Pool boiling in deep and shallow vessels and the effect of surface nano-texture and self-rewetting

Abstract

Pool boiling of ethanol and self-rewetting fluids on bare copper surface and copper surface with polymer nanofibers were studied experimentally. No significant effect of the depths of ethanol layer on the heat removal rate was found. That indicates that only the heat transfer in the liquid microlayer near the heater surface is a dominant factor. As a result, one can expect that selfrewetting fluids can significantly affect boiling performance. Accordingly, several alcohol solutions including the self-rewetting ones were investigated as working fluids in the boiling chamber. It was found that at the 0.1% (v/v) concentration, only the high carbon-alcohol, nheptanol in aqueous solution, improved boiling heat transfer considerably. Furthermore, the experimental study of the effect of surface nano-texture on boiling characteristics was undertaken. For that aim, polyacrylonitrile (PAN) nanofibers were deposited onto the copper heater surface. Measurements of the boiling curve revealed a detrimental effect of such nano-texture in the case of such working fluids as ethanol and self-rewetting n-heptanol solutions. On the other hand, when polystyrene (PS) nanofibers were deposited onto the copper heater surface instead of PAN nanofibers, a significant improvement in boiling heat transfer was observed. The more hydrophobic nature of PS compared to copper is responsible for this effect, i.e. is the reason of the heat transfer enhancement on such a nano-textured surface compared to the pure copper one. In addition, the critical heat flux in the case of n-heptanol solution was found to be reduced considerably on the PS nano-textured surface compared to the pure copper one. This stems from the increased propensity of the heater surface to be covered by vapor, while the rewetting is insufficiently effective at high heat fluxes in presence of PS nanofibers.

Hydrophilic and Hydrophobic Nanostructured Copper Surfaces for Efficient Pool Boiling Heat Transfer with Water, Water/Butanol Mixtures and Novec 649

“…Most research is focused on developing functionalized surfaces for boiling enhancement through surface engineering methods, by changing the surface morphology and their interaction with the boiling fluid [3,[5][6][7]. Boiling surfaces tailored toward enhancement of boiling performance are highly required in industrial applications [8,9]. The performance of boiling surfaces depends on a variety of interconnected factors, such as roughness, topology, porosity, wettability, wickability and others [10][11][12][13] and might vary in time due to surface aging, fouling, oxidation or contamination [3,11,[14][15][16][17].…”

“…Although negligible in the boiling of single-component fluids [52], Marangoni flow significantly affects the boiling performance in dilute mixtures of alcohols (with four and more carbon atoms) in water [53,54]. These mixtures were named self-rewetting fluids (SRFs), as solutal Marangoni flow is expected to be further enhanced by thermal Marangoni flow, and the inflow of colder liquid spontaneously rewets hot places during boiling due to surface tension gradients induced by temperature inhomogeneities inside the boiling liquid [9,52,54].…”

“…Zhou et al [60] experimented with the boiling of 1-butanol/water mixtures on a flat copper surface bonded with copper foam and achieved minor enhancement of heat transfer using their densest foam. Sankaran et al [9] studied the boiling of 1-heptanol/water mixtures on flat copper surfaces with polymer nanostructures. Although a small HTC improvement was achieved, CHF was lower compared with the boiling of pure water.…”

Pool Boiling Performance of Water and Self-Rewetting Fluids on Hybrid Functionalized Aluminum Surfaces

“…It is a cheap method of surface functionalization, simple to apply and suitable for various materials and geometries. Sankaran et al [25] emphasize that low-cost, simple and straightforward methods for the manufacturing of micro-or nanoscaled textured surfaces for boiling applications with a scale-up potential are highly desired. Kim et al [26] point out that microstructures are rarely applied in the industry due to the higher costs and need for special materials.…”

“…Zhou et al [51] investigated boiling enhancement using a combination of a copper foam and a 1-butanol-based SRF, finding that a very slight heat transfer intensification could be achieved using the densest foam in combination with the SRF. Sankaran et al [25] covered copper surfaces with polymer nanofibers and found that using a 1-heptanol-based SRF on such surfaces can provide a small additional HTC enhancement but decreased the CHF values compared with the boiling of pure water. In general, the results of previous studies suggest that a small additional enhancement of boiling performance can be achieved using SRFs on modified surfaces, but no tests seem to have been performed on surfaces with hydrophobic properties.…”