Patent Publication Number: US-2010124676-A1

Title: Managing gas bubbles in a liquid flow system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to liquid flow systems, and, more particularly, to managing bubble sizes in liquid flow systems. 
     2. Background Information 
     There are many types of devices that are configured to receive liquid that are sensitive to bubbles within the liquid. For instance, bubbles may form that are intolerably large in size, and may cause problems for the liquid receiving device, whose degree of severity varies with the particular type of liquid receiving device and the occurrence of bubbles within the liquid flow to that device. 
     One example device sensitive to bubbles in liquid flow are electrochemical energy conversion devices, such as fuel cells. Many fuel cell systems utilize pumps to move fluids/liquids within the system, e.g., from a reactant/fuel source to the fuel cell. Various types of pumps are well known to those skilled in the art. Often, these pumps may generate gases, which, under certain conditions of the reactant, such as temperature, pressure, viscosity, and the saturation state, may evolve into gas bubbles that occupy volume in an exit stream from the pump that would otherwise be occupied by liquid reactant. For example, electroosmotic (EO) pumps may generate some gases during the process of moving fluids such as water and methanol. Other causes of bubbles, such as mechanical, thermal, chemical, and electrical causes may also create bubbles within the liquid reactant flow system. 
     As a consequence, in a reactant flow system (or “feed manifold”) to a fuel cell, the gas bubbles represent voids or absences of the reactant in the reactant (fuel) flow. This leads to dropouts in the fuel cell power generation, such dropouts being proportional in their severity to the size of the bubbles, and the amount of time that passes before the fuel line begins to again deliver liquid reactant (e.g., methanol fuel) to the electrochemical energy conversion device (e.g., fuel cell). In particular, the gas bubbles at the fuel cell (device  150 ), while possibly being a gaseous reactant, generally have a much lower energy density (e.g., negligible) than the liquid reactant (e.g., gaseous hydrogen or vapor methanol versus liquid methanol), so if the bubble is particularly large, it may be minutes before reactant again reaches the fuel cell (due to a slow rate of fuel delivery). Larger bubbles are particularly burdensome for the flow system  100 . 
     In addition, many other liquid receiving devices are also sensitive to bubbles in the liquid flow, such as various medical devices, paint supply systems, power plants, etc. Air bubbles flowing within a medical device may have particularly severe consequences, such as fatality of a patient or other less sever outcomes, as may be appreciated by those skilled in the art. Also, paint supply systems may suffer from bubbles, such as where finely detailed paint projects (e.g., automotive finishes) may become uneven, costing time and money to remedy the situation. 
     Moreover, bubbles passing through any flow measuring device for these systems may generate perturbations in the flow measurement, making such measurements more difficult and less precise. This is particularly true at low liquid flow rates, such as those typically found in reactant for a low power direct oxidation fuel cell (e.g.,  1  cubic centimeter per hour). Often, such flow measurements are used to control operation of the pumps, to accommodate for changes in the flow. However, with difficulty properly determining the flow, and by not reacting quickly enough (slow feedback), the pump may not only frequently adjust its settings in an attempt to cope with flow fluctuation caused by the bubbles, but may also be potentially out of synchronization with the actual amount of liquid reaching the receiving device. These constant flow changes, in addition, may cause undue damage to the pumps over time. Also, the increased stresses on the pump may create more bubbles, leading to worse fluctuations in flow. 
     Various schemes have been attempted to eliminate the bubbles, such as by separating the gas (bubbles) from the liquid, separating the liquid from the gas, shunting the liquid by gravitometric or centrifugal traps and so on. In all cases, the complexity of the mechanisms, the additional flow path, and the ability of the scheme to accommodate a wide range of gas content in the fluid stream are less than sufficient to provide a smooth and continuous flow of liquid, e.g., reactant to a fuel cell, or other liquid to other types of systems. There remains a need, therefore, for efficient management of gas bubbles in a liquid flow system. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to techniques for managing (or mitigating) gas bubbles in a liquid flow system. According to the one aspect of the present invention, novel systems and methods may be used to reduce a volume of cavities in the liquid flow system and limit a cross-sectional area of the liquid flow system to a maximum cross-sectional area of tolerably sized bubbles. In this manner, by reducing the cavity volumes and limiting cross-sectional areas, the formation of intolerably sized bubbles and the aggregation of tolerably sized bubbles into intolerably sized bubbles are each substantially prevented. 
