Daimler AG and
Ford Global Technologies, LLC
Apparatus and method for separation of a liquid from a gas flow, as well as a fuel cell system
The invention relates to an apparatus and a method for separation of a liquid from a gas flow, having a flow channel which has a first subelement and a second subelement in the flow direction of the gas flow, which second subelement has a larger internal size than the first subelement at least in places, and a transition between the second subelement and a third subelement is designed to be stepped. The invention also relates to a fuel cell system having an apparatus such as this, and to a method for separation of a liquid from a gas flow flowing in a flow channel.
In fuel cell systems, water enters the fuel cell stack in specific operating states, with this occurring both on the cathode side and on the anode side of a fuel cell. This may be caused either by condensed water or else by water from the fuel cell, which has been recirculated. Particularly when sudden load changes occur, water accumulations are detached from components or tubes, and are dragged along. They then land in the fuel cell stack and can lead to an adverse effect on the operation of a fuel cell, and/or to a negative influence on restarting, or even on starting in cold environmental conditions, particularly close to below the 0-degree limit.
In principle, however, a problem such as this can also occur independently of a fuel cell system in all other systems in which a two-phase mixture with liquid and gas is transported as a gas flow.
In conventional separator devices, the normal procedure is to use a wire mesh, and/or an interwoven mesh or a cyclone in order to make it possible to achieve an improvement in the liquid separation, by forming larger droplets. One disadvantage of these embodiments
is that an undesirable additional pressure loss and an additional necessary value are required, particularly when this device is provided in a fuel cell system.
DE 101 20 018 A1 discloses a fuel cell system having a water separator which, in a first embodiment, has an inlet tube which is arranged at right angles to a collecting tube and opens into the latter. The water-loaded flow is introduced via the inlet tube. This vertical arrangement of these two tubes with respect to one another results in the water-loaded flow flowing into the collecting tube being swirled along the inner face of the collecting tube, with the water in the flow being forced against the wall of the collecting tube, by centrifugal force. A curved off-gas tube is also introduced into the collecting tube the same end at which the inlet tube opens into the collecting tube, with the axis of the off-gas tube in the opening area likewise extending at right angles to the axis of the collecting tube. The off-gas tube is tapered at the end which is located in the collecting tube. Furthermore, this water separator has a sump for collecting the separated water, with this sump being formed separately and independently from the inlet tube and the off-gas tube, at the opposite end of the collecting tube. The sump therefore has no operative connection to the off-gas tube and the inlet tube. In a further embodiment, a water separator is provided in which the collecting tube is arranged first of all in the flow direction of the gas flow, and the off-gas tube, which is tapered in places, is introduced at its end. In this embodiment, the off-gas tube and the collecting tube are arranged one behind the other along the longitudinal axis, with the important factor being that the collecting tube is open at the end at which the off-gas tube extends into the collecting tube, thus allowing the gas flow and the water droplets to emerge there and allowing them to enter a perforated cylinder or screen in the form of a wire mesh, in order to be collected there. The water then drips from this cylinder or screen into a sump arranged underneath it.
Furthermore, DE 10 2004 022 245 A1 discloses a moisture exchange module for a fuel cell system, which is designed to moisturize the oxidant that is supplied through the cathode area of a fuel cell. In addition to this functionality, the moisture exchange module is also designed with means for separation of liquid from the gas flow emerging from the fuel cell. For this purpose, a groove formed on the inner face is provided in a housing of the moisture exchange module, such that two subareas with different internal diameters are formed in the housing. The groove is in this case arranged in an area in which the gas flow flows along the housing of the inlet flow area because of its swirling movement, such
that liquid droplets located in the gas flow are collected, by virtue of the centrifugal force, in the area of the housing in which the groove is arranged. The liquid that is collected can then be removed from the area of the groove in the inlet flow area, via a valve or a run-off channel. The widening of the cross section may be stepped or continuous. However, the cross section of the groove is in each case in the form of a stepped increase in diameter.
The object of the present invention is to provide an apparatus and a method by means of which the liquid separation can be improved. Particularly when used in a fuel cell system, the aim is to be able to separate liquid in this way from a gas flow without any major pressure loss and with a minimized physical space.
This object is achieved by an apparatus which has the features according to Claim 1 , and by a method which has the features according to Claim 13. Furthermore, the object is achieved by a fuel cell system which has the features according to Claim 12.
