Abstract
A combined experimental and numerical study on the laminar-to-turbulent transition in microchannels using gas flow is presented. The effects of two geometric parameters, namely aspect ratio (height to width) of microchannels and inlet manifold shape, are considered on the value assumed by the critical Reynolds number linked to the laminar-to-turbulent transition. To study the effect of aspect ratio, seven rectangular microchannels having an aspect ratio between 0.25 and 1.04 are micro-milled in PMMA plastic with a constant length of 100 mm. Four rectangular microchannels with different inlet shapes, namely sudden contraction, rounded entrance, V shape and bellmouth, are fabricated to analyze the effects of inlet shape. Pressure loss analyses are then performed for all 11 microchannels by evaluating both average and semi-local friction factors. The Reynolds number in correspondence of which the transition takes place is determined by observing the trend of the friction factor. In parallel, numerical simulations using an intermittency-based transitional turbulence model are also performed and results are compared with the experiments. Experimental and numerical results have demonstrated that both of the investigated geometrical characteristics (aspect ratio and inlet manifold shape) play an important role on the range of the Reynolds number between the onset of transition and the onset of fully turbulent regime for gas microflows. Experimental critical Reynolds numbers show a good agreement with the predictions of the conventional theory and are in the range of 1863–3470 for all the tested microchannels. The role of gas compressibility on the laminar-to-turbulent transition is also discussed.Graphic abstract
Highlights
Laminar-to-turbulent flow transition in microchannels (MCs) is useful to enhance mixing and heat transfer in microsystems
Obot (1988) demonstrated that the Rec is a function of the crosssectional geometry of the channel and, for rectangular channels, it tends to increase if the channel aspect ratio ( :height to width) is decreased
By observing the trend of the friction factor as a function of the Reynolds number shown by Fig. 6a, it is well evident that the geometry of the inlet is able to change significantly the extension of the transition region between laminar and turbulent regimes
Summary
Dh Hydraulic diameter ff Fanning friction factor fR,lam Laminar friction factor for rectangular cross section fB Blasius friction factor Ġ Mass flux h Height of the microchannel Hin Height of the reducer ṁ Mass flow rate p Pressure R Gas constant Re Reynolds Number T Temperature u Velocity of the fluid w Width of the microchannel L Length of the microchannel/microtube x Axial distance from the inlet of the MC y+ Non-dimensional distance between the wall and first node element. Subscripts av Averaged over the whole length of the microtube/microchannel c Critical in At the inlet of the microtube/microchannel out At the outlet of the microtube/microchannel SL Semi-local value of the evaluated parameter tot Total (static and dynamic) part of the flow quantity. RANS Reynolds-averaged Navier–Stokes equations RE Rounded entrance SC Sudden contraction SST Shear stress transport S&L Shah and London SL Semi-local VS V-shaped
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
