Pressure Drop in Microchannels

Estimates of pressure drops can be made using the traditional theory and correlations. However, accurate prediction of pressure drops in microchannels for nonslip flows is not possible [1]. Eor better prediction many models have been proposed [2]. Shockwave phenomena in microchannels have been investigated analytically and numerically, and experimental work is under way. 

Friction factor in microchannels Pressure drop in microchannels Transition in microchannels... 

Model (1) One of the most commonly used models to characterize the pressure drop in microchannels is that proposed by Lockhart and Martinefli [63] iov gas-liquid horizontalflow in pipes, which is used for all regimes. It employs two friction multiphers for gas and liquid, 4>g and , as given by the following equation ... 

R. Revellin, J. R. Thome. Adiabatic two-phase frictional pressure drops in microchannels, Exp. Thermal Fluid Sci., 2007, 31, 673-685. 

A. Kawahara, P. M.-Y. Chung, M. Kawaji, Investigation of two-phase flow pattern, fraction and pressure drop in microchannel, Int. J. Multiphase Flow,... 

Parallel flow Instant separation of immiscible phase after extraction Rapid extraction requires narrow channel, which leads to high pressure drop in microchannel... 

Sobhan CB, Garimella SV (2001) A comparative analysis of studies on heat transfer and fluid flow in micro-channels. Microscale Thermophys Eng 5 293-311 Steinke M, Kandlikar SG (2003) Flow boiling and pressure drop in parallel flow micro-channels. In Kandlikar SG (ed) Proceedings of 1st International Conference on Micro-channels and Mini-channels, Rochester, 24-25 April 2003, pp 567-579 Thome JR (2006) State-of-the-art overview of boiling and two-phase flows in microchannels. Heat Transfer Eng 27(9) 4-19... 

The data on pressure drop in irregular channels are presented by Shah and London (1978) and White (1994). Analytical solutions for the drag in micro-channels with a wide variety of shapes of the duct cross-section were obtained by Ma and Peterson (1997). Numerical values of the Poiseuille number for irregular microchannels are tabulated by Sharp et al. (2001). It is possible to formulate the general features of Poiseuille flow as follows ... 

The measurements show that the pressure drop in circular and rectangular microchannels depend strongly on the mass and heat fluxes (Tran et al. 2000 Yu et al. 2002 Shuai et al. 2003). 

Numbering up microchannels to large-scale capacity reactors is driven by a rigorous understanding of pressure drop in every parallel circuit Passive flow distribution permits sufficient flow to each channel. No serious evaluation of microvalves or actuators has been undertaken for high-capacity systems with thousands to tens... 

Chen, Y., Chen, P., Heat transfer and pressure drop in fractal tree-like microchannel nets, Int. J. Heat Mass Transfer 45 (2002) 2643-2648. 

Development of additional models for pressure drop in noncircular channels, and for heat transfer coefficients and transition criteria based on nondimensional parameters is underway. This integrated approach using flow visualization, pressure drop and heat transfer measurements, and analytical modeling, is yielding a comprehensive understanding of condensation in microchannels. 

Garimella, S., Killion, J. D., and Coleman, J. W. (2003) An Experimentally Validated Model for Two-Phase Pressure Drop in the Intermittent Flow Regime for Noncircular Microchannels, Journal cf Fluids Engineering, Vol. 125(5), pp. 887-894. 

Garimella, S., Agarwal, A., and Killion, J. D. (2004) Condensation Pressure Drops in Circular Microchannels, Proceedings of the Second International Conference on Microchannels and Minichannels (ICMM2004), Rochester, NY, United States, American Society of Mechanical Engineers, New York, NY 10016-5990, United States, pp. 649-656. 

Another important point that should be considered in industrial applications of microflow systems is an increase in the flow rate to increase productivity, because productivity depends on the flow rate as well as the cross-sectional area of the microchannel. Therefore, it is necessary to construct a system that avoids an increase in the pressure drop with increasing flow rate. The pressure drop in a system strongly depends on the structure and size of the micromixer used. As shown in Figure 10.7 the use of a T-shaped mixer leads to a small pressure drop, whereas the use of a... 

G. Croce, P. D Agaro, and A. Filippo, Compressibility and rarefaction effects on pressure drop in rough microchannels. Heat Transfer Engineering 28, 688-695 (2007). 

S. M. Senn and D. Poulikakos. Laminar mixing, heat transfer and pressure drop in tree-like microchannel nets and their application for thermal management in polymer electrolyte fuel cells. J. Power Sources, 130 178-191, 2004. 

In Figure 11.11, the pressure drop in a packed bed is compared with that in microchannel of different section geometries. It turns out that the pressure drop in a packed bed is two to three times higher compared to that in microchannels. 

