Spray Characteristics Of Nanofluid Jets In Subsonic Crossflow

Nanofluids have received widespread attention due to their outstanding performances in enhancing heat transfer and improving combustion,and are considered to be secondary energy carriers which could change the future global energy landscape.In this study,the objective was to discuss the potential mechanism for nanofluids improving combustion from the perspective of atomization.The primary breakup mechanism,the macro and micro spray characteristics of nanofluid jets in a subsonic crossflow were experimentally and theoretically studied by focusing on the additional effects of nanoparticle additives.The preparation of nanofluids and characterization of physical properties were first completed as groundwork.The design and establishment of a transverse jet experimental system were completed to provide research support.A jet feature analysis program was established by combining the image grayscale standard deviation statistical processing and fast Fourier transform(FFT)spectrum analysis.Firstly,the effects of nano-alumina additives on the instability of cylinder jets without crossflow were experimentally studied by flow visualization inside and near the exit of transparent nozzles.An additional-effect distribution diagram of nanoparticles were drawn and attributed to the two mechanisms of viscosity enhancement and cavitation promotion.The experiment results show that the nanoparticles promoted the flow cavitation inside and outside the sharp-edged nozzle,and had three different additional effects on the jet instability according to the cavitation state.1)For the no-cavitation flow,the nanoparticles exerted a frequency-reduction effect owing to the viscosity increase.2)For the partial cavitation state,the nanoparticles had an increasing-frequency effect owing to the internal cavitation enhancement.3)For supercavitation flow,the nanoparticles increased the jet instability owing to the enhancements in the external cavitation and aerodynamic force.Secondly,the basic features of nanofluid transverse jets,such as morphological change,primary breakup modes,surface wavelengths and jet instability were discussed based on the statistical analysis of high-speed shadow images.A schematic diagram of the primary breakup of nanofluid jets in a crossflow at a small gas Weber number(Weg)was proposed.A new power index of nanoparticles Py was proposed by considering the solvation-layer effects,as well as the nanoparticle properties,such as concentration and particle size.The experiment results show that the addition of low-concentration nanoparticles hardly affected the breakup regimes of transverse jets.As the nano-alumina concentration increased,the amplitude of jet oscillation increased significantly,and the surface wavelength decreased relatively.The upwind wavelength had a negative exponential relationship with and Pv.Relative to that of basic fluid,the small addition of nano-alumina at a volume fraction of 0.51%reduced the characteristic wavelength by up to 18%.Finally,a high-speed camera,a particle image velocimetry(PIV)and a laser particle sizer were used to obtain the macroscopic and microscopic characteristics of jets,such as vertical penetration,lateral diffusion,column fracture,and droplet size/velocity distribution.And empirical correlations were proposed for the quantitative description of liquid column fracture distance and average droplet diameter.The experiment results show that the addition of low-concentration nano-alumina had little effect on the jet trajectory.When the particle size was close to the submicron level or above,larger droplets were produced in the primary breakup,and the nanofluid jet showed stronger penetration ability downstream.It was found that the nanoparticles promoted cavitation inside the jet,which increased the liquid column burst frequency.The addition of nano-alumina diminished the average turbulence intensity in the downstream area of the transverse jet as a whole,but increased the average turbulence intensity in the shear vortex area around the jet.The column fracture distance and the average equivalent diameter of near-field droplets diminished with the increase in nano-alumina concentration,and enlarged with the increase in nanoparticle size.In far field,the fineness and uniformity of nanofluid droplets were significantly improved compared to those of basic fluid.The Sauter mean diameter(SMD)had a negative exponential relationship with Weg,PV and the jet momentum flux ratio q.