An apparatus according to the invention for separation of a liquid from a gas flow comprises a flow channel which has a first and a second subelement in the flow direction of the gas flow. The second subelement is designed such that it has a larger internal size than the first subelement, at least in places. A transition of the inner faces between the second subelement and a third subelement, which is adjacent to the second subelement in the flow direction, of the flow channel is stepped. The second subelement is widened at least in places on its inner face, seen in the longitudinal direction. This configuration of the flow channel makes it possible to ensure better liquid separation, wherein the apparatus can be designed to minimize the physical space required. The undesired liquid, in particular water, in the gas flow is moved primarily along the inner faces of the subelements, and thus in particular against the walls of the tubes. Liquid droplets in the gas flow itself occur only to a minimal extent in comparison thereto.
With the apparatus according to the invention, there is therefore no longer any need to base the apparatus on a separator principle which first of all causes the liquid droplets to be connected to one another to form larger droplets, in order then to join them together, as is the case for example with the corresponding interwoven meshes in wire mesh separators, for example.
A wire mesh such as this is therefore no longer provided, in particular for the apparatus according to the invention.
In fact a type of chamber in which the liquid located in the gas flow can collect can effectively be formed by the specific arrangement of the subelements of the flow channel and their internal configuration, by virtue of the stepped design between the second and third subelement. In particular, the liquid component moving along the inner faces is separated and collected particularly effectively in this way.
The second subelement is designed in particular such that it widens in the direction of the third subelement. Widening means an internal size which increases when viewed along the longitudinal axis. In particular, the widening may therefore also be conical or in the form of a funnel.
The second subelement is preferably designed such that it widens continuously on its inner face. In particular, therefore the second subelement first of all widens continuously, and therefore not in a stepped form, in the direction of the third subelement, but t here is then another step at the transition to the third subelement, with a reduction in cross section, in particular, therefore being provided at this transition from the second subelement to third subelement.
The second subelement preferably has its largest internal size at its end adjacent to the third subelement. In particular, this allows relatively large collecting areas to be formed at the transition between the second and the third subelement, thus making it possible to collect a relatively large amount of liquid as well without this being dragged along again with the gas flow and being passed on through the third subelement.
The internal size of the third subelement at the transition to the second subelement preferably corresponds to the internal size of the first subelement at the transition to the second subelement. This then opens at both ends of the second subelement into further subelements, which therefore have essentially the same internal size.
The rear wall, which opens to the third subelement, of the second subelement preferably has an essentially vertical inner face. This also allows a compact configuration minimizing the physical space, to be achieved with a relatively short configuration, and furthermore
with the very abrupt transition preventing the water that has already been collected from being dragged along. The inner face of the rear wall may, however, also be inclined with respect to the vertical, at least in places.
That end of the third subelement which faces the second subelement preferably extends into the interior of the second subelement. A configuration such as this allows a collecting area to be created in a particularly preferred manner for the separated liquid at the transition between the second and the third subelement. That end of the third subelement which is located in the second subelement is therefore used effectively as a base or cover for the collecting area and thus making it possible to particularly effectively prevent liquid from being dragged along in the direction of the third subelement, and further. Furthermore, this configuration allows a very low pressure loss to be achieved.
The end of the third subelement therefore preferably extends into the interior of the widened section of the second subelement such that an air space in which the liquid which can be separated from the gas flow can be collected is formed between the outer face of the third subelement and the inner face of the second subelement. The flow channel is therefore effectively formed with an integrated collecting space, and this can be achieved effectively automatically because of the very skilful positioning of the subelements with respect to one another.
Multifunctionality of the flow channel can be ensured with a minimum number of components.
The three subelements of the flow channel are preferably arranged in a row with respect to one another. It has been found to be particularly preferable for the three subelements to be arranged such that they have a common straight longitudinal axis. Arranging them one behind the other in this way actually allows the functional principle to be implemented particularly effectively.
A line to carry away the liquid that has been separated is preferably tapped off from the second subelement. In particular, the line is tapped off at the transition between the second and the third subelement, in particular by providing a tapping on the lower face of the transition. This also makes it possible to make use of the effect of gravity.
An air space in which the separated water can be collected is formed in particular by the shape and arrangement of the second and of the third subelement at the transition between these components.
The flow channel with its subelements is preferably formed integrally.
A fuel cell system according to the invention comprises an apparatus according to the invention or an advantageous refinement thereof. The fuel cell system is designed in particular as a mobile fuel cell system and can preferably be used in a motor vehicle. The fuel cell system has at least one fuel cell, in particular a fuel cell stack with a plurality of fuel cells, with the fuel cells preferably being in the form of PEM fuel cells.
