We know a reversible adiabatic process is an isentropic process......but  is an isentropic process necessarily a reversible adiabatic process. Why or why not?

Equal amounts of Water at 80 deg C and water at 0 deg C are mixed together. What would be the final temperature of mixture? What will be change in answer if water at 0 deg C is in form of ice?

What is the effect on power generated, efficiency when we make a boiler super critical? 

A power plant working based upon Rankine cycle, is designed to run in winters. Now it is operated in summers. Now what would happen to efficiency; it decrease, increase on remain same?