Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
  • F28F13/00—Arrangements for modifying heat-transfer, e.g. increasing, decreasing
  • F28F13/06—Arrangements for modifying heat-transfer, e.g. increasing, decreasing by affecting the pattern of flow of the heat-exchange media
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28D—HEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
  • F28D15/00—Heat-exchange apparatus with the intermediate heat-transfer medium in closed tubes passing into or through the conduit walls ; Heat-exchange apparatus employing intermediate heat-transfer medium or bodies
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L23/00—Details of semiconductor or other solid state devices
  • H01L23/34—Arrangements for cooling, heating, ventilating or temperature compensation ; Temperature sensing arrangements
  • H01L23/46—Arrangements for cooling, heating, ventilating or temperature compensation ; Temperature sensing arrangements involving the transfer of heat by flowing fluids
  • H01L23/473—Arrangements for cooling, heating, ventilating or temperature compensation ; Temperature sensing arrangements involving the transfer of heat by flowing fluids by flowing liquids
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
  • F28F2250/00—Arrangements for modifying the flow of the heat exchange media, e.g. flow guiding means; Particular flow patterns
  • F28F2250/08—Fluid driving means, e.g. pumps, fans
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
  • F28F2260/00—Heat exchangers or heat exchange elements having special size, e.g. microstructures
  • F28F2260/02—Heat exchangers or heat exchange elements having special size, e.g. microstructures having microchannels
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
  • H01L2924/0001—Technical content checked by a classifier
  • H01L2924/0002—Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00

Definitions

• Heat exchange is an important design consideration. Heat exchangers such as heat sinks or heat emitters benefit many systems and methods by transferring heat away from, or to, a heat source. Improved heat sinks provide several benefits, such as reduced size, mass, cost, improved performance or increased reliability, among others.
• heat exchanger performance is a function of the materials used, the geometry of the heat exchanger, and the heat exchanger's heat transfer coefficient. In an example, the more efficient a heat exchanger is, the greater its Nusselt number is. In an example, microchannel heat exchangers, possessing a manifold of channels through which either a liquid (single-phase) or a gas and a liquid (two-phase) are flown transfer more heat than single-phase heat exchangers, as measured by their higher Nusselt number, but can be difficult to manage.
• Some examples disclosed here represent a manageable and efficient two-phase flow heat exchanger, with a Nusselt number double that of single- phase flow heat exchangers.
• An example uses segmented flow, a type of two- phase flow where bubbles segment the flow of water through the channels.
• segmented flow is more readily controlled than a boiling two-phase flow approach, where the water boils to produce the second phase while flowing.
• segmented flow increases convection, thereby increasing the Nusselt number.
• gas is removed from a channel such as a microchannel using a hydrophobic porous membrane.
• dynamic bubble traps and diffusion-based gas separators remove gas.
• one or more heat exchangers are configured to cool electronic equipment, such as CPUs. Soldering irons can reach temperatures greater than 400°C.
• a heat exchanger is configured to reduce damage to transistors by absorbing heat.
• one or more laser diodes such as in continuous and pulsed mode lasers, benefit from cooling. In an example, twenty- five or more diodes are fixed to a small bar, generating heat fluxes of up to 107 W/m 2 , are cooled using subject matter disclosed herein.
• deaerating liquids are used in the operation of equipment such as analytical equipment.
• gas such as air is removed from a supply influent to a pump to prevent bubble formation.
• a gas such as air can cause reactions in samples used in clinical testing if not properly removed from a circulating loop.
• deaeration is a part of an industrial purification of water to produce deionized water or water for nuclear power plants.
• Example 1 can include a heat exchange system for heat exchange with a heat source and a cold source.
• the system can include a circulation loop.
• the circulation loop can include a heat emission portion configured to exchange heat with the cold source and a heat absorption portion configured to exchange heat with the heat source, the heat absorption portion comprising a channel.
• Example 1 can include a liquid pump configured to circulate a liquid through the circulation loop, from an inlet of the channel to an outlet of the channel and a bubble injector coupled to the circulation loop proximal to the inlet of the channel and configured to flow a gas to form a plurality of gas bubbles in the channel, with each of the plurality of gas bubbles monodispersed across the channel, with segments of liquid separating successive gas bubbles of the plurality of gas bubbles.
• a liquid pump configured to circulate a liquid through the circulation loop, from an inlet of the channel to an outlet of the channel and a bubble injector coupled to the circulation loop proximal to the inlet of the channel and configured to flow a gas to form a plurality of gas bubbles in the channel, with each of the plurality of gas bubbles monodispersed across the channel, with segments of liquid separating successive gas bubbles of the plurality of gas bubbles.
• Example 2 the subject matter of Example 1 can optionally include a gas circulation loop coupled to the bubble injector and can include a gas separator coupled to the circulation loop proximal to the outlet of the channel, the gas separator configured to remove the gas from the liquid.
• Example 3 the subject matter of any one of Examples 1-2 can include a gas circulation loop coupled to the gas separator, with the bubble injector configured to draw the gas from the gas separator, through the gas circulation loop.
• Example 4 the subject matter of any one of Examples 1-3 can optionally be configured such that the gas separator includes a hydrophobic membrane.
• Example 5 the subject matter of any one of Examples 1-4 can optionally be configured such that the channel is sized such that at least one of the plurality of gas bubbles has a bond number below around 3.6.
• Example 6 the subject matter of any one of Examples 1-5 can optionally be configured such that at least one of the plurality of bubbles has an aspect ratio of length to width less than or equal to 4:1.
• Example 7 the subject matter of any one of Examples 1-6 can optionally be configured such that the channel is sized to maintain a wetted channel between successive segments of liquid.
• Example 8 the subject matter of Example 7 can optionally be configured such that the bubble injector is configured to disperse the segments of liquid over regular intervals.
