Experimental Study On Charges Carried By Individual Sand Grains Under The Influence Of Electric Field

Sand grains are commonly electrified due to collisions,rubbing and induction.The charges of sand grains moving in air generate an electric field which called aeolian electric field,which will in turn influence the sand grains to change the sand grains motion,such as lift-off and transport trajectory.Even the charging mechanism may be affected by the aeolian electric field.Therefore,the experimental measurement of the amount of charges of an individual sand grain under the influence of electric field can help to understand the charging mechanism of sand grains;it can also help to study the influence of the charges on the grain trajectory,sand transport rate;what's more,it's important to study the enhanced electromagnetic wave attenuation due to the charges carried by sand grains.There are only the results of a field experiment are available for reference.However the field experimental conditions are uncontrollable,and the effects of the electric field strength on the charges of a single sand grain cannot be well studied.Therefore,in this thesis,two experimental studies are carried out to investigate the charges carried by individual sand grain under the influence of electric field,including the measurement of charges carried by individual sand grains in the wind-sand system under the influence of electric field,and the charge exchange of individual sand grain due to collisions between the grains and the electrode plate.The experimental principles of the two experiments based on grain trajectory images are firstly elaborated,and the expressions of charges are reversely derived.The experimental errors caused by the forces and the image identification of grain size are analyzed.Then a sand blowing device was designed and built with its design principle and relevant parameters elaborated.By using this device and the silicone oil tank sand grain collection device,the sand grains in the wind and sand system under different conditions were collected,and the charges of an individual sand grain were reversely calculated,and it was found that the strength of the ambient electric field does affect the amount of charges carried by individual grain in the wind blown sand system.In order to well investigate how the electric field and collisions between the grains and the electrode plate affect the charges transfer,experiments were carried out to measure the charge exchange of sand grains colliding with the electrode plate,and further the transferred the charges between the grains and the electrode plate are calculated.It was found that(1)wind speed has a significant effect on the amount of charges carried by individual sand grain in the wind-sand system.When the wind speed increased from 5.41m/s to 6.97m/s(an increase of about 28.84%),the charge to mass ratio of positively charged sand grains increased from 3.59?C/kg to 7.60?C/kg(an increase of about 111.70%).(2)The electric field strength in the vertical direction of the wind-sand system has a large influence on the charges of sand grains.The higher the electric field strength is,the larger the charges of grains with corresponding polarity(corresponding polarity indicates that if the electric field direction is vertical upward,the corresponding grains are charged positively)is.When the electric field is directed upward,the electric field strength increases from 22.26 k V/m to 111.32 k V/m,and the average charge to mass ratio of positively charged grains increases from 4.72 ?C/kg to12.24 ?C/kg(an increase of about 159.32%).(3)When the electric field strength is greater than a critical electric field strength,there is no significant correlation of charges loss caused by the collision of sand grains with the electric field strength.The charges of sand grains transferred to electrode plate is linearly with the charges of grains before collisions and the charge transferred rate is negatively related to the electric field.