Heat transfer surface with a microstructure of projections...

Details Heat transfer surface with a microstructure of projections... Heat transfer surface with a microstructure of projections...

2002-11-27

2004-05-18

McKinnon, Terrell (Department: 3743)

Heat exchange

Heat transmitter

C165S905000, C165S133000, C361S704000, C361S703000, C361S707000

Reexamination Certificate

active

06736204

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a heat transfer surface on tubular or plate-like bodies having a microstructure of projections protruding out of the base surface, the microstructure being galvanized onto the base surface with a minimum height of 10 &mgr;m, as well as a method of producing such heat transfer surfaces.
According to the state of the art, heat transfer surfaces are used in a variety of shapes and sizes in evaporators and condensers. Their structural design will depend on the type of evaporation (convective evaporation, nucleate boiling or film evaporation) and condensation (dropwise or film condensation).
The area of nucleate boiling is of the greatest importance. The formation of vapor bubbles takes place on the heat transfer surfaces. The growth, size and number of bubbles per unit of heat transfer surface and time are determined by essentially three parameters:
a) the properties of the boiling liquid,
b) the material of the heating wall as well as the structure of the heating surface,
c) the heat flow density.
In order for vapor bubbles to be able to develop and grow in a liquid, certain physical conditions must be met. The model concepts for describing these conditions are usually based on homogeneous nucleation, which in turn is usually attributed to fluctuations in density. Once it has formed, a vapor bubble requires an environment that allows it to grow. A simple equilibrium analysis yields the following relationship in evaporation:
T
-
T∞
=
2
⁢
σ
⁢
 
⁢
T∞
Δ
⁢
 
⁢
hςvr
(equation I)
where:
r=bubble radius,
&sgr;=surface tension of the liquid,
&Dgr;h=enthalpy of evaporation,
&dgr;v=vapor density,
T=temperature of the liquid,
T∞=the equilibrium temperature at a planar phase boundary.
The temperature difference T−T∞ may thus be interpreted as the minimum required overheating of the boiling liquid at the prevailing bubble size having radius r. It may be reduced by the fact that bubbles of large dimensions—i.e., with a large r—are produced through suitable measures. The heating heat transfer surface plays a central role. A favorable design of this heat transfer surface can greatly increase the efficiency of heat transport in boiling. The goal here is to achieve a heat transfer surface having a microstructure, which leads to the highest possible bubble density with a large bubble radius at the smallest possible temperature difference. This is a prerequisite for efficient heat transfer from the heat transfer surface to the liquid.
Essentially microstructures having cavities which are not flooded by the surrounding liquid after the bubbles break away are essentially suitable for this purpose. Vapor bubbles formed in the cavities expand during the growth phase into the liquid adjacent to the heat transfer surface and break way from this heat transfer surface when a system-dependent critical variable is exceeded; this takes place in such a manner that vapor residues remain in the cavities and serve as nucleation seeds for subsequent bubbles.
In the area of condensation, we encounter essentially film condensation and heat transfer devices, where the primary purpose is to keep the thicker condensate film away from the cooling heat transfer surface, which should also be provided with suitable microstructures. The driving force for the runoff of condensate can be linked to the capillary pressure
Δ
⁢
 
⁢
p
=
2
⁢
σ
r
(equation II)
where &sgr; is the surface tension and r is the radius of curvature of the phase boundary.
U.S. Pat. Nos. 4,288,897, 4,129,181 and 4,246,057 have disclosed microstructures as heat transfer surfaces on tubular bodies, where smooth tubes are wrapped with layers of polyurethane foam with a thickness of approximately 0.00025″ to 0.0025″ (approximately 6.35 &mgr;m to 63.5 &mgr;m), their open pore structures first being metal plated in a chemical process. Then the tube is connected to the metal-plated polyurethane sheathing as the cathode and to the base surface of the tube as the anode, and the galvanic deposition is begun. The electrolyte penetrates through the foam to the cylindrical surface of the tube, permitting a uniform deposition of metal ions on the tube and also in the interior of the foam structure. After achieving a suitable layer thickness, the galvanic process is terminated and the foam material is removed by burn-off (pyrolysis, leaving a porous metallic structure that is highly cross-linked and intermeshed on the base surface. It contains completely irregular thicknesses of the webs and completely different cavities and thus completely irregular, unordered structures, leaving the formation of vapor bubbles, e.g., in evaporation, up to chance. In cooling, impurities in the coolant remaining behind in the microfine cavities can have an extremely negative effect on the heat transfer.
U.S. Pat. No. 4,219,078 discloses a heat transfer surface in which a porous film to be wrapped around a tube contains copper particles with a diameter of 0.1 mm to 0.5 mm which are applied to the base surface in multiple layers and are joined by a galvanic process to an entire surface structure. Although this surface structure has a certain regularity, this cannot conceal the fact that bubbling is hindered more than promoted by the multilayered nature of the particles. The numerous cavities also counteract good heat transfer efficiency with regard to film condensation.
To make heat transfer surfaces porous and thus provide them with a certain uniformity in ordered structure with regard to their surface, non-generic mechanical machining processes are often used, such as those disclosed in German Patent 197 57 526 C1, U.S. Pat. No. 4,577,381, German Patent 27 58 526 A1 and European Patent 0 713 072.
Thus, for example, the tubes disclosed in German Patent 197 57 526 C1 and in European Patent 0 057 941 are worked with special rolling and upsetting tools to achieve a special, very rough, knurled surface structure. However, this surface structure is not in the micro range but instead is in the millimeter range, the thickness of the ribs being approximately 0.1 mm and their pitch approximately 0.41 mm with a tube diameter of 35 mm, which does not correspond to the generic microstructure. Although the channel-like cavities beneath the base surface can promote the development of bubbles in evaporation, they counteract the goal of keeping the cooling surfaces free in film condensation. The same thing is also true of the objects of the other publications cited above.
In addition to this previously known state of the art, there are also a number of types of coating by means of a sintering technique, a sprayer technique, flame spraying and sandblasting. None of these are generic methods and none of them attempt to solve the problem on which the present invention is based.
BRIEF SUMMARY OF THE INVENTION
The object of this invention is to create a heat transfer surface of the generic type defined in the preamble as well as a method for producing such a heat transfer surface, which is characterized by an increase in the heat transfer efficiency of its heat transfer surfaces at the lowest possible temperature differences T−T∞ and an optimum thermal efficiency and is suitable for nucleate boiling as well as film condensation at a justifiable manufacturing cost.
This complex object is achieved with regard to the heat transfer surface in combination with the above-mentioned generic term by the fact that the base surface is entirely or partially covered with projections; these projections are applied in the form of ordered microstructures and they have a pin shape, their longitudinal axis running either perpendicular or at an angle between 30° and 90° to the base surface. This feature creates for the first time a heat transfer surface in the microstructure range whose projections are shaped like pins and extend with their longitudinal axis perpendicular or transversely to the base surface. T

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