We establish first- and
second-order gradient estimates for positive solutions of the heat equations under
general geometric flows. Our results generalize the recent work of S. Liu, who
established similar results for the Ricci flow. Both results can also be considered as
the generalization of P. Li, S. T. Yau, and J. Li’s gradient estimates under
geometric flow setting. We also give an application to the mean curvature
flow.