Diffuser Orifice Plate Effects on ACSC Performance (Validation)
This project validates the ACSC system in a 600 MW power plant based on the article by CFD simulation.
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This project simulates the ACSC system in a 600 MW power plant based on the article “Effects of diffuser orifice plate on the performance of air-cooled steam condenser” and the results of the current numerical CFD simulation work are compared and validated with the reference paper. The main purpose of designing and using these systems in power plants is to prevent energy waste; In this way, these systems condense the hot steam coming out of the steam turbine and transfer the water from the distillation process to the pump section of the steam turbine.
The power plant studied in the present work consists of 7 rows of ACSC system. The fourth row is studied in this CFD simulation. Each of these rows of ACSC systems consists of eight fans, and each of these fans is located under two diagonally porous plates. The operation of these systems is such that the hot and low pressure steam of the turbine outlet passes through each of the pipes and then is transferred to the interior space of the diagonal plates installed on both sides of that pipe.
The underside of the pipe and the interior of the diagonal plates have a porous structure; Because, the use of a porous medium increases the contact levels between the hot steam and the cooling fluid, and as a result, the heat transfer is increased and the condensation phenomenon is amplified. On the other hand, the air blown by the fans installed under both diagonal plates acts as a cooling fluid and causes the condensation of the vapor. In fact, the fan sucks the cool air in the space below and transfers it to the plates, and the cool air is heated after heat exchange and leaves the system as hot air.
The diagonal structure of these plates causes the water resulting from condensation to move down due to the gravity of the earth and eventually leaves the system. In the reference paper, two modes of fan with and without diffuser have been studied. The system is located within a larger computing area; So that the velocity at the inlet of this area acts as the velocity of free flow. The different velocities studied in these two modes include 0, 3, 6, 9 m.s-1. The present numerical work only simulates the fan without diffuser mode at a speed of 6 m.s-1 for the validation.
In the present model, the lower part of the hot steam flow pipes and two diagonal plates connected to each pipe are made of porous materials. The viscous resistance in a porous medium is the inverse of the fluid permeability within the porous medium. Since according to the defined geometry, the direction of the air flow blown from the fans is in the vertical direction (y axis), it is assumed that the value of the viscous resistance in this direction is zero. While the viscous resistance along the other two axes is 4465730 m-2.
The total inertial resistance is equal to 139.6 m-1 which is applied perpendicular to the porous plates; Therefore, according to the angle of position of the porous plates, the inertial resistance in the direction of the x-axis is equal to 120.9 (1330.6 * cos30) and in the direction of the y-axis is equal to 69.8 (1360.6 * cos60). Also, since the flow in the x and z directions is assumed to be much more limited than the y-axis, the amount of inertial resistance in these two directions is assumed to be 1000 times their normal state.
The purpose of the problem is the distillation process; Therefore, a constant saturation temperature for steam must be defined; Because the condensation process takes place at saturation temperature at a constant pressure. This value is 319.75 K. Also, for each of the eight installed fans, the fan boundary condition is considered. In the fan boundary condition, a pressure jump must be used; Because 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 pressure jump path. The amount of pressure jump is obtained based on a polynomial function in terms of the axial velocity passing through the fan surface; Hence, polynomial is imported in this section. The main purpose of this work is to investigate the effect of open air flow velocity on the performance of the fans of the system in terms of volume transfer from the cooling air flow.
To investigate the volumetric flow rate of the fans, a dimensionless parameter equivalent to the ratio of the volumetric flow rate transferred from the fans resulting from the numerical solution to the volumetric flow rate flowing from the fans in ideal state, is defined. The ideal volume flow rate is 428 m.s-1 and the dimensionless parameter is defined as volumetric effectiveness. The following figure shows an example of a study model.
ACSC Geometry & Mesh
The present 3-D model is designed using SOLIDWORKS and Design Modeler software. The geometry is considered as the environment with dimensions of 500 m * 160 m * 300 m in which air flows in this area; Thus, a section is defined as an inlet, a section as an outlet, a floor as a ground surface, and other surfaces as symmetrical surfaces. Inside, there is a cooling system consisting of eight rows of hot steam pipes, diagonally porous plates on either side of each pipe, and eight fans on one platform. The following figure shows a view of the geometry.
The meshing of the present model is done using ANSYS Meshing. The mesh type of the model is hybrid and the element number is 2668772. The following figure shows the mesh.
ACSC CFD Simulation Settings
To simulate the present model, several assumptions are considered:
- We perform a pressure-based solver.
- The simulation is steady.
- The gravity effect on the fluid is equal to -9.81 m.s-2 along the Y-axis.
A summary of the defining steps of the problem and its solution is given in the following table:
|near-wall treatment||standard wall function|
|Boundary conditions (ACSC)
|wall motion||stationary wall|
|gauge pressure||0 Pascal|
|momentum||first order upwind|
|turbulent kinetic energy||first order upwind|
|turbulent dissipation rate||first order upwind|
|energy||first order upwind|
|gauge pressure||0 Pascal|
ACSC Paper Validation Results
At the end of the solution process, the graph of volumetric effectiveness changes in terms of the fan number in the system is obtained and validated with the results of the source paper. The diagram under consideration of the article is related to Figure 5-c of the article. The result of the comparison is shown in the following figure.
There are a Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.