Electrochemical sensors for flow measurements

Objetivo

A.BACKGROUND

There are only a few methods for measuring fluid velocity very close to a wall. The polarographic method which implies a redox reaction at a metal and measures the mass flux exchanged between the fluid and the wall, provides for an electrode of small area (an electrochemical sensor), the wall velocity gradient value from which the velocity profile can be assessed.

This method represents thus the "mass" equivalent of the Hot Wire Anemometer (HWA), widely used in Fluid Mechanics for measuring velocities in liquids or gas.

Except for the fact that this method cannot be used in a gas, it has several advantages over the HWA, among others that point electrodes can in fact be used for local measurements and the method is not intrusive.

The basics of the method have been known for several decades. However, the topic has gained a renewal of interest with some significant breakthroughs. The strongest incentive to start a COST Action has been the organization of the Workshop series "Electrodiffusion Diagnostics of Flows" held in 1990 in Prague, in 1991 in Dubna (Russia) and the third one in 1993 in Dourdan (France). These Workshops were successful because the relevant scientific community was well identified from different disciplines: Chemical Engineering, Electrochemistry, Fluid Mechanics and Biomechanics.

The existence of a network such as a COST cooperation can be an appropriate forum to help an easy transfer of information and know-how from the theoreticians, methodologists or those who utilize the up-to-date technologies for producing new and performing sensors, and the users who could take advantage of the anticipated falls-out.

In Europe, activities based on those sensors are underway with some qualitative differences in the countries that have participated in the preparation of the Action.

There are several French groups, in Paris, Nancy, Saint-Nazaire, Nantes and Valenciennes. The investigations are twofold: Upstream, kinetic studies are performed (impedance techniques), modelling the dynamic response of single or segmented sensors to a flow modulation. Downstream, there are many applications oriented towards Chemical Engineering with emphasis put on multi-phase flows (packed beds, fluidized beds, gas-fluid reactors), Energetics (mass/heat exchangers for process industries) and Mechanics (momentum/mass/heat transfer analogies in turbulence).

In parallel, a research for processing photolithography devised sensors is in progress.

The Spanish group is involved in wall turbulence studies. The German, Belgian, Swiss and Greek groups carry out investigations mostly concerned with multi-phase flows (composites, electrode position, gas/liquid flows, corrosion, etc.) The Swedish group has already designed microtechnology based sensors for measuring pressure and shear stress. The Czech group has an expertise in segmented electrodes.

There are few groups in USA working with this method but one in the university of Missour-Rolla had a leading role (Pr Hanratty) in the field. Present research themes concern the flow reconstruction for non-linear (large amplitude) flow motions from the sensor signal.

There are also few teams in Great Britain involved in Electrochemical Engineering (reactors).

Russia has strong groups, especially the Frumkin Institute in Moscow and the Thermophysics Institute in Novo-Sibirsk but the present low level of fundings and above all the move of some theoreticians abroad, considerably weakened their activity in this field.

There is already some informal (sometimes formalized) cooperation existing between laboratories that have expressed interest in the Action. For example cooperation between Paris and Prague or Prague and Meinsberg are based on the development of segmented electrodes. Other cooperation between Paris and Saint-Nazaire or Saint-Nazaire and Prague concern the use of the sensors in turbulent flows or in non-newtonian fluids. Another example is cooperation between Saint-Nazaire and Tarragona on the use of the pattern recognition technique to characterize large scale structures in turbulence.

However, no common planned action exists so far and this COST Action will be very useful for a better coordination of all individual efforts.

There is no equivalent COST Action so far. However, this new COST Action should obviously try to establish bridges with the COST Action F1 "Fluid Dynamics": Three-dimensional complex viscous fluids: Prevision, Modeling, Experiment and Control", because some of its anticipated results could be useful for COST F1.

B.OBJECTIVES AND BENEFITS

Main Objective

The main objective of the Action is the development of improved electrochemical sensors for flow measurements by coordinating and integrating both theoretical research and technological R&D in this field.

Secondary Objectives

-Progress in different fields of basic and applied research is anticipated such as:

Chemical and Biomedical (or Biomechanical) Engineering.

Flows in confined volumes.

Non-Newtonian fluids.

Two-phase flows in reactors.

-Manufacturing of sensors working in industrial reactors or in usual fluids (fresh or sea water).

-The possibility to transfer to mid-size industries the know-how of the sensors processing with the following anticipated advantages over comparable sensors (e.g. HWA):

Low cost processing.

No calibration stage.

Simplicity of signal conditioning.

-Federate the different groups participating in the Action by interdisciplinary actions.

-Develop cooperation at a deeper level between European laboratories in view of preparing future projects in EC programs.

-Rationalize the field by defining standardized procedures for using the sensors and carry out normalization actions.

-Increase the size of the present scientific community to new fields by promoting a policy of diffusion of information.

