Numerical investigation of air curtain jet effect on the upper layer temperature evolution of a compartment fire and its transition

Highlights: • Air curtain effect on the compartment fire evolution investigated. • Inside temperature obtained for different opening size and air curtain conditions. • Upper layer temperature transition due to the presence of air curtain observed. • Correlation of the critical heat release rate with the new Froude numer proposed. This paper investigates the upper layer temperature evolution of a compartment fire and its transition behavior with air curtain jet conditions. Based on FDS simulation, a series of full scale models with a fire compartment of 4 m × 4 m × 4 m are set up. During simulation, different heat release rates, opening sizes and air curtain jet velocities are considered, while the upper layer temperature evolution inside the compartment is recorded by thermocouple trees. It could be seen from the results that the evolution of upper layer temperature could be distinguished into the rapid growth regime and the moderate developing regime, indicating the combustion transition inside the compartment. The normalized critical heat release rates of such transition are all around 0.6 for the compartment fire cases without air curtain jet condition, however the presence of air curtain would change the compartment flows resulting in the difference of the temperature inside and the critical condition of the transition. A new Froude Number $\frac{v_{\mathrm{jet}}}{\sqrt{\mathrm{gH}}}$ in relation to the air curtain jet velocity and the characteristic outflow velocity is defined, and finally a linear correlation of the normalized critical heat release rate with Froude Number $\frac{v_{\mathrm{jet}}}{\sqrt{\mathrm{gH}}}$ is well proposed.