# Patent

## Title
Two-phase cross-connected micro-channel heat sink

## Abstract

An improved two-phase micro-channel heat sink has a plurality of flow micro-channels extending in parallel in a longitudinal direction for accommodating a flow of liquid coolant therein, and a plurality of cross-connected micro-channels extending in parallel in a transverse direction formed at regular intervals along the longitudinal direction for cross-connecting with the flow micro-channels. The cross-connected micro-channels enable a uniform pressure field to be maintained for two-phase flow boiling of liquid coolant in the heat sink. In a preferred embodiment, the heat sink contains flow micro-channels with width of 100 μm, wall thickness of 100 μm, and height of 600 μm, and cross-connected micro-channels of similar width and height cut at 1 mm intervals along the flow direction.

## Background
This U.S. patent application claims the priority of Provisional Patent Application 61/004,821 filed on Nov. 30, 2007, by the same inventor.


TECHNICAL FIELD
The present invention is directed to a heat sink device for efficient cooling of high-power-density electronic devices and, more particularly, to an improved design for a two-phase micro-channel heat sink.
BACKGROUND OF INVENTION
The ceaseless pursuit of improvement in performance of electronic devices with simultaneous reductions in weight and volume has sought improvements in efficient heat sink designs for ever-increasing dissipative waste heat densities. Current and near future high-performance commercial/defense electronic devices require heat fluxes on the order of 100 W/cm2 to be dissipated. As the reliability and life span of electronics are both strongly affected by temperature, it is necessary to develop highly efficient cooling techniques to reduce the maintenance costs and increase the service life of high-power-density electronic devices.
Two-phase micro-channel heat sinks are an attractive solution for removing waste heat from high-power-density devices, as described for example in “Thermal Design Methodology for Low Flow Rate Single-Phase and Two-Phase Micro-Channel Heat Sinks,” by Scott Lee and Weilin Qu, published in IEEE Transactions on Components and Packaging Technologies, December 2007. Two-phase micro-channel heat sinks feature micro-size parallel channels as coolant flow passages for flow boiling of liquid coolant. Key merits include low thermal resistance to heat dissipation, large surface-area-to-volume ratio, small heat sink weight and volume, small liquid coolant inventory and flow rate requirement, and relatively uniform temperature distribution along the stream-wise direction. However, one practical concern that hinders the implementation of this powerful cooling scheme is the temperature and pressure oscillation due to flow instability. It would be desirable to provide an improved design which can prevent severe flow instability and therefore ensure safe operation and predictable cooling performance.
SUMMARY OF INVENTION
In accordance with the present invention, an improved two-phase micro-channel heat sink comprises: a plurality of flow micro-channels extending in parallel in a longitudinal direction and having channel walls substantially symmetrical with each other of a given wall thickness and defining a flow channel width and height for accommodating a flow of liquid coolant therein; and a plurality of cross-connected micro-channels extending in parallel in a transverse direction normal to the longitudinal direction and being substantially symmetrical with each other and formed at regular intervals along the longitudinal direction for cross-connecting with the flow micro-channels, thereby enabling a uniform pressure field to be maintained for two-phase flow boiling of liquid coolant in the heat sink.
In a preferred embodiment, the heat sink contains flow micro-channels with width of 100 μm, wall thickness of 100 μm, and height of 600 μm, and cross-connected micro-channels of similar width and height cut at 1 mm intervals along the flow direction.
The cross-connection design prevents the severe flow oscillation that can occur in parallel channel designs due to feedback interaction between flow channels through common inlet and outlet plenums, by promoting the communication of flow between channels and creating a uniform pressure field in the heat sink. The novel two-phase cross-connected micro-channel heat sink can therefore deliver more stable flow and ensure safe operation and predictable cooling performance.
The two-phase cross-connected micro-channel heat sinks are capable of dissipating high heat fluxes from small areas, and hence, are suitable for thermal management of high-power-density commercial/defense electronic devices such as high-performance microprocessors, laser diode arrays, high-power components in radar systems, switching components in power electronics, x-ray monochromator crystals, avionics power modules, and spacecraft power components.
Other objects, features, and advantages of the present invention will be explained in the following detailed description of the invention having reference to the appended drawings.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 illustrates a section of the cross-connected micro-channel heat sink design.
FIG. 2 shows a top view of an overall cross-connected micro-channel heat sink layout.
FIG. 3 shows a perspective view of a typical construction of a conventional micro-channel heat sink design.
FIG. 4 is a graph showing a temporal record of inlet and outlet pressures for a conventional two-phase micro-channel design without cross connection.
FIG. 5 is a graph showing a comparable temporal record of inlet and outlet pressures for the new cross-connected design.



