Methodology for the Evaluation of Natural Ventilation in ... - Cham

applications involving full-scale buildings, scaled models using water or air, and wind tunnels.These are the three areas that are currently used to evaluate building design, and each has its ownunique requirements for flow visualization. There are three methods of flow visualizationdefined by Merzkirch: one which introduces foreign material into a flow (indirect method), onewhich records the varying density (optical method), and one which introduces energy into a flow.The latter two normally apply to compressible fluids, and are therefore not presented here. Theformer is applied to incompressible fluids, including air, and does not release further energy intothe flow, which may influence the initial flow.Flow visualization is commonly used to evaluate mechanical systems to ensure that air is beingintroduced and exhausted properly, and that the occupants have a healthy and comfortableenvironment in which to work. Several studies, including one sponsored by the USDOEBuilding Technologies Program (McWilliams 2002) cover the types of airflow measurementtechniques used in evaluating indoor environments. Fogging machines and smoke pencils havebeen used extensively in evaluating such systems as displacement ventilation and under-floorsystems. The airflow rate is controlled, with high enough velocities so that media can beintroduced and flow patterns traced and captured on film.When air is the fluid used in the experimental procedure at both small and full scale, issues ofvisualization and neutral buoyancy are important, in addition to the dissipation rate, or ‗hangtime‘, of the tracer material. This is of note particularly in full-scale buildings when largevolumes of air and turbulent mixing quickly dilute the tracer material and make visualization ofairflow patterns on a large scale difficult. Localized flow patterns in full-scale buildings canmake use of methods such as smoke pencils; if the local air velocities are relatively slow (lessthan 0.5 m/s).In evaluating fluid flow patterns in air models, in full-scale spaces and reduced-scale models, theability of a tracer material to visibly follow the airflow, without effecting or changing it, isimportant. This requires the tracer material to be neutrally buoyant, be visible, and not dispersetoo quickly. For air models, these requirements leave only a few alternatives that were evaluatedfor use in both the full-scale prototype building and the reduced-scale air model. They includedDraeger smoke pencils, fog generating machine, and helium bubbles. In developing themethodology for evaluating and designing naturally ventilated buildings, the ability not only toanalyze the temperature profile throughout the occupied space, but also to visualize the flowpatterns within the space provides a stronger impetus for using the methodology. With thecapability to map the path of outside air as it is introduced, move through the space beingventilated, and finally be exhausted, the methodology combined with flow visualizationtechniques can further the understanding and improve the ventilation effectiveness, therebyimproving the design of naturally ventilated buildings.4.2.3 Methods UsedFor use in the MIT test chamber with the reduced-scale air model, a marker was needed thatwould be neutrally buoyant, non-toxic, and highly visible. Neutrally buoyant helium bubbleswere initially used for both a full-scale and reduced-scale room test case. This method involveda single-head Sage Action Helium Bubble Generator, which connected a tank of helium gas to areservoir of bubble fluid and an air compressor. Through trial and error, neutrally buoyant79