What happens to the air after it is expanded across the turbine in an air-cycle cooling system?

In an air-cycle cooling system, the air expands across the turbine, which leads to a decrease in pressure and results in cooling. This process is based on the principles of thermodynamics, where the expansion of the air does work on the turbine, causing the temperature of the air to drop as it loses energy. As the air expands, it transitions from a higher pressure state to a lower pressure state, which not only cools the air but also decreases its density. This cooling effect is vital in aircraft systems, particularly for maintaining comfortable cabin temperatures at high altitudes where outside temperatures are extremely low.

The other options do not accurately describe the process that occurs in this context. The air does not increase in temperature after expansion; rather, it is defined by the cooling that takes place. Humidity is not directly affected in the expansion process, and the air does not mix with combustion gases, as this would pertain to a different operational context pertaining to propulsion rather than the air-cycle cooling system. Overall, the loss of pressure and cooling of the air are essential aspects of how the air-cycle cooling system efficiently operates.