Finned surfaces natural convection with

As we have discussed earlier, the buoyancy force is caused by the density difference between the healed (or cooled) fluid adjacent to the surface and tiie fluid surrounding it, and is proportional to this density difference and the volume occupied by the warmer fluid. It is also well knowu ll at whenever Iwc bodies in contact (.solid--solid, solid-fluid, or fluid-fluid) move relative to cacf other, a friction force develops at the contact surface in the direction opposite ic that of the motion. This opposing force slows down the fluid and thus reduce the flow rate of the fluid. Under steady conditions, the airflow rate driven b buoyancy is established at the point where these two effects balance each othet The friction force increases as more and more solid surfaces are introduced, se tiously disrupting the fluid flow and heat transfer. For that reason, heat sink with closely spaced fins are not suitable for natural convection cooling. 

When extended surfaces such as fins are used to enhance natural convection heat transfer between a solid and a fluid, the flow rate of the fluid in the vicinity of the solid adjusts itself to incorporate the changes in buoyancy and friction. It is obvious that this enhancement technique will work to advantage only when the increase in btroyancy is greater than the additional friction introduced. One does not need to be concerned with pressure drop or pumping power when studying natural convection since no pumps or blowers are used in (his case. Therefore, an enhancement technique in natural convection is evaluated on heat transfer performance alone. 

C Why are heal sinks with closely packed fins not suitable for natural convection heat transfer, although they increase the heat transfer surface area more ... 

A 15 -cm-widc and 18-cm-high vertical hot surface in 25 C air is to be cooled by a heat sink with equally spaced fins of rectangular profile. The fins are 0.1 cm thick and 18 cm long in the vertical direction. Determine the optimum fin height and the rate of heat transfer by natural convection from the heat sink if the base temperature is SS C. 

Even smaller stacks, with relatively large outside surface area (relative to the active area inside), may be passively cooled by natural convection and radiation. If necessary, the surface area may be enlarged by fins. 

Tlie fin is the most common fonn of extended surface, associated with both tubular and plate-type heat exchangers - the car radiator is a highly compact finned unit. Most heat exchangers used in gas streams have fins to improve the gas-side heat transfer. Compact units such as the plate-fin heat exchanger have fins, or secondary surfaces , between each pair of plates. Fins are used in both natural and forced convection. They are also used, when they are sometimes called ribs, to aid boiling or to provide drainage... 

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