Air is compress from 100 kPa and 50°C to 2000 kPa and 500°C. Assuming constant specific heats, determine the change in the specific entropy of air.Group of answer choices-0.0478 kJ/kg.K1.767 kJ/kg.K0.0478 kJ/kg.K-1.767 kJ/kg.K
Question
Air is compress from 100 kPa and 50°C to 2000 kPa and 500°C. Assuming constant specific heats, determine the change in the specific entropy of air.Group of answer choices-0.0478 kJ/kg.K1.767 kJ/kg.K0.0478 kJ/kg.K-1.767 kJ/kg.K
Solution 1
To solve this problem, we need to use the formula for change in entropy for an ideal gas with constant specific heats:
Δs = cp * ln(T2/T1) - R * ln(P2/P1)
where:
- cp is the specific heat at constant pressure,
- R is the specific gas constant,
- T1 and T2 are the initial and final temperatures in Kelvin,
- P1 and P2 are the initial and final pressures.
For air, cp = 1.005 kJ/kg.K and R = 0.287 kJ/kg.K.
First, convert the temperatures from Celsius to Kelvin: T1 = 50°C + 273.15 = 323.15 K, T2 = 500°C + 273.15 = 773.15 K.
Then, substitute these values into the formula:
Δs = 1.005 * ln(773.15/323.15) - 0.287 * ln(2000/100)
Calculate the natural logarithms and the products:
Δs = 1.005 * 0.857 - 0.287 * 2.303 = 0.862 - 0.661 = 0.201 kJ/kg.K
So, the change in the specific entropy of air is 0.201 kJ/kg.K. This is not one of the answer choices you provided, so there may be a mistake in the problem or the answer choices.
Solution 2
To solve this problem, we need to use the formula for change in entropy for an ideal gas with constant specific heats:
Δs = cp * ln(T2/T1) - R * ln(P2/P1)
where:
- cp is the specific heat at constant pressure,
- R is the specific gas constant,
- T1 and T2 are the initial and final temperatures in Kelvin,
- P1 and P2 are the initial and final pressures.
For air, cp = 1.005 kJ/kg.K and R = 0.287 kJ/kg.K.
First, convert the temperatures from Celsius to Kelvin: T1 = 50°C + 273.15 = 323.15 K, T2 = 500°C + 273.15 = 773.15 K.
Then, substitute these values into the formula:
Δs = 1.005 * ln(773.15/323.15) - 0.287 * ln(2000/100)
Calculate the natural logarithms and the products:
Δs = 1.005 * 0.857 - 0.287 * 2.303 = 0.862 - 0.661 = 0.201 kJ/kg.K
So, the change in the specific entropy of air is 0.201 kJ/kg.K. This is not one of the answer choices you provided, so there may be a mistake in the problem or the answer choices.
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