• Example 9 the subject matter of any one of Examples 1-8 can optionally be configured such that the bubble injector includes a jet pump.
• the subject matter of Example 9 can optionally be configured such that the liquid pump is peristaltic pump.
• Example 11 the subject matter of any one of Examples 1-1 1 can optionally be configured such that the bubble injector is configured to flow the plurality of gas bubbles, substantially free of bubbly flow.
• Example 12 includes a method for heat exchange between a heat source and a heat sink.
• Example 12 can includes affixing the heat sink to the heat source, with the heat sink thermally communicative with the heat source, flowing a liquid through a channel in the heat sink, at a determined flow rate and liquid pressure, flowing a gas into the liquid and forming a plurality of gas bubbles in the liquid, with each of the plurality of gas bubbles monodispersed in the liquid, in the channel, with segments of liquid separating successive gas bubbles of the plurality of gas bubbles and exchanging heat between the heat sink and the heat source.
• Example 13 the subject matter of Example 12 can optionally include separating the gas from the liquid after exchanging heat between the heat sink and the heat source.
• Example 14 the subject matter of any one of Examples 13-14 can optionally be configured such that flowing the gas includes flowing gas separated from the liquid and circulated to a gas pump.
• Example 15 the subject matter of any one of Examples 12-14 can optionally include extracting the gas from the liquid after exchanging heat between the heat sink and the heat source.
• Example 16 the subject matter of any one of Examples 12-15 can optionally be configured such that segmenting the bubbles includes controlling the segmenting by adjusting at least one of a gas pressure of the gas, a gas-flow rate of the gas, a wetting angle of the liquid, a liquid flow rate of the liquid and a liquid pressure of the liquid.
• Example 17 the subject matter of any one of Example 12-16 can optionally include circulating liquid through a circulation loop, and cooling the liquid along a heat emission portion of the circulation loop, can optionally be configured such that exchanging heat between the heat sink and the heat source includes heating the liquid along a heat absorption portion of the circulation loop.
• Example 18 can include a heat exchange system for heat exchange with an integrated circuit.
• Example 18 can include a heat sink comprising a close circulation loop.
• the heat sink of Example 18 can include a heat emission portion configured to exchange heat with a cold source and at least one heat absorption portion configured to exchange heat with the integrated circuit, the heat absorption portion including a microchannel.
• Example 18 can include a liquid pump configured to circulate a liquid through the circulation loop, from an inlet of the microchannel to an outlet of the microchannel, a bubble injector coupled to the microchannel and configured to flow a gas to form a plurality of gas bubbles in the microchannel, with each of the plurality of gas bubbles monodispersed across the microchannel, with segments of liquid separating successive gas bubbles of the plurality of gas bubbles and a closed gas circulation loop, with a hydrophilic membrane coupled to the circulation loop proximal to the outlet of the microchannel and between the circulation loop and the gas circulation loop, the hydrophilic membrane configured to remove the gas from the liquid, the bubble injector configured to draw the gas from the hydrophilic membrane to the bubble injector.
• a liquid pump configured to circulate a liquid through the circulation loop, from an inlet of the microchannel to an outlet of the microchannel
• a bubble injector coupled to the microchannel and configured to flow a gas to form a plurality of gas bubbles in the microchannel, with each of the pluralit
• Example 19 the subject matter of Example 18 can optionally be configured such that the heat sink defines a plurality of microchannels, each coupled to the circulation loop in parallel.
• Example 20 the subject matter of any one of Examples 18-20 can optionally be configured such that the integrated circuit forms a part of a computer comprising a random access memory coupled to the integrated circuit.
• FIG. 1 illustrates a heat sink such as a microchannel heat sink in direct contact with a heated substrate (such as an integrated circuit chip), according to some examples.
• Figure 2 illustrates a thermal resistances and temperature profile along the flow path for constant heat flux conditions, according to some examples.
• Figure 3 A illustrates LB &ndL s i ug , according to some examples.
• Figure 3B illustrates a definition of LB, L s i ug , and L ce n, according to some examples.
• Figure 4 illustrates a cross-sectional view of a bubble in a square channel at low Ca, with partitions, according to some examples.
• Figure 5 illustrates a thermal resistance plotted for a heat sink with 7 parallel 500 ⁇ channels, 25 mm in length, according to some examples.
• Figure 6 illustrates thermal resistances plotted for a heat sink with 70 parallel 50 ⁇ channels, 25 mm in length, according to some examples.
• Figure 7 illustrates thermal resistances plotted for a mini channel heat sink with 3 parallel 2mm channels, 25 mm in length, according to some examples.
• Figure 8A illustrates a heat sink and an o-ring, according to some examples.
• Figure 8B illustrates an example with heated substrate showing thermocouple locations, with a heat sink on top, with units in mm, according to some examples.
• Figure 8C illustrates a cross-sectional view of an example section.
• Figure 9 illustrates a set up for energy balance verification, with corresponding Equations, according to some examples.
• Figure 10A illustrates a graphical representation of experimental setup, according to some examples.
• Figure 10B illustrates a flow visualization, according to some examples.
• Figure IOC illustrates a flow visualization, according to some examples.
• Figure 10D illustrates a flow visualization, according to some examples.
• Figure 1 OE illustrates a flow visualization, according to some examples.
• Figure 11 illustrates theoretical and measured values of Nusselt number for single phase and segmented flow versus the Reynolds number based on the mass velocity of water, according to some examples.
• Figure 12 illustrates theoretical and measured values of pressure drop for single phase and segmented flow versus the Reynolds number based on the mass velocity of water, according to some examples.
• Figure 13 illustrates a Nusselt number expressed as a function of the pressure drop for single phase and segmented flow, according to some examples.
• Figure 14 illustrates surface temperatures, Ts, along the flow direction, for a water flow rate of 75 mL/min, in the single-phase and segmented flow cases, according to some examples.
• Figure 15A illustrates an assembly of a gas separator, according to some examples.
