Skip to main content

Über dieses Buch

Increasing possibilities of computer-aided data processing have caused a new revival of optical techniques in many areas of mechanical and chemical engi­ neering. Optical methods have a long tradition in heat and mass transfer and in fluid dynamics. Global experimental information is not sufficient for de­ veloping constitution equations to describe complicated phenomena in fluid dynamics or in transfer processes by a computer program. Furthermore, a detailed insight with high local and temporal resolution into the thermo­ and fluiddynamic situations is necessary. Sets of equations for computer program in thermo dynamics and fluid dynamics usually consist of two types of formulations: a first one derived from the conservation laws for mass, energy and momentum, and a second one mathematically modelling transport processes like laminar or turbulent diffusion. For reliably predicting the heat transfer, for example, the velocity and temperature field in the boundary layer must be known, or a physically realistic and widely valid correlation describing the turbulence must be avail­ able. For a better understanding of combustion processes it is necessary to know the local concentration and temperature just ahead of the flame and in the ignition zone.



1. Introduction

Optical measurement techniques have been receiving an increasing attention during the last years. Although most of the techniques which are described in this book were developed decades ago, their application to the analysis of phenomena in heat- and mass-transfer becomes more and more common. This originates from several reasons. The most important one - common to all optical techniques — is that they work non-intrusively and therefore do not influence the investigated process. Even highly transient and/or sensitive phenomena can be analyzed with a high spatial and temporal resolution. Furthermore, in most cases it is possible to visualize directly (on-line) the processes in the test section. This yields the very high accuracies of the measurements and new insights in heat- and mass-transfer phenomena. Both are required to analyze and to improve processes in chemical and power engineering. Secondly, the rapid improvements in computer technology allow to process and to store the huge amount of data within a reasonable time that is obtained by applying optical measuring techniques. High development rates can also be found in the field of light detector, laser, and semiconductor technology.
Franz Mayinger, Oliver Feldmann

2. The Schlieren Technique

The measuring-principle of the schlieren technique is based on the deflection of a collimated light beam crossing gradients of the index of reflection in a transparent medium. It is, therefore, suited for applications in which deviations of light are intended to be visualized as they appear e.g. at refraction- gradients due to density-discontinuities in a fluid. This classical non-invasive optical measuring technique was established by August Toepler [1,2] in 1864. It is often applied at heat- and mass-transfer phenomena, such as convection, mixing processes of gases or fluids, flame-propagation phenomena, or the investigation of (super-) sonic flows, where the density-gradients of the gas are strong enough for a sufficiently high deviation of the light.
Andreas Eder, Martin Jordan

3. Fundamentals of Holography and Interferometry

Holography allows for the use of various interferometric methods for measuring processes of heat and mass transfer. In this chapter the principles of optical arrangements for such experiments are shown. The techniques of data acquisition and evaluation are discussed. Examples demonstrate the advantages of the techniques used. Holographic interferometry has displaced Mach-Zehnder interferometry completely due to its greater cost effectiveness, simplicity of operation and convenience. Holographic interferometry does not require machining or manufacturing of test section windows, mirrors or lenses with special precision or accuracy, because imperfections are automatically balanced by the holographic two step procedure.
Franz Mayinger

4. Holographic Interferometry

Holography can be used to store the amplitude and phase distribution of wave fronts on a photographic plate. In order to take a hologram the object is illuminated by coherent light. The diffuse reflected light can be stored on the hologram and reconstructed by illuminating the hologram with the reference beam.
Robert Tauscher

5. Short Time Holography

Holography is an imaging method based on the interference ability of coherent light. Apart from artistic and commercial applications, holograms are often used as storage media in databank systems and in devices for optical measurement.
Oliver Feldmann, Peter Gebhard, Anselmo Chavez

6. Evaluation of holograms by digital image processing

The rapid development of computer technology and the mass production of computer chips in the past decade have resulted in the application of image analysis and image processing in many technological and scientific areas. (s. Table 6.1). Numerous problems of pattern recognition, data handling of digitized pictures and computer graphics, formerly reserved for computing centers, TV stations or military organizations, can now be solved on a personal computer (PC).
Oliver Feldmann, Robert Tauscher

7. Light Scattering

The term light scattering refers to physical processes which involve the interaction of light and matter. Light that incidents on an ensemble of particles-crystals, aerosols, molecules, atoms etc.-is partially “deflected” or absorbed. In addition to a change in direction a change in frequency is also possible depending on the scattering process. The evaluation of the scattered light with regard to its intensity, wavelength, and direction often yields valuable information about the scattering matter. A set-up for scattering experiments is shown schematically in Fig. 7.1. Beside the light source and the detectors, also other optical elements, such as mirrors, filters, lenses etc., may have to be placed in the beam path to fulfil the requirements of the specific application.
Boris Kruppa, Gernoth Strube, Christof Gerlach

8. Laser-Doppler Velocimetry — Principle and Application to Turbulence Measurements

The Laser-Doppler Velocimetry (LDV) is an optical measuring method which allows the determination of the velocity of a fluid with a very high temporal resolution. The velocity is measured virtually at one single point, referred to as measurement volume. Steady state as well as transient turbulent flow fields can, therefore, be investigated with a very high spatial accuracy. The measuring principle of LDV is based on the physical effect of Mie scattering (s. Chap. 7). Therefore, the flow has to be seeded with particles capable to follow the flow such that their movement reflects the motion of the flow well.
Andreas Eder, Bodo Durst, Martin Jordan

