Effects of Low Uniform Relative Roughness on Single-Phase Friction Factors in Microchannels and Minichannels

Abstract

Nikuradse’s [1] work on friction factors focused on the turbulent flow regime in addition to being performed in large diameter pipes. Laminar data was collected by Nikuradse, however only low relative roughness values were examined. A recent review by Kandlikar [2] showed that the uncertainties in the laminar region of Nikuradse’s experiments were very high, and his conclusion regarding no roughness effects in the laminar region is open to question. In order to conclusively resolve this discrepancy, we have experimentally determined the effects of relative roughness ranging from 0–5.18% in micro and minichannels on friction factor and critical Reynolds numbers. Reynolds numbers were varied from 30 to 7000 and hydraulic diameters ranged from 198μm to 1084μm. There is indeed a roughness effect seen in the laminar region, contrary to what is reported by Nikuradse. The resulting friction factors are well predicted using a set of constricted flow parameters. In addition to higher friction factors, transition to turbulence was observed at decreasing Reynolds numbers as relative roughness increased.

Investigation on liquid flow characteristics in microtubes

“…It is obvious that early transition happened in microtubes. This phenomenon of early transition is also observed by other literature …”

“…After that, f versus Re relationship (in the log–log plot) is not in linear relation anymore and the Poiseuille number rises with the increase of Reynolds number (Re c < Re < Re t , Re t is the Reynolds number where turbulent flow begins). Attention should be paid that the critical Reynolds number is defined as the point that friction factor starts to deviate from the linear portion of f versus Re plot in laminar flow region, that is, the point that Poiseuille number begin to rise with the increase of Reynolds number, in the present work as well as other literature, not the point that friction factor starts to increase as the increase of Reynolds number. When the Reynolds number becomes larger (Re > Re t ), the friction factor decreases slowly with the increase of Reynolds number.…”

“…Unfortunately, most of the references did not mention how they obtained the values of critical Reynolds number. A few references considered the Reynolds number where the deviation from the linear portion of the f versus Re in the log–log plot achieved some certain values, such as 1 42 or 5%, as the critical Reynolds number. It seems like that the deviation criterion is difficult to determine.…”

“…Kandlikar et al also suggested that the laminar to turbulent transition was seen to occur at lower Reynolds number with an increase in the roughness. In their later work, they did more experiments to support their conclusion. The details about the references mentioned above are listed in Table .…”

“…Some researchers found that the critical Reynolds number was around 1800–2000 and considered that the same as conventional theory. However, most researchers thought that early transition exists. Broadly speaking, there are two views on the cause of early transition.…”