Optimization study of a Coanda ejector

Abstract

The Coanda effect has long been employed in the aerospace applications to improve the performances of various devices. This effect is the ability of a flow to follow a curved contour without separation and has well been utilized in ejectors where a high speed jet of fluid emerges from a nozzle in the ejector body, follows a curved surface and drags the secondary flow into the ejector. In Coanda ejectors, the secondary flow is dragged in the ejector due to the primary flow momentum. The transfer of momentum from the primary flow to the secondary flow takes place through turbulent mixing and viscous effects. The secondary flow is then dragged by turbulent shear force of the ejector while being mixed with the primary flow by the persistence of a large turbulent intensity throughout the ejector. The performance of a Coanda ejector is studied mainly based on how well it drags the secondary flow and the amount of mixing between the two flows at the ejector exit. The aim of the present study is to investigate the influence of various geometric parameters and pressure ratios on the Coanda ejector performance. The effect of various factors, such as, the pressure ratio, primary nozzle and ejector configurations on the system performance has been evaluated based on a performance parameter defined elsewhere. The performance of the Coanda ejector strongly depends on the primary nozzle configuration and the pressure ratio. The mixing layer growth plays a major role in optimizing the performance of the Coanda ejector as it decides the ratio of secondary mass flow rate to primary mass flow rate and the mixing length.

Simulation Analysis of a Coanda-Effect Ejector Using CFD

“…Qualitatively, the behavior of the present simulation corresponds to the experimental results used in [7]; however, a difference of velocity is noted between the zone of interface C close to the tube of compressed air above 412 m⁄s and the lower part of 348 m⁄s for the configuration of c = 0.3 mm and compressed air pressure of 0.3 MPa (black line). For the configuration of c = 0.3 mm and compressed air pressure of 0.2 MPa, we have values of 246 m⁄s and 307 m⁄s.…”

“…From literature and experimental tests, it may be possible to obtain a similar behavior of the influence of the separationc from the interface and ejector efficiency, verifying improvement in the speed profiles in the mixture zone, mainly finding increased velocities as the separation-c diminishes at the output of the high-pressure chamber [7]. Although several works have studied the influence of geometric and operational parameters, a correlation that permits estimating the behavior of the fluid within the ejector (fields of velocity and pressure) was not found in the state-of-the-art.…”

“…The flow regime in this configuration becomes supersonic, which generates a restriction that impedes the output of the primary air contained in the high-pressure region toward the mixture region and, hence, a decrease in the relation between the secondary mass flow with respect to the primary (Table 1 Given that the scope of this work does not integrate experimental results on the commercial device used, a qualitative comparison is made of the numerical results obtained through the computational simulation process with respect to the experimental and numerical results available in [7]. Although these results correspond to a geometry for which a separation-c equivalent to 0.2 and 0.4 mm is presented, along with a primary air pressure (compressed air) of 0.6 MPa, it is found that the velocity profiles obtained through computational simulation for the different configurations of this study (Table 1) are similar to the profiles obtained experimentally for Coandă-effect ejectors with similar dimensions.…”

“…According to [7] the w factor determines, under an energy perspective, the efficiency of the Coandă-effect ejector, which is defined as the relation between the entrained or secondary mass flow (m_s) with respect to the injected or primary mass flow (m_p).…”

Improving the Performance of a Thermal Compressor in a Steam Evaporator via CFD Analysis

“…The sonic and supersonic nozzles were adopted as primary driving nozzles, and a movable cone cylinder, inserted into a conventional ejector-diffuser system, was used to change the ejector throat area ratio. Kim et al [11] investigated the influence of various geometric parameters and pressure ratios on the Coanda ejector performance. In Coanda ejectors, the secondary flow is dragged into the ejector, following a curved contour without separation, due to the primary flow momentum.…”