WO2009089901A1

WO2009089901A1 - Apparatus and method for determining the thermal conductivity of a fluid

Info

Publication number: WO2009089901A1
Application number: PCT/EP2008/011115
Authority: WO
Inventors: Uwe Pasera; Harald Teves; Simon Steinhübl; Mathias Zilly; Karl Schaufler
Original assignee: Daimler AG; Ford Global Technologies LLC
Current assignee: Mercedes Benz Group AG; Ford Global Technologies LLC
Priority date: 2008-01-17
Filing date: 2008-12-24
Publication date: 2009-07-23
Anticipated expiration: 2010-07-17
Also published as: DE102008004812A1

Abstract

The invention relates to an apparatus (10) for determining the thermal conductivity of a fluid, in particular of a service fluid for a fuel cell, having a feed means (14), by means of which the fluid may be fed to a main body (12), with which there are associated a temperature sensor (24) for measuring and a temperature control element (22) for adjusting the temperature of the main body (12), wherein a further temperature sensor (26) for measuring the temperature of the fluid in the feed means (14) is provided for detecting the presence of a liquid phase in the fluid. The invention further relates to a method for determining the thermal conductivity of a fluid.

Description

APPARATUS AND METHOD FOR DETERMINING THE THERMAL CONDUCTIVITY OF

A FLUID

The invention relates to an apparatus for determining the thermal conductivity of a fluid, having a feed means, by means of which the fluid may be fed to a main body, with which there are associated a temperature sensor for measuring and a temperature control element for adjusting the temperature of the main body. The invention further relates to a method for determining the thermal conductivity of a fluid.

DE 10 2005 033 867 A1 describes a thermal conductivity sensor with a main body on which a membrane is arranged. In this case, a cavity is formed between the membrane and the main body. The membrane is additionally screened relative to the area surrounding the main body by a cap-shaped cover, the cover comprising an opening as feed means for feed of the fluid. A heating element and a temperature sensor are arranged on the membrane. To determine the thermal conductivity of the fluid, the membrane is heated by means of the heating element and the temperature thereof is measured by means of the temperature sensor. By relating the heating power applied by the heating element to the temperature of the membrane achieved by the heating power, a statement may be made about the thermal conductivity of the fluid surrounding the membrane.

The object of the present invention is to provide an apparatus and a method of the above- mentioned type by means of which the presence of a liquid phase in the fluid may be detected in an improved manner.

This object is achieved according to the invention by an apparatus having the features of patent claim 1 and by a method having the features of patent claim 11. Advantageous configurations with convenient further developments of the invention are indicated in dependent patent claims.

In the apparatus according to the invention for determining the thermal conductivity of a fluid, having a feed means, by means of which the fluid may be fed to a main body, with which there are associated a temperature sensor for measuring and a temperature control element for adjusting the temperature of the main body, a further temperature sensor for measuring the temperature of the fluid in the feed means is provided for detecting the presence of a liquid phase in the fluid.

By means of the further temperature sensor, the temperature of the fluid in the feed means may thus be provided as a reference variable for the measured temperature of the main body. The invention is based on the recognition that the fluid has a greater thermal capacity if a liquid phase is present in the fluid. For instance, the fluid with the liquid phase brings about a greater change in the temperature of the main body established by means of the temperature control element than would be the case with a fluid without a liquid phase present therein.

For instance, by comparing the temperature of the fluid measured in the feed means and the temperature established in the main body, temperature-adjusted by means of the temperature control element, as a consequence of feeding the fluid to the main body, it is possible to detect the presence of a liquid phase in the fluid.

In a fuel cell system, for instance of a motor vehicle, the apparatus may be used, for instance, to detect the presence of liquid water in a service fluid of the fuel cell system. Appropriate countermeasures may then be taken, by means of which the liquid water is removed from the fuel cell system, so as to guarantee functioning of the fuel cell system even at temperatures of below 0° C.

