Accurately measuring the airflow delivered by tunnel ventilation fans during site acceptance testing can be difficult due to the fan configuration and constrained sizes of adjoining plenums. Most axial flow tunnel ventilation fans are reversible with drive motors attached directly to the fan impeller. Obstructions created by the motor supports preclude performing pitot tube traverses in the fan section and the presence of transition ductwork and sound attenuators on each side of the fan dictate that airflow measurements be taken at the interface of fan equipment trains and adjoining plenums. Point air velocity measurements taken with an anemometer at one end of the fan equipment train are averaged to calculate the fan airflow. Limited floor space in tunnel ventilation buildings may result in narrow plenums adjacent to fan rooms that produce non-uniform flow with high velocities concentrated on one side, making the accuracy of the anemometer point traverses problematic. This paper presents the results of a study of optimum grid spacing of point anemometer measurements to yield more accurate field measurements using CFD modeling of sample fan room and plenum arrangements. Alternative grid spacings are evaluated against the modeled air velocity distribution to determine recommended spacing of measurement points.
