PHYSICAL MECHANISM OF CONVECTION

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Tue 8:54:24 AM Slide Nr. 0 of 33 Slides PHYSICAL MECHANISM OF CONVECTION Heat transfer through a fluid is by convection in the presence of bulk fluid motion and by conduction in the absence of it. Chapter 3: FUNDAMENTALS OF CONVECTION

Tue 8:54:24 AM Slide Nr. 1 of 33 Slides PHYSICAL MECHANISM OF CONVECTION Experience shows that convection heat transfer strongly depends on the fluid properties dynamic viscosity m, thermal conductivity k, density r and specific heat Cp, as well as the fluid velocity v. It also depends on the geometry and the roughness of the solid surface, in addition to the type of fluid flow (such as being streamlined or turbulent). Thus, we expect the convection heat transfer relations to be rather complex Chapter 3: FUNDAMENTALS OF CONVECTION

Tue 8:54:24 AM Slide Nr. 2 of 33 Slides PHYSICAL MECHANISM OF CONVECTION Chapter 3: FUNDAMENTALS OF CONVECTION

Tue 8:54:24 AM Slide Nr. 3 of 33 Slides PHYSICAL MECHANISM OF CONVECTION An implication of the no-slip and the no-temperature jump conditions is that heat transfer from the solid surface to the fluid layer adjacent to the surface is by pure conduction, Chapter 3: FUNDAMENTALS OF CONVECTION

Slide Nr. 4 of 33 Slides Nusselt Number A Nusselt number of Nu = 1 for a fluid layer represents heat transfer across the layer by pure conduction.

Slide Nr. 5 of 33 Slides CLASSIFICATION OF FLUID FLOWS Viscous versus Inviscid Flow Internal versus External Flow Compressible versus Incompressible Flow Laminar versus Turbulent Flow

Slide Nr. 6 of 33 Slides CLASSIFICATION OF FLUID FLOWS Natural (or Unforced) versus Forced Flow Steady versus Unsteady (Transient) Flow One-, Two-, and Three-Dimensional Flows

Slide Nr. 7 of 33 Slides VELOCITY BOUNDARY LAYER

Slide Nr. 8 of 33 Slides Surface Shear Stress

Slide Nr. 9 of 33 Slides THERMAL BOUNDARY LAYER

Slide Nr. 10 of 33 Slides Prandtl Number The relative thickness of the velocity and the thermal boundary layers is best described by the dimensionless parameter Prandtl number, defined as Pr about 1 for gases Pr << 1 for liquid metals Pr >> 1 for oil

Slide Nr. 11 of 33 Slides LAMINAR AND TURBULENT FLOWS

Slide Nr. 12 of 33 Slides CONVECTION COEFFICIENTS where m and n are constant exponents (usually between 0 and 1), and the value of the constant C depends on geometry. Sometimes more complex relations are used for better accuracy.

Slide Nr. 13 of 33 Slides EXTERNAL FORCED CONVECTION the fluid properties are usually evaluated at the so-called film temperature, defined as

Slide Nr. 14 of 33 Slides PARALLEL FLOW OVER FLAT PLATES

Slide Nr. 15 of 33 Slides PARALLEL FLOW OVER FLAT PLATES

Slide Nr. 16 of 33 Slides PARALLEL FLOW OVER FLAT PLATES

Slide Nr. 17 of 33 Slides Example Engine oil at 60 C flows over the upper surface of a 5-m-long flat plate whose temperature is 20 C with a velocity of 2 m/s. Determine the rate of heat transfer per unit width of the entire plate.

Slide Nr. 18 of 33 Slides FLOW ACROSS CYLINDERS AND SPHERES

Slide Nr. 19 of 33 Slides FLOW ACROSS CYLINDERS AND SPHERES

Slide Nr. 20 of 33 Slides FLOW ACROSS CYLINDERS AND SPHERES

Slide Nr. 21 of 33 Slides Exxample A long 10-cm-diameter steam pipe whose external surface temperature is 110 C passes through some open area that is not protected against the winds. Determine the rate of heat loss from the pipe per unit of its length when the air is at 1 atm pressure and 10 C and the wind is blowing across thepipe at a velocity of 8 m/s.

Slide Nr. 22 of 33 Slides INTERNAL FORCED CONVECTION

Slide Nr. 23 of 33 Slides THE ENTRANCE REGION

Slide Nr. 24 of 33 Slides INTERNAL FORCED CONVECTION

Slide Nr. 25 of 33 Slides INTERNAL FORCED CONVECTION

Slide Nr. 26 of 33 Slides INTERNAL FORCED CONVECTION

Slide Nr. 27 of 33 Slides INTERNAL FORCED CONVECTION

Slide Nr. 28 of 33 Slides Example Water enters a 2.5-cm-internal-diameter thin copper tube of a heat exchanger at 15 C at a rate of 0.3 kg/s, and is heated by steam condensing outside at 120 C. If the average heat transfer coefficient is 800 W/m2 C, determine the length of the tube required in order to heat the water to 115 C

Slide Nr. 29 of 33 Slides Laminar Flow in Pipes

Slide Nr. 30 of 33 Slides Laminar Flow in Pipes

Slide Nr. 31 of 33 Slides Turbulent Flow in Pipes where n = 0.4 for heating and 0.3 for cooling of the fluid flowing through the tube. The fluid properties are evaluated at the bulk mean fluid temperature

Slide Nr. 32 of 33 Slides Turbulent Flow in Pipes

Tue 9:03:41 AM Slide Nr. 33 of 33 Slides

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