Chapter 2 Section 2

1. Chapter 2 Section 2 Work in fluid systems

2. Objectives • Differentiate between open and closed fluid systems. • Explain the relationship between work and pressure in a fluid system • Differentiate between positive and negative work • Solve fluid work problems in SI & English units

3. Open & Closed Fluid Systems • A closed fluid system recirculates the working fluid in the system. The fluid stays in the system. Examples include a hydraulic brake system or the body’s circulatory system. • An open system allows the fluid to leave the system. It usually circulates only one time. Examples include an irrigation system, city water system or a fire truck water system.

4. Work Done in Fluid Systems • Work done by a fluid in a moving boundary in a closed system equals a constant pressure times a change in volume • W = PDV • Work done by a fluid in a steady flow process where density is constant equals the pressure change times the volume moved. • W = -DPV

5. Positive and negative work • If the pressure difference, DP, is negative, work is positive. The fluid does work. • If the pressure difference is positive, work is negative. Work is done on the fluid.

6. Pressure difference • Fluid flows from high pressure to low pressure. • DP = P2 – P1 • If P1 > P2 , DP is negative. • If P1 < P2 , DP is positive.

7. Summary • Fluid flows from high pressure to low pressure • Closed system - recirculates fluid • Open system – fluid circulates once • W = PDV for closed system with changing boundary • W = -DPV for steady flow, constant density process • Fluid work can be positive or negative