Patent Description:
Advanced two-phase cooling devices, e.g. 3D-cooling devices, like 2D vapor chambers, and/or 2D-cooling devices, like 2D heat pipes or thermosyphons, for power electronic applications are made from a material having a high thermal conductivity, e.g. from metal, e.g. copper, and rely on using a cooling medium, e.g. water, as a working fluid for its optimal thermal properties.

<FIG> shows an example of a two-phase cooling device <NUM>, in particular a heat pipe or a 2D-vapor chamber according to the prior art. The cooling device <NUM> has a housing <NUM> enclosing a cavity <NUM>, wherein an inner wall <NUM> of the housing <NUM> surrounds the cavity <NUM>. The cavity <NUM> has a top region <NUM> and a bottom region <NUM>. In <FIG>, the cooling device <NUM> is oriented as it would be during normal usage of the cooling device <NUM>. With this "normal" orientation, the top region <NUM> is arranged above the bottom region <NUM>. A cooling medium <NUM> is in the cavity <NUM>. Further, a porous structure may be arranged within the cavity <NUM>. The porous structure may cover at least a part of the inner wall <NUM> of the housing <NUM> and may extend from the top region <NUM> to the bottom region <NUM>. The porous structure may be arranged for guiding condensed cooling medium <NUM> from the top region <NUM> to the bottom region <NUM>.

The cooling device <NUM> is symmetric with respect to a symmetry axis <NUM>. In case of the 2D-cooling device, the cooling device <NUM> is rotationally symmetric with respect to the symmetry axis <NUM>. In case of the 3D-cooling device, the cooling device <NUM> may be mirror-symmetric, wherein the symmetry axis <NUM> may be representative for a corresponding symmetry plane.

In case of e.g. water as the cooling medium <NUM>, when cooled below <NUM>, the cooling medium <NUM> freezes. While undergoing this phase change, if there is more water inside the cavity <NUM> as may be contained within the porous structure, the water accumulates in the bottom region <NUM> because of gravity and may form a liquid water pocket, i.e. a completely enclosed amount of liquid cooling medium at some point.

<FIG> shows the bottom region <NUM>, wherein the cooling medium <NUM> is present in the bottom region <NUM> as liquid cooling medium <NUM> and as frozen cooling medium <NUM>. Because of the housing <NUM> having the high thermal conductivity, the cooling medium <NUM> freezes from the peripheral region of the bottom region <NUM> towards a center of the bottom region <NUM>, wherein, in <FIG>, the center of the bottom region of <NUM> corresponds to the symmetry axis <NUM> in the bottom region <NUM>. So, if the cooling medium <NUM> freezes, firstly a pool of liquid cooling medium <NUM> is formed within the middle of the frozen cooling medium <NUM>. The liquid cooling medium <NUM>, in particular the pool, has an upper surface <NUM>. As long as there is the upper surface <NUM> of liquid cooling medium <NUM>, the expansion of the frozen cooling medium <NUM> pushes the remaining liquid cooling medium <NUM> towards the top region <NUM> and does not subject it to high pressure. So, even if there is the pool of water, the housing <NUM> would not bulge.

However, if the liquid cooling medium <NUM> begins to be surrounded by frozen cooling medium <NUM>, in particular if the upper surface <NUM> of the liquid cooling medium <NUM> is closed, the pool of liquid cooling medium <NUM> is transformed into a pocket of enclosed liquid cooling medium <NUM> and high pressures on the inner walls <NUM> of the housing <NUM> appear, which ultimately cause a bulging of the housing <NUM> and/or a leakage of the cooling medium <NUM> in the bottom region <NUM>, and as a consequence a loss of cooling performance.

<FIG> shows the bottom region <NUM> of <FIG>, wherein the cooling medium <NUM> is frozen such that the pocket of liquid cooling medium <NUM> is enclosed by the frozen cooling medium <NUM>. The pressure acting on the housing <NUM> because of the expansion of the frozen cooling medium <NUM> and the presence of the pocket of liquid cooling medium <NUM> completely within the frozen cooling medium of <NUM> is represented by two arrows illustrating the direction <NUM> of the corresponding expansion force. The bulging of the housing <NUM> may in turn cause the leakage, through which the liquid cooling medium <NUM> may leak outside of the housing <NUM>, and/or a loss of contact with the electronic component to be cooled, because of a deformed outer wall of the housing <NUM>. Both cases will result in a failure of the cooling device <NUM> and/or the electronic component to be cooled.

The correct amount of liquid cooling medium <NUM> is a tradeoff between the cooling performance and the reliability of the cooling device <NUM>. The cooling performance has its optimum with a specific predefined amount of water, wherein the reliability of the cooling device <NUM> drops, if the amount of liquid cooling medium <NUM> is more than may be accommodated by the porous structure. The manufactures of the cooling devices <NUM> tend to fill a little bit more cooling medium <NUM> into the cavity <NUM>, as needed in order to be on the safe side regarding the cooling performance of the cooling device <NUM>. However, if the cooling device <NUM> is overfilled, the above disadvantages may arise. Therefore, it is important to find out whether the correct amount of liquid cooling medium <NUM> is within the cavity <NUM>. Thus, there is a need for a method for testing a two-phase cooling device <NUM>, in particular for testing whether the cooling device <NUM> is overfilled or not.

<CIT>, which is considered as the closest prior art, describes thermal energy storage systems comprising battery assemblies containing phase change materials and a monitoring system therefor. In addition thermal stores comprising battery assemblies having integral control means for management of the thermal energy provided by the battery assembly are described.

Therefore, it is an objective of the present invention to provide a method for testing a two-phase cooling device, in particular for testing whether the cooling device is overfilled or not, which may be carried out easily, which may be carried out without the need to open the cooling device and/or to measure the amount of cooling medium within the cooling device, e.g. by removing the cooling medium and measuring its volume, and/or which is very accurate.

