Question

W12 for part C should be -1.48 kJ, but I am not getting that. Please show your work so I can learn!! Thanks!

Given: A vertical piston-cylinder device contains air, which is initially at 900 kPa and 250°C and occupies a volume of 0.005 m². The frictionless piston is anchored by a hard stop in the expansion direction, but moves freely in the compression direction. The piston weighs 200 kg and has an area of 0.005 m². Heat is removed from the piston- cylinder device until the volume is 40% of the initial volume. The atmospheric pressure acting on the piston is 100 kPa. Neglect the area of the stops in comparison to the area of the piston. The gravitational acceleration g=9.8 m/s². Find: b) Calculate the pressure at which the piston just starts to move from the stops, in kPa. Hint: draw a free-body diagram of the piston, 1 Pa = 1 J/mº. c) Determine the boundary work during the heat transfer process, in kJ.

Answer 1

Patm XA ThI 212 =2009 121. Initial condition of system PL=900kPa Ti = 250°C miti 0.005 m3 Area of piston(A)=0.005 m2 Weight of piston (W) = 200kg X9.8mis? Final condition after removal of 'Q' amount of heat from System: So Final volume (Hz) = 0.4x+1=0.4x0.005 F. (Hz) m² <b> Piston just starts to move when the force due to atmospheric pressure and weight of piston balances the internal pressure force p’ system of the . Patm XA 7W = PXA Patm + W P * А P = 100x1037 200X9.81 O-005 :: Aston starts to move when the pressure of system reduces to 492.4 kPa CSS ned with caricanner

C> Heat rejected from system in two stages Stage-1: Constant volume heat rejection from Pressure R = 900kPa to P2 = 492.4 kPa stag-2 . constant pressure heat rejection at P = = 492.4 kPa from t = 0.005 m² to H2 = 2x103 m3. Anos OVA Total boundary wook during heat tromsger process is done only during stage-2 coo CE Work (W) = P2(t- = = 492:4X103( 2x105 0.005) W=-1.477 KJ 2421 CS Scanned with CamScanner