Defining Flow Work
Flow work, also known as flow energy, is the work done on a fluid to push it into or out of a control volume (an imaginary boundary in a system). It represents the energy associated with the bulk movement of a fluid, specifically the work done by the fluid's pressure as it enters or exits a system. Unlike shaft work, which involves a moving boundary like a piston, flow work occurs as fluid crosses a fixed boundary due to a pressure difference.
Key Principles and Calculation
The core principle of flow work is that a fluid element entering or leaving a control volume carries energy due to its pressure and volume. It is calculated as the product of the fluid's pressure (P) and its volume (V) – specifically, PV. This PV term is critical in the energy balance equation for open systems, as it accounts for the energy carried by the mass flowing into or out of the system.
A Practical Example
Consider a pump moving water through a pipe. As water is pushed into the pump, work is done *on* the water by the external pressure at the inlet. As water exits the pump, the water does work *on* the surroundings by pushing against the external pressure at the outlet. The flow work at the inlet might be P_in * V_in, and at the outlet P_out * V_out, contributing to the overall energy changes within the pumping system.
Importance in Open Systems
Flow work is fundamental to analyzing open thermodynamic systems, such as turbines, compressors, pumps, and nozzles, where mass continuously flows across the system boundaries. It helps engineers and scientists accurately account for all forms of energy transfer when performing energy balances, leading to the definition of enthalpy (H = U + PV, where U is internal energy), which conveniently lumps internal energy and flow work together for flowing fluids.