Wet Steam Simulation for Condensation inside a Steam Ejector, ANSYS Fluent
The present problem simulates the process of condensation steam inside an ejector using ANSYS Fluent software.
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Wet Steam Project Description
The present problem simulates the process of condensation steam inside an ejector using ANSYS Fluent software. This steam condensation process is modeled by defining a multiphase flow of wet steam. When the wet steam multiphase model is used, two sets of transport equations are solved: the mass fraction of the condensed liquid phase and the number or concentration of droplets per unit volume. This phase change model involves forming liquid droplets during a homogeneous non-equilibrium condensation process and is based on the classical non-isothermal nucleation theory.
This model causes the superheated dry steam to cool first after a rapid expansion of the steam and then to form a core, consisting of a two-phase mixture of saturated steam and liquid droplets, to which the mixture is called wet steam. The system in which the occurrence of the condensation process is investigated belongs to an ejector. An ejector is a mechanical device that uses an actuator fluid to suck in the secondary material. Finally, the two actuator fluids and the suction substance are mixed and removed from the system. Ejectors have two main functions, including vacuuming and suctioning gases, as well as mixing fluids.
The ejector structure is in the form of a convergent-divergent tube which, by entering the driving fluid into it and passing through the converging part of the nozzle, according to the continuity equation, causes the velocity of the passing fluid to increase by reducing the cross-sectional area of the flow; In fact, the potential energy of the fluid is converted into kinetic energy. Then, according to Bernoulli’s law, the fluid pressure decreases with increasing velocity and consequently the kinetic energy of the fluid. The resulting pressure drop creates a compressive vacuum inside the ejector, which causes the secondary material to suck into the ejector.
The primary stimulation fluid and the suction secondary material are now mixed and compressed in the diffuser section.
Ejector Geometry & Mesh
The current model is designed in two dimensions using Design Modeler software. The model corresponds to a two-dimensional plane of an ejector; So that it has a length of 0.411 m and an exit with a width of 0.02 m and the first entrance with a width of 0.0038 m, and a second entrance with a width of 0.0165 m. Also, the lower edge of the model is defined as an axis; That is, the two-dimensional geometry of the model can be rotated around this axis and turned into a three-dimensional structure. In fact, due to the perfectly symmetrical structure of the geometry of this ejector and to reduce the computational cost, the geometry is defined in two dimensions.
The meshing of the present model has been done using ANSYS Meshing software. The mesh type is structured and the number of production cells is equal to 25984.
We consider several assumptions to simulate the present model:
- We perform a density-based solver.
- The simulation is steady.
- We ignore the gravity effect on the fluid.
The following table represents a summary of the defining steps of the problem and its solution:
|near wall treatment||standard wall functions|
|Multiphase Model||Wet Steam|
|Inlet 1 (primary)||Pressure Inlet|
|gauge total pressure||198670 Pascal|
|Inlet 2 (secondary)||Pressure Inlet|
|gauge total pressure||1228 Pascal|
|gauge pressure||3000 pascal|
|wall motion||stationary wall|
|heat flux||0 W.m-2|
|flow||second order upwind|
|turbulent kinetic energy||second order upwind|
|turbulent dissipation rate||second order upwind|
|wet steam||first order upwind|
Results & Discussions
At the end of the solution process, we obtain two-dimensional contours related to pressure, velocity, temperature, turbulent kinetic energy, and the rate of mass production of liquid vapor (same as the density rate). It is also possible to obtain a three-dimensional contour of these parameters by rotating this two-dimensional contour around the central axis.
There are a Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.