In the case of a method according to the invention for separation of a liquid from a gas flow flowing in a flow channel, the liquid is moved along the inner face of a first subelement in the flow direction of the gas flow, and a second subelement which follows the first subelement and widens in the flow direction on its inner face, at least in places. This longitudinal movement is produced in particular by the gas flow itself. The liquid is collected at a stepped transition on the inner faces between the second subelement and a third subelement which follows it. All three subelements are designed to guide the gas flow. A procedure such as this allows liquid to be separated from a gas flow better, and in particular allows this to be done without a cyclone separator or a wire-mesh separator.
Three subelements of the flow channel are preferably arranged one behind the other in the axial direction, in particular being arranged such that they have a common straight longitudinal axis. The collected liquid is preferably let out at the transition between the second and third subelements.
Advantageous refinements of the apparatus according to the invention may be regarded as advantageous refinements of the method according to the invention.
Exemplary embodiments of the invention will be explained in more detail in the following text with reference to schematic drawings, in which:
Figure 1 shows a schematic section illustration through a first exemplary embodiment of an apparatus according to the invention; and
Figure 2 shows a schematic section illustration through a second exemplary embodiment of an apparatus according to the invention.
Identical or functionally identical elements are provided with the same reference symbols in the figures.
Figure 1 shows a schematic illustration in the form of a cross section through an apparatus I for the separation of a liquid from a gas flow. The apparatus I is illustrated in Figure 1 with sub-components, with the apparatus I being associated with a mobile fuel cell system in a motor vehicle. The gas flow flowing through the apparatus I is in this case an off-gas flow from the cathode area and/or the anode area of a fuel cell, or a fuel cell stack.
The apparatus I comprises a flow channel 1 which comprises a first subelement 2 and a second subelement 3, which is arranged downstream from it and is adjacent to it in the flow direction, as well as a third subelement 4, which is once again directly adjacent to the second subelement 3. The three subelements 2, 3 and 4 are thus arranged one behind the other in a row in the flow direction which is characterized by the arrow P, and are therefore designed and arranged such that they have a common straight longitudinal axis A.
The first subelement 2 is tubular and, in the exemplary embodiment, has a round cross section, such that an internal size d1 is characterized by an internal diameter. The second subelement 3 is likewise analogously a tubular section, which has an internal diameter as the internal size d2. The second subelement 3 is designed such that it widens continuously in the direction of the longitudinal axis A, and therefore has a conical internal structure. The widening is formed in the direction of the third subelement 4. The internal size d2 therefore increases continuously in the exemplary embodiment, starting from the transition 8a between the first subelement 2 and the second subelement 3 up to the transition 8b between the second subelement 3 and the third subelement 4.
The third subelement 4 is likewise tubular and likewise has an internal diameter as the internal size d3.
In addition to a circular cross section of the first subelement 2 and/or of the second subelement 3 and/or of the third subelement 4, at least one of these subelements 2 to 4 may also have a different cross-sectional geometry and, for example, may also be polygonal, oval or the like.
The first subelement 2 has an inner face 5, the second subelement 3 has an inner face 6, and the third subelement 4 has an inner face 7. The transition 8a between the inner face
5 of the first subelement 2 and the inner face 6 of the second subelement 3 is formed effectively continuously, without any sharp sudden change, in the illustrated embodiments.
A transition 8b between the second subelement 3 and the third subelement 4 is in the form of a sharp discrete step in the illustrated embodiment, in this case with the inner face
6 opening essentially vertically to the horizontally oriented inner face 7.
The second subelement 3 may also be in the form of a groove in the flow channel 1. In particular, the flow channel 1 is preferably formed integrally, with this second subelement 3 therefore being conceived as a groove. In particular, this second subelement 3 is formed completely circumferentially around the axis A and can thus be referred to in particular also as an annular groove. A line 9 for carrying away the collected separated water from the gas flow is tapped off from the second subelement 3 on its lower face.
It has been observed in the fuel cell system that the uπdesired water in the gas flow is moved along in the subelements 2 to 4, which altogether are intended to carry the gas flow, in particular on the inner faces 5, 6 and 7, and virtually no droplet formation occurs in the interior of the gas flow. In this case as well, there is therefore no longer any need to use a wire mesh or a cyclone which first of all causes the droplets to be connected to one another to form larger droplets, and then joins them together.
In Figure 1 , the water 10 is carried by the gas flow along the inner face 5 of the first subelement 2, and lands in the annular groove or the second subelement 3 which is formed with the larger internal size d2. This occurs because the liquid or the water 10 attempts to adhere to the respective inner face 5 or 6. This effect is referred to as the Coanda effect.
The water 10 is forced out with the aid of the force of gravity and/or the pressure difference between the internal pressure in the flow channel 1 and the pressure at the end of the line 9.
