Project ForgeA gas turbine with air as the working fluid has an isentropic efficiency of 0.70 when operating at a pressure ratio of 3. The pressure ratio is increased to 5 while maintaining the same inlet conditions. Assuming air as a perfect gas with γ=1.4, if the specific work output remains the same for both cases, the isentropic efficiency of the turbine at a pressure ratio of 5 is (round off to two decimal places)
Explanation
Ideal (isentropic) fractional temperature drop is 1−(1/PR^((γ−1)/γ)). At PR=3: 1−1/3^0.2857 ≈ 0.2695; actual work ∝ η1×0.2695 = 0.70×0.2695 ≈ 0.1886. At PR=5: 1−1/5^0.2857 ≈ 0.3686. Equal specific work gives η2 = 0.1886/0.3686 ≈ 0.51.
Source: GATE ME 2019 Session 1 (Set 1) (2019)
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