• Figure 15B illustrates a micrograph of a porous hydrophobic membranes with a 0.2 ⁇ pore size, according to some examples.
• Figure 15C illustrates a micrograph of a porous hydrophobic membranes with a 1.2 ⁇ pore size, according to some examples.
• Figure 15D illustrates a micrograph of a porous hydrophobic membranes with a 10 ⁇ pore size, according to some examples.
• Figure 16 illustrates bubble dynamics during a extraction process, according to several examples.
• Figure 17 illustrates shapes of vanishing bubbles at different Weber numbers, according to several examples.
• Figure 18 illustrates pressure drop across the membrane as a function of the volume flux of the gas-flow through the membrane, according to some examples.
• Figure 19 illustrates a comparison between some examples and examples using Equation 18 for criteria 2, according to several examples.
• Figure 20 illustrates bubble travel distance and theoretical film thickness as functions of bubble travel speed, according to several examples.
• Figure 21 illustrates a gas-liquid meniscus that is held at an entrance of a pore as the surface tension holds the pressure difference across the meniscus and discourages or prevents water from leaking through the pore.
• Figure 22 illustrates a method, according to some examples.
• the apparatus, systems and methods described here are for enhancing heat exchange between a heat source and a liquid.
• Various examples include a heat sink including liquid flowing through or more channels such as one or more microchannels. Bubbles are created in the liquid, such as to produce a segmented or slug flow pattern. Segmented flow increases the heat transfer coefficient in some instances, even in instances where the bubbles have a lower thermal conductivity than the liquid in which the bubbles are formed. In some examples, the heat transfer coefficient is increased by more than 100% over a single-phase liquid flow. Some examples remove the bubbles after heat transfer.
• bubbles are extracted from liquid in a channel using a separator such as a gas separator.
• a separator includes a hydrophobic porous membrane.
• bubbles travel along a hydrophobic porous membrane where they are extracted through the membrane, with liquid remaining in the channel. Removing bubbles from the channel is useful, such as for preventing the bubbles from reaching some elements of a closed-loop system, such as a pump, where they can cause undesirable effects, such as pump damage or bubbly flow.
• Liquid cooling is an efficient way to remove heat fluxes, such as those with magnitudes of up to 10,000 W/cm 2 .
• One limitation of heat sinks including a single-phase fluid flow through a channel such as a microchannel is their relatively low Nusselt number, due in some instances to laminar flow. Examples discussed here enhance the Nusselt number, such as by introducing segmented flow.
• a segmented flow pattern is created by periodic or regular injection of bubbles such as gas bubbles into one or more liquid filled channels, such as by injecting gas through a T-junction, such as into one or more water-filled channels.
• Various injection methods can be used, such as the opening of a valve that constrains a pressurized gas, jet pump injection, and other injection forms.
• Some examples include a polycarbonate heat sink.
• Some heat sinks include an array of parallel channels such as an array of parallel microchannels. In some instances, each channel has a square cross-section that is around 500 ⁇ wide. Some examples increase the Nusselt number of laminar flow by more than a hundred percent, such as in instances when the mass velocity of the liquid is within the range 330 - 2000 kg/m 2 s.
• A area (m 2 )
• Ca capillary number ( ⁇ / ⁇ )
• G mass velocity (kg/m s)
• n number of bubbles
• N number of channels
• Pr Prandtl Number (v/a)
• Nu Nusselt Number (hd/k)
• Re Reynolds Number (pUd/ ⁇ )
• Q heat flow (W)
• V volume (m 3 )
• Various approaches include a heat sink for the purpose of cooling, such as cooling electronics.
• a heat sink that removes heat, Q , such as by flowing fluid such as liquid or liquid-gas mixes in channels over a heated substrate (e.g. a computer chip).
• Some approaches improve or optimize the dimensions of one or more channels in terms of width and height for single-phase flow of water under the constraint of maximum allowable pressure drop and substrate surface temperature.
• One approach demonstrates that single-phase water-cooling removes up to 790 W/cm .
• a heat flux required a mass velocity, G, of 5700 kg/m 2 s and a pressure drop of 220 kPa.
• an optimization process is done to minimize the pressure drop under the constraints of a given heat flux and maximum substrate temperature.
• a water pressure drop below 10 kPa is sufficient to remove 100 W/cm 2 with optimum channel geometry.
• One problem with single-phase flow heat transfer in one or more channels such as one or more microchannels is the low Nusselt number obtained in laminar flow, on the order of 4.
• Methods for increasing the Nusselt number include: surface area enhancement by geometric obtrusions, tree-like bifurcating channels, large aspect ratio channels, serpentine channels to promote mixing and turbulence, short channels where the entrance region dominates, nano-fluids, and two-phase flow, among others.
• two-phase flow is desirable, due at least in part to a very high heat of vaporization.
• flow boiling can dissipate up to 10,000 W/cm 2 , which in some cases is 10 times more heat than single-phase flow.
• flow boiling is beneficial in some instances because it delivers high heat flux at the constant temperature of the phase change, it is difficult to control due to backflow and instabilities.
• Some approaches have attempted to control the instabilities and backflow, such as by manufacturing artificial nucleation sites and inlet restrictions.
• a drawback of boiling flow where water is the working liquid, is that the saturation temperature is higher than the operating temperature of most electronics.
• refrigerants as working fluids, since the boiling temperature is lower than water. Refrigerants, however, offer lower cooling capabilities due to a lower specific heat and heat of vaporization.
• segmented flow is a periodic pattern of non- condensable bubbles and liquid slugs.
• the bubbles are created at a T-junction by the injection of a gas such as air or nitrogen.
• bubbles are injected into liquid-filled channels such as microchannels.
• the bubbles are longer than the channel diameter.
