The Confinement Effects on Jet Kinetic Momentum Flux Quantified by Measuring the Reaction Force

A turbulent jet is the most important flow element in mechanical ventilation. Mixing ventilation is based on the properties of turbulent jets. By entrainment into the jet the ambient air is set into motion. For a jet supplied within a room the enclosure may affect the jet in several ways, through:

1. Coanda effect which is the tendency of a fluid to be attracted to a nearby surface. A free jet is turned into a wall jet and the momentum flux of the jet decreases by friction against the room surfaces.
2. The jet collides with the opposing wall and the jet is transformed into a wall jet.
3. The size of the cross sectional area relative to the supply opening will affect the flow pattern within the enclosure. One can expect the direction of the inflow (entrainment) to the jet to be affected.
4. Location of supply and extract. The location of the supply is a factor that influences the pressure gradient within the room.

This paper considers the items 2,3,and 4.

The main characteristic of a jet is its momentum flux, but determining the momentum flux is not an easy task and has lead to contradicting results. Standard methods require velocity field measurements which have their restrictions and uncertainties. To overcome these problems a direct and more reliable method was used by recording the flow force, caused by an impinging jet, with a digital balance. The tests were carried out both for unenclosed (free jet) and enclosed cases. In the latter case tests were conducted with supply and extract both located on the same wall and located on opposite walls. Detailed pressure measurements were conducted to describe the details of the reaction force. There was a clear effect of the confinement on the reaction force and a Reynolds number dependence.