Abstract:
The invention relates to a method and a device for the continuous measurement ( 30 ) of the thermal conductivity of a multifunctional fluid. The inventive method consists of: placing a sample of the multifunctional fluid in a space ( 31 ) which is defined by an inlet face and an outlet face; transmitting at least one very brief pulse of a heat flux to the sample via the inlet face, using a laser ( 40 ); measuring the heat wave at least three points which are spaced out inside the sample; using at least three temperature sensors (S 1 , S 2 , S 3 ) in order to determine the change in the temperature of the multifunctional fluid as a function of time at the three spaced-out points inside the sample; deducing the thermodynamic characteristics of the sample from the aforementioned temperature change and calculating the thermal conductivity from equation (I), wherein T represents thermal conductivity which is dependent on temperature, t represents thermal diffusivity which is dependent on k and which is equal to k(T)/ρ*Cp, ρ and Cp representing mass density and specific heat.

Description:
TECHNICAL FIELD  
       [0001]     The present invention concerns a method for the continuous measurement of the thermal conductivity of a multi-functional fluid in which a sample of the multi-functional fluid is passed through a space delimited by a first face, called the entry, and a second face, called the exit, and in which an increase in temperature of the sample of multi-functional fluid is generated and this increase in temperature is measured.  
         [0002]     It also concerns a device for the continuous measurement of thermal conductivity of a multi-functional fluid consisting of means for passing a sample of the multi-functional fluid through a space delimited by a first face, called entry, and a second face, called exit, of the sample, means of heating to vary the temperature of this sample and means designed to measure the variation of this temperature.  
       EARLIER TECHNIQUE  
       [0003]     A multi-functional fluid is a fluid which can be comprised of several components which can be in different phases, liquid, solid or gaseous. A simple example of a multi-functional fluid is blood. Other multi-functional fluids are, for example, biphasic mixtures consisting of phase change materials, currently called PCMs, in suspension in a liquid and an ice slurry.  
         [0004]     In order to be able to resolve the various problems of heat transfer, fluid flow or other, the numerical values of the physical and thermo-physical properties of fluids are of great importance.  
         [0005]     Thermal conductivity in particular defines the degree of propagation of heat in a material as a function of the temperature gradient. Conductivity is essentially a transfer of energy under the effect of movement, notably the vibrations of particles. The coefficient of conductivity k (W/m.K) is dependent on the crystalline structure of solids, on the homogeneity, temperature, pressure, of the liquid, solid or gaseous phases and/or the composition.  
         [0006]     It is noted that liquids are better conductors than gases, and solids are better conductors than liquids. The conductivity of liquids depends in the first instance on their temperature.  
         [0007]     The precise measurement of the coefficient of conductivity is a difficult operation. In fact, the materials which are presently used are not always similar. This leads to differences in the experimental results established by different research laboratories. Thus, the precision related to the coefficient of conductivity does not exceed 5%.  
         [0008]     For simple fluids, without a phase change, methods for the measurement of thermal conductivity already exist.  
         [0009]     In order to characterize a multi-functional fluid with or without a change of phase, practically no direct, reliable method of measuring thermal conductivity exists.  
         [0010]     The German publication DE 199 49 327 A1 describes a method and a device for the implementation of this method for determining the concentration of a gas in a gaseous mixture comprised of several components. The method is based on the measurement of thermal conductivity of a gaseous mixture which is subjected to an increase in temperature between a minimum and maximum value determined by a temperature/time function. An analysis of the curve of temperature variation as a function of time permits the determination of the concentration of a gas contained in the mixture. The device includes a temperature sensor which transmits a signal to a Fourier analyzer. Such a device is not adapted to the measurement of thermal conductivity of a multi-functional fluid.  
       DESCRIPTION OF THE INVENTION  
       [0011]     The objective of the present invention is to alleviate this problem by providing a method as well as a device which enables the determination in a rapid, effective and economical manner of the thermodynamic characteristics of a multi-functional fluid, and to deduce the thermal conductivity there from.  
         [0012]     This objective is attained by a method as defined in the preamble, and characterized by the facts that: 
        through the sample, through the first input face, at least one very brief impulse of heat flux is transmitted,     the temperature is measured at least three separate points within this sample,     by means of this measurement, the evolution of the temperature of the multi-functional fluid is measured at these three points as a function of time,     as a function of this evolution, the thermodynamic characteristics of the sample of the multi-functional fluid is determined, and the thermal conductivity of this sample is determined.        
 
         [0017]     According to one preferred method of implementation, the impulses of heat flux are transmitted in a repetitive manner and a thermogram is established which consists of curves of the temperature evolution as a function of time passing between the sending of a heat flux through the first input face and the increase in temperature determined at the at least three separated points within the sample.  
         [0018]     By preference, the thermal conductivity is deduced from the following equation:  
             ∂   T       ∂   t       +       α   ⁡     (   k   )       ⁡     [           1   k     ·       ⅆ   k       ⅆ   t         ⁢       (       ∂   T       ∂   x       )     2       +         ∂   2     ⁢   T       ∂     x   2           ]         =   0       
        where: T is the temperature     k is the thermal conductivity dependent upon the temperature     t is the time     á is the thermal diffusivity dependant upon k and which is equal to:     k(T)ρ*Cp     with ρ and Cp being the volume mass and the specific heat.        
 