• the presence of recirculating wakes requires surface tension to dominate over gravity, which occurs when the Eotvos or Bond number pgd 2 /o ⁇ 3.6. In some examples, the Bond number pgd 2 /a ⁇ 3.368.
• a finite-element simulation is used to determine the liquid to solid mass transfer, for the case of catalyst removal from a monolith wall.
• the approach determined that the rate of mass transfer in the liquid slugs is 10 times the rate of laminar flow.
• the presence of bubbles increases the pressure drop in the channel due to the Laplace pressure at the liquid gas interface, but the present subject matter is not so limited.
• Some examples including some industrial applications, have a closed loop system, where water from the outlet is circulated through a pump and a heat sink to an inlet of a channel.
• bubbles are extracted before the pump and re-injected after the pump.
• bubbles are extracted from segmented flow by the addition of smaller capillary vessels or tubes downstream which remove the bubbles due to the difference in interfacial tension.
• Some examples use a hydrophobic porous membrane forming part of a circulation loop or flow channel to remove bubbles.
• Figure 1 illustrates a heat sink 102 couplable to a heated substrate 104.
• heated substrates include, but are not limited to integrated circuits, power supplies, radiators, reactors and other heat sources.
• the heated substrate 104 is one type of heat source that the heat sink 102 is coupleable to, as the heat sink 102 is also couplable to a cold source such as a finned heat sink.
• the heat sink 102 and the heat source are adjacent, in some examples, and are not necessarily touching one another. In some examples they abut. In some examples, the heat sink 102 defines a plurality of channels 106 extending through the heat sink 102.
• the ID equivalent resistances method used in some approaches is used to calculate the performance of a micro heat sink, such as the heat sink 102 illustrated in Figure 1.
• the heat resistance in Equation 1 is due to the heating of the fluid as it passes through the heat sink; it depends on volumetric flow rate/ ⁇ and specific heat capacity of the fluid (c . earV(Clfl) (1)
• Equation 2 gives ⁇ ⁇ , which is the resistance of the coolant fluid to heat convection, according to some examples.
• L c is the channel length
• w is the heat sink width
• w c is the channel width
• w w is the width between one or more channels
• H c is the channel height
• 2H c /(w w +w c ) is the fin enhancement factor.
• the outlet temperature of the fluid and the maximum temperature of the substrate surface are found, according to one approach, based on the power dissipated Q and the inlet temperature of the fluid, as shown in Equation 3 and 4.
• the Nusselt number of single phase flow is expected to increase at higher Re due to the increasing thermal entry length, in some examples. According to one approach, when the thermal entry length is less negligible or no longer negligible, Nu is found using a first correlation approach.
• this correlation is valid for rectangular channels of any aspect ratio, constant heat flux boundary conditions, and laminar, hydrodynamically developed flow.
• one or more correlations according to a second correlation approach are used to find Nu. This approach is valid for Pr > 0.1, uniform heat flux and constant surface temperature, and any channel cross-section.
• Equation 6 The pressure drop for single-phase flow, in one approach, is found valid for both laminar and turbulent flow, given in Equation 6, where d is the hydraulic diameter, K is the minor loss term and UL is the liquid velocity.
• a channel 306 is illustrated with a bubble 302 monodispersed in the channel 306.
• Equation 7 is valid for well-defined bubbles when d is on the order of mm, Pr > 1 , Re seg is on the order of 1000, and 300 K ⁇ (7L) mea n ⁇ 340 K.
• multiphase flow simulations revealed two mechanisms that increase Nu: the generation of the bubbles and the circulation in the liquid slugs.
• a segmented flow with recirculating wakes are generated provided the Bond number pgd 2 /o ⁇ 3.6 and the capillary number Ca ⁇ 0.04.
• other correlations for determining the Nusselt number of segmented flow are possible.
• one approach provides an expression for Nu from numerical simulation in square channels.
• the pressure drop in segmented flow is described with Equation 8, where a pressure drop term across the bubbles is added to the single-phase pressure drop for the liquid slugs.
• the pressure drop depends on two measurable quantities: the number of bubbles in a channel, n, and slug length, L s i ug , as per the example of Equation 8.
• the capillary number, Ca is determined by the bubble velocity, in several approaches.
• Equation 7 through 9 such as L s i ug , LB and L3 ⁇ 4, are available from high-speed visualizations, as set forth herein. Values of pressure drop and Nu compare with the correlations, in some examples.
• Equation 7 through 9 are used to design a heat sink, in some examples.
• a cross-section of the bubble is constant along its length, and according to some approaches, the bubble velocity is expressed by mass conservation.
• U B /U slug A c /A B (10)
• the slug velocity defined as U s i ug Gi epL, used in
• Figure 4 illustrates a cross-sectional shape of the bubble 402 monodispersed in a channel 404 for Ca ⁇ 0.04.
• the film thickness ⁇ is a function of the capillary number, Equation 9, and is expressed by Equations 12 and 13 using a correlation based on simulations according to some approaches.
• ⁇ 0.00332w c (12)
• Ca ⁇ 0.04 ⁇ -0.0423e ( - c ⁇ 5'3092) -0.1018e W3343) + 0.1761 (13)
• the Re transition represents the change from laminar to turbulent flow for the liquid flow rate.
• Figure 5 illustrates that the Reynolds number of the liquid flow is greater than 60 in a regime where ⁇ ⁇ dominates over dheat-
• Figure 6 and Figure 7 show examples including the thermal resistances plotted for different channel widths, with the length remaining unchanged.
• ⁇ heat hs the dominant resistance so that changes in Nu would not significantly modify the surface temperature, which is used to experimentally determine the Nusselt number.
• values of B conv are higher than in the 500 ⁇ channel case, resulting in a less efficient heat sink.
• Various examples are contemplated in which at least one of a plurality of bubbles has an aspect ratio of length to width of 4: 1 or smaller.
• a heat sink 802 including channels such as microchannels is coupled or affixed to a heat source such as a heated substrate 804.
