Steam Ejector in Refrigeration Cycle Simulation, Paper CFD Validation, ANSYS Fluent

Paper Description

The present problem simulates the water vapor flow inside a steam ejector by ANSYS Fluent software. This numerical simulation is based on the reference paper “CFD simulation on the effect of primary nozzle geometries for a steam ejector in refrigeration cycle” and the results of the present numerical work are compared and validated with the results in the reference article. An ejector is a mechanical device that uses an actuator fluid to suck a secondary material (gas, liquid, or solid particles), and finally the actuator fluid and the suction substance are mixed together and exit from the system.

Ejectors have two main functions, including vacuuming and suctioning gases, as well as mixing fluids. The ejector modeled in the present work has a convergent-divergent structure and inside it, a convergent-divergent nozzle is located. The operating mechanism of this ejector is such that the water vapor enters the ejector as the primary fluid and by passing through the convergent part of the nozzle, which has a smaller cross section, it accelerates and according to Bernoulli’s law, the pressure decreases.

Paper Description

On the other hand, this pressure drop causes a pressure vacuum inside the ejector and as a result causes the secondary fluid (suction) to suck into the ejector. The primary and secondary fluids are then mixed together in the diffuser section and compressed. The present ejector is part of a refrigeration cycle; In this way, the primary inlet fluid comes from the boiler outlet, the secondary inlet fluid comes from the evaporator outlet, and the outlet goes to the condenser.

Water vapor from the boiler outlet in the cycle with a saturation temperature of 403.15 K and at a relative pressure corresponding to this temperature of 270,000 Pascal goes to the input of the primary fluid of the ejector. On the other hand, water vapor is sucked into the ejector from the evaporator outlet with a saturation temperature of 280.65 K and a pressure corresponding to this temperature of 1000 Pascal. Finally, the resulting mixed steam is discharged from the ejector outlet to the condenser with a saturation pressure of 3000 Pascal and a corresponding temperature of 297.23 K.

Steam Ejector Geometry & Mesh

The present model is designed in two dimensions using Design Modeler software. Due to the symmetrical structure of the ejector nozzle and diffuser, only half of the ejector is designed. The geometry consists of two inlet sections for floes related to the primary and the secondary fluid, one outlet for the mixed output fluid and an internal part as the nozzle for the primary fluid flow.

In terms of geometric scales, the model designed based on Table 1 of the article (d2M4); So that the throat diameter of the nozzle related to the primary fluid is equal to 2 mm and the ratio of the ejector area (the ratio of the throat diameter of the ejector to the throat diameter of the primary fluid nozzle) is equal to 90. The following figure shows a view of the geometry.

The meshing of the model has been done using ANSYS Meshing software and the mesh type is structured. The element number is 51990. The following figure shows the mesh.

Steam Ejector CFD Simulation

To simulate the present model, several assumptions are considered:

• The simulation is density-base; Because in models such as the convergent-divergent nozzle, the fluid is compressible and the flow Mach number is high.
• The simulation is steady.
• The gravity effect on the fluid is ignored.

A summary of the defining steps of the problem and its solution is given in the following table:

	Models
Viscous model		k-epsilon
	k-epsilon model	realizable
	near-wall treatment	standard wall function
Energy		on
	Boundary conditions
Inlet – primary steam		Pressure inlet
	gauge pressure	270000 Pascal
	temperature	403.15 K
Inlet – secondary steam
	gauge pressure	1000 Pascal
	temperature	280.65 K
Outlet		Pressure outlet
	gauge pressure	3000 Pascal
	temperature	297.23 K
walls		Wall
	wall motion	stationary wall
	heat flux	0 W.m-2
	Solution Methods
Formulation		Implicit
Spatial discretization	flow	second order upwind
	turbulent kinetic energy	second order upwind
	turbulent dissipation rate	second order upwind
	Initialization
Initialization method		Hybrid

Paper Validation

At the end of the solution process, the results of the present numerical simulation are compared and validated with the results in the article. For this purpose, the input ratio of the ejector (entrainment ratio) has been used. This expression is equivalent to the ratio of the mass flow rate of the secondary fluid to the mass flow rate of the primary fluid. The mass flow rates of the primary and secondary fluids at the input points of the primary and secondary fluids were obtained using the Fluent software report and the ratio was calculated.

This ratio has been compared and validated with the value in Table 3 of the reference article (the d2M4 mode in the table).

mass flow rate for inlet – primary steam	kg.s-1
mass flow rate for inlet – secondary steam	kg.s-1

 

Entrainment Ratio in reference paper	0.2849
Entrainment Ratio in present simulation	0.286

Also, after the completion of the solution process, we obtain two-dimensional contours related to Mach number, pressure, velocity, and temperature.

You can obtain Geometry & Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.