A dimensionless number, Da, used to relate the rate of a chemical reaction to certain phenomena that occur in the reaction. There are several types of numbers that are used whose definition depends on whether the chemical reaction is related to momentum transfer or to heat transfer. For example, for an nth order of reaction, the Damköhler number is:

where k is the rate constant, C_o is the initial concentration of reactant, and t is the reaction time. It is used to indicate the extent of conversion within continuous flow reactors. In continuous chemical processes, the Damköhler number is defined as the ratio of the chemical rate of reaction to the convective chemical reaction time. A second Damköhler number is defined as the ratio of the chemical reaction rate to the rate of mass transfer:

(p. 95) where k_g is the mass transfer coefficient and a is the interfacial area. There are two further Damköhler numbers related to heat transfer. They are all named after the German chemist Gerhard Damköhler (1908–44).