Please refer me to some paper or book if you know any. A graph showing the adiabatic flame temperature vs equivalence ratio would be ideal.
In 2014 it appears that Glaude, Sirjean, Fournet et al. published a paper that gives exactly the data you are looking for. See Figure 4 from their paper, which I have also included below.
Possibly relevant text from the paper:
maximum flame temperature is obtained for slightly rich
mixtures, at an equivalence ratio around 1.05-1.1, because of
the lower impact of dilution by nitrogen when the amount of
air decreases. At the strict stoichiometric conditions, the rate
of the endothermal molecular dissociations is also at its
maximum which tends to depress the temperature.
Also this next bit:
The results show that alcohols have flame temperatures
comparable to those of conventional fuels, contrary to DME
that develops significantly higher temperatures. This is
consistent with the respective values of the heating values of
these fuels and thermochemical equilibrium data.
I couldn't find anything on google.
I found the paper by Googling
dme adiabatic flame temperature.