  Nonperturbative flow equations within an effective linear sigma model coupled
to constituent quarks for two quark flavors are derived and solved. A heat
kernel regularization is employed for a renormalization group improved
effective potential. We determine the initial values of the coupling constants
in the effective potential at zero temperature. Solving the evolution equations
with the same initial values at finite temperature in the chiral limit, we find
a second order phase transition at T_c \approx 150 MeV. Due to the smooth
decoupling of massive modes, we can directly link the low-temperature
four-dimensional theory to the three-dimensional high-temperature theory. We
calculate the equation of state in the chiral limit and for finite pion masses
and determine universal critical exponents.
