1 / 18

CHE/ME 109 Heat Transfer in Electronics

CHE/ME 109 Heat Transfer in Electronics. LECTURE 19 – NATURAL CONVECTION FUNDAMENTALS . NATURAL CONVECTION MECHANISMS. NATURAL CONVECTION IS THE RESULT OF LOCALIZED DENSITY DIFFERENCES THESE CAN BE DUE TO DIFFERENCES IN COMPOSITIONS

nigel
Download Presentation

CHE/ME 109 Heat Transfer in Electronics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. CHE/ME 109 Heat Transfer in Electronics LECTURE 19 – NATURAL CONVECTION FUNDAMENTALS

  2. NATURAL CONVECTION MECHANISMS • NATURAL CONVECTION IS THE RESULT OF LOCALIZED DENSITY DIFFERENCES • THESE CAN BE DUE TO DIFFERENCES IN COMPOSITIONS • FOR HEAT TRANSFER THEY ARE GENERALLY RELATED TO TEMPERATURE DIFFERENCES • CONCENTRATION BASED CONVECTION INCLUDES CLOUD FORMATIONS • WATER HAS A LOWER MOLECULAR WEIGHT THAN AIR • CONCENTRATIONS OF WATER WILL TEND TO RISE THROUGH AIR DUE TO CONVECTION TO FORM CLOUDS • CUMULONIMBUS CLOUD FORMATION AS A RESULT OF CONVECTION. THE CLOUD TRACES THE PATH OF THE CONVECTION CURRENTS. http://blogs.sun.com/staso/resource/cumulonimbus-cloud-akbhhf-sw.jpg

  3. CLOUD FORMATION H • THIS MECHANISM IS BASED ON NATURAL CONVECTION

  4. CLOUD FORMATIONS • TEMPERATURE DIFFERENCES WILL ALSO RESULT IN ADVECTION, A HORIZONTAL TRANSFER OF HOT AIR OVER COLD AIR • NIMBOSTRATUS CLOUDS FORMED DUE TO ADVECTION. CLOUDS SHOW THE HORIZONTAL CURRENTS http://cimss.ssec.wisc.edu/satmet/modules/clouds/lowclouds2.html

  5. SURFACE WINDS • THE RESULT OF PRESSURE DIFFERENCES. • THE FLOW OF COOL AIR FROM THE OCEAN TO THE COAST IS THE RESULT OF THIS TYPE OF NATURAL CONVECTION • THE MOST EXTREME EXAMPLES OF THESE FLOWS CAN RESULT IN THE FORMATION OF TORNADOES, CYCLONES AND HURRICANES http://www.berkeleycitycollege.edu/faculty/rhaberlin/images/pwppthl.gif

  6. SEA AND LAND FLOWS • THESE ARE BASED ON DENSITY DIFFERENCES THAT RESULT IN PRESSURE VARIATIONS http://www.free-online-private-pilot-ground-school.com/images/sea-land-breeze.gif

  7. DENSITY DIFFERENCES • DEFINED IN TERMS OF VOLUME EXPANSION COEFFICIENT • DERIVATION OF CHANGES IN DENSITY FOR FLUIDS: • VOLUME EXPANSIVITY: • ISOTHERMAL COMPRESSIBILITY:

  8. DENSITY DIFFERENCES • FOR IDEAL GASES: • SO AROUND AMBIENT TEMPERATURE β = 3.3x10-3 K-1 = 1.8x10-3 R-1 • FOR LIQUIDS THE VALUES ARE ON THE ORDER OF β = 3x10-4 K

  9. GRASHOF NUMBER • FLUID MOTION OCCURS DUE TO BOUYANCY EFFECTS AS PER (FIGURE 9-6) • ONCE THE FLUID IS IN MOTION, THEN VISCOUS EFFECTS OCCUR • COMPLETING A MOMENTUM BALANCE FOR A NATURAL CONVECTION FLOW WITH VELOCITIES IN THE x AND y DIRECTION (u AND v RESPECTIVELY) CONSIDERED YIELDS (9-13):

