Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
2001-05-01
2002-12-24
Chervinsky, Boris (Department: 2835)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S699000, C361S701000, C257S714000, C257S715000, C174S015100, C165S080400, C165S104210, C165S104220, C165S104310, C165S104330
Reexamination Certificate
active
06498725
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to improved cooling systems for devices that need to be actively cooled to function reliably and efficiently. A primary application of this invention is microelectronic chips, devices, or systems that generate large quantities of heat in small volumes, thereby requiring high heat flux removal techniques. Other applications include rapid quenching of metals, thermal control of mirrors, and cooling of lasers.
As microelectronics continue to develop, there is an increasing trend of performing more functions at a faster rate in a smaller package volume. The net effect is that more heat is generated and it must be removed from a smaller surface area for efficient operation and reliability. It is not only desirable to remove the heat, but also to control the chip temperature independently of ambient conditions. It is well known that in certain microelectronic devices, such as monolithic microwave integrated circuit (MMIC) chips, that by lowering chip temperature, greater throughput may be allowed without damaging the chip and more efficient amplification is possible. These chips find application in electronically scanned active aperture antennas and have potential application in next-generation, electronic warfare systems and other equipment requiring high performance, electronically-steered antennas.
Common heat removal techniques such as conductive heat transfer through the use of heat sinks, natural or forced convective heat transfer, or combinations thereof, limit chip temperature to temperatures slightly above ambient. Active cooling systems provide the flexibility to cool microelectronics to temperatures below ambient and provide high heat flux removal. While there are several high heat flux cooling techniques, the present invention focuses on spray cooling, the advantages of which will become apparent in the following discussion.
There have been numerous investigations into the use of spray cooling for microelectronic systems. In most cases, practical application or systems to control the temperature and spray of the issuing spray have been limited. This is partially due to the lack of understanding of the contributions from each of the mechanisms behind spray cooling that lead to an optimized spray for high heat flux removal.
The first condition for high heat flux spray cooling is atomization of the fluid spray. Prior to spray cooling, early studies were performed using liquid jet impingement, where a narrow jet of fluid is directed upon the cooling surface. Later studies confirm that a finely atomized spray increases the heat flux removal capability of the fluid. Atomized sprays provide a more equal distribution of heat flux thereby maintaining more uniform temperature distributions across the cooling surface and preventing localized hot spots. This in turn prevents burnout and allows for higher critical heat flux, i.e. the point where increasing the temperature difference between the cooling surface and the spray is no longer associated with an increase in heat flux. Additionally, spray cooling allows for lower flow rates for equivalent average heat flux, thus reducing cooling system size. Spray atomization occurs when the magnitude of aerodynamic disruptive forces exceeds the consolidating surface tension forces and stabilizing viscous forces.
A wide variety of spray atomizers exist. Pressure-type atomizers are the most compact and therefore beneficial for compact microelectronics cooling packages. Within pressure-type atomizers, plain orifice and simplex pressure swirl are the most compact, simplistic, and rugged. To enhance spray cooling, several atomization conditions including uniform spray, complete cooling surface coverage, minimal momentum losses, and minimum spray evaporation losses are preferred.
Plain orifice atomizers produce uniform, full cone sprays. Simplex (or pressure-swirl) type atomizers generally produce a hollow cone that can be modified to produce a full cone by using an axial jet or some other device to inject droplets in the center of the hollow conical spray pattern. Hollow cone sprays can potentially lead to burnout at the cone center, whereas, full cone sprays created with axial jets or injections typically produce a bimodal distribution of drop sizes with the droplets at the center of the spray being larger than those near the edge leading to burnout at the spray edge. To prevent burnout, the entire heat-producing surface must be covered. “Simplex” atomizers produce a spray angle that is highly dependent on pressure differential across the nozzle. Practical application would require moving the nozzle axially to maintain c overage with changes in pressure differential or a control system to maintain nozzle pressure differential regardless of other system parameters such as cooling load or heat rejection temperature. Plain orifice atomizers produce a more constant spray angle that is mostly dependent on fluid properties such as viscosity and surface tension as well as turbulence of the issuing spray and therefore do not require axial locational control. Axial momentum improves spray cooling heat transfer by holding a thin liquid layer on the cooling surface. This is particularly important in adverse-gravity environments that may be encountered in space cooling applications or aboard aircraft during flight maneuvers. Therefore, it is desirable to minimize axial momentum losses that are typically increased as the radial spray component increases (i.e. increased spray angle). Plain orifice nozzles have a narrower spray cone (typically 5 to 15 degrees) than simplex nozzles (typically 30 to 180 degrees) and therefore reduced axial momentum losses. Additionally, narrow cone sprays are less susceptible to evaporation due to mixing of ejected liquid droplets and entrained vapor, thus providing more evaporation due to impingement of saturated liquid on the hot surface to be cooled.
These atomization conditions lead to an associated spray property that also enhances spray cooling. Prior art approaches typically use a spray that is “at or near” saturated liquid conditions. This allows for two-phase boiling heat transfer at the heated surface and takes advantage of the high heat transfer associated with latent heat of vaporization. Two-phase heat transfer is typically at least an order of magnitude greater than single phase heat transfer. However, the condition of “at or near” saturated liquid is not optimal. If the spray fluid is slightly subcooled (i.e. only near saturated liquid conditions) then nucleation for boiling must occur at the heated surface. This means the entire process of creating a nucleation site, allowing bubble growth, and removing the bubble to allow new nucleation sites must occur at the chip surface after the liquid droplets impinge on the surface.
It is not only important to initiate nucleation prior to contacting the surface to be cooled, it is also important that a system control the temperature at which this nucleation occurs and a system be capable of accommodating varying heat loads at the desired temperature. For instance, while it is desirable to constrain a spray to a uniformly atomized spray, that is two phase and is issued at an adjustable temperature, practical controls to perform these functions must be incorporated into a complete cooling system. Atomization in pressure-type spray nozzles only occurs if the pressure differential across the nozzle is sufficient.
At very low pressure differentials the flow begins as a “dribble” or “thin distorted pencil”. At intermediate pressure differentials additional stages such as “onion” or “tulip” stages may occur prior to fully developed sprays at higher differentials. It is doubtful that the early stages will provide adequate cooling and will most likely lead to premature burnout. Therefore, control of the spray must be incorporated in a practical design. Optimum chip operating temperature may be determined a priori or through the use of chip performance monitoring plus feedback. For instance, high power microwave amplifiers
Cole Gregory S.
Scaringe Robert P.
Chervinsky Boris
Crowell & Moring LLP
Mainstream Engineering Corporation
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