The flow over a backward facing step (BFS) has been taken as a useful proto-type to investigate intrinsic mechanisms of separated flow with heat transfer.However,to date,the open literature on the effect of Richardson number on entropy generation over the BFS is absent yet,although the flow pattern and heat transfer characteristic both will receive significant influence caused by the variation of Richardson number in many practical applications,such as in microelectromechanical systems and aerocrafts.The effect of Richardson number on entropy generation in the BFS flow is reported in this paper for the first time.The entropy generation analysis is conducted through numerically solving the entropy generation equation.The velocity and temperature,which are the inputs of the entropy generation equation,are evaluated by the lattice Boltzmann method.It is found that the distributions of local entropy generation number and Bejan number are significantly influenced by the variation of Richardson number.The total entropy gen-eration number is a monotonic decreasing function of Richardson number,whereas the average Bejan number is a monotonic increasing function of Richardson number.