The average Nusselt numbers were plotted against superficial
Reynolds numbers, as presented in Figs. 9 and 10. As seen in
Fig. 9, the results, as expected, showed monotonous increases of
the Nusselt numbers with the liquid superficial Reynolds numbers.
As also shown in the figure, the Nusselt number increases with
increasing the gas superficial Reynolds number. Hetsroni et al.
[19] studied heat transfer characteristics in the channels of
0.1 mm in size and reported that an increase in gas superficial Reynolds
number involved a decrease in heat transfer, which was
opposite to the present results corresponding to micro-channels
of 0.4 mm in size. The behavior observed by Hetsroni et al. [19]
may be mainly due to, as suggested by Chung and Kawaji [2], significant
effects of surface tension as well as liquid viscosity, which
prohibit agitation of the gas–liquid interface in very small channels.
Such different observations were also reported by Choo and
Kim [21]. Based on their work, the heat transfer results for the
channels with diameters of 0.14 mm and 0.22 mm revealed the
trend similar to that of Hetsroni et al. [19]. On the other hand,
for the 0.33 mm and 0.5 mm channels, Choo and Kim [21] reported
the Nusselt number increasing with increasing the air flow rate.