and hydrophilic stripes, are investigated to control and enhance pool boiling hydrodynamics. Surfaces are fabricated such that half of the surface area on each surface is superhydrophobic but stripe widths differ acrosssurfaces. Superhydrophobic regions are brought into the Wenzel state prior to boiling to prevent premature film boiling. Boiling occurs preferentially on the superhydrophobic regions, demonstrating control over the location of vapor generation by the surface patterning. Both the critical heat flux and heat transfer coefficient are shown to increase as the stripe size decreases, indicating performance enhancement due to changes in the hydrodynamic ordering during boiling. Ultimately, a uniform superhydrophobic surface in the Wenzel state provides better performance than any of the patterned surfaces. The boiling behavior of hydrophobic surfaces is further investigated to elucidate the surface properties responsible for vapor spreading and premature onset of film boiling. Smooth and textured hydrophobic surfaces with high and low contact angle hysteresis are studied. The results show that the receding contact angle dominates bubble growth dynamics, rather than the static contact angle which is typically considered. These bubble dynamics, in turn, indicate whether premature critical heat flux will occur. Hydrophobic surfaces with low receding contact angles are shown to decrease surface temperatures compared with hydrophilic surfaces while reaching similar critical heat fluxes. The boiling behavior of parahydrophobic surfaces, those with high static contact angle as well as high contact angle hysteresis, is investigated for the first time, revealing untapped potential for these surfaces to minimize surface temperatures during boiling.

Degree

Ph.D.

Advisors

Weibel, Purdue University.

Subject Area

Fluid mechanics|Electrical engineering|Mechanical engineering|Mechanics|Thermodynamics

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