Refrigeration – Automatic control – Gas-liquid contact cooler – fluid flow
Reexamination Certificate
2003-03-07
2004-11-16
Jiang, Chen Wen (Department: 3744)
Refrigeration
Automatic control
Gas-liquid contact cooler, fluid flow
C062S259200, C165S104330, C361S699000
Reexamination Certificate
active
06817196
ABSTRACT:
The present invention relates generally to spray-cooling systems for heat-generating devices and, more particularly, to spray-cooling regime detection in cooling methods and apparatus.
BACKGROUND OF THE INVENTION
With the advent of semiconductor devices having increasingly large component densities, the removal of heat generated by the devices has become an increasingly challenging technical issue. Many higher-dissipation semiconductor chips require substantially greater dissipation than air-cooled and liquid convection heat sinks can reasonably provide. Because liquids typically have a high latent heat of vaporization, immersion (i.e., boiling liquid off a submerged device) and spray cooling (i.e., boiling a sprayed liquid cooling fluid off a device) provide a high heat-transfer efficiency, absorbing a large quantity of heat at a constant temperature.
FIG. 1A
depicts the cooling regimes that can occur during immersion, over various wall excess temperatures (i.e., the temperature difference between the chip wall temperature T
w
and the fluid saturation temperature T
Sat
, i.e., the pressure-specific boiling point of the fluid). As depicted, an immersed chip's heat flux varies with excess temperature. When considering the performance of a cooling system with respect to an independent parameter, a local maximum dissipation level within a reasonable excess temperature range is referred to as a critical heat flux (CHF). With a good coolant, the cooling capability via immersion, with respect to heat generation, has a local maximum power density that is at reasonable temperature, the immersion CHF. When an immersed device generates heat at a rate greater than the immersion CHF, the vaporized cooling fluid forms a vapor barrier insulating the device from the liquid cooling fluid, allowing the wall temperature of the device to increase greatly from that of the immersion CHF to a level where it radiates enough energy to dissipate heat at the generated rate.
The variation in an immersed chips heat flux occurs over a number of cooling regimes, numbered
1
-
5
in the figure. In regimes
1
and
2
, the amount of heat flux via spray cooling increases with the excess temperature. Simple convective cooling occurs in regime
1
. In regime
2
, nucleate boiling occurs. As discussed above, in regime
3
, a vapor zone forms inside a pool of liquid, and heat dissipation reaches a maximum, the immersion CHF. The cooling regime then passes through regime
4
, a transition boiling regime where additional wall temperatures result in lower heat flux, to regime
5
, a film boiling regime where radiation becomes the dominant mode of heat transfer. As noted above, in regime
3
, should the heat flux increase beyond CHF, the cooling would jump to regime
5
, causing a large increase in the wall temperature (and likely a device failure).
When conducted at a given mass-flow rate, spray cooling can be characterized by a graph somewhat similar in shape to that of immersion cooling. A typical spray-cooling sprayer is used to spray a chip such that the cooling fluid forms a thin film on the chip that immediately vaporizes in nucleate boiling. This formation of a nucleate boiling film is similar to regime
2
(the nucleate boiling regime) for immersion cooling, but it is characterized by substantially higher dissipation levels. Nucleate boiling for spray cooling can only be accomplished at certain mass-flow rates. The spray-cooling graph varies depending on the mass-flow rate of the cooling fluid.
FIG. 1B
depicts the cooling regimes that can occur during both spray cooling (at three different cooling-fluid mass-flow rates) and immersion, over varied excess temperatures. In the figure, the solid portion of the curve for each mass-flow rate represents the nucleate boiling regime. The dissipation levels for spray cooling at relatively low wall excess temperatures can reach well over an order of magnitude higher than the immersion CHF, so long as the cooling fluid is sprayed at a rate to maintain the nucleate boiling regime.
With the second mass flow rate curve (i.e., the center spray cooling curve) used as an example, with respect to heat generation, spray cooling is limited to a reasonable-temperature local-maximum power density CHF
m2
, its dry-out CHF. As wall excess temperature increases, the dissipation level increases up to this maximum. If heat is generated at a rate greater than the dry-out CHF, an inadequate amount of sprayed cooling fluid is available to dissipate the heat, and the cooling regime jumps to a radiation regime (similar to regime
5
for immersion), where the wall temperature increases substantially.
Likewise looking at the second mass flow rate curve, with respect to heat generation, spray cooling is limited to a local-minimum power density at Q
min,2
. Below that point, a build up of liquid can occur that will initiate immersion boiling, moving to the immersion curve with a substantially higher excess temperature. At such a low heat flux, a decrease in mass flow rate is required to revert back to a spray cooling regime, such as is depicted for moving from the immersion curve back to the first (and lowest depicted) mass flow curve.
Typically, the objective of spray cooling is to achieve a device wall temperature close to a coolant's saturation temperature, e.g., a vaporization of low boiling point fluid such as 3M FC-72 (with a boiling point of 56° C. at one atmosphere), which can achieve a chip wall temperature close to 70° C. This wall temperature is generally useful and/or necessary for keeping the device junction temperature at 85° C. This need for the chip wall temperature to remain at 70° C. stems in part from the irregular distribution of power on a chip. High power densities, reaching 200 W/cm2, for a 50 W source distributed over 0.5 cm by 0.5 cm, cannot generally be addressed by a conventional mechanical interfacing means. The temperature gradients are prohibitive with conventional interfacing, and thus spray cooling is preferable.
In a typical spray-cooling system, (see FIG.
2
), an inert spray coolant from a reservoir
11
is uniformly sprayed by a group of one or more sprayers
13
onto an aligned group of one or more chips
15
mounted on a printed circuit board
17
. The coolant preferably evaporates, dissipating heat within the chip. The sprayers and chips, and the board, are mounted within sealed cases
19
fixed within an electronic device such as a computer system. The vaporized sprayed coolant is typically gathered and cooled within a condenser
21
, and then routed back to the reservoir by a pump
23
. Any runoff or subsequently condensed fluid remaining around the circuit board is also cycled back to the reservoir.
The nozzle design is a key component of spray cooling. Sprayer designs commonly employ either pressurized liquid (i.e., pressure-assisted spraying) or pressurized gas (i.e., atomized spraying). These types of spraying can be difficult to control, which can be important in maintaining a high dissipation level, as is pointed out above. For spray-cooling systems to function at optimal efficiency, the sprayers' mass-flow rate (s) should be adjusted to avoid having the semiconductor device experience either the dry-out or pool boiling regimes (i.e., become either dry or immersed). For controllable sprayers this rate is controlled by having a controller adjust the rate that the sprayers are sprayed. Alternatively, the mass-flow rate could be adjusted to allow having the semiconductor device experience either the dry-out or pool boiling regimes, but to enforce a limit on how close to approach the dry-out and/or pool boiling CHF.
Over time, chips will generally have different activity levels, and thus have different dissipation requirements. The optimum cooling-fluid mass-flow rate changes as the heat flux of the semiconductor device changes. Thus, for a controller to correctly control the mass-flow rate, semiconductor-device and/or cooling-system parameters that indicate the cooling regime need to be sensed.
To monitor the cooling regime, tempera
Bash Cullen E.
Malone Christopher G.
Patel Chandrakant D.
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