The Use of a Nano- and Microporous Surface Layer to Enhance Boiling in a Plate Heat Exchanger

Abstract

Presented research is an experimental study of the performance of a standard plate heat exchanger evaporator, both with and without a novel nano- and microporous copper structure, used to enhance the boiling heat transfer mechanism in the refrigerant channel. Various distance frames in the refrigerant channel were also employed to study the influence of the refrigerant mass flux on two-phase flow heat transfer. The tests were conducted at heat fluxes ranging between 4.5 kW/m2 and 17 kW/m2 with 134a as refrigerant. Pool boiling tests of the enhancement structure, under similar conditions and at various surface inclination angles, were also performed for reasons of comparison. The plate heat exchanger with the enhancement structure displayed up to ten times enhanced heat transfer coefficient in the refrigerant channel, resulting in an improvement in the overall heat transfer coefficient with over 100%. This significant boiling enhancement is in agreement with previous pool boiling experiments and confirms that the enhancement structure may be used to enhance the performance of plate heat exchangers. A simple superposition model was used to evaluate the results, and it was found that, primarily, the convective boiling mechanism was affected by the distance frames in the standard heat exchanger. On the other hand, with the enhanced boiling structure, variations in hydraulic diameter in the refrigerant channel caused a significant change in the nucleate boiling mechanism, which accounted for the largest effect on the heat transfer performance.

Flow boiling in vertical narrow microchannels of different surface wettability characteristics

“…Most experimental investigations presented in the literature so far, concern micro/nano-scale structured surfaces focusing on microscale structures such as micro-roughness, micro-cavities and micro-porosity for heat transfer enhancement [18][19][20][21]. However, the underlying physical mechanisms as for how micro/nano-scale structures enhance HTC (heat transfer efficient) and CHF are still not well understood.…”

Surface engineering for phase change heat transfer: A review

“…Control of surface texture to enhance CHF: Several attempts have been made to use micro and nanostructures, to delay the occurrence of dry-out, or minimize its effects, as reviewed by Lu and Kandlikar 131 . The mechanism by which these nanostructures enhance heat flux is still a matter of debate used to enhance CHF are copper nanowires 35,[232][233][234] , silicon nanowires 231,232 , carbon nanotubes 235 , zinc oxide nanoparticles 236 , nanoporous copper 237,238 , nanoporous zirconium 239 , nanoporous silicon 240 , and nanoporous aluminum oxide 241 . Figure 16 presents a micrograph of superhydrophilic silicon nanowires which achieved a 100% increase in the CHF value for water, as compared with smooth silicon 232 .…”

Boiling heat transfer on a dendritic and micro-porous surface in R134a and FC-72

“…In this study, detailed heat transfer and bubble data has been gathered and analyzed, with the aim of deepening the understanding of the boiling mechanism in a unique nano-and micro-porous copper structure (nµp) used to enhance boiling, previously reported on by Furberg et al [10,11] and later by El-Genk and Ali [12]. The studied structure has earlier been shown to enhance boiling of R134a in pool-and convective boiling applications in both small and large channels, as reported by Furberg et al [13,14]. In an effort to gain more insight into the enhancement mechanisms of the dendritic and micro-porous structure, pool boiling tests were conducted in FC-72 and R134a, visualized with a high speed imaging system.…”