Analysis of laminar to turbulent transition and inlet loss coefficient of compressible flow in rectangular microchannels

Laminar to turbulent transition in rectangular microchannels (MCs) has been numerically investigated using a modified K - ω SST transitional turbulence model, namely γ −Reѳt model. The model has been first validated on two MCs of 100mm in length. These two samples have hydraulic diameter (Dh) 295µm and 215µm whereas aspect ratio (AR = Height/ Width) is 0.7 and 0.24 respectively. The results obtained for friction factor showed a good agreement between simulation, experiments and classical correlations of macro theory. Moreover, the model was able to predict critical Reynolds number (Rec) with sufficient accuracy and shows sensibility to geometrical changes. After the validation, a set of parametric simulations are performed to analyze the effects of AR and Dh on laminar to turbulent transition of the gas flows. It was found that AR strongly influenced the Rec, indeed varying the AR of the MCs from 1 to 0.1, Rec increased from 2000 to 4000 for all the three Dh (100,300 and 500µm). The effect of Dh on Rec instead, was less pronounced. Furthermore, an experimental and numerical campaign, to analyze the effects of inlet manifold shape on both laminar to turbulent transition and inlet loss coefficients (KL in) has been performed. Four samples, whit same dimensions but different inlet design, have been milled on PMMA plastic. Four different shapes have been tested namely: Sudden contraction, V-shape, Rounded entrance and Bellmouth. Both numerical and experimental friction factor highlighted that the Bellmouth inlet significantly delays Rec, whereas for the other three shapes the influence of inlet manifold shape is not so significant. For what concerns KL in at least for fully turbulent regimes a close match between numerical and experimental values is observed. From this analysis it shown that Sudden contraction induces the highest losses, followed by the V-shape, whereas for Rounded entrance and Bellmouth, the cofficient is similar and it is the smallest of all the cases tested.