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Particular features of fluid motion in tubes

Heat transfer on fluid motion in tubes

Two patterns of flow: laminar and turbulent are distinguished for fluid forced motion in tubes.

Laminar pattern of flow. Heat transfer process occurring on fluid motion in tubes is more complicated as compared with flow of non-limited flux over a surface.

Tube cross-section has finite dimensions. As a result starting at a certain distance from tube inlet all the fluid is exposed to slowing down action of viscous forces. Tube finite dimensions cause fluid temperature change in every cross-section and along the tube which influences heat transfer processes.

Pattern of flow is determined by the value of similarity number

,

where –average fluid flow;

d –tube inside diameter.

If , then flow is laminar (figure 4.15). Value of similarity number is the lower critical value of Reynolds number. Developed turbulent flow occurs in tubes with

.

The flow with similarity number value

is called transient. It is also characterized by transient type of heat transfer.

If fluid is introduced into the tube from a significant volume and tube edges are curved then velocity distribution in the initial sections will be uniform. Hydrodynamic boundary layer with gradually increasing thickness is formed on motion along the walls. In long tubes increase of boundary layers thickness leads to their joining. So constant velocity distribution typical to concrete pattern flow is maintained.

Distance counted from the tube inlet to section in which constant velocity distribution is established is called length of hydrodynamic initial section l_in or hydrodynamic stabilization section.

If the value of is less than the critical one then along all the initial section hydrodynamic boundary layer is of laminar pattern. With Re>Re_cr, first laminar boundary layer is formed in the vicinity of tube inlet which is gradually transited into turbulent and after turbulent boundary layers joining stable turbulent pattern of flow is established (figure 4.16). Only in very thin viscous sublayer near the surface flow pattern remains laminar.

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