DETAILED DESCRIPTION OF INVENTION
In the following detailed description of the invention, certain preferred embodiments are illustrated providing certain specific details of their implementation. However, it will be recognized by one skilled in the art that many other variations and modifications may be made given the disclosed principles of the invention.
Micro-channel heat sinks have emerged as a prime contender for thermal management of next generation high-power-density electronic devices, whose key technical merits include low thermal resistance, small coolant inventory requirement, small coolant flow rate requirement, and small heat sink mass and volume. A heat sink is classified as a micro-channel heat sink by the characteristic dimensions of the flow passages. FIG. 3 illustrates the typical construction of a conventional micro-channel heat sink design. The micro-channel heat sink has a length L and width W and is comprised of parallel channel walls on a planar base substrate defining flow channels in a longitudinal direction (of length L). The heat sink base substrate is typically fabricated from a high thermal conductivity solid material such as copper or silicon. The flow channels are formed as a series of parallel rectangular micro-slots machined into the substrate to form flow channels of a width Wch defined by channel walls with a width of Ww and height Hch. The micro-channels are closed using a cover plate on the top side to form a heat sink of overall width W, length L, and height H. Inlet and outlet plenums at inlet and outlet ends introduce and discharge liquid coolant into and from the flow passages. During operation, the heat sink is attached to the top surface of a high-heat-flux electronic device. Heat generated by the device is first transferred to the micro-channels through the solid substrate by heat conduction, and then carried away by the liquid coolant that is forced to flow through the micro-channels.
Micro-channel heat sinks can be classified as single-phase or two-phase. For a fixed dissipative heat flux and relatively high coolant flow rate, the coolant may maintain its liquid single-phase state throughout the micro-channels, which leads to a single-phase heat sink. When the coolant flow rate is relatively low, the liquid coolant may reach its boiling point while still flowing in the micro-channels and flow boiling occurs, which results in a two-phase heat sink. While single-phase heat sinks generally require less complicated flow delivery and control systems to operate, two-phase heat sinks offer additional advantages over their single-phase counterparts as a result of enhanced heat transfer associated with flow boiling, including much smaller coolant flow rate requirements, much higher convective heat transfer coefficients, and better stream-wise temperature uniformity.
Previous studies, such as the previously referenced “Thermal Design Methodology for Low Flow Rate Single-Phase and Two-Phase Micro-Channel Heat Sinks,” IEEE Transactions on Components and Packaging Technologies, December 2007, have shown that two-phase micro-channel heat sinks can have advantages of low thermal resistance to heat dissipation, large surface-area-to-volume ratio, small heat sink weight and volume, small liquid coolant inventory and flow rate requirement, and relatively uniform temperature distribution along the stream-wise direction. However, one practical concern that hinders the implementation of this powerful cooling scheme is the temperature and pressure oscillation due to flow instability between flow channels through the common inlet and outlet plenums.
In accordance with the invention, an improved design for a two-phase micro-channel heat sink is shown in FIGS. 1 and 2. As in conventional micro-channel heat sinks, the heat sink layout has a plurality of flow micro-channels 10 extending in parallel in a longitudinal direction L and having channel walls substantially symmetrical with each other of a given wall thickness and defining a flow channel width and height for accommodating a flow of liquid coolant therein. The improved design provides a plurality of cross-connected micro-channels 20 extending in parallel in a transverse direction W normal to the longitudinal direction. The micro-channels 20 are substantially symmetrical with each other and formed at regular intervals along the longitudinal direction L for cross-connecting with the flow micro-channels, thereby enabling a uniform pressure field for two-phase flow boiling of liquid coolant in the heat sink.
In a preferred embodiment, the heat sink contains flow micro-channels with width of 100 μm, wall thickness of 100 μm, and height of 600 μm, and transverse micro-channel cross-connections of similar width and height cut at 1 mm intervals along the flow direction (at every 5 flow channels) to allow for communication between channels to reduce pressure oscillations.
The improved two-phase cross-connected micro-channel heat sink may be fabricated from silicon or high-thermal-conductivity metal such as copper. The flow micro-channels can have a width dimension ranging from 10 to 1000 micrometers. These micro-channels serve as passages for flow boiling of liquid coolant such as water and dielectric fluorochemical coolants. In application, a high-power-density electronic device is attached to the base surface of the heat sink. The heat generated by the device is first transferred to the micro-channels by heat conduction through the solid substrate, and then removed by the flow boiling of liquid coolant in the micro-channels.
The cross-connection of flow channels in the improved design provides uniform pressure distribution and prevents severe flow oscillation. It thus can provide marked technical advantages in low thermal resistance to heat dissipation, large surface-area-to-volume ratio, small heat sink weight and volume, small liquid coolant inventory and flow rate requirement, relatively uniform temperature distribution along the stream-wise direction, and stable flow. These attributes enable two-phase cross-connected micro-channel heat sinks to dissipate high heat fluxes from small areas, and hence, are suitable for thermal management of high-power-density electronic devices such as high-performance microprocessors, laser diode arrays, high-power components in radar systems, switching components in power electronics, x-ray monochromator crystals, avionics power modules, and spacecraft power components.
The improved stability of flow boiling of liquid coolant in the cross-connected micro-channel design can be illustrated by comparing the amplitude of the instantaneous pressure fluctuation at the heat sink inlet and outlet. FIG. 4 shows a temporal record of inlet and outlet pressures for a conventional two-phase micro-channel design, having 231×713 micron micro-channels without cross connection and using water as coolant, and FIG. 5 shows a comparable temporal record for the new cross-connected design using dielectric fluorochemical FC-72 as coolant. The new cross-connected design showed markedly improved flow stability. It should be noted that while the operating conditions of inlet temperature and flow rate are comparable, the input heat flux for the conventional design in FIG. 4 is higher than that for the new design in FIG. 5, which is because water can transfer more heat than FC-72 during flow boiling process.
It is to be understood that many modifications and variations may be devised given the above description of the principles of the invention. It is intended that all such modifications and variations be considered as within the spirit and scope of this invention, as defined in the following claims.

