Study On The Gas Flow Field In Spray Deposition And The Breakup Mechanism

The design of the atomizer is a key technology in both spray forming and theatomization of the liquid, and affects the gas-only and the gas-liquid flow field.This research about the gas flow field is based on the atomizer designed byourselves. Theory analysis is combined with numerical simulation and experimentto analyze the flow fields generated by the single layer atomizer and the doublelayer atomizer, and the fluctuation and breakup of the liquid in the gas flow field.All these are the foundation to control the gas-only and gas-liquid flow field.Both k-ε turbulence model and Reynolds stress turbulence model (RSM)provided by FLUENT, a commercial CFD software, are applied to calculate the gasflow field formed by four atomizers with different intersection angles. Thescattering angle of the main part of gas spray increased from15.0°to17.4°givenby k-ε model and17.2°to19.0°given by RSM model, as the nozzle intersectionangle increased from5°to65°at the operating gas pressure of10atm, and the twomodels give similar results. Comparing the simulation results with the experimentaldata, they agree well with each other, and RSM model is more accurate. In the gasflow field, the gas sprayed from the nozzle is reflected by the out surface of theprotector of the delivery tube, and an annular speed peak comes up downstream ofthe delivery tube. The results by both k-ε model and RSM model show that theposition where the annular speed peak merged into one single peak moves fartherfrom the delivery tube as the nozzle intersection angle increased. There is arecirculation zone in front of the delivery tube, and the gas moves towards thedelivery tube in the center of this area. The confluence of opposite gas flow is theso-called ‘stagnation point’ of the gas speed, where the gas speed nearly drops tozero. Increasing the intersection angle affects the position of the recirculation, butthere is only a limited effect. Dynamic analysis has also been done to the gas flowfield. Observing different gas flow fields under different gas inlet pressures, it’sfound that the increasing of the gas inlet pressure doesn’t change the structure ofgas flow field and the position of the recirculation zone, but only makes the gasmove faster. A slight disturbance is introduced from the center inlet, which leads toa periodic jet fluctuation. In the study about double layer atomizer, the assembly relation between uplayer and under layer nozzles is determined by both simulation and experiment. Thedistance between up layer and under layer nozzles is no more than15mm, and thediameter of under layer central opening is65mm. When both two nozzles areworking together, the gas spray from up nozzle can restrain the scope of the largerecirculation between the two nozzles. When these two nozzle are working under10atm operating pressure and the scanning angle of the under nozzle is5°, thedeflection angle of gas flow in the main part is5°from the axis. But when onlyunderlayer nozzle is working under10atm operating pressure and the scanningangle of the under nozzle is also5°, the deflection angle of gas flow in the mainpart is6°.The primary and secondary breakups have been analyzed in the paper. Duringthe primary breakup process, when the Weber number is in the range of130.1~160.9, vibration breakup is transforming into sheet breakup. The vibrationbreakup is based on the Kelvin-Helmholtz instability, and the disturbances on theinterface make big droplets separate from the liquid column and lead the liquidcolumn to break into fragments. The results indicate that sheet breakup is the resultof gas static pressure and dynamic pressure, which made the liquid columntransform into liquid sheet then break into little droplets. Those droplets generatedafter sheet breakup are smaller than those after vibration breakup. Observing thepowders after breakup, it’s found that hollow powder is out of the vibration of thelocal liquid sheet after breakup and its own surface tension.The trajectories of droplets with different diameters have been calculated inthe paper. The movements of droplets are affected by the recirculation. When theadmission velocity of droplets is50m/s, the ones with diameters larger than35μmcan go through the recirculation zone easily without being influenced. For thedroplets with same diameter, the faster get into the flow field, the easier go throughthe recirculation zone. It’s analyzed that the whole flow field and temperature fieldin the droplets with50μm,100μm and150μm diameters moving within the gasflow of100m/s and200m/s relative velocity. The cooling speed of the droplets areof the order of104~105just in the context of the given situation. When thetemperature of droplets is below the liquidus, the differences of the temperaturewithin droplets are no more than2℃in gas flow with100m/s relative velocity, and no more than1℃in gas flow with200m/s relative velocity.