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CHE/ME 109 Heat Transfer in Electronics

CHE/ME 109 Heat Transfer in Electronics. LECTURE 16 – EXTERNAL CONVECTION IN SPECIFIC SYSTEMS. FLOW ACROSS CYLINDERS AND SPHERES. FLUID FLOW ACROSS A CYLINDER PASSES THROUGH SEVERAL DIFFERENT FLOW REGIMES. VELOCITY PATTERNS.

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CHE/ME 109 Heat Transfer in Electronics

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  1. CHE/ME 109 Heat Transfer in Electronics LECTURE 16 – EXTERNAL CONVECTION IN SPECIFIC SYSTEMS

  2. FLOW ACROSS CYLINDERS AND SPHERES • FLUID FLOW ACROSS A CYLINDER PASSES THROUGH SEVERAL DIFFERENT FLOW REGIMES

  3. VELOCITY PATTERNS • THE VELOCITY AT THE CENTERLINE IS ZERO AT THE FORWARD STAGNATION POINT • A BOUNDARY LAYER FORMS AS THE FLUID ACCELERATES AROUND THE CYLINDER WITH DECREASING PRESSURE • THERE IS A POINT WHERE • AND THE VELOCITY REACHES A MAXIMUM Incropera & DeWitt, Fundamentals of Heat and Mass Transfer, 4th Edition, Wiley, 1996

  4. VELOCITY PATTERNS • BEYOND THIS POINT, THERE IS A REVERSAL OF THE PRESSURE AND THE MATERIAL SEPARATES FROM THE BOUNDARY LAYER • VORTICES ARE FORMED AND THE WAKE DEVELOPS SEPARATION POINTS • FOR LAMINAR FLOW (Re < 2x10^5) THE SEPARATION OCCURS AT ~ 80◦ FROM THE STAGNATION POINT • AS Re INCREASES TO HIGHER LEVELS, THE SEPARATION POINT MOVES AROUND TO A MAXIMUM OF ~ 140∘ • THE SIZE OF THE WAKE IS INVERSELY PROPORTIONAL TO THE FORM DRAG

  5. SURFACE ROUGHNESS EFFECTS ON DRAG • BREAKING UP THE BOUNDARY LAYER (USING DIMPLES ON GOLF BALLS OR SPOILERS ON HIGH SPEED VEHICLES) WILL REDUCE THE PRESSURE DRAG. • THE DRAG COEFFICIENT CAN BE REDUCED BY INDUCING TURBULENCE AT A LOWER REYNOLD’S NUMBER

  6. CFD SIMULATIONS OF VELOCITY PROFILES • LAMINAR FLOW • TURBULENT FLOW http://www.math.rug.nl/~veldman/figures/dns-zoom.jpg

  7. FLOW PARALLEL TO THE CYLINDER AXIS • MOMENTUM AND HEAT TRANSFER IS MODELED USING THE FLAT PLATE CORRELATIONS • FOR SPHERES THE SAME EFFECTS ARE PRESENT IN THREE DIMENSIONS • PRESSURE DROP CORRELATIONS ARE SHOWN IN FIGURE 7-17

  8. HEAT TRANSFER COEFFICIENTS • HEAT TRANSFER COEFFICIENTS FOR CYLINDERS AND SPHERES ARE OF THE FORM: • EXAMPLES ARE (7-35) AND (7-36) • PROPERTIES ARE EVALUATED AT FILM TEMPERATURES, EXCEPT FOR THE WALL VISCOSITY • THESE CORRELATIONS INCLUDE A LAMINAR AND A TURBULENT PORTION

  9. FLOW ACROSS A RANGE OF EXTERNAL FORMS • A MORE GENERAL FORM IS Nu = CRemPrn • VALUES FOR FLOW ACROSS A RANGE OF EXTERNAL FORMS ARE SHOWN IN TABLE 7-1 • ALL FLUID PROPERTIES ARE BASED ON THE FILM TEMPERATURE • A VARIATION OF THIS EXPRESSION IS: • FOR THIS VERSION ALL PROPERTIES EXCEPT THE PrSurf ARE EVALUATED AT THE MEAN STREAM TEMPERATURE

  10. LIMITATIONS FOR CORRELATIONS • THESE CORRELATIONS ARE ALL BASED ON: • A SPECIFIC FLUID • SPECIFIC FLOW REGIMES • SPECIFIC SURFACE ROUGHNESS • SPECIFIC RANGES OF Pr AND Re • EXPECTED ACCURACY IS + 20%

  11. FLOW ACROSS TUBE BANKS • EXTERNAL FLOWS OVER BANKS OF TUBES ARE INFLUENCED BY THE RELATIVE POSITIONS OF THE TUBES • THERE CAN BE SHADOWING - WHERE THE WAKE OF AN UPSTREAM TUBE AFFECTS THE BOUNDARY LAYER FORMATION ON A DOWNSTREAM TUBE • THERE CAN BE ADDITIONAL MOMENTUM INTERACTIONS BETWEEN ADJACENT TUBES

  12. FLOW ACROSS TUBE BANKS • TUBE PATTERNS • TUBES CAN BE INSTALLED WITH ALIGNED OR STAGGERED LAYOUTS (SEE FIGURE 7-25) • LOCATIONS ARE CHARACTERIZED IN TERMS OF PITCH (SEE FIGURE 7-26) OR DISTANCE BETWEEN TUBE CENTERS • TRANSVERSE PITCH, ST, IS THE DISTANCE BETWEEN TUBES NORMAL TO FLOW • LONGITUDINAL PITCH, SL, IS THE DISTANCE • DIAGONAL PITCH, SD, IS BASED ON THE DISTANCE BETWEEN TUBE CENTERS IN ALTERNATE ROWS FOR STAGGERED CONFIGURATIONS

  13. CROSS TUBE HEAT TRANSFER CORRELATIONS • THE CORRELATIONS ARE BASED ON THE MAXIMUM FLUID VELOCITY IN THE TUBE BUNDLE, WHICH IS USED TO CALCULATE Re FOR THE FLUID • THIS CAN BE RELATED TO THE FREE STREAM VELOCITY AS FOR ALIGNED LAYOUTS AS: • FOR STAGGERED LAYOUTS • THE SAME EXPRESSION APPLIES WHEN 2(SD - D)>(ST - D) • FOR 2(SD - D)<(ST - D) THEN THE MAXIMUM IS CALCULATED:

  14. CROSS-TUBE HEAT TRANSFER CORRELATIONS • THE GENERAL FORM OF THE CORRELATION IS • FLUID PROPERTIES ARE CALCULATED AT THE MEAN TEMPERATURE WHICH IS THE AVERAGE BETWEEN THE ENTRY AND EXIT OVER THE TUBE BANK • RECOMMENDED EQUATIONS ARE SHOWN IN TABLE 7-2

  15. CROSS-TUBE PRESSURE DROP CORRELATIONS • CONSIDER TUBE CONFIGURATION AS WELL AS Re NUMBER • THESE GRAPHS ALSO USE THE MAXIMUM VELOCITY TO CALCULATE THE Re

  16. HEAT TRANSFER IN ELECTRONICS COOLING • SIGNIFICANCE OF PRINTED CIRCUIT BOARDS ARE EXAMPLES OF FLAT PLATE INSTALLATIONS • IN HIGH HEAT LOAD SERVICES, THE HEAT TRANSFER MAY INCLUDE PASSING COOLING AIR OVER TUBE BUNDLES TO REDUCE TEMPERATURE http://www.flir.com.hk/images/SPLi_ir_PCboard_108.jpg

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