This document summarizes a CFD analysis of a new spiral microreactor design for catalytic propane combustion. The analysis showed that the spiral design improved heat recirculation and stability over straight channel designs. Higher inlet velocities, thermal conductivity, number of turns, and equivalence ratios led to higher combustion temperatures and conversion. The spiral microreactor showed better performance than planar helical and U-bend microreactor designs in terms of operating range and maximum temperature.

1. CFD Analysis of New Heat Recirculating
Microreactor for Propane Catalytic Combustion
Amit Kunte and Niket Kaisare
Department of Chemical Engineering
Indian Institute of Technology Madras

2. Motivation
• Hydrocarbons have high gravimetric and volumetric energy density
• Catalytic combustion of hydrocarbons in micro-channels
 Energy generation
 Provide energy for fuel processing, reforming and synthesis
 Micro-propulsion
• Sustaining combustion at small channels is challenging
• “Excess Enthalpy” microburners show improved stability

3. Objectives
• Introduce new “Excess Enthalpy” geometry – Spiral Microburner
Y. Ju and K. Maruta. (2011) Progress in Energy and Combustion Science

4. Objectives
• Introduce new “Excess Enthalpy” geometry – Spiral Microburner
• Analyze extinction / blowout in spiral microburner using CFD
• Analyze thermal behavior of the PHR vis–à–vis ks , Φ , uo, and N.
• Compare with straight channel microburner
• Compare with the countercurrent heat recirculation ‘U-Bend’
combustor
Y. Ju and K. Maruta. (2011) Progress in Energy and Combustion Science

5. Super-adiabatic combustion in Planar Helical Micro-reactor
900
1100
1300
1500
1700
1900
2100
2300
0.25 0.35 0.45 0.55 0.65 0.75 0.85 0.95
Adiabaticreactiontemperature(K)
Equivalence ratio
Super-adiabaticity
~200K

6. Proposal for a “New” Geometry: Spiral Microburner
Pt washcoat on internal walls
Outlet channels shield reaction zone
Simpler than Swiss Roll (double-spiral)
Only geometry with heat recirculating
channel co-current to reaction channel

7. Model Description
• 2D CFD laminar model solved in Fluent
• Mass inlet at center; pressure outlet at periphery
• 600 μm channels; 200 μm walls; ~2.6 cm linear length
• Propane catalytic combustion (Deshmukh and Vlachos, 2007) as
UDF

8. Temperature contours Conversion Contours
1200 K
300 K
600 K
900 K
k = 1 W/mK
h = 10 W/m2K
u0 = 0.5 m/s
Φ = 0.65
100 %
80 %
60 %
40 %
20 %
0 %

9. Axial temperature and Mass fraction profiles
300
500
700
900
1100
1300
Temperature(K)
channel centerline
wall 1
wall 2
0 200 400 600
Massfraction
Dimensionless axial location
0.04
0.03
0.02
0.01
Turn1 Turn 2 Turn 3
a
uo = 0.5 m/s
b

10. Comparison between the 2D and the 3D temperature
contours at the inlet
2D 3D c/s

11. Effect of inlet velocity (uo)
1150
850
638
300
1553
1180
676
300
683
530
415
300
uo= 0.5 m/s
k = 1 W/mK
h = 10 W/m2K
Φ = 0.65
uo= 0.11 m/s uo= 6 m/s

12. Axial propane and mass fraction profiles at different uo
uo = 0.11 m/s uo = 6 m/s
0 200 400 600
Massfraction
Dimensionless axial location
300
400
500
600
700
Temperature(K)
channel centreline
wall 1
wall 2
0.04
0.03
0.02
0.01
Turn 1 Turn 2 Turn 3
0
0.01
0.02
0.03
0.04
0 100 200 300 400 500 600
Massfraction
Dimensionless axial location
300
600
900
1200
1500
1800
Temperature(K)
channel centreline
wall1
wall2
Turn 1
Turn 2
Turn 3
k = 1 W/mK
h = 10 W/m2K
Φ = 0.65

13. Transverse temperature profiles at different uo
uo = 0.5 m/s uo = 0.11 m/suo = 6 m/s

14. Axial catalytic reaction rate profiles at different Φ

15. Axial location of max reaction rate at various uo
0
100
200
300
0.1 0.5 2.5 12.5
Dimensionlessaxiallocation
Inlet velocity, uo (m/s)
ф=0.65
ф=0.45
ф=0.35
Extinction
Blowout

