The present disclosure relates to a pump configured to pump a magnetic mixture in a flow direction. The disclosure also relates to a magnetocaloric refrigerator comprising such pump and to a method of pumping a magnetic mixture through a channel comprising an ascending portion and a descending portion, downstream of the descending portion.

A magnetic mixture comprises a carrier liquid and magnetic particles. When a magnetic particle is exposed to a magnetic field gradient, it experiences a body force given by equation 1.

Herein, po is the permeability of free space [H/m], H is the applied magnetic field [A/m] and M is the magnetization of the mixture [A/m]. Pumps that achieve flow in magnetic mixtures by utilizing this body force are called magnetic pumps.

Two well-known types of magnetic mixtures are ferrofluids and magneto-rheological fluids. Ferrofluids include magnetic particles of a size in the order of nanometres. Magneto- rheological fluids include magnetic particles of a size in the order of micrometres. In such types of fluids, particles are generally stably suspended in the carrier liquid. However, for the purpose of this application, the term magnetic mixtures should be understood as also including mixtures in which magnetic particles sediment over time.

Magnetic pumps known from the art generate time-varying magnetic fields to expose the magnetic particles to a gradient of the magnetic field continuously. These magnetic fields vary with frequencies of anywhere between 100 to 10,000 hertz. The resulting body force is utilized to achieve the desired fluid motion.

Pumping a magnetic mixture using this approach is not very efficient. The amount of energy transferred from the magnetic field to the magnetic mixture is proportional to the frequency, so relatively high frequencies are necessary to pump a magnetic mixture with any relevant flow rate. (In the context of this application, a flow rate may be expressed in a volume of magnetic mixture pumped over time, e.g. litre per second.) At the same time, the extent to which certain types of losses occur is also proportional to the frequency of the time-varying magnetic field: The physically rotating magnetic particles generate heat due to viscous dissipation. Also, when an electrically conducting carrier liquid is used (e.g. Galistan), eddy currents are generated therein, which results in further heat losses.

It is an object of the present application to provide a more efficient magnetic pump. It is a further object to provide a magnetic pump that generates less excess heat, and/or that may be operated at lower frequencies. It is a further object to provide a magnetic pump that relies less on high frequency magnetic fields to achieve liquid motion, and/or in which the abovementioned disadvantages are at least less prevalent. It is a further object to provide a more efficient magnetocaloric refrigerator.

At least one of the abovementioned objects is at least partially achieved by providing a magnetic pump configured to pump a magnetic mixture in a flow direction according to appended claim 1. The magnetic pump comprises a channel and a magnetic field generating unit. The channel comprises an ascending portion and a descending portion, downstream of the ascending portion. The magnetic field generating unit is arranged adjacent to and at least partially enclosing adjoining sections of the ascending portion and the descending portion. The magnetic field generating unit is configured to generate a magnetic field through the enclosed sections, and to vary a magnetic field strength of the magnetic field between a first average field strength, and a second average field strength, lower than the first average field strength. The magnetic field strength is varied such that:

- when the magnetic field has the first average field strength, a first volume of the mixture present in the ascending portion of the channel is urged, under the influence of the magnetic field, to flow in the flow direction, from the ascending portion into the descending portion, thereby at least partially filling the enclosed section of the descending portion, and,

- when the magnetic field strength is lowered to the second average field strength, the first volume of mixture present in the enclosed section of the descending portion is allowed, under the influence of gravity, to flow in the flow direction, out of, and thereby at least partially draining the enclosed section of the descending portion.

The magnetic field exerts a magnetic body force on the magnetic particles suspended in the magnetic mixture which are translatory in nature (i.e. which displace the magnetic particles, rather than rotate them), and this force draws magnetic mixture present in the ascending portion, into the sections of the channel enclosed by the magnetic field generating unit.

When the magnetic field strength is lowered, magnetic mixture present in the enclosed sections will again flow out, and because one of the enclosed sections is a section of the descending portion of the channel, the magnetic mixture that is present therein will flow in the flow direction.

By varying between the first average field strength and the second average field strength, a pulsating flow is created in the flow direction using only the aforementioned translatory magnetic body force. This magnetic pump does not rely on high frequency time-varying fields and therefore does not experience the same aforementioned disadvantages.

In the case of magnetic-torque driven flow, such as in magnetic pumps known from the art, due to the superparamagnetic nature of suspensions, only a fraction of the suspended particles contributes to the flow. Therefore, another advantage of relying on translatory forces is that particles of all sizes (including superparamagnetic particles) when exposed to a magnetic field gradient contribute to the flow of magnetic mixture through viscous drag. In some embodiments of the magnetic pump, the magnetic field exerts a magnetic body force on magnetic particles suspended in liquid present at either end of the magnetic field generating unit. In such embodiments, the magnetic body force exerted at an upstream end of the magnetic field generating unit preferably points in the flow direction and the magnetic body force exerted at a downstream end of the magnetic field generating unit preferably points in a direction opposite of the flow direction.

In some embodiments of the magnetic pump, the magnetic field has a non- zero gradient at either end of the magnetic field generating unit. In such embodiments, the component of the non-zero gradient at the upstream end of the magnetic field generating unit pointing in the flow direction is preferably positive; and the component of the non-zero gradient at the downstream end of the magnetic field generating unit pointing in the flow direction is preferably negative.

