Federico Mazzelli

The surface roughness effect on the performance of supersonic ejectors

The paper presents the numerical simulation results of the surface roughness influence on gas-dynamic processes inside flow parts of a supersonic ejector. These simulations are performed using two commercial CFD solvers (Star- CCM+ and Fluent). The results are compared to each other and verified by a full-scale experiment in terms of global flow parameters (the entrainment ratio: the ratio between secondary to primary mass flow rate - ER hereafter) and local flow parameters distribution (the static pressure distribution along the mixing chamber and diffuser walls). A detailed comparative study of the employed methods and approaches in both CFD packages is carried out in order to estimate the roughness effect on the logarithmic law velocity distribution inside the boundary layer. Influence of the surface roughness is compared with the influence of the backpressure (static pressure at the ejector outlet). It has been found out that increasing either the ejector backpressure or the surface roughness height, the shock position displaces upstream. Moreover, the numerical simulation results of an ejector with rough walls in the both CFD solvers are well quantitatively agreed with each other in terms of the mean ER and well qualitatively agree in terms of the local flow parameters distribution. It is found out that in the case of exceeding the “critical roughness height” for the given boundary conditions and ejector’s geometry, the ejector switches to the “off-design” mode and its performance decreases considerably.

Ejector CFD Modeling

The prediction of the supersonic ejector dynamics implies the accurate description of all the complex flow features discussed in Chap. 2. Unfortunately, the theoretical modeling approach necessitates a number of simplifying assumptions and empirical constants that introduce significant uncertainty and reduce the capability of capturing a number of relevant flow features. In this respect, computational fluid dynamics may represent a tool to overcome these difficulties and analyze the flow details of arbitrary ejector geometries.

Physics of the Ejectors

The global behavior of the ejector results from a combination of complex flow features including shock trains, turbulent mixing layers bounded by wall regions, shock-induced separations, boundary layers subject to adverse pressure gradients, non-equilibrium phase change, etc. It is because of this complexity that ejector design and performance have thus far been difficult to characterize and optimize.

Ejector Chillers for Solar Cooling

Ejector chillers are being studied at Department of Industrial Engineering of Florence (DIEF) since 2000, both theoretically and experimentally. This chapter discusses the application of solar-powered chillers in air conditioning and details the fundamental parameters. This technology is not far from being a technically feasible alternative to commercially available single-stage absorption chillers, but obviously a huge effort is still needed to improve its performance and gain access to the market.