These objectives are achieved by the subject-matter of the independent claims. Further exemplary embodiments are evident from the dependent claims and the following description.

In particular, an objective of the present invention is achieved by a method for testing a two-phase cooling device. The cooling device has a housing surrounding a cavity and a cooling medium within the cavity. The method comprises: controlling a temperature of ambient air of the cooling device such that the cooling medium within the cavity transitions from its liquid state to its solid state and/or from its solid state to its liquid state, while monitoring a first temperature of the cooling device over a predetermined amount of time; determining whether the monitored first temperature fulfills a predetermined criterion; and determining that the cooling device is overfilled with the cooling medium if the criterion is fulfilled.

As explained above with respect to <FIG>, if the cooling device is overfilled, the pool of liquid cooling medium is formed in the bottom region. If the cooling device is cooled down to a freezing temperature of the cooling medium or below, the enclosed pocket of liquid cooling medium is formed within the frozen cooling medium. The pool as well as the pocket of liquid cooling medium result in that the cooling device has a much larger latent heat within the bottom region than in the top region. This effect can accurately be seen from the monitored first temperature at the phase transitions. In addition, measuring and monitoring the first temperature may be carried out in an easy way. Further, the method may be carried out without the need to open the cooling device and/or to measure the amount of cooling medium within the cooling device, e.g. by removing the cooling medium and measuring its volume.

Therefore, the above method for testing the two-phase cooling device, in particular for testing whether the cooling device is overfilled or not, may be carried out easily, may be carried out without the need to open the cooling device and/or to measure the amount of cooling medium within the cooling device, e.g. by removing the cooling medium and measuring its volume, and/or is very accurate.

The two-phase cooling device may be a 3D-cooling device, like a 3D vapor chamber, and/or a 2D-cooling device, like a 2D heat pipe or thermosyphon. The housing may comprise or may be made of a metal having a high thermal conductivity, e.g. copper. During normal usage of the two-phase cooling device, the top region is arranged above the bottom region. The cooling medium may for example be water. The predetermined criterion may be a predetermined temperature pattern over time. The first temperature of the cooling device may be monitored by receiving a signal from a first sensor, which may be arranged at the outside of the housing in thermal contact with the housing, which may be thermally isolated against the ambient air, and which may be arranged at the bottom region.

If the predetermined criterion is the predetermined temperature pattern, a neuronal network may be trained and used to differentiate temperature patterns of cooling devices, which are overfilled, from temperature patterns of cooling devices, which are not overfilled. The training may be carried out with the help of a training set of temperature patterns of correspondingly overfilled and properly filled cooling devices.

According to an embodiment, it is determined whether the monitored first temperature fulfills the at least one predetermined criterion by determining at least one graph, which represents the monitored first temperature over the time, during which the first temperature is monitored, and determining whether the graph fulfills the predetermined criterion. For example, if the first temperature is measured for a give duration, e.g. during at least one of the phase transitions, the first temperature may not be measured continuously but at regular time intervals, e.g. at time intervals of a view seconds, e.g. every <NUM> or <NUM> seconds. Then, a trendline or average line may be determined from the measured temperature values, wherein the graph may correspond to the determined trendline or, respectively, average line. The graph enables in an easy way to see the temperature behaviour of the cooling device on the first side. Further, the graph enables an easy mathematical analysis of the temperature behaviour of the cooling device. The graph may be composed of a plurality of temperature measurements at consecutive time points.

According to an embodiment, the graph of the monitored first temperature comprises a first section with falling temperature indicating an supercooling of the cooling medium in its liquid state inside the cooling device, after that the graph comprises a jump to a freezing temperature of the cooling medium indicating the start of freezing of the cooling medium, after that the graph comprises a second section at the freezing temperature of the cooling medium indicating the freezing of the cooling medium, and after that the graph comprises a third section with falling temperature; wherein the predetermined criterion is searched in the third section. Searching the predetermined criterion in the third section only enables to ignore measured temperature values outside of the third section or enables to only measure the first temperature within the third section. In both cases the amount of measured temperature values is decreased. Therefore, a smaller amount of data has to be handled and analysed. So, searching for the predetermined criterion in the third section only contributes to the simple and easy testing of the cooling device.

According to an embodiment, it is determined whether the graph fulfills the predetermined criterion by determining a first derivative of the graph; and determining an amount of extreme values of the derivate of the graph during one of the phase transitions and/or within the third section; wherein the predetermined criterion is fulfilled, if the derivate of the signal has at least two extreme values. The positions of the extreme values of the derivative correspond to the positions of turning points of the original graph. The extreme values each may be a local maximum or a local minimum of the derivative, and/or may be referred to as "peak". In an optimally filled cooling device the cooling medium is absorbed within the porous structure having a relatively even thickness and porosity. Therefore, the cooling medium is evenly distributed over the cavity and the phase transition also takes places quite evenly and the derivate of the graph normally has only one extreme value. In contrast, if the cooling device is overfilled, firstly the above described pool and then the above described pocket of liquid cooling medium are formed. In this case, the cooling medium is not evenly distributed over the cavity and correspondingly the phase transition of the cooling medium also does not take place evenly. This effect is reflected in the graph and may be seen quite easily from the derivative of the graph having the at least two extreme values. So, determining the first derivative of the graph and counting the amount of extreme values may contribute to accurately and easily find out, whether the cooling device is overfilled or not.