In some embodiments, when the pump is positioned as it would be in use and when considered in the flow direction, the ascending portion, with respect to the direction of gravity, inclines upwards and/or the descending portion, with respect to the direction of gravity, inclines downward.

In preferred embodiments, the magnetic mixture comprises a carrier liquid and at least one of ferromagnetic, anti-ferromagnetic, or paramagnetic particles suspended in the carrier liquid.

To provide a pulsating flow, the magnetic field generating unit may be configured to repeatedly cycle through generating the magnetic field having the first average field strength for a first amount of time and generating the magnetic field having the second average field strength for a second amount of time. The first amount of time may correspond to the amount of time it takes to at least partially fill the enclosed section of the descending portion, and, to achieve a higher flow rate, preferably corresponds to the amount of time it takes to completely fill the enclosed section of the descending portion. The second amount of time may correspond to the amount of time it takes to at least partially drain the enclosed section of the descending portion, and, to achieve a higher flow rate, preferably corresponds to the amount of time it takes to completely drain the enclosed section of the descending portion.

Repeated cycling through generating the magnetic field having the first average field strength (more specifically, a first, constant magnetic field strength) for a first amount of time and generating the magnetic field having the second average field strength (more specifically, a second, constant magnetic field strength) for a second amount of time may be done with a frequency of 10 Hz or lower, or even with a frequency of 1 Hz or lower.

In some embodiments, the first constant magnetic field strength is about 30 mT to about 50 mT. The applicant finds that the field strength influences the mass flow rate that can be pumped for a given pumping height. In some exemplary embodiments, around 0.3 gram per second can be obtained using 32 mT to pump the fluid over a height difference of 2 mm with a pipe internal diameter of 6 mm. Other magnetic field strengths could also be used.

The second field strength only has to be such that the force of gravity overcomes the magnetic body force at the downstream end of the enclosed section of the descending portion. However, it is much simpler and more efficient to just set the second average field strength to zero - e.g. turn off the magnetic field generating unit. For such an embodiment, it may be said that the magnetic field generating unit is further configured to intermittently generate the magnetic field having the first field strength.

Because magnetic pumps according to the invention rely less, or do not rely on timevarying, high frequency magnetic fields, in some embodiments the magnetic field generating unit is further configured to generate the magnetic field when provided with a direct current, ‘DC,’ and wherein the pump preferably comprises a DC source configured to provide DC to the magnetic field generating unit. When a constant current (DC) is provided to the magnetic field generating unit, this results is a constant (average) magnetic field strength. The DC source may be configured to vary the magnetic field strength between a first constant magnetic field strength value and a second constant magnetic field strength value. The frequency of variation between the first constant magnetic field strength value and a second constant magnetic field strength value can be very low, typically a frequency of 10 Hz or lower, or even 1 Hz or lower.

In some embodiments, the magnetic field generating unit comprises a solenoid wound around the channel and wherein the magnetic field is generated by running a DC current through the solenoid. These solenoids are relatively easy to manufacture, and their behaviour is well known to the skilled person.

When the magnetic field strength is lowered, the mixture present in the ascending portion flows back in a direction opposite the flow direction. This mixture flowing back into the ascending portion is called a backflow. This backflow is limited in a magnetic pump wherein, in a cross- sectional plane perpendicular to the flow direction, the ascending portion has a first area, and the descending portion has a second area, larger than the first area. This is also limited in a magnetic pump wherein the ascending portion inclines upwards at a first angle and wherein the descending portion inclines downward at a second angle, steeper than the first angle.

In some embodiments, the magnetic field has, from an upstream end up to a downstream end of the magnetic field generating unit, a magnetic field strength that is substantially constant and/or that has a gradient of which a component pointing in the flow direction is substantially zero. Such a magnetic field provides an inward pointing magnetic body force at either end of the magnetic field generating unit. In some embodiments, when the pump is positioned as it would be in use, a lowest point of the descending portion is at a height, equal to or higher than a lowest point of the ascending portion. This may provide the magnetic mixturewith at least some potential energy.

In some magnetic pumps, the channel further comprises a loop section connecting, in the flow direction, a downstream end of the descending portion to an upstream end of the ascending portion. The mixture then has to be pumped in one direction only, since said magnetic mixturecan flow back through the loop section to be pumped again.

For some embodiments of such one-directional magnetic pumps, when the magnetic mixture is at rest, a level in the ascending portion and a level in the descending portion are equal. The upstream end of the magnetic field generating unit is at a height lower than, or approximately equal to the level of the magnetic mixture at rest, and the downstream end of the magnetic field generating unit is at a height higher than the level of the magnetic mixture at rest. In this embodiment, when starting from the rest position, mixture may be urged into the upstream end of the magnetic field generating unit - i.e. in the flow direction - and mixture may not be urged into the downstream end of the magnetic field generating unit - i.e. in a direction opposite the flow direction.

In embodiments of the present application the pump is pump is a ferrohydrodynamic pump. In a ferrohydrodynamic pump the Lorentz force is not the (primary) driving force. A ferrohydrodynamic pump is one in which pumping is achieved by utilizing magnetic body force (cf. eq. 1) experienced by the magnetic entity of interest (for example, the fluid to be pumped or more specifically the magnetic particles in the fluid to be pumped) when it is exposed to a magnetic field gradient. This magnetic body force may take the form of translatory force causing a translational movement of the fluid.

