Passive Flow Control

Passive gas control systems control gas movement by altering the paths of flow without the use of mechanical components. There are generally two types, high-permeability and low-permeability. 

Because of the enormous diversity in components it is difficult to describe a straightforward design-path for components for the MCB concept. Here we focus on the modeling and the design of the fluid control modules and specific on the thermo-pneumatic actuated micropump used (twice) in the demonstrator. An elaborated model of this micropump is given by van de Pol et al. [21]. The main functions of the fluid control in micro analysis systems are the switching function and the direct flow and/or pressure control. Building blocks are hydraulic inertances, resistors, capacitors and passive and control-valves. Very often an active element like a micropump is needed. 

Several techniques for miniaturization of simple chemical and medical analysis systems are described. Miniaturization of total analysis systems realizes a small sample volume, a fast response and reduction of reagents. These features are useful in chemical and medical analysis. During the last decade many micro flow control devices, as well as the micro chemical sensors fabricated by three dimensional microfabrication technologies based on photofabrication, termed micromachining, have been developed. Miniaturized total analysis systems (pTAS) have been studied and some prototypes developed. In microfabricated systems, microfluidics , which represent the behavior of fluids in small sized channels, are considered and are very important in the design of micro elements used in pTAS. In this chapter microfluidics applied flow devices, micro flow control devices of active and passive microvalves, mechanical and non-mechanical micropumps and micro flow sensors fabricated by micromachining are reviewed. 

Recent progress in microflow devices and systems is described in this chapter. Examples of passive and active flow control methods applicable in practical pTAS are described in Sect. 2. Multiple flow control systems, i.e., arrayed microvalves, for advanced high-throughput microflow systems are then introduced in Sect. 3. Examples of microflow devices and systems for chemical and biochemical applications are described in Sect. 4. 

Liquid flow is incompressible, so, in micrometer-scale channels, the flow has a small Reynolds number (Re), usually less than 1, and the flow in simple microchannels is laminar, thus chaotic or turbulent flows are not observed [1]. Many types of microfluidic device have been developed on the basis of this flow behavior. Functional flow control methods based on laminar flow profiles have been proposed and applied in microflow devices and systems. Passive and active flow control methods and their applications are introduced in this section. 

Fig. 9 Continuous flow control microvalves using horizontally moved PDMS structures [31, 32], (a) Schematic of passive microvalve, (b) Schematic and photograph of active microvalve top view). Reprinted with kind permission from [31] Copyright 2008 IEEE...

The research presented opens a new area for flow control in combustion systems — mixing by active feedback control. Considered herein is a two-dimensional (2D) jet, and simple control strategies are proposed that, with small control effort, generate a large increase in turbulent kinetic energy of the jet flow. The control is applied by microjets or microflaps at the lip of the jet and requires only the meeisurement of pressure at the jet lip. It is demonstrated that the controller enhances mixing of massless particles, particles with mass, and a passive scalar. 

Gupta and his coworkers at the University of Maryland obtained for the first time 3D PIV images of a flame and used these images to interpret airflow characteristics associated with the high-shear regime of the flow (Chapter 12). These features — flame plume and spray flame — are considered to manipulate passive flame control of swirl-stabilized combustion systems. 

Easley, C.J., Karlinsey, J.M., Leslie, D.C., Begley, M.R., and Landers, J.R Directional and frequency-dependent flow control in microfluidic circuits using passive elastomeric components. Proceedings of the mTAS 2006 Conference, 10th International Conference on Miniaturized Systems for Chemistry and the Life Sciences (Tokyo), 2006, 2, 1064—1068. 

Instead of merely exploiting microscale physical phenomena using simple straight-channel patterns with constant cross section, it is certainly feasible to make use of well-developed microfabrication methods for introducing flow control functionality into individual components. Although various MEMS approaches have been utilized for this purpose (as reviewed by Oh and Ahn ), many of these techniques require complex fabrication and will not be discussed here. This section focuses on more practical passive components that can be fabricated in PDMS or PDMS-glass hybrid devices, requiring minimal fabrication complexity in addition to the typical microchannel assembly methods. 

FIGURE 40.7 Characteristic frequencies could be shifted by an order of magnitude by locally altering PDMS membrane thicknesses ( fluidic capacitors ). Different curves represent different combinations of the thicknesses of two fluidic capacitor components in the same fluidic network. These fluidic bandpass filters provide the proof-of-concept of a new paradigm in microfluidic flow control, where actuation frequency could be used to passively control relative flow rates. 

Set stope floor is composed of N layers of rock, i is rock number, see Eigure 1, for the one-dimensional passive flow, while ignoring the volume and compressibility of water, the control equation for the rock dynamic system (Shu J S et al 2004, Schiavi C Guglielmone F. 2012, Miao X Y et al 2007, Sun M G et al 2005)... 

This chapter has been structured into four sections firstly, the principles governing the centrifugal hydrodynamics on rotational platforms are briefly discussed. Next, externally actuated (active) and rotationally controlled ( passive ) valving schemes are outlined. To form fully integrated centrifugal LoaD platforms that can operate in a sample-to-answer fashion with low-complexity instrumentation, we finally present process automation of networked LUOs through novel, rotationally, and/ or event-triggered flow control schemes as well as common detection techniques. 

Leslie DC, Easley CJ, Seker E, Karlinsey JM, Utz M et al (2009) Frequency-specific flow control in microfluidic circuits with passive elastomeric features. Nat Phys 5 231-235... 

The flow control is broadly classified as (a) passive control, where no auxiliary power source is required, and (b) active control, where there is expenditure of energy. In passive control, parameters like geometry, compliance, temperature, and porosity, etc. are varied. Boundary oscdlation, acoustic waves, blowing, sucticai, etc. are used for active control. The active control schemes use actuators for manipulating the flow behavior. The size of these actuators depends on the nature of the flow. When the Reynolds number is increased, the required size of the actuator is reduced. The availability of MEMS fabrication technique has contributed toward small-scale actuator development. 

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