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Prediction of heat and mass transfer in canister filters
Tony SmithS & C Thermofluids Limited PHOENICS User Conference
Melbourne 2004
Co-authors - Martin Smith, Dstl, Porton Down
Kate Taylor, S & C Thermofluids
Overview• Introduction to S & C Thermofluids• Canister filters • Porous media modelling• Voidage distribution • Geometry • Pressure drop calculation • Adsorption • Results • Conclusion and recommendations
S & C Thermofluids
• Formed in 1987• Research into fluid (gas/liquid) flow
and heat transfer • Based in BATH, U.K.
www.thermofluids.co.uk
S & C Thermofluids
Use combination of analysis (mainly CFD) and experimental validation and demonstration
RR Gnome engine test rig
CFD prediction of Gnome exhaust
Experimental facilities
• RR Gnome turbojet and turboshaft engines
• Universal jet flow rig• Water tunnel • JPX turbojet• Ejector performance test
rig• Catalyst research engines
CFD modelling
• External aerodynamics• Propulsion system
(nozzle flows)• Exhaust plume mixing • Exhaust reactions• Interactions • Catalytic converters • Filters
From vacuum cleaners to supersonic aircraft
From green houses to nuclear reactors
From leaf blowers to rockets
Canister filters for respirators
Drivers for porous media modelling
• Pressure drop• Flow distribution • Performance
– Adsorption– Break-through– Conversion (reactions) – Minimise use of materials
Modelling approach
Typical filter monolith
Porous media such as catalytic converters and packed bed filters often contain very high surface areas which are difficult to represent in detail whilst modelling the bulk flowfield
Modelling philosophy• Continuum
approach – macroscopic
model of complete system
• Single channel – detailed model of
one flow path
Continuum methodology
• Solving gas and solid (adsorbed) species separately but within the same computational space with mass transfer
• Gas and solid energy can also solved separately with heat transfer
• This methodology has been described in earlier papers relating to filter performance prediction
Canister geometry
Air Flow
Impregnated granular
activated carbon
Glass Fibre
Filter
VOIDAGE DISTRIBUTION IN CYLINDRICAL FILTER BEDS
• Radial voidage distribution in ‘snowstorm’ packed filter beds is a function of the ratio:
particle size/bed diameter• Affects the velocity distribution within the filter bed• Measurements made of voidage distribution for
range of particle sizes• Fitted to modified ‘Mueller’ model
Voidage distribution
Radial voidage distribution - 4mm beads
0
0.2
0.4
0.6
0.8
1
1.2
0
1.9
6
3.9
2
5.8
8
7.8
4
9.8
11
.8
13
.7
15
.7
17
.6
19
.6
21
.6
23
.5
26
.5
30
.4
34
.3
38
.2
42
.1
46
.1distance from the edge of the bed
Vo
ida
ge
= b + (1-b)e-brJo(ar*)
Geometry
• Canister key dimensions converted to FEMGEN geometry input
• Mesh generated in FEMGEN
• Output as PHOENICS 2D, axisymmetric BFC mesh using Phirefly
• PHOENICS Q1 file written out
Voidage distribution - canister
• Grid fixed by geometry
• Voidage calculated locally according to modified Mueller equation
• Voidage set in ground coding
radial distribution of void fraction
0.00E+00
1.00E-01
2.00E-01
3.00E-01
4.00E-01
5.00E-01
6.00E-01
7.00E-01
8.00E-01
9.00E-01
0.00E+00 1.00E+01 2.00E+01 3.00E+01 4.00E+01 5.00E+01 6.00E+01
radius (mm)
Pressure drop
• Local voidage distribution coupled to Ergun-Orning equation for pressure loss through bed:
p/L = 5 So2(1-)2U/3 + 0.29 So(1-)U2/3
| |
viscous loss turbulent loss
• This pressure drop is applied to both axial and radial velocities
• Earlier work using this equation have given rise to good agreement with experimental data for pressure drop.
Pressure drop
Predicted pressure drop 275Pa
Measured pressure drop 110Pa
Filter paper section pressure drop 40Pa
Flowrate 30l/min
Adsorption model
• Transient model to predict ‘breakthrough’• Steady state flowfield used as initial
conditions• Adsorption rate source term:
-C/t = 1/ So k (C - Ci)
• Sh = 1.15 (Rep/)0.5 Sc0.33 for Rep >1• Sh = k dp/D
Adsorption model
• Rate of uptake in adsorbent: m/t = /(1-) (-C/t)/z
• Maximum uptake from isotherm equation• Cumulative uptake is calculated :
m/t. /t • Uptake value stored
• Interface concentration Ci set to be in local equilibrium with uptake value
Velocity distribution
Predicted uptake of contaminant after 10
minutes
Conclusions • A CFD model of a canister filter has been
produced• The model provides predictions of
pressure drop, flow distribution and adsorption in transient conditions
• The model uses PHOENICS as the main solver with additional ground coding for voidage distribution, pressure drop and adsorption
Conclusions (2)
• FEMGEN is used to create the BFC grid for use in PHOENICS
• Pressure drop predictions show some discrepancy with measurement – unlike earlier packed bed filter work
• Early predictions of contaminant adsorption look realistic but require validation
Recommendations • Investigate pressure drop prediction
discrepancies • Improve adsorption model • Include heat of adsorption • Provide axial variations of voidage • Modify aspects of canister model (eg gap
at rear wall) • Provide full transient input of contaminant
concentration as well as flowrate• Provide validation
Acknowledgements
• Martin Smith, Dstl, Porton Down• Kate Taylor, S & C Thermofluids