The depth (vertical direction) and/or length (horizontal direction) of the second subelement 3 may be designed on the basis of how much water 10 is expected. It is likewise also possible to provide for the second subelement 3 not to have a pointed configuration in cross section but for the cross-sectional shape also, for example, to be designed to be free of corners, for example formed by curves. It is also possible for the second subelement 3 to be formed circumferentially around the axis A only in places, and therefore only in places to have a radius (d2/2) which extends beyond the radius (d1/2) of the first subelement 2 and beyond the radius (d3/2) of the third subelement 4.
As already mentioned, the internal size of the second subelement 3 increases continuously from this end facing the first subelement 2, and has its maximum internal size at its end facing the third subelement 4. The transition 8b between the second subelement 2 and the third subelement 4 is once again formed stepped, with the inner face 6 in this context merging into a vertically oriented inner face 12 of the rear wall of the second subelement 3, with this vertical inner face 12 ending at an outer face 13 of the third subelement 4. The internal size d3 of the third subelement 4 at this transition 8b is smaller than the maximum internal size of the second subelement 3 at this transition 8b.
Furthermore, the third subelement 4 is arranged such that its end 11 which faces the first and the second subelements 2 and 3, respectively, extends into the interior of the second subelement 3. This refinement results in a collection space in the form of an air space 14 being formed between the outer face 13 and the inner face 6, and being closed at the rear by the inner face 12. It is therefore impossible for a flow to pass through the inner face 12. The third subelement 4 in this refinement is therefore effectively in the form of a "submerged tube".
The transition 8a, which is not sharply stepped, between the first subelement 2 and the second subelement 3 makes it possible to prevent the water 10 flowing on the inner face 5 from being dragged along by the gas flow, thus becoming separated from the respective inner face 5 or 6 and in consequence not being able to enter the air space 14. This separation effect can be prevented by the continuous widening and thus the
obliquely positioned inner face 6, in comparison to a sharp discrete step system, and even after the transition 8a, the water 10 flows along the inner face 6 in the direction of the air space 14. The third subelement 4, which extends into the interior of the second subelement 3, makes it possible to particularly effectively prevent the water which has been collected in these air spaces 14 from being removed by the flow again, and also being dragged along through the third subelement 4.
The geometry of the second subelement 3 with respect to the widening relating to the size of the air space 14 is merely an example. Furthermore, the length of that part of the third subelement 4 which extends into the interior of the second subelement 3 is merely an example and can also be configured differently, in contrast to the illustration in Figure 1.
Gravitation allows water 10 to be collected effectively, virtually like a container, in particular on the lower face of the space 14. Since the third subelement 4 extends effectively like a cover over this space 14, this area of the third subelement 4 effectively also makes it possible to ensure protection against sloshing over. Furthermore, this refinement as illustrated in Figure 1 makes it possible to ensure very low pressure loss.
Figure 2 shows a further exemplary embodiment of a flow channel 1 for an apparatus I which, in contrast to the illustration shown in Figure 2 has a third subelement 4 in the cross section, ending at the transition 8b to the second subelement 3. In this embodiment, the third subelement 4 therefore does not extend into the interior of the second subelement 3.
The embodiments shown in Figures 1 to 3 are preferably all designed to be rotationally symmetrical about the axis A. However, of course, it is also possible to provide, as has already been mentioned with respect to the exemplary embodiment in Figure 1 , for no such rotationally symmetrical configuration to be provided. For example, in the refinements illustrated in Figures 2 and 3, it is also possible to provide for the inner faces 12 above and below the axis A to have different vertical lengths in the illustrated cross-sectional figures. In this case, the design would then be based on asymmetric configuration with respect to the axis A. It is likewise possible to provide for the second subelement 3 to be widened over only part of its length.
Particularly for use in a fuel cell system, the apparatus I allows liquid to be separated effectively from a gas flow and in particular, in the event of sudden load changes on the fuel cell system, makes it possible to prevent water collections from being separated from components or the flow channel, and being dragged along. This also makes it possible to prevent water that has been dragged along from entering the fuel cell stack, where it leads to an adverse effect on operation and/or to a negative influence on restarting or on starting when the fuel cell system is in low environmental temperatures.
The use of the apparatus I is, however, not restricted to a fuel cell system and in principle it can be used for all systems in which the aim is to separate liquid from a gas flow.
List of reference symbols
1 Flow channel
2 First subelement
3 Second subelement
4 Third subelement
5, 6, 7, 12 Inner face
8a, 8b Transition
11 End of the third subelement
13 Outer face of the third subelement
14 Air space
A Longitudinal axis d1 , d2, d3 Internal side