According to an embodiment, it is determined whether the graph fulfills the predetermined criterion by determining a width of the graph during at least one of the phase transitions of the cooling medium and/or within the third section; wherein the predetermined criterion is fulfilled, if the determined width is larger than a predetermined width threshold. As explained above, in an optimally filled cooling device the cooling medium is evenly distributed over the cavity. Therefore, a latent heat density along the cooling device is relatively constant. During cooling of the cooling device by ambient air convection, the heat flux is relatively evenly distributed across the surface of the housing of the cooling device also. In contrast, because of the formation of the pool and the pocket within an overfilled cooling device the cooling device has a much larger latent heat within the bottom region, where the pool and the pocket are formed. Therefore, during cooling or heating across the freezing temperature of the cooling medium, even when the phase transition is already complete for the cooling medium in the porous structure, the phase transition continues in the bottom region of the cooling device. The more excess cooling device is collected in the bottom region, the longer take the phase transitions. So, overfilled cooling devices have a tendency to go along with a long phase transitions, during freezing as well as during melting. The duration of the phase transition is reflected in the width of the graph showing the phase transition. So, determining the width of the graph during at least one of the phase transitions of the cooling medium and/or within the third section, and determining that the predetermined criterion is fulfilled, if the determined width is larger than a predetermined width threshold, contributes to accurately and easily determining whether the cooling device is overfilled or not.

According to an embodiment, the first temperature is sensed at the bottom region of the housing, the method further comprising: monitoring a second temperature of the cooling device, wherein the second temperature is sensed at an upper part of the housing, while monitoring the first temperature; and determining a difference between the monitored first and second temperature during at least one of the phase transitions of the cooling medium and/or within the third section; wherein the predetermined criterion is fulfilled, if the determined difference is larger than a predetermined difference threshold. As explained above, in an optimally filled cooling device the latent heat density along the cooling device is relatively constant and the heat flux is relatively evenly distributed across the surface of the housing of the cooling device also. Therefore, the temperature along the cooling device is relatively constant and a difference between the first temperature and the second temperature would be quite small. In contrast, because of the formation of the pool and the pocket within an overfilled cooling device, the cooling device has a much larger latent heat within the bottom region than in the top region. Therefore, with an overfilled cooling device, the difference between the first temperature and the second temperature is quite big. So, it may be determined that the cooling device is overfilled, if the difference is larger than the predetermined difference threshold.

According to an embodiment, the first temperature is monitored for a predetermined amount of time. For example, the first temperature may be monitored from the beginning of the test, when the cooling of the cooling device starts, to the end of the test, when the cooling device has its initial temperature again. Alternatively, the first temperature may be monitored only from the beginning of the freezing until the ending of the melting of the cooling medium. Alternatively, the first temperature may be monitored only from the beginning of the freezing until the ending of the freezing of the cooling medium. Alternatively the first temperature may be monitored only from the beginning of the melting until the ending of the melting of the cooling medium. If it is determined whether the cooling device is overfilled or not by counting the amount of extreme values of the first derivative or by analysing the width of the graph, it is advantageous, if the first temperature is only monitored for one of the phase transitions or if the first temperature is monitored all over the test but only those values are analysed which are monitored during one of the phase transitions. If it is determined whether the cooling device is overfilled or not by counting the amount of extreme values of the first derivative, the criterion of having at least two extreme values refers to one of the phase transitions only. In other words, when analysing the first derivative of the graph during both phase transitions the corresponding derivative must have at least four extreme values in order to being able to classify the cooling device is being overfilled.

According to an embodiment, the method further comprises arranging the cooling device in a temperature chamber, which encloses the ambient air and which enables the control of the temperature of the ambient air, prior to controlling the temperature of the ambient air.

According to an embodiment, the method further comprises arranging the first sensor at the housing such that the first sensor has the thermal contact to the housing and is thermally isolated against the ambient air, prior to controlling the temperature of the ambient air.

According to an embodiment, the first sensor is arranged at a bottom region of the housing. The effects of the overfilling of the cooling device may be seen at best within the bottom region, because of the formation of the pocket and the pool. Therefore, the effects on the first temperature are the strongest within the bottom region. Therefore, arranging the first sensor at the bottom region and correspondingly sensing the first temperature in the bottom region may contribute to a very easy and accurate determination of whether the cooling device is overfilled or not.

The above features, advantages and/or effects of the method for testing the two-phase cooling device may be transferred to the controller for testing the two-phase cooling device, the computer program for operating the controller, and/or to the computer-readable medium on which the computer program is stored, as explained in the following. Therefore, a repetition of these features, advantages and/or effects is omitted and it is referred to the above explanations only in order to provide a concise description.

An objective of the present invention is achieved by the controller for testing the two-phase cooling device. The controller comprises a processor and a memory. The processor is configured to carry out the above method.

An objective of the present invention is achieved by the computer program for operating the controller, which, when being executed by a processor, is adapted for performing the above method.

An objective of the present invention is achieved by the computer-readable medium on which the above computer program is stored.

The subject matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings.

<FIG> schematically shows a two-phase cooling device <NUM> according to the prior art. The cooling device <NUM> may be a 3D-cooling device, e.g. a 3D vapor chamber, or a 2D-cooling device, e.g. a 2D heat pipe or a thermosyphon. The cooling device <NUM> may be used for cooling an electronic component (not shown). The electronic component may be an electronic circuit, an integrated circuit, a chip, a transformer, etc..

