A dimensionless group used in the study of diffusion of a solid material A into a stream of vapor initially composed of substance B, equal to

where M_A and M_B are the molecular weights of components A and B, M_V and M_W are the molecular weights of the mixture in the vapor and at the wall, and Y_AW and Z_A are the mole fractions of A at the wall and in the diffusing stream, respectively. Symbolized N_D .