HW_8_Soln

# HW_8_Soln - ME 309 Fall 2008 Section 2(Merkle Homework 8...

This preview shows pages 1–3. Sign up to view the full content.

ME 309 Fall 2008 Section 2 (Merkle) Homework # 8 Due Mon 15 Sep 2008 Problem 8-1 A frictionless 3000- N piston maintains a gas at constant pressure in a cylinder. The cross-sectional area of the piston is 5200 mm 2 and the atmospheric pressure acting on this area is 0.1 MPa . A process occurs in which a paddle wheel transfers 6800 N · m of work to the gas, 10 kJ of heat is transferred from the gas, and the internal energy of the gas decreases by 1 kJ . Determine the distance ( mm ) moved by the piston. Solution : Known : piston = 3000 N, p=0.1Mpa (const), A = 05200 mm 2 , Paddle wheel work— 6800 n m Q=10 kJ Δ U=1 kJ . Find: Distance piston moves ( mm ) Sketch : Assumptions : Perfect gas, all work input to paddle goes into fluid; no change in pe or ke Analysis : Note : 1 N-m = 1 J P gas = 100,000 + 3000/5.2e-3 = 676,900 N/m 2 Energy equation Q – W = Δ U gas Q is removed—sign is negative: Q = -10,000 J Work done on system by paddle = 6800 N-m (= J ) Work done by piston against ambient pressure (force times distance) Note : Fluid inside piston pushes against both the piston weight and the ambient pressure, so the pressure acting is the pressure inside the cylinder. = p cyl A * d = N-m Energy equation: -10,000 + 6800 + 676,900*5.2e-3*d = -1000 J Solve for distance traveled by piston: = 0.625 m Piston Travel : 625 mm Problem 8-2 A well-insulated, 0.3028 m 3 (80-gal), electric hot-water heater initially contains water at 322 K (120 F). Hot water is withdrawn from the tank at 1.893e-4 m 3 /s (3 gal/min) while cold water at 283 K (50 F) enters to keep the tank full. The cold water mixes completely with the water in the tank at each instant. Because of a power outage,

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
there is no energy input to heat the incoming cold water. Estimate the time required for
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

### Page1 / 4

HW_8_Soln - ME 309 Fall 2008 Section 2(Merkle Homework 8...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online