Forced Convection In Laminar And Turbulent Flow In Flat Plated Tubes And Pipes

Presented To:Mam. Aisha kousar

Group Members:

13CH03 13CH10 13CH101

13CH18 13CH146 13CH110

13CH128 13CH156 13CH138

13CH126 13CH137

In order to understand the presentation ,following concepts should be clear

Forced ConvectionTurbulent FlowLaminar FlowNusselt NumberPrandtl numberBoundary LayerFriction Factor

Turbulent And laminar flow

Laminar flow: Where the fluid moves slowly in layers in a pipe, without much mixing among the layers. Turbulent flow Opposite of laminar, where considerable mixing occurs, velocities are high.

Reynolds Number: The Reynolds number is defined as the ratio of inertial forces

to viscous forces and consequently quantifies the relative importance of these two types of forces for given flow conditions .

They are also used to characterize different flow regimes within a similar fluid, such as laminar or turbulent flow:

laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion;

turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce eddies, vortices and other flow instabilities.

where: is the mean velocity of the object relative to the fluid (SI units: m/s) is a characteristic linear dimension, (travelled length of the fluid; hydraulic

diameter when dealing with river systems) (m) is the dynamic viscosity of the fluid (Pa·s or N·s/m² or kg/(m·s)) is the kinematics viscosity ( ) (m²/s) is the density of the fluid (kg/m³).

Nusselt Number:

In heat transfer at a boundary (surface) within a fluid, the Nusselt number (Nu) is the ratio of convective to conductive heat transfer across (normal to) the boundary.

A Nusselt number close to one, namely convection and conduction of similar magnitude, is characteristic of "slug flow" or laminar flow. A larger Nusselt number corresponds to more active convection, with turbulent flow typically in the 100–1000 range.

Prandtl Number The Prandtl number is a dimensionless number, named after the German

physicist Ludwig Prandtl, defined as the ratio of momentum diffusivity (kinematics viscosity) to thermal diffusivity. That is, the Prandtl number is given as:

where: : kinematics viscosity, , (SI units : m2/s) : thermal diffusivity, , (SI units : m2/s) : dynamic viscosity, (SI units : Pa s = N s/m2) : thermal conductivity, (SI units : W/(m K) ) : specific heat, (SI units : J/(kg K) ) : density, (SI units : kg/m3 )

Pr<1 means thermal diffusivity dominates., Pr>1momentum diffusivity dominates

Dependence Of transition and laminar flow

The transition from laminar to turbulent flow depends on the surface geometry, surface roughness, upstream velocity, surface temperature, and the type of fluid, among other things, and is best characterized by the Reynolds number

Laminar and Turbulent Flow In Tubes Flow in a tube can be laminar or turbulent, depending on

the flow conditions. Fluid flow is streamlined and thus laminar at low

velocities, but turns turbulent as the velocity is increased beyond a critical value.

Transition from laminar to turbulent flow does not occur suddenly; rather, it occurs over some range of velocity where the flow fluctuates between laminar and turbulent flows before it becomes fully turbulent.

Most pipe flows encountered in practice are turbulent. Laminar flow is encountered when highly viscous fluids

such as oils flow in small diameter tubes or narrow passages.

Temperature profile in forced convection

Now consider a fluid at a uniform temperature entering a circular tubewhose surface is maintained at a different temperature. This time, the fluidparticles in the layer in contact with the surface of the tube will assume thesurface temperature. This will initiate convection heat transfer in the tube andthe development of a thermal boundary layer along the tube. The thickness ofthis boundary layer also increases in the flow direction until the boundarylayer reaches the tube center and thus fills the entire tube, as shown inFigure

Laminar And Turbulent Flow over Flat plates

PARALLEL FLOW OVER FLAT PLATESConsider the parallel flow of a fluid over a flat plate of length L in theflow direction, as shown in Fig. 7–6. The x-coordinate is measured alongthe plate surface from the leading edge in the direction of the flow. The fluidapproaches the plate in the x-direction with a uniform velocity V and

temperature T`. The flow in the velocity boundary layers starts out as laminar,but if the plate is sufficiently long, the flow becomes turbulent at a distancexcr from the leading edge where the Reynolds number reaches its criticalvalue for transition.The transition from laminar to turbulent flow depends on the surface geometry,surface roughness, upstream velocity, surface temperature, and the type offluid, among other things, and is best characterized by the Reynolds number.

Forced Convection In Laminar And Turbulent Flow In Pipes

For forced convection in pipes: