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Gas Flow in Complex Microchannels
Amit AgrawalDepartment of Mechanical EngineeringIndian Institute of Technology Bombay
TEQIP Workshop
IIT Kanpur, 6th October 2013
Motivation• Devices for continual monitoring the health
of a patient is the need of the hour• Microdevices are required which can
perform various tests on a person• Microdevice based tests can yield quick
results, with smaller amount of reagents, and lower cost
From internet 2 2
Motivation
CPUTj
GraphicsTj
ComponentTcase
ExhaustTemperature
(thermo limit)
Chassis Temperature
AcousticsAcousticsFan noiseFan noise
*Slide taken from Dr. A. Bhattacharya, Intel Corp
Motivation: Electronics cooling
• Heat Flux > 100 W/cm2*: IC density, operating frequency, multi-level interconnects
• Impact on Performance, Reliability, Lifetime
• Hot spots (static/dynamic) and thermal stress can lead to failure
• Strategy to deal with Very High Heat Flux (VHHF) Transients required
*2002 AMD analyst conference report @ AMD.com
Channel length 130 nm 90 nm 65 nmDie Size 0.80 cm2 0.64 cm2 0.40 cm2Heat flux 63 W/cm2 78 W/cm2 125 W/cm2
Applications
• Several potential applications of microdevices– Example: Micro air sampler to check for
contaminants– Breath analyzer, Micro-thruster, Fuel cells– Other innovative devices
• Relevant to space-crafts, vehicle re-entry, vacuum appliances, etc
5
Introduction
Flow regimes Kn Range Flow Model
Continuum flow Kn < 10-3 Navier-stokes equation with no slip B. C.
Slip flow 10-3 ≤ Kn < 10-1 Navier-stokes equation with slip B. C.
Transition flow 10-1≤ Kn < 10 Navier-stokes equation fails but iner-
molecular collisions are not negligible
Free molecular flow 10 ≤ Kn Intermolecular collisions are negligible as
compared to molecular collision to the
wall
Gas flow mostly in slip regime – regime of our interest
KnLλ
=Knudsen numberλ is mean free path of gas
(~ 70 nm for N2 at STP)L is characteristic dimension
Kn ~ 7x10-3 for 10 micron channel
Introduction
Maxwell’s model (1869)
Sreekanth’s model (1969)
2slip
wall
duudy
σ λσ−⎛ ⎞= ⎜ ⎟
⎝ ⎠
22
1 2 2λ λ= − −slipwall wall
du d uu C Cdy dy
How to determine the slip velocity at the wall?
σ is tangential momentum accommodation coefficient (TMAC) = fraction of molecules reflected diffusely from the wall
C1, C2: slip coefficients
7
Maxwell’s Slip Model
Specular reflection(τr = τi; σ = 0)
Diffuse reflection(τr = 0; σ = 1)
i r
i
τ τστ−
=
τi = tangential momentum of incoming molecule
τr = tangential momentum of reflected molecule
In practice, fraction of collisions specular, others diffuse
σ is fraction of diffuse collisions to total number of collisions
2slip
wall
duudy
σ λσ−⎛ ⎞= ⎜ ⎟
⎝ ⎠
Introduction
• New effects with gases– Slip, rarefaction, compressibility
• Continuum assumptions valid with liquids– Conventional wisdom should apply– Similar behavior possible at nano-scales
9
Outline and Scope• Flow in straight microchannel
– Analytical solution– Experimental data
• Applicability of Navier-Stokes to high Knudsen number flow regime?
• Flow in complex microchannels (sudden expansion / contraction, bend)– Lattice Boltzmann simulation– Experimental data
• Conclusions
10
Solution for gas flow in microchannelAssumptions:Flow is steady, two-
dimensional and locally fully developed
Flow is isothermalx, u
y, v
Governing Equations:2( )
.
wAdp dx d u dA
p RTp Kn const
τ ρ
ρ
− − =
==
∫ Integral momentum equationIdeal gas law
From,2RT
pμ πλ =
11
Solution for gas flow in microchannel(contd.)
Boundary conditions:
0
0
slipu u
p pKn Kn
=
=
=
at y = 0, H
} specified at some reference location, x = x0Solution procedure:1. Assume a velocity profile (parabolic in our case)
2. Assume a slip model (Sreekanth’s model in our case)
3. Integrate the momentum equation to obtain p
4. Obtain the streamwise variation of u
5. Obtain v from continuity12
Solution for gas flow in microchannel(contd.)
