Development of an automated calibration system for hotwire anemometers

Development of an automated calibration system for hotwire anemometers

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Inhaltsangabe:Introduction: In experimental fluid dynamic measurements hot-wire anemometry is used to record information about flow fields. Furthermore one can obtain the magnitude, the direction and even the time dependant behaviour of the fluid flow, if multiple-wire probes are in operation. The hot-wire measurement technique is based on the convective heat transfer from a heated element to the fluid flow, which is actually proportional to the velocity of the flow. So HWA is an indirect measurement technique. There are miscellaneous sensors which work properly in water or other liquids, air or in gas flows. As an example, Fig. 1.1 shows a cross-wire probe in a fluid flow, which can detect the velocity and its direction in two components, if the main flow direction is in one plane (2D flow). Predominantly HWA is a research tool for turbulent flow studies, especially transient procedures. Turbulence models have to be built to represent the characteristics of the flow in numerical simulations (CFD). Therefore only detailed experimental measurements lead to reliable information about the local velocity of a turbulent flow. This can be provided by HWA on the basis of its very high spatial and temporal resolution. Although the development of HWA started at the beginning of the 19th century and new techniques like PIV or LDA (direct methods) have been established, it is still a common device in all wind tunnel labs. The analogue output signal can be optimized by filters before signal processing. It can also be deployed to arrange a spectrum analysis, due to the high temporal resolution. Moreover, unlike the digital devices the analogue signal is densely packed. The range of application is large and leads from sub- and supersonic flows, the independency of the medium to high-temperature measurements. HWA is also affordable in contrast to LDA and PIV systems. In spite of these advantages the natural contamination of the hot-wire probe increases by and by, since the particles in the fluid flow mature themselves to the probe and finally isolate it. As this effect of disturbance causes measuring errors, the hot-wire probes have to be calibrated at frequent intervals - best before and after every data acquisition series. However, HWA is an intrusive measurement technique, thus disturbing the flow. Another disadvantage is that it is not applicable in separation and backward flow regions. The aim of this thesis is to develop an automated calibration system to implement an in situ calibration. A traverse system will move the hot-wire probe to the calibrator. Immediately after accomplishing the calibration, the traverse will adjust the probes position directly in the wind tunnel to fulfil the measuring task. Reasons for applying the in situ calibration are to minimize mistakes and to accelerate this time-consuming procedure. The disturbance of the flow, caused by the probe and its fixation is the same during calibration and experiment. Disconnecting cables between calibration and measurement, which avoids additional errors, is not necessary anymore. Furthermore it is not needed to move the computer system and the pressure supply. This thesis consists of the mechanical design of the calibration facility, of the composition of the computer programs needed and of an experimental validation. All parts of the calibration and of the measurement program are generated in LabVIEW 8.5 - a development environment for a visual programming language by National Instruments [FD], [HD]. Inhaltsverzeichnis:Table of Contents: Declaration2 Abstract3 Acknowledgement4 Abbreviations8 1.Introduction9 2.Theory of hot-wire-anemometry11 2.1Hot-wire probes11 2.1.1Measuring chain11 2.1.2Probe selection12 2.1.3Coordinate system13 2.2Heat transfer of HWA15 2.2.1Ohmic resistance of a hot-wire15 2.2.2Thermal balance15 2.2.3Influences on the sensor signal18 2.3Electrical circuit of HWA18 2.3.1CCA mode18 2.3.2CTA mode20 2.3.3Ohmic and complex resistance of a hot-wire20 2.4Anemometer setup21 2.4.1Overheat adjustment21 2.4.2Square wave test21 2.4.3Low-pass filtering22 2.5Reference velocity for hot-wire calibrations22 2.5.1Calculation of the fluid velocity23 2.5.2Adjusting a defined mass flow in the calibrator24 2.6Calibration methods26 2.6.1Velocity calibration26 2.6.2Calibration of cross-wire sensors according to the DANTEC method27 2.6.3Look-up matrix method30 2.6.4Temperature correction32 2.7Spatial resolution errors33 3.Design of the automated calibration facility34 3.1Initial requirements34 3.2Dimensioning of the mechanical components34 3.3Design of the calibration device37 3.4Important components of the measuring system40 3.5Positioning accuracy40 4.Experimental approach during calibration and measurement42 4.1Measurement devices42 4.1.1Measuring chain42 4.1.2Arrangement of the measuring system44 4.1.3Controller card ISEL IT 11645 4.1.4Pressure transducer SETRA 23945 4.1.5Hot-wire sensors46 4.1.6CTA unit DISA Type 55M01/55M1047 4.1.7Analogue-to-digital converter48 4.2Calibration equipment DISA Type 55D9050 4.3Description of the settings52 4.3.1Overheat setup52 4.3.2Dynamic bridge balancing53 4.3.3Precise positioning of the hot-wire probe54 4.3.4Calibration of the pressure transducer54 4.4LabView programs57 4.4.1Calibration program57 4.4.2Measurement program64 4.4.3step motor control66 5.Measurement results 67 5.1Hot-wire calibration67 5.1.1Single-wire probe (steady flow)67 5.1.2Single-wire probe (transient flow)68 5.1.3Cross-wire probe (steady flow)69 5.1.4Cross-wire probe (transient flow)71 5.2Hot-wire validation73 5.2.1Single-wire probe (steady state flow)73 5.2.2Single-wire probe (transient flow)74 5.2.3Cross-wire probe (steady state and transient flow)75 5.3Summary of validation78 5.4Discussion and limitations78 5.4.1Single-wire78 5.4.2Cross-wire79 6.Perspective82 Bibliography85 List of symbols86 Figures and Pictures93 AAppendix: About hot-wire measurements94 A.1Spatial resolution error - turbulent velocity distribution94 A.2Table for quick selection of HW probes96 BAppendix: Datasheets98 B.1Hot-wire probes98 B.1.1Single-wire probe DISA type 55P198 B.1.2Cross-wire probe DANTEC type 9055P051198 B.1.3Specifications99 B.1.4 SEL IT 116100 B.2NI 6036E101 B.3SETRA 239103 Textprobe:Eine Textprobe erhalten Sie auf Anfrage. Bitte senden Sie dazu eine Mail an LabVIEW-based control and measurement program was written, allowing manual and automatic calibration of one- and two-wire probes. With this newly developed system it is possible to calibrate up to two component hot-wire probes.

Title:Development of an automated calibration system for hotwire anemometers
Author: Constantin Schosser - 2010-01-21

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