Head losses in pipes:

• The reduction in the total head (sum of elevation head, velocity head, and pressure head) of the fluid as it moves through a fluid system is referred to as head loss.
• In real fluids, head loss is unavoidable. It is present because of:
  • The friction between the fluid and the walls of the pipe.
  • The friction between adjacent fluid particles as they move relative to one another.
  • The turbulence is caused by such components as piping entrances and exits, pumps, valves, flow reducers, and fittings.
• Although the head loss represents an energy loss, it does not represent a loss of the fluid's total energy. As a result of the law of conservation of energy, the total energy of the fluid conserves. In reality, the frictional head loss causes a corresponding increase in the fluid's internal energy (temperature).
• Head loss of the piping system is divided into two main categories:
  • Major Head Loss – due to friction in pipes and ducts.
  • Minor Head Loss – due to components as valves, fittings, bends and tees.
• In most engineering flows, the major head loss is roughly proportional to the square of the flow rate and is given by Darcy–Weisbach equation.

$\frac{{\Delta h}}{L} = \frac{{f{V^2}}}{{2gD}}$

where

Δh is the head loss in m

f is Darcy friction factor

L is the pipe length

D is the hydraulic diameter

V is the mean flow velocity

• An increase in velocity causes the flow to transition to turbulent flow and this turbulence causes the formation of eddies of many different length scales. These formations come at the expense of the available energy loss of the flow.
• The large-scale eddies contain the majority of the kinetic energy of turbulent motion. Some portion of this energy is dissipated when these large-scale eddies are converted into small scale due to the viscous effects.