ANSYS-CFD Analysis of Condensation Process Occurring Inside High Efficiency Boilers
Transcript of ANSYS-CFD Analysis of Condensation Process Occurring Inside High Efficiency Boilers
© 2008 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary
2008 International ANSYS Conference
CFD Analysis of Condensation Process Occurring Inside High Efficiency Boilers
Jenny Cheng, P.Eng and Alexander Ene, P.EngGSW Water Heating CompanyFergus, ON. Canada
© 2008 ANSYS, Inc. All rights reserved. 2 ANSYS, Inc. Proprietary
Modeling Strategy
• Finned tube simplificationFinned tube area modeled using porous media; a sub-model is used to determine the proper C2 (pressure drop) and porosity (conductive heat transfer) values.
• Coupled water-gas analysis Simultaneous modeling of water flow on water side and combustion coupled heat transfer on the flue gas side.
• Condensation modelingHeat transfer enhancement on the flue gas side due to water vapor condensing on the heat exchanger surface modeled using in-house UDF.Condensing technology: The recovery of the water vapor latent heat contained in the flue by condensing the vapors in the cold part of the water heater heat exchanger • Hydrocarbon combustion lead to 14% water vapor in flue gases
© 2008 ANSYS, Inc. All rights reserved. 3 ANSYS, Inc. Proprietary
Combustion chamber
Boiler Solid Model
• Thermal efficiency of current boiler is 84% without condensing input.
• By using condensing technology, thermal efficiency can exceed 90%
Wire mesh burner
Water flow passes
Finned tube
© 2008 ANSYS, Inc. All rights reserved. 4 ANSYS, Inc. Proprietary
Combustion Chamber
Flue outlet
Burner ports (Fuel-air mixture inlet)
Water outlet
Water inlet
CFD Modeling
Water domain
Simplified finned tubes
Porous material: -Solid-Fluid porosity (heat transfer)-Radial pressure drop through finned tubes
© 2008 ANSYS, Inc. All rights reserved. 5 ANSYS, Inc. Proprietary
Simplified fin: Porousflue & copper
Detail fin:Solid copper
Detail SimplifiedCells: 579,955 147,455Faces: 1,345,868 319,893Nodes: 208,231 42,209
Air outlet
Hot air inletWater inlet
Water outlet
Finned tube
Finned Tube Simplification
Mesh size
Sub-model
© 2008 ANSYS, Inc. All rights reserved. 6 ANSYS, Inc. Proprietary
Temperature (oF)
Velocity (m/s)
Pressure (Pa)
Detail fin (Solid copper)
• Porosity – related to heat transfer (0.68) • Pressure drop - Inertial resistance C2 (750 [1/m])
Finned Tube Simplification
Simplified fin (porous)
© 2008 ANSYS, Inc. All rights reserved. 7 ANSYS, Inc. Proprietary
Detail fin
Temperature distribution (oF) in vertical section
Simplified fin
Finned Tube Simplification
Detailed Simplified
Outlet-water (ºF) 77.1 75.9
Outlet- flue (ºF) 1363 1785
Pressure drop (Pa) 0.054 0.053
© 2008 ANSYS, Inc. All rights reserved. 8 ANSYS, Inc. Proprietary
Steady state, combustion on flue gas path and radiation effects considered, phase change on the water side (boiling) not modeled, fin space simplified as a porous material, no wall roughness effect considered
Simulation Methodology
• UDF code was developed to model coupled water-gas (combustion included) process:– Defining the different material properties (density,
viscosity and thermal conductivity) on combustion-mixture and water, separately
• Computational models:– Turbulence – k- epsilon standard & wall function– Combustion – Finite-Rate/Eddy-Dissipation– Radiation – DO (Discrete Ordinates)
© 2008 ANSYS, Inc. All rights reserved. 9 ANSYS, Inc. Proprietary
Nominal Input Rate: 500,000 Btu/hrThermal efficiency tests
Test A- Test methodology for water heater (ΔT=70oF); Test B- Test methodology for boiler (ΔT=100oF);
Boundary Conditions
• Flue-gas side boundary conditions– Inlet
• Mass flow rate – based on nominal input rate• Mass fraction (CH4 & O2) – based on air-excess requested
– Outlet: Pressure • Water side boundary conditions
– Inlet• Mass flow rate - tuned to meet the temperature increase ΔT requested
– Outlet: Pressure• Porous (fin) zone conditions
– Flue & copper porous material, flue porosity, inertial resistance in radial & circumferential directions
© 2008 ANSYS, Inc. All rights reserved. 10 ANSYS, Inc. Proprietary
CFD Results – Flow Path Lines
Water flow path lines colored by temperature distribution (oF)Test A Test B
© 2008 ANSYS, Inc. All rights reserved. 11 ANSYS, Inc. Proprietary
Test A (water heater). Flow pattern in two vertical sectionsSection Y=0 Section x=0
CFD Results – Flow Field
© 2008 ANSYS, Inc. All rights reserved. 12 ANSYS, Inc. Proprietary
CFD Results - Temperature Field
Results of Test A (Water heater)
Temperature (oF) in a vertical plane Heat flux (W/m2) in inner finned tubes
© 2008 ANSYS, Inc. All rights reserved. 13 ANSYS, Inc. Proprietary
Test A (water heater)
Test B (boiler)
Lab CFD Lab CFDWater temp. increase (oF) 69 69.2 100 99
Mass flow rate of water(kg/s) 0.7630 0.7625 0.5234 0.5230
Fuel consumption(ft3/s) @ STD
0.13125Natural gas
0.1367CH4
0.13124Natural gas
0.1367CH4
Thermal Efficiency (%) 84.31 83.78 83.88 82.48
Comparison between CFD and Lab
Note: When operating as a water heater (Test A), the outlet water temperature will be 140oF, while operating like a boiler (Test B) the outlet temperature will be 180 oF. There is no actual boiling (phase change) occurring within the water path of the boiler.
