دانلود رایگان مقاله گرفتن ترموفورتیک از ذرات زیر - میکرونی توسط یک قطره

abstract

Thermophoresis is an important mechanism for submicron particle capture by droplets. The thermophoretic deposition efficiencies under varying Reynolds (Re) numbers and temperature differences are obtained from the direct numerical simulation of the submicron particle flowing around the droplet. Comparison of the results calculated under the same conditions through the classical thermophoretic deposition efficiency formula and by numerical simulation shows that the Davenport formula always returns greater values than the numerical simulation, by a relative deviation of 19.8%e63.8%. The relative deviation decreased first and then increased with increasing difference in temperature, and increased gradually with increasing Re. The deviation resulted from the assumption that the particle concentration on the droplet surface is equal to that of the incoming flow in the formula deduction process. The convection of the gas and the thermophoresis of the particles together determined the migration of the particles in the boundary layer, and so determined the particle concentration distribution on the surface of droplets. Thus, the particle concentrations on the surface of droplets are actually lower than those of the incoming flow and are distributed bimodally on the surface. The dimensionless particle concentration on the surface of droplets decreased with increasing Re, and increased first then decreased later with increasing difference in temperature. The dimensionless thermophoretic driving velocity and Re were adopted to correct the formula. The results calculated by the corrected formula were consistent with the numerical simulation employed in this paper, such that the maximum relative deviation was reduced from the original 66.8% to less than 8%.

4. Conclusion

The law of change in the particle thermophoretic deposition efficiencies of droplets under different Re and temperature differences was obtained by direct numerical simulation. Increasing Re enhanced the action of the gas on the downstream transport of particles and reduced the thermophoretic deposition efficiency. Increasing temperature difference increased thermophoretic strength and thermophoretic deposition efficiency. The deposition efficiency obtained through the thermophoretic deposition effi- ciency formula was 19.8%e63.8% higher than that obtained through numerical simulation. The relative deviation decreased initially then increased with the increase in difference in temperature, and also gradually increased with the increase in Re. From the deduction process of the Davenport formula the deviation is found to be caused by the assumption that the particle concentration on the droplet surface is equal to that of the incoming flow. The particle concentrations on the actual surface of the droplets are actually lower than those of the incoming flow and are bimodally distributed on the surface. The dimensionless particle concentrations on the surface of droplets decreased with increasing Re, and increased first then decreased later with increasing difference in temperature. Re and temperature difference are thus key parameters in determining the particle concentration on the surface of droplets. Increasing Re decreased particle transport volume on the surface of droplets and increased the thermophoretic driving velocity, thus reducing the particle concentration on the surface of droplets. The difference in temperature increased both the particle transport volume on the surface of droplets and the thermophoretic driving velocity. The competition between these factors determines the distribution of the particle concentration on the surface of droplets. The dimensionless thermophoretic driving velocity and Re were adopted to correct the formula based on the law of change in the relative deviation with the key parameters, temperature difference and Re. The results calculated from the corrected formula were highly consistent with the numerical simulation and experimental results. The maximum relative deviation was reduced from the original 66.8% to less than 8%, which demonstrates that the corrected formula predicts the thermophoretic deposition efficiency more accurately.