CFD simulation
Combustion system
Due to instability of the hot air mass flow and basing on results coming from survey, a preliminary fluid dynamic analysis has been done.
The analysis of the combustion system from physical point of view is very complex and difficult because it involves many phenomena, such as turbulent flow of humid and dry air, chemical reactions, forced convection, irradiation and diffusion. Also from a geometric point of view it is complicated to analyze the system, because of its differents order of magnitude regarding the dimensions. The the ducts dimension magnitude is about 10^3 mm, far from methane output holes, that is of several mm. The scope of the present analysis is to show the temperature and velocity distribution near the burner and near the wet-dry ducts ramification. Temperature and velocity values are important not just for their magnitude, but specially for their distribution profile insed the ducts. Starting from this point of view, we have done a simplified CFD model. Below are reported all the simplification done.
CFD Geometric model
As explained in introduction, the lenght scale of the problem is a main study parameter. The assumption done for the drawing of the 3D geometry, for the scope of computational fluid dynamic analysis, is that the minimum dimension of the problem is 6 mm. This assumption have an impact on burner thickness and combustion air holes. The big thickness means combustion air velocity inside burner higher than real. but we are not interested in the computation of the velocity and pressure drop of combustion air inside burner ducts. Other components involved in this study are a flow rectifier and grid. Luckily the existing mesh of the rectifier are very large so, the big thickness doesn’t have impact on the flow analysis. About grid we change the number of holes fixing the free surface equal to the real one. The grid is very small so from global point of view the effect on global result are not influenced by assumptions.
Flow turbolece model
The K- ω model is one of the most commonly used turbulence models. It is a two equation model, that means, it includes two extra transport equations to represent the turbulent properties of the flow. This allows a two equation model to account for history effects like convection and diffusion of turbulent energy. The first transported variable is turbulent kinetic energy Ƙ. The second transported variable in this case is the specific dissipation, ω. It is the variable that determines the scale of the turbulence, whereas the first variable, Ƙ, determines the energy in the turbulence. For the flow model the inlet velocity of process air and combustion air are imposed according with survey data and process fan data sheet. For the process air we have imposed a inlet velocity profile according with Maxwell Boltzmann distribution described on process fan paragraph of this document.
Conjugate Heat Transfer and Nonisothermal Flow
Used Heat Transfer Module contains features for modeling conjugate heat transfer and non isothermal flow effects. The temperature transition at the fluid-solid interface is automatically handled using continuity. Thermal condition are, imposed inlet temperature for process air 300°C and imposed combustion air of 1300°C. The high value imposed for combustion air respect the real one of 200°C is due to reach a process air temperature of 370°C at the end of mixing. The assumption is very strong and don’t take in account the irradiation contribution of the flame. Thermal radiation in grid burner (without combustion chamber) is one order or magnitude less than convection effect so for a preliminary analysis done in order to have a fast response this assumption could be appropriate.
Result coming from simulation
Existing configuration
Follows results coming from existing geometry of combustion system apparatus as picture 12.
Follows parameters evaluated on the outlet surface of wet and dry ducts:
Parameters | Wet | Dry |
Average velocity on surface [m/s] | 20,6 | 13,2 |
Average temperature on surface [°C] | 351 | 357 |
Mass flow [kg/h] | 39013 | 20818 |
Velocity and temperature profile are evaluated as follow :
Table 14 shows that in the existing configuration there is a difference of average temperature between wet and dry of seen degrees and a difference in mass flow at same pressure condition of 16.854 kg/h.
It will be better to investigate what happen if velocity of process air through burner will increase to recommended value of 15 m/s. The following simulation is done adding a laterals frame on burner duct.
MODIFIED configuration WITH LATERAL FRAMES
The 3D geometry with the lateral frame is shown below.
Follow result coming from modified geometry of combustion system apparatus as following picture.
Follows parameters evaluated on the outlet surface of wet and dry ducts:
Parameters | Wet | Dry |
Average velocity on surface [m/s] | 20 | 13,5 |
Average temperature on surface [°C] | 348 | 379 |
Mass flow [kg/h] | 38346 | 21492 |
Velocity and temperature profile evaluated in the same lines of previous configuration, are the following :
The difference of the average temperature between wet and dry is increased respect to the original configuration. Increasing the velocity of process air through burner is not recommended for this particular layout because there isn’t enough space for air to mix after burner, ramification wet-dry is very close to burner outlet.