Microchannel evaporation

Yoshida, K, Tanaka, S, Hiraki, H, Esashi, M. A micro fuel reformer integrated with a combustor and a microchannel evaporator. J. Micromech. Microeng. 2006 16 191-197. 

Bubble dynamics in microchannels is of great interest in a number of applications, including microchannel evaporators, high-flux heat removal systems for chip cooling applications, ink-jet printers, atomization nozzles and bubble pumps. The high heat transfer coupled with multidimensional heat conduction in the channel wall, and the rapid growth of the bubble at the microscale make it difficult to verify the analytical models. Future research is needed in understanding the heat transfer mechanism... 

In Proceedings of 21st SemiTherm Symposium, San Jose, 15-17 March 2005, pp 1-7 Copeland D, Behnia M, Nakayama W (1997) Manifold micro-channel heat sinks isothermal analysis. IEEE Trans Comp Packag Manuf Technol A 20 96-102 Dupont V, Thome JR, Jacobi AM (2004) Heat transfer model for evaporation in microchannels. 

Thome JR, Dupont V, Jacobi AM (2004) Heat transfer model for evaporation in microchannels. 

The forced fluid flow in heated micro-channels with a distinct evaporation front is considered. The effect of a number of dimensionless parameters such as the Peclet, Jacob numbers, and dimensionless heat flux, on the velocity, temperature and pressure within the liquid and vapor domains has been studied, and the parameters corresponding to the steady flow regime, as well as the domains of flow instability are delineated. An experiment was conducted and demonstrated that the flow in microchannels appear to have to distinct phase domains one for the liquid and the other for the vapor, with a short section of two-phase mixture between them. 

One way to achieve miniaturization using this solution-based procedure is through incorporation of the system into a microchannel. Microchannels are particularly attractive for sensing purposes because they provide a convenient platform for rapid analysis and detection, as has been shown both for biological samples [67] and, more recently, for chemical analytes [68— 70]. Furthermore, it eliminates problems such as solvent evaporation in the preparation of the monolayer and in the analysis, as well as pollution or contamination. 

Figure 2.82 Schematic of the integrated microchannel combustor/ evaporator [129] (by courtesy of Taylor Francis Ltd).

Wegeng, R. S., MicroChannel combustor/evaporator thermal process, Microscale Therm. Eng. 1997, 1, 321-332. 

Liquid evaporation was employed for liquid pumping in Si-glass microchannels. With hydrophobic patterning at the outlet reservoir, the evaporation rate at the liquid meniscus was controlled to produce a flow rate of 5 nL/min. The hydro-phobic region was patterned by an A1 mask using a silane solution (FDTS) [146]. 

Figure 6 Chip component of the MicroChannel assay system, (a) Overview of the entire chip, (b) detail of capillary pumps, which provide autonomous filling of each channel by capillary action, (c) each capillary retention valve prevents drainage of a channel s reaction zone after the fill port has emptied, and (d) a chip with an attached PDMS cube is fdled by a standard pipette. By placing the chip on 2 aluminum blocks, whose temperature is individually controlled by 2 Peltier elements, differential evaporation allows extremely low flow rates...

For an extended review of experimental work on mini and microchannels, the reader is refered to the Thome (2004) and Kandlikar (2002) papers. This brief review covers a representative selection of heat transfer studies in minichannels and its aim is to illustrate the tendencies observed in the presented data. Recently Kandlikar (2004) developed a new general correlation adapted to minichannels which gives very good results for low qualities but fails to take dry-out into account, as noted by the author in question. Lately Thome et al. (2004) and Dupont et al. (2004) proposed a semi-empirical three zone model which is the only published work to predict the unique trends observed in minichannels. In this model the dominant boiling mechanism is the evaporation of the liquid film pressed under confined bubbles. 

V. Dupont, J. Thome, and A. Jacobi. Heat transfer model for evaporation in microchannels, part 11 comparison with the database . International Journal of Heat and Mass Transfer, 47, pp. 3387-3401 (2004). 

Kandlikar, S.G., (2001), Two-phase flow patterns, pressure drop and heat transfer during boiling in minichannels and microchannels flow passages of compact evaporators. Keynote Lecture presented at the Engineering foundation Conference on Compact Heat Exchangers, Davos, Switzerland, July 1-6. 

Jacobi, A.M., Thome, J.R., (2002), Heat transfer model for evaporation of elongated bubble flows in microchannels, J HEAT TRANSFER, 124, 6, pp. 1131-1136. 

Jacobi, A. M. and. Thome J. R., (2002) Heat Transfer Model for Evaporation of Elongated Bubble Flows in Microchannels, HSA7E Jowraa/ of Heat Transfer, Vol.124, pp.1131-1136. 

Appendix 1 Summary of Investigation on Evaporation in Mini and Microchannels (Adapted... 

The nature of boiling heat transfer in a channel with the gap less than the capillary is also studied and presented. The condensation flow mechanisms, pressure drop and heat transfer in microchannels, role of microscale heat transfer in augmentation of nucleate boiling and flow boiling heat transfer, binary-fluid heat and mass transfers in microchannel geometries for miniaturized thermally activated absorption heat pumps, evaporation heat... 

Heat transfer can also be enhanced by application of an electric field. In boiling heat transfer, electric fields have been successfully used to control nucleation rates and achieve a continuous rise in heat transfer coefficient. Up to a sevenfold heat transfer enhancement by the electric field in falling film evaporators has been reported. In the presence of an electric field, both AC and DC, the mixing length in microchannels is shortened considerably, by a factor 30 or more. 

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