I was under the impression that the entropy reduction in a system only works with either mass or heat transfer.
While I was researching intercooling of compressors I stumbled across a peculiar sketch (Figure 8-46 from Çengel: Introduction to Thermodynamics and Heat Transfer). I cross-checked with my thermodynamics textbook (Figure 9-43 from Çengel: Thermodynamics - An Engineering Approach).
The entropy gets smaller throughout the complete compression process. While I do understand that it gets smaller when cooled I do not understand why the entropy reduces prior and after the intercooling. Figure 9-43 shows the compression as isentropic (which is often done to simplify the analysis).
What am I missing? The only idea would be a heat-loss during the compression. I then realised that a bit later in the book a similar graph shown in Fig. 7-34:
What is the reason for a reduction in entropy other that heat-loss in these cases? Is it maybe a didactic choice?