Chapter 2 Section 2 Work in fluid systems

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

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.

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 = P  V Work done by a fluid in a steady flow process where density is constant equals the pressure change times the volume moved. W = -  PV

Positive and negative work If the pressure difference,  P, is negative, work is positive. The fluid does work. If the pressure difference is positive, work is negative. Work is done on the fluid.

Pressure difference Fluid flows from high pressure to low pressure.  P = P 2 – P 1 If P 1 > P 2,  P is negative. If P 1 < P 2,  P is positive.

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