Abstract: In order to study the impact of air gap on the heat transfer of solid fuel arrays, a reduced-scaled experimental platform was established, and a series of solid fuel arrays with different air gaps were designed to analyze the variation of parameters such as total heat flux, ignition time and flame height. It was found that with the increase of air gap, the heat flux released from the fuel array shows a trend of increasing and then decreasing, and when the air gap is no more than 8 cm (when the fuel coverage is less than or equal to 50%), the heat flux released by the fuel arrays is higher than that of the condition without air gap. When the air gap exceeds 8 cm, with a fuel coverage rate above 50%, the heat flux is lower than that of the condition without air gap. By introducing the concept of ignition delay time, the impact of air gaps on ignition delay time is quantitatively described. The ignition delay time increases with the increase of air gap and shows a good exponential relationship, based on which a model for the heat flux received by solid fuel arrays under different air gaps was established. This paper also quantitatively analyzes the effect of air gaps on the heat flux released by the fuel arrays, showing that air gaps influence the released heat flux by altering flame height. As the air gap increases, the flame height first increases and then decreases. When the air gap is no more than 10 cm, with a fuel coverage rate not exceeding 55.6%, the flame height of the fuel arrays is higher than that in the condition without an air gap. However, when the air gap exceeds 10 cm, with a fuel coverage rate above 55.6%, the flame height decreases. The heat flux released by solid fuel arrays decreases with the increase of relative flame height, which decreases sharply when the relative flame height is less than 1, and starts to decrease slowly when the relative flame height is greater than 1, and the heat flux is related to the relative flame height in a power function, and based on this, a model for the released heat flux under different air gap conditions is proposed.

Key words: air gap, solid fuel array, heat transfer, heat flux