C.SCIENTIFIC PROGRAMME

There will be basically three types of activities in the frame of the Action which will be developed in parallel or in a sequence:

-A theoretical activity oriented towards the knowledge of the response of existing (or new) sensors in permanent or dynamic flow conditions, including the case of non-linear perturbations. This will also cover their behaviour in turbulent or 2-phase flows encountered in complex media (Two-phase fluids, non-newtonian fluids) for single or multi-segmented sensors.

The problem of data processing will be addressed by elaborating both adapted softwares and hardwares (conditioners).

All the work belonging to this first type of activity will be upstream and of common interest for all participants. It will be grouped together in a Work Package called:

WP 1: Theory of the sensors/data treatment/complex flows

-The second type of activity will be a technological effort to produce optimized sensors, for example with the help of Microtechnologies, ensuring:

Perfect geometrical characteristics.

Robustness and durability for use in industrial or natural environment.

Reproducibility for avoiding the calibration stage necessary with the present sensors.

Devising sensors which allow to take advantage of their capabilities is a critical issue. The groups already involved here are academic which work in cooperation with laboratories equipped with Microtechnology facilities. This environment is not usual for academic institutions and there is a real challenge for attracting the interest of industry. In fact, industrials are not yet engaged at this step of the COST Action though some of them are already identified as future partners. For an industrial, the motivation must come from a well-identified need at the level of application, from which a specific sensor, and hence a tailor-made Microtechnology process can be developed.

The work within this second type of activity will form a Work Package called:

WP 2: Sensors manufacturing

-The third type of activities will cover applications which will be carried out independently but which will take advantage of the progresses of the first two points: The possible concerned domains are Chemical, Electrochemical, Mechanical and Biochemical Engineerings. They may address various applications as for example in Fluid Mechanics or in Corrosion. Therefore, the following is just given here as a framework:

Analysis of time and space correlations in turbulence (1-d or 2-d spectra). This will concern specific flow geometrics in reactors.

Flow measurements in confined volumes (flow recirculation after a step). Here, the use of segmented electrodes will be necessary.

Creeping flows (natural convection).

Measurement of the normal component of the velocity (stagnation or separation points) with segmented electrodes.

Local flow measurements in non-newtonian fluids or in packed bed reactors.

Solids/liquids two-phase flows (simultaneous measurements of mass flux and ohmic drop).

Gas/liquid two-phase flows.

On-line data treatment (software).

These activities of the third kind will be grouped together in a third Work Package:

WP 3: Applications/multiple-phase flows in reactors

D.ORGANIZATION AND TIMETABLE

The diagram below gives an idea of the important steps of the life of this COST Action. There are already a limited number of existing cooperations between some laboratories involved in this Action. They imply individual exchanges of researchers and students. This COST Action will offer an opportunity to increase the exchange of researchers (this can be done for short periods with the help of the institutional channels for each country) and also informations. The most efficient way to manage this process will be to organize meetings concerning some or all the working groups. These meetings will have the form of workshops and will be held once a year. At the end the Workshop in Dourdan in 1993, it was decided by the participants that a next edition of the workshop should be organized. The next edition has been planned last year for March 1996 in Lahnstein (Germany): it is desirable to take advantage of the presence there of most of the presumptive participants in the Action to devote one session involving the three working groups for starting the process.

The most critical issues to be discussed in Lahnstein will be:

to define the required characteristics for preparing the sensors (geometry, substrate, nature of the electrode material) for common testing.

Planning of the sensors preparation and deadlines for delivery. This action will require internal funding of the laboratories. In a preliminary phase, it will be interesting to compare the behaviour of the same sensors in different experimental situations.

During the last general Workshop in 1999, the form of contributions could be review papers in view of publishing a book.

The above diagram defines the contributions of the different laboratories to the working groups.

Each number in the square corresponds to the number defining each laboratory appearing in the first column of the table in Section E.

Each of the working groups should have a coordinator to follow the progress of the different actions and to make a concise report every six months to emphasize the breakthroughs or difficulties, one between two workshops for helping the preparation of the next one, and one after each workshop to make a general evaluation. The mid-term assessments could be done during a meeting of coordinators together with the main coordinator (if this structure is agreed) and the report sent to all the participants.

This Action is scheduled for a period of four years.

E.ECONOMIC DIMENSION

Many COST countries such as Belgium, the Czech Republic, Germany, Greece, Spain, Sweden and Switzerland have actively participated in the preparation of this Action.

The overall cost of the research to be carried out under the Action has been estimated at 1995 prices, at roughly ECU 8,5 million over 4 years, equivalent to around 45 man/years.

Programa(s)

IC-COST - European cooperation in the field of scientific and technical research (COST), 1971-

Tema(s)

15 - Fluid dynamics

Convocatoria de propuestas

Data not available

Régimen de financiación

Data not available

Coordinador

N/A

Aportación de la UE

Sin datos

Dirección

Francia

Coste total