Particle nucleation in turbulent gas jets

Abstract

Experiments were conducted to test the theory of nucleation-controlled growth of spherical particles in a turbulent jet. Dibutyl phthalate (DBP) particles were formed in a bench-scale jet apparatus with nozzle diameters 0.235 cm and 0.375 cm. Size distributions and number concentrations of the particles were measured at different DBP vapor concentrations, jet velocities, and positions.There is evidence that the DBP particle nucleation was confined to the shear layer of the jet and that the particles grew by condensation as they moved away ffom the nozzle. Trends in the data suggest that for low rates of particle formation in jets, the final aerosol concentration can be predicted fiom simple scaling laws. IntroductionHomogeneous nucleation is a temperature-and concentration-dependent process that frequently occurs in turbulent flow. Examples of particle formation by nucleation are emissions from industrial, vehicular and aircraft sources, and process gas mixing. The complexity of turbulent flow fields has made it necessary to make simplifying assumptions, often oversimplifications, in predicting nucleation rates. Turbulent fluctuations have frequently been neglected and calculations based on mean velocity, temperature, and concentration profiles. As methods have been developed to incorporate turbulent fluctuations into models, the effects of these fluctuations have been found to be significant.A condensable vapor at high temperature may become supersaturated as it undergoes turbulent mixing with a gas at lower temperature. When the vapor attains only moderate saturation ratios such that rates of nucleation are in the range lo4 -10' particles/cm3 s, the resulting particle number concentrations are low. Particle-particle collisions are infrequent; growth is by heterogeneous condensation and the particle concentration changes only by dilution. In such cases, the growth is said to be nucleation-controlled because the final particle size and concentration are determined by the rates and duration of homogeneous nucleation in the initial mixing stage.This study focuses on a specific flow system of great practical interest, the turbulent jet. As shown in Figure 1, the flow pattern of a turbulent jet has two distinct regions, the initial and similarity regions. The initial region consists of the potential core, a cone of undisturbed nozzle fluid, surrounded by the shear layer. For aerosol formation studies, the shear layer is very important since it is where incipient turbulent mixing, supersaturation, and homogeneous nucleation occur. Hidy and Friedlander (1964) showed qualitatively that processes occurring in the shear layer of a jet strongly affect particle formation by homogeneous nucleation. Lesniewski and Friedlander (1995) hypothesized that particle formation occurs in the shear layer of the jet, but is quenched as the particles move down the axis. Quenching may occur by three mechanisms: (1) the condensable vapor is diluted by ambient fluid entrained into the jet; (2) the number concentration of newly formed partic...

Aerosol nucleation and growth in a turbulent jet using the Stochastic Fields method

“…This illustrates the difficulties of using classical theory to predict nucleation rates in complex situations. Lesniewski (1997) suggests that the results can be reconciled to the experimental observation by using classical nucleation theory, but reducing the calculated surface tension by 10 -15%. Following this, the surface tension was initially reduced by 14%.…”

“…The co-flow mass inlet extends from a radius of 1.35 cm to 7.5 cm (taken from scale drawing in Lesniewski (1997)) and a flow rate of 4.0833 × 10 −3 kg s −1 of air at a temperature of 299 K. A turbulence intensity of 10% is used as measured in the experiment and a length scale of 1.5 cm, equal to the diameter of the glass spheres placed upstream of the co-flow.…”

“…This corresponds to the conditions in trial 824 of Appendix D of Lesniewski (1997) for which centre-line measurements are available at several axial locations. Lesniewski (1997) and Okuyama et al (1987) found that classical theory (Eq. (2)) under-predicts nucleation rates of DBP droplets by several orders of magnitude.…”

“…Lesniewski and Friedlander (1998) and Lesniewski (1997) studied the homogeneous nucleation and growth of Dibutyl Phthalate (DBP) droplets in a turbulent axi-symmetric jet. DBP was chosen as it will condense at room temperature but will remain as vapour at temperature not far above this.…”

“…The objectives of this paper are (i) to implement the Stochastic Fields PDF method for aerosol nucleation and growth and (ii) to assess the importance of taking into account turbulent fluctuations on predictions of laboratory data (Lesniewski and Friedlander, 1998;Lesniewski, 1997), especially in comparison to sensitivity of predictions to the expression used for particle surface tension. The next section details the method used and its implementation here.…”