A semi-empirical investigation of transient ventilation rates in retail stores

Using the tracer gas technique to establish an accurate field measurement of the ventilation rate in a big-box retail store requires a significant investment in terms of labor, time, and instrumentation. Although a numerical method such as the multi-zone model has the potential to calculate airflow characteristics across building components, the accuracy of a calculation derived in this way depends on the quality of the model input The present study extends the experiments in ASHRAE RP1596 to develop a quick and accurate calibrated multi-zone ventilation model to enable further investigations into the ventilation performance of retail stores including the mechanical airflow rate and unintentional airflow. Specifically, this study (1) investigates air infiltration across frequently used automatic doors and (2) establishes significant parameters that quickly improve the accuracy of the simulation model. For the infiltration study, an airflow element was modified based on hourly door traffic passage through automatic entrance doors to predict physical conditions including differential pressure and direction across frequently used automatic entrance doors. The calibration of transient differential pressure across automatic entrance doors was achieved using a multi-zone model to calculate the transient airflow rate across the doors. In terms of establishing significant parameters for the ventilation model, the study developed a calibration procedure based on a number of measured data used in the model setting starting from the least effort to the greatest effort. The simulation result of the SF6 concentration decay at each calibration step was then calibrated with the field-measured data.;For the modified airflow element in the leakage area model, the frequency fluctuation of the differential pressure across the automatic entrance doors agreed well with that reported in the field-measured data. It was found that the automatic entrance doors constituted the main path of the unintentional airflow, which comprised 76--92% of the infiltration throughout the entire building. The greatest improvement in terms of model accuracy with the time-effective method was effected by more accurately measuring the outdoor air fraction measured from the rooftop units. The model uncertainties were improved by 42--119% over the uncertainties of the base case model, and the estimated total effort for this task was 3--15 hours depending on the number of rooftop units subjected to monitoring. Then the corrected airflow characteristics across the automatic entrance doors further improved the model accuracy by 29--56% but only in the building with infiltration at the automatic doors. The proposed calibration procedure including the recommended significant input parameters can reduce uncertainties in the simulation models. However, it is also necessary to improve on previous models by using high accurate sensors and measurement techniques to in turn obtain more accurate measurements of the outdoor air fraction.