• the heated substrate includes an aluminum block.
• a layer such as a
• PDMS polydimethylsiloxane
• the heated substrate made from aluminum in some instances, is configured to provide uniform heat flow from the cartridge heater 814.
• Foam such as melamine foam 816 insulates the heated substrate, and Teflon 812 provides insulation between the heated substrate 804 and the fasteners 810.
• An o-ring 818 provides a seal to discourage flow leakage.
• a heat sink 802 is extracted, and the top surface of the aluminum block 804 is exposed to an gaseous atmosphere such as air.
• a remainder of the heated substrate 804 is insulated with melamine foam 816, in some examples approximately 2 cm thick, as shown in Figure 9.
• Some examples include a base such as an acrylic base 850.
• an infrared pyrometer is to measure the heated substrate 804 temperature. Since the emissivity of aluminum is very low, in some examples 0.05, the surface of aluminum is painted black so that the emissivity is in the range of the pyrometer (in some examples 0.95).
• the natural convection heat transfer h is obtained by a correlation specific to small geometries and dependent on surface temperature, ranging from 15-25 W/m K, according to some approaches.
• five rows of three K-type thermocouples, as shown in Figure 8B, are used to determine the surface temperature and temperature gradient using linear extrapolation.
• the procedure is run at 5 substrate temperatures: 50, 75, 90, 115, 150°C.
• the maximum heat loss through the insulation Q loss is found to be less than 1W. In some cases, this is negligible in instances where a 40 W heat flux is applied during measurements involving fluid flow.
• An example describing Figure 9 includes one or more of the following calculations:
• a surface temperature is recorded with a pyrometer and corresponds within 0.5°C to the surface temperatures extrapolated from a set of 5 thermocouple measurements perpendicular to the surface. The standard deviation of the extrapolated values along the surface is less than 0.5°C, in some examples.
• the heat sink 802 is milled from a polycarbonate slab. In some examples, the heat sink 802 has a glass transition temperature of 150°C. In some examples, the heat sink 802 includes seven parallel square channels 820, although examples with another number are contemplated. In some examples, one or more channels 820 of the heat sink 802 have a respective length and width of 25 mm and 500 ⁇ .
• Benefits of polycarbonate include that it is transparent and that it is easy to manufacture; some examples include other materials.
• the heat sink 802 is pressed on top of the heated substrate 804 and sealed with the O-ring 818.
• the heat sink 802 is heated with a constant power of 40W and the water flow rate is varied from 238 - 3095 kg/m 2 s.
• the heated substrate 804 and heat sink 802 are insulated with melamine foam, as shown in Figure 8C.
• a liquid such as water is pumped with a pump, such as a peristaltic pump, and the liquid mass velocity G L is found by measuring the fluid volume at the outlet over time.
• bubbles are generated by introducing, such as through injecting, gas through an opening such as a slit, in various examples.
• introducing bubbles in the liquid includes shearing the gas.
• a bubble injector includes a jet pump.
• a bubble injector is configured to introduce a plurality of bubbles at a gas pressure of from around 2 kilopascals to 10 kilopascals.
• a bubble injector is configured to pressurize gas to a determined pressure and wherein the bubble injector is configured to flow the gas at a determined flow rate.
• bubbles are introduced at constant pressure using a pressure regulator, such as a DRUCK DPI 530 pressure regulator.
• the pressure is varied depending on G L to produce a liquid fraction close to 0.5.
• a pressure drop along the channel 820 is measured with a pressure transducer (e.g.
• thermocouples e.g., type K, 0.5 mm diameter, OMEGA, 100ms response time, ⁇ 0.5°C uncertainty
• a bubble injector is configured to disperse the segments of liquid over regular intervals, that is, at repeating rates. Successive bubbles flow one after another, in some examples.
• convective heat transfer recordings are made using the heat sink described in the previous section at constant heating power of 40W, and with water flow rates between 35-300 mL/min, corresponding to water mass velocities Gi of 300-3000 kg/m 2 s and ReL from 160-1580, where ReL is defined as Gid/ ⁇ iL for both single phase and segmented flow. Neglecting the low thermal losses through the insulation, as discussed in relation to Figure 8, enthalpy change of the fluid is replaced by the power supplied by the heater, according to some examples.
• An energy balance surrounding the channel provides the convection coefficient, h:
• thermocouples ⁇ 2 H c + w c )Nh [ dx (14)
• ⁇ , r ⁇ and N are the temperature difference between the substrate and the fluid, the fin efficiency and the number of channels, respectively, according to one approach.
• the first and last row of the thermocouples on the heated substrate correspond to the fluid inlet and outlet, the integral is discretized along the fluid flow direction into four sections using the trapezoidal rule.
• Equation 14 is rearranged to solve for h as a function of the 5 surface temperatures and the inlet and outlet temperature of the fluid, according to some examples.
• a determined or maximum uncertainty of the Nusselt number is ⁇ 4 % due to the propagation of uncertainties in the temperature, geometry and thermal losses through the insulation.
• a summary of uncertainties and their sources is found in Table 1 , according to several examples.
• thermocouple measurements and the heat flow measurement.
• the average temperature difference ⁇ is 20°C with an uncertainty of ⁇ 0.5°C, or ⁇ 2.5 %.
• the uncertainty of Q HEATER due to heat losses is less than 1 W, i.e. 2.5 %.
• Figure 1 1 illustrates that segmented flow increases the dimensionless heat transfer coefficient Nu up to 140% over single phase flow, for values of GL ⁇ 2000 kg/m 2 s, in agreement with the numerically obtained correlation of at least one approach.
• transition starts at flow rates where the capillary number reaches the transition value of 0.04 (shown by a vertical bar), and the flow visualizations in Table 2, which illustrates an example transition to churn flow.
• Table 2 includes example visualization of four cases spanning three flow regimes with corresponding liquid mass velocity GL, according to some examples.
