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

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

Motivation

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

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

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

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)

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)

A 2D example

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

2D: pressure contours

2D: shell-side temperature

2D: Tube-side temperature

2D: Wall (tube) temperature

2D: Shell-side heat-transf coef

2D: Tube-side heat-transf coef

Geometrical modelling

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

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)

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

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...

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

Mathematical modelling

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

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)

Results

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)

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

Single-bank: Test cases

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

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

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

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

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

Computational mesh n75x64x142 nApprox 680,000 cells

Shell-side temperature

Flow field (velocity vectors)

Pressure field

Shell temperature

Tube-side temperature

Tube-wall temperature

Heat-transfer rate nNB per cell

Tube-side heat-transfer coeff

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

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