Velocity profile:2 2
1 22
1 2
/ ( / ) 2 8( , ) ( )1/ 6 2 8
y H y H C Kn C Knu x y u xC Kn C Kn
− + +=
+ +
Pressure can be obtained from:
{
22
1 0 2 00 0 0
2 2 20 0 01 0 2 0
20
2 2 2 21 2 1 1
1 24 1 96 log
2 Re 12 1 24 [ 1] log } 48Re
2 2Re ; ;
1/ 30 2 / 3 8 / 3 4 321
o
p p pC Kn C Knp p p
p p x xpC Kn C Knp p p H
where
uH Kn
C Kn C Kn C Kn Cu dAA u
βχ β
ρ βμ π
χ
⎛ ⎞ ⎛ ⎞ ⎛ ⎞− + − + +⎜ ⎟ ⎜ ⎟ ⎜ ⎟
⎝ ⎠ ⎝ ⎠ ⎝ ⎠⎛ ⎞⎛ ⎞ ⎛ ⎞ −
− + − − = −⎜ ⎟⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠ ⎝ ⎠
= =
+ + + +⎛ ⎞= =⎜ ⎟⎝ ⎠∫
3 2 42 2
2 21 2
64(1/ 6 2 8 )
C Kn C KnC Kn C Kn
++ +
13
Solution for VelocityStreamwise velocity continuously increases –flow accelerates
Pressure drop non-linear
Lateral velocity is non-zero (but small in magnitude)
Normalized Volume Flux versus Knudsen Number
First-order slip-model fails to predict the Knudsen minima
Navier-Stokes equations seems to be applicable to high Kn with modification in slip-coefficients
Dongari et al. (2007) Analytical solution of gaseous slip flow in long microchannels. International Journal of Heat and Mass Transfer, 50, 3411-3421
Extension of Navier-Stokes to high Knudsen number regime
15
Questions?
• Can we extend this approach to other cases?
• What are the values of slip coefficients? – or tangential momentum accommodation
coefficient (TMAC)
• Why does this approach work at all?
12C σσ−
=
Extension of Navier-Stokes to high Knudsen number regime (contd.)
Idea extended to
- Gas flow in capillary (Agrawal & Dongari, IJMNTFTP, 2012)
- Cylindrical Couette flow (Agrawal et al., ETFS, 32, 991, 2008)
Conductance versus Kn
Experimental Data and Curve Fit to Data by Knudsen (1950)
Extension of Navier-Stokes to high Knudsen number regime (contd.)
N. Dongari and A. Agrawal, Modeling of Navier-Stokes Equations to High Knudsen Number Gas Flow. International Journal of Heat and Mass Transfer, 55, 4352-4358, 2012
( )( )e
e yy μλμλ
=Normalized volume flux versus inverse Knudsen number
Pr: Pressure ratio
0.4
0.6
0.8
1
0 0.1 0.2 0.3 0.4 0.5
U/U
cent
relin
e
y/H
Present model
DSMC (Karniadakis et al. 2005)Kn = 1
Normalized Velocity Profile
Determination of Tangential Momentum Accommodation Coefficient (TMAC)
What does TMAC depend upon?Surface material - For same pressure drop, will flow rate in glass
& copper tubes be different?Gas - Will nitrogen and helium behave differently, in
the same material tube?Surface roughness and cleanlinessTemperature of gasKnudsen number
Three main methods to determine TMAC:Rotating cylinder methodSpinning rotor gauge methodFlow through microchannels
19
Specular reflection(τr = τi; σ = 0)
Diffuse reflection(τr = 0; σ = 1)
Recall
TMAC versus Knudsen number for monatomic & diatomic gases
Monatomic Diatomic: 0.71 log(1 )Knσ = − +
• TMAC = 0.80 – 1.02 for monatomic gases irrespective of Kn and surface material– Exception is platinum where σmeasured = 0.55 and σanalytical = 0.19– TMAC = 0.926 for all monatomic gases
• TMAC = 0.86 – 1.04 for non-monatomic gases – Correlation between TMAC and Kn– Different behavior for di-atomic versus poly-atomic gases?
