Mixing, Reaction and Heat Transfer in a Pulsatile Flow Microreactor: Infrared Measurements

Single-phase microreactors and micro-heat-exchangers have been widely studied over the last decade. Greater safety, better temperature control and hence better reaction products are provided by the high surface-to-volume ratios and compactness of microscale devices, making them more attractive than conventional systems for future industrial applications. Since the flow in microfluidic devices is predominantly laminar long mixing channels or complex geometries are needed for molecular diffusion and completion of the reaction. Hoping to remove this drawback, we demonstrate experimentally the merits of flow pulsation in enhancing mixing efficiency in a simple T channel (50 μm high, 500 μm wide and 40 mm long). For this purpose, a U-shaped PDMS microchannel is enclosed by a 0.1 mm glass plate coated with an opaque layer that constitutes the fourth wall of the microchannel. We investigate experimentally the effect of flow pulsation on mixing of an acid-base neutralization reaction. The mixing of HCl and NaOH reactants, both of 0.8 mol/L concentration, in a T-shaped microchannel is assured by time-pulsed flows at small Reynolds numbers. The effect of mixing is observed at the location along the microchannel at which the temperature reaches its maximum value. For this purpose, infrared thermography, a non-intrusive temperature measurement technique with spatial resolution of a few tens of microns, is used. The mixing efficiency is shown to depend strongly on the ratio between the peak amplitude and the mean flow rate (between 0.4 and 1 mL/h). Saturation is observed for values of this ratio greater than 2.5. Mixing also appears to be enhanced for frequencies of the periodic inlet flows increasing from 1.25 Hz to 5 Hz, i.e. for increasing Strouhal number.