Briefly answer the following: a) system consisting of 1 kg of saturated water va

Question

Briefly answer the following:

a) system consisting of 1 kg of saturated water vapor at 100KPa and 101.3C is cooled unitl the system exists a saturated liquid. The change of entropy for this process is:(-6.0476kj/(kg k)) (i.e a decrease of 6.0476 kj/(kg k)). Does this process violate the ''Increase in Entropy Principle"? explain why or why not.

b) In going from thermodynamic state 1 to thermodynamic state 2 for a pure substance, through a heat addition process, what is the significance of evaluating dQ/T using an isentropic process path between these two states?

c) For a given compression ratio, which cycle is more efficient, Diesel or Otto? Explain.

d) What is the fundamental theoretical difference affecting overall thermal efficiency between a carnot cycle process and an ideal cycle process (such as a Diesel or Otto cycle)?

e) why is the condenser of a Rankine cycle process run under vacuum conditions?

Explanation / Answer

a) It does not because the process is reversible. Increase in entropy principle is applicable for irreversible processes.

b)

dQ / T for an isentropic process should be zero. That is dQ should be zero. That means no heat transfer.

dQ / T gives the entropy difference s2 - s1.

c)

For Otto cycle, efficiency e = 1 - r^(1 - k) where k = ratio of specific heats and r = compresson ratio

For Diesel cycle, e = 1 - r^(1-k)*[(r_c^k - 1) / k*(r_c - 1)].........where r_c = cut-off ratio

Therefore, if (r_c^k - 1) / (k*(r_c - 1)) > 1, Diesel cycle would be less efficient than Otto cycle.

Taking k = 1.4 for air, above expression would be true. Hence, Diesel cycle would be less efficient than Otto cycle.

d)

In Carnot cycle, heat transfer occurs at constant temperature.

e)

The lower the temperature, the higher the efficiency.

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