The cooling device <NUM> comprises a housing <NUM>. The housing <NUM> may comprise or may be made of a material having a high thermal conductivity, e.g. a metal, e.g. copper. The housing <NUM> encloses a cavity <NUM>. The cavity <NUM> is surrounded by an inner wall <NUM> of the housing <NUM>. In <FIG>, the cooling device <NUM> is orientated as it would be during normal usage, i.e. when cooling the electronic component. With this "normal" orientation, a top region <NUM> of the cooling device <NUM> is above a bottom region <NUM> of the cooling device <NUM>. A cooling medium <NUM>, e.g. water, is arranged within the cavity <NUM>. Further, the cooling device <NUM> may comprise a porous structure (not shown), which may be arranged at the inner wall <NUM> and/or which may be arranged all over the cavity <NUM>. The cooling device <NUM> is symmetric with respect to a symmetry axis <NUM>. In case of the 2D-cooling device, the cooling device <NUM> may be rotationally symmetric with respect to the symmetry axis <NUM>. In case of the 3D-cooling device, the cooling device <NUM> may be mirror-symmetric, wherein the symmetry axis <NUM> may be representative for a corresponding symmetry plane.

Prior to the use of the cooling device <NUM>, the cooling medium <NUM> is in a liquid state. During normal usage of the cooling device <NUM>, the cooling device <NUM> has thermal contact to the electronic component to be cooled at least in the bottom region <NUM> and absorbs at least in part the thermal energy generated by the operation of the electronic component. If the cooling device <NUM> absorbs sufficient thermal energy from the electronic component, the cooling medium <NUM> transitions at least in part from the liquid state to a gaseous state. The cooling medium <NUM> in the gaseous state rises to the top region <NUM> and condenses in the top region <NUM>. The condensed cooling medium <NUM> is guided by the porous structure from the top region <NUM> to the bottom region <NUM>. If all of the cooling medium <NUM> is in the liquid state, at least a part of the cooling medium <NUM> is absorbed within the porous structure. If there is more liquid cooling medium <NUM> in the cavity <NUM> than may be absorbed by the porous structure, the rest of the cooling medium <NUM> accumulates within the bottom region <NUM>.

<FIG> schematically shows a detailed view of a two-phase cooling device <NUM> according to the prior art in a first state, e.g. the cooling device <NUM> according to <FIG>. In the first state, a part of the cooling medium <NUM> is frozen and the rest of the cooling medium <NUM> is still liquid. In particular, a pool of liquid cooling medium <NUM> having an upper surface <NUM> is partly embedded within frozen cooling medium <NUM>. If the freezing process proceeds, the liquid cooling medium <NUM> may be pressed upwards by an expansion of the frozen cooling medium <NUM> as long as the upper surface <NUM> of the pool is open and as such exists. However, because of the higher thermal conductivity of the material of the housing <NUM>, the upper surface <NUM> of the pool may be closed by the frozen cooling medium <NUM> and as such may disappear.

<FIG> schematically shows the two-phase cooling device <NUM> according to <FIG> in a second state. In the second state, a pocket of a liquid cooling medium <NUM> is formed. The pocket of liquid cooling medium <NUM> is completely enclosed by the frozen cooling medium <NUM>. If the freezing process proceeds further, the frozen cooling medium <NUM> increases and expands further, but the liquid cooling medium <NUM> cannot be pressed upwards, because the pocket is closed. Then, an expansion force is created, which acts on the inner wall <NUM> of the housing <NUM> in a direction <NUM> pointing outward. This expansion force may lead to a bulging of the housing <NUM> and may, if the bulging proceeds, lead to a leakage in the housing <NUM>.

The formation of the pocket of liquid cooling medium <NUM> and the associated bulging of the housing <NUM> and in case the leakage of the housing <NUM> may be avoided, if only those cooling devices <NUM> are used for applications at or below the freezing temperature of the cooling medium <NUM>, which are not overfilled and/or in which the whole liquid cooling medium <NUM> may be absorbed by the porous structure. In this context it should be noted that a very small amount of excess cooling medium, i.e. more liquid cooling medium <NUM> than may be absorbed by the porous structure, may be tolerable. The corresponding cooling device <NUM> may be classified as not being overfilled, because such a very small amount of excess cooling medium does not harm the cooling device <NUM>, if the cooling medium <NUM> freezes, because of the elasticity of the material of the housing <NUM> of the cooling device <NUM>. For example, a cooling device <NUM> having an amount of <NUM>% to <NUM>%, e.g. <NUM>% to <NUM>%, e.g. <NUM>% of excess of the cooling medium <NUM> still may be regarded as not being overfilled. In contrast, this additional amount may be provided in order to guarantee the expected cooling performance without damaging the housing <NUM> of the cooling device <NUM>.

The inventors of the present invention have recognized that a temperature behaviour of the cooling device <NUM> during a phase transition of the cooling medium <NUM> from its liquid state to its frozen state or vice versa may be representative for whether the cooling device <NUM> is overfilled or not.

<FIG> schematically shows an exemplary embodiment of a two-phase cooling device <NUM> in a test situation, according to the present invention. The cooling device <NUM> may correspond to the above cooling device <NUM>. The cooling device <NUM> in the test situation may be arranged in the temperature chamber. The temperature chamber comprises ambient air, wherein the temperature of the ambient air may be controlled. In particular, the temperature may be controlled such that the ambient air and the cooling device <NUM> may be cooled below the freezing temperature of the cooling medium <NUM>.

A first sensor <NUM> for measuring a first temperature of the cooling device <NUM> may be arranged at an outer circumference of the housing <NUM>. In particular, the first sensor <NUM> may be arranged at the housing <NUM> in the bottom region <NUM>. The first sensor <NUM> may be in thermal and/or direct contact with the housing <NUM>. For example, the first sensor <NUM> may be fixed to the housing <NUM> by a thin layer of a thermally conductive glue. The first sensor <NUM> may be thermally isolated against the ambient air by a first insulation <NUM>.

