Proceedings of ICNMM2006 Proceedings of ICNMM2006 Fourth International Conference on Nanochannels, Microchannels and Minichannels Fourth International Conference on Nanochannels, Microchannels and Minichannels June 19-21, 2006, Limerick, Ireland June 19-21, 2006, Limerick, Ireland
ICNMM2006-96248 ICNMM2006-96248 INVESTIGATION OF FOULING IN MICROCHANNELS Jeffrey L. Perry,
[email protected] Microsystems Engineering Rochester Institute of Technology Rochester, NY 14623
ABSTRACT Particulate fouling in microchannels is a subject that is largely unexplored. It does, however, have significant implications for all microchannel flows since the hydraulic diameters are very small and consequently are susceptible to excessively large pressure drops. The significant forces for dilute solutions of silica particles ranging from 3 to 10 µm are studied in rectangular microchannels made in silicon with a hydraulic diameter of 106 µm. The effects of zeta potential which is pH driven, lift force on the particulates and their fouling characteristics are evaluated by measuring the pressure drop across the microchannel test section. INTRODUCTION Microfluidics is an increasingly studied field because of its use in practical applications in both biology and microelectronics. In recent years, the proliferation of MEMS has resulted in the use of microchannels for many applications. In particular, with microchannels there has been a great emphasis on semiconductor chip cooling. Since large pressure drops are inherent with the use of microchannels it is desired to minimize further increases in pressure drop that can arise due to fouling which can severely limit the efficiency of a microchannel heat exchanger. The smallest hydraulic diameters, Dh, that have been previously used in fouling studies that the authors are aware of were by Yiantsios et al. (1995, 1998, 2003) and Niida et al. (1989). Yiantsios et al. studied fouling in channels with Dh = 952 µm. Their test section consisted of two glass plates separated by Teflon spacer strips to create a channel that was 10 mm by 0.5 mm. Similarity, Niida et al. used an observation cell, made of borosilicate glass with a cross-section of 4 mm by 0.4 mm to provide a Dh of 727 µm. Based on the definitions provided by both Kandlikar & Grande (2002) and Kandlikar & Kuan (2006) these are categorized as minichannels. In the
Satish G. Kandlikar,
[email protected] Department of Mechanical Engineering Rochester Institute of Technology Rochester, NY 14623
present fouling studies microchannels are being used which have dimensions of 70 µm x 220 µm (Dh = 106 µm). In general particulate fouling may be considered a two-step process. It consists of a transport step, in which particles are transferred to the channel wall, and a subsequent adhesion step, which is dominated by the interaction forces between particles and the wall. An important parameter in both steps is the particle size. Particles in the colloidal size range (< 1 µm) tend to be strongly controlled by Brownian diffusivity. However, particles which are several microns in size are less likely to experience random displacement caused by Brownian motion. Table 1 illustrates this by providing the mean displacement, in one direction for particles ranging from 10 nm to 10 µm. Consequently, particles in the size range used in this work (3 – 10 µm diameter) will not be affected by random displacements due to Brownian motion; and therefore fouling as it relates to this phenomenon may be neglected. Moreover, it needs to be stated that particle size is a critical parameter in determining their attachment efficiency. It affects the magnitude of physicochemical interactions between particles and channel wall as well as the hydrodynamic forces that tend to cause particles to detach or prevent them from adhering. Table 1: Diffusion coefficients and Brownian displacements calculated for uncharged spheres in water at 25oC.
1
Diameter
Diffusivity at 25oC (m2/s)
10 nm 100 nm 3 µm 5 µm 10 µm
4.9x10-11 4.9x10-12 1.6x10-13 9.8x10-14 4.9x10-14
Average displacement after 1 hour (µm) 590 190 34 26 19
Copyright © 2006 by ASME
THEORECTICAL APPROACH A. Forces on a particle When a particle is in contact or near contact with a horizontal channel wall, many forces around the particle are involved in determining whether or not adhesion will occur. In this study the forces are categorized into two main groups. The first group is the adhesive forces which are due to van der Waals forces, Fvdw, and gravity, Fg. The second group is the removal forces which include the hydrodynamic lift force, FL, and electro static forces, Fel. The electrostatic forces in actuality may be attractive or repulsive in nature. However, in these experiments the conditions were adjusted such that they were always repulsive by proper pH adjustment. A schematic showing of all these forces is given in Figure 1. The total force, FT, on a particle is the sum of the adhesive and removal forces which is expressed as (1) F = F +F −F −F T
vdw
g
el
L
The more negative FT is, the greater the chance that the rate of fouling will be less. This translates into smaller pressure drops across a microchannel test section. However, because Eq. 1 can be somewhat awkward to represent graphically, the ratio of the removal forces to adhesive forces is used. This ratio is defined as RM with
RM =
FL + Fel Fg + Fvdw
(2)
B. Van der Waals force particle/wall The van der Waals or London dispersion force is an attractive force between two atoms or non polar molecules that arises due to fluctuating dipole moments. In this phenomenon a fluctuating dipole moment in one molecule induces a dipole moment in the other which results in a net attraction between them. The van der Waals force between a sphere and a plate which takes into account retardation effects is given by Gregory (1981).
Fvdw
d A132 d p 1 1− , h