     In other words, according to one aspect of the novel invention, the presence of bubbles in the liquid flow system is accepted, but techniques are in place to minimize the effect of the bubbles on uniform liquid flow by dividing the gas bubbles as finely as possible and distributing the bubbles as uniformly as possible throughout the liquid. As such, a substantially reduced likelihood of intolerably sized bubbles exists in the liquid flow. For example, according to an embodiment described herein, long dropout periods where no liquid reactant is reaching an electrochemical energy conversion device, e.g., fuel cell, may be alleviated accordingly. 
     In addition, while formation of intolerably sized bubbles and the aggregation of tolerably sized bubbles into intolerably sized bubbles are each substantially prevented, provisions may be in place to accumulate and remove/release any intolerably sized bubbles from the liquid flow system. Thus, fewer bubbles need be managed by the other techniques described herein. 
     Advantageously, the novel system manages bubbles in a liquid flow system. In particular, by substantially preventing formation of intolerably sized bubbles and aggregation of tolerably sized bubbles into intolerably sized bubbles, the novel technique provides solutions to various problems associated with bubbles in liquid flow systems. For example, finely divided and distributed bubbles in the liquid reactant flow of a fuel cell have been demonstrated to reduce power fluctuations in the presence of given gas amounts within the liquid as contrasted with such amounts of gas agglomerating into one or more large bubbles that pass at one time through the system. In addition, the highly distributed and finely divided bubbles create smaller perturbations on the flow measurement of the liquid flow, enabling more precise control. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which like reference numerals indicate identically or functionally similar elements, of which: 
         FIG. 1  is a simplified schematic illustration of one embodiment of a liquid flow system that may be advantageously used with the present invention; 
         FIG. 2  is a schematic illustration of one embodiment of a flow channel that may be advantageously used with the present invention; 
         FIG. 3  is a schematic illustration of one embodiment of micro flow channels that may be advantageously used with the present invention; 
         FIG. 4  is a schematic illustration of one embodiment of volume-reduced flow channels that may be advantageously used with the present invention; 
         FIG. 5  is a schematic illustration of one embodiment for capillary pathways that may be advantageously used with the present invention; 
         FIG. 6  is a schematic illustration of one embodiment for a flow system with a break-up device that may be advantageously used with the present invention; 
         FIG. 7  is a flowchart illustrating a procedure for managing gas bubble size in a liquid flow system in accordance with one or more embodiments of the present invention; 
         FIGS. 8A-D  are schematic illustrations of one embodiment for a flow system with a bubble accumulation and removal chamber that may be advantageously used with the present invention; 
         FIG. 9  is a flowchart illustrating another procedure for managing gas bubbles in a liquid flow system in accordance with one or more embodiments of the present invention; 
         FIG. 10  is a schematic illustration of one embodiment of a micro porous flow channel that may be advantageously used with the present invention; and 
         FIG. 11  is a flowchart illustrating another procedure for managing gas bubbles in a liquid flow system in accordance with one or more embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT 
       FIG. 1  is a simplified schematic illustration of one embodiment of a liquid flow system  100  that may be advantageously used with the present invention. The system  100  comprises a liquid source  110  interconnected to a liquid receiving device  150  via flow channel/conduit  130  through which liquid  180  may flow. To move the liquid, a pump  120  (e.g., electrical, mechanical, etc.) may be placed along the flow channel  130 . Also, one or more flow sensors  140  may be placed along the flow channel  130  to monitor various conditions of the flow, such as rate, volume, temperature, pressure, etc. 
