# Two-phase flow patterns in micro- and minichannels

The need for reliable, high-performance, price-competitive electronic devices, most notably electronic chips, has created demand for comparably small heat transfer devices capable of removing the required heat load within a limited temperature range. Closed two-phase devices, such as miniature/micro heat pipes and capillary-pumped loops, have been and are being used successfully for this application. Typically, these use a spreading strategy and feature a large number of small circular channels arranged in parallel rows in a rectangular body.

Whatever configuration is used, the heat energy removed at the chip is transported away and rejected from the system by condensation at a remote location (Begg et al., 1999; Zhang et al., 2001). Therefore, it is necessary to gain a fundamental understanding of two-phase flow and phase change heat transfer in miniature/micro channels. Another area of application is in the refrigeration industry, since miniaturization of the heat exchanger can (a) reduce plant size, (b) lower materials costs, and (c) reduce fluid inventory (Vlasie et al., 2004). Reducing refrigerant charge in the refrigeration system can also minimize possible leakage.

The transport phenomena occurring in different scales can vary significantly. In general, the scales of two-phase flow and heat transfer in channels can be classified according to the hydraulic diameter of the channels, although it also depends on the properties of the fluid and flow conditions. One classification is based on the hydraulic diameter of the channels:

Macro channels: Dh > 6mm

Minichannels: $200\text{ }\!\!\mu\!\!\text{ m}<{{D}_{h}}\le 6\text{mm}$

Microchannels: $10\text{ }\!\!\mu\!\!\text{ m}<{{D}_{h}}\le 200\text{ }\!\!\mu\!\!\text{ m}$

Transitional channels: $100\text{nm}<{{D}_{h}}\le 10\text{ }\!\!\mu\!\!\text{ m}$

Nanochannels: ${{D}_{h}}\le 100n\text{m}$

Flow regimes for two-phase flow in both horizontal and vertical tubes have been studied intensively, as outlined in the preceding section. However, most flow regime studies on two-phase flow were performed in conventionally sized (macroscale) tubes with hydraulic diameters greater than 6 mm. Unlike in conventionally-sized passages, in which surface tension effects are limited, surface tension in miniature/micro channels can have significant effects on flow pattern transitions, on overall hydrodynamics, and in particular on the thin films that are believed to be the dominant mechanism controlling the heat transfer characteristics. The flow regime maps for condensation and boiling in conventionally sized tubes may not be relevant to flow in miniature circular tubes, where the surface tension has a significant effect on the hydrodynamics. At present, the data regarding basic flow patterns for two-phase flow, with or without heat transfer in miniature/micro circular tubes, is still very limited. It follows that little is known about the mechanisms associated with transition in these flows.

The classifications based on the size of the channel do not necessarily address the threshold where the characteristics of two-phase flow and heat transfer depart from the macroscale descriptions. For example, Barnea et al. (1983) found that the effect of surface tension is important for stratified-slug transition in horizontal flow in small-diameter (4 – 12 mm) tubes. The best threshold criterion for distinguishing the microscale from the macroscale is bubble growth. With its diameter confined by the channel, a bubble can grow only in length rather than diameter. This criterion should be a function of geometry, size and fluid properties and does not exist currently.

Kawaji and Chung (2004) presented a thorough review of adiabatic two-phase flow in minichannels and microchannels, and suggested that the transition from minichannels to microchannels occurred between 100 and 250 μm. The flow pattern for an air-water system in circular and semi-triangular minichannels with hydraulic diameters between 1.09 and 1.49 mm was investigated by Triplett et al. (1999). They identified five flow patterns for all test sections: bubbly, slug, churn, slug-annular, and annular flow. Since the gravitational force no longer dominates the two-phase flow in minichannels, the stratified flow and stratified-wavy flow that appeared in the horizontal conventional channels were not observed in minichannels. Other than that, the flow patterns in the minichannels and conventionally-sized channels are similar.

The studies of flow patterns in microchannels are still quite limited at this time. The flow patterns for an air-water system in 25 and 100 μm microchannels were studied by Feng and Serizawa (1999), Serizawa and Feng (2001), and Serizawa et al. (2000). They identified five flow patterns: dispersed bubbly flow, gas slug flow, liquid-ring flow, liquid lump flow, and liquid droplet flow. Chung and Kawaji (2004) investigated in detail nitrogen gas-water two-phase flow patterns in microchannels with diameters between 50 and 500 μm; they identified slug flow, liquid-ring flow, gas core flow with a serpentine liquid film, and semi-annular flow patterns; dispersed bubbly and droplet flows were not observed. Bubbly flow does not appear in Chung and Kawaji (2004) because it requires bubbles smaller than the channel diameter and would occur at extremely low gas flow rates in adiabatic systems. Another flow pattern that can be observed in microchannels but not minichannels is liquid ring flow, which occurs when the liquid bridge between two consecutive gas slugs becomes unstable at higher flow rates.

## References

Barnea, D., Luninski, Y., and Taitel, Y., 1983, “Flow Pattern in Horizontal and Vertical Two Phase Flow in Small Diameter Pipes,” Canadian Journal of Chemical Engineering, Vol. 61, pp. 617-620.

Begg, E., Khrustalev, D., and Faghri, A., 1999, “Complete Condensation of Forced Convection Two-Phase Flow in a Miniature Tube,” ASME Journal of Heat Transfer, Vol. 121, pp. 904-915.

Chung, P.M.Y., and Kawaji, M., 2004, “The Effect of Channel Diameter on Adiabatic Two-Phase Flow Characteristics in Microchannels,” International Journal of Multiphase Flow, Vol. 30, pp. 735–761.

Faghri, A., and Zhang, Y., 2006, Transport Phenomena in Multiphase Systems, Elsevier, Burlington, MA

Feng, Z.P., and Serizawa, A., 1999, “Visualization of Two-Phase Flow Patterns in an Ultra-Small Tube,” Proceedings of the 18th Multiphase Flow Symposium of Japan, pp. 33–36.

Kawaji, M., and Chung, P.M.Y., 2004, “Adiabatic Gas-Liquid Flow in Microchannels,” Microscale Thermophysical Engineering, Vol. 8, pp. 239-257.

Serizawa, A. and Feng, Z.P., 2001, “Two-Phase Flow in Micro-Channels,” Proceedings of the 4th International Conference on Multiphase Flow, New Orleans, LA.

Serizawa, A., Feng, Z., and Kawara, Z, 2000, “Two-Phase Flow in Microchannels,” Experimental Thermal and Fluid Science, Vol. 26, pp. 703-714.

Triplett, K.A., Ghiaasiaan, S. M., Abdel-Khalik, S. I., and Sadowski, D. L., 1999, “Gas-Liquid Two-Phase Flow in Microchannels – Part I: Two-Phase Flow Pattern,” International Journal of Multiphase Flow, Vol. 25, pp. 377–394.

Vlasie, C., Macchi, H., Guilpart, J., and Agostini, B., 2004, “Flow Boiling in Small Diameter Channels,” International Journal of Refrigeration, Vol. 27, pp. 191-201.

Zhang, Y., Faghri, A., and Shafii, M.B., 2001, “Capillary Blocking in Forced Convective Condensation in Horizontal Miniature Channels,” ASME Journal of Heat Transfer, Vol. 123, pp. 501-511.