  We present 3D hydrodynamical simulations aimed to study the dynamical and
chemical evolution of the interstellar medium in dwarf spheroidal galaxies.
This evolution is driven by the explosions of Type II and Type Ia supernovae,
whose different contribution is explicity taken into account in our models. We
compare our results with detailed observations of the Draco galaxy. We assume
star formation histories consisting of a number of instantaneous burst
separated by quiescent periods. Because of the large effectiveness of the
radiative losses and the extended dark matter halo, no galactic wind develops,
despite the total energy released by the supernovae is much larger than the
binding energy of the gas. This explains why the galaxy is able to form stars
for a long period (> 3 Gyr), consistently with observations. In this picture,
the end of the star formation and gas removal must result from external
mechanisms, such as ram pressure and/or tidal interaction with the Galaxy. The
metallicity distributions of the stars found in our models agree very well with
the observed one. We find a mean value <[Fe/H]>=-1.65 with a spread of ~1.5
dex. The chemical properties of the stars derive by the different temporal
evolution between Type Ia and Type II supernova rate, and by the different
mixing of the metals produced by the two types of SNe. We reproduce
successfully the observed [O/Fe]-[Fe/H] diagram. However, our interpretation of
this diagram differs from that generally adopted by previous chemical models.
In fact, we find that the chemical properties of the stars derive, besides the
different temporal evolution of the SNe II and SNe Ia rates, from the spatial
inhomogeneous chemical enrichment due to the different dynamical behaviour
between the remnants of the two types of supernovae.