In a n advantageous development of the invention, the main body is constructed such that the fluid may flow through it. In this way, a particularly large contact surface may be provided between the fluid and the main body, which enables particularly quick and efficient heat transfer between the fluid and the main body. In this case, the temperature sensor for measuring the temperature of the main body does not have any direct contact with the fluid. In a further advantageous development of the invention, the temperature control element takes the form of a heating element and/or a cooling element. The temperature control element in the form of a heating element may increase the temperature of the main body above the temperature measured in the fluid in the feed means. If, in the case of identical heating element heating power, a marked difference arises between two temperatures measured at the main body by means of the temperature sensor during feed of the fluid, it may be concluded from the lesser heating of the main body effected by means of the heating element that a liquid phase is present in the fluid.

Alternatively or in addition, it is possible, for instance as a function of the measured temperature of the fluid in the feed means, for the main body to be actively cooled by means of the temperature control element taking the form of a cooling element. In this case, a temperature may be established in the main body which is lower than the temperature of the fluid in the feed means measured by means of the further temperature sensor. In the case of feed of the fluid with the liquid phase, greater heating by the fluid may then be measured in the main body than would be the case with the fluid without the presence of a liquid phase. This advantageously makes it possible, depending on the operating state of the fuel cell system and/or depending on ambient temperature, to bring about active heating or active cooling of the main body by means of the temperature control element.

It is additionally advantageous for the main body to have a thermal conductivity which is greater than the thermal conductivity of the feed means. In this way, the amount of heat lost from the feed means into the area surrounding the apparatus may be kept low, while good heat transfer is made possible in the main body. In this way, the temperature control element may establish a comparatively small difference between the temperature of the main body and the temperature of the fluid in the feed means.

It is additionally advantageous for the main body to comprise a metallic material, in particular an aluminum alloy and/or a ceramic material. Such a main body comprises a particularly high level of thermal conductivity, whereby particularly efficient heat transfer between the main body and the fluid fed thereto may be achieved. In a further advantageous development of the invention, the main body is at least in part thermally insulated from its surroundings. In this respect, an insulating material such as polyurethane and/or polystyrene may be used. In this way, ambient influences on the temperature of the main body may be kept slight. A change in the temperature of the main body may thus be regarded as being brought about at least substantially by the fluid fed thereto and/or the temperature control element.

It is additionally advantageous for a pressure sensor to be provided for measuring the pressure in the fluid. By taking account of the pressure in the fluid, it is possible, for instance, to determine whether an elevated thermal capacity of the fluid is attributable to an elevated fluid pressure.

In a further advantageous development of the invention an evaluation unit is provided, by means of which the humidity of the fluid may be determined as a function of measured values detected by means of the temperature sensors. It is possible, for instance, especially when taking account of the pressure in the fluid, to determine whether a fluid humidity threshold value has been exceeded, the exceeding of said value meaning that a liquid phase is present in the fluid. The threshold value may here be dependent on the type and load situation of the fuel cell system. Thus, for instance, in the fuel cell system the fluid may have a relative humidity of 80 % to 90 % and a liquid phase may here be present in the fluid.

It has furthermore proven advantageous for a humidifier to be provided for humidifying the fluid, in particular the service fluid for the fuel cell, by means of which the humidity of the fluid may be modified as a function of detection of the presence of the liquid phase in the fluid. It is thus possible, by means of the humidifier, to expose the fluid to a lower level of humidity if the presence of the liquid phase has been detected in the fluid. It is likewise possible, by means of the humidifier, to adjust the fluid to a specific relative humidity below the threshold value at which the occurrence of a liquid phase is to be expected in the fluid. In this way, it is possible by means of the humidifier to adjust the humidity of the fluid in accordance with the load situation of the fuel cell system comprising the fuel cell.

Finally, it has proven advantageous for dry operation to be established by means of the humidifier, dry operation being established as a function of an ambient temperature, a date and/or a control input. Dry operation is in this case distinguished by a comparatively low humidity of the fluid, which lies so markedly below the threshold value of the moisture content of the fluid that it cannot be expected that a liquid phase will occur in the fluid.

