Study Of The Effect Of Central Tube Thickness On Round Liquid Jet Breakup

Effect of central tube thickness on round liquid jet Breakup has been studied experimentally and numerically on the basis of coal water slurry (CWS) gasification. Using a high-speed camera, the entrainment behavior at the nozzle exit and the wave frequency of the liquid jet were analyzed. The efficient method for the numerical investigation of coaxial atomizers with various dimensions was presented. The main contents and results are summarized as follows:1. The recirculating flow field and liquid entrainment behavior at the near-field of coaxial nozzles have been investigated. The "bulge" structure which is axisymmetric has been found on the surface of the near-field liquid column when the gas velocity is bigger than a selected value. The size of the "bulge" depends mainly on fluid velocity and the thickness of the central tube. For water media, the size and location of the "bulge" are stable when gas and liquid velocities are fixed for a selected atomizer. When CWS is the test media, the liquid "bulge" will periodically fluctuate when gas velocity is bigger than a specified value, which is the main difference from water. Critical situations in the process of liquid entrainment to the exit surface of central tube have been proposed for different test media, and the prediction formulas of critical gas velocities for different situations have been deduced on the basis of total pressures conservation theory.2. The analysis on the effect of coaxial nozzle structure (especially for the thickness of central tube) on the instability and breakup of liquid jet has been conducted for water and CWS. The wave frequency of liquid jet increases with the increase of the gas jet velocity and the liquid jet velocity. As L=a/b increases, the wave frequency gradually decreases when u/and Do are equal. The relationship has been obtained which is f(?)Uc/δr. The equation δr=δo+c2a could be used to describe the vorticity layer thickness at gas-liquid merged point, where the proportionality coefficients c2are0.071and0.047for water and CWS, respectively. The vorticity layer magnification coefficient Cm has been defined to assess the effect of central tube on atomization.3. LES/VOF has been performed on the liquid primary breakup process of twin-fluid atomizers with various dimensions from laminar to turbulent regimes. The ratio K=δo/a has been defined as a parameter to assess the relative importance of central tube thickness on the setup of boundary conditions. It was observed that the liquid primary breakup morphology of large K value nozzle is more sensitive to the liquid inlet velocity distribution than small K. Based on the parameter K, the reliable and efficient method for the numerical investigation of coaxial atomizers with various dimensions has been presented. The inner flow field of liquid jet has been analyzed. The breakup of liquid column is found to be a gradually accelerating process. In a surface tension dominated process, once the surface wave appears, the liquid column will eventually breakup in the wave trough, which is an irreversible process. The evolution and formation characteristics of wave trough and "satellite" droplet have been analyzed in detail.4. The mechanism of liquid entrainment behavior in the nozzle near-field has been numerically presented using LES/VOF. The region in liquid column near the gas-liquid merged point is of high pressure. With the increase in gas jet velocity, the gas-liquid merged point moves downstream, and the gas-liquid interface gradually closes to the gas inlet. Therefore, the gas reverse flow-field region gradually becomes into a long and narrow band. The velocity of outer liquid layer in the near-field transforms from negative to positive, therefore a liquid stagnation point appears in the outer layer. Recirculating motion in the liquid "bulge" is induced by the shear effects of liquid core flow and recirculating gas flow, which increases with gas velocity. The time-averaged maximum recirculating liquid velocities of initial entrainment and full entrainment have been found to be almost-ul and-3ul, respectively.