9. Phase Doppler Anemometry (PDA)

Phase Doppler Anemometry (PDA), also known as Particle Dynamics Analysis (PDA), Phase Doppler Particle Analysis (PDPA), Phase Doppler Difference Method (PDDM) and Phase Doppler Interferometry (PDI), is a laser optical instrument for simultaneous measurement of the velocity and size of spherical particles. Additionally it can be used for concentration and mass flux measurements.
Bernd Ofner

10. Dynamic Light Scattering

As the name implies, Dynamic Light Scattering analyzes the temporal behavior of light scattered by a sample fluid, enabling a number of properties of the sample to be determined. The theoretical foundations of this method were laid at the beginning of the century, but the high resolution required to detect and measure the small frequency shifts and narrow spectral linewidths of the scattered light imposed stringent demands on both the light source and the detection system.
Boris Kruppa, Martin Pitschmann, Johannes Straub

11. Raman Scattering

Raman spectroscopy is a non-intrusive method for measuring species concentrations, concentration ratios and temperatures of molecules. While Smekal had predicted the Raman effect in 1923 based on theoretical considerations, it was the Indian scientist C.V. Raman who first observed the effect now carrying his name since 1928. Today, after only 60 years, the development of measuring methods and suitable systems based on the Raman effect has resulted in a wide selection of measuring systems which are available for various applications of Raman spectroscopy. The main reason for this rapid development is the tremendous progress in laser technology and electronics.
Gernoth Strube

12. Laser induced Fluorescence

Among the newest measurement techniques are those based on laser induced fluorescence. The first successful application of fluorescence imaging was reported in 1982. Despite this short history LIF techniques have become very powerful and are the most widely used non-intrusive techniques for gas measurement. This is mainly due to their high signal strength as compared to the main competitors (i.e. Rayleigh scattering and Raman scattering) as shown in Chap. 7 (Techniques Based on Light Scattering) of this book. This high signal strength make two-dimensional measurements with excellent time and space resolution possible. The principal applications of LIF are the measurement of minority species concentrations and temperatures. In special arrangements of the LIF probe, however, pressure and velocity distributions have also been obtained by fluorescence measurements.
Peter Andresen

13. Absorption

Absorption spectroscopy is one of the oldest techniques of non-intrusive investigation of gaseous media. In this technique the radiation emitted by a source towards a radiation detector is absorbed along its way to the detector. This loss is monitored and analyzed for its dependence on wavelength. Absorption is widely used for industrial gas analysis because of its simplicity, low cost and effectiveness. Nevertheless it is still an area of active research. Up to the late sixties most spectroscopic data was gained only with the help of broadband sources and monochromators or filters, however, with the development of lasers, research gained an important tool to improve the quality of the data and, more importantly, to access completely new areas for the application of non-intrusive optical measurements. These new possibilities are based on the superior properties of laser radiation which include high spectral resolution, very high spectral power density and directivity of the radiation. Chemical analysis and determination of temperature by optical methods in some cases has already been made possible or could be carried out much more specifically with these features, while signal to noise and sensitivity could be enhanced dramatically.
Volker Ebert, Jürgen Wolfrum

14. Pyrometry and Thermography

Herschel1 discovered the infrared spectral region of optical rays about the year 1800. Although the first applications for thermography devices date back to 1900, the field of pyrometrics took a different course of development. An exact definition of the two measurement methods shall not be made at this point. The following characteristics will serve to illustrate the differences between the two methods. Pyrometry is a method which allows measurements of temperature to be recorded from surfaces emitting temperature radiation also known as heat radiation or thermal radiation. These temperature radiations are recorded by a pyrometer, a “radiation thermometer” resulting in point like information. In contrast, when the information obtained is optically recorded as an entire scene ie. buildings, operational equipment etc. and displayed as a “temperature picture” this is known as thermography. A firm grasp of the difference between infrared photography and similar methods -which record only the reflected spectral portion of the optical spectrum (only in the near region of the infrared spectrum) and which generally do not permit any temperature intepretations of the recorded scene to be made-is essential to understanding this phenomena.
Udo L. Glückert, Robert Schmidt

15. Tomographic Measurement and Reconstruction Techniques

Tomographic measurement techniques which allow the measurement of three dimensional concentration, temperature and velocity fields within an investigated volume without influencing physical processes have been developed in the past fifty years within the fields of medicine, electron microscopy and radio astronomy. Tomographic measurement techniques are especially well suited to the analysis of unsteady phenomena and their application to the analysis of materials and to the field of chemical engineering has risen markedly in recent years. This chapter describes the different measurement techniques used in tomography. The mathematical methods implemented in the reconstruction of the measured physical properties are reviewed and the quality of the reconstruction is critically evaluated. Some applications of tomographic techniques are also discussed.
Mathias Buchmann, Dieter Mewes

16. Particle Image Velocimetry

Particle Image Velocimetry (PIV) belongs to the class of optical “whole- field” measuring techniques. The name of the method is self-explaining: The velocity distribution in a whole field of a fluid flow is determined by measuring the displacements Δs that the images of tracer particles experience during a time interval Δt. Local, instantaneous velocity values (and directions) w(x,y) = Δs / Δt are measured simultaneously at many positions (x, y) in the field of view. The idea of determining flow velocities by measuring the displacement of tracer particles is not quite new and was in practical use long before the name Particle Image Velocimetry appeared. The new name arose from the possibility of registering the particle images in digital form and efficiently handling the large amount of planar, quantitative data with the techniques of digital image processing. The state of the art for the period prior to the PIV era is well documented in the article by Emrich [414].
Wolfgang Merzkirch


Weitere Informationen