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HEAT TRANSFER BY CONVECTION

HEAT TRANSFER BY CONVECTION. CONDUCTION Mechanism of heat transfer through a solid or fluid in the absence any fluid motion. CONVECTION Mechanism of heat transfer through a fluid in the presence of bulk fluid motion Natural (free) Convection Forced Convection

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HEAT TRANSFER BY CONVECTION

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  1. HEAT TRANSFER BY CONVECTION Dr. Şaziye Balku

  2. CONDUCTION Mechanism of heat transfer through a solid or fluid in the absence any fluid motion. CONVECTION Mechanism of heat transfer through a fluid in the presence of bulk fluid motion • Natural (free) Convection • Forced Convection (depending on how the fluid motion is initiated) Dr. Şaziye Balku

  3. CLASSIFICATION OF FLUID FLOWS • Viscous-inviscid • Internal flow- External flow • Open-closed channel • Compressible- Incompressible • Laminar- Turbulent • Natural- Forced • Steady- Unsteady • One-,two-,three-dimensional Dr. Şaziye Balku

  4. VISCOSITY When two fluid layers move relative to each other, a friction force develops between them and the slower layer tries to slow down the faster layer. internal resistance to flow • cohesive forces between the molecules in liquid • molecular collisions in gases. Viscous flows: viscous effects are significant Inviscid flow regions: viscous forces are negligibly small compared to inertial or pressure forces. measure of stickness or resistance to deformation • Kinematic viscosity • Dynamic viscosity Dr. Şaziye Balku

  5. VISCOSITY DEPENDS ON • TEMPERATURE • PRESSURE For liquids dependence of pressure is negligible For gases kinematic viscosity depends on pressure since its relation to density Dynamic viscosity (kg/m.s or poise) Kinematic viscosity, m2/s or stroke Dr. Şaziye Balku

  6. Convection heat transfer • Dynamic viscosity • Thermal conductivity • Density • Specific heat • Fluid velocity • Geometry • Roughness • Type of fluid flow Dr. Şaziye Balku

  7. NEWTON’S LAW OF COOLING (W) Convection heat transfer coefficient (W/m2.0C) The rate of heat transfer between a solid surface and a fluid per unit surface area per unit temperature difference Dr. Şaziye Balku

  8. GENERAL THERMAL ANALYSIS Dr. Şaziye Balku

  9. FORCED CONVECTION Dr. Şaziye Balku

  10. LAMINAR FLOW Smooth streamlines Highly- ordered motion (highly viscous fluids in small pipes) • TURBULENT FLOW Velocity fluctuations Highly-disordered motion • TRANSITIONAL FLOW Dr. Şaziye Balku

  11. REYNOLDS NUMBER Ratio of the inertial forces to viscous forces in the fluid Flow Regime: Geometry Surface roughness Flow velocity Surface temperature type of fluid Mean flow velocity Characteristic length of the geometry Kinematic viscosity Dr. Şaziye Balku

  12. Definition of Reynolds number • Critical Reynolds number (Recr) for flow in a round pipe Re < 2300  laminar 2300 ≤ Re ≤ 4000  transitional Re > 4000  turbulent • Note that these values are approximate. • For a given application, Recrdepends upon • Pipe roughness • Vibrations • Upstream fluctuations, disturbances (valves, elbows, etc. that may disturb the flow) Dr. Şaziye Balku

  13. HYDRAULIC DIAMETER • For non-round pipes, • the hydraulic diameter Dh = 4Ac/P Ac= cross-section area P = wetted perimeter Dr. Şaziye Balku

  14. Velocity No-slip condition Dr. Şaziye Balku

  15. THERMAL BOUNDARY LAYER Flow region over the surface in which the temperature variation in the direction normal to the surface Velocity profile influences temperature profile Dr. Şaziye Balku

  16. VELOCITY A flow field is best characterized by the velocity distribution, and velocity may vary in three dimension in rectangular in cylinderical coordinates One dimensional flow in a circular pipe In which direction does the velocity change in this figure??? Dr. Şaziye Balku

  17. NUSSELT NUMBER (Dimensionless number) Dr. Şaziye Balku

  18. PRANDTL NUMBER • Boundary layer theory Pr<<1 heat diffuses very quickly in liquid metals, tbl thicker Pr>>1 heat diffuses very slowly in oils relative to momentum, tbl thinner than vbl Dr. Şaziye Balku

  19. PARALLEL FLOW OVER FLAT PLATES laminar turbulent Dr. Şaziye Balku

  20. NATURAL CONVECTION Dr. Şaziye Balku

  21. CONVECTIVE HEAT TRANSFER COEFFICIENT Coefficient of volume expansion Grashof number Rayleigh number Prandtl number Nusselt number viscosity Table 20-1 Dr. Şaziye Balku

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