     Illustratively, the liquid receiving device  150  is an electrochemical energy conversion device or fuel cell system, e.g., a direct oxidation fuel cell, direct methanol fuel cell (DMFC), liquid or vapor feed fuel cell (fed by liquid in flow channel  130 ), portable fuel cell, transportable reformer-based fuel cell system, or other devices powered by a liquid fuel or other reactant, as will be understood by those skilled in the art. Notably, while an example receiving device  150  is a fuel cell, the techniques described herein are applicable to other liquid receiving devices that may be sensitive to bubbles in the liquid flow, and the illustrative example of a fuel cell should not be limiting on the scope of the present invention. Moreover, the system  100  embodying the invention may include a number of other components, or may omit certain components shown (including but not limited to conduits, interfaces, cartridges, and/or pumps) while remaining within the scope of the present invention. The example view shown herein is for simplicity, and is merely representative. 
     Also, the illustrative embodiment of the invention describes liquid and its use within system  100  generally, such as in fluid form. However, it should be understood that the liquid itself may be in the form of a higher viscosity liquid (e.g., gel), a liquid, or a combination of any of these fluidic forms, and the invention is not limited to use with any particular type and/or form. Also, the liquid may change from one form to another through the system, such as storing a supply of liquid to be vaporized for introduction to a receiving device  150  (e.g., a vapor-feed fuel cell, etc.). 
     As noted, certain components of the system  100  may generate gases, such as from pumps  120  (e.g., due to cavitations from mechanical pumps), which may evolve into gas bubbles  190  under certain conditions of the liquid  180  (temperature, pressure, viscosity, saturation state, etc.). Other causes, such as electrical, mechanical, chemical, and thermal causes within the liquid flow system may also cause bubbles. The bubbles  190  may occupy volume in the flow channel  130  that would otherwise be occupied by liquid  180 , thus creating voids or absences of the liquid in the flow channel  130 . As mentioned above, these bubbles  190  lead to various problems in the system  100 , such as dropouts in power generation (at device  150  when illustrative a fuel cell), difficulties in flow measurement (sensor  140 ), notably causing damage to the pumps  120 , as well as possible creation of more bubbles  190 . 
     According to one embodiment of the novel invention, the presence of bubbles  190  in the liquid flow system  100  is accepted, but techniques are in place to minimize the effect of the bubbles on uniform flow by dividing the gas bubbles as finely as possible and distributing the bubbles as uniformly as possible throughout the liquid, so that there is a substantially reduced likelihood of intolerably sized bubbles (for example, causing a long dropout period in fuel cells where no liquid reactant is reaching the fuel cell). In particular, the novel techniques described herein are directed to substantially preventing formation of intolerably sized bubbles and aggregation of tolerably sized bubbles into intolerably sized bubbles. That is, simply preventing bubbles from forming may not be sufficient, since bubbles may reform and/or collect and rejoin further down the flow system  100 . 
     In order to achieve this desirable outcome, the flow path (channel  130 ) from the reactant source  110  (more particularly, from the pump  120 ) to the receiving device  150  is carefully designed so that there are no cavities in which bubbles can collect, and no channels whose diameter is greater than the largest tolerable bubble diameter. Cavities, generally, are defined as a region of space within the system  100  occupying a volume that is not necessary or beneficial to the liquid flow  180 . For instance, portions of the system  100  may have a greater volume than the smallest flow channel, thus allowing for liquid  180  and/or bubbles  190  to collect within the cavity volumes. Example cavities often exist within the pump  120  and at various joints of the flow channel  130 . 
     Illustratively, the flow channels  130  may have cross-sectional areas limited to a maximum cross-sectional area of tolerably sized bubbles. For instance,  FIG. 2  illustrates a simplified schematic illustration of system  200  having a portion of a flow channel  130  showing cross-sectional area  210 , and a diameter  220 . (Note that where diameter is used, the implied meaning is simply a distance across the channel  130 . As such, while a cylindrical channel is shown, other shapes, regular or irregular, may be used with the teachings described herein, such as square, hexagonal, etc., and the use of a circular cross-section and associated diameter are merely illustrative examples.) Assuming that a tolerably sized bubble  230  has a known (configured/planned) cross-sectional area, cross-sectional area  210  of the flow channel  130  may be designed such that it is limited to that of the tolerably sized bubble  230 . For example, an illustrative diameter  220  of the flow channel may be between fifteen (15) and twenty (20) thousandths of an inch. 