16. Effect of thermal conductivity (ks)
ks = 1 W/mK ks = 10 W/mK ks = 50 W/mK
uo = 0.5 m/s
h = 10 W/m2K
Φ = 0.65
1150
850
638
300

17. Axial temperature profiles at various ks
ks = 50 Wm/K
300
500
700
900
1100
0 100 200 300 400 500 600
Temperature(K)
Dimensionless axial location
channel centerline
wall 1
wall 2
Turn 1
Turn 2
Turn 3
300
500
700
900
1100
0 100 200 300 400 500 600
Temperature(K)
Dimensionless axial location
channel centreline
wall 1
wall 2
Turn 1
Turn 2
Turn 3
ks = 1 Wm/K uo = 0.5 m/s
h = 10 W/m2K
Φ = 0.65

18. Axial catalytic reaction profiles at different ks
0 100 200 300 400 500 600
Reactionrate(mol/m2s)
Dimensionless axial location
ks= 1W/mKTurn 1
Turn 2
Turn 3
ks= 10 W/mK
ks= 50 W/mK
0
0.0025
0.005
0.0075
0.001
0.0125
0.015

19. Axial location of max reaction rate at various ks
0
100
200
300
0.1 0.5 2.5 12.5
Dimensionlessaxiallocation
Inlet velocity, uo (m/s)
ks=50
ks= 10
ks= 1
Extinction
Blowout
Re >750

20. Stability plot (Φ vs uo ) at various ks
0.05
0.5
5
0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9
Inletvelocity,uom/s
Equivalence ratio (ф)
Re > 750

21. Effect of uo on peak temperature and conversion at different ks
30
40
50
60
70
80
90
100
0.05 0.5 5
Percentageconversion
(%)
Inlet velocity, uo (m/s)
ks= 10 W/mK
0.210.160.11
500
750
1000
1250
1500
1750
Temperature(K)
ks= 50 W/mK
ks= 1 W/mK
9.4 m/s
6 m/s
a
b
Re > 750

22. Effect of Number of turns (N)
1070
827
610
300
1150
850
638
300
1190
876
650
300
N=2 N=3 N=4k = 1 W/mK
h = 10 W/m2K
u0 = 0.5 m/s
Φ = 0.65

23. Stability plot (Φ vs uo ) at various no. of turns (N)
0.05
0.5
5
0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9
Inletvelocity,uom/s
Equivalence ratio (Φ)

24. Effect of uo on peak temperature and conversion at different N
N=
3
65
75
85
95
0.05 0.5 5
Percentageconversion(%)
Inlet velocity, uo (m/s)
N=4
N=3
b
0.1
0.12
500
750
1000
1250
1500
1750
Temperature(K) N=2
N=4
a
N=3
N=2
0.095
4 m/s
6
7.9

25. Axial location of max reaction rate at various N
0
100
200
300
400
500
0.1 0.5 2.5 12.5
Dimensionlessaxiallocation
Inlet velocity, uo (m/s)
N=2
N=3
N=4
Extinction
Blowout

26. Comparison with SCR (ceramic materials, 1 W/m/K)
0.05
0.5
5
50
0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9
Inletvelocity(uo)m/s
Equivalence Ratio (ф)
Tmax > 1500K
operating region SCRoperating region PHR
conversion < 99%

27. Effect of Φ and uo on peak temperature and conversion

28. Comparison with UBR (ceramic materials, 1 W/m/K)
0.05
0.5
5
0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9
Inletvelocity(uo)m/s
Equivalence Ratio(ф)
U-bend microburner
Re > 750

29. Effect of Φ and uo on peak temperature and conversion

30. Comparison of 2D and 3D extinction limits
0
0.1
0.2
0.3
0.4
0.5
0.6
0.3 0.4 0.5 0.6 0.7 0.8 0.9
Inletvelocity,uo(m/s)
Equivalence ratio
3D PHR
2D PHR

31. Conclusion
• Spiral microburners show significant heat recirculation effect and
superadiabaticity
• Higher stability than straight channel (though lesser blowout stability than U-
Bend)
• Good propane breakthrough behavior
• Better operating characteristics compared to SCR