CFD tools and lower order modeling for regenerative ...

CFD tools and lower order modeling for regenerative chambers with

gas recirculation system Ing. Alessandro Spoladore Prof. Carlo Cravero

Università di Genova

Facoltà di Ingegneria

DIME

LIFE12 ENV/IT/001020 - PRIME GLASS

Scientific Conference

Università di Genova – Savona Campus – January 24th 2017

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Computational techniques for the analysis of the strategic gas recirculation system

CFD

Lower order modelling

1. Regenerator general models

2. Single flow channel models

3. Strategic gas recirculation configuration models

1. Single chamber model

2. Regenerator system transient model

3. Radiation effect parametric tool




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The CFD analysis on regenerative system is focus on:• Calculate pressure drops in the stackers;• Calculate the temperature profile of the COLD and HOT flows;• Evaluate how different configurations of the gas recirculation system affect the system.

It is not feasible the direct use of CFD methods in the a real configuration of the chamberequipped with the stackers because of the enormous demand for computational resources,and then the RAM memory and the CPU. To this end the Ansys software provides differenttechniques to simplify the modeling and save resources.

CFD systematic approach to the regenerator system




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To limit the computational costs the checkers zone is emulatedby means of a porous domain, a mathematical tool, withwhich it is possible to associate to the selected zone specificssolid-to-fluid interaction laws, both in terms of pressure dropsthat in terms of heat exchange.

From the thermal point of view, two different type of porousmodels exists:

• the Equilibrium model

• the Non-equilibrium model.





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• In the Equilibrium model the thermal interaction between solid and fluid is treated by setting the specific thermal flux exchanged; it is therefore necessary to know some experimental data, such as mass flows and inlet/outlet temperatures of the flows. Consequently the latter can not be used directly in the design process.

• On the other hand, in the Non-Equilibrium model, the solid-fluid thermal interaction occurs by setting the wall temperature and the heat exchange coefficient profiles. In this case it is possible to characterize these quantities by means of a transient simulation of a single flow channel which requires only the knowledge of the flow inlet temperatures. The model is therefore suitable to be directly used in the design process.





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Single flow channel model

A single flow channel is the volumeformed between two adjacent bricks, forexample of the cruciform type.The transient analysis is obtained byalternating the air and fumes flows every20 minutes.

0

2

4

6

8

10

0 10 20 30 40

h [

W/m

2K

]

z/d

010203040506070

0 10 20 30 40

h [

W/m

2K

]

z/d

COLD phase HOT phase

Air phase Flue gas phase

Comparison of porous domain models




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CFD











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CFD simulation approach for strategic gas recirculation system design optimization

• The strategic gas recirculation system is a technique for the removal of pollutants in newor existing glass furnaces.

• The system has a bypass of flue gas that is fed into the regenerator operating with air(cold phase). The flue gas is channeled into the stackers and then it is introduced in thecombustion chamber.

• To ensure a good performance of the system is fundamental that the flue gas flowremains concentrated in the primary combustion zone of the combustion chamber.

By means of the CFD analysis, the recirculation system can be optimized and calibrated tobe coupled to a given furnace geometry.




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Case 1«short square duct»

Case 2«long square duct»

Case 3«divergent duct»

Examples of exhaust gas feeding system optimization design

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Systematic data post-processing methodology

121110987654321

0 5 10 15 20

S-1

S-2

S-3

S-4

S-5

S-6

S-7

S-8

S-9

S-10

S-11

S-12

Mass flowdistribution

Case 3

Case 2

Case 1

0% 10% 20% 30% 40% 50%

S-1

S-2

S-3

S-4

S-5

S-6

S-7

S-8

S-9

S-10

S-11

S-12

Exhaust mass fractiondistribution

Case 3

Case 2

Case 1

% % % % %




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CFD











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Thermal regenerators unsteady 1-D model

CFD Ansys-Fluent

SUB-SYSTEMs

Single channel models

Gas recirculation system models

• Heat transfer coefficients profile (convective or total) for the Hot and Cold periods

• Mass flow distribution

• Flue gas fraction distribution

Matlab-Simulink

MAIN System

• thermofluidodynamic properties;• H2O and CO2 emissivity model;• Energy balance transient solver;• «reverse cycle» algorithm;

Fluid and refractorytemperature profile

Regenerator systemperformance

Results




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CFD











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Regenerator chamber model

A single regenerator chamber is modelled as a solid and a fluid domain, eachone is divided in a fixed number of cells. For each cell is defined a transientenergy balance equation that describes the processes of heat exchange of thecell with the neighboring ones.

The model use the following assumptions:• fluids are a mix of N2,O2,CO2 and H2O;• fluid thermal properties variable with the chemical composition and

temperature;• no heat exchange between chamber and outside environment;• no mass flow infiltration or leak in the chamber casing.