## Claims

1. A two-phase micro-channel heat sink comprising:
a first plurality of juxtaposed channel walls, each wall having a wall thickness, wherein the first plurality of channel walls defines a corresponding first plurality of flow micro-channels extending longitudinally between adjacent walls, wherein each micro-channel in the first plurality of micro-channels defines a flow channel width and a flow channel height configured to accommodate a flow of coolant therein;
a second plurality of juxtaposed channel walls spaced from the first plurality of juxtaposed channel walls, wherein the second plurality of juxtaposed channel walls defines a corresponding second plurality of flow micro-channels extending longitudinally between adjacent walls; and
a substrate defining a boiling region adapted to accommodate flow boiling of the coolant and being thermally coupled to the first plurality of juxtaposed channel walls and the second plurality of juxtaposed channel walls, wherein said flow boiling of the coolant generates a two-phase flow of coolant, wherein at least one gap between the first plurality of channel walls and the second plurality of channel walls defines at least one corresponding transverse micro-channel extending transverse to the flow micro-channels such that the first and second pluralities of flow micro-channels are cross-connected and thereby configured to facilitate pressure equalization in the two-phase flow of coolant among the flow micro-channels of the first plurality and the second plurality of flow micro-channels and inhibit unstable flow oscillations among the flow micro-channels, wherein the at least one gap comprises a plurality of gaps defining a corresponding plurality of transverse micro-channels and the flow micro-channels define a flow channel width of about 100 μm, a wall thickness of about 100 μm, and a flow channel height of about 600 μm, and the transverse micro-channels have a similar width and height and are spaced apart at about 1 mm intervals along the flow direction.

2. The two-phase micro-channel heat sink according to claim 1, wherein the transverse micro-channel defines a cross-channel width and a channel height similar to those of the flow micro-channel.

3. The two-phase micro-channel heat sink of claim 1, wherein the at least one gap comprises a plurality of gaps defining a corresponding plurality of transverse micro-channels, wherein the plurality of transverse micro-channels are spaced apart at substantially regular intervals measuring about five flow micro-channel widths.

4. The two-phase micro-channel heat sink of claim 1, wherein the second plurality of juxtaposed channel walls is positioned such that at least one of the channel walls in the second plurality of channel walls is substantially longitudinally aligned with and longitudinally spaced from a corresponding channel wall in the first plurality of juxtaposed channel walls so as to define one of the at least one gap.