• the liquid fraction, e is calculated precisely for slug flow using the method described in section 2, according to some examples.
• segmented flow with a wetted channel that is a liquid film between the bubble and the channel, enhances heat transfer provided the film between bubbles and wall does not become too thick, which can weaken the recirculation wakes, as explained in the discussion of Figure 2, and according to some additional approaches.
• Some examples produce segmented flow for values of GL between 330 and 2850 kg/m s.
• segmented flow is replaced by bubbly flow (i.e. bubbles with diameters smaller than the channel diameter), and at higher GL values a churn flow appears (fast bubbles with thick films, Ca reaching 0.04 and above, no heat transfer enhancement), in agreement with some approaches.
• Visualization of these flow regimes is seen in Figures 10B-E, according to several examples.
• a bubble injector is configured to flow a plurality of gas bubbles, substantially free of bubbly flow.
• FIG. 10 illustrates one configuration for single-phase and segmented flow apparatus, according to some examples.
• a pump 1002 such as a peristaltic pump is coupled in fluid communication to a pressure sensor 1020 and a water reservoir 1018 such as a DI water reservoir, in some examples.
• the pressure sensor 1020 is electronically coupled with a DAQ board 1010, which is electronically coupled to a PC 1008, in some examples.
• a CMOS camera 1006 is to record images of flow in a heat sink 1004, to which the pressure sensor 1020 is coupled in fluid communication, in some examples.
• the heat sink 1004 is in fluid communication with a water outlet 1012, which is some examples that circulate fluid, is coupled in fluid communication with the pump 1002, in some examples.
• a heater controller 1014 is configured to heat liquid in the heat sink 1004, in some examples.
• a bubble injector 1016 is coupled in fluid
• a reservoir 1018 is coupled in fluid communication with pump 1002 to provide water to the pump 1002, in some examples.
• the water outlet 1012 is in fluid communication with the reservoir 1018 as part of a closed-loop circulation system including a circulation loop.
• Some examples include a gas separator 1028, as discussed herein in relation to bubble extraction.
• Figures 10B-10E show four operational modes for segmented flow, each showing a bubble 1022, a liquid slug 1024 and a channel 1026.
• the increasing values of the Nusselt numbers for single phase flow at larger flow rates are due to the non-negligible thermal and hydrodynamic entry lengths.
• the first correlations approach is used to calculate Nu for thermally developing flow.
• the second correlation approach discussed in relation to Figure 2 is used. Predictions from this correlation agree with physical examples set forth herein, with Re > 100.
• the pressure drop and Nusselt number are compared to single phase and evaporative flow measurements with similar flow rate and heat flux. According to some approaches, values are produced by an aluminum heat sink with 21 channels and a hydraulic diameter of 348.8 ⁇ . Table 3 illustrates examples in which segmented flow provides Nusselt number values in a range that is between single-phase and evaporative cooling.
• Certain examples demonstrate that segmented flow enhances heat transfer by up to 140% in a heat sink, in comparison with single-phase flow at the same liquid flow rate.
• the Nusselt number demonstrates the improvement in heat transfer, in some examples.
• segmented flow delivers a higher Nusselt number than single phase flow.
• segmented flow provides an intermediate step between single-phase and boiling flow for the purpose of electronic cooling.
• the heat transfer enhancement occurs for a specific range of flow rates and capillary numbers. According to various examples, at lower or higher capillary numbers, no significant heat transfer enhancement is observed because segmented flow is replaced by bubbly or churn flow respectively.
• Various examples provide a simple and efficient way to remove gas bubbles from liquid-filled channels such as microchannels.
• Various examples integrate or combine a gas separator such as a membrane or a portion including one or more capillary vessels or tubes, with channels such that liquid in one or more channels passes over the separator.
• a gas separator such as a membrane or a portion including one or more capillary vessels or tubes, with channels such that liquid in one or more channels passes over the separator.
• a hydrophobic porous membrane Several examples include a chip is manufactured in hard, transparent polymer filters gas plugs out of a segmented flow at rates up to 7.4 ⁇ /s per mm 2 of gas separator area.
• Several examples include a bubble generation section and a gas separation section.
• the bubble generation section includes a T-junction is used to generate a train of gas plugs into a water stream. In various examples, these gas plugs are transported towards a gas separation section, and slide along a hydrophobic membrane until extraction is sufficient or
• a gas separation process occurs provided four criteria are met, in some cases simultaneously.
• the first criterion is that the bubble diameter is larger than the channel diameter.
• the first criterion is that the bubble cross sectional width is larger than the channel width.
• the second criterion is that the gas plug remain on the gas separator for a time sufficient to transport a determined, e.g., 100%, of the gas through the separator.
• the third criterion is that the gas plug travel speed remain lower than a determined or critical value. In some examples, if the speed rises above the critical value, a stable liquid film between the bubble and the membrane reduces or prevents mass transfer.
• the fourth criterion is that the pressure difference across the membrane remain below the Laplace pressure to prevent water from leaking through the membrane.
• Bubbles are generated in systems such as fluidic systems such as fluidic systems such as microfluidic systems, in some approaches, such as continuously by flow-focusing and T-junction configurations, among others, or on-demand by thermal heating and piezo actuation, among others. Unwanted gas pockets, in some examples, form accidentally due to priming or cavitation.
• these bubbles are sometimes useful, e.g. for enhancing heat and mass transfer according to some approaches, creating streaming or microstreaming according to one approach, providing a platform for biochemical synthesis according to one approach, or enhancing mixing for chemical reaction and cell lysis according to some approaches.
• bubbles are associated with disturbances in fluidic device such as microfluidic devices. For instance, they clog channels or reduce the dynamic performance of a fluidic device such as microfluidic device. Therefore a gas separation process, such as one integrated on a chip, is of interest in fluidics such as microfluidics.
