Thermal counterflow of superfluid 4He: Temperature gradient in the bulk and in the vicinity of the heater

We report measurements of longitudinal temperature gradient in thermal counterflow of superfluid He-4 performed directly in the 17.5 cm long channel of a square cross section of 7 mm side using static temperature sensors placed on its wall and one small sensor movable along its axis. For various temperatures and heat inputs, we verify linear temperature gradient in the bulk and detect a "boundary layer"-a region with increased temperature gradient-adjacent to the planar heater. We verify this result using three different heater geometries, all of which display similar thickness of this region but differ in exact details of the temperature profile. We propose that the observed temperature gradient enhancement is at least partly due to an increase of the density of the vortex tangle induced by the inhomogeneity of areal heat input distribution and support this claim by numerical simulations.