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Modelling the convective zone of a utility boiler Norberto Fueyo Antonio Gómez Fluid Mechanics Group University of Zaragoza

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Contents +Motivation +2D example +Geometrical modelling +Mathematical modelling +2D validation +Application to a 350 MW(e) boiler +Conclusions +Further work

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Motivation

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Furnace modelling Aim: Modelling Simulation Validation of Multiphase flow (including turbulence), Heat transfer (including radiation) Pollutant (NOx) formation in Furnace of power-production utilities

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Strategy (divide and conquer) = Furnace Convective zone + (Model coupling through boundary conditions)

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Convective-zone modelling Aim: Modelling Simulation Validation of Fluid flow (including turbulence) and Thermal fields (gas and tube sides) Heat transfer in Convective zone of boiler In Out

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Model input nGeometrical data (tubes, banks, etc) nFluid (shell-side and tube-side) and solid (tube) properties nOperating conditions (inlet mass-flow rates, inlet temperatures, etc)

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Model output nDetailed fields of:- nVelocity nPressure nTurbulence nShell fluid, tube fluid and wall temperature nShell-to-wall and tube-to-wall heat-transfer coefficients nHeat-transfer rate (W/m3) nOverall heat-transfer rate, per tube-bank (W)

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A 2D example

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Complex 2D case Vapour in/out Hotter gas in Colder gas out Manifold

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2D: pressure contours

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2D: shell-side temperature

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2D: Tube-side temperature

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2D: Wall (tube) temperature

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2D: Shell-side heat-transf coef

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2D: Tube-side heat-transf coef

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Geometrical modelling

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The problem nGeometrically complex problem nTubes nTube-banks nInterconnections nTubes representented as distributed, sub-grid features nSpecify geometry in ASCII file nSubordinate mesh to geometry

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Strategy (schematic) Convective-zone database (ASCII) Geometrical data, mesh, etc Simulation parameters (Q1) Parser program (in-house made) Simulation (Earth) Numerical results Graphical results: (PHOTON, TECPLOT)

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Element types nGeneral data n2D tubebanks (tube wall) n3D tube banks nBank arrays (2D, usually) nManifolds (virtual) nInternal nInlets nOutlets

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Data required for each element nFeature name nPosition and dimensions nTube orientation nInternal and external tube diameter nTube pitch nTube material nFluid velocity nFluid Cp, Prandtl number, density, viscosity nTube-bank conectivity nSome others...

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Typical database entry [tubebank] type = 3D long_name = Lower_Economizer_1 short_name = Ecoinf1 [[descrip]] posi = (14.323,1,22.61) dime = (6.34,8.24,2.3) alig = +2 diam = 50.8 pich = (146.26,0,83.3) poro dint = 46 velo dens enul pran mate = SA.210.A1 [[connect]] From_bank = ent1 In_face = South Out_face Link

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Mathematical modelling

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Main physical models - shell side nFull Navier-Stokes equations, plus enthalpy equation, plus turbulence statistics (typically, k- epsilon model) nFull account of volume porosity due to tube-bank presence nShell-side pressure-loss via friction factors in momentum equations nShell-side modification of turbulent flowfield due to presence of tubes nEmpirical heat-transfer correlations, based on tube-bank geometry (diameters, pitch, etc) nSimple (but flexible) account of shell-side fouling

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Main physical models - tube side nOne-directional enthalpy equation (along the tube direction) nMass-flow rates in the tubes obtained from mass balance nEmpirical heat-transfer correlations, based on tube geometry (diameter)

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Results

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Applications n2-D, multiple tube-bank configuration (functional validation) n2-D, single tube-bank configuration (numerical validation) n3-D convective zone (validation in real-case application)

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2D validation nValidation with single-bank configuration: SLSL Air V T1T1 D STST NLNL NTNT TwTw T2T2

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Single-bank: Test cases

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Single-bank: thermal results nTheory: Log Mean Temp Difference method (1-4) and Number of Transfer Units method (5)

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Single-bank: pressure loss nTheor 1: Grimison correlation nTheor 2: Gunter and Shaw correlation

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350 Mw boiler nNB: still not fully converged, but nevertheless... nPhysically plausible nResults follow

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Boiler layout L V Flue gas Gases Vapour Turbine Vapour Turbine LE UE Reheater 2SH 1SH Dividing walls Final reheater 1SHPrimary Superheater 2SH Secondary superheater UEUpper economizer LELower economizer

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Typical geometry nAs interpreted by the graphics program from database nSome bounding walls not plotted for the sake of clarity

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Computational mesh n75x64x142 nApprox 680,000 cells

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Shell-side temperature

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Flow field (velocity vectors)

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Pressure field

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Shell temperature

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Tube-side temperature

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Tube-wall temperature

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Heat-transfer rate nNB per cell

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Tube-side heat-transfer coeff

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Comparison with measurements nResults not fully converged nEffect of fouling to be studied nGeometry not 100% accurate

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Computational details nFinite-volume formulation of equations nNumber of cells: approx 670,000 (75x64x142) nNumber of dependent variables: 8 (pressure correction, 3 shell-side velocity components, k, epsilon, tube-side and shell-side enthalpy) nRunning time: nAround 12 minutes CPU time per sweep (PENTIUM 300) nAround 1500 iterations to convergence

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