• dynamic bubble traps are often used in extracorporeal blood flow circuits: they use 3D spiral channels to accelerate the flow radially and focus the bubbles towards one location, where extraction proceeds.
• a significant amount of liquid is extracted together with the gas.
• Diffusion-based bubble extraction uses a gas- permeable membrane, such as a thin PDMS layer, according to some approaches.
• gas separation rates are relatively low, e.g. lxlO "4 ⁇ ,/s per mm 2 .
• a porous membrane is used to separate immiscible fluids: some approaches complete separation of organic-aqueous and fluorous-aqueous liquid/liquid systems in a fluidic device such as microfluidic device.
• Gas/liquid separation in an example illustrates that hydrophobic and hydrophilic membranes used together in the end of a channel such as a microchannel achieve gas/liquid separation, but the approach demonstrates incomplete separation when using a hydrophilic membrane in a channel flown with a gas/water mixture.
• the present subject matter provides an integrated gas separator in a channel such as a microchannel flow circuit. Specific examples provide a hydrophobic membrane integrated into a microfluidic chip and configured to separate gas plugs from a segmented flow.
• the present subject matter provides example conditions for bubble extraction, and provides four criteria associated with separation of gas from liquid.
• Figure 15 illustrates an example assembly of a gas separator.
• one or more channels 1516 are 500 ⁇ wide and 500 ⁇ deep.
• one or more channels 1516 are milled from PMMA 1508 (polymethylmethacrylate) using a mill such as a Minitech CNC milling machine.
• cut portions exhibit less than 500 nm surface roughness.
• one or more channels 1516 are sealed with PMMA 1502, with the 200 ⁇ thick hydrophobic acrylic copolymer membrane, and 70 ⁇ thick double-sided tape 1506, as shown in Figure 15.
• membrane 1504 such as porous hydrophobic membrane.
• the three tested porous hydrophobic membranes are made of acrylic copolymer and have three respective typical pore sizes, 0.2, 1.2 and 10 ⁇ (e.g., as illustrated in Figure 15).
• a 500 ⁇ wide slit is cut through the tape 1506, and aligned on top of the channel 1516 such as a main channel.
• the bubble generation section has four walls made of PMMA while the gas removing section has a channel 1516 made of three PMMA walls and one membrane 1504 wall, not including fasteners such as tape 1506.
• gas plugs are generated at a T-junction, where water is pushed by syringe pumps 1510 (e.g., KDS 210) and the gas pressure is controlled by a pressure controller 1512 (e.g., DRUCK DPI 530, 2 bar gauge, precision ⁇ 1% FS).
• a pressure controller 1512 e.g., DRUCK DPI 530, 2 bar gauge, precision ⁇ 1% FS.
• generated bubbles are transported to the bubble extraction section, where bubble extraction takes place.
• a pressure sensor 1514 such as a piezoresistive pressure sensor (e.g., HONEYWELL ASCX15DN, 103.4 kPa differential, repeatability ⁇ 0.2% FS) is used to monitor the pressure difference between the atmosphere and the fluid upstream of the hydrophobic membrane, in some examples.
• the measured void fraction ranges from 0.25 to 0.78.
• the picture sequence in Figure 16 illustrates bubble 1602 dynamics during a extraction process.
• Figure 16 illustrates a sequence of a bubble extraction process, using a membrane with 1.2 ⁇ pores.
• bubbles travel at or around a speed of 0.62 m/s and are extracted or completely extracted from the channel through the membrane.
• the receding contact angle at the bubble front increases during the vanishing period as the bubble travels through the channel 1604, as shown in frames 0 to 4.4ms
• the vanishing bubble first reduces its length, for example the bubble at 2.8 ms is about half of the original length. Then, after 3.2 ms, the height of the bubble starts to decrease, according to some examples.
• the remaining part of the bubble seen from the side assumes a sharp triangular shape, before it fully disappears (see e.g. frames at 0.8 and 1.2ms).
• this curvature change occurs when the Weber number is greater than unity, as shown in Figure 17.
• the shape at large Weber numbers is due to the pressure drop along the bubble length. According to some examples, a reason for this is that competition of inertial forces and surface tension forces over the bubble create a pressure difference across the bubble length large enough to induce a change of curvature.
• a syringe pump fails at 46 mL/m and a gage pressure of 81 kPa, and leakage of water through the membrane is found to occur at 40 and 21 mL/m of water flow rate for 1.2 and 10 ⁇ pore sizes respectively, and the respective gage pressures, measured at the location upstream from the membrane, are 41 and 20 kPa.
• the gage pressure is kept lower than these critical pressures to prevent water leakage, in some examples.
• h is the membrane thickness
• Ap is the pressure difference across the membrane
• P is the gas permeability.
• Nitrogen is used in some examples, and P is 1.34xl0 "16 kmol/(Pa s m 2 ) for nitrogen.
• gas transport through a porous membrane is caused at least in part by a viscous flow in the parallel pores, and the steady-state gas volume flux q (m 3 /m 2 /s) is estimated from Darcy's law according to some approaches:
• ⁇ is the gas viscosity
• Kr is permeability of the membrane
• Table 4A theoretical mass flow rate across a porous membrane and a PDMS membrane.
• Table 4B theoretical mass flow rate across a porous membrane and a PDMS membrane.
• two outcomes are unsatisfactory for a gas separation device: membrane leakage and incomplete extraction.
• incomplete extraction the outflow is not pure water but an gas-liquid mixture.
• membrane leakage water and gas go through the membrane.
• criteria as listed in Table 5 and discussed herein are satisfied, in some cases simultaneously, to provide gas extraction, such as complete gas extraction, without membrane leakage.
• the geometry of a bubble trap such as one or more of the bubble traps disclosed herein, uses a bubble diameter larger than the channel height for making degassing possible.
• this is the first criterion for complete gas extraction, criterion 1 in Table 5.