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𝑚𝑠,𝑖𝑐𝑣𝑠,𝑖 𝑑𝑇𝑠,𝑖𝑑𝑡

=

𝑈𝑠𝑠,𝑖−1𝑆𝑢𝑟𝑠𝑠,𝑖−1 𝑇𝑠,𝑖−1 − 𝑇𝑠,𝑖 + 𝑈𝑠𝑠,𝑖+1𝑆𝑢𝑟𝑠𝑠,𝑖+1 𝑇𝑠,𝑖+1 − 𝑇𝑠,𝑖 +

+ 𝑈𝑠𝑓,𝑖𝑆𝑢𝑟𝑠𝑓,𝑖 𝑇𝑓,𝑖 − 𝑇𝑠,𝑖

𝑚𝑓,𝑖𝑐𝑣𝑓,𝑖 𝑑𝑇𝑓,𝑖

𝑑𝑡=

𝑚𝑓 𝑐𝑝𝑓,𝑖−1𝑇𝑓,𝑖−1 − 𝑐𝑝𝑓,𝑖𝑇𝑓,𝑖 + 𝑈𝑠𝑓,𝑖𝑆𝑢𝑟𝑠𝑓,𝑖 𝑇𝑠,𝑖 − 𝑇𝑓,𝑖

Energy balance equation for the ith fluid cell:

Energy balance equation for the ith solid cell:

Regenerator chamber model structure




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𝑈𝑠𝑓,𝑖 =1

1ℎ𝒄𝒐𝒏𝒗

+1

ℎ𝒓𝒂𝒅+𝑡ℎ𝑘𝑠

Regenerator chamber model structure

Total thermal trasmittance between ith solid and ith fluid cells:

𝑈𝑠𝑓,𝑖 =1

1ℎ𝒕𝒐𝒕

+𝑡ℎ𝑘𝑠

02468

10

h [

W/m

2K

]

Regenerator height0

20

40

60

80

h [

W/m

2K

]

Regenerator height

Active with the gas recirculation system only

Model with polynomicregressions of the Hottel’semissivity charts of H2Oand CO2.

CFD result CFD result

COLD Phase HOT Phase




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Regenerator chamber validation

The chamber model validation is made by the comparison with the CFD’s analysis results on a conventional regenerator system. The model is set to reproduce the Non Equilibrium steady model.

the same boundary conditions in the CFD for the COLD and the HOT phases are set :• the energy equation in solid is replaced by a constant linear temperature profile;• it is set a linear total heat transfer coefficient profile;• the characteristic geometrical parameters affecting heat transfer are given (heat

exchange surfaces, solid and fluid volumes……)




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0

10

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30

40

50

60

70

80

90

100

400 600 800 1000 1200 1400 1600

Re

gen

era

tor

he

igh

t %

Temperature [K]

Solid B.C.

Fluid-CFD Non Eq.

Fluid-Matlab Non Eq0

10

20

30

40

50

60

70

80

90

100

800 900 1000 1100 1200 1300 1400 1500

Re

gen

era

tor

he

igh

t %

Temperature [K]

Solid B.C.

Fluid-CFD Non Eq

Fluid-Matlab Non Eq

COLD phase (air only) HOT phase

Regenerator chamber validation

Outlet temperature difference <5%

Outlet temperature difference <1%




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CFD











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Regenerator system model

Switch every 20’

• The regeneration system model is constructed bycoupling two specular chambers, the first fed airwhile the second by exhaust gases.

• Every 20‘ the temperature profile of the solidmatrices is switched between the chambers.

• Each chamber is divided longitudinally into threezones to simulate different flow rates andchemical concentrations typical of the gasrecirculation system.




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Transient regenerator model applied to a conventional regenerator system

0

10

20

30

40

50

60

70

80

90

100

700 900 1100 1300 1500

Re

gen

era

tor

he

igh

t%

Temperature [K]

HOT phase

Flue gas initial

Flue gas final

Solid initial

Solid final

𝜼𝑻,𝒂𝒗𝒈 =𝑻𝒂𝒊𝒓,𝒐𝒖𝒕𝑻𝒈𝒂𝒔,𝒊𝒏

= 𝟎. 𝟖𝟖0

10

20

30

40

50

60

70

80

90

100

400 600 800 1000 1200 1400

Re

gen

era

tor

he

igh

t%

Temperature [K]

COLD phase

Air initial

Air final

Solid initial

Solid final




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0 % 10% 35%

25% 25% 50%

S-1 S-2 S-3

Flue gas mass fraction

Mass flow distribution

Transient regenerator model applied to the gas recirculation system

The regenerative chambers model is divided into three geometrically equal sectors.The three sectors of the COLD chamber work with different flow rates, flue gas fractions and inlet temperature in order tosimulate the effects of the strategic gas recirculation system.