         [0025]     This objective is also attained by the device as defined in the preamble and characterized in that it consists, among other things, of means designed to transmit to the sample, through the first input face, at least a very brief impulse of heat flux, means designed to measure the heat wave at least three separated points within this sample, means designed to determine on the basis of the measured values, the evolution of the temperature of the multi-functional fluid as a function of time at the separated points within the sample, means designed to deduce from this evolution the thermodynamic characteristics of the sample of the multi-functional fluid and means designed to calculate the thermal conductivity of this sample.  
         [0026]     According to one preferred method of implementation, the means designed to pass a sample of the multi-functional fluid through the space delimited by the first and second faces includes an enclosure with an insulating lining and an interior coating of polished metal, through which is continually passed the multi-functional fluid.  
         [0027]     The means (are) designed to transmit to the sample at least one very brief impulse of heat flux comprised of at least one laser.  
         [0028]     According to one particular preferred method of implementation, the means designed to transmit to the sample at least one very brief impulse of heat flux can be comprised of an emitter tube.  
         [0029]     The means designed to measure the heat wave which has passed through the sample is comprised preferably of a receiver tube.  
         [0030]     According to one particularly advantageous construction, the means designed to determine the evolution of the temperature of the multi-functional fluid as a function of time is comprised of at least three temperature probes designed to measure the temperature of the sample of multi-functional fluid at the at least three points.  
         [0031]     The means designed to deduce, from the evolution of the temperature at the three separated points in the sample of the multi-functional fluid, the thermodynamic characteristics of this sample and to calculate its thermal conductivity, preferably comprised of an arithmetic unit designed to receive from the temperature probes signals corresponding to the values measured. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]     The present invention and its advantages will become more apparent in the following description of the various modes of implementation of the invention, by making reference to the appended drawings, in which:  
         [0033]      FIG. 1  is a sketch of the principle illustrating the application of the method according to the invention,  
         [0034]      FIG. 2  is a view illustrating schematically a mode of implementation of the invention device,  
         [0035]      FIG. 3  is a section view of an advantageous mode of implementation of the invention device, and  
         [0036]      FIG. 4  represents a cross sectional view of a measuring probe used in the invention device. 
     
