Question

Problem 1 [15 pts] Air (R=0.287 kJ/kgK), initially at 300 K and 100 kPa is heated to 450 K in an internally reversible closed system process. You do NOT need to show a system sketch for this problem. Using variable (temperature dependent) specific heats a. Find the specific entropy change (As= S2-S1) of air if the process occurs at constant pressure b. Find the specific entropy change (As= S2-S1) of air if the process occurs at constant volume c. Show the processes in part (a) and (b) on the same P-v diagram (including the necessary constant temperature lines). Also show the processes in part (a) and (b) on the same T-s diagram (including the necessary constant pressure lines)

Answer 1

$s_{2-1}=s^{0}_{2}-s^{0}_{1}-R\ln (P_{2}/P_{1})$

ait: initially; state 1 I = 300k P2 = 180kPa state 2 finally; T = 450k (9) process 1–2; (constant pressure) using air property table (appendic, A-17) = 1.78203 KJ/Kg K de T= 350k similarly 2 = 2.11161 KJ/Kg -klo T = 450K - B = (-2y) - Rp ln & for constant pressure P2= P2 & S = 2.11161 - 1.70203 = 0.40958 kJ / kg k (6) process 1-2; (constant volone - Az = - - Pfu (12/9) process.) = 2.11161- 1.70203 - 0.28 PY = MRT (ideal gas equ) or constant Whine: 1 2 = 1 / 2

- IxP,= (450) (100) = iso kle 12-64 = 0.40958 - 0.289 ln (S) = 0.29321 KI/kg-K 2, (at constart wlane) 2 (at constant pressure = 100kpan const volume pressure conet 450k N 300k -17 I