Behaviour of rarefied gas flow near the junction of a suddenly expanding tube

This paper presents an experimental study of isothermal rarefied gas flow through a tube with sudden expansion in the slip flow regime. The measurements reported here are for nitrogen flowing at low pressures in conventional tubes with sudden expansion 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 < 0.075) falls in the slip flow regime; the Reynolds number in the smaller section (Re-s) ranges between 0.2 and 837. The static pressure along the wall is measured for different mass flow rates controlled by a mass flow controller and analysed to understand the flow behaviour. The velocity profiles are obtained through a momentum balance and using the pressure measurements. A discontinuity in the slope of pressure at the sudden expansion junction is noted and given special attention. The absence of flow separation is another key feature observed from the measurements. The streamlines are found to be concave near the junction. It is demonstrated that the flow 'senses' the oncoming sudden expansion junction and starts adjusting itself much before reaching the junction; this interesting behaviour is attributed to an increased axial momentum diffusion and wall slip. The additional acceleration of the central core of the gas flow causes an increase in the wall shear stress and a larger pressure drop as compared with a straight tube. These results are not previously available and should help in improving understanding of gaseous slip flows

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

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 (Re-s) 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 thewall, 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. (C) 2014 AIP Publishing LLC

Slip flow through a converging microchannel: experiments and 3D simulations

An experimental and 3D numerical study of gaseous slip flow through a converging microchannel is presented in this paper. The measurements reported are with nitrogen gas flowing through the microchannel with convergence angles (4 degrees, 8 degrees and 12 degrees), hydraulic diameters (118, 147 and 177 mu m) and lengths (10, 20 and 30 mm). The measurements cover the entire slip flow regime and a part of the continuum and transition regimes (the Knudsen number is between 0.0004 and 0.14); the flow is laminar (the Reynolds number is between 0.5 and 1015). The static pressure drop is measured for various mass flow rates. The overall pressure drop increases with a decrease in the convergence angle and has a relatively large contribution of the viscous component. The numerical solutions of the Navier-Stokes equations with Maxwell's slip boundary condition explore two different flow behaviors: uniform centerline velocity with linear pressure variation in the initial and the middle part of the microchannel and flow acceleration with nonlinear pressure variation in the last part of the microchannel. The centerline velocity and the wall shear stress increase with a decrease in the convergence angle. The concept of a characteristic length scale for a converging microchannel is also explored. The location of the characteristic length is a function of the Knudsen number and approaches the microchannel outlet with rarefaction. These results on gaseous slip flow through converging microchannels are observed to be considerably different than continuum flow

Early onset of flow separation with rarefied gas flowing in a 90 degrees bend tube

This paper presents experimental and three-dimensional numerical study of early onset of separation with rarefied gas flow through a tube with single sharp 90 bend. Experiments are conducted for nitrogen gas flowing at low pressures in three conventional size tubes. The flow is dynamically similar to gas flow in a microchannel as the Knudsen number range (0.0003 < Kn < 0.0385) covers part of the continuum and the slip flow regime while maintaining the Reynolds number between 0.27 and 418.5. The static pressures along the inner, outer and top walls are measured for different mass flow rates and analyzed to understand the flow behavior. The static pressure measurement indicates adverse pressure gradient near the bend along the inner and outer walls of the tube at much lower value of Reynolds number as compared to conventional flow. The numerical solution of the Navier-Stokes equations with the Maxwell's slip boundary condition shows good agreement with experimental data and helps bring out the complex flow behavior near the bend. The adverse pressure gradient, velocity profile, flow streamlines and velocity vectors in the bend plane clearly indicates secondary flows near the bend at as low a Reynolds number as unity. The flow acceleration and the presence of secondary flows near the bend causes a larger pressure drop as compared with a straight tube. Empirical correlations for Poiseuille number and additional pressure drop coefficient are proposed as part of this work. It is noted that limited experimental data exists in the literature for such flows; these results should therefore help enhance the fundamental understanding of gas flow in microchannels with bend. (C) 2015 Elsevier Inc. All rights reserved

Velocity measurement in low Reynolds and low Mach number slip flow through a tube

This paper presents an experimental procedure and results of velocity measurement in the slip flow regime. The measurements are for nitrogen gas flowing at low pressure (flow Reynolds number: 4-198, Mach number: 0.05-0.12) in a conventional tube of 16 mm internal diameter. The static pressure is measured at the wall of the tube and the stagnation pressure is measured through a Pitot tube, for different mass flow rates. The velocity is estimated by applying stagnation pressure correction proposed by earlier researchers, as the Bernoulli equation cannot be applied in gas flow for Re-P <30. In the slip regime, the stagnation pressure correction is found to be a function of Reynolds number and a weak function of rarefaction. A new correlation for stagnation pressure correction is also proposed. Using this correction, velocity within +/- 8% with reference to the analytical solution is obtained. These results are not previously available and should help in velocity measurement ability of low Reynolds number slip flows. (C) 2014 Elsevier Inc. All rights reserved

Low Mach number slip flow through diverging microchannel

This paper presents experimental and three-dimensional numerical study of gaseous slip flow through diverging microchannel. The measurements are performed for nitrogen gas flowing through microchannel with different divergence angles (4 degrees, 8 degrees, 12 degrees and 16 degrees), hydraulic diameters (118, 147 and 177 mu m) and lengths (10, 20 and 30 mm). The Knudsen number falls in the continuum and slip regimes (0.0005 <= Kn <= 0.1; Mach number is between 0.03 and 0.2 for the slip regime) while the flow Reynolds number ranges between 0.4 and 1280. The static pressure drop is measured for various mass flow rates; and it is observed that the pressure drop decreases with an increase in the divergence angle. The viscous component has a relatively large contribution in the overall pressure drop. The numerical solution of the Navier-Stokes equations with the Maxwell's slip boundary condition shows absence of flow reversal (due to slip at the wall), larger viscous diffusion and lower kinetic energy in the diverging microchannel. The centerline velocity and wall shear stress decrease with an increase in the divergence angle. The numerical results further show three different flow behaviors: a nonlinear pressure variation with rapid flow deceleration in the initial part of the microchannel; uniform centerline velocity with linear pressure variation in the middle part, and flow acceleration with nonlinear pressure variation in the last part of the microchannel. A characteristic length scale for diverging microchannel is also defined. The location of the characteristic length is a function of the Knudsen number and shifts toward the microchannel inlet with rarefaction. Mass flow rate and pressure distribution along the channel are also obtained numerically from the direct simulation Monte Carlo (DSMC) method and compared suitably with the experimental data or Navier-Stokes solutions. Empirical relations for the mass flow rate and Poiseuille number are suggested. These results on gaseous slip flow through diverging microchannels are considerably different than their continuum counterparts, and are not previously available. (C) 2015 Elsevier Ltd. All rights reserved