Flow and Separation Characteristics of Three-dimensional Negatively Buoyant Wall Jets

Abstract

This paper reports a study of the behavior of cold air jets, specifically three-dimensional, incompressible, negatively buoyant, turbulent wall jets, and their separation from horizontal surfaces. With a cold-air distribution system, both the supply air temperature and flow rate are lower than in conventional systems for the same cooling load. The cold air is supplied to a zone at temperatures between 39°F and 49°F (3°C and 9°C) instead of the conventional 55°F (13°C). As the building's ventilation and cooling airflow supply temperature and flow rate are reduced below the conventional values, indoor thermal environment considerations become increasingly important. An analysis using integral forms of the momentum and energy equations is presented for determination of the separation point of buoyant turbulent wall jets. The analysis shows that the separation point is a function of the outlet Richardson number in addition to the geometry of the outlet diffuser or nozzle. Also reported in this paper is an experimental investigation of the mixing and separation of three-dimensional buoyant turbulent wall jets of air discharging into a large room. Velocity and temperature difference profiles and half-width growth rates for these jets are presented. The separation distances are also measured, and compared to the predicted results.