     Further, due to the reduced size of the flow channel, which may become clogged or substantially reduce liquid flow (depending on how much of a reduction in size is the diameter  220 ), one or more embodiments of the present invention combine a plurality of micro channels into a large conglomerate flow channel. For instance,  FIG. 3  illustrates a system  300  having a flow channel  130  comprising a plurality of micro channels  310 , each sized appropriately to prevent intolerably sized bubbles. The reactant flow  380  through the micro channels  310  is substantially similar to the flow  180  through a conventional flow channel  130  (e.g., of  FIG. 1 ), however the divisions created by the micro channels  310  maintain a plurality of corresponding “micro liquid flows”  380 , each individually separated from one another to prevent bubbles  190  from multiple channels  310  from combining. 
     Another design feature that may be used to reduce cavity volumes within the flow channel  130  is to manufacture the system  100  and flow channels  130  with reduced cavity volumes. For example,  FIG. 4  illustrates portions of a liquid flow system  100  with reduced cavity volumes. In particular, flow channel  130  may be designed with joints  420  having substantially no stagnant volume and very small swept volume (e.g., “zero-volume” or “0-volume” joints  420 ). A zero-volume joint  420 , for example, may be a fixture or component that interconnects or redirects liquid flow  180  without creating additional (and unnecessary) volume. For instance, typical 90-degree square “elbow” joints, as will be appreciated by those skilled in the art, actually create a cavity at the crest or peak of the bend, such as the top joint  430  in  FIG. 4  (shown with volume-reducer  440 , described below). To remove the cavity, a zero-volume joint  420  may be designed and utilized that removes this excess volume from the flow channel  130 , leaving no available room for tolerably sized bubbles to accumulate and aggregate into intolerably sized bubbles. Also, in addition to appropriately designing the reactant flow channels  130 , other components of the system may also be designed with reduced cavities, such as flow sensor  140 B. (Note further, that the length of flow channels  130  may also be shortened, thus reducing the volume in which bubble generation and accumulation may occur.) 
     In addition, according to one or more embodiments of the present invention, where it is not possible (or simple, or desired, etc.) to eliminate a cavity volume where bubbles  190  might collect through design, such as in the cavities of commercial devices such as the pump  120  itself, the volume may be filled with a volume-reducing material (“volume reducer”). For instance, if there are any cavities (e.g., those that cannot be eliminated through design/manufacture as mentioned above), those cavities may be filled with the volume-reducing material to reduce the volume of the cavities, accordingly. By reducing the cavity volumes in system  100 , areas in which smaller bubbles may accumulate and aggregate (combine) into intolerably sized bubbles are reduced and/or eliminated. 
     Referring again to  FIG. 4 , flow system  400  may also comprise volume reducer  440  strategically placed in cavities, such as within the pump  120 , in certain joints/areas  430  of the flow channel  130  (as noted above), etc. Note that while the volume reducer  440  is shown in certain configurations/locations within the system  400 , such locations are merely a simplified example, and are not meant to signify actual locations or configurations, and are not meant to be to scale. For example, an outlet (exit plenum) of the pump  120  may have a large chamber, as may be appreciated by those skilled in the art, and the volume reducer  440  is used to fill the cavity volumes of the large chamber and to divide the bubbles exiting the pump into tolerably sized bubbles. 
