CFD tools and lower order modeling for regenerative chambers with gas recirculation system

Transcription

1 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 1

2 Computational techniques for the analysis of the strategic gas recirculation system 1. Regenerator general models CFD 2. Single flow channel models 3. Strategic gas recirculation configuration models Lower order modelling 1. Single chamber model 2. Regenerator system transient model 3. Radiation effect parametric tool 2

3 CFD systematic approach to the regenerator system 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 chamber equipped 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 different techniques to simplify the modeling and save resources. 3

4 CFD systematic approach to the regenerator system To limit the computational costs the checkers zone is emulated by means of a porous domain, a mathematical tool, with which it is possible to associate to the selected zone specifics solid-to-fluid interaction laws, both in terms of pressure drops that in terms of heat exchange. From the thermal point of view, two different type of porous models exists: the Equilibrium model the Non-equilibrium model. 4

5 CFD systematic approach to the regenerator system 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. 5

6 h [W/m 2 K] h [W/m 2 K] LIFE12 ENV/IT/ PRIME GLASS A single flow channel is the volume formed between two adjacent bricks, for example of the cruciform type. The transient analysis is obtained by alternating the air and fumes flows every 20 minutes. Single flow channel model COLD phase HOT phase z/d z/d 6

7 Comparison of porous domain models Air phase Flue gas phase

8 Computational techniques for the analysis of the strategic gas recirculation system 1. Regenerator general models CFD 2. Single flow channel models 3. Strategic gas recirculation configuration models Lower order modelling 1. Single chamber model 2. Regenerator system transient model 3. Radiation effect parametric tool 8

9 CFD simulation approach for strategic gas recirculation system design optimization The strategic gas recirculation system is a technique for the removal of pollutants in new or 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 the combustion chamber. To ensure a good performance of the system is fundamental that the flue gas flow remains concentrated in the primary combustion zone of the combustion chamber. By means of the CFD analysis, the recirculation system can be optimized and calibrated to be coupled to a given furnace geometry. 9

10 Examples of exhaust gas feeding system optimization design Case 1 «short square duct» Case 2 «long square duct» Case 3 «divergent duct» 10

11 Systematic data post-processing methodology Mass flow distribution Exhaust mass fraction distribution S-12 S-11 S-10 S-9 S-8 S-7 S-6 S-5 S-4 S-3 S-2 S-1 Case 3 Case 2 Case 1 S-12 S-11 S-10 S-9 S-8 S-7 S-6 S-5 S-4 S-3 S-2 S-1 Case 3 Case 2 Case 1 0% 5% 10% 15% 20% 0% 10% 20% 30% 40% 50% 11

12 Computational techniques for the analysis of the strategic gas recirculation system 1. Regenerator general models CFD 2. Single flow channel models 3. Strategic gas recirculation configuration models Lower order modelling 1. Single chamber model 2. Regenerator system transient model 3. Radiation effect parametric tool 12

13 CFD Ansys-Fluent SUB-SYSTEMs Single channel models LIFE12 ENV/IT/ PRIME GLASS Thermal regenerators unsteady 1-D model Matlab-Simulink MAIN System Heat transfer coefficients profile (convective or total) for the Hot and Cold periods Gas recirculation system models Results Fluid and refractory temperature profile Regenerator system performance Mass flow distribution Flue gas fraction distribution thermofluidodynamic properties; H 2 O and CO 2 emissivity model; Energy balance transient solver; «reverse cycle» algorithm; 13

14 Computational techniques for the analysis of the strategic gas recirculation system 1. Regenerator general models CFD 2. Single flow channel models 3. Strategic gas recirculation configuration models Lower order modelling 1. Single chamber model 2. Regenerator system transient model 3. Radiation effect parametric tool 14

15 Regenerator chamber model A single regenerator chamber is modelled as a solid and a fluid domain, each one is divided in a fixed number of cells. For each cell is defined a transient energy balance equation that describes the processes of heat exchange of the cell with the neighboring ones. The model use the following assumptions: fluids are a mix of N 2,O 2,CO 2 and H 2 O; 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. 15

16 Regenerator chamber model structure Energy balance equation for the i th fluid cell: dt f,i m f,i cv f,i dt = m f cp f,i 1 T f,i 1 cp f,i T f,i + U sf,i Sur sf,i T s,i T f,i Energy balance equation for the i th solid cell: dt s,i m s,i cv s,i dt = U ss,i 1 Sur ss,i 1 T s,i 1 T s,i + U ss,i+1 Sur ss,i+1 T s,i+1 T s,i + + U sf,i Sur sf,i T f,i T s,i 16

17 h [W/m 2 K] h [W/m 2 K] LIFE12 ENV/IT/ PRIME GLASS Regenerator chamber model structure Total thermal trasmittance between i th solid and i th fluid cells: U sf,i = COLD Phase th h conv h rad k s HOT Phase 1 U sf,i = 1 + th h tot k s CFD result Regenerator height Active with the gas recirculation system only Model with polynomic regressions of the Hottel s emissivity charts of H2O and CO CFD result Regenerator height 17

