  This study presents a numerical analysis of the impact of hydrogen addition
on the consumption speed of premixed lean methane-air laminar flames exposed to
combined strain and heat loss. Equivalence ratios of 0.9, 0.7, and 0.5 with
fuel mixture composition ranging from pure methane to pure hydrogen are
considered to cover a wide range of conditions in the lean region. The 1-D
asymmetric counter-flow premixed laminar flame aCFPF with heat loss on the
product side is considered as a flamelet configuration that represents an
elementary unit of a turbulent flame and the consumption speed is used to
characterize the effect of strain and heat loss. Due to the ambiguity in the
definition of the consumption speed of multi-component mixtures, two
definitions are compared. The definition of the consumption speed based on the
heat release results in lower values of the stretched flame speed and even an
opposite response to strain rate for some methane-hydrogen-air mixtures
compared to the definition based on the fuel consumption. Strain rate leads to
an increase in the flame speed for the lean methane-hydrogen mixtures, reaching
a maximum value after which the flame speed decreases with strain rate. Heat
loss decreases the stretched flame speed and leads to a sooner extinction of
the flamelet due to combined strain and heat loss. Hydrogen addition and
equivalence ratio significantly impact the maximum consumption speed and the
flame response to combined strain rate and heat loss. The effect of hydrogen on
the thermo-diffusive properties of the mixture, characterized by the Zel'dovich
number and the effective Lewis number, are also analyzed and related to the
effect on the consumption speed. Two definitions of the Lewis number of the
multi-component fuel mixture are evaluated against the results from the aCFPF.
