Hydrodynamic Characteristics of Mixed Flow in Liquid-Gas Ejectors

Introduction. The aim of this study is to identify the regularities governing mixed-flow formation and to determine the influence of ejector design elements on hydrodynamic characteristics.

Materials and methods.Ejection processes in ejectors with a cylindrical mixing chamber and active nozzles (jet and centrifugal-jet nozzles) are studied experimentally on a hydraulic stand and using numerical modeling tools in the academic version of the ANSYS 2020 R2 program with a maximum number of calculation cells of 512,000.

Results and discussion.It was found that the vacuum generated in a conventional jet sulfitator with a geometric characteristic of 126 and a standard jet nozzle is relatively low, reaching only 20–25 Pa. In contrast, replacing the standard nozzle with a centrifugal jet nozzle markedly increased rarefaction: at a distance of 20 mm from the nozzle outlet, the vacuum reached 425–555 Pa. Replacing the jet nozzle with a centrifugal jet nozzle while maintaining the same geometric characteristic of the ejector resulted in stable operation of the sulfitation station. Further analysis identified a rational range of the ejector geometric characteristic of m = 25–40 when using a centrifugal jet nozzle, which ensured stable rarefaction in the range of 78–100.4 kPa and reliable gas-phase ejection. Under a liquid pressure of 0.225 MPa at the nozzle, the ejection coefficient reached 2.5, which is sufficient to achieve the required technological pH of the liquid during the diffusion process.

At a liquid pressure in the nozzle channel of the nozzle of 0.25 MPa, its maximum speed reaches 20 m/s along the ejector axis. Due to the loss of energy for dispersion and resistance to the movement of droplets in a gas environment in the mixing chamber, the liquid speed at the ejector outlet decreases to 13.8 m/s. Two zones of mixed flow formation in the ejector have been established. In the first zone at a distance of up to 1.5 diameters of the mixing chamber, primary formation occurs with the formation of a homogeneous flow. In the second zone at a distance of up to 3 diameters of the mixing chamber, phase redistribution occurs and a ring flow regime is established. Based on these results, a rational design of a sulfitator for sugar production liquids was proposed to improve controllability and process stabilization. The working nozzle must provide sufficient surface area for phase contact, which is achieved using centrifugal-jet type nozzles, and the geometric characteristic of the ejector should remain within the identified range of 25–40 to ensure optimal vacuum generation and ejection efficiency.

Conclusions.Experimental and numerical (CFD) studies of ejection processes in liquid-gas sulfitators allowed us to establish the regularities of mixed flow formation and recommend rational size ratios when designing ejectors.