The discussed magnetic pumps may be included in magnetocaloric refrigerators, such as ones comprising:

- one of the magnetic pumps,

- a further portion of channel, connecting a downstream end of the descending portion to an upstream end of the ascending portion, wherein the magnetic pump is configured to pump the magnetic mixture through the further portion of channel;

- a first heat exchanger arranged at a first position of the further portion of channel, a second magnetic field generating unit configured to generate a magnetic field through the first heat exchanger; and,

- a second heat exchanger, arranged at a second position of the further portion of channel at a distance of the first position; wherein the magnetic mixture preferably comprising magnetocaloric particles and/or a heat transfer liquid.

Some embodiments of the magnetic pump are further configured to pump the magnetic mixture in a direction opposite to the flow direction. Such magnetic pumps may be called bidirectional pumps. These are magnetic pumps wherein the magnetic field generating unit is further configured to generate a further magnetic field through the enclosed sections of which the field lines point in a direction opposite to those of the magnetic field and to vary a further magnetic field strength of the further magnetic field between a third average field strength, and a fourth average field strength, lower than the third average field strength, such that:

- when, while the further magnetic field has the third average field strength, a second volume of mixture present in the descending portion of the channel is urged, under the influence of the further magnetic field, to flow in the direction opposite to the flow direction, from the descending portion into the ascending portion, thereby at least partially filling the enclosed section of the ascending portion, and,

- when the further magnetic field strength is lowered to the fourth average magnetic field strength, the second volume of mixture present in the enclosed section of the ascending portion is allowed, under the influence of gravity, to flow in the direction opposite to the flow direction, out of, and thereby at least partially draining the enclosed section of the ascending portion.

In some embodiments of these bidirectional pumps, the magnetic field has, from the upstream end and up to the downstream end of the magnetic field generating unit, at least one of a monotonically increasing magnetic field strength or a gradient of which a component pointing in the flow direction is positive. This allows for urging the magnetic mixture in the flow direction not just at the upstream end of the magnetic field generating unit, but throughout almost all of the magnetic field generating unit, up to its downstream end.

In some embodiments of these bidirectional pumps, the further magnetic field has, from the upstream end up to the downstream end of the magnetic field generating unit, at least one of a monotonically decreasing magnetic field strength, or a gradient of which a component in the flow direction is negative. This allows for urging magnetic mixture in a direction opposite the flow direction, not just at the downstream end of the magnetic field generating unit, but throughout almost all of the magnetic field generating unit, up to its upstream end.

To pump magnetic mixture back and forth, the magnetic field generating unit of a bidirectional magnetic pump may be further configured to repeatedly cycle through generating:

- the magnetic field having the first average field strength for the first amount of time;

- the magnetic field having the second average field strength for the second amount of time; - the further magnetic field having the third average field strength for a third amount of time; and

- the further magnetic field having the fourth average field strength for a fourth amount of time.

In such embodiments, the third amount of time may approximately correspond to the amount of time it takes to at least partially fill the enclosed section of the ascending portion, and, to achieve a higher flow rate, preferably approximately corresponds to the amount of time it takes to completely fill the enclosed section of the ascending portion.

In such embodiments, the fourth amount of time may approximately correspond to the amount of time it takes to at least partially empty the enclosed section of the ascending portion, and, to achieve a higher flow rate, preferably approximately corresponds to the amount of time it takes to completely empty the enclosed section of the ascending portion.

In some embodiments of the bidirectional magnetic pump, the magnetic field generating unit comprises at least two solenoids wound around the channel, wherein a first solenoid from the two solenoids is configured to generate the magnetic field and wherein a second solenoid from the two solenoids is configured to generate the further magnetic field. Solenoids are simple to manufacture and their behaviour is well-known to the skilled person.

To provide the earlier discussed magnetic field, the first solenoid may have, in the flow direction, an increasing number of windings per unit length. To provide the earlier discussed further magnetic field, the second solenoid may have, in the flow direction, a decreasing number of windings per unit length.

Alternatively, the magnetic field generating unit may comprise a plurality of coil units sequentially arranged along the channel, each coil unit comprising two or more coils arranged adjacent to, and at least partially enclosing the channel, and preferably, for each coil, a metal core arranged in said coil. The individual coil units allow for generating a desired magnetic field more accurately.

Such magnetic pumps may be included in a magnetocaloric refrigerator, further comprising:

- a first heat exchanger, arranged at an upstream end of the ascending portion,

- a second heat exchanger, arranged at a downstream end of the descending portion;

- porous magnetocaloric material, arranged in between the pump and the second heat exchanger; and,

- a second magnetic field generating unit configured to generate a magnetic field through the magnetocaloric material;

- wherein the magnetic mixture preferably comprising heat transfer liquid. The abovementioned objects are at least partially achieved by pumping a magnetic mixture through a channel comprising an ascending portion and a descending portion, downstream of the descending portion, using a method comprising:

- urging, by generating a magnetic field through adjoining sections of the ascending portion and the descending portion, and the magnetic field having a first average magnetic field strength, a first volume of the mixture present in the ascending portion of the channel to flow in the flow direction, from the ascending portion into the descending portion, thereby at least partially filling the section of the descending portion in which the magnetic field is generated, and,

- allowing, by lowering the magnetic field strength of the magnetic field to a second average magnetic field strength, the first volume of mixture present in the section of the descending portion through which the magnetic field is generated, to flow, under the influence of gravity, in the flow direction, thereby at least partially draining the section of the descending portion in which the magnetic field is generated.