Setting the fuel cell system to dry operation is sensible in a motor vehicle in particular in winter. When the fuel cell system, arranged for instance in the motor vehicle, is brought into service in winter, during which the fuel cell system displays temperatures of below 0° C, it is advantageous for the humidity in the fuel cell system to be as low as possible. Moreover, no liquid phase must be present in the service fluid for the fuel cell, so as to allow the fuel cell system to be brought into service at temperatures of below 0° C.

For instance, dry operation may be established when the ambient temperature of the fuel cell system falls below a value close to freezing. Provision may likewise be made for dry operation to be established during a particular season of the year in which particularly low ambient temperatures are known from experience to occur. Furthermore, it may be indicated to a user of the fuel cell system that dry operation should be established by means of a control input as a result of current ambient temperatures and/or as a result of the season.

In dry operation, which may also be known as winter operation, a relative humidity of 80 % may be set for the fluid, for instance. Since dry operation may reduce the service life of the fuel cell system, provision is made for dry operation not to be set as the standard operating situation of the fuel cell system.

The preferred embodiments and advantages described in connection with the apparatus according to the invention for determining the thermal conductivity of a fluid also apply to the method according to the invention for determining the thermal conductivity of a fluid.

Further advantages, features and details of the invention are revealed by the following description of a preferred exemplary embodiment and by the drawings, in which elements which are the same or have the same function are provided with identical reference signs and in which:

Fig. 1 shows an apparatus for detecting the presence of a liquid phase in a fluid, which comprises a service fluid for a fuel cell system of a motor vehicle; and Fig. 2 shows two diagrams with examples of curves by means of which the presence of a liquid phase in the fluid may be detected.

An apparatus 10 for detecting the presence of a liquid phase in a fluid comprises, according to Fig. 1 , a main body 12, to which the fluid may be fed by means of a feed means 14. The main body 12, which in the present case consists of an aluminum alloy, comprises an inlet opening 16, to which the feed means 14, here in the form of a line, is connected. After flowing through the main body 12, the fluid exits via an outlet opening 18 in the main body 12, to which a discharge means 20 is connected for discharging the fluid. The feed means 14 and the discharge means 20 consist in the present case of a material with a very low thermal conductivity relative to the material of the main body 12, for instance of plastics.

By means of a temperature control element 22, which in the present case takes the form of an electrically operated heating element, thermal energy may be fed to the main body 12 and the temperature of the main body 12 may thereby be adjusted. The temperature of the main body 12 is measured by means of a temperature sensor 24, which does not come into direct contact with the fluid flowing through the main body 12. The main body 12 is thermally insulated relative to its surroundings in a manner not described in any more detail here, for instance by means of an insulating material such as polyurethane and/or polystyrene.

The temperature of the fluid in the feed means 14 may be measured by means of a further temperature sensor 26.

Electrical power is applied to the temperature control element 22, such that the temperature of the main body 12 is markedly higher than the temperature of the fluid at the inlet opening 16, provided that no liquid phase is present in the fluid. Depending on the geometry of the main body 12 and depending on the configuration of a contact surface between the fluid and the main body 12, the main body 12 may be heated by means of the temperature control element 22, and thereby exhibit a temperature which is for example 5 % to 25 %, preferably around 15 %, higher than that of the fluid in the feed means 14. If, therefore, no liquid phase is present in the fluid, the temperature of the main body 12 measured by means of the temperature sensor 24 is markedly higher than the temperature of the fluid measured in the feed means 14 by means of the further temperature sensor 26.

If, on the other hand, a liquid phase is present in the fluid, the temperature of the main body 12 determined by the temperature sensor 24 is markedly lower despite an identical heating power being provided by the temperature control element 22 to adjust the temperature of the main body 12. It may thus be detected from the difference between the temperature of the main body 12 and the temperature of the fluid in the feed means 14 whether a liquid phase is present in the fluid.

The apparatus 10 shown in Fig. 1 comprises in the present case an evaluation unit 28, which is designed to receive and process measured values and signals from the temperature sensors 24, 26, the temperature control element 22 and a pressure sensor 30. The pressure sensor 30 is designed in the present case to measure a pressure in the fluid in the feed means 14.

