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What is Laminar Flow Viscous Flow Definition

Laminar flow is characterized by smooth or in regular paths of particles of the fluid. The laminar flow is also referred to as streamline or viscous flow. Thermal Engineering

In fluid dynamics,laminar flowis characterized bysmooth or in regular pathsof particles of the fluid, in contrast to turbulent flow, that is characterized by the irregular movement of particles of the fluid. The fluid flows inparallel layers(with minimal lateral mixing), with no disruption between the layers. Therefore the laminar flow is also referred to asstreamline or viscous flow.

The term streamline flow is descriptive of the flow because, in laminar flow, layers of water flowing over one another at different speeds with virtually no mixing between layers, fluid particles move in definite and observable paths or streamlines.

When a fluid is flowing through a closed channel such as a pipe or between two flat plates, either of two types of flow (laminar flow or turbulent flow) may occur depending on thevelocity,viscosityof the fluid and thesize of the pipe.Laminar flowtends to occur atlower velocitiesandhigh viscosity. On the other hand turbulent flow tends to occur at higher velocities and low viscosity.

Since laminar flow is common only in cases in which the flow channel is relatively small, the fluid is moving slowly, and its viscosity is relatively high, laminar flow is not common in industrial processes. Most industrial flows, especially those in nuclear engineering are turbulent. Nevertheless laminar flowoccurs at any Reynolds numbernear solid boundaries in a thin layer just adjacent to the surface, this layer is usually referred to as thelaminar sublayerand it is very important in heat transfer.

Despite the small thickness of thelaminar sublayer(usually much less than 1 percent of the pipe diameter), since it strongly influences the flow in the rest of the pipe. Any irregularity or roughness on the surface disturbs this layer and significantly affects the flow. Therefore, unlike laminar flow,the friction factorin turbulent flow is a strong function of surface roughness.Reynolds Number

The Reynolds numberis the ratio ofinertial forcestoviscous forcesand is a convenient parameter for predicting if a flow condition will belaminar or turbulent. It can be interpreted that when theviscous forcesare dominant (slow flow, low Re) they are sufficient enough to keep all the fluid particles in line, then the flow is laminar. Even very low Re indicates viscous creeping motion, where inertia effects are negligible. When theinertial forces dominateover the viscous forces (when the fluid is flowing faster and Re is larger) then the flow is turbulent.

It is a dimensionless numbercomprised of the physical characteristics of the flow. An increasing Reynolds number indicates an increasing turbulence of flow.

D is acharacteristic linear dimension, (travelled length of the fluid;hydraulic diameteretc.)

Laminar Flow:Re 2000low velocityFluid particles move instraight linesLayers of water flow over one another at different speeds withvirtually no mixingbetween layers.The flow velocity profile for laminar flow in circular pipes is parabolic in shape, with a maximum flow in the center of the pipe and a minimum flow at the pipe walls.The average flow velocity is approximately one half of the maximum velocity.Simple mathematical analysis is possible.Rare in practice in water systems.

All fluid flowis classified into one of two broad categories or regimes. These two flow regimes are:Single-phase Fluid FlowMulti-phase Fluid Flow(orTwo-phase Fluid Flow)

This is abasic classification. All of the fluid flow equations (e.g.Bernoullis Equation) and relationships that were discussed in this section (Fluid Dynamics) were derived for the flow of asingle phaseof fluid whether liquid or vapor. Solution of multi-phase fluid flow isvery complex and difficultand therefore it is usually in advanced courses of fluid dynamics.

Laminar flowis characterized bysmoothor inregular pathsof particles of the fluid. Therefore the laminar flow is also referred to asstreamline or viscous flow. In contrast to laminar flow,turbulent flowis characterized by theirregular movementof particles of the fluid. The turbulent fluid does not flow in parallel layers, the lateral mixing is very high, and there is a disruption between the layers.Most industrial flows, especially those in nuclear engineeringare turbulent.

The flow regime can be also classified according to thegeometry of a conduitor flow area. From this point of view, we distinguish:Internal FlowExternal Flow

Internal flowis a flow for which the fluid is confined by a surface. Detailed knowledge of behaviour of internal flow regimes isof importance in engineering, because circular pipes can withstand high pressures and hence are used to convey liquids. On the other hand,external flowis such a flow in which boundary layers develop freely, without constraints imposed by adjacent surfaces. Detailed knowledge of behaviour ofexternal flowregimes isof importance especially in aeronauticsandaerodynamics.Reynolds Number Regimes

Transitional flow.At Reynolds numbersbetween about 2000 and 4000the flow is unstable as a result of the onset of turbulence. These flows are sometimes referred to as transitional flows.

Turbulent flow.If the Reynolds number isgreater than 3500, the flow is turbulent. Most fluid systems in nuclear facilities operate with turbulent flow.

Example: Reynolds number for a primary piping and a fuel bundleIt is an illustrative example, following data do not correspond to any reactor design.

The primary circuit of typical PWRs is divided into4 independent loops(piping diameter ~ 700mm), each loop comprises asteam generatorand onemain coolant pump. Inside the reactor pressure vessel (RPV), the coolant first flows down outside the reactor core (through thedowncomer). From the bottom of the pressure vessel, the flow is reversed up through the core, where the coolant temperature increases as it passes through the fuel rods and the assemblies formed by them.

Assume:the primary piping flow velocity is constant and equal to 17 m/s,the core flow velocity is constant and equal to 5 m/s,thehydraulic diameter of the fuel channel,Dh, is equal to 2 cmthe kinematic viscosity of the water at 290C is equal to 0.12 x 10-6m2/s

See also:Example:Flow rate through a reactor core

Determinethe flow regime and the Reynolds number inside thefuel channelthe flow regime and the Reynolds number inside theprimary piping

The Reynolds number inside the primary piping is equal to:

ReD= 17 [m/s] x 0.7 [m] / 0.1210-6[m2/s] =99 000 000

This fully satisfies theturbulent conditions.

The Reynolds number inside the fuel channel is equal to:

ReDH= 5 [m/s] x 0.02 [m] / 0.1210-6[m2/s] =833 000

This also fully satisfies theturbulent conditions.Laminar Flow Heat Transfer CoefficientExternal Laminar Flow

The averageNusselt numberover the entire plate is determined by:

This relation gives the averageheat transfer coefficientfor the entire plate when the flow islaminarover the entire plate.Internal Laminar Flow

Inlaminar flowin a tube with constant surface temperature, both the friction factor and theheat transfer coefficientremain constant in the fully developed region.

Therefore, for fully developedlaminar flowin a circular tube subjected to constant surfaceheat flux, the Nusselt number is a constant. There is no dependence on theReynoldsor thePrandtl numbers.

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