ANSYS-CFD Analysis of Condensation Process Occurring Inside High Efficiency Boilers

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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


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


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


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


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


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])


Simplified fin (porous)


Detail fin

Temperature distribution (oF) in vertical section

Simplified fin


Detailed Simplified

Outlet-water (ºF) 77.1 75.9

Outlet- flue (ºF) 1363 1785

Pressure drop (Pa) 0.054 0.053


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)


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


CFD Results – Flow Path Lines

Water flow path lines colored by temperature distribution (oF)Test A Test B


Test A (water heater). Flow pattern in two vertical sectionsSection Y=0 Section x=0

CFD Results – Flow Field


CFD Results - Temperature Field

Results of Test A (Water heater)

Temperature (oF) in a vertical plane Heat flux (W/m2) in inner finned tubes


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.


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 )


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


• 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


Velocity (m/s) and temperature (oF) in a vertical section of flue-gas side

TemperatureVelocity vector

Evaluation of Proposed Boiler Features


Mass fraction of possible condensed water liquid in the porous (finned tubes) zone

Evaluation of Condensation Model

Existed model Improved high-efficiency concepts


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)


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.

Documents

ANSYS-CFD Analysis of Condensation Process Occurring Inside High Efficiency Boilers