    
     BEST METHOD OF IMPLEMENTING THE INVENTION  
       [0037]     With reference to  FIG. 1 , the method consists firstly of selecting a sample  10  of a multi-functional fluid to be studied, for example, by having it circulate between two linings  11  and  12  which are thermally insulated from a conduit or an enclosure of a form appropriate to define a first face, called input face,  13  and a second face, called exit face,  14 . The fluid is preferable subjected to an increase in temperature by conventional means. In addition, at least one very brief impulse of heat flux is transmitted across the first input face  13 , illustrated by the arrow  15 , for example, by means of a laser. Following this impulse, a heat wave propagates across the sample  10  and crosses the second exit face  14 . It is represented by arrow  16  and measured by a device  17 . At least three separate probes S 1 , S 2  and S 3  within the sample permit the tracing of the temperature evolution curve of the multi-functional fluid as a function of time by providing a thermogram. An arithmetic unit enables the deduction from this evolution of the thermodynamic characteristics of the sample of the multi-functional fluid, and the calculation of the thermal conductivity of this sample. The method preferably includes the repeated emission of heat flashes and the measurement is conducted in a repetitive manner.  
         [0038]     Device  20  for the implementation of the method of measuring the thermal conductivity of a sample of a multi-functional fluid, illustrated by way of a non-limiting example, in the form of an advantageous implementation by  FIG. 2 , consists of a first emitter tube  21  and a second receiver tube  22 , set up in such a way that the space separating their respective extremities  21   a  and  22   a  define the first input face  23  and the second output face  24  of this sample. An impulse, called a flash of heat flux, is emitted by the emitter tube  21 , crosses the sample in the form of a heat wave and is captured by the receiver tube  22 . The two tubes are advantageously several centimeters in length and have a diameter of less than 0,01 m. They contain the electronic components required to control the impulses and manage the measurements. They are mounted respectively on two supports  21   b  and  22   b  comprised of rigid conducting wires.  
         [0039]      FIG. 3  is a cross sectional view of a measuring device  30  according to the invention. It is mainly comprised of an enclosure  31  with an insulating lining  32  and an interior coating of polished metal  33 . This enclosure is traversed continually by a multifunctional fluid, such as for example an ice slurry for which we wish to know the thermal conductivity. This fluid enters enclosure  31  by means of a conduit  34  and leaves this enclosure by a conduit  35 . It is in addition equipped with a chamber  36  containing heating elements  37  which are designed to vary the temperature of the sample of multi-functional fluid. In addition, impulses of heat flux, represented by an arrow  38 , are generated preferably in a repetitive manner, across the input face, for example, by a laser  40 . The heat waves generated traverse the sample of fluid contained in the enclosure  31 , exiting from the enclosure (arrow  39 ) and are measured by at least three temperature probes S 1 , S 2  and S 3  separated from one another and located within the sample. The thickness e of the enclosure  31  is known precisely. This thickness can be variable to enable variation of the measurement parameters. To this end, device  30  is equipped with instrumentation (not shown) comprised of a micrometer which allows the precise determination of the thickness e of the enclosure  31 . The two conduits  34  and  35  are respectively equipped with a valve  41 ,  42  which allows continuous control of the input, exit and circulation of the multi-functional fluid in the enclosure.  
         [0040]     The probe  50 , schematically represented by  FIG. 4 , corresponds to an advantageous form of implementation of the temperature probes S 1 , S 2  and S 3  mentioned above. In fact, it combines the measurement of the temperature and the measurement of the electrical conductivity. It is immersed in the multi-functional fluid  51 . It is comprised of a temperature sensor  52  and an electrical conductivity measurement sensor  53  of the multi-functional fluid. These two sensors are for example, mounted on the interior lining of a tubular element  54  carried by a support  55  which is immersed in the multi-functional fluid.  
         [0041]     The device according to the invention functions advantageously in the following manner. The means, for example the enclosure  31 , permits the insulation of a sample of the multi-functional fluid. The means, consisting of, example, instrumentation comprising a micrometer, enables the determination of the thickness of the enclosure. The means, for example, consisting of heating elements  37 , enabling the generation and raising of the sample temperature. The means such as the laser  40  enabling the generation and transmission through the sample of at least one very brief impulse of heat flux and preferably, a series of such impulses. The means such as the receiver tube  22 , illustrated in  FIG. 2 , enabling the measurement of the heat wave which has traversed the sample. The temperature sensor  52  of  FIG. 4  allows the determination of the temperature evolution of the multi-functional fluid as a function of time. An arithmetic unit (not shown) enables the deduction from this evolution of the thermodynamic characteristics of the sample of the fluid, and the calculation of the thermal conductivity of this sample.  
         [0042]     To determine the thermal conductivity, it is advisable to solve the heat equation by considering that thermal conductivity is a function which is dependent on the temperature. This equation is the following:  
             ∂   T       ∂   t       +       α   ⁡     (   k   )       ⁡     [           1   k     ·       ⅆ   k       ⅆ   T         ⁢       (       ∂   T       ∂   x       )     2       +         ∂   2     ⁢   T       ∂     x   2           ]         =   0       
        where: T is the temperature     k is the thermal conductivity dependent on the temperature     t is the time     a is the thermal diffusivity dependent on k, and equals: k(T)/ρ*Cp     with ρ and Cp the volume mass and the specific heat.        
 
         [0048]     By discretising this equation with the help of appropriate software and by using the values for thermal conductivity given by a model called the Jeffrey model, a family of curves is obtained which constitute a thermogram.  
         [0049]     The thermal conductivity can be determined by using the thermogram which is constituted on the basis of the only experimental data available. In this regard, it is advisable to rewrite the heat equation by bringing out two temperature dependant coefficients:  
           ∂   T       ∂   t       =       a   ⁢         ∂   2     ⁢   T       ∂     x   2           +       b   ⁡     (       ∂   T       ∂   x       )       2           
        in which:  
         a   =     k     ρ   ⁢           ⁢     C   F           ,     b   =         1   k     ·       ⅆ   k       ⅆ   T         ⁢   a           
       
 
         [0051]     By writing this equation twice for two very close locations, the first at the point x and the second at the point x+dx, a system of two equations in two unknowns is obtained. It is assumed that the coefficients a and b at points x and x+dx are equal. By putting this system into matrix form, it can be solved very simply by means of appropriate software, and the thermal conductivity of the sample can be found.  
         [0052]     The phase change materials currently called PCMs (Phase Change Material) are alkane polymers with a solid-liquid phase change temperature varying between 0 and 65° C. The PCMs offer an advantage for static uses, for example, storage, and dynamic uses, for example, the transport of thermal energy.  
         [0053]     The addition of microcapsules (10 μm to 1,000 μm) of PCM materials such as for example, naphthalene in the solid phase in a liquid in suspension gives a biphasic mixture in liquid form currently called &lt;&lt;PCMS)&gt; which can be put into circulation by use of conventional methods, for example, a pump. This aqueous solution allows the combining in an ecological and economical manner of the advantages of storage and distribution of energy in the form of heat and cold, and of indirect systems.  
         [0054]     Such a PCMS is constituted by the ice slurry. The addition of small grains or flakes of ice into an aqueous solution yields a mixture in the liquid form which can be pumped. This mixture offers the possibility of combining in an ecological and economical manner the advantages of storing of cold and of indirect cooling with the high power refrigerating of direct expansion.  
         [0055]     With respect to probe  50  in particular, other methods of construction can be envisaged. The sensors for temperature and the measurement of a conductivity are available on the market. Their arrangement on an immersion support in the multi-functional fluid could be adapted as a function of requirements and applications.