Plasmonic Photothermal Conversion And Phase Change Heat Transfer Of Nanofluid

Plasmonic photothermal conversion of nanofluid is one of the international hot research topics,which also meets the major national demand.It can be widely applied in industrial fields such as solar power generation,seawater desalination,and water purification.In fields of functional application,photothermal conversion of nanofluid has great application potential,due to the unique advantages of optofluidics.Particles in the nanofluid could couple with incident light to generate the localized surface plasmon resonance(LSPR),realizing photothermal conversion,which can be regarded as moving heat sources at the nanoscale.Therefore,the core scientific issue of plasmonic photothermal conversion and phase change heat transfer of nanofluid is "the strong coupling between the moving heat sources and the flow field,temperature field and concentration field in nanofluid".In this thesis,we focus on this scientific issue and introduce our comprehensive study about the heat generation and phase change of nanofluids from three aspects:1)theoretical research in the heat generation characteristics of the nanoparticles,2)experimental research in the relationship between nanofluid droplets evaporation process and light irradiation heating,3)applied research in the functional application of nanofluid photothermal conversion.At the aspect of the heat generation characteristics of the nanoparticles,a complete scattering model is developed based on the Maxwell’s electromagnetic theory to describe the interaction between nanoparticles cluster and incident radiation.We discussed the coupling between nanoparticles and electromagnetic waves in the aspect of photothermal conversion in a one-dimensional ordered nanoparticles chain and analyzed how the number,gap and size of nanoparticle affect photothermal conversion.We found that as the gap increases,the heat generation of nanoparticles decreases as index law.As the number of particles and the gap increase,and the size decreases,the boundary effect of heat generation distribution for nanoparticles chain would be weakened.With larger nanoparticles,larger absorption cross-section emerges so that more radiation energy can be converted into thermal energy.This research laid a theoretical foundation for the following experiments and theoretical studies.Based on the above research,this thesis would discuss the relationship between nanofluid droplets evaporation process and light irradiation heating.We mainly analyzed the effect of the nanofluid initial concentration on droplets evaporation dynamics.The evaporation rates are found to behave an increased regime and a constant regime when crossing a critical concentration:below the critical concentration,the evaporation rate increased as the increase of the concentration;after the initial concentration passed the threshold,the evaporation rate became constant.In order to understand the critical concentration phenomenon,infrared thermography was applied to measure the droplet surface temperature.According to the droplet surface temperature distribution,a contact line region(CLR)and a bulk volume region(BVR)are identified,contributing to the multi-scale effect and the boundary layer effect respectively.The total light energy absorption is separated into two parts to study,the visible light part and the infrared part,in terms of the optical bands.Our data revealed that a multi-scale matching pattern exists between the absorption of incident light energy and the absorbing sites of the droplet:CLR contributes more than 99%of the irradiation energy absorption in the visible region,alternatively,BVR contributes almost all of the infrared energy absorption.Furthermore,the mechanism behind this critical concentration phenomenon is revealed.As initial nanofluid concentration increases,the droplet evaporation rate is increased due to more heat generated by the enhanced nanoparticle deposition in CLR.The evaporation rate stops increasing at a critical concentration due to saturated nanoparticle deposition.Above the concentration threshold,the redundant nanoparticles will be buried under the effective particle layers to be useless.This research gives a new clue to use dilute nanofluid to realized high photothermal conversion efficiency.Additionally,the droplet evaporation induced coffee-ring is a new fabrication method to deposit nanoparticles on a surface.The nanofluid droplets evaporation with asymmetric light heating is also briefly discussed.We found that the droplet surface temperature distribution,flow field,and particle deposition displayed asymmetrical characteristics.Light pressure occurs due to the momentum transfer of photons when light"particles" impact a surface.The associated force is too small to be useful for micron/millimeter-sized machines,Here,the concept of "indirect light pressure" is firstly proposed,originated from the momentum transfer of light-induced liquid-vapor interface movement due to phase change.To verify the indirect light pressure,the theoretical and experimental studies are implemented.We derived the formula to calculate Fm,indicating that Fm is proportional to the square of the incident light intensity,which is different from the classic light pressure with a linear relationship between the light pressure and light intensity.Also,a scale analysis is derived between the size and its forces of the bubble in the nanofluid.An effective bubble size range is identified for Fm to suppress other forces,such as buoyancy Fb and drag force Fd.Then an experiment about pulse laser-controlled bubble movement is performed.Our experiment supports the theoretical analysis regarding the scale effect of various forces on a bubble.There is a bubble size range to let indirect light pressure successfully suppress the bubble underneath the water surface.Due to the periodicity of the laser pulse,the indirect light pressure drives down the bubble with laser pulses to generate the bubble oscillation movement.So far.our findings have great reference value for people to understand the mechanism of photothermal conversion and phase change heat transfer of the nanofluid,which also provide a new idea for optofluidics.At the aspect of functional application of nanofluid photothermal conversion,a new type of light-driven pneumatic piston actuator is proposed and manufactured,which is controlled by the spatial switching frequency of laser focus points.Benefit from the high conversion efficiency of gold nanofluid to convert the light energy to steam,the drive assembly and movement part are integrated into one device.By considering the pressure generated by the expansion of water steam as the spring force and introduced the stiffness coefficient,we obtained the equation of motion for the liquid column.The operating characteristics of the actuator are investigated,and we found an exponential relationship between the amplitude of fluid column motion and the motion frequency(la=a·fab)The coefficient a is co-defined by the laser power,the length of the liquid column and the nanofluid concentration while the index b is only dependent upon the nanofluid concentration.Our finding here reveals that the nanofluid concentration is a key factor affecting the performance of the actuator.This research provides a practical and reliable actuator solution in application fields of mechanical transmission,signal control and so on,which enlarge the application fields of the nanofluids photothermal conversion.