Entrainment and its implications on microclimate ventilation systems: Scaling the velocity and temperature field of a round free jet

Near-Field Flow Structure and Entrainment of a Round Jet at Low Exit Velocities: Implications on Microclimate Ventilation

This paper explores the flow structure, mean/turbulent statistical characteristics of the vector field and entrainment of round jets issued from a smooth contracting nozzle at low nozzle exit velocities (1.39–6.44 m/s). The motivation of the study was to increase understand of the near field and get insights on how to control and reduce entrainment, particularly in applications that use jets with low-medium momentum flow like microclimate ventilation systems. Additionally, the near field of free jets with low momentum flow is not extensively covered in literature. Particle image velocimetry (PIV), a whole field vector measurement method, was used for data acquisition of the flow from a 0.025 m smooth contracting nozzle. The results show that at low nozzle exit velocities the jet flow was unstable with oscillations and this increased entrainment, however, increasing the nozzle exit velocity stabilized the jet flow and reduced entrainment. This is linked to the momentum flow of the jet, the structure characteristics of the flow and the type or disintegration distance of vortices created on the shear layer. The study discusses practical implications on microclimate ventilation systems and at the same time contributes data to the development and validation of a planned computational turbulence model for microclimate ventilation.

Potential application of using vortex ring for personalized ventilation

A novel vortex ring personalized ventilation system (VRPV) is proposed for efficiently supplying fresh air to room occupants. A vortex ring generator with a piston-cylinder is developed for an experimental study of the formation, transportation, and ventilation characteristics of the VRPV. The translational velocity, volume, and fresh air ratio of the vortex rings are studied using high-speed cameras and tracer gas experiments. According to the results, the categories of the vortex ring volume in the formation stage are studied. It is observed that the velocity of the piston determines the initial translational velocity of the vortex ring, and a fitting equation is proposed to predict the evolution of the translational velocity. The deviation range of the VRPV over different distances is studied, and it is shown to be affected by interference from both the generator and the environment. Finally, the total volumes, fresh air volumes, and fresh air ratios of the VRPV are studied at different distances. The results indicate that, as a personalized ventilation system, the fresh air ratio of the VRPV is up to 159.3% higher than that of a symmetrical round jet within a 0-4 m range. This shows the excellent application potential of the VRPV for providing high-efficiency personalized ventilation with lower fresh airflow rates.