• TMAC depends on surface roughness & cleanliness– Effect (increase/ decrease) is inconclusive
• Temperature affects TMAC– Effect is small for T > room temperature
( )7.01log1 Kn+−=σ
Summary of Tangential Momentum Accommodation Coefficient
Agrawal et al. (2008) Survey on measurement of tangential momentum accommodation coefficient. Journal of Vacuum Science and Technology A, 26, 634-645
Boltzmann Equation
5 October 201322
O(Kn) Resulting Equations0 Euler Equations1 Navier Stokes Equations2 Burnett Equations3 Super Burnett Equations
Generalized Equations
5 October 201323
Burnett Equations
5 October 201324
Higher Order Continuum Models
• Have been applied to Shock Waves • Here, want to apply to Micro flows
Conventional Burnett Equations (1939)
BGK-Burnett Equations (1996,1997)
Super Burnett Equations Augmented Burnett Equations (1991)
Grad’s Moment Equation R13 Moments Equation
5 October 201325
Ref: Agarwal et al. (2001) Physics of Fluids, 13:3061–3085; Garcia-Colin et al. (2008) Physics Report, 465:149–189
Analytical Solution from Higher Order Continuum Equations
5 October 201326
Numerical Solution of Higher-Order Continuum Equations
• Agarwal et al. (2001): Planar Poiseuille flow;Convergent solution till Kn = 0.2; solution diverged beyond it
• Xue et al. (2003): Couette flow; Kn < 0.18• Bao & Lin (2007): Planar Poiseuille flow; Kn <
0.4 • Uribe & Garcia (1999); Planar Poiseuille flow;
Kn < 0.1
5 October 201327
Present Approach• Start with the solution of the Navier-Stokes
equations; • Substitute the solution in the Burnett equations
and evaluate the order of magnitude of additional terms in the Burnett equations;
• Identify the highest order term and augment the governing equation with this additional term, and re-solve the governing equations;
• Repeat above two steps, till convergence is achieved.
5 October 201328
Solution of the Equation
5 October 201329
Results
Error (in %) versus Knudsen number as given by Burnett equation and Navier-Stokes at the wall (C1 = 1.066, C2 = 0.231; Ewart et al. (2007))
5 October 201330
Slip Velocity
0.2
0.4
0.6
0.8
0 0.2 0.4 0.6 0.8 1
Us/U
in
x/L
DSMC (Karniadakis et. al. 2005)
Burnett Analytical Solution
Knout = 0.20Maout = 0.212
Pr = 2.28
Slip velocity comparison with DSMC data (Karniadakis et al. 2005)
N. Singh, N. Dongari, A. Agrawal (2013) Analytical solution of plane Poiseuille flow within Burnett hydrodynamics, Microfluidics and Nanofludics, to appear
Complex Microchannels
32
Gas – Nitrogen (Helium to a limited extent)
Developing length is 10% greater than required from calculations
Test section for flow through sudden expansion / contraction
Test section for flow through 90º bend
Lattice Boltzmann SimulationMicrochannel with Sudden Contraction or Expansion
x/L
P/P o
0 0.25 0.5 0.75 11
1.5
2
2.5
3
x/w
y/w
46 48 50 52 540
1
2
x/w
y/w
46 48 50 52 540
1
2
1. Pressure drop in each section follows theory
2. Secondary losses are small
3. Limited transfer of information in microchannel
Possible to understand complex microchannels in terms of primary units?
Agrawal et al. "Simulation of gas flow in microchannels with a sudden expansion or contraction," Journal of Fluid Mechanics, 530,135-144, (2005). 33
34
Experimental SetupVacuum pumpRotary with speed of 350 lpmMaximum vacuum 0.001 mbar
MFC0 - 20 sccm0 - 200 sccm0 - 5000 sccm
APT0 – 1 mbar0 – 100mbar0 – 1 bar
1
23
4567
8
To vacuum system
To display 1 – Gas cylinder
2 – Pressure regulator
3 – Particle filter
4 – Mass flow controller
5 – Inlet reservoir
6 – Test section
7 – Outlet reservoir
8 – Absolute pressure transducer
The leakage is ensured to be less than 2 % of the smallest mass flow rate. The absolute
static pressure along the wall is measured for different mass flow rates of N2 at 300 K.
