| The droplet quality of hydrodynamic ultrasonic atomization nozzle is better than the ordinary two-phase nozzle.And the atomization amount is larger than the piezoelectric atomization nozzle.High-quality droplet and high atomization amount are both required in the field of areoponics.Therefore,it is necessary to develop a hydrodynamic ultrasonic atomizing nozzle suitable for large-scale aeroponics.Based on the basic principle of the Hartmann resonator,the mechanism of ultrasonic vibration of resonant cavity and the atomization mechanism of resonant cavity supersonic nozzle are analyzed theoretically.The Hartmann low-frequency ultrasonic atomization nozzle with stepped resonator and adjustable structural parameters is designed,including the Laval tube,the whirlpool flow impeller,the stepped tube,conical shield.The influence of the structural parameters on the resonant state of the resonator is studied by means of CFD transient numerical simulation.The key dimensions of the stepped resonator are determined accordingly.When depth ratio of the stepped tube exceeds 2,its resonance frequency will reaches 1.6 to 1.7 times of the cylindrical one under the same working parameters;the conical shield can make the pressure oscillation amplitude in the cavity bigger;start-up properties of oscillation can be optimized due to the main frequency unaffected by the conical shield;the diameter ratio of stepped resonance tube is a quite sensitive parameter influencing the resonance state.According to the above results,the optimal design parameters were determined.Namely,the aperture ratio D1 / D2 = 2 and the optimum Laval tube and the resonant distance X = 5.5mm.The spray test was conducted as follows:Firstly,the atomization properties were tested contrastively under different conditions.And the flow characteristics of the nozzle two-phase flow system were measured.The atomization properties of Hartmann atomization nozzle with a stepped resonance tube is better than those of Hartmann atomization nozzle with a cylindrical one;if the air supply pressure is low,the droplet size is more sensitive with the pressure after adding a conical shield,while the gap of the average droplet size between the nozzle with stepped tube and that with traditional tube is not obvious under the condition of high air supply pressure.The distance between the Laval tube exit and the resonance tube is another sensitive parameter influencing the droplet size.An optimal distance,where the minimum droplet size can be acquired,is 6.5 mm.Secondly,sound pressure level is an important parameter that affects the atomization effect of stepped-type resonant cavity ultrasonic atomization nozzle.The sound field characteristics of the stepped tube nozzle were tested,and the factors influencing the sound field characteristics were studied.The conical shield can effectively bring the sound field energy of the nozzle into the range of the conical cover.The sound pressure of the nozzle with conical shield level increases 13 dB compared with the nozzle without conical shield.The supply pressure is the most important factor affecting the sound pressure level.The results of sound field characteristic test and the results of droplet size test results are compared.It is concluded that the SPL is an important parameter affecting the droplet size.Thirdly,the influence of spray vortex impeller on spray angle and spray distance was tested.And the two-stage aeroponic system was designed.The feasibility of the application of nozzle with vortex impeller in atomization cultivator was analyzed.The rotation of the whirlpool impeller makes the droplet group more evenly diffuse in the radiation range of the conical shield,which also makes the spray distance longer.The maximum value of atomization angle occurs when the supply pressure is in the range of 0.15 MPa to 0.25 MPa.What was studied above has some guiding significance for the layout of the nozzles in the aeroponic cultivator and the setting of the spray time. |
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