Effects of inlet manifold geometry on the laminar to turbulent transition of gas microflows in adiabatic rectangular microchannels
Résumé
ln this work, a combined experimental and numerical campaign to investigate the effects of inlet manifold shape on the laminar to turbulent transition of gas microflows is carried out. Four rectangular microchannels are micromilled in PMMA plastic with Dh ranging from 250-330 µm and constant length of 54 mm. An attempt has been made to keep the aspect ratio (α = h/ω) constant, however in manufactured microchannels il varies between 0.4-0.48. Four different inlet shapes namely sudden contraction (SC), rounded entrance (RE), V shape (VS) and bellmouth (BM) are investigated in currenl work. ln parallel, validation of an intermittency based transitional turbulence model is also performed using experimental results. Experimental results show that laminar to turbulent transition is delayed the most for BM inlet manifold shape with critical Reynolds number (Rec ) of ~3470. lt was followed by SC and VS. The smallest Rec resulted in case of RE where laminar to turbulent transition initiated around Re ~ 2090. Numerical results also showed the same pattern with BM having the highest Rec followed by SC, whereas no appreciable difference is observed between VS and SC. Experimental results have also demonstrated that gas flow with BM entrance shape microchannel undergoes the longest transitional regime with ΔReLT ~ 5687 between onset of transition to fully turbulent flow, followed by RE where ΔReLT ~ 4554. SC and VS entrances show relatively abrupt transition with ΔReLT of ~3200 and ~1680 respectively. Both experimental and numerical results have demonstrated that inlet manifold shape plays a determining role on the onset as well as length of transitional regime for gas microflows. A good agreement between experimental and numerical Rec, enables the use of intermittency based transitional turbulence model for hydraulic and thermal design of micro heat exchanging systems in product development stages.
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