K-factor data of interacting duct fittings based on measurement and CFD modelling

Energy use and the environment in mechanically-ventilated buildings are strongly influenced by the performance of heating, ventilation and air-conditioning (HVAC) systems, which is in turn governed by the accurate prediction of pressure loss. This paper presents the results of an investigation of pressure loss and associated loss coefficient (k-factor) for a number of interacting duct fittings in close proximity as commonly found in HVAC systems. The constant-injection tracer-gas technique and pitot-tube were used to measure the mean air velocity in ducts and the pressure distribution along the ducts was measured using static pressure tappings. k-Factors were calculated from the measured pressure loss and air velocity for each interacting duct fitting. Computational fluid dynamics (CFD) was used for prediction of air flow and pressure distribution in ducts. Predictions were compared with results obtained experimentally and CFD was found to be a useful method for the prediction of the k-factors of duct fittings. It will also be shown that duct fittings in close proximity interact in such a way that it is difficult to predict their behaviour based on evidence of the fittings' individual characteristics. The behaviour of interacting duct fittings as described in this investigation appears to be isolated to fittings of similar individual loss coefficients placed in close proximity without the effect of spacers. To establish this characteristic and fully to understand the complexity of interacting duct fittings there is an obvious need for further work in this area.