  Size-dependence of energy transport and the effects of reduced dimensionality
on transport coefficients are of key importance for understanding
nonequilibrium properties of matter on the nanoscale. Here, we perform
nonequilibrium and equilibrium simulations of heat conduction in a 3D fluid
with the multiparticle collision dynamics, interacting with two thermal-walls.
We find that the bulk 3D momentum-conserving fluid has a finite non-diverging
thermal conductivity. However, for large aspect-ratios of the simulation box, a
crossover from 3D to one-dimensional (1D) abnormal behavior of the thermal
conductivity occurs. In this case, we demonstrate a transition from normal to
abnormal transport by a suitable decomposition of the energy current. These
results not only provide a direct verification of Fourier's law but also
further confirm the validity of existing theories for 3D fluids. Moreover, they
indicate that abnormal heat transport persists also for almost 1D fluids over a
large range of sizes.
