Note that the above observation applies to flow

The secondary flows at the bend occurs at a relatively small value of Reynolds number (Re = 1) and Knudsen number in the slip flow regime. Note that slip at the wall leads to finite momentum near the wall; this low momentum fluid reverses its direction in presence of an adverse pressure gradient near the bend, leading to an early appearance of secondary flow. These results can be compared with that of related studies in the literature. Maharudrayya et al. [39] noted that for a sharp bend, LCZ696 at the inner corner appears at Re of 100 while significant recirculation at the outer corner appears at Re of about 200. Xiong and Chung [38] noted that in the continuum regime there is no flow separation for Re < 100 rather flow separation occurs at Re = 300. Lee et al. [42] reported the possibility of flow separation in a 90° bend microchannel. The present results are in qualitative agreement with the numerical simulations of Agrawal et al. [45] where an early flow separation (Re = 2.14) was first noted in presence of wall slip. A direct comparison with these results is however not possible due to difference in the geometry (planar/rectangular cross section in the earlier works whereas circular cross section in the present work). The present results conclusively show that the flow behavior in the slip regime is substantially different than the continuum behavior, in that rarefaction leads to an early onset of flow separation.