Effect of droplet characteristics on liquid-phase distribution in spray zone of internal mixing air-mist nozzle

In continuous casting production,droplet characteristics are important parameters for evaluating the nozzle atomization quality,and have a significant impact on the secondary cooling effect and the slab quality.In order to study the behavior of atomized droplets after reaching the slab surface and to optimize the spray cooling effect,the influence of droplet diameter and droplet velocity on the migration behavior of droplets in the secondary cooling zone was analyzed by FLUENT software.Results show that the droplets in the spray zone and on the slab surface are mainly concentrated in the center,thus,the liquid volume fraction in the center is higher than that of either side.As the droplet diameter increases,the region of high liquid volume fraction on the slab surface becomes wider,and the liquid phase distribution in the slab width direction becomes uneven.Although increasing the droplet velocity at the nozzle exit has little effect on droplet diffusion in the spray zone,the distribution becomes more uneven due to more liquid reaches the slab surface per unit time.A prediction formula of the maximum water flow rate on the slab surface for specific droplet characteristics was proposed based on dimensionless analysis and validated by simulated data.A nozzle spacing of 210 mm was recommended under the working conditions in this study,which ensures effective coverage of the spray water over the slab surface and enhances the distribution uniformity of water flow rate in the transverse direction.

Numerical Simulation of Droplet Generation in Coaxial Microchannels

In this study,numerical simulations of the pinching-off phenomena displayed by the dispersed phase in a continuous phase have been conducted using COMSOL Multiphysics(level-set method).Four flow patterns,namely“drop flow”,“jet flow”,“squeeze flow”,and“co-flow”,have been obtained for different flow velocity ratios,channel diameter ratios,density ratios,viscosity ratios,and surface tension.The flow pattern map of two-phase flow in coaxial microchannels has been obtained accordingly,and the associated droplet generation process has been critically discussed considering the related frequency,diameter,and pinch-off length.In particular,it is shown that the larger the flow velocity ratio,the smaller the diameter of generated droplets and the shorter the pinch-off length.The pinch-off length of a droplet is influenced by the channel diameter ratio and density ratio.The changes in viscosity ratio have a negligible influence on the droplet generation pinching frequency.With an increase in surface tension,the frequency of generation and pinch-off length of droplets decrease,but for small surface tension the generation diameter of droplet increases.

Development of droplet characteristics prediction models for air induction nozzles based on wind tunnel tests

Nozzle flowrate and spray pressure are two of the most important factors influencing on droplet characteristics.With the aim to develop prediction models for air-induction nozzles(AINs),a series of Billericay Farm Services(BFS)AINs with different orifice diameters in combination with tap water were tested.0.2 MPa,0.3 MPa,0.4 MPa,0.5 MPa,0.6 MPa and 0.7 MPa of spray pressures and 2 m/s,3 m/s,4 m/s and 5 m/s of air speeds were setup.Based on the wind tunnel tests data,prediction models with input variables of nozzle flowrate and spray pressure and output variables of D_(v0.1),D_(v0.5),D_(v0.9),%<150μm(proportion of spray volume contained in droplets with diameter below 150μm),relative span(RS)and coefficient of variation(CV)of D_(v0.5) were developed.The developed models were validated based on wind tunnel experimental data.Results showed that:for D_(v0.1),D_(v0.5),D_(v0.9) and%<150μm,R^(2) were equal to 0.768,0.823,0.868 and 0.811,indicating that the predictive ability for these four parameters is strong.For RS and CV,R^(2) were equal to 0.100 and 0.113,respectively,indicating that the predictive ability for these two parameters is poor.The models developed in the present study are helpful for facilitating the use of AIN in agricultural spray application.

Influence of UAV flight speed on droplet deposition characteristics with the application of infrared thermal imaging

A plant protection unmanned aerial vehicle(UAV)applied for spraying pesticide has the advantages of low cost,high efficiency and environmental protection.However,the complex and changeable farmland environment is not conductive to perform spray test effectively.It is therefore necessary to carry out spray test under controlled conditions.The current study aimed to illuminate the variation law of droplet deposition characteristics under different UAV flight speeds,and to verify the feasibility for applying infrared thermal imaging in detection of droplet deposition effects.A UAV simulation platform with an airborne spray system was established and an analysis program Droplet Analysis for dealing with water-sensitive paper was developed.The results showed that,when the flight speed was set at 0.3 m/s,0.5 m/s,0.7 m/s,0.9 m/s and 1 m/s,respectively,the droplet deposition density,droplet deposition coverage and arithmetic mean of droplet size D0 decreased as the UAV flight speed increased.On the contrary,the droplet diameter variation coefficient CV increased with the increase of UAV flight speed,resulting in the worse uniformity of sprayed droplet distribution as well.The results can provide a theoretical support for optimizing the spraying parameters of plant protection UAV,and demonstrate the practicability of infrared thermal imaging in evaluating the droplet deposition in the field of aerial spraying.

Effect of droplet superficial velocity on mixing efficiency in a microchannel

In this study, droplet characteristics including droplet length and formation time, and mixing efficiency in droplets were investigated via the volume of fluid(VOF) method coupled with a user defined scalar(UDS) model. A cross-shaped junction with a square cross-section was designed and used for droplet formation. An initial arrangement which differed from that of a conventional operation was adopted. Results show that when the droplet superficial velocity is constant, the exchange between the dispersed phase velocity and the continuous phase velocity has a marginal effect on the droplet formation time.However, the exchange has a great effect on droplet length. These findings provide a valuable guide for future operation of droplet formation. In addition, the results show that the mixing efficiency in the droplet forming stage can be classified into time-dominated and length-dominated regimes according to the droplet superficial velocity. When a droplet flows in a microchannel,a higher droplet superficial velocity increases mixing efficiency due to the faster inner circulation and shorter droplet length.