Optionally, a second sensor <NUM> for measuring a second temperature of the cooling device <NUM> may be arranged at an outer circumference of the housing <NUM>. In particular, the second sensor <NUM> may be arranged at the housing <NUM> above the first sensor <NUM>, e.g. in the top region <NUM>. The second sensor <NUM> may be in thermal and/or direct contact with the housing <NUM>. For example, the second sensor <NUM> may be fixed to the housing <NUM> by a thin layer of a thermally conductive glue. The second sensor <NUM> may be thermally isolated against the ambient air by a second insulation <NUM>.

The first and in case the second sensor <NUM>, <NUM> may be electrically coupled to a controller <NUM>. The controller <NUM> may be arranged for receiving a first signal from the first sensor <NUM> and in case a second signal of the second sensor <NUM>. The first signal may be representative for the first temperature. The second signal may be representative for a second temperature. The controller <NUM> may be configured to monitor the first and/or second temperature over the predetermined amount of time, e.g. during the whole test, during a transition of the cooling medium from its liquid state to its solid state and afterwards from its solid state to its liquid state, or during one of these phase transitions only. Further, the controller <NUM> may be configured to analyse the monitored first and/or second temperature regarding whether the cooling device <NUM> is overfilled or not. In other words, the controller <NUM> may be configured to carry out a method for testing the cooling device <NUM>, in particular regarding whether the cooling device <NUM> is overfilled or not. Furthermore, the controller <NUM> may be configured to control the temperature of the ambient air in the temperature chamber. The controller <NUM> may comprise a processor and a memory. The controller <NUM> may be regarded as and/or may be coupled to a computer. If the controller <NUM> is coupled to a computer, one or more of the above tasks of the controller <NUM> may be outsourced to the computer.

<FIG> shows a first diagram comprising several graphs, which represent temperatures of the cooling devices <NUM> of the same type, which have different amounts of cooling medium <NUM>, during freezing and melting of the corresponding cooling medium <NUM>. The cooling devices <NUM> may correspond to the above cooling device <NUM>. For example, the cooling devices <NUM> are arranged in the above temperature chamber, in which the temperature of the ambient air may be controlled.

The cooling devices <NUM> may be cooled from <NUM> to -<NUM> starting at t=<NUM>. The temperatures of the ambient air and of the cooling devices <NUM> are represented by corresponding graphs within the first diagram. According to <FIG>, the temperature of the ambient air drops from <NUM> to -<NUM> within the first five minutes. All other graphs, representing the temperatures of the cooling devices <NUM>, roughly show a similar behaviour. In particular, all temperatures except for the temperature of the ambient air drop down to -<NUM> to -<NUM> in first sections of the corresponding graphs. This behaviour may be called supercooling effect and takes place before the cooling medium <NUM> starts freezing. The amount of supercooling does not correlate with the amount of the cooling medium <NUM> within the cooling devices <NUM> and may slightly vary for different test cycles.

At some moment during cooling, in <FIG> at about t=<NUM>, the freezing of the cooling medium <NUM> starts and the temperatures sensed by the sensors <NUM>, <NUM> simultaneously jump up to <NUM>. This jump is followed by a second section of the graphs, which represents a period of evenly freezing of the cooling medium <NUM> and during which the temperatures remain close to <NUM>.

After this second section, in <FIG> between t=<NUM> and t=<NUM>, the temperatures start going down again in a third section. At the beginning of the third section, freezing is complete in the vicinity of the first sensor <NUM> but not yet complete in the bottom region <NUM>, where there is the pool of liquid cooling medium <NUM>. The housing <NUM>, which normally has a high thermal conductivity and/or which for example may be made from copper, provides a thermal link between the firs sensor <NUM> and the freezing liquid cooling medium <NUM> at the bottom. That is why the temperature measured by the first sensor <NUM> never reaches the ambient temperature as long as some cooling medium <NUM> is in its liquid state at the bottom. After the third section, the freezing is completed everywhere within the cavity <NUM>, and there is no more liquid cooling medium <NUM> in the cavity <NUM>, signalling the completion of freezing. The curves of temperatures within this third section strongly vary depending on the amount of cooling medium <NUM> within the cooling devices <NUM>, if the cooling devices <NUM> are overfilled. Therefore, this third section may be used for determining whether the cooling devices <NUM> are overfilled or not, as explained below.

After the third section, the temperature is held at below -<NUM> for a predetermined duration, in <FIG> until about t=<NUM>. Then, the temperature of the ambient air is raised to <NUM>. All temperatures show an inverse but similar behaviour during this heating process as they showed during cooling, except for the supercooling effect. In particular, the temperature of the ambient air jumps to <NUM> within <NUM> (t=<NUM>). The temperatures of the cooling devices <NUM> rise quite evenly until <NUM>, corresponding to the first section of the cooling process.

Then, the melting of the cooling medium <NUM> starts and the temperatures stay around <NUM> for about <NUM> to <NUM> minutes, corresponding to the second section of the cooling process.

Afterwards, in <FIG> between about t=<NUM> and t=<NUM>, the temperatures start going up again, corresponding to the third section, signalling the completion of melting, but not simultaneously. The curse of temperatures within this section strongly varies depending on the amount of cooling medium <NUM> within the cooling devices <NUM>, if the cooling devices <NUM> are overfilled. Therefore, this section of the melting process, corresponding to the third section of the cooling process may be used for determining whether the cooling devices <NUM> are overfilled or not, as explained below. After this section, the test may be stopped and the results may be evaluated.

<FIG> shows a second and a third diagram each comprising several graphs, which represent temperatures of the same cooling devices <NUM> having different amounts of cooling medium <NUM> during freezing of the cooling medium. In particular, the second diagram shown in <FIG> above the third diagram is a detailed view of the temperature graphs of an overfilled cooling device <NUM>, i.e. a first cooling device S1, shown in <FIG>. The third diagram is a detailed view of the temperature graphs of a properly filled cooling device <NUM>, i.e. a second cooling device S2, shown in <FIG>. These detailed views refer to the cooling process only. The detailed views of the melting process are shown in <FIG>.

