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• 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
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)
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.
• 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
• 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
• 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.