0

2

4

6

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10

12

14

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18

0 10 20 30 40 50 60 70 80 90 100

Heattransfer

coef. COLD phase

[W/m2K]

Regenerator height %

h_tot (S-1)

h_tot (S-2)

h_tot (S-3)

h_rad (S-2)

h_rad (S-3)Radiative (S-2)

Total (S-3)




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TotalVolume

[m3]Surface

[m2]Mass flow

[kg/s]Flue gas fraction

Temperature [K] Heatflux

[kW]

SpecificHeat flux[kW/m3]Inlet Outlet

Reference Case 100 2900 3.77 0 420 1241 3095 30.95

Case with gas recirculation

system

TOT 100 2900 4.52 0.20 478.1 1291 3746 37.46

S-1 33.3 966.7 1.13 0 420 1256 945 28,37

S-2 33.3 966.7 1.13 0.1 459.5 1325 991 29,76

S-3 33.3 966.7 2.26 0.35 516.4 1292 1810 54,08

𝜼𝑻,𝒂𝒗𝒈 = 𝟎. 𝟗𝟏(ref case 𝜼𝑻,𝒂𝒗𝒈=0.88)

Transient model applied to the gas recirculation system




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CFD











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Impact analysis of the refractory brick length on the thermal flux transmittedThe portion of the air regenerator interested by the exhaust flow is characterized by a mass flow rategreater than the surrounding areas. Secondly the high concentration of CO2 and H2O involves positiveeffects on heat transfer.In this area, an increase of the flow passage section can have a positive effect for the regenerationsystem because:1. It increases the emissivity of the radiant gas with a possible increase of the exchanged heat flow;2. It reduces the mass of refractory material and thus reduces the cost of the system;3. It reduces the flow velocity reducing the load losses and the erosion of channels.

Analysis hypothesis:• Investigation on 1m x 1m regenerator base area;• Refractory temperature profile from 800 to 1350 K;• Fluid temperature profile from 450 to 1250 K;• Flue gas fraction equal to 20% of the total mass flow;• Convection heat transfer coefficient profile from 8.9 to 3.2 W/m2K.




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𝑙𝑏𝑟𝑖𝑐𝑘 = 𝐿 4𝑝𝑤𝑒𝑡 = 16𝐿

𝑙𝑏𝑟𝑖𝑐𝑘 = 𝐿 2𝑝𝑤𝑒𝑡 = 8𝐿

𝑙𝑏𝑟𝑖𝑐𝑘 = 𝐿𝑝𝑤𝑒𝑡 = 4𝐿

Wetted perimeter 𝑝𝑤𝑒𝑡 = 4𝐿𝑙𝑏𝑟𝑖𝑐𝑘

Influence of the brick length on the thermal exchange surface

L=1

m

0

5

10

15

20

25

30

35

40

45

0,00 0,20 0,40 0,60 0,80 1,00

Wet

ted

pe

rim

ete

r[m

/m2]

Brick length [m]

Square bricksTotal cross section =1 m2

𝑙𝑏𝑟𝑖𝑐𝑘 ↑↑↑




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Influence of the brick length on the radiant heat trasfer coefficient

The emissivity of the gas increases with the increasing of the flow channel cross section i.e. with the brick length.

𝜀𝑔𝑎𝑠 = 𝑓(𝑝𝑔𝑎𝑠, 𝑙𝑏𝑟𝑖𝑐𝑘, 𝑇𝑠, 𝑇𝑓)

Consequently, the radiant heat transfer coefficient increases with the increase of the length of the brick.

ℎ𝑟𝑎𝑑 = 𝜎 𝜀𝑓𝑇𝑓

4 − 𝛼𝑓𝑇𝑠4

𝑇𝑓 − 𝑇𝑠

0

5

10

15

20

25

30

35

40

0,00 0,20 0,40 0,60 0,80 1,00

Ave

rage

he

attr

ansf

er

coe

ffic

ien

t[W

/m2K

]

Brick length [m]

Flue gas mass fraction 20%




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Influence of the brick lenght on the total heat transfer

The graph shows the trend of the average heatflux per 1m3 of stacking volume varying thelength of the refractory brick. It is show that theheat flux increases with the length of the brick.

This preliminary analysis shows that anappropriate sizing of the regenerator zone flowedby a significant exhaust fraction cansimultaneously optimize the thermalperformance reducing the refractories material.

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110

120

0,00 0,20 0,40 0,60 0,80 1,00

Tota

l he

atfl

ux

pe

r vo

lum

e [

kW/m

3]

Brick length [m]




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Computational techniques in a nutshell

We have seen:

• a CFD approach to the regenerator chamber simulation;• a standardized CFD approach and post-processing tools for the strategic

gas recirculation system design optimization;• a radiation model for co2 and h2o gases;• a lower order model of the regenerator system for the plant analysis also

in transient mode for preliminary evaluation of the recirculation gas system.




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Conclusion

We have observed:

1. By experimental evidence it is clear that the flue gas recirculation system isa good solution for the reduction of pollutants. The system needs to betailored for a given chamber design and geometry to get the bestperformance.2. The exhaust gas recirculation strategy can be beneficial even for theoverall design optimization of the chamber (heat transfer and refractoryvolumes) as demonstrated in a preliminary application of a model.




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