BREAKUP PHENOMENA IN COAXIAL AIRBLAST ATOMIZERS

The breakup of a liquid jet with length-to-diameter ratio of 22 surrou nded by a coaxial how of air has been examined by a combination of hig h-speed photography and phase-Doppler velocimetry. The air-to-liquid m omentum and kinetic energy ratios, the Reynolds number of the coaxial water and air jet flows and the exit-plane Weber number have been vari ed over extensive ranges and the results examined in terms of the brea kup length, frequency, droplet size distributions and velocity charact eristics. The photographs reveal the deterministic nature of the liqui d flow at Reynolds numbers which are sufficient to guarantee turbulent flow, with the formation of a wave-like structure for a short distanc e followed by the formation of a, liquid cluster and subsequent breaku p into ligaments and droplets, with the entire process repeated in a p eriodic manner. Attempts are made to relate the breakup length and the frequency of the process to the air-to-liquid momentum and energy rat ios, the exit Weber number and the slip velocity between the two strea ms at the nozzle exit. The results confirm that the ratio of the frequ encies of the wave-like structures and breakup decreased with the slip velocity between the two streams and asymptotically approached a valu e of around one for values higher than 150 m s(-1). The photographs in dicate that the droplet sizes in the sprays are due mainly to disinteg ration of liquid clusters produced after the initial breakup of the li quid jet and the phase Doppler measurements confirm that most of the l iquid remained close to the centreline, where the mean diameter reache d a maximum and the slip velocity between the droplets and the air flo w was low. An atomization model based on the value of the local Weber number on the centreline of the sprays is used to explain the size cha racteristics of the sprays. The atomization process was affected by th e air-to-liquid momentum ratio at the nozzle exit, the annular width o f the coaxial atomizer, the liquid-to-air density ratio, the surface t ension and the kinematic viscosity and density of the air. The rate of spread of the sprays close to the nozzle reduced with increase of the air and liquid flowrates and was affected by the initial breakup of t he liquid jet and the amplitude of the wave-like structure of the liqu id jet during breakup rather than by the air flow turbulence.