Patent Application
20100124676
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.
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.
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:
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,
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,
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,
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
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,
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
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.
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,
As shown in
Further,
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 130d 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 130e 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 130e 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.
