A New Device for Tracer Gas Dispersion in Ducts for Very Short Mixing Distance

The uniform tracer gas dispersion for very short distances, when measuring airflow by the constant emission method in HVAC systems is of great interest. In the present paper, the development of a new device for the injection of tracer gas is discussed with the objective of practical application in the field of air flow measurements. This injection device has a compact tubular shape, with magnetic fixation to be easy to apply to duct walls. One row of fifty-nine high speed micro jets in counter current direction, with the possibility of angular movement according to its main axle, makes the injection possible. A second separate device, with an effective integration function, feeds the sampling system for analysis. The device was tested in one special installation with a wind tunnel of approximately 21 m total length. The work section dimensions were 0.3m x 0.3 m. The distances between injection device and integration device considered were: X/Dh=22; X/Dh=4; X/Dh=2; and X/Dh=1. For the very short distances of X/Dh=2 and X/Dh=1, correction expressions were found, and a good repetition of air flow rate values was obtained. The final tests conducted demonstrated that the practical implementation of tracer gas techniques in HVAC systems, for measuring air flow rate with a very short mixing distance, is possible.