Flow measurement technology

The flow measurement technology is concerned with the determination of physical quantities of fluid flows (gas or liquid). These can for example be pressures , flow velocities , temperatures , gas densities , etc.

Areas of application for flow measurement technology are research and development, where, for example, flow processes are investigated or optimized using models (e.g. minimizing the air resistance of vehicles using experiments in the wind tunnel ). Flow measurement technology is also an essential component for process control in industrial plants (example: flow meters and pressure sensors in a chemical plant or pipeline).

Flow visualization

Different types of flow visualized using smoke probes

The flow visualization or flow visualization is used to research the topology of the flow field. Measures to improve the flow conditions can often be derived from the qualitative statements of the flow visualization. For example, the streamlines can be made visible by smoke probes in the wind tunnel or by color probes in water currents and thus areas with undesirable flow separation can be identified.

With Schlieren optics , pressure gradients in flows can be made visible. This method is particularly suitable for visualizing shock waves in supersonic flows .

With the help of paint pictures, the direction of the shear stresses acting on the surface and the position of separation lines can be shown on bodies around which flow is flowing.

Measurement of two- and multi-phase flows

Two- or multiphase flows are flows of different substances and / or states of aggregation. They require special measuring techniques to determine the proportions of the individual phases .

Measurement of pressure

Functional diagram of a can barometer

The pressure is an important scalar quantity in flowing fluids. It is defined as the quotient of the normal force that a fluid exerts on a surface and this surface: ${\displaystyle p}$ ${\displaystyle {\vec {F}}_{\perp }}$ ${\displaystyle A}$

${\displaystyle p={\frac {|{\vec {F}}_{\perp }|}{A}}\,}$

Pressure gauges can be divided into, based on their measuring methods

• direct pressure gauges (direct measurement of force): e.g. B. Can barometer
• indirect pressure measuring devices (deriving the pressure from another substance size, e.g. particle density): e.g. B. Ionization vacuum gauges

Measurement of the flow velocity

This is often referred to as anemometry . One distinguishes between

• Methods that influence the flow ( anemometer , hot wire anemometer , measuring orifice ), as well as
• Methods without influencing the flow ( laser Doppler anemometry , particle image velocimetry (PIV), magnetic resonance velocimetry (MRV), particle tracking velocimetry (PTV), Doppler global velocimetry (GDV), laser two-focus (L2F), spatial filter )

Flow measurement

A flow meter measures the amount that flows through a defined flow cross section (for example a pipe) within a certain time interval. The flow can be expressed as a volume flow

${\displaystyle {\dot {V}}={\frac {\Delta V}{\Delta t}}\,}$

or as a mass flow

${\displaystyle {\dot {M}}={\frac {\Delta M}{\Delta t}}\,}$

be measured.

Depending on the operating principle, a distinction is made between the following types of flow meters:

• Volumetric method
• Differential pressure method ( Venturi tube )
• Inductive methods, and
• Ultrasonic procedure

Method of measuring fluid temperature

The temperature is a scalar (undirected) physical state variable.

Suitable probes ( thermometers ) can be positioned in the flow field to measure the temperature . In technical applications, thermocouples or resistance thermometers are mostly used for this.

The pyrometer measurement is a non-contact process. The pyrometer measures the frequency of the emitted electromagnetic waves and uses this to determine the temperature at the measuring point. Thanks to the indirect measurement method, it can also be used to measure very high temperatures (e.g. temperature measurement in boilers). Thermography cameras are in principle spatially resolving pyrometers and allow the temperature distribution to be measured in the entire flow field.

With laser-induced fluorescence (LIF) , the temperature distribution can be determined spatially resolved in a plane. For this purpose, temperature-dependent fluorescence tracers are added to the fluid, e.g. B. Rhodamine B . The temperature distribution in the flow can be determined from the ratio of the fluorescence intensities of two spectral ranges, one being temperature-dependent and the other temperature-independent.

If you add special liquid crystals to the fluid , which change their color depending on the temperature, the temperature distribution in the flow field can also be visualized. The temperature dependence of the color of a substance is called thermochromism . The targeted use of thermochromism for temperature measurement is limited to special applications.

Measurement of the wall shear stress

The wall shear stress describes the friction of the liquid on a wall. It is defined as the quotient of the parallel force that a fluid exerts on a wall and the area of ​​the wall: ${\displaystyle \tau }$

${\displaystyle \tau ={\frac {{\vec {F}}_{||}}{A}}\,}$

With

${\displaystyle \tau }$: Wall shear stress

${\displaystyle {\vec {F}}_{||}}$: Force parallel to the wall

${\displaystyle A}$: Area

In contrast to pressure, the wall shear stress is a vectorial (directed) quantity.

It is determined, for example, by:

• Wall shear stress scales
• Hot film

Measurement of two-phase flows

Measurement techniques for two- or multiphase flows can be differentiated into techniques for

• Measurement of the disperse phase (phase Doppler measurement technology, laser diffraction measurement technology, rainbow measurement technology, shadow Doppler measurement technology, in-line holography, interferometric laser imaging droplet sizer (ILIDS))
• Measurement of concentration or mixture ( laser-induced fluorescence (LIF), quantitative Mie scattering / quantitative light scattering (QLS))

literature

• Helmut Eckelmann: Introduction to flow measurement technology. Teubner, Stuttgart 1997, ISBN 3-519-02379-2 .
• Wolfgang Nitsche, André Brunn: Flow measurement technology . Springer-Verlag Berlin Heidelberg 2006, ISBN 3-540-20990-5