Active Techniques

Abstract

Active techniques require external power, such as electric or acoustic fields, surface/fluid vibrations, suction and jet impingement. In general, active techniques are not as common as passive techniques in industry as active techniques need external energy. The majority of commercially interesting techniques are mainly limited to passive techniques. However, active techniques such as electrohydrodynamic (EHD) enhancement of boiling and condensation indicate significant potential (Akira and Hiroshi 1988; Seyed-Yagoobi and Bryan 1999). EHD enhancement of heat transfer in two-phase flow features several advantages: (a) able to control the heat transfer coefficient by changing the applied voltage, (b) significant enhancement, (c) no moving parts, and (d) the electric power input is usually negligible. The heat transfer enhancement is highly dependent on quality, flow regime, heat flux, mass flux, and the strength of the radial EHD forces relative to the flow axial momentum. The EHD phenomena involve the interaction of electric fields and flow fields in a dielectric fluid medium. This interaction, under certain conditions, results in electrically induced fluid motion and/or interfacial instabilities, which are caused by an electric body force. When this force is enhancing heat transfer it is thinning and/or destabilizing the liquid layer, depending on the mass flux. However, this force can thin the liquid layer to a point of removing it and can drastically reduce the heat transfer, especially at low mass fluxes and high heat fluxes (Bryan and Seyed-Yagoobi 2001). The electric body force density acting on the molecules of a fluid in the presence of an electric field consists of three terms, as shown below (Melcher 1981):

$$ \bf{f}_e={\rho}_eE-\frac{1}{2}{E}^2\nabla \varepsilon +\frac{1}{2}\nabla \left[{E}^2\rho {\left(\frac{2\varepsilon }{\partial \rho}\right)}_T\right] $$

The three terms in Eq. (1) stand for the electrophoretic, dielectrophoretic, and electrostrictive components of the electric force. For two-phase flows, the dielectrophoretic force dominates because the gradient in the dielectric permittivity, ∇ε, is very high at the vapor-liquid interface, resulting in a large EHD force acting on the interface. This force can cause interfacial instabilities that force the liquid with higher permittivity to move to the regions of higher electric field. This phenomenon is usually referred to as the liquid extraction phenomenon and is believed to be the primary mechanism responsible for flow regime transitions which cause heat transfer enhancement. Sadek et al. (2006) studied in-tube condensation of R134a in a horizontal, single-pass, counter-current heat exchanger with a rod electrode placed in the centre of the tube. As shown in Fig. 10.1, the heat transfer coefficient was enhanced by a factor up to 3.2 times for applied voltage of 8 kV. The pressure drop was increased by a factor of 1.5 at the same conditions of the maximum heat transfer enhancement. The EHD force extracts sufficient liquid from the liquid stratum at the bottom region to cause flow regime transition from stratified flow to annular flow. The decrease in the liquid layer thickness and introduction of droplets into the vapor core (Cotton et al. 2001) result in a large improvement in heat transfer coefficient and a relatively moderate increase in pressure drop.