Is it possible for an open system to reach thermal equilibrium? why/why not?
If you have an open system (fixed control volume) operating at steady state, and shaft work is being done to compress the gas, if the system is adiabatic (insulated), then the enthalpy per unit mass of the gas at the exit is higher than at the entrance. So the temperature of the gas increases. If heat is being removed from the control volume as the gas passes through, such that the heat removed is equal to the shaft work, then the inlet and outlet enthalpies per unit mass are the same, and the temperatures are the same. But, I'm not sure you would call this thermal equilibrium.You could define an system in which a flow at T=400K goes in, a flow at T=400K goes out, while inside it is compressed isothermally, couldn't you? It remains in thermal equilibrium while work is done, and it's an open system since there is mass flow in and out.