Experimental application of particle imaging to fluid velocity analysis in building drainage systems

Abstract

Previous research into the hydraulic and pneumatic conditions in the pipework associated with building drainage waste and ventilation (DWV) systems has been interpreted as suggesting that the flow regime within the vertical stack consists of an annular water flow entraining a central air core with an associated pressure drop. However, previous evidence had suggested that a significant proportion of the water was found in the air core by Pink [A study of the effect of stack length on airflow in drainage stacks. BRE Current Paper, 1973; 38/73] and Wyly and Eaton [1961. Capacity of stacks in sanitary drainage systems for buildings. U.S. Department of Commerce, National Bureau of Standards, Monograph 3]. A new model has recently been proposed by Campbell and MacLeod [Investigation of the causative factors of airflow entrainment in building drainage-waste-ventilation (DWV) systems. Building Services Engineering Research Technology 1999; 20(3): 99—104] which suggests that the airflow entrainment is due to the distribution of work between the water annulus and the droplets falling in the central air core. This article describes the investigation of the velocity profile of mixed-phase fluid flow in building DWV systems utilising the particle tracking velocimetry (PTV) flow measurement technique previously established by Campbell [The application of particle tracking velocimetry as a velocity measurement technique. Building Services Engineering Research Technology 2006; 27(4): 327—40]. PTV is a non-intrusive optical flow measurement technique that supplies an instantaneous sample of velocity throughout a two-dimensional plane in the flow field. The results of the PTV investigation support the assumption that water droplets exist in the air core and have a velocity greater than that of the water annulus.

Practical application: This article describes the application of particle tracking velocimetry (PTV) to the interaction between air and water in typical building drainage waste and ventilation (DWV) systems. Results indicate an unexpected distribution of mass across the stack discharge which, together with the nature of the air/water interface, governs air movement. The technique of PTV is particularly suited to this area, and represents the first steps towards eventual refinement of mathematical models designed to simulate building DWV system behaviour.