Ejector Design Calculation.pdf
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How to Design an Ejector for Efficient Refrigeration
Ejectors are devices that use a high-pressure fluid to entrain and compress a low-pressure fluid, creating a mixed fluid that can be used for various applications. One of the most common uses of ejectors is in refrigeration systems, where they can improve the performance and efficiency of absorption chillers. However, designing an ejector is not a trivial task, as it involves complex thermodynamic and fluid dynamic phenomena. In this article, we will review some of the basic principles and methods for ejector design, as well as some of the challenges and opportunities for optimization.
Types of Ejectors
According to the position of the primary nozzle, which is the component that expands and accelerates the high-pressure fluid, ejectors can be classified into two categories[^1^]:
Constant-area mixing ejectors: The nozzle exit is located within the cylindrical part of the ejector, and the mixing of primary and secondary fluids occurs here. This type of ejector has a simpler geometry, but it is less efficient than the other type.
Constant-pressure mixing ejectors: The nozzle exit is located in a suction chamber (usually conical) in front of the cylindrical part, where the secondary fluid is entrained. The mixing process takes place in a converging duct formed by the expanding primary flow and the suction chamber wall. This type of ejector has a more complex geometry, but it offers superior performance and flexibility.
In both cases, the mixed fluid exits the ejector through a diffuser, which recovers some of the kinetic energy into pressure.
Design Parameters
The main design parameters that affect the ejector performance are[^2^]:
Primary nozzle area ratio: The ratio between the nozzle throat area and the nozzle exit area. This parameter determines the degree of expansion and acceleration of the primary fluid.
Ejector area ratio: The ratio between the nozzle exit area and the diffuser throat area. This parameter determines the degree of compression and deceleration of the mixed fluid.
Mixing zone length: The distance between the nozzle exit and the diffuser throat. This parameter determines the extent of mixing and momentum transfer between the primary and secondary fluids.
Secondary flow inlet angle: The angle between the secondary flow direction and the axis of the ejector. This parameter affects the entrainment and pressure recovery of the secondary fluid.
Other geometrical details, such as fillets, curves, or bends, may also have an influence on the flow field and should be considered in a detailed analysis.
Design Methods
Ejector design can be performed at various levels of complexity, depending on the accuracy and completeness required. Some of the most common methods are[^1^]:
Zero-dimensional design: This method is based on thermodynamic equations that relate the flow properties at different sections of the ejector. It assumes ideal gas behavior, adiabatic and reversible processes, constant specific heats, and negligible friction losses. It is a simple and fast method, but it does not account for shock waves, boundary layer effects, or geometrical details.
One-dimensional design: This method is based on fluid dynamic equations that describe the variation of flow properties along streamlines. It accounts for real gas behavior, irreversible processes, variable specific heats, and friction losses. It also considers different flow regimes that may occur in the ejector, such as subsonic, supersonic, or choked flow. It is a more accurate and realistic method than zero-dimensional design, but it still neglects shock waves, boundary layer effects, or geometrical details.
Computational Fluid Dynamics (CFD) design: This method is based on numerical simulations that solve the governing equations for mass, momentum, energy, and species conservation in three dimensions. It accounts for all physical phenomena that occur in the ejector, such as shock waves, boundary layer effects, turbulence, heat transfer, or chemical reactions. It also allows for a detailed 061ffe29dd