The second as well as the third diagram show graphs of an ambient temperature T_amb within the temperature chamber, an internal temperature T_in within the corresponding cooling device <NUM>, and first, second, third, and fourth temperatures T1, T2, T3, T4 measured with sensors at the housing <NUM> of the corresponding cooling device <NUM>. In particular, the first temperature T1 and the second temperature T2 may be sensed by the corresponding first sensors <NUM> and, respectively, the corresponding second sensors <NUM>. The third and the fourth temperature T3, T4 may be sensed with corresponding further sensors (not shown) attached at the housings <NUM> in the top regions <NUM> with increasing distance to the corresponding bottom regions <NUM>.

Firstly, the cooling devices <NUM> may be cooled from <NUM> to -<NUM> starting at t=<NUM>, as explained above. In both diagrams, the ambient temperature T_amb drops to -<NUM> within the first five minutes. All other temperatures T_in, T1, T2, T3, T4 drop down to -<NUM> to - <NUM> in first sections of both diagrams due to the above supercooling effect.

At about t=<NUM>, the freezing of the cooling medium <NUM> starts and the temperatures T_in, T1, T2, T3, T4 simultaneously jump up to <NUM>. This jump is followed by the second sections of the graphs, which represents the period of evenly freezing of the cooling medium <NUM> and during which the temperatures remain close to <NUM>. In the second diagram, representing the temperatures of the overfilled cooling device <NUM>, the duration of the second section varies depending on position of the corresponding sensor <NUM>, <NUM> at the housing <NUM>. This is because the cooling medium <NUM> finishes freezing first within the top region <NUM> of the cooling devices <NUM>, in which the cooling medium is evenly distributed within the porous structure. In contrast, within the bottom region <NUM>, where the additional cooling medium <NUM> is accumulated, e.g. with the pool, the freezing finishes last. As may be seen from a comparison of the second and third diagrams, this difference is significant only for the overfilled cooling device <NUM> represented by the second diagram. In contrast, for optimally charged cooling device <NUM> represented by the third diagram, the freezing completes almost simultaneously along the whole height of the cooling device <NUM>, except for the very top at which the fourth temperature T4 is measured.

Similarly, within the third sections, in which the temperatures T_in, T1, T2, T3, T4 drop down again, there is a significant difference between the properly filled cooling device <NUM> represented by the third diagram and the overfilled cooling device <NUM> represented by the second diagram. In particular, in the third diagram, the temperatures T_in, T1, T2, T3, T4 drop down quite evenly. In contrast, in the second diagram, the first to fourth temperatures T1, T2, T3, T4 measured at the housing <NUM> of the corresponding cooling device <NUM> drop down unevenly, wherein the corresponding graphs show at least two turning points.

A likely explanation of the observed differences between the differently filled cooling devices <NUM> is the following: In an optimally filled cooling device <NUM> the liquid cooling medium <NUM> is basically absorbed within the porous structure having a relatively even thickness and porosity. Therefore, a latent heat density along the cooling device <NUM> is relatively constant. During the cooling of the cooling device <NUM> by the ambient air convection, the heat flux is relatively evenly distributed across an outer surface of the cooling device <NUM>. Therefore, the freezing of the liquid cooling medium <NUM> or the melting of the frozen cooling medium <NUM> (explained with respect to <FIG>) within the porous structure begins and ends approximately simultaneously. In contrast, liquid cooling medium <NUM> within the vertically oriented overfilled cooling device <NUM> forms the pool or the pocket of liquid cooling medium <NUM> within the bottom region <NUM>. Therefore, the cooling device <NUM> has a much larger latent heat within the bottom region <NUM> compared to the top region <NUM> of the same cooling device <NUM>. During the cooling or heating across <NUM>, even if the phase transition is already completed for cooling medium <NUM> within the porous structure, the freezing/melting continues within the bottom region <NUM> of the cooling device <NUM>. Therefore and due to high thermal conductivity of the walls and the porous structure of the cooling device <NUM>, the temperature of the wall cannot change freely until the pool and/or the pocket within the bottom region <NUM> is completely frozen or, respectively, melted. This leads to the broadening of the third sections, in particular of the graphs within the third sections, to the two or more turning points of the graphs within the third sections, and to the differences of the graphs of the temperatures T1, T2, T3, T4 measured by the corresponding sensors <NUM>, <NUM> at the outside of the housing <NUM> of the cooling devices <NUM> within the third sections. The more excess liquid cooling medium <NUM> is collected within the bottom region <NUM>, the broader are the third sections showing the freezing transitions, and the broader are the melting transitions also.

So, it may be determined whether one of the cooling devices <NUM> is overfilled or not by analysing the width of the first to fourth temperature graphs T1, T2, T3, T4, the differences of the graphs of the temperatures T1, T2, T3, T4, and/or the amount of turning points of the graphs, each within the third section. For example, a width threshold may be determined in advance such that it may be determined that the cooling device <NUM> is overfilled, if the width of at least one of its temperature graphs within the third section is larger than the predetermined width threshold. Alternatively or additionally, a difference threshold may be determined in advance such that it may be determined that the cooling device <NUM> is overfilled, if a difference between the temperatures measured in the top region <NUM>, e.g. the fourth temperature T4, and the temperatures measured in the bottom region <NUM>, e.g. the first temperature T1, within the third section is larger than the predetermined difference threshold. Alternatively or additionally, the amount of turning points of at least one of the temperature graphs corresponding to the temperature in the bottom region <NUM>, e.g. the first temperature T1, may be determined and it may be determined that the cooling device <NUM> is overfilled, if there are two or more turning points of the corresponding temperature graph within the third section. The turning points may be determined directly or may be derived from the first derivative of the corresponding temperature graph, wherein the positions of local extreme values of the derivative correspond to the positions of the turning points of the original graph.