Some methods for pumping the magnetic mixture also allow for pumping the magnetic mixture in a direction opposite to the flow direction. Such methods include:

- urging, by generating a further magnetic field of which the field lines point in a direction opposite to those of the magnetic field through the adjoining sections, and having a third average magnetic field strength, a second volume of the mixture present in the descending portion to flow in a direction opposite to the flow direction, from the descending portion into the ascending portion, thereby at least partially filling the section of the ascending portion in which the magnetic field is generated, and,

- allowing, by lowering the magnetic field strength of the further magnetic field to a fourth average magnetic field strength, the second volume of mixture present in the section of the ascending portion through which the further magnetic field is generated, to flow, under the influence of gravity, in the direction opposite the flow direction, thereby at least partially draining the section of the ascending portion in which the further magnetic field is generated.

The following description concerns a number of embodiments of a magnetic pump according to the invention. Further details and advantages of these embodiments will be elucidated by referring to the accompanying figures, in which likewise reference numbers refer to likewise elements, and wherein: figure 1 shows a magnetocaloric refrigerator that includes a magnetic pump according to the present disclosure; figure 2a shows a cross-section of an embodiment of a magnetic pump; figure 2b shows a cross-section of another embodiment of a magnetic pump; figures 3a-b show graphs describing aspects of the magnetic field generated by the magnetic pump of figure 2b; figures 4a-c show the magnetic pump of figure 2b during various stages of operation; figure 5 shows a cross-section of another embodiment of a magnetic pump; figures 6a-d show the magnetic pump of figure 5 during various stages of operation; figure 7 shows a further embodiment of a magnetic pump; figures 8a-b and 9a-b show graphs describing aspects of magnetic fields generated by the magnetic pump of figure 7; figure 10 shows another magnetocaloric refrigerator including an embodiment of a magnetic pump; figures lla-b and 12a-b show coil units which may be included in the magnetic pump shown in figure 10; figures 13a-c show cross-sections of various embodiments of magnetic pumps and indicate a general direction of magnetic field lines of magnetic fields generated therein; figure 14 shows a cross-section of another embodiment of a magnetic pump.

Reference is made to figure 1, which shows a magnetocaloric refrigerator. Specifically, the refrigerator includes a magnetic pump 1, a magnet 10, a first heat exchanging unit 11, second heat exchanging unit 12, and channel 100. Magnetic pump 1 includes a magnetic field generating unit 2.

Magnetocaloric material exhibits a temperature rise (under adiabatic condition) on the application of a magnetic field (typically in the order of 1 tesla), and a corresponding temperature drop on the removal of this magnetic field. In magnetocaloric refrigerators, such as the one shown in figure 1, the magnetic mixture may specifically be a magnetocaloric mixture. As magnetic particles, the magnetocaloric mixture comprises magnetocaloric material such as MnFe(P,Si) or La(Fe,Mn,Si)H. As carrier liquid, the magnetocaloric mixture may comprise a suitable heat transfer liquid, such oil, alcohol or Galinstan.

At the position of hot heat exchanger 11, magnet 10 applies a magnetic field to the magnetocaloric mixture. The magnetocaloric mixture undergoes magnetization and the resulting heat is transferred to the environment through first heat exchanger 11. Because, to the environment, first heat exchanger 11 is hot, it may also be referred to as hot heat exchanger 11. Meanwhile, pump 1 pumps the magnetocaloric mixture through channel 100. Thereby the magnetocaloric mixture will move outside the magnetic field, and due to demagnetization, its temperature reduces. The resulting colder magnetocaloric mixture is pumped towards second heat exchanger 12, and there absorbs heat from the environment. Because, to the environment, second heat exchanger 12 is cold, it may also be referred to cold heat exchanger 12. At cold heat exchanger 12, the magnetocaloric mixture will return to the same temperature as the environment and then flows back to first heat exchanger 11, thus completing a cycle.

The skilled person will appreciate that the magnetocaloric mixture, when having a temperature different from its immediate surroundings, will always, however minor, exchange thermal energy with said immediate surroundings. However, for the purpose of this application, this exchange of thermal energy is facilitated at the cold and hot heat exchangers to such a degree, that further loss or absorption of thermal energy throughout channel 100 may be neglected.

Reference is made to figures 2a and 2b, which show embodiments of a magnetic pump according to the disclosure. For reasons that will become clear later in this description, the magnetic pump shown in figures 2a-b, 4a-c may be called a one-directional pump. In both of the embodiments shown, magnetic pump 1 comprises a channel and a magnetic field generating unit 20 (fig. 2b) 2 (fig. 2a). Pump 1 is shown in an orientation that it would have, when in use. Arrow g in figure 2a indicates the direction of gravity. Pump 1 is configured to pump the magnetic mixture through the channel in a flow direction, which, in the embodiments shown in figures 2a and 2b, is generally from left to right.

In figure 2a specifically, an ascending portion of the channel is indicated by L _pr and a descending portion of the channel is indicated by L _P f. The ascending portion has an angle 0 _r with respect to a horizontal plane. The descending portion has an angle 0f with respect to a horizontal plane. Magnetic field generating unit 20 encloses a section of the channel. Specifically, a section of the ascending portion indicated by L _S1 and a section of the descending portion indicated by L _S f.

