Lesson 21 Laminar and Turbulent Flow DESCRIBE the characteristics and flow velocity profiles of laminar flow and turbulent flow. DEFINE the property of viscosity. DESCRIBE how the viscosity of a fluid varies with temperature. DESCRIBE the characteristics of an ideal fluid. DESCRIBE the relationship between the Reynolds number and the degree of turbulence of the flow.

Flow Regimes Two flow regimes laminar flow turbulent flow Important in the design and operation of any fluid system. Fluid friction, which determines the amount of energy required to maintain the desired flow, depends upon the mode of flow. Affects heat transfer to the fluid.

Laminar Flow Streamline or viscous flow Characterized by layers of water flowing over one another at different speeds with virtually no mixing between layers fluid particles moving in definite and observable paths or streamlines Type of flow observed by viscous (thick) fluid in which viscosity of the fluid plays a significant part.

Turbulent Flow Characterized by the irregular movement of particles of the fluid. There is no definite frequency as there is in wave motion. The particles travel in irregular paths with no observable pattern and no definite layers.

Flow Velocity Profiles Not all fluid particles travel at the same velocity within a pipe The shape of the velocity curve (the velocity profile across any given section of the pipe) depends upon whether the flow is laminar or turbulent. Laminar The velocity distribution at a cross section will be parabolic in shape Maximum velocity at the center is twice the average velocity in the pipe. Turbulent flow Relatively flat velocity distribution across the section of pipe, The entire fluid flows at a given single value. In each case, velocity of the fluid in contact with the pipe wall is essentially zero and increases the further away from the wall.

Laminar and Turbulent Flow Velocity Profiles

Average (Bulk) Velocity A single average velocity is often used to represent the velocity of all fluid at that point in the pipe. Assume that the average velocity is the same as the velocity at the center of the pipe. If the flow regime is laminar use an average value that is half of the center line value.

Viscosity A fluid property that measures the resistance of the fluid to deforming due to a shear force. Is the internal friction of a fluid which makes it resist flowing past a solid surface or other layers of the fluid. Can also be considered to be a measure of the resistance of a fluid to flowing. The unit of measurement for absolute viscosity is μ = absolute viscosity of fluid (lbf-sec/ft2) Usually dependent on the temperature of the fluid - as the temperature of the fluid increases Usually independent of the pressure

Ideal Fluid A fluid that is incompressible and has no viscosity. Ideal fluids do not actually exist, but concept is useful to simplify some problems

Reynolds Number NR = ρv D / μ gc where: NR = Reynolds number (unitless) v = average velocity (ft/sec) D = diameter of pipe (ft) μ = absolute viscosity of fluid (lbf-sec/ft2) ρ= fluid mass density (lbm/ft3) gc = gravitational constant (32.2 ft-lbm/lbf-sec2)

Reynolds Number Dimensionless number comprised of the physical characteristics of the flow. Can be conveniently determined using a Moody Chart Reynolds number ranges based on type of flow Laminar if the Reynolds number is less than 2000 Turbulent if the Reynolds number is greater than 3500 Transitional flows - Reynolds numbers between 2000 and 3500 Most fluid systems in nuclear facilities operate with turbulent flow