<FIG> shows a fourth and a fifth diagram each comprising several graphs, which represent temperatures of the same cooling device <NUM> having different amounts of cooling medium <NUM> during melting of the cooling medium <NUM>. In particular, the fourth diagram shown in <FIG> above the fifth diagram is a detailed view of the temperature graphs of the overfilled cooling device <NUM> shown in <FIG>. The fifth diagram is a detailed view of the temperature graphs of the properly filled cooling device <NUM> shown in <FIG>. These detailed views refer to the melting process only. The detailed views of the freezing process are shown above in <FIG>. The different sections of the graphs during the melting process widely correspond to the inverse freezing process. In particular, the graphs of the first to fourth temperatures T1, T2, T3, T4 show the same uneven behaviour in the section corresponding to the third section, i.e. between about t=<NUM> and about t=<NUM>, as they show during freezing. Therefore, it may be determined whether the cooling device <NUM> is overfilled or not by analysing the temperatures, in particular the graphs representing the temperatures, during the melting process, in accordance with the above determination during the freezing process.

So, it may be determined whether one of the cooling devices <NUM> is overfilled or not by analysing the width of the first to fourth temperature graphs T1, T2, T3, T4, the differences of the graphs of the temperatures T1, T2, T3, T4, and/or the amount of turning points of the graphs, each within the "melting" section corresponding to the third section during freezing. For example, a width threshold may be determined in advance such that it may be determined that the cooling device <NUM> is overfilled, if the width of at least one of its temperature graphs within the corresponding melting section is larger than the predetermined width threshold. Alternatively or additionally, a difference threshold may be determined in advance such that it may be determined that the cooling device <NUM> is overfilled, if a difference between the temperatures measured in the top region <NUM>, e.g. the fourth temperature T4, and the temperatures measured in the bottom region <NUM> within the corresponding melting section is larger than the predetermined difference threshold. Alternatively or additionally, the amount of turning points of at least one of the temperature graphs corresponding to the temperature in the bottom region <NUM>, e.g. the first temperature T1, may be determined and it may be determined that the cooling device <NUM> is overfilled, if there are two or more turning points of the corresponding temperature graph within the corresponding melting section. The turning points may be determined directly or may be derived from the first derivative of the corresponding temperature graph, wherein the positions of local extreme values of the derivative correspond to the positions of the turning points of the original graph.

<FIG> shows a sixth diagram comprising several graphs, which represent temperatures of different cooling devices <NUM> having different amounts of cooling medium <NUM> during freezing. In particular, the first cooling device S1 is overfilled, the second cooling device S2 has the proper amount of cooling medium <NUM>, and a third cooling device S3 has an amount of cooling medium <NUM> as provided from the manufacturer of the cooling devices <NUM>. The cooling devices S1, S2, S3 correspond to the cooling device <NUM> and only differ from each other by the contained amount of cooling medium <NUM>. As may be seen from <FIG>, only the temperature graph of the overfilled first cooling device S1 shows the broad phase transition and the two turning points within the third section.

<FIG> shows a seventh diagram comprising first derivatives of the graphs representative for the temperatures of the different cooling devices <NUM> according to <FIG>. As may be seen from <FIG>, only the derivative of the graph of the overfilled first cooling device S1 shows the two extreme values, in this case local minima, within the third section.

<FIG> shows an eighth diagram comprising several graphs, which represent temperatures of different cooling devices <NUM> having different amounts of cooling medium <NUM> during melting. The first to third cooling devices S1, S2, S3 correspond to the above first to third cooling devices S1, S2, S3. As may be seen from <FIG>, only the temperature graph of the overfilled first cooling device S1 shows the broad phase transition and the two turning points within the third section.

<FIG> shows a nineth diagram comprising first derivatives of the graphs representative for the temperatures of the different cooling devices <NUM> according to <FIG>. As may be seen from <FIG>, only the derivative of the graph of the overfilled first cooling device S1 shows the two extreme values, in this case local maxima, within the third section.

The first to fourth temperature graphs T1, T2, T3, T4 and their derivatives shown in <FIG> correspond to different temperature patterns. These patterns vary depending on whether the corresponding cooling device <NUM> is overfilled or not and on the position of the housing <NUM> at which the corresponding temperature is measured. Therefore, all temperature graphs of overfilled cooling devices <NUM> with the corresponding temperature not being measured at the top or close to the top of the corresponding cooling device <NUM> show the same or at least a similar temperature pattern. A template of the temperature pattern may be determined in advanced and may be used for differentiating temperature patterns of overfilled cooling devices <NUM> from temperature patterns of properly filled cooling devices <NUM>, e.g. by a neural network. The predetermined temperature pattern may comprise the width of the corresponding graph being larger than the above width threshold and/or the two or more turning points, each within the third section.

<FIG> shows a flow chart of an exemplary embodiment of a method for testing a two-phase cooling device, according to the present invention, in particular for testing one of the above cooling devices <NUM>, S1, S2, S3. The method may be used for testing whether the corresponding cooling device <NUM> is overfilled by the cooling medium <NUM> or not. It is noted in this context, that in some cases an overfilling up to about <NUM>% may be regarded as not being critical and/or as not being overfilled, because in these cases the pool and/or the pocket of liquid cooling medium <NUM>, which is formed in the <NUM>% overfilled cooling device <NUM>, do not harm the cooling device <NUM> during freezing. For example, the elasticity of the material of the housing <NUM> may allow a slight deformation caused by the pool and/or pocket of the housing such that the corresponding disadvantages and/or damages may not occur.