Figure 2a shows an embodiment which also comprises a channel and a magnetic field generating unit 2, and in which the channel also comprises an ascending portion 101 and a descending portion 102. Similarly to the embodiment shown in figure 2a, magnetic field generating unit 2 encloses a first enclosed section 103 of ascending portion 101, as well as a second enclosed section 104 of descending portion 102. In this embodiment, the channel further comprises an upstream portion 105 and a downstream portion 106.

Portion 101 of the channel may be called ascending because when the magnetic mixture flows through portion 101 in the flow direction, it ascends. Alternatively said, portion 101 slopes upwards. Portion 102 of the channel may be called descending because when the magnetic mixture flows through portion 102 in the flow direction, it descends. Alternatively said, portion 102 slopes downwards. Ascending portion 101 and descending portion 102 adjoin at respectively a downstream end of ascending portion 101 and an upstream end of descending portion 102. First enclosed section 103 and second enclosed section 104 adjoin at respectively a downstream end of first enclosed section 103 and an upstream end of second enclosed section 104. In figure 2a the level of the magnetic mixture in rest is shown. In ascending portion 101 this level is referred to as level L _r and in descending portion 102 this level is referred to as level Lf.

While the pumps shown in figures 2a and 2b may be included in any one of a plurality of apparatuses, in figures 2a and 2b the pumps are shown as included in a magnetocaloric refrigerator similar to the one discussed in relation to figure 1. In some apparatuses, such as the magnetocaloric refrigerator shown in figure 1, a downstream end of descending portion 102 may be connected to an upstream end of ascending portion 101. For the embodiment of pump 1 shown in, and when implemented as in figure 2a, it may be more specifically said that channel 100 comprises a loop portion, and be said that the loop portion includes downstream portion 106 and upstream portion 105.

In these embodiments the magnetic mixture, when at rest, will have the same level LI in ascending portion 101, as in descending portion 102.

In some embodiments, like those shown in figures 2a and 2b, magnetic field generator 2 may be implemented as a single solenoid wound around, and thereby enclosing the first and second enclosed sections 103 and 104. In some embodiments, such as the ones shown in figures 2a and 2b, pump 1 may further comprise a direct current, DC, source configured to provide magnetic field generating unit 2 with a direct current.

Operation of pump 1 shown in figures 2a-b will now be explained, further referring to figures 3a-b, and figures 4a-c. Figures 3a-b describe properties of the magnetic field that is generated when the solenoid is provided with a direct current. Figures 4a-c show pump 1 in various stages of operation. Figure 3a specifically shows a graph with the magnetic field strength |H| along axis Al on the y-axis. Figure 3B specifically shows a graph of the gradient of the magnetic field strength along axis Al in the flow direction on the y-axis. For both graphs shown in figures 3a and 3b, the x- axis corresponds to positions on axis Al in the flow direction (e.g. the left side of the x-axis of the graphs shown in figures 3A and 3B corresponds to the left side axis Al shown in figure 2a).

When observed along axis Al, in the flow direction, the magnetic field strength of the generated magnetic field increases substantially when input I is approached, remains substantially constant when input I is passed, and when approaching output O, and decreases substantially when output O is passed. When observed along axis Al, in the flow direction, the magnetic field gradient has a gradient component which, in the flow direction, is positive around input I and negative around output O. This gradient, specifically a spatial gradient, results in the magnetic body force according to equation 1. Therefore, a magnetic field having the properties shown in figures 3a and 3b will subject magnetic particles in magnetic mixture present at input I and output O to an inward body force. This force may be called ‘inward’ because it points into magnetic field generating unit 2 both at input I (e.g. where the force points stream downwards) and at output O (e.g. where the force points stream upwards).

While magnetic mixture is urged from first encircled section 103 into second encircled section 104, a gas bubble (for example, an air bubble) may be trapped in an upper part of first encircled section 103 and an adjacent upper part of second encircled section 104. This is because magnetic mixture may pass over point of no return Pl and fill a downstream end of second encircled section 104 completely before an upstream end of first enclosed section 103 or second enclosed section 104. For so far that the aforementioned inward forces at output O and input I allow, this gas bubble may be at least partially compressed. If a gas bubble is indeed present and at least partially compressed, potential energy stored in this compression may provide a force, in addition to the force of gravity, that urges magnetic mixture present in enclosed sections 103 and 104 downwards. In so far that ascending portion 101 is completely filled with magnetic mixture and that descending portion 102 is not and/or is filled with air, the skilled person will appreciate that the outward force exerted by the compressed gas bubble will primarily urge the magnetic mixture present in the second enclosed section stream downward.

As mentioned earlier, figure 2a shows pump 1 when the magnetic mixture is at rest.

Figure 4a shows pump 1 of figure 2a shortly after magnetic field generating unit 2 started generating the magnetic field having the properties shown in figures 3a and 3b. Magnetic field generating unit 2 is arranged such that input I is below level L _r . Magnetic mixture present at input I, under the influence of the aforementioned body force, flows into magnetic field generating unit 2 (Fl). The skilled person will appreciate that input I does not even have to exactly coincide with liquid level L _r and that input I may be arranged somewhat higher than liquid level L _r as long as the gradient of the magnetic field generated by the magnetic field generating unit reaches L _r . That is, as long as the magnetic mixture can experience the magnetic body force given by equation 1, the mixture can be urged into the magnetic field generating unit. So even in this case, flow can be achieved. It may be less efficient.

