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Steam Ejector in Refrigeration Cycle CFD Simulation

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The ejector is a mechanical device that uses a primary (motive) fluid to suck up a secondary fluid (gas, liquid, or solid particles). Eventually, the ejector mix the two primary and secondary fluids and they exit from the outlet. Ejectors have two major tasks, including creating vacuum and gas suction as well as fluid mixing. The basis of the ejector is to create a vacuum for the suction of the desired fluid (such as powder, granule, sludge, etc.), which is based on the continuous conversion of kinetic and pressure energy.

 

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Description

Ejectors

Ejector performance is like vacuum pumps, but because of the lack of moving mechanical components inside the ejector building, they are more economical. Industries like food industries (to improve fruit concentrate quality), refineries (to separate heavy oils at high temperatures), refrigeration cycles (to increase steam pressure as a compressor replacement) and … apply ejector.

The ejector structure is a convergent-divergent nozzle. By entering the primary fluid through the convergent section of the nozzle – according to the continuum equation – causes the fluid velocity to increase by decreasing the cross-sectional area of the flow, in fact converting the potential energy of the fluid into kinetic energy. Then, due to Bernoulli’s equation, the fluid pressure decreases with increasing velocity and consequently the kinetic energy of the fluid. The resulting pressure drop causes a vacuum inside the ejector, which suck the secondary fluid. The primary fluid and the suctioned secondary fluid are then mixed and compacted in the diffuser section.

Steam Ejector Project Description

The present problem deals with the flow of water vapor as the main fluid (primary) and the secondary fluid (suction) within a convergent-divergent ejector. The purpose of the present simulation is to investigate the behavior of primary and secondary fluid after passing through the internal convergent-divergent nozzle and the ejector diffuser. In the present model, due to the vacuum pressure difference between the two inlet fluids, the suction phenomenon for the secondary fluid has to occur. The Mach number corresponding to the fluid flow inside the ejector increases as well. To analyze this model, we investigate parameters such as Mach number, velocity, and pressure based on the motion of the fluid flow along with the ejector.

Assumption

  • The simulation is STEADY.
  • The solver is density-based because in models such as the above-mentioned convergent-divergent nozzle, the fluid is compressible and the flow Mach number is high (more than 3).
  • We use the axisymmetric method and draw half of the geometry of the steam ejector.
  • We don’t consider the effect of Earth’s gravity on the simulation.

Geometry & Mesh

We design the present 2-D model by the Design Modeler software. Due to the symmetrical structure of the nozzle and the ejector diffuser, we design only half of the ejector. The desired geometry consists of two inlet sections for the flows related to the actuator and the secondary fluid, an outlet for the mixed output fluid, and the nozzle wall. Meshing was performed by ANSYS Meshing software. The mesh type is unstructured and the element number is equal to 51990. Due to the sensitivity of the fluid to the cross-sectional area, manual meshing was used at each of the defined edges to increase mesh accuracy at more sensitive points. Face Meshing has also been used to uniform the structure of the mesh.

Staem Ejector CFD Simulation Steps:

Here is a summary of the steps to define and solve the problem in the table:

 (Model)
k-epsilon
k-epsilon Realizable
Standard wall function
energy equation on
 (boundry conditions) steam ejector
inlet pressure inlet
primary fluid gauge pressure 270000 Pa
supersonic pressure 3000 Pa
temperature 403.15 K
secondary fluid gauge pressure 1000 m.s-1
temperature 280.65 K
supersonic pressure 3000 Pa
outlet Pressure outlet
pressure 3000 Pa
backflow temperature 297.23 K
Wall
 (Methods) steam ejector
flux Roe-FDS
discretization momentum Second order upwind
kinetic First order upwind
dissipation First order upwind
(initialization)
Hybrid

Density-Based Solver for Ejector CFD Simulation

As the present model uses a convergent-divergent nozzle to drive the fluid, the fluid velocity is significantly increased, so that the fluid velocity exceeds the velocity of sound within the fluid. The actuator fluid and the secondary fluid then mix and compress. In such problems with a Mach Number more than .3, a Density-Based solver is used.

Boundary Condition for Ejector CFD Simulation

The operating pressure is defined as zero, so in this case, there will be no difference between absolute pressure and relative pressure. Since the mechanism of the ejector is based on the suction of a secondary fluid by converting the pressure energy of a driving fluid into kinetic energy, a pressure condition should be used to define the boundary conditions at the injector inlet and outlet.

How to select the total relative pressure at the two ejector inlets?
The larger pressure at the primary fluid inlet causes the suction pressure for the secondary fluid. The Pressure Total Gauge option is used to define the pressure difference for driving the fluid flow into the system, while the Supersonic Gauge Pressure option is used to define the static pressure where the flow is supersonically localized. The Initial Gauge Pressure is applied to define the pressure used at the inlet boundary when initializing based on this boundary. It should be added that in incompressible flows the “P_total = P_static + 0.5⍴V2” equation 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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