Analysis of makeup air in a natural smoke vent system in a tall space using numerical simulation and Schlieren technique

Abstract

Purpose The purpose of this paper is to analyze the phenomenon of makeup effect using numerical simulation and model experiments on seven different natural smoke extraction patterns of tall space. Airflow distribution and heat accumulation phenomenon in different cases are compared. The natural smoke exhaust system for tall spaces has many advantages, including low cost, no power and low maintenance cost. It is more advantageous than the mechanical type of exhaust. However, the internal air distribution is complicated since the large span spatial character. Effective and correct verification method is very important for the analysis of flow fields in tall spaces. Design/methodology/approach This study used fire dynamics simulator (FDS) software to simulate the fire scene. The model experiments are conducted to determine if the numerical simulation results are reasonable. A single-mirror Schlieren system, including an 838 (H) × 736 mm (W) square concave mirror, as well as the focal length of 3,100 mm was adopted to record the dynamic flow of hot gas. Six smokeless candles were burned in a 1/12.5 model in experiments to record the distribution of inflow, accumulation and outflow of airflow in the space. In addition, the thermocouple lines were mounted in the model for temperature measurement. Findings The results of numerical simulation and model experiments have proved that makeup air has a significant effect on the effectiveness of a natural smoke vent system. Larger areas of smoke vents will produce more heat accumulation phenomenon. In this study, the air inlet and vent installed on the same side have a better heat removal effect. Moreover, Schlieren photography technique is proved to be an accurate measurement method to record the dynamic flow of hot air immediately, directly and accurately. The dynamic flow behavior of hot gas in the model has been visualized in this paper. Originality/value At present, there is no examination method other than checking the smoke vent area to validate the effectiveness of a natural smoke vent system in Taiwan, as well as no requirements regarding the makeup inlet. The effect of makeup air in generating the effective push-pull phenomenon of airflow has been analyzed. In addition, the post-combustion hot gas distributions were visualized by using Schlieren photography technology in the model space, compared with the FDS simulation result and thermocouple recorded temperature. A verification method in the model experiments is established to determine if the numerical simulation results are reasonable.