Magnetic field generating unit 2 is arranged such that output O is above level Lf. Therefore, at this stage, there is no mixture present at output O and no magnetic mixture is urged into magnetic field generating unit 2 from that side.

That output O is above level Lf is, however, not essential. At least some volume is pumped when mixture present in the ascending portion is urged into the descending portion of the channel. If the enclosed section of the descending portion is also partially filled by urging in magnetic mixture present around level Lf the magnetic pump still functions according to the same principle, but simply less efficient. Once a first volume VI of magnetic mixture is pushed up into first enclosed section 103, other magnetic mixture will be present at input I. This other magnetic mixture will itself also be pushed up into first enclosed section 103, and thereby push first volume of magnetic mixture VI further through channel VI, across a point of no return Pl, and into the second enclosed section 104.

Figure 4b shows pump 1 of figures 2a and 4a in a steady state, while the magnetic field is still generated. Magnetic mixture has filled up first and second enclosed sections 103 and 104. Due to the inward force at output O, the first volume of magnetic mixture VI is kept inside of second enclosed section 104, against the force of gravity.

Figure 4c shows pump 1 of figures 2a, 4a, and 4b shortly after magnetic field generating unit 2 stopped generating the magnetic field. Magnetic mixture present in the channel is again subject to just the force of gravity. Magnetic mixture present in first enclosed section 103 flows back through ascending portion 101 (F2). The first volume of magnetic mixture VI present in the second enclosed section 104 flows, in the flow direction, through descending portion 102 (F3).

By repeating this process (e.g. alternatingly turning the magnetic field on and off) a pulsating flow is provided.

In some embodiments, the magnetic field may be completely turned off but this is not essential. The magnetic field strength only has to be lowered to a point that the force of gravity overcomes the inward force at output O, thereby allowing first volume VI to flow from second enclosed section 104, draining enclosed section 104.

In some embodiments ascending portion 101 has a smaller cross-section than descending portion 102. Thereby, less mixture is required to fill ascending portion 101 before being able to start filling second enclosed section 104. Alternatively said, second enclosed portion 104 can be filled with relatively more magnetic mixture, allowing for a larger first volume VI, and thus for a larger flow rate.

In preferred embodiments 0 _r is smaller than Of. In a steeper portion of the channel, the magnetic mixture will, under the influence of gravity, flow faster. Thus, when descending portion 102 is steeper than ascending portion 101, the magnetic mixture will be drained from second enclosed section 104 at a faster rate than from first enclosed section 103. Thus, the magnetic mixture will reach level Lf in descending portion 102 faster than level L _r in ascending portion 101. When the magnetic field strength is increased back to the first average magnetic field strength before the magnetic mixture reaches level Lr in ascending portion 101, less potential energy is lost and/or it will take less time to fill the first enclosed section 104 in the next cycle, thus making it possible to pump faster and thus at a higher rate.

Additionally, a steeper descending portion 102 will reduce the backflow of the magnetic mixture through ascending portion 101, thus contributing to improved pumping performance. When using the unidirectional pump of figures 2a-b in a refrigerator as shown figure 1, it is preferred that a downstream end of the pump is at a position larger than an upstream end. Then, when passing through the loop section of the channel, the magnetic mixture can partially and/or continuously flow downwards, improving flow characteristics.

Ideally the amount of time that the magnetic field is generated with the first average magnetic field strength corresponds to the amount of time it takes to fill second enclosed portion 104. Since magnetic mixture had to be urged through ascending portion 101 first, there is a small period of time after starting to generate the magnetic field at the first average field strength in which magnetic mixtureis not being urged cross point Pl into second enclosed section 104. Therefore, maintaining the first average magnetic field strength for a longer period of time decrease, percentage wise, the amount of time in which no magnetic mixtureis urged into second enclosed section 104. However, when second enclosed section 104 is full, no more magnetic mixture is urged beyond point Pl after that, so the magnetic field strength should be lowered to drain second enclosed section 104.

Ideally the amount of time that the magnetic field is generated with the second magnetic field strength corresponds to the amount of time it takes to completely drain second enclosed portion 104. If it wasn’t completely drained before increasing the average magnetic field strength, then magnetic mixture remaining in second enclosed portion 104 reduces the amount of magnetic mixture that may be pumped from ascending portion 101, across point Pl, into descending portion 102.

Reference is made to figure 5, schematically shows another magnetocaloric refrigerator. This refrigerator includes a channel 100 and a magnetic pump according to the invention. The magnetic pump shown in figure 5, figures 6a-d, and figure 7 may be called a bi-directional pump. The magnetocaloric refrigerator shown in figure 5 furthermore comprises magnetocaloric material 10, a first heat exchanging unit 11, a second heat exchanging unit 12.

The variant of embodiment one that may be included in the refrigerator of figure 5 is shown by itself in figure 6A. Therein, it is shown that magnetic pump 1, again, comprises channel 100 and magnetic field generating unit 2. Channel 100 again comprises ascending portion 101, descending portion 102, first and second enclosed portions 103/104. In such bi-directional pumps an upstream end of ascending portion 101 may be closed off and a downstream end of descending portion 102 may be closed of as well.