In optional step A2, the temperature sensors, e.g. the first and second sensor <NUM>, <NUM> are attached to the housing <NUM> of the cooling device <NUM> to be tested, e.g. as explained above with respect to <FIG>. If the test is carried out by analysing the width of the temperature graphs and/or the amount of turning points of the graphs or the extreme values of the corresponding derivates, the first sensor <NUM> is sufficient for carrying out the method. If the test is carried out by analysing the differences of the temperatures measured at different positions of the housing <NUM>, the second sensor <NUM> is needed for carrying out the method.

In optional step A4, the cooling device <NUM> comprising the sensor(s) <NUM>, <NUM> may be arranged in the temperature chamber. In particular, the cooling device <NUM> is arranged vertically, as it would be during normal usage and as it is shown with respect to <FIG>.

In step A6, the temperature of the ambient air, e.g. the above ambient temperature T_amb, is controlled. In particular, the temperature of the ambient air may be controlled such that the cooling medium <NUM> within the cooling device <NUM> to be tested undergoes a phase transition from its liquid state to its solid state and afterwards from its solid state to its liquid state, from its liquid state to its solid state only, or from its solid state to its liquid state only. In the following, it is assumed, that only the phase transition from the liquid state to the solid state is monitored and analysed for testing the cooling device <NUM>. However, the method easily may be extended to monitoring and analysing both phase transitions, or only the phase transition from the solid state to the liquid state. For example, in case of water as the cooling medium <NUM>, the ambient temperature T_amb may be controlled such that it crosses <NUM>, e.g. from <NUM> to -<NUM>, e.g. by the controller <NUM>.

In step A8, the temperature of the cooling device <NUM> may be monitored, e.g. the first temperature T1. Additionally, another temperature of the cooling device <NUM> may be monitored, e.g. the second, the third, and/or the fourth temperature T2, T3, T4. In particular, the temperature(s) may be monitored during the third section, which corresponds to the freezing section of the corresponding graph.

In optional step A10, the graph(s) representing the monitored temperature(s) may be determined. For example, the graph representing the first temperature T1 may be determined.

In step A12, it is determined whether the monitored temperature fulfills the predetermined criterion. The different options for determining, whether the monitored temperature fulfills the predetermined criterion or not, are explained below with respect to <FIG>, and <FIG>. If the predetermined criterion is fulfilled, the method proceeds in step A14. If the predetermined criterion is not fulfilled, the method proceeds in step A16.

In step A14, it is determined that the cooling device <NUM> under test is overfilled.

In step A16, it is determined that the cooling device <NUM> under test is not overfilled.

<FIG> shows a flow chart of an exemplary embodiment of a method for determining whether the predetermined criterion is fulfilled, according to the present invention. The method of <FIG> may be implemented, e.g. as a subroutine, in the method explained with respect to <FIG>. In particular, the method of <FIG> may be carried out as the step A12 of the method of <FIG>.

In step A20, the derivative of the graph, which has been determined in step A10 of the method of <FIG>, may be determined, e.g. by a corresponding mathematical application.

In step A22, the amount of extreme values of the derivative is determined, e.g. within the duration of at least one of the phase transitions, e.g. within the third section.

In step A24, it is determined whether the amount of extreme values is <NUM> or more, in case the extreme values are determined for one phase transition only. If the amount of extreme values is <NUM> or more, the method proceeds in step A26. If the amount of extreme values is less than <NUM>, the method proceeds in step A28. Alternatively, if both phase transitions are monitored, it is determined whether the amount of extreme values is <NUM> or more.

In step A26, it is determined that the predetermined criterion is fulfilled.

In step A28, it is determined that the predetermined criterion is not fulfilled.

In step A30, the width of the graph, which has been determined in step A10 of the method of <FIG>, during at least one of the phase transitions, e.g. within the third section, is determined.

In step A32, it is determined whether the width of the graph is larger than the predetermined width threshold. If the width of the graph is larger than the predetermined width threshold, the method proceeds in step A34. If the width of the graph is not larger than the predetermined width threshold, the method proceeds in step A36.

In step A34, it is determined that the predetermined criterion is fulfilled.

In step A36, it is determined that the predetermined criterion is not fulfilled.

In step A40, the second temperature T2, which has been sensed by the second sensor <NUM> in the top region <NUM>, may be monitored.

In step A42, the difference between the monitored first and second temperature T1, T2 is determined, e.g. within the duration of at least one of the phase transitions, e.g. within the third section.

In step A44, it is determined whether the determined difference is larger than the predetermined difference threshold. If the determined difference is larger than the predetermined difference threshold, the method proceeds in step A46. If the determined difference is not larger than the predetermined difference threshold, the method proceeds in step A48.

In step A46, it is determined that the predetermined criterion is fulfilled.

In step A48, it is determined that the predetermined criterion is not fulfilled.

Claim 1:
Method for testing a two-phase cooling device (<NUM>), the cooling device (<NUM>) having a housing (<NUM>) surrounding a cavity (<NUM>) and a cooling medium (<NUM>) within the cavity (<NUM>), the method comprising:
controlling a temperature of ambient air of the cooling device (<NUM>) such that the cooling medium (<NUM>) within the cavity (<NUM>) transitions from its liquid state to its solid state and/or from its solid state to its liquid state, while monitoring a first temperature (T1) of the cooling device (<NUM>) over a predetermined amount of time;
determining whether the monitored first temperature (T1) fulfills a predetermined criterion; and
determining that the cooling device (<NUM>) is overfilled with the cooling medium (<NUM>) if the predetermined criterion is fulfilled.