0.1 – 45 mbar
4 – 5000 sccm
N2Flow parameters range
Re = 0.2 – 837
Kno = 0.0001 – 0.0605
Validation(Flow in tube)
Kn
fRe
10-4 10-3 10-2 10-1 100 1010
10
20
30
40
50
60
70
Sreekanth (Brass-N2)Ewart ( Fused slilica- N2)Ewart ( Fused slilica- He)Ewart ( Fused slilica- Ar)Brown ( Copper- Air)Brown ( Copper- H2)Brown ( Iron- Air)Choi ( Fused slilica- N2)Tang ( Fused silica- N2)Tang ( Fused silica- He)Demsis (Copper- N2)Demsis (Copper- O2)Demsis (Steel- O2)Demsis (Steel- Ar)
Present} Verma et al. Journal of Vacuum Science and Technology A, 27(3) 584-590 (2009).
641 14.89
fKn
=+
Re
11 different gas-solid combinations; Knrange 0.0004-2.77 and Re range of 0.001-227
fRe for different combinations of gas-surface material seems small as compared to experimental scatter.
35
Experimental setup and test section geometry for sudden expansion
36
Figure 1a. Schematic diagram of the experimental set up
Figure 1b: Test section geometry for sudden expansion
Parameter Maximum uncertainty
Mass flow rate ± 2% of full scale
Absolute pressure ± 0.15 % of the reading
Diameter ± 0.1 %
Reynolds number ± 2 %
Knudsen number ± 0.5 %
Pressure loss coefficient ± 6 %
Temperature ± 0.3 K
Flow parameters rangeRe = 0.2 – 837Kno = 0.0001 – 0.075
Flow through tube with sudden expansion
37
Oliveira and Pinho (1997) noted separation at Re = 12.5 (AR = 6.76) for liquid flow
Goharzadeh and Rodgers (2009) noted separation at Re = 100 (AR = 1.56) for liquid flow
No flow separation at the junction
Static pressure variation along wall for area ratio = 64
X/L
P/P
o
0 0.25 0.5 0.75 10
2
4
6
8
10
12Knj = 0.036Knj = 0.007Knj = 0.003Knj = 0.001
Sudden Expansiond = 4 mm, D = 32 mmAR = 64, Nitrogen
PoPi
Junction
Radial pressure variation
38
local radius/radius of smaller section
(P/P
w) st
atic
0 1 2 3 4 5 6 7 80.95
0.96
0.97
0.98
0.99
1
1.01
1.02
Knj = 0.036, Just upstream of junctionKnj = 0.036, Just downstream of junctionKnj = 0.001, Just upstream of junctionKnj = 0.001, Just downstream of junction
Sudden ExpansionAR = 64, Nitrogen
(c)Radial static pressure variation just upstream and just downstream of the junction for AR = 64.
Comparison with straight tube
39
Static pressure variation (normalized with the outlet pressure) along the wall.
X/L
P/P
o
0 0.25 0.5 0.75 10.9
1
1.1
1.2
1.3
1.4
1.5
StraightSE
Sudden Expansion, AR = 12.43Res = 11.3, Knj = 0.0112, Nitrogen
Junction
Lu Ld
Variation in Lu and Ld
40
Variation in Ld (normalized with smaller section tube diameter d) versus Knudsen number at junction (Knj).
KnjL d
/d
0 0.01 0.02 0.03 0.04 0.05 0.06 0.070
1
2
3
4
5
6
7
8
AR = 3.74AR = 12.43AR = 64Curve fit
Sudden ExpansionNitrogen
Ld/d = AR1.35/(400*Knj
0.45)
Variation in Lu (normalized with smaller section tube diameter d) versus Knudsen number at junction (Knj).
Knj
L u/d
0 0.01 0.02 0.03 0.04 0.05 0.06 0.070
5
10
15
20
25
30
35
AR = 3.74AR = 12.43AR = 64Curve fit
Sudden ExpansionNitrogen
Lu/d = 15.45(AR×Knj)0.52
Velocity profile
41
(a)
Straight tubeSudden expansion
Analysis of velocity distribution
42
p1, p2, known from experimental measurements, Need velocity distribution for calculating shear stress and momentum M1 and M2 .
