A mixing chamber combines two or more flowing streams into one outlet stream.

At steady state, conservation of mass gives

$$\sum_i\dot m_i=\dot m_{out}.$$

If the chamber is adiabatic, has no shaft work and has negligible kinetic and potential-energy changes, the energy balance becomes

$$\sum_i\dot m_i h_i=\dot m_{out}h_{out}.$$

For two equal mass-flow rates with inlet enthalpies $h_1=200$ and $h_2=400\ \mathrm{kJ/kg}$,

$$h_{out}=\frac{h_1+h_2}{2}=300\ \mathrm{kJ/kg}.$$

The outlet temperature is not necessarily the arithmetic mean of inlet temperatures because enthalpy can depend nonlinearly on temperature and may include phase changes.

Mixing is generally irreversible. Even when the energy balance looks like a weighted average, entropy can be generated as streams with different temperatures, compositions or velocities equilibrate.