Air-Cooled Steam Condenser (ACSC) CFD Simulation
Air-Cooled Steam Condensers (ACSC) are progressively used to decline heat in pioneer power plants.
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The Air-Cooled Steam Condenser (ACSC) is a Direct Dry Cooling system. The power plant is a power station that converts the heat energy to the electrical power and uses ACSC. A diffuser orifice plate implementation has some advantages for ACSC systems such as using kinetic energy of wind and making the ambient flow more uniform, as the ambient wind has negative effects on the ACSC efficiency. Various columns of fans, implement to intensify the buoyancy flow due to the heat generation of power plant’s equipment, to increase the ACSC’s efficiency.
How does ACSC operate?
Air-Cooled Steam Condenser, condense the steam with air applying fans. Cold ambient air, condenses the steam through the tubes, and this cooling system applies in power plants where the outlet steam from the turbines, condenses and the liquid enters Boiler in a closed cycle, know as Rankine Cycle. This is the best cooling system for the power plants which suffer water-scarce. Also, a lot of modules made with finned tubes arrange in parallel arrows to make a heat transfer. These finned tubes are as a porous zone in this simulation. By the way, we model fans in this project.
Problem Description for ACSC Simulation
In this simulation, the hot vapor condensation is investigated by the embedded fan system. The hot, low-pressure steam from the turbine’s outlet passes through each of the pipes and then moves to the interior of the diagonal plates embedded on either side of the pipe. The underside of the pipe and interior of the diagonal plates have a porous structure because the use of the porous medium increases the contact surfaces between the hot steam and the cooling fluid, resulting in increased heat transfer and condensation phenomena. On the other hand, the blown air by cooling fans beneath both diagonal plates acts as a cooling fluid and condenses the vapor. The diagonal structure of these plates causes condensed water to move downward due to the gravitational force. This process is carried out in all eight rows of fans and pipes and eventually condensed water is discharged.
Geometry and Mesh
The ACSC geometry and grid designed and meshed by Design Modeler and ANSYS Meshing software, respectively. Part of the geometry is defined as the system environment in which air flows in this area. Inlet, outlet, and floor are defined as the ground surface and other surfaces as symmetrical surfaces. In the interior of this domain, there is a cooling system consisting of eight rows of hot steam carrier tubes, oblique porous plates on either side of the pipe and eight fans on one platform. The meshing is a hybrid model with 2668772 cells.
There are some specific terms of condition for Air-Cooled Steam Condenser (ACSC) CFD simulation. The project is a Pressure-Based solver considering gravity force, and the solver is Steady-State.
Air-Cooled Steam Condenser CFD Simulation
Here is a summary of the steps to define and solve the problem in the table:
|Standard wall function|
|air inlet||velocity||6 m.s-1|
|air outlet||relative pressure||0 kPa|
|discretization||First order upwind|
The underside of the hot steam flow pipes and the two diagonal plates attached to each pipe consist of porous materials. The use of porous media is due to increased contact surfaces between hot steam and cold airflow to enhance the steam distillation process. The viscous resistance in the porous medium is the inverse of the fluid permeability inside the porous medium. Since, according to the defined geometry, the direction of air blown by the fans is perpendicular to the y-axis, it is assumed that the amount of viscous resistance in this direction is zero; in other words, The air penetration in the porous media is infinite in this respect. The viscous resistance of the other two axes is 4465730 m-2.
The total inertia resistance is 139.6 m-1 applied perpendicular to the porous planes, so, considering the layout angle of the porous planes, the inertia resistance along the x-axis equals 120.9 (139.6 * cos30) and for the y-axis is 69.8 (139.6 * cos60). Also, since the flow in the x and z directions is assumed to be much more restrictive than the y-axis, the inertial resistance value in these two directions is assumed to be 1000 times their normal state. It should be noted that by defining the values of viscous resistance and inertia, there is no need to define the amount of porosity.
In general, the ratio of pressure drop to length characteristic of the system is equivalent to the momentum sink which consists of two terms related to viscous resistance and inertial resistance. In the following formula, 1% corresponds to the viscous resistance and C is the coefficient of inertia resistance.
S_i = -ΔP / l = – (µ / α_i v_i + C_i 1/2 ⍴_a | v | v_i)
Fixed Values (ACSC)
Since the purpose of the problem is the distillation process, therefore, the constant saturation temperature for the vapor must be defined because the condensation process takes place at the saturation temperature at constant pressure. This value is considered to be 319.75 K. In fact, this constant value of temperature is related to the saturation temperature at the steam pressure of the turbine output.
Boundary Condition Definition (ACSC)
All walls defined in geometry are considered insulated. For each of the eight embedded fans, the fan boundary condition is taken into account. In the fan boundary condition, the pressure jump should be used, as there will be a pressure difference on both sides of the fan in the specified direction. Since the fan is blowing in the vertical direction, the y-axis is considered as the compressive jump path. The value of the pressure jump is obtained by a polynomial function in terms of the axial velocity passing through the surface of the fans; hence, the polynomial is used.
All files, including Geometry, Mesh, Case & Data, are available in Simulation File. By the way, Training File presents how to solve the problem and extract all desired results.
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