Spraying atomization performance by pulse width modulated variable and droplet deposition characteristics in wind tunnel

Abstract: Pulse width modulation (PWM) technology is an important means to achieve variable spray, and is attracting more and more researchers' attention due to its short reaction time, fast response, large flow adjustment range and good spray characteristics using conventional nozzles. But during the actual spraying process, its working parameters and environmental conditions could influence the spray accuracy seriously. In order to investigate the atomization and deposition characteristics of the PWM variable-rate nozzle, a pulse width modulation variable spraying system was designed to study the spraying atomization and deposition characteristics of TR80-005C hollow cone spray nozzle commonly used in agricultural application. In order to maintain a stable environmental condition to produce setting wind speed, the experiments were carried out in the type IEA-II wind tunnel designed by Agricultural Intelligent Equipment Technology Research Center. A dot matrix placement capacitive droplet deposition monitoring sensor was used to detect spray deposition in real time. To effectively evaluate the ground deposition properties of the wind tunnel environment, deposition evaluation index (DEIX) was deduced based on drift potional index (DIX). DEIX is inversely proportional to DIX, the smaller the DEIX value, the smaller the potential of droplet deposition, which means that the possibility of drift loss is greater. The Spraytec droplet size meter was sued to test the droplet volume median diameter (VMD) and the relative span of the droplets (RS) to determine the relationship between duty cycle and spraying atomization performance. The experiment was carried out at the Xiaotangshan National Precision Agriculture Research Station in Changping district of Beijing city. The test devices were mainly composed of PWM variable-rate spraying system, IEA-II conventional-speed wind tunnel, laser particle size analyzer and deposition measurement sensor network system. Before tests, all systems were powered on and warm up for 30 minutes. In the droplet size tests, the nozzle was placed 0.5 m directly above the droplet size analyzer, the test pressure was set to 0.4 MPa, the PWM frequency was set to 1 Hz, and the duty cycle was set to 10%-60% at the interval of 10%. Tap water was used as the spraying solution, and each setting repeated 5 times. For the droplet deposition characteristic tests, droplet deposition sensor was arranged at the bottom of the wind tunnel, the sensors were arranged in 5 rows (1 m spacing) and 3 columns (0.55 m spacing) on the vertical wind direction, and were numbered 1 to 15 starting from the upper side to the bottom of the upper side. The nozzle was fixed at the top of the wind tunnel, and the height of the relative deposition sensor was set to 1and 1.5 m respectively, and the horizontal distance between the nozzle and the first column deposition sensor was 1.3 m, the wind speed was set to 1-5 m/s, PWM frequency was set to 1 Hz and duty cycle was 10%-60%, spraying time was set as 10 s, the spraying pressure was set as 0.4 MPa. At the beginning of the test, the sensors saved datas in real time and transmitted it back to the computer. The test results showed that when the duty cycle was between 10%-40%, the VMD decreased with the increases of duty cycle, VMD was 122.3 μm at 60% duty cycle, which increased by 1.8 μm compared with that of at 40% duty cycle. When the PWM duty cycle was 60%, the RS was the smallest, compared with that of duty cycle at 20%, the RS decreased by 9.52%, that means that the droplet spectrum was the narrowest, and droplet size distribution was the most concentrated. In the deposition test, under the condition of wind speed at 1 m/s, droplets were mainly deposited within 3.3 m from the nozzle, which accounted for 95.7% of the total deposition. When the wind speed exceeded 3 m/s, the droplet settling distance increased under the action of the airflow, which may increase the possibility of spray drift. With increase of the duty cycle, DEIX value decreased and the drift rate of the droplets increased. Under the same working conditions, the larger the wind speed and the nozzle height, the smaller the DEIX and the easier spray drift. This study provides a basis for the practical application of pulse width modulation variable application techniques and PWM working condition parameters selection in agricultural field production, and provides a theoretical basis for further optimization of PWM variable adjustment devices.