     In one embodiment described herein, the volume-reducing material allows for flow of liquid and gas, such as through capillary micro pathways, but divides the liquid/gas, and more particularly, divides any bubbles, and keeps any small bubbles from aggregating into larger bubbles. Example volume-reducing material may comprise, inter alia, frit, open-cell foam, fibrous material, sintered polyethylene, etc. Frit, generally a loose powder or very fine porous block (e.g., ceramic), may be created by heating dust/beads for fusion into a porous material. Also, fibrous material may comprise wick felt, cotton wool, or other known fibrous material, particularly that is acceptable for use within a particular flow system  100  (e.g., within particular chemicals, solvents, reactants, etc.). Illustratively, the volume-reducing material (e.g., the frit) may comprise a grain size substantially smaller than a tolerably sized bubble, that is, to reduce the likelihood that bubbles larger than a tolerably sized bubble (i.e., intolerably sized bubbles) will have the opportunity to form. 
     Due to the micro capillary pathways formed by certain volume-reducing materials (e.g., frit, foam, etc.), it may also be beneficial to dispose the volume-reducing material within the flow channel(s)  130  of the liquid flow system  100 . For instance,  FIG. 5  illustrates a simplified schematic diagram of a system  500  having a flow channel  130  substantially filled with volume-reducer  540 . In this manner, liquid flow  580  may traverse a series of micro capillary pathways  510  that are formed by the volume-reducer  540  in a similar manner to the micro channels  310  above. Bubbles  190  again may be dispersed throughout the channel  130 , and kept separate to prevent combination into larger, e.g., intolerably sized, bubbles. 
     Notably, the capillary micro channels (e.g., micro channels  310  and/or pathways  510 ) may serve a secondary purpose that is additionally useful in distributing the bubbles  190  throughout the liquid flow  380 / 580 . In particular, for a given flow, the smaller diameter of the flow channel may increase the flow velocity such that a given rate of bubbles will be more widely spaced along the flow channel, in addition to being small. In other words, while the flow rate remains relatively the same, the velocity of the liquid through the micro channels may increase, as will be appreciated by those skilled in the art. Accordingly, it may thus be additionally beneficial to fill the entire flow channel  130  with volume-reducing material (suitable to create capillary pathways). 
     In addition to volume reducers that allow for the flow of liquid and gas, however, certain volume-reducing materials may be impenetrable to bubbles, i.e., preventing any reactant or bubbles from entering the cavities. In this manner, the volume-reducer does not divide bubbles, but instead simply removes cavity volumes in which bubbles may agglomerate into larger bubbles. 
     According to one or more additional embodiments of the present invention, intolerably sized bubbles that form in the system may be “broken up” (split, divided, busted, burst, etc.). Such breaking up may be performed by a suitable break-up device, as illustrated in simplified example system  600  of  FIG. 6 . In particular, bubbles  190  that may form within the flow channels may be broken-up by break-up device  610  prior to entry into the liquid receiving device  150  (e.g., after pump  120 ). The breakup-device may be configured in a variety of suitable manners, such as, e.g., a blender to blend the bubbles, an ultrasonic frequency generator to ultrasonically break bubbles, an atomizer applied to the bubbles, etc., Also, the break-up device  610  may be configured as a series of micro capillaries, through which the bubbles may be directed such that any larger bubbles are forced to divide into smaller bubbles for traversal of the micro capillaries, such as described above (e.g.,  FIG. 5 ). 
     By breaking up the larger bubbles (e.g., intolerably sized), smaller bubbles (e.g., tolerably sized) are created and allowed to flow within the channel  130  to the liquid receiving device  150 . Also, in addition to simply reducing the size of the bubbles, i.e., by dividing large bubbles into a plurality of smaller bubbles, the overall surface area of the smaller bubbles may be increased as compared to the surface area of the larger bubble. This increased surface area, along with a suitable solubility factor of the liquid, may allow the smaller gas bubbles to molecularly mix with the liquid; that is, the liquid may absorb the smaller bubbles. 
