Study On The Enhancement Mechanism Of Radiation-induced Evaporation Process In Nanofluids

Nanofluid,as a new type of heat transfer medium,has been widely used in many fields due to their higher thermal conductivity and enhanced radiation absorption capacity.In particular,compared to pure liquids,nanofluids can exhibit obviously enhanced evaporation processes,accompanied by the emergence of many new phenomena and mechanisms.This makes nanofluids having great potential applications in solar evaporation,desalination,phase change energy storage,optofluidic,lightdriven motors,drug delivery and release,and photothermal treatment of tumor tissue.When nanoparticles interact with the incident light with the specific wavelength,nanoparticles in fluids not only act as “localized heat sources”,which diffuse the absorbed energy to surrounding medium by conductive and convective manners,but also as “charged particles”.The localized enhanced electric field(EEF)excited on the nanoparticle surface can be directly imposed on surrounding water molecules.Under the EEF,water molecules can be transformed from the liquid state to the vapor state.Therefore,the radiation-induced liquid-gas phase change process of nanofluids is a multiphase,multiscale and complex energy transfer process involving both thermal diffusion and EEF.The two physical mechanisms interact with each other to jointly promote the evaporation and boiling process of nanofluids.Thermal-mediated mechanism has been clearly investigated,however,the effect of plasma-mediated mechanism on phase change process has rarely been mentioned.Hence,the thesis focused on this scientific issue and conducted an in-depth study of the phase change process of nanofluids under the “non-thermal” plasma-mediated mechanism at different aspects: experiment,simulation and theoretical analysis.This can provide clear guidelines for relevant applications.Firstly,monochromatic incident lights with different wavelengths were used to heat different nanofluids,and the evaporation mass and temperature were comparatively measured.It is found that the LSPR effect can significantly enhance the evaporation process of the nanofluid,especially for silver nanofluids.In order to explore the mechanism,we combined the microscale radiative theory,the electromagnetic theory and the molecular dynamics(MD)simulation to develop a calculation model to study the motion of nanoparticles and surrounding water molecules under the plasma-mediated effect.Using the model,it is found that the localized EEF can directly perturb the motion of water molecules result in dramatically increasing the evaporation mass.Hence,under the irradiation of incident light with plasmon resonant wavelength,there are two energy conversion pathways in nanofluids:light energy→thermal energy of nanoparticle→kinetic energy of molecules and light energy→EEF of nanoparticle→kinetic energy of molecules.In particular,the localized EEF can increase the evaporation ratio by at least 20%.Besides,some necessary conditions for exciting the plasma-mediated mechanism are also proposed,which can provide firm foundations for the following studies.On the basis of revealing that plasma-mediated mechanisms,we further demonstrated the influence of the plasma-mediated mechanism on the latent heat of vaporization(LHV),which is a thermal property closely related to the boiling process.In experiments,monochromatic incident lights with plasmon resonant wavelength were also used to heat the gold nanofluids with different concentrations.It is found that the LHV of nanofluids can be dramatically reduced with the LSPR effect based on the Clausius-Clapeyron equation by measuring the temperature and pressure of vapor.Moreover,the degree of reduction is closely related to nanofluid concentration.The LHV of the gold nanofluids with high concentration can be obviously reduced by nearly60% under the LSPR effect.Meanwhile,combined with the MD simulation,it is revealed that under the perturbation of the EEF excited on the surface of particle clusters in gold nanofluids with high concentration,water molecules can rapidly transform from the liquid to the gaseous state.Hence,the plasma-mediated mechanism plays a significant role in the reduction of the LHV of nanofluids.In addition,the LHV of nanofluids can be flexibly manipulated under the LSPR effect,which make nanofluids having a more attractive prospect in various fields.In order to clarify the complex heat transfer process within the plasmonic nanofluid under the combined effect of both thermal-mediated and plasma-mediated mechanisms,this thesis investigated the effect of the plasma-mediated mechanism on the thermal transport process between nanoparticles and surrounding base fluids.It is found that the LSPR-induced EEF can enhance the heat transfer process between a nanoparticle and the surrounding water by reducing the interfacial thermal resistance.This demonstrates that a modification of the thermal diffusion model is necessary to precisely analyze the complex heat transfer process in nanofluids with the LSPR effect.Thus,we have re-investigated the heat transfer process between nanoparticles and surrounding base fluids by modifying the interfacial thermal resistance based on the thermal diffusion equation.This also formed the basis for subsequent studies on the fluence threshold of nanobubble nucleation.The issue of high fluence threshold of bubble nucleation under continuous incident light irradiation limited the applications of small nanoparticles in some emerging fields such as optofluidic and biomedicine.In order to solve this problem,nanoparticle arrays were formed based on the plasma-mediated mechanism.It is found that the fluence threshold can be effectively manipulated by adjusting the spacing and size of nanoparticles.The significant reduced method in the nucleation threshold was obtained by reducing the particle spacing and simultaneously increasing the size.Compared to an individual nanoparticle,when the radius is increased from 2 nm to 3 nm in an array with the gap of 1 nm,the corresponding fluence threshold can be reduced by 87.5%.In addition,the particle array formation and the excitation of LSPR-induced EEF also dramatically accelerate the nucleation rate of surface nanobubbles.This precise,noncontact,active control of nanobubble nucleation based on the plasma-mediated mechanism can open a new door for applications such as optofluidic,drug delivery and release,as well as signal transmission.