ANALYSIS OF DUCTS

     CFD is one of the branches of fluid  mechanics  that uses In the duct system leading flue gases from Air-heater to ESP, general problems regarding high-pressure drop, turbulence, erosion, leakages and imbalanced flow in branching ducts are regularly observed. The flue gases carry coarse ash particles, which under certain operating conditions hit the duct walls and other physical surfaces present within the ducts causing erosion on walls. Turbulence is another problem that commonly occurs in the duct due to sharp turns or sudden change in the duct geometry. It results in noise and vibration. Hence it is also necessary to kill the turbulence in order to reduce noise and vibration within the duct.

     The flow of flue gases through the ducting system is analyzed to investigate means for mitigating the erosion of walls of the ducts and reducing the pressure drop through the system. To achieve this, a model is developed which is simulated for the flow of flue gases through the duct. The focus of the modifications done hereafter is to improve the flow characteristics and to make the flow as uniform as possible. The results obtained after modifications are verified by comparing the pressure and velocity readings taken at site prior to and after the modifications are done. CFD analysis of the duct is carried out to get an idea about flow distribution, turbulence and erosion caused by flow of flue gases through the duct. For the analysis of flow the CFD commercial code CFX, FLUENT is used.

FLOW SEPARATION

     Flow separation occurs when a bend or expansion is too severe to allow the flow to follow the contour of the duct. This causes a wake to form, resulting in an effective reduction in the ducts cross sectional area (known as the vena contracta). This area reduction results in a pressure loss that can be significant. At times, flow separation creates pulsations throughout a system. These pulsations have been known to cause flow-induced vibrations that may damage ductwork and/or fan bearings. Separated flow also typically results in highly non-uniform gas velocity profiles. This can degrade efficiency of downstream heat exchangers, particle collectors, mixers, instrumentation, etc.

 

 

 

 

 

 

 

 

 

 

 

     From above results it can be seen that, recirculation zone is totally avoided in the modified duct. Flow separation can usually be eliminated or controlled through the addition of optimally designed turning vanes or by altering duct geometry. Existing duct shows turbulent, re-circulating flow region downstream of the corner. By implementing properly designed turning vanes, the flow exiting the corner exhibits smooth, low turbulence, attached flow as seen in modified duct.

              Given the complexity of the flow field erosion of the duct is hard to avoid. Introduction of suitable mechanisms that alter particulate and gas flow characteristics can possibly minimize turbulence and erosion at those places where the maximum value occurs. By introducing such mechanisms we can substantially delay the formation of holes on the duct walls. The objective is to protect the duct wall from erosion by reducing the level of erosion at places that experience maximum erosion rate and achieving a uniform and minimized overall erosion of the duct wall.