Oscillatory Transient Flow Experiments and Analysis in Circular Microchannels

In this research purely oscillation fluid flow in two microtubes 150 and 250 μm (3.5 mm length) is studied using computational fluid dynamic (CFD) approach and utilizing a new experimental setup developed for dynamic interfacial tension measurement (capillary pressure technique) in the frequency range between 0.2 and 80 Hz. The experiments are done with pure water at a mean temperature of about 25 °C. The results of oscillatory conditions for microtubes of 0.5 mm in diameter have been compared with experimental results for several frequencies. The computational approach was validated by comparison with experimental data of the continuous constant flow through microtubes and also with experimental results of an oscillatory flow through the same tubes at up to 25 Hz. For evaluation of the effects of hydrodynamic relaxation time t_h = R² / ν on the amplitude of the pressure gradient, CFD simulation of the oscillatory flow through microtubes of 0.3 and 0.5 mm (diameter) with t_h =0.0225 s and 0.0625 s have been provided to compare with own corresponding maximum continuous flow (CMCF) experimental data for each frequency which occurs at maximum speed of sinusoidal motion of the piezo. The comparison demonstrates that for a microtube of 0.5 mm and t_h=0.0625 s for frequencies F〈(1/t_h) ≤ 16 Hz the computational results for amplitude of pressure gradient is in relatively good agreement with own CMCF experimental data, while for microtubes of 0.3 mm in diameter this agreement is observed for frequencies lower than F〈(1/t_h) ≤ 44 Hz. CFD simulations of the velocity profile of oscillatory flow through these microtube support these findings and show a parabolic velocity profile (like Poiseuille flow) for frequencies ≤ 10 Hz for microtube of 0.5 mm diameter while this situation is observed below 40 Hz for microtubes of 0.3 mm diameter. Although for a smaller microtube size a relatively developed flow occurs in a higher frequency range, turbulence effects can appear sooner due to the higher flow rates and consequently higher Reynolds numbers. The combination of these two opposite effects would have to be considered when comparing the flow field through microtubes of different size.