  10. GRASHOF NUMBER • GRASHOF NUMBER IS THE RATIO OF THE BOUYANCY FORCES TO THE VISCOUS FORCES • VALUE OF THE GRASHOF NUMBER CAN BE LINKED TO FLOW REGIMES FOR NATURAL CONVECTION

  11. NATURAL CONVECTION OVER SURFACES • FOR NATURAL CONVECTION HEAT TRANSFER PROCESSES • THE CORRELATIONS FOR HEAT TRANSFER COEFFICIENTS ARE BASED ON THE RAYLEIGH NUMBER: Ra = GrPr • Ra IS THE NATURAL CONVECTION EQUIVALENT OF THE PECLET NUMBER, Pe = RePr FOR FORCED CONVECTION

  12. NATURAL CONVECTION OVER SPECIFIC SHAPES • VERTICAL FLAT PLATES • BOUNDARY LAYER STAYS AGAINST THE SURFACE AND THE FLOW REGIME CHANGES WITH DISTANCE. • TRANSITION TO TURBULENCE IS GENERALLY DEFINED IN TERMS OF THE Ra NUMBER AT Ra > 109. • EQUATIONS ARE DEVELOPED • FOR CONSTANT TEMPERATURE OR CONSTANT HEAT RATE • BASED ON FILM TEMPERATURE EQUAL TO (Ts - T )/2 • APPLY EQUALLY TO HOT OR COLD WALLS, RELATIVE TO T∞

  13. NATURAL CONVECTION OVER SPECIFIC SHAPES • VERTICAL CYLINDERS CAN BE ANALYZED WITH THE VERTICAL PLATE EQUATIONS AS LONG AS THE DIAMETER IS LARGE ENOUGH

  14. INCLINED PLATES AND FLAT PLATES • HAVE DIFFERENT FLOW PATTERNS FOR PLATE TEMPERATURES • GREATER THAN THE SURROUNDINGS • LOWER THAN THE SURROUNDINGS

  15. INCLINED PLATES AND FLAT PLATES • HAVE DIFFERENT CORRELATIONS FOR TOPS OF PLATES AND BOTTOMS OF PLATES • )INCLINED PLATES CAN USE VERTICAL • PLATE CORRELATIONS BY REPLACING g IN THE Gr NUMBER WITH g(cos θ): • FOR THE TOP OF COOLED PLATES OR THE BOTTOM OF HEATED PLATES • FOR θ < 60 • FOR OTHER SITUATIONS, THE BOUNDARY LAYER BREAKS UP AND A SINGLE CORRELATION IS NOT PRACTICAL

  16. HORIZONTAL PLATES • USE DIFFERENT CORRELATIONS BASED ON Lc = As/P FOR • THE UPPER SURFACE OF A HEATED PLATE OR THE LOWER SURFACE OF A COOLED PLATE (9-22 & 9-23) • THE LOWER SURFACE OF A HEATED PLATE OR THE UPPER SURFACE OF A COOLED PLATED (9-32)

  17. HORIZONTAL CYLINDERS • THE BOUNDARY LAYER FORMS AROUND THE RADIUS AS SHOWN IN FIGURE 9-12 • SINGLE CORRELATION IS PROVIDED (9-25) APPLIES TO • LAMINAR CONDITIONS Ra < 1012 • FOR TURBULENT FLOW Ra > 109:

  18. OTHER CORRELATIONS • FOR CONSTANT SURFACE TEMPERATURE , VALUES ARE BASED ON THE GENERAL FORMULATION: • SPHERES ARE MODELED USING (9-26) FROM IRVINE & HARTNETT (Eds), ADVANCES IN HEAT TRANSFER, Vol 11, 1975, Pp. 199-264

More Related