  We derive nonperturbative flow equations within an effective constituent
quark model for two quark flavors. Heat-kernel methods are employed for a
renormalization group improved effective potential. We study the evolution of
the effective potential with respect to an infrared cutoff scale $k$ at
vanishing temperature. At the first stage we omit corrections coming from the
anomalous dimension. This investigation is extrapolated to finite temperature,
where we find a second order phase transition in the chiral limit at $T_c
\approx 130$ MeV. Due to a smooth decoupling of massive modes, we can directly
link the low-temperature four-dimensional theory to the three-dimensional
high-temperature theory and can determine universal critical exponents.