© 2008 ANSYS, Inc. All rights reserved. 14 ANSYS, Inc. Proprietary
Flue outlet
Bottom chamber
Proposed High- efficiency Boiler
Expected condensing regions with low temperature (< dew point)
Condensing in finned tubes
Condensing in bottom surfaces
(T <T_dew )
© 2008 ANSYS, Inc. All rights reserved. 15 ANSYS, Inc. Proprietary
Condensation Modeling MethodologiesModeling options
• Input a transport/reaction model – Use DEFINE_VR_RATE micro to specify a custom
volumetric reaction rate for H2O(vapor) H2O (liquid) to let the reaction takes place where the cell temperature is lower than the vapor dew point (T_dew)
– Challenge: Difficult to arrive at a “realistic” set of parameters without going through a few combinations
• Input source/sink terms– Apply DEFINE_SOURCE micro to specify custom
source terms for energy and H2O vapor/liquid species mass fraction transport equations
– Technically easier to approach, but need to “manually” calculate amount of H2O (vapor) mass to be condensed
© 2008 ANSYS, Inc. All rights reserved. 16 ANSYS, Inc. Proprietary
• In a finned tube (porous) domain:
• On a flat wall (bottom chamber):
C0; T0 C1; T1 C2; T2m0 m1 m2 m3
Non-condensation zone: T0 >T_dewCondensation zone: T1, T2<T_dew
• The mass flow m1 coming into the condensation zone is subjected to condensation
• Condensing takes place in cells satisfying T_cell <T_dew
• The mass flow m going through the condensation zone being in the first layer of boundary mesh is subjected to condensation
• Condensation takes place in cells satisfying – T_wall < T_dew
m
Non-condensation zone: the cell is not in the adjacent of the wall or T_wall >T_dew Condensation zone: at least one face of the cell is on the wall and T_wall <T_dew
T_wall
Condensation Modeling MethodologiesCondensing Vapor mass flow calculation
© 2008 ANSYS, Inc. All rights reserved. 17 ANSYS, Inc. Proprietary
Velocity (m/s) and temperature (oF) in a vertical section of flue-gas side
TemperatureVelocity vector
Evaluation of Proposed Boiler Features
© 2008 ANSYS, Inc. All rights reserved. 18 ANSYS, Inc. Proprietary
Mass fraction of possible condensed water liquid in the porous (finned tubes) zone
Evaluation of Condensation Model
Existed model Improved high-efficiency concepts
© 2008 ANSYS, Inc. All rights reserved. 19 ANSYS, Inc. Proprietary
Evaluation of Condensation Model
CFD Lab
Condensing rate (kg/s) 0.0 0.00181 0.00196
Thermal Efficiency (%) < 90.0 93.0 92.0
Comparison between CFD and Lab
Condensing region in the bottom chamber with low temperature (< dew point)
© 2008 ANSYS, Inc. All rights reserved. 20 ANSYS, Inc. Proprietary
Conclusions
• The developed CFD methodology is capable to predict the thermal efficiency of boilers with reasonable accuracy by validating with test. Modeling simplification assumptions related to the finned tubes and wire mesh burners are effective
• Condensing heat transfer modeling has been developed and the methodology has been refined by comparing its predictions with further experimental data.
• Based on the current condensation modeling methodology, the optimized design solutions have been explored to achieve the target high efficiency. The significant contribution to the overall heat transfer enhancement was found as a result of the water vapor condensation process on the flue gas side.