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CHE/ME 109 Heat Transfer in Electronics

CHE/ME 109 Heat Transfer in Electronics. LECTURE 13 – CONVECTION FUNDAMENTALS. MECHANISM FOR CONVECTION. CONVECTION IS ENHANCED CONDUCTION FLOW RESULTS IN MOVEMENT OF MOLECULES THAT WILL EFFECTIVELY INCREASE THE VALUE OF THE DRIVING FORCE (dT/dX) FOR CONDUCTION

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CHE/ME 109 Heat Transfer in Electronics

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  1. CHE/ME 109 Heat Transfer in Electronics LECTURE 13 – CONVECTION FUNDAMENTALS

  2. MECHANISM FOR CONVECTION • CONVECTION IS ENHANCED CONDUCTION • FLOW RESULTS IN MOVEMENT OF MOLECULES THAT WILL EFFECTIVELY INCREASE THE VALUE OF THE DRIVING FORCE (dT/dX) FOR CONDUCTION • CONVECTION OCCURS AT A SURFACE • NEWTON’S LAW OF COOLING APPLIES

  3. MECHANISM FOR CONVECTION • HEAT FLUX AT THE SURFACE IS BASED ON THE TEMPERATURE PROFILE AT THE SURFACE (WHERE A ZERO VELOCITY FOR THE FLUID IS ASSUMED: • THE RESULTING DEFINITION OF h IS:

  4. NUSSELT NUMBER • PROVIDES A RELATIVE MEASURE OF HEAT TRANSFER BY CONDUCTION VERSUS HEAT TRANSFER BY CONVECTION • THE VALUE OF THE L TERM IS ADJUSTED ACCORDING TO THE SYSTEM GEOMETRY

  5. TYPES OF FLOWS • THERE ARE A WIDE RANGE OF FLUID FLOW TYPES • VALUES OF h ARE BASED ON CORRELATIONS • CORRELATIONS ARE BASED ON FLUID FLOW REGIME, GEOMETRY, AND FLUID CHARACTERISTICS

  6. VISCOUS/INVISCID (FRICTIONLESS) INTERNAL/EXTERNAL COMPRESSIBLE/NON-COMPRESSIBLE LAMINAR/TURBULENT/TRANSITION NATURAL/FORCED CONVECTION STEADY/UNSTEADY ONE-TWO-THREE DIMENSIONAL FLOWS TYPES OF FLOWS H

  7. VELOCITY BOUNDARY LAYER • THERE IS A VELOCITY GRADIENT FROM THE HEAT TRANSFER SURFACE INTO THE FLOW REGIME. • AS THE FLOW INTERACTS WITH THE SURFACE, MOMENTUM IS TRANSFERRED INTO VELOCITY GRADIENTS NORMAL TO THE SURFACE

  8. BOUNDARY LAYER • DEFINED AS THE REGION OVER WHICH THERE IS A CHANGE IN VELOCITY FROM THE SURFACE VALUE TO THE BULK VALUE • THE TYPE OF FLOW ADJACENT TO THE SURFACE IS CHARACTERIZED AS • LAMINAR – TURBULENT OR TRANSITION

  9. BOUNDARY LAYER FLOWS • LAMINAR - SMOOTH FLOW WITH MINIMAL VELOCITY NORMAL TO THE SURFACE • TURBULENT - FLOW WITH SIGNIFICANT VELOCITY NORMAL TO THE SURFACE • THE TURBULENT LAYER MAY BE FURTHER SUBDIVIDED INTO THE LAMINAR SUBLAYER, THE TURBULENT LAYER, AND THE BUFFER LAYER • THE BREAKS OCCURS AT VALUES RELATIVE TO THE CHANGES IN VELOCITY WITH RESPECT TO DISTANCE • TRANSITION - THE REGION BETWEEN LAMINAR AND TURBULENT

  10. VISCOSITY • DYNAMIC VISCOSITY - IS A MEASUREMENT OF THE CHANGE IN VELOCITY WITH RESPECT TO DISTANCE UNDER A SPECIFIED SHEAR STRESS • KINEMATIC VISCOSITY IS THE DYNAMIC VISCOSITYDIVIDED BY THE DENSITY AND HAS THE SAME UNITS AS THERMAL DIFFUSIVITY

  11. FRICTION FACTOR • IS A VALUE RELATED TO THE SHEAR STRESS AS A FUNCTION OF VELOCITY AND VISCOSITY FOR A SYSTEM: • IT IS RELATED TO THE VELOCITY BOUNDARY LAYER AND HAS UNITS N/m2

  12. THERMAL BOUNDARY LAYER • GENERAL CHARACTERIZATION IS THE SAME AS FOR THE VELOCITY BOUNDARY LAYER • THE PRANDTL NUMBER (DIMENSIONLESS RATIO) IS USED TO RELATE THE THERMAL AND VELOCITY BOUNDARY LAYERS:

  13. CHARACTERIZATION OF FLOW REGIMES • REYNOLD’S NUMBER (DIMENSIONLESS) IS USED TO CHARACTERIZE THE FLOW REGIME: • THE CHANGES IN FLOW REGIME ARE CORRELATED WITH THE Re NUMBER

  14. REYNOLD’S NUMBER PARAMETERS • THE VALUE FOR THE LENGTH TERM, D, CHANGES ACCORDING TO SYSTEM GEOMETRY • D IS THE LENGTH DOWN A FLAT PLATE • D IS THE DIAMETER OF A PIPE FOR INTERNAL OR EXTERNAL FLOWS • D IS THE DIAMETER OF A SPHERE OR THE EQUIVALENT DIAMETER OF A NON-SPHERICAL SHAPE

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