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

6.5 ExperimentsTo evaluate the model under the different cases of natural ventilation, experiments wereconducted. This was completed for buoyancy, wind, and combined buoyancy-wind drivennatural ventilation experiments using the reduced-scale air model. The summary of modelvariables that were altered is listed in Table 23. These experiments were carried out at steadystate conditions, and did not account for variations in external or ambient temperature or windconditions that might occur in the prototype building, but were run multiple times to ensurerepeatability. This section describes the sets of buoyancy and wind-driven experiments that werecompleted during the process of developing and evaluating this modeling methodology.Table 23. Matrix of Model VariablesVariable Option 1 Option 2 Option 3Number of Heated Zones 1 2 4Status of Heaters On Off ----Location of Open Windows Used Lower Upper Lower and UpperStack Status All Closed All Open Some Open6.5.1 BuoyancyUsing natural ventilation alone to passively cool and ventilate a building under the buoyancydriven case is the critical design situation for applying this ventilation strategy in buildings. Thehighest internal temperatures occur on hot summer days when there is no wind driving theairflow. The analysis of the buoyancy driven case is complicated by the need to considermultiple and interdependent design parameters including the size of inlets and outlets, the heightof the space, the strength of the heat sources driving the airflow, and the resulting temperaturedifference between the interior and exterior spaces. It is this complexity and the lack ofunderstanding of the physical mechanisms involved in buoyancy driven natural ventilation thatreduce the effective use of natural ventilation in building design. Much research has beencompleted in the theoretical aspects of buoyancy driven natural ventilation, using a variety ofmethods, including physical and computer modeling of single story and multiple story spaces.Analyses are most often done under steady-state conditions to simplify otherwise complexphenomena and investigate the impact of the above parameters on the airflow through the spaceof interest.Assumptions found in many buoyancy driven natural ventilation models should be understoodbefore using the modeling method. Among the issues that arise, include the assumption of a wellmixed, or uniform interior temperature and uniform velocity across inlet and outlet openings.The restrictiveness of the zones under consideration affects the effectiveness and behavior of theairflow for a building, which in turn helps to determine the location of and number of neutralplanes within the building or space under investigation. Li investigated natural ventilation inbuildings with large openings and defined internal pressures for each zone, relative to the outsidepressure. This method created neutral planes for each internal zone (Li 2000). However, inexperimental modeling, measuring the internal and external pressures can be difficult, so massflow balances were used in determining the neutral plane in the reduced-scale model.106