Microflow hydrodynamics in slits: Effects of the walls relative roughness and spacer inter-filaments distance

A slit designed with very tight dimension tolerances was manufactured to perform pressure drop measurements in fully developed laminar flows, viewing the isolation of the effects of the walls roughness from those of the entrance developing flow and from uncertainties in the channel height. For that, five different values of the slit height, 0.7, 1.0, 1.2, 1.5 and 2.0 mm, were used together with five differently finished surfaces for the channel bottom wall, possessing average roughness values of 31.4, 11.4, 4.1, 1.3 and below 1 μm, which is the sensor detection limit of the contact profilometer used for the measurements. The slit allows also for flow visualization and can accommodate cylindrical-type spacers, positioned transversally to the flow and in direct contact with the bottom wall, with diverse inter-filament distances to study the critical Reynolds numbers marking the departure from the steady-state laminar flow regime. Results with deionised water and 5.4-13.9% of uncertainty for the pressure drop in the open channel showed that surface phenomena due to roughness, irrelevant at macroscale flows, are present in flows like those studied herein. Flow visualization for the same fluid and pressure drop measurements in the channel filled with ladder-type spacers showed that its height is crucial in the set up of the flow instabilities marking the departure from steady laminar flow.