• a second criterion is formulated by considering the time needed to fully extract the gas bubble.
• the shrinking rate of the bubble dV/dt is substantially equivalent to the gas-flow rate Q through the membrane, which is estimated by Darcy's law (i.e., Equation [16]):
• the x-axis plots the term ln(/( / / ) and the y-axis plots the product of the extraction time and the pressure.
• the extraction time are larger than a determined value, which in some cases is explained by a liquid film formed between the wall and the gas plug, as analyzed in association with Criteria 3.
• a static gas/water interface contacts the wall with a contact angle ⁇ 3 ⁇ 4 and forms a clear triple line.
• an interface moving along the wall exhibits a dynamic contact angle ⁇ , which decreases for increasing bubble velocities.
• there is a critical velocity where the wetting angle approaches zero, and above which a film appears between the plug and the wall because the triple line cannot find a stable position anymore.
• the critical velocity v c is estimated by:
• a 20 is a dimensionless coefficient that only weakly depends on v.
• v c is calculated to be 0.38 and 2.3 m/s, respectively, using contact angles measured in the experiments (e.g., 68° for PMMA, 124° for the porous membrane).
• experimental determined or maximum bubble travel distance and theoretical film thickness are functions of bubble travel speed.
• gas is extracted or completely extracted at determined locations in the channel, a situation that is not achieved for bubbles that are faster than
• Vc membrane in some examples.
• a bubble vanishing location distance increases with the bubble travel speed, as illustrated according to some examples with scattered data points.
• scattering of data points is associated with nonuniformity of pore size or nonhomogeneous distribution of pores on a membrane surface, as pictured in Figure 15. According to some examples, bubbles travel along the PMMA wall and then on the membrane so that the corresponding film situations occur as in Table 6:
• a stable film between the bubble and the membrane can reduce or prevent gas separation.
• the film might become unstable on top of the membrane and rupture so that gas is extracted, provided the membrane is long enough.
• the bubble vanishing location generally increases with the bubble velocity, visualized with scattered data points. In some examples this is associated with nonuniformity of the pore sizes and nonhomogeneous distribution of the pores on the membrane surface, as pictured in Figure 15.
• the third criterion is that the bubble speed v be lower than a critical value, and the corresponding Equation is listed in Table 5 to describe this criterion.
• a porous hydrophobic membrane reduces or prevents a water-gas meniscus from passing through the pores because of interfacial tension, a situation analyzed in one approach for a liquid/liquid system. According to that approach, several examples provide a criterion necessary to prevent water from leaking through a porous hydrophobic membrane.
• Figure 21 illustrates a bubble 2106 in a channel 2108 including a gas-liquid meniscus 2102 that is held at an entrance of a pore 2104 as the surface tension holds the pressure difference across the meniscus 2102 and discourages or prevents water from leaking through the pore.
• the angle between the meniscus and the inner wall of the pores reaches a determined or maximum value of the equilibrium wetting angle ⁇ .
• a meniscus holds a pressure difference up to a maximum value of
• Ap LP -4 ⁇ COS c9 / i (21) where yis the surface tension between gas and water, #is the contact angle and d is the pore size, according to one approach.
• y is the surface tension between gas and water
• # is the contact angle
• d is the pore size, according to one approach.
• a pore size is associated with a membrane, such as by being given by the manufacturer.
• the Laplace pressures Ap LP are calculated as 804, 134 and 16 kPa for 0.2, 1.2 and 10 ⁇ membrane respectively.
• water begins to leak at > 81 (e.g., where a syringe pump fails), at about 41 and 20 kPa respectively.
• these values are associated with uncertainties on the pore shape and size (see Figure 15) or on the wetting angle, in one approach.
• a fluidic device such as microfluidic device is manufactured to separate gas from water in a segmentation flow.
• four operating criteria are determined and explained in association with physical examples and achieve a complete separation of the gas from the liquid.
• Figure 22 illustrates a method, according to some examples.
• a method for heat exchange between a heat source and a heat sink includes affixing the heat sink to the heat source, with the heat sink thermally
• the method includes flowing a liquid through a channel in the heat sink, at a determined flow rate and liquid pressure.
• the method includes flowing a gas into the liquid and forming a plurality of gas bubbles in the liquid, with each of the plurality of gas bubbles monodispersed in the liquid, in the channel, with segments of liquid separating successive gas bubbles of the plurality of gas bubbles.
• the method includes exchanging heat between the heat sink and the heat source.
• Some methods include separating the gas from the liquid after exchanging heat between the heat sink and the heat source. Some methods include flowing the gas includes flowing gas separated from the liquid and circulated to a gas pump. Some methods include extracting the gas from the liquid after exchanging heat between the heat sink and the heat source. Some methods include segmenting the bubbles includes controlling the segmenting by adjusting at least one of a gas pressure of the gas, a gas-flow rate of the gas, a wetting angle of the liquid, a liquid flow rate of the liquid and a liquid pressure of the liquid. Some methods include circulating liquid through a circulation loop, and cooling the liquid along a heat emission portion of the circulation loop, wherein exchanging heat between the heat sink and the heat source includes heating the liquid along a heat absorption portion of the circulation loop.
• Method examples described herein can be machine or computer- implemented at least in part. Some examples can include a computer-readable medium or machine-readable medium encoded with instructions operable to configure an electronic device to perform methods as described in the above examples.
• An implementation of such methods can include code, such as microcode, assembly language code, a higher-level language code, or the like. Such code can include computer readable instructions for performing various methods. The code can form portions of computer program products. Further, the code can be tangibly stored on one or more volatile or non-volatile computer- readable media during execution or at other times.
• These computer-readable media can include, but are not limited to, hard disks, removable magnetic disks, removable optical disks (e.g., compact disks and digital video disks), magnetic cassettes, memory cards or sticks, random access memories (RAMs), read only memories (ROMs), and the like.

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