Drop coalescence in turbulent liquid-liquid dispersions

Ramachandran Muralidhar, Purdue University

Abstract

The evolution of drop size spectra of a purely coalescing system can be predicted using the population balance equation provided the size specific coalescence frequencies of drop pairs are available. Small amounts of surface active matter can have a profound effect on the behavior of a dispersion. A proper understanding of the dependence of coalescence frequencies on physical parameters such as drop-pair sizes, interfacial tension, turbulence energy dissipation and surface viscosities is vital for the control of drop size distributions to meet delicate stipulations in chemical reaction conversion and selectivity, product quality and stability, etc. in many applications. The coalescence frequency is written as the product of the collision frequency and the coalescence efficiency. In the past, models have been developed to derive expressions for the coalescence efficiency based on severe assumptions and as such do not represent a realistic picture of the coalescence process. The coalescence of drops is viewed as the drainage of a continuous phase film separating the drops under the action of forces arising from the contiguous turbulent flow field. A detailed time-scale analysis of competing dynamical processes is employed to derive realistic descriptions of the film drainage process. Such a timescale analysis shows the force acting on the drop pair is in general a random process. The effects of different physical parameters are systematically analyzed by developing models corresponding to limiting situations. Analysis of the models reveals that drop deformation and interfacial mobility influence the coalescence process most significantly. The coalescence efficiency can show reversed dependencies on physical parameters in different situations. The models yield functional forms for the efficiency in terms of dimensionless groups with coefficients to be identified from transient coalescence data. Dynamic simulations of agglomerating populations are performed using expressions of the coalescence efficiency derived from different models. The results indicate that different drop size spectra differ quite significantly emphasizing the need for detailed models for drop coalescence. These size spectra are also investigated for self-preserving behavior by a method developed in this thesis. Self-preserving size spectra present an alternative approach to arrive at the coalescence frequency by the inverse problem approach. (Abstract shortened with permission of author.)

Degree

Ph.D.

Advisors

Ramkrishna, Purdue University.

Subject Area

Chemical engineering

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