def Le(tC,hC,rho,DAB):
    """
    Lewis number: 
        thermal conductivity/(heat capacity)/(humid vapor density)/(DAB)
    
    Paramters:
        tC, thermal conductivity in W/m/K
        hC, heat capacity in J/mol/K
        rho, molar density of humid vapor in mol/m^3
        DAB, diffusion of component A in B in m^2/s

    Returns:
        Lewis number (dimensionless)
        
    """
    return tC/hC/rho/DAB