As a zero-carbon fuel, ammonia (NH \(_3\) ) has attracted great interests recently. Due to its slow propagation speed, NH \(_3\) flames are strongly affected by gravity and radiation. This study investigates the propagation of spherically expanding NH \(_3\) /air flames under the combined effects of gravity and radiation using two-dimensional simulations with detailed chemistry and transport models. The results show that gravity significantly deforms the flame front, leading to a mushroom-shaped structure in which the local flame displacement speed varies along the front due to local stretch effects. This phenomenon becomes more pronounced at lower equivalence ratios as a result of the reduced flame speed. Overall, gravity enhances the global flame propagation speed. On the other hand, radiation slows the flame propagation by lowering the flame temperature and inducing an inward flow velocity. This makes the flame more susceptible to the influence of gravity and amplifies the deformation of the flame front. Finally, the performance of various approaches for determining the unstretched laminar flame speed from spherically expanding flames under gravitational and radiational conditions is assessed. It is found that when both gravity and radiation effects are significant, the spherically expanding flame method using flame radius history is not applicable, regardless of the definition of equivalent radius, and the surface-averaged method is the only reliable approach. This study provides insights into the understanding and accurate measurement of NH \(_3\) /air flame propagation characteristics.
