Evaporation Mechanism Of Photothermal Boundary Layer And Application For Solar Steam Generation

As a new research direction of solar thermal utilization,solar steam generation technology has developed rapidly in recent ten years.It has broad application prospects in the fields of seawater desalination,wastewater treatment,distillation sterilization and steam power generation.Using nanofluids as absorbers is one of effective ways to improve the performance of photo-thermal-vapor conversion.Nanoparticles in the working medium can improve the absorption of incident sunlight by plasmon-effect.Meanwhile,due to the large specific surface area and size effect of nanoparticles,the heat transfer capacity of the working medium can be significantly improved.However,the bulk temperature increasing of fluids causes thermal loss,which in turn reduces the phase-change evaporation at air-water interface.Based on the above issues,a concept of light scattering bubbles coupled with light absorption nanoparticles to improve solar steam generation is proposed in the present study.Then,the characteristics and mechanism of solar steam generation coupled with bubbles are investigated from optics,heat transfer,and mass transfer.Furthermore,assembling nanoparticles at the droplet interface to form three-dimensional ordered photothermal nanostructures,the experiment of photothermal boundary layer evaporation is performed and the mechanism of photothermal boundary layer evaporation at the vapor-liquid interface is revealed.Finally,the regulation of optical absorption and thermal conversion at micro scale is explored by numerical simulation.The purpose is to provide scientific basis and theoretical guidance for the regulation of phase transition based on photothermal evaporation.To overcome the bulk temperature increasing,a concept of light scattering bubbles coupled with light absorption nanoparticles is proposed to improve solar steam generation.Bubbles are introduced into the conventional nanofluids evaporation system.The effects of the interaction between nanoparticles and bubbles on solar steam generation are verified from optics,multiphase flow,heat and mass transfer.These dynamic bubbles not only act as light scattering centers to extend the incident light path and amplify the light flux,but also provide large gas-liquid interfaces for moisture capture as well as kinetic energy from bubble bursting to accelerate vapor diffusion.Thus,solar photothermal steam generating efficiently is realized,where the comprehensive steam generation rate is more than three times that of pure water evaporation.Plasmonic nanoparticles have the inherent defect that the plasmonic effect only induces strong light absorption around its nature resonance peak,which is not desirable for broadband solar absorption.To improve the efficiency of steam generation,the finite element method is employed to simulate the optical properties of nanoparticles for revealing the regulation of optical absorption and scattering properties.After establishing the regulation method of mixing particles with different materials,the absorption of incident light can be effectively broaden by selecting nanoparticles with different resonance peaks.According to numerical results,we propose a composite nanofluids composed of three different kinds of particles with distinguishing absorbance peaks for collaborative light absorption over the entire solar spectrum.Dynamic bubbles are further introduced into the nanofluids to promote the solar vapor generation.An appreciable evaporation rate can be achieved,which is 3.61 times higher than the pristine water.The mechanism of photothermal boundary layer solar evaporation is explored to surmount the heat loss during the photothermal response of nanoparticles.The hydrophobic Fe3O4 nanoparticles are prepared by co-precipitation method and assembled on the surface of the droplet.Then,the experiments in solar evaporation of droplet coating with photothermal nanoparticles are conducted under controlled condition to verify the performance of steam generation.It is found that the temperature measured at the interface of the assembled droplet is higher than its internal temperature,which can achieve a great heat localization effect on the gas-liquid interface.The particles form three-dimensional photothermal nanostructures on the interface,which can rapidly drive the phase change evaporation of surrounding liquid,and the enhancement rate of droplet evaporation can reach 1.88 times.The photo-thermal response process of nanoparticles at the vapor-liquid interface is investigated by the finite element method.Simultaneously,the mechanism and regulation law of local temperature rise are also revealed.The results show that the nanoparticles are arranged orderly on the interface and the particle spacing is effectively reduced,which triggers the coupled plasmon resonance mode between particles.Consequently,the generated heat is confined at the vapor-liquid interface and a thermal boundary layer is formed on the nanoscale,which highly suppresses the heat transfer loss to the underlying water body.In order to regulate the photothermal response of particles assembled on the droplet interface,nanoparticles with different materials and structures are selected to assemble nanostructures with obvious roughness differences on the droplet interface.The effects of roughness on heat localization and evaporation are investigated experimentally.It is found that the improvement of evaporation performance by roughness is better than that of material absorption.Roughness of Fe3O4 nanoparticles is the largest at the droplet interface,and the highest evaporation rate is achieved with 6.55 μg/s.The thermal properties of nanoparticles with different roughness are simulated by finite element method,and then the correlation between roughness and local temperature rising is revealed at the nanoscale.Nanostructures with higher roughness have more enhancement effect on electromagnetic field.At the same time,the temperature rising effect of the high roughness nanostructures at the interface is more significant owing to the heat collection effect of the coupling between particles.The numerical results show that photothermal conversion efficiency can be improved though improving the surface roughness of the absorber when solar thermal collector is designed.The photothermal synergistic mechanism of plasmon nanoparticles and bubbles and the photothermal boundary layer evaporation mechanism proposed in this paper have distinct pioneering characteristics,which expands the frontier of solar evaporation.The investigation can provide reference and theoretical guidance for the regulation of photothermal evaporation.The regulation methods proposed in terms of optical absorption and thermal confinement in the present study,can lay a theoretical foundation for its industrial application,and is expected to promote the development of solar photothermal evaporation theory and technology.