In some embodiments, such as the one shown in figures 5 and 6a-c, magnetic field generating unit 2 may be implemented using two solenoids 21 and 22. When observed from left to right, solenoid 21 has an increasing number of windings and solenoid 22 has a decreasing number of windings.

When a direct current is passed through solenoid 21 , a magnetic field whose magnetic field strength increases from the left end of magnetic field generating unit 2 almost up to the right end of the magnetic field generating unit 2 (a to b in figure 6a, aO to bO in figure 7) is generated. This increase, as observed along axis A2 defined throughout channel 100, is shown in figure 8A.

Similar to the one-directional pump of figure 2a, in the bidirectional pump the magnetic mixture will experience an inward force at either end of magnetic field generating unit 2.

What is different in this embodiment is that due to the continuous increase in magnetic field strength, a component of the spatial gradient in the flow direction, as shown in figure 8B, is positive throughout almost all of the magnetic field generating unit 2.

In the one directional pump, the magnetic body force was experienced by the magnetic mixture primarily at input I, and when magnetic mixture was urged beyond input I it could only be moved further up into ascending portion 101 (and eventually, past point Pl, into second enclosed section 104) by being pushed forward by other magnetic mixture that, at that point, occupied input I. That is, to fill second enclosed section 104 all of the first enclosed section had to be filled as well.

This is not required in the pump shown in figures 6a-c and figure 7. Magnetic mixture, even when urged beyond point a, will still experience the magnetic body force described in equation 1. Therefore, in a bi-directional magnetic pump according to the present application it is possible to pump all of, or at least most of a limited amount of magnetic mixture from the left end to the right end.

Furthermore, when a direct current is passed through solenoid 22, a magnetic field whose magnetic field strength increases from the right end of magnetic field generating unit 2 almost up to the left end of magnetic field generating unit 2 (b to a in figure 6a, bO to aO in figure 7) is generated. This increase, as observed along axis A2 defined throughout channel 100, is shown in figure 9 A. Opposite to the magnetic field generated by solenoid 21, a component of the spatial gradient in the flow direction, as shown in figure 9B, is negative throughout almost all of the magnetic field generating unit 2. Hence, magnetic mixture urged beyond point b (or bO) in a direction opposite the flow direction, will be urged further through descending section 102, into ascending section 101.

The skilled person will appreciate that the flow direction as a defined in relation to the one directional pump is just a reference direction and does not necessarily indicate that that is the direction in which the magnetic mixture has to flow. Hence, as described in relation to figures 6C, 6D, 9 A, and 9B, a magnetic mixture can flow in a direction opposite the flow direction.

Relying on the magnetic field that may be generated by solenoids 21 and 22, a bidirectional pump may operate as shown in figures 6A-D.

Figure 6a shows an embodiment of a bidirectional magnetic pump 1 as included in the refrigerator of figure 5. Magnetic mixture, when at rest, may be present at a bottom end a2 of ascending portion 101, up to a level L2. Solenoid 21 is arranged and configured such that the spatial gradient is preferably positive up to the point where magnetic mixture, when at rest, is present (i.e. level L2, or point a3 in channel 100) and more preferably up to the closed upstream end a2 of ascending portion 101.

It is not essential that the magnetic body force affects all of the magnetic mixture present in closed end of ascending portion 101. If the magnetic body force urges only some upper part of the magnetic mixture in the flow direction, the remaining magnetic mixture will be pulled along thereby.

Figure 6b shows pump 1 of figure 6a in a steady state, sometime after having started generating the magnetic field having the properties shown in figures 8a and 8b. For various positions in channel 100 (indicated by circles) the magnetic body force that the magnetic mixture present at said position is indicated with arrows in figure 6b. By urging a second volume of magnetic mixture beyond point of no return P2, second enclosed section 104 is at least partially filled. Due to the inward force at position b (or bO), the second volume of magnetic mixture is kept inside of second enclosed section 104, against the force of gravity.

When magnetic field generating unit 2 stops generating this magnetic field, magnetic mixture present in the channel is again subject to just the force of gravity. Magnetic mixture present in second enclosed section 104 flows, in the flow direction, through descending portion 102, thereby draining second enclosed section 104.

Figure 6c shows the bidirectional magnetic pump 1 when magnetic mixture is at rest, and may be present at a bottom end b2 of descending portion 102, up to a level L3.

Solenoid 22 is arranged and configured such that the spatial gradient is preferably positive up to the point where magnetic mixture, when at rest, is present (i.e. level L3, or point bl) in channel 100) and more preferably up to the closed downstream end b2 of descending portion 102.

It is not essential that the magnetic body force affects all of the magnetic mixture present in closed end of descending portion 102. If the magnetic body force urges only some upper part of the magnetic mixture in the flow direction, the remaining magnetic mixture will be pulled along thereby.

Figure 6d shows pump 1 of figures 6a-c in a steady state, sometime after having started generating the magnetic field having the properties shown in figures 9a and 9b. For various positions in channel 100 (indicated by circles) the magnetic body force that the magnetic mixture present at said position is indicated with arrows in figure 6d. By urging a third volume V3 of magnetic mixture, preferably equal to the second volume V2, beyond point of no return P2, first enclosed section 103 is at least partially filled. Due to the inward force at position a (or aO), the second volume of magnetic mixture is kept inside of first enclosed section 103, against the force of gravity.

