Effect of Vent Area Ratio on Vent Flow Characteristics and Temperature D istribution in a C ompartment F ire with T wo Ceiling Vents

In this study, the effect of the vent area ratio on vent flow characteristics and temperature distribution in a compartment during a fire in the compartment with two ceiling vents was examined. The area ratio (A1/A2) between ceiling vent 1 (CV1), with a duct height of 0.19 m, and ceiling vent 2 (CV2), with a duct height of 0.05 m, was varied from 0.052 to 0.542, 1, 5.587, and 19.36, and numerical simulation was performed under these conditions. As the A1/A2 ratio increased, the flow pattern changed from unidirectional inflow to bidirectional flow for CV1 and from bidirectional flow to unidirectional inflow for CV2. When the A1/A2 ratio was 0.542, the outflow and inflow mass flow rates for CV1 and CV2 were found to be nearly equal. The outflow and inflow mass flow rates at CV1 were higher than those at CV2 when the A1/A2 ratio was high, while the outflow and inflow mass flow rates at CV2 were higher than those at CV1 when the A1/A2 ratio was low. Furthermore, it was observed that the temperature was lower and the temperature difference between the higher and lower regions in the upper part of the measurement position decreased near the lower part of the vent with a larger area compared to the vent with a smaller area. This was probably because the mixing effect increased near the vent with a larger area due to the occurrence of the active bidirectional flow pattern. When the flow pattern at CV1 was bidirectional, the previous correlation for predicting mass flow rate was examined. It was found that the previous correlation underestimated the mass flow rate of the numerical simulation conducted in this study.