About modern methods of optical diagnostics
The Bauman Moscow State Technical University tested a modern method of optical diagnostics of velocity flows in linear diffusers of Flow Engineering.
Modern methods of diagnosing the parameters of the movement of the working fluid in the flow part include such a method as anemometry from images of the part or the more recognizable name PIV (Particle Image Velocimetry) measurement of velocity from images of particles.
Fig. 1 Schematic diagram of 2D PIV flow diagnostics method
Measuring systems are usually two-component 2D PIV, three-component 3D PIV. The diagnostic method under consideration is based on working with images, the correctness of the results directly depends on the quality of the photos obtained. The images should clearly show individual particles or homogeneous structures-clusters of particles. It is also necessary to minimize the illumination, shading, blurring of the frame areas and other negative phenomena leading to a partial loss of the frame's informativeness. Among other tools for studying the structure of flows, it occupies a special place due to the possibility of registering instantaneous spatial velocity distributions. The measurement of the instantaneous velocity field is based on the measurement of the movement of particles located in the cross-sectional plane over a fixed time interval. The measuring area is considered to be a plane "cut out" by a light knife.
Fig. 2 How the velocity vector field appears in the software
Particles in the measuring area are illuminated at least twice. Particle images are recorded on a digital cross-correlation camera. Subsequent processing makes it possible to calculate particle displacements during the time between flashes of light and to construct a two-component velocity field. The obtained two-component vector values are projections of real vectors on a plane perpendicular to the optical axis of the equipment registering particle images.
The use of field methods makes it possible to obtain information about the dynamics of structures, their scales, calculation of differential characteristics, spatial and spatio-temporal correlations. The main measurement results include instantaneous fields with basic consideration of two components of the 2D PIV velocity and three components in the case of the 3D PIV method. With good repeatability of the experiment, it allows you to obtain very high-quality statistical data in various modes.
The laboratory is armed with the latest sample of a world-class foreign speed field diagnostic system manufactured by Dantec Dynamic, with the following main technical characteristics:
- FlowSense EO 2M cross-correlation camera with a resolution of 1600 x 1200 pixels and a shooting frequency of 35 pairs of frames per second;
- optics of the Zeiss 50 mm f/1.4 ZF camera.2;
- dual pulsed laser DualPower 145-15, 145 MJ, wavelength 532 nm.
Fig. 3 General view of 2D velocity field diagnostics system
Fig. 4 Layout of the laboratory stand (elements are shown simplified)
Figure 4 shows a typical case of diagnostics of air movement parameters in the flow part of a polymer transparent model of an air distribution unit. The equipment should be positioned in such a way that the laser plane of the knife highlights the flow areas of interest.
Fig. 5 Example of an illuminated flow area using a laser knife
In Figure 5, it is possible to observe the movement of the air flow with the help of introduced special microparticles of glycerin vapor with a characteristic size of 1-5 microns. The use of such small particles in the flow visualization process is due to the short time of dynamic relaxation of particles, in simple words, microparticles track the flow lines of moving air in the best way.
Some practical diagnostic results:
Fig. 6 Finding the main area of flow dissipation
Fig. 7 Diagnostics of vortex zones of different signs of rotation
Fig. 8 Vector field of a flooded air jet behind a slice of the output linear section
Fig. 9 Vector field inside the flow part of the air distribution unit
Fig. 10 Determination of the area of the greatest turbulent losses in the flow part
Fig. 11 Image of the instantaneous velocity fields per section of the deturbulating grid device
Fig. 12 A time-averaged ensemble of vector maps for illustrating the effect of deturbalization