When magnetic field generating unit 2 stops generating this magnetic field, magnetic mixture present in the channel is again subject to just the force of gravity. Magnetic mixture present in first enclosed section 103 flows, in a direction opposite the flow direction, through descending portion 101, thereby draining first enclosed section 103.

Similar to the one directional pump, for this bi-directional pump the magnetic field may be completely turned off to drain second enclosed section 104 (i.e. at figure 6b) of first enclosed section 103 (i.e. at figure 6d) but this is not essential. The magnetic field strength only has to be lowered to the point where the force of gravity overcomes the inward force at position b (for second enclosed section 104 in figure 6b) or at position a (for first enclosed section 103 in figure 6d), thereby allowing the respective volumes to flow from, and thus draining the enclosed section in question.

In the previously discussed magnetic pumps magnetic field generating unit 2 was implemented by one or two solenoids. The skilled person will appreciate that the magnetic field generated in the embodiment shown in figure 2a, and by solenoid 21 in the embodiment shown in figure 5, when observed along axis A2, would look something like what is shown in 13 A. The direction of the magnetic field is substantially parallel to axis A2 and, considered from left to right, increases in strength throughout most of magnetic field generating unit 2.

The skilled person will appreciate that there may be many other implementations of a magnetic field generating unit 2 that can provide magnetic fields having a spatial gradient shown in figure 3B, 8B or 9B.

It is for example possible to implement magnetic field generating unit comprising a plurality of coil units 25A-E as shown in figure 10. Figure 10 shows an embodiment of a refrigerator, similar to the one discussed in relation to figure 5. Magnetic pump 1 may be embodiment as exemplified in the context of the refrigerator shown in figure 10, but may also be used in other applications.

Coil units 25A-E may for example be configured to each generate a magnetic field through channel 100 of approximately the same magnetic field strength. That would provide a combined magnetic field of which the spatial gradient in the flow direction is comparable to what is shown in figure 3b.

Alternatively, coil unit 25A-E may be configured, in increasing order of enumeration, to generate a magnetic field through channel 100 stronger than a preceding coil unit. That would provide a combined magnetic field of which the spatial gradient in the flow direction is comparable to what is shown in figure 8b.

Alternatively, coil unit 25A-E may be configured, in decreasing order of enumeration, generate a magnetic field through channel 100 stronger than a preceding coil unit. That would provide a combined magnetic field of which the spatial gradient in the flow direction is comparable to what is shown in figure 9b. When magnetic field generating unit 2 is implemented with these coil units 25A-E, the increase of the magnetic field strength, and thus the spatial gradient, throughout the magnetic field generating unit 2 can be controlled more accurately.

Coil units 25A-E may be implemented by two half-circle coil units 26A, each comprising a half-circle core 27A made of a magnetizable material such as a metal, and a number of windings 28A wound around half-circle core 27 A. Specifically, a first half-circle coil unit 26A’ and a second half-circle coil unit 26A” may together effectively form a complete circle as shown in figure 11B. Semi-dotted lines in figure 1 IB indicate a schematic path field lines passing through the half-circle coil units 26 A’, and 26 A” may take. The arrow in figure 11B shows a direction that the magnetic field lines may have and A2 indicates the direction of axis A2, being into figure 11B.

The skilled person will appreciate that the magnetic field at axis A2 generated by halfcircle coil units 26A’ and 26A’ ’ could look something like that is shown in figure 13B. The direction of the magnetic field is substantially perpendicular to axis A2 and, considered from left to right, increases in strength throughout most of magnetic field generating unit 2.

Coil units 25A-E may be implemented by four quarter circle coil units 26B, each comprising a quarter circle core 27B made of a magnetizable material such as a metal, and a number of windings 28B wound around quarter-circle core 27B. Four quarter-circle coil units 26B’ through 26B”” may together effectively form a complete circle as shown in figure 12B. Semi-dotted lines in figure 12B indicate a schematic path field lines passing through some two of the quarter circle coil units 26B’-26B”” may take. The arrows in figures 12B show a direction that the magnetic field lines may have. A3 and A4 indicate the direction of axis A3 and A4

The skilled person will appreciate that the magnetic field at axes A3 and A4 generated by quarter-circle coil units 26B’-26B”” could look something like that is shown in figure 13C. The direction of the magnetic field is substantially perpendicular to axes A3 and A4, go into figure 13C at axis A3 and come out of figure 13C at axis 4 A, and, considered from left to right, increases in strength throughout most of magnetic field generating unit 2.

Any of coil units 25A-E may also be implemented using other coils units, preferably some multiple of two times a ‘one-over-this-multiple-of-two’ coil unit. Like the coil units shown in figures 11 A and 12A, another example would be use of six ‘one-sixth’ coil units. The skilled person will also appreciate that in all coil units, cores, like the cores 27A and 27B, while advantageous, are not essential.

Figure 14 shows a cross-section of another embodiment of a magnetic pump. This embodiment is, theoretically, particularly efficient since most of the ascending portion 101 and most of the descending portion 102 is arranged substantially vertically. Preferably, the horizontal distance between the upper end of the ascending portion and the upper end of the descending portion is minimized. In this embodiment, the magnetic field generating unit 2 (e.g. a solenoid) makes two 90 degree angles. The skilled person may find that this is challenging, but not impossible to make.