       FIG. 7  is a flowchart illustrating a procedure for managing gas bubble size in a liquid flow system in accordance with one or more embodiments of the present invention. The procedure  700  starts at step  705 , and continues to step  710 , where the volume of cavities are reduced in the liquid flow system  100 . For example, in step  712 , the liquid flow system may be manufactured with reduced cavity volumes, such as O-volume joints  420 , or other means for reducing cavity volume, as described above. Alternatively or in addition, in step  714  cavity volumes may be filled with volume-reducing material  440 , such as in certain components (e.g., pump  120 ) or areas of the flow channels (e.g., joints with excess volume cavities). 
     In addition, in step  720 , the cross-sectional area ( 210 ) of the liquid flow system  100  may be limited to a maximum cross-sectional area of tolerably sized bubbles ( 230 ), such as limiting diameters of flow channels and any components to a certain value, e.g., 15-20 thousandths of an inch. As described above, one option to limit the cross-sectional area of flow channels is to provide a plurality of micro channels  310  through which the liquid may flow in step  722 . Another option in step  724  is to dispose volume-reducing material  540  within one or more flow channels  130  of the liquid flow system  100  to form a series of micro capillary pathways  510 . 
     According to one or more embodiments described herein, and additional step  730  may break up intolerably sized bubbles that form, such as with a break-up device  610  (e.g., blender, ultrasonic frequencies, etc.) as noted above. The procedure  700  ends in step  740 , notably with substantially prevented formation of intolerably sized bubbles and aggregation of tolerably sized bubbles into intolerably sized bubbles, accordingly. 
     In addition, while formation of intolerably sized bubbles and the aggregation of tolerably sized bubbles into intolerably sized bubbles are each substantially prevented by the techniques described above, it may also be helpful to reduce the number of bubbles within the liquid flow system as a whole. For instance, according to one or more embodiments of the present invention, provisions may be in place to accumulate and remove/release any intolerably sized bubbles from the liquid flow system. Thus, fewer bubbles need be managed by the other techniques described herein. 
     In particular,  FIGS. 8A-8D  illustrate schematic diagrams of an additional and/or alternative embodiment of the present invention, while  FIG. 9  illustrates an example procedure  900  (described in parallel). The procedure  900  starts in step  905 , and continues to step  910  where the bubbles are separated from the liquid through gravity and/or dividing mechanisms. For example, in  FIG. 8A , the liquid flow system  800  may comprise a pump  120  to force the liquid flow  880  from a flow channel  130   a  into an illustratively larger flow channel  130   b  (larger than channel  130   a ), where one or more bubbles  190  (e.g., tolerably and/or intolerably sized bubbles) may be formed, e.g., by the pump. Due to the lightness of the bubbles (that is, the density of the bubbles as compared to the density of the liquid), the bubbles may hit a wall and flow ( 830 ) into an accumulation chamber  810 , stopped by a check valve  820 . The liquid flow  880  may then continue into the reduced-size flow channel  130   c  with fewer bubbles (e.g., to other bubble size management devices, as mentioned above). Alternatively, as in  FIG. 8B , the flow channel  130  may remain substantially the same size, however a bubble/liquid divider, such as a liquid permeable and gas semi/impermeable membrane, for example, to relocate at least some bubbles into the accumulation chamber  810 , regardless of orientation of the system  800 . 
     As shown in  FIG. 8C , once the pressure of the accumulated bubbles reaches a pre-determined amount, a check valve  820  (e.g., one way) may be opened (step  915 ) to release the bubbles out of the liquid flow system (e.g., into the surrounding environment, or to a bubble collection mechanism, not shown). In  FIG. 8D , once the pressure inside the collection chamber is reduced below a certain amount (e.g., external pressure to prevent backflow into the system), the check valve  820  may be closed, to allow bubbles to continue to accumulate until subsequent releases in this manner (step  920 ). This way, the number of bubbles that remain in the liquid flow system may be substantially reduced, which may either be the only bubble management technique in the system, or, illustratively, an additional measure used to manage bubbles and bubble size within the liquid flow system. 
