Experimental study of rarefied gas flow near sudden contraction junction of a tube

Abstract

An experimental study of nearly isothermal rarefied gas flow near the sudden contraction junction of a tube is presented in this paper. The measurements are performed with nitrogen gas flowing at low pressures in conventional tubes with sudden contraction area ratios of 1.48, 3.74, 12.43, and 64. The flow is dynamically similar to gas flow in a microchannel as the Knudsen number (0.0001 < Kn < 0.032) falls in the slip flow regime. The Reynolds number in the smaller section (Res) ranges between 0.2 and 837. The static pressure measurements are analyzed to understand the flow behavior. The static pressure variation along the wall and uniform radial pressure profile near the junction indicates absence of flow separation and vena contracta. The static pressure variation in both the tubes approaches the pressure variation as that of an isolated straight tube at a certain critical Knudsen number for a given area ratio. The velocity field is obtained through a momentum balance and using the flow measurements. The effect of larger momentum diffusivity and slip at the wall, restricts any deviation in velocity profile from its parabolic nature at the junction and suppresses flow separation and vena contracta. The larger inertia force at the sudden contraction junction causes larger acceleration of the flow near the junction in the smaller tube as compared to that of the straight tube. The larger pressure drop in the sudden contraction is a result of the extent of flow compression and additional acceleration near the junction in the smaller tube as compared to the straight tube. This paper reports a set of new results that are expected to help in improving understanding of gaseous slip flows.

On mechanisms of choked gas flows in microchannels

“…Although the expansion wave has not been caught in our simulations, it is reasonable to believe that it is the expansion wave downstream the channel outlet that holds the effective pressure ratio (p i /p o ) when the outlet pressure p o decreases lower than the critical value. A recent experiment and analysis on expansion wave of rarefied gas flow confirms our suspicion on the role of expansion wave for reforming the pressure at the outlet region [40,41]. Further studies are still necessary to discover the mechanism of expansion wave in subsonic flows.…”

Low Mach number slip flow through diverging microchannel

“…The experimental facility is similar to that employed by Varade et al [28,45,46]. It consists of a vacuum system, inlet reservoir, outlet reservoir and mass flow controller as shown in Fig.…”

Simulation of Water Flow in a Nanochannel with a Sudden Contraction or Expansion

“…Type A: the pressure variation is continuously linear, decreasing along with the flow direction, same as its flow in a single straight pore. This type of pressure distribution was observed by Varade et al, 15,16 who conducted experiments on rarefied gas flow through sudden contraction junction or expansion junction in a tube, and the requirement is that the contraction and expansion coefficients should be smaller than 2. Type B1 and Type B2: these two types of pressure distributions show the discontinuity in slope before and after the contraction or expansion, respectively.…”

“…They showed that Re determines the size of the vortex. 15,16 Meanwhile, as indicated above, the wettability of the nanopore surface has dominated the water flow behaviors in the nanoscale contraction−expansion nanofluidic system. Before moving to the wettability effect on flow patterns and vortex characteristics, the effect of Re on flow patterns and vortex lengths is explored first.…”

“…Similar results can also be observed for gas flow in nanoscale, 19 indicating that the velocity profile close to the junction is distorted by the sudden contraction or expansion. 16 The flow enhancement factor is always an important parameter that attracts researchers' interest. The previous studies of water transport in infinite-length nanopores showed that, in the hydrophilic nanopore, because the structured nearwall water molecules constrict the flow capacity, the enhancement factors are nearly equal to but below 1.…”