I want to model flow and convective heat transfer in 2d nano-channel with linear temperature profile. My model is shown as Fig.1. The red point represent wall atoms and the green points denote fluid atoms. All the outmost wall atoms are fixed and the remaining wall atoms are constrained to their initial positions by using fix spring/self command. Wall temperature along the flow direction (x direction) , as the yellow line shown in Fig.2, is control by the Langevin thermostat. The fluid domain is divided as four zones: buffer zone, body force zone, thermostat zone and sample zone. External body force is applied to fluid atoms in the body force zone to drive fluid atoms flow in x direction (periodic boundary condition). For fluid atoms in the thermostat zone, langevin thermostat may control the temperature to the specified value 100K. So, my question is that why the mean cross section temperature of the fluid along the x direction is much higher than the wall temperature. Thanks in advance.