Segmented Nusselt number

Theoretical and experimental studies demonstrated that segmented gas/liquid flow enhances the heat transfer considerably compared to a single flow. Multiphase flow simulations revealed mainly two mechanisms explaining the increase of the Nusselt number, namely the circulation within the liquid slugs and the disturbing and renewing of the fluid layer near the wall by the gas bubbles. Detailed multiphase simulations for cylindrical chaimels by Lakehal et al. [19] lead to the... 

Figure 5.14 Estimated Nusselt numbers for segmented flow [19] and hydrodynamically developing single-phase flow In cylindrical pipes Pry = 7 dy,/L, = 0.02) [20]. (Experimental values taken from Ref. [20]) (squared channel, Pry = 7 dy /L, = 0.02).

Figure 5.15 Nusselt number at various flow rates. Comparison between segmented and single-phase flow in squared microchannels [21]. (Adapted with permission from Elsevier.)...

The figure confirms the significant Nusselt number enhancement for segmented flow in microchannels. Up to fourfold increase was observed for atotalthroughput of 130 il min , corresponding to a linear velocity of 0.217 m s (Figure 5.15). 

C., and Poulikakos, D. (2011) Significant Nusselt number increase in microchan-nels with a segmented flow of two iimniscible liquids An experimental study. Int J. Heat Mass Transfer, 54 (7-8), 1456-1464. 

The cooling data (positive B values) also show large deviations from the theoretical predictions. These data can also be used to estimate actual Nusselt numbers for B values higher than the experimental data. In the case of the cooling, the short segments for B of 0.078 and 0.108 can be utilized for the... 

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