Figure 11.11 Pressure drop in packed beds and different microchannels (circular, quadratic, and parallel plates (slits)) as a function of mean velocity in the channel, respectively, interstitial velocity in the packed bed (air, 20°C).

Knowledge of pressure drop during two-phase flow in microchannels is essential for the design of energy efiicient systems. However, despite its importance only relatively few studies are available in literature concerning pressure drop in liquid-liquid microchannel flows (Chakrabarti et al. 2005 Kashid and Agar 2007 Salim et al. 2008 Jovanovic et al. 2011), compared to those available for gas-liquid flows (Triplett et al. 1999 Chen et al. 2002 Kawahara et al. 2002 Kreutzer et al. 2005). 

Kashid and Agar (2007) investigated the effects of various operating conditions on pressure drop in a PTFE microchannel reactor with a Y-junction as mixing zone. They developed a theoretical prediction for pressure drop based on the capillary pressure and the hydrodynamic pressure drop without the presence of a continuous film and for a constant contact angle between the dispersed plug and the channel wall (Fig. 2.11a). 

Kawahara, A., Chung, P.-Y., Kawaji, M. (2002). Investigation of two-phase flow pattern, void fraction and pressure drop in a microchannel. International Journal of Multiphase Flow, 28, 1411-1435. 

Fig. 4.8 Comparison of non-dimensional pressure drop in the FEP, Tefzel and glass microchannels with internal diameter of 220, 270 and 200 pm, respectively, with a T-junction as inlet, at a constant ionic liquid flow rate, Qn. = 2.26 cm h ...

Fig. 5 Microscale cooling device applications for pure water (a) schematics of single-pass and split-flow arrangements showing fluid flow through microchannels (b) comparison of pressure drops for microchannels with offset strip-fins (1 = 0.5 mm) in single-pass and split-flow arrangements on a 10 mm x 10 mm chip [17]...

Croce G, D Agaro P (2005) Numerical simulation of roughness effect on microchannel heat transfer and pressure drop in laminar flow. J Phys D Appl Phys 38(10) 1518-1530... 

In summary, even though the characteristic length scales of turbulent flows in microchannels are several orders of magnitudes smaller than their counterparts in macroscale pipes and channels, experimental evidence suggests the flows are statistically and structurally similar. As such, long established correlations for pressure drop in pipes and channels and computational tools available for the study of turbulent pipe and chaimel flows should be equally applicable to turbulent microchannel flows. 

Example 6.2 Comparison of pressure drop in microstructured packed bed reactor, microchannel reactor and foam reactor... 

Now we can estimate the pressure drop in all devices with the presented relations Equations 6.5 and 6.7 for the foam Equations 6.9 and 6.10 for the microchannel reactor and Equation 6.4 for the packed bed with spherical particles. For the microchannel reactor we suppose that 60% of the cross section of the reactor is occupied by the channel walls and catalytic layer (see Figure 6.7). Therefore, the channel volume available for the fluid corresponds to the void volume in the packed bed i.e. =0.4 = e. For a given superficial fluid velocity u, the velocity in the void volume is given by = u/e. From Figure 6.10 it becomes evident that the pressure drop in packed bed reactors are several times higher than in foam reactors. The difference can be explained by the high porosity in the foam (efoam = .9) compared to the packed bed = 0.4). The lowest pressure and, therefore, the lowest energy dissipation is found for the multichannel microreactor. 

Commenge, J.-M., Falk, L., Corriou, J.-P, and Matlosz, M. (2002) Optimal design for flow uniformity in microchannel reactors. AIChE /., 48 (2), 345-358. Wirth, K.-E. (2010) Pressure drop in fixed beds. Part L1.6, in VDI-Heat Atlas, Springer, Berlin, New York, Heidelberg. Dietrich, B., W.S., Kind, M., and Martin, H. (2009) Pressure... 

A liquid-liquid system flows in a circular cross section capillary with < ( = 0.8mm diameter forming a continuous water phase and a dispersed toluene phase. The flow rate of the continuous phase is Smlmin while the flow rate of the dispersed phase is 4mlmin"k Estimate the pressure drop in the microchannel assuming dispersed slug velocity equals the two-phase velocity. 

Most databases present coherent trends in the experimental data. As expected, pressure drops in microscale channels were generally much larger than in macroscale channels. The pressure drop increases with increasing mass velocity whereas it decreases with increasing saturation temperature and internal diameter. Unfortunately, there are hardly any data obtained under similar experimental conditions by independent authors to check for quantitative inconsistency among data from different authors. Furthermore, some microchannel two-phase flow data are recorded for unsteady, fluctuating test conditions (such as with intermittent back flow) and these data are often not clearly earmarked as such. 

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