Dense spray characterization in airblast atomization using laser-based techniques

Abstract

Laser Sheet drops-sizing (LSD) technique for planar Sauter Mean Diameter (SMD) and liquid volume fraction measurement is improved with experimental and numerical methods. The methodology is used to study dense spray characteristics in airblast sprays at various gas-to-liquid mass ratio (GLR) conditions. A methodology is proposed to measure optical depth (OD) in a spray using the planar SMD and liquid volume fraction distributions. The OD increases with increase in GLR. The maximum OD ( 6) is observed for GLR 4 along the axis of the spray. The OD represents number of scattering events, which has signi cant impact on the conventional LSD measurements. A comparison of SMD measurements from particle/droplet imaging analysis (PDIA) and phase doppler particle analyzer (PDPA) revealed signi cant disagreement along the axis of the spray. An empirical correlation is developed to predict SMD at a point in airblast sprays. The proposed correlation can predict SMD at a point in airblast sprays for various liquids within an error band of 20%. Further, the empirical correlation is used to calculate calibration constant in the LSD technique. The conventional LSD measurements are improved when the location for calibration constant is determined using the spray cone angle and the axial distance below the nozzle tip.The structured laser illumination and planar imaging (SLIPI) technique is used to reduce the contribution of multiple scattering in Mie and planar laser induced

uorescence (PLIF) signals. It is observed that contribution of multiple scattering in Mie signal and PLIF signal is not the same and cannot be neglected in LSD measurements even in dilute sprays (OD 1.5). The SLIPI-based LSD measurements are in better agreement with PDPA. The contribution of the numerical corrections for laser sheet absorption and scattering is marginal in dilute sprays (OD 1.5), however, signi cant contribution is observed in dense sprays with OD higher than 4. The liquid volume fraction distributions are obtained using the corrected SLIPIPLIF signal and the PDIA measurements. It is observed that the contribution of multiple scattering in PLIF signal is signi cant. The methodology is extended to study airblast sprays of Jatropha Straight Vegetable Oil (SVO) and its ethanol blends. Uniformly distributed liquid in the spray plane with SMD distribution in the range of 40 m is observed for E30 fuel (blend of 30% ethanol and 70% Jatropha oil on volumetric basis) at GLR 5 condition. The SLIPI-LSD technique is further used to study airblast sprays in cross- ow of air in a wide range of momentum ratio. The SMD distribution is improved with increase in cross- ow velocity at low GLR conditions. The e ect of cross- ow velocity on SMD values is marginal at high GLR conditions. Spatial bifurcation is observed in liquid volume fraction distributions for GLR 3 and 5 with two-phase momentumratio (q2)>13.06. Improved SMD and liquid volume fraction distributions in the axial plane are obtained for q2>25.6.