Assuming velocity distribution as second order parabolic velocity profile
The mass flow rate can be calculated as
At r = R, velocity at the wall u = us
1 2 w 2 1(p p )A Ddx [M M ]τ π− − = −
2crau +=
∫=R
rdrum0
2πρ
Streamlines near sudden expansion junction
43
X/L
Tube
radi
us(m
)
0.45 0.5 0.55 0.6 0.650
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
99 %95 %90 %70 %50 %30 %10 %5 %1 %0.5%
Sudden Expansiond = 9.5 mm, D = 33.5AR = 12.43, NitrogenRes = 2.8, Knj = 0.03
% of mass flow rate
Streamlines near the junction for rarefied gas flow.
Streamlines near sudden expansion junction
44
Varade et al.; JFM (under review)
Bend Microchannel(Lattice Boltzmann Simulations)
Streamlines near the bend for Kn = 0.202
Presence of an eddy at the corner for Re = 2.14!
Mass flow rate
Mass flux in bend to mass flux in straight
Mass flux in bend can be up to 2% more than that in straight under same condition
White et al. (2013) found same result using DSMC
Agrawal et al. Simulation of gas flow in microchannels with a single 90 bend. Computers & Fluids 38 (2009) 1629–1637
Experimental data for bend
Test section for flow through 90º bend
Heat Transfer Coefficient(Experimental setup)
To vacuum pump
2 3 4 5
1
6
7
Port for Inlet Temperature probe (Tci)
Hot Water Inlet (Thi)
Port for Outlet Pressure gauge (Po)
Nitrogen Outlet
Port for Outlet Temperature probe (Tco)
Water Outlet (Tho)
Port for Inlet Pressure gauge (Pi)
Nitrogen Inlet
Nusselt number measurements(Circular tube with constant wall temperature BC)
Knm
Nu
10-4 10-3 10-2 10-10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Re range (0.5-124.3)
Re
Nu
0 50 100 1500
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Knm range 0.01-10-4
Nu versus Kn
Nu versus Re
Gr / Re2 < 10-4
Comparison against theoretical values (at Kn = 0.01)
0.00166Present (Experimental)8.58Hooman [21]3.55Aydin and Avci [20]3.55Tunc and Bayazitoglu [12]3.75Ameel et al. [10]3.6Larrode et al. [5]NuSource
Our experimental data predicts 3 orders of magnitudesmaller value as compared to theoretical analysis!!
Kn
Nu
10-3 10-2 10-110-4
10-3
10-2
10-1
Run 1Run 2Run 3Run 4
Validation & Repeatability
1125.028.084201238.334.310870
Percentage difference
Nu from the present
experiment
Nu from Dittus-Boelter
Correlation
Re
Why is heat transfer coefficient so small?
• Rarefaction reduces ability of gas to remove heat from wall
• Temperature jump at wall
• Radial convection may be important (i.e.,term comparable to ), but ignored in theoretical analysis
• Reason for difference is still not fully understood
wT
Twg n
TKnTT ⎟⎠⎞
⎜⎝⎛∂∂
⎥⎦
⎤⎢⎣
⎡+
−=−
Pr122γγ
σσ
rTv ∂∂
xTu ∂∂
Conclusions• Possible to derive solution for microchannel flow
for higher-order slip-model• Possible to extend the applicability of N-S
equations by modification of slip coefficients• TMAC of monatomic gas = 0.93 (independent of
temperature and surface and Kn)• Flow in complex microchannel exhibits non-
intuitive behavior
AcknowledgementsColleagues: Dr. S. V. Prabhu, Dr. A.M. Pradeep
Students: Nishanth Dongari, Abhishek Agrawal, Vijay Varade, Bhaskar Verma, Gaurav Kumar, Anwar Demsis
Funding: IIT Bombay, ISRO
Effect of sudden expansion on velocity distribution
55
Rate of increase in shear stress and momentum is more in sudden expansion than in
straight tube towards the expansion junction
Shear stress variation along the wall Momentum variation along the length
Varade et al. (2012) (to be submitted)
Maxwell’s Slip Model
Specular reflection(τr = τi; σ = 0)
Diffuse reflection(τr = 0; σ = 1)
i r
i
τ τστ−
=
τi = tangential momentum of incoming molecule
τr = tangential momentum of reflected molecule
In practice, fraction of collisions specular, others diffuse
σ is fraction of diffuse collisions to total number of collisions
2slip
wall
duudy
σ λσ−⎛ ⎞= ⎜ ⎟
⎝ ⎠
DSMC Simulations(Couette flow)
Slip from DSMC/ Slip from Maxwell model
versus
Knudsen number
57
DSMC Simulations(Couette flow)
Slip from DSMC/ Slip from Maxwell model
versus
Knudsen number
58
x/L0 0.25 0.5 0.75 1
Kn
Kn
Microchannel with Sudden Contraction or Expansion
(Pressure distribution)
x/L
P/P o
0 0.25 0.5 0.75 11
1.5
2
2.5
3
Microchannel with sudden Contraction or Expansion
(Velocity distribution)
x/L
U
0.48 0.49 0.5 0.51 0.52
0.02
0.03
0.04
0.05
0.06
x/L
U
0 0.25 0.5 0.75 10.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
Kn
What do we learn?
