Violation of D²- Law in Droplet Combustion under Gravity: New Shrinking Kinetic Law Due to Flame-Driven Buoyant Convection

D²-law is a long-established and widely used relationship for describing the shrinking behavior of a vaporizing droplet in fuel spray and combustion processes, stating that square of the droplet instantaneous diameter D decreases linearly with time t. Here we show both experimentally and theoretically that this classical law is not the true law for a burning droplet under the influence of gravity. Experimentally, we find that such a droplet actually obeys a Dⁿ-law with the exponent n =2.53±0.30-2.69±0.19 for a variety of common liquid fuels such as alkanes and alcohols. On the theoretical side, we are able to identify mechanisms involved to show n=8/3 well capturing the experimental values. This new D^8/3-law originates from the additional inherent buoyancy length determined solely by fuel properties. It is a consequence of simultaneous momentum, heat, and mass transfer arising from buoyant convection set up by the blazing flame around the droplet, insensitive to combustion chemistry and detailed reaction kinetics.