18 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 ) 18

19 Regenerator height % Regenerator height % LIFE12 ENV/IT/ PRIME GLASS COLD phase (air only) Regenerator chamber validation HOT phase Solid B.C. 20 Fluid-CFD Non Eq. 10 Fluid-Matlab Non Eq Temperature [K] Solid B.C. 10 Fluid-CFD Non Eq Fluid-Matlab Non Eq Temperature [K] Outlet temperature difference <5% Outlet temperature difference <1% 19

20 Computational techniques for the analysis of the strategic gas recirculation system 1. Regenerator general models CFD 2. Single flow channel models 3. Strategic gas recirculation configuration models Lower order modelling 1. Single chamber model 2. Regenerator system transient model 3. Radiation effect parametric tool 20

21 Regenerator system model Switch every 20 The regeneration system model is constructed by coupling two specular chambers, the first fed air while the second by exhaust gases. Every 20 the temperature profile of the solid matrices is switched between the chambers. Each chamber is divided longitudinally into three zones to simulate different flow rates and chemical concentrations typical of the gas recirculation system. 21

22 Regenerator height % Regenerator height % LIFE12 ENV/IT/ PRIME GLASS Transient regenerator model applied to a conventional regenerator system COLD phase Air initial Air final Solid initial Solid final Temperature [K] η T,avg = T air,out T gas,in = HOT phase Flue gas initial Flue gas final Solid initial Solid final Temperature [K] 22

23 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 to simulate the effects of the strategic gas recirculation system Total (S-3) S-1 S-2 S-3 Mass flow distribution 25% 25% 50% Flue gas mass fraction 0 % 10% 35% Heat transfer coef. COLD phase [W/m 2 K] Regenerator height % Radiative (S-2) h_tot (S-1) h_tot (S-2) h_tot (S-3) h_rad (S-2) h_rad (S-3) 23

24 Transient model applied to the gas recirculation system Total Temperature [K] Heat Specific Volume [m3] Surface [m2] Mass flow [kg/s] Flue gas fraction Inlet Outlet flux [kw] Heat flux [kw/m3] Reference Case Case with gas recirculation system TOT S ,37 S ,76 S ,08 η T,avg = (ref case η T,avg =0.88) 24

25 Computational techniques for the analysis of the strategic gas recirculation system 1. Regenerator general models CFD 2. Single flow channel models 3. Strategic gas recirculation configuration models Lower order modelling 1. Single chamber model 2. Regenerator system transient model 3. Radiation effect parametric tool 25

26 Impact analysis of the refractory brick length on the thermal flux transmitted The portion of the air regenerator interested by the exhaust flow is characterized by a mass flow rate greater than the surrounding areas. Secondly the high concentration of CO2 and H2O involves positive effects on heat transfer. In this area, an increase of the flow passage section can have a positive effect for the regeneration system 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/m 2 K. 26

27 L=1 m Wetted perimeter [m/m 2 ] LIFE12 ENV/IT/ PRIME GLASS Influence of the brick length on the thermal exchange surface Square bricks Total cross section =1 m l brick = L p wet = 4L l brick l brick = L 2 p wet = 8L Wetted perimeter p wet = 4L lbrick l brick = L 4 p wet = 16L 5 0 0,00 0,20 0,40 0,60 0,80 1,00 Brick length [m] 27

28 Average heat transfer coefficient [W/m 2 K] LIFE12 ENV/IT/ PRIME GLASS Influence of the brick length on the radiant heat trasfer coefficient ,00 0,20 0,40 0,60 0,80 1,00 Brick length [m] Flue gas mass fraction 20% The emissivity of the gas increases with the increasing of the flow channel cross section i.e. with the brick length. ε gas = f(p gas, l brick, T s, T f ) Consequently, the radiant heat transfer coefficient increases with the increase of the length of the brick. σ ε h rad = f T 4 f α f T4 s T f T s 28

29 Total heat flux per volume [kw/m 3 ] LIFE12 ENV/IT/ PRIME GLASS Influence of the brick lenght on the total heat transfer ,00 0,20 0,40 0,60 0,80 1,00 Brick length [m] The graph shows the trend of the average heat flux per 1m 3 of stacking volume varying the length of the refractory brick. It is show that the heat flux increases with the length of the brick. This preliminary analysis shows that an appropriate sizing of the regenerator zone flowed by a significant exhaust fraction can simultaneously optimize the thermal performance reducing the refractories material. 29

30 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. 30

31 We have observed: Conclusion 1. By experimental evidence it is clear that the flue gas recirculation system is a good solution for the reduction of pollutants. The system needs to be tailored for a given chamber design and geometry to get the best performance. 2. The exhaust gas recirculation strategy can be beneficial even for the overall design optimization of the chamber (heat transfer and refractory volumes) as demonstrated in a preliminary application of a model. 31

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