1. Good agreement with theory
2. Pressure drop in each section follows theory
3. Secondary losses are small
4. Compressibility and rarefaction have opposite effects
5. Limited transfer of information in microchannel
Possible to understand complex microchannels in terms of primary units?
TMAC versus Knudsen number
62
Flow Rate Vs Kn
5 October 201363
0
2
4
6
0.01 0.1 1 10
G
Kn
Boltzmann Solution
Exp. data (Pr = 5)
Exp. Data (Pr = 4)
Exp. data (Pr = 3)
Burnett Analytical Solution (C1 = 1.466, C2 = 0.5)
Figure 2. Comparison of proposed sol. with the exp. data of Ewart et al.(2007) and the sol. of linearized Boltzmann equation by Cercignani et al.(2004). (C1 = 1.1466 and C2 = 0.5, Chapman & Cowling (1970).
Introduction (contd.)(Compressibility)
For flow to be compressible: (Gad-el-Hak, 1999)
i.e., both spatial and temporal variations in density are small as compared to the absolute density
Compressibility effect is important because of large pressure drop in microchannel
1 0DDtρ
ρ=
Gas versus liquid flow
• New effects with gases– Rarefaction, slip, compressibility
• Continuum assumptions valid with liquids– Conventional wisdom should apply– Similar behavior possible at nano-scales
Velocity measurement in rarefied gas flow through tube
29 - February - 2012 Study of rarefied gas flow through microchannel 66
For low Re the magnitude of viscous forces are comparable with inertia forces hence
Bernoulli formula needs correction
Schematic diagram of experimental set up
221 u
PPCp soρ−
=
Stagnation pressure correction coefficient
Velocity measurement in rarefied gas flow through tube
29 - February - 2012 Study of rarefied gas flow through microchannel 67
Author Cp
(Pitot tube
geometry)
Re range (Re is based on
pitot tube ID)
Remark
Homann (1952)1+[8/(Re+0.64Re1/2)]
Cylinder3.2 - 120 Experimental
Hurd et al. (1953)11.2/Re
Circular blunt nosed1.7 - 100 Experimental
Lester (1961)2.975/Re0.43
Circular1- 10 Numerical
Chebbi and
Tavoularis (1990)
4.2/Re
Circular< 1 Experimental
Velocity measurement in rarefied gas flow through tube
29 - February - 2012 Study of rarefied gas flow through microchannel 68
Variation in Cp with Re (based on Pitot tube ID) Variation in central velocity with Re (based on Pitot tube ID)
Velocity measurement in rarefied gas flow through tube
29 - February - 2012 Study of rarefied gas flow through microchannel 69
% deviation in central velocity with reference to Sreekanth (1969)
Velocity measurement in rarefied gas flow through tube
29 - February - 2012 Study of rarefied gas flow through microchannel 70
(a) (b)
(c) (d)Comparison of experimental velocity measurement with Sreekanth (1969), Homann’s correction
Velocity measurement in rarefied gas flow through sudden expansion using Pitot tube
29 - February - 2012 Study of rarefied gas flow through microchannel 71
Continuum flow
Velocity measurement in rarefied gas flow through sudden expansion using Pitot tube
29 - February - 2012 Study of rarefied gas flow through microchannel 72
Slip flow