The evaluation unit 28 makes it possible to determine whether a relative humidity threshold value has been exceeded in the fluid, the exceeding of which value means that a liquid phase has arisen in the fluid.

Also associated with the apparatus 10 is a humidifier 32, by means of which a service fluid of a fuel cell system 34 may be exposed to humidity. In this way, the apparatus 10 may be used to monitor and/or adjust the correct setting for a desired relative humidity in the fuel cell system 34. Provision may also be made, depending on the load situation of the fuel cell system 34, to establish a given relative humidity of the fluid by means of the humidifier 32.

The humidifier 32 is in the present case arranged on an inlet side of the fuel cell system 34 and may be provided for humidifying a cathode gas and/or for humidifying an anode gas. The feed means 14 for feeding the fluid to the main body 12 is arranged in the present case on the output side of the fuel cell system 34. Dry operation of the fuel cell system 34 may be established by means of the humidifier 32. To this end, a detection means 36 is associated with the apparatus 10, said detection means being designed to detect an ambient temperature of the fuel cell system 34, a date and to detect a control input. Dry operation may be established manually by means of the control input.

Provision may likewise be made for dry operation to be established by means of the humidifier 32 when the ambient temperature of the fuel cell system 34 falls below a temperature of 0° C or when the date indicates a cold season.

In an alternative embodiment of the apparatus 10, the temperature control element 22 may be designed as a cooling element, by means of which the main body 12 may be cooled to a temperature which is lower than the temperature of the fluid measured in the feed means 14. In this case, the fluid brings about greater heating in the main body 12 if a liquid phase is present in the fluid than in the case of a purely gaseous fluid.

The above-described apparatus 10 uses the temperature sensors 24, 26, the pressure sensor 30 and the temperature control element 22, i.e. components which have proven to be particularly highly fit for service in motor vehicles, to determine the relative humidity of the fluid. Furthermore, the above-described apparatus 10 is very robust, structurally simple and cost-effective due to the measuring principle used to determine humidity.

To detect the amount of liquid phase in the fluid, it is possible to use an apparatus which is not shown here for determining through-flow through the main body 12.

Fig. 2 shows a first diagram 38, in which a curve I represents a temperature difference yT in K over the time in minutes plotted on an x-axis 40. The temperature difference yT is here a difference between the temperature measured at the main body 12 by means of the temperature sensor 24 and the temperature T1 of the fluid measured in the feed means 14 by means of the further temperature sensor 26.

The temperature difference yT is plotted in the first diagram 38 in accordance with values on a first y-axis 42. A second y-axis 44 illustrates a scaling of values of a water mass flow rate W in grams per second, which is illustrated in a second curve Il as a function of the time in the first diagram 38. It is clear from the first diagram 38 in Fig. 2 that the temperature difference yT has negative values, as long as a water mass flow rate W is established in the fluid which oscillates by a value of 0.02 g/sec. The temperature of the main body 12 is thus lower than the temperature T1 of the fluid in the feed means 14 if a liquid phase is present in the fluid.

At a time 46, the water mass flow rate W is reduced so significantly in the fluid that no liquid phase is present in the fluid. From the time 46 the curve I shows an increase in the temperature difference yT, until the latter reaches a value of approximately 15 K after approximately 5 mins. The slow increase in the temperature difference yT is caused by the main body 12 being heated up comparatively slowly by the fluid flowing therethrough.

Negative values for the temperature difference yT are brought about, in the present case, in that heat was lost to the surroundings of the apparatus 10 between a measuring point of the temperature sensor 26 in the feed means 14 and the inlet opening 16 of the main body 12.

Fig. 2 shows a second diagram 48, in which the time in minutes is plotted on the x-axis 40 and the temperature T1 of the fluid in the feed means 14 in degrees Celsius and a relative humidity rF in percent are plotted on the y-axes 42, 44, with different scaling.

A curve III shows the temperature T1 as a function of time, wherein the temperature T1 of the fluid is measured by means of the further temperature sensor 26 in the feed means 14.