     Further,  FIG. 10  illustrates a schematic diagram of an additional and/or alternative embodiment of the present invention, while  FIG. 11  illustrates an example procedure  1100  (described in parallel). The procedure  1100  starts in step  1105 , and continues to step  1110  where the bubbles are separated from the liquid through another example dividing mechanism. For example, in  FIG. 10 , the liquid flow system  1000  may comprise a liquid source  110  to provide liquid through channel  130  to one or more bubble creating devices  1050  (e.g., pumps, flow channels, etc.). Bubbles  190  may then traverse flow channels  130  into a bubble separation component  1060  having an inlet and two outlets. Illustratively, the component  1060  comprises a liquid flow filter tube  1062 , which may be made from a micro porous tube material, such as a liquid permeable and gas semi/impermeable membrane. 
     When the liquid flows from the flow channel  130  into the tube  1062  of the bubble separation component  1060 , due to, for example, surface tension of the bubbles, the bubbles (particularly, intolerably sized bubbles) generally will not pass through walls of the filter tube  1062 . As such, only liquid that is basically bubble-free (or at least bubble-lean) passes through the tube  1062  and into a collection chamber  1064 , which then feeds to a flow channel  130   d  to bubble sensitive components  1070  (e.g., sensors, receiving devices, outputs, etc.), as in step  1115 . 
     Bubbles  190  continue to flow down the tube  1062  and eventually reach the outlet of the bubble separation device  1060 . Illustratively, the bubbles  190  and an amount of liquid (e.g., having a higher concentration of bubbles) traverse flow channel  130   e  in step  1120 , e.g., on a return path to the liquid source  110 , which may reuse the unused liquid, and may have provisions for collecting or removing the bubbles  190  (e.g., gas release outlets, collection volumes/voids created as liquid is removed from the source, etc.). (Notably, other flow channels  130   e  may also be used, such as sending the bubble-rich liquid to bubble removal devices before returning the liquid to the bubble sensitive components  1070 .) In this manner, bubbles  190  may be removed regardless of orientation of the system  1000 . 
     Advantageously, the novel system manages bubbles in a liquid flow system. In particular, by substantially preventing formation of intolerably sized bubbles and aggregation of tolerably sized bubbles into intolerably sized bubbles, the novel technique provides solutions to various problems associated with bubbles in liquid flow systems. For example, finely divided and distributed bubbles in the liquid reactant flow of a fuel cell have been demonstrated to reduce power fluctuations of given gas amounts within the liquid as contrasted with such amounts of gas agglomerating into one or more large bubbles that pass at one time through the system. In addition, the highly distributed and finely divided bubbles (e.g., homogenized with the liquid) create smaller perturbations on flow sensing of the liquid flow, enabling more precise control and measurement sensitivity. Also, by removing intolerably sized bubbles from the system according to one or more aspects of the invention, fewer intolerably sized bubbles need be managed by other techniques described herein. 
     While there has been shown and described an illustrative embodiment that delivers liquid to a liquid receiving device, it is to be understood that various other adaptations and modifications may be made within the spirit and scope of the present invention. For example, the invention has been shown and described herein using a fuel cell (or other electrochemical energy conversion device) as receiving device  150 . However, the invention in its broader sense is not so limited, and may, in fact, be used with other devices, and is not limited to use with electrochemical energy conversion devices. For example, any liquid flow system that is concerned with flow of the liquid and gas bubbles that may occur within the liquid. In particular, certain devices that are sensitive to bubbles in liquid, such as for flow rates and/or pressure monitoring of the fluid, or simply to reduce bubbles for other reasons (e.g., paint systems, medical devices, power plants, etc.), may also make use of the novel techniques described herein. Accordingly, any references to size (e.g., “micro capillaries”) are merely relative and scaled within a particular system, for example, in accordance with the size of tolerably sized bubbles suitable for a respective system. 
     The foregoing description has been directed to specific embodiments of the invention. It will be apparent, however, that other variations and modifications may be made to the described embodiments, with the attainment of some or all of the advantages of such. Accordingly this description is to be taken only by way of example and not to otherwise limit the scope of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.