It may here be detected that the temperature T1 exhibits an abrupt, slight increase at the time 46. The reason for this is that, when the liquid phase is present in the fluid, evaporating liquid water leads to slight cooling of the fluid. With removal of the liquid phase from the fluid due to the significant reduction in the water mass flow rate W, the fluid is fed to the main body 12 with the lower relative humidity rF and the higher temperature T1. A curve IV in the diagram 48 shows a corresponding drop in the relative humidity rF. After the time 46 the curve IV of the relative humidity rF oscillates about a value of 80 %, while before the time 46 it oscillates about a value of 82 %.

Moreover, the second diagram 48 shows a curve V, which represents a dew point temperature of the fluid as a function of time. The curve V is largely constant in a period illustrated in the second diagram 48 and is lower than the temperature T1 , which was determined by means of the further temperature sensor 26 in the feed means 14 for the fluid. The relative humidity rF is determined by means of the evaluation unit 28 and derived from the temperature difference yT.

It goes without saying that the curves I to V may differ from the curves I to V shown in Fig. 2, depending on the arrangement and configuration of the components of the apparatus 10 shown in Fig. 1.

List of reference signs

10 Apparatus

12 Main body

14 Feed means

16 Inlet opening

18 Outlet opening

20 Discharge means

22 Temperature control element

24 Temperature sensor

26 Temperature sensor

28 Evaluation unit

30 Pressure sensor

32 Humidifier

34 Fuel cell system

36 Detection means

38 Diagram

40 X-axis

42, 44 Y-axes

46 Time

48 Diagram rF Relative humidity yT Temperature difference

T1 Temperature

W Water mass flow rate

I, II, III, IV, V Curves

Claims

Patent Claims

1. An apparatus for determining the thermal conductivity of a fluid, in particular of a service fluid for a fuel cell, having a feed means (14), by means of which the fluid may be fed to a main body (12), with which there are associated a temperature sensor (24) for measuring and a temperature control element (22) for adjusting the temperature of the main body (12), characterized in that a further temperature sensor (26) for measuring the temperature (T1) of the fluid in the feed means (14) is provided for detecting the presence of a liquid phase in the fluid.

2. The apparatus as claimed in claim 1 , characterized in that the main body (12) is constructed such that the fluid may flow through it.

3. The apparatus as claimed in claim 1 or 2, characterized in that the temperature control element (22) takes the form of a heating element and/or a cooling element.

4. The apparatus as claimed in one of claims 1 to 3, characterized in that the main body (12) has a thermal conductivity which is greater than the thermal conductivity of the feed means (14).

5. The apparatus as claimed in one of claims 1 to 4, characterized in that the main body (12) comprises a metallic material, in particular an aluminum alloy, and/or a ceramic material.

6. The apparatus as claimed in one of claims 1 to 5, characterized in that the main body (12) is thermally insulated at least in part from the surrounding area.

7. The apparatus as claimed in one of claims 1 to 6, characterized in that a pressure sensor (30) is provided for measuring a pressure in the fluid.

8. The apparatus as claimed in one of claims 1 to 7, characterized in that an evaluation unit (28) is provided, by means of which the humidity of the fluid may be determined as a function of measured values detected by means of the temperature sensors (24, 26).

9. The apparatus as claimed in one of claims 1 to 8, characterized in that a humidifier (32) is provided for humidifying the fluid, in particular the service fluid for the fuel cell, by means of which the humidity of the fluid may be modified as a function of detection of the presence of the liquid phase in the fluid.

10. The apparatus as claimed in claim 9, characterized in that dry operation may be established by means of the humidifier (32), dry operation being established as a function of an ambient temperature, a date and/or a control input.

11. A method for determining the thermal conductivity of a fluid, in particular of a service fluid for a fuel cell, having the following steps: adjusting a temperature of a main body (12) by means of a temperature control element (22); measuring the temperature of the main body (12) by means of a temperature sensor (24); measuring the temperature (T1) of the fluid in a feed means (14) by means of a further temperature sensor (26); feeding the fluid to the main body (12) by means of the feed means (14); to detect the presence of a liquid phase in the fluid: remeasuring the temperature of the main body (12) by means of the temperature sensor (24).