Laser Diagnostic Techniques for Engine Research

Transcription

1 Laser Diagnostic Techniques for Engine Research IIT Kanpur Kanpur, India (208016) Dr. Avinash Kumar Agarwal, Associate Professor, Engine Research Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur Optical Diagnostics Increasing Environmental problems - more stringent Emission Control Norms Demand to minimize fuel consumption Better understanding of in-cylinder processes required To simulate fuel injection and combustion in the cylinder To use optical diagnostic techniques to visualize the in-cylinder processes Even the simulation results need to be verified experimentally using Optical Diagnostic Techniques 1

2 Optical Access: Major Challenges Very high non-steady pressure and temperature conditions; high mechanical and thermal stresses proper lubrication not possible, liner heating fowling of optical access window; need frequent cleaning supporting structure should not block the optical access requirement of a flat optical window aberration due to unwanted scattering maintaining realistic engine geometry Optical Access Optical access is usually obtained by: Full Optical Access: Transparent Piston Head, and Transparent Cylinder Liner Endoscopic Access: Optical Fiber based Endoscopic windows Common Materials used: Quartz Sapphire 2

3 Full Optical Access vs. Endoscopic Access Full Optical Access: The optical access is maximized to allow application of complex optical diagnostic techniques while maintaining minimum necessary operability of the engine or engine components Full optical access allows a wide range of diagnostics to be applied Endoscopic Access: Full engine operability is maintained while optical access and diagnostic techniques are tailored to the diagnostic demand and the restraints of engine operation Endoscopic access puts the emphasis on organizing and extending realistic engine operation conditions Full Optical Access Transparent Cylinder Liner 3

4 Endoscopic Access Arrangement Optical Diagnostic Techniques Particle Image Velocimetry (PIV) Scattering Mie/Raman/Rayleigh Scattering Self Emission Spectroscopy LASER Induced Fluorescence (LIF) Planar LASER Induced Fluorescence (PLIF) LASER Induced Incandescence (LII) Laser Holography Laser Doppler Velocimetry 4

5 Particle Image Velocimetry (PIV) for IC Engines What is PIV? Technique Study flow of a Fluid. The flow is illuminated with a double pulsed light sheet and the positions of a large number of tracer particles are recorded with a photographic camera viewing normal to the plane of the sheet. Advantages of PIV Non-intrusive into the flow field being studied. 2D or 3D full-field flow measurements can be made. Instantaneous velocity fields are obtained. Capability for studying multiphase flows. 5

6 PIV can be Used to Study: Single and multi-phase channel flows. Steam bubble collapse Flow around cylinders in a channel Bubbly pipe flows Free surface experiments Sprays Heated cavity flows Principle of PIV PIV measures whole velocity fields by taking two images shortly after each other and calculating the distance individual particles travelled within this time. From the known time difference and the measured displacement the velocity is calculated. 3-D PIV based on the principle of stereoscopic imaging: two cameras capture the image of the illuminated particles from different angles and then the images are digitally combined to obtain a 3-D images. 6

7 Components Seeding: The flow medium is seeded with particles, droplets or bubbles Double Pulsed Laser: Two laser pulses illuminate these particles with short time difference Light Sheet Optics: Laser light is formed into a thin light plane guided into the flow medium CCD Camera: A fast frame-transfer CCD captures two frames exposed by laser pulses Software: Calculates the velocities and makes Velocity Maps Principle of Particle Image Velocimetry (PIV) PIV is a technique for the measurement of instantaneous planar velocity fields. Experimental Arrangement for PIV in a Wind Tunnel 7

8 Particle Image Velocimetry (PIV) Definition An optical imaging technique to measure fluid or particulate velocity vectors at many (eg. Thousands) points in a flow field simultaneously. Measurements (2 or 3 components of velocity) usually made in Planar slices of the flow field. Accuracy and Spatial resolution Comparable to LDV and HWA. Particle Image Velocimetry 8

9 PIV - Principle Cross-correlation Interrogation region Interrogation region frame 1 Crosscorrelation frame 2 particle displacement Crosscorrelation Vector field 9

10 Components Needed for PIV: an illumination source optical system for illuminating the test section. digital imagers for capturing the flow field a system for image processing, particle identification, particle tracking, and vector field cleaning The laser is synchronized with the digital imagers, the laser light is positioned to illuminate the test volume, the scattered light from the tracer particles is recorded with the digital cameras, and then image analysis is performed. General Aspects of PIV Non-intrusive velocity measurement Indirect velocity measurement Whole field technique Velocity lag Illumination Duration of illumination pulse Time delay between illumination pulses Distribution of tracer particles in the flow Density of images of tracer particles on the PIV recording Low image density (PTV) Medium image density High image density (LSV) Number of illumination per recording Number of components of the velocity vector Extension of observation volume Extension in time Size of interrogation area 10

11 Light Source (LASER): Laser Material, Pump Source, Mirror Arrangement Schematic of a laser Various Kinds of interactions between atoms and electromagnetic radiation Three level laser system Four level laser system 11

12 Particle Generation and Supply For seeding gas flow: Air jets, condensation generator, atomizers, smoke generators, Laskin nozzle generators. Table: Seeding material for liquid flows Types Material Mean Diameter ( m) Solid Polystyrene Aluminum Glass Sphere Granules for synthetic coating Liquid Different oils Gaseous Oxygen bubbles Table: Seeding material for gas flows Types Material Mean Diameter ( m) Solid Polystyrene Aluminum Magnesium Glass micro-balloons Granules for synthetic coating Dioctylphathalate Smoke < Liquid Different oils Light Sheet Optics Light sheet optics using two spherical lenses Light sheet optics using three cylindrical lenses (one of them with negative focal length) Light sheet optics using three cylindrical lenses 12

13 PIV Recording Techniques Single frame/multi-exposure PIV Multi-frame/single exposure PIV Types of CCD Camera Full-frame CCD Frame transfer CCD Interline transfer CCD Full-frame interline transfer CCD POST-processing of PIV Data Replacement of incorrect data Data reduction Analysis of the information Presentation and animation of the information Study of IC Engine Charge Motion Using PIV : Charge motion within a IC Engine has a Significant effect on: Power Output Fuel Efficiency Exhaust Emissions The combustion behavior of internal-combustion spark-ignition (SI) engines is strongly dependent on: The quality of the mixture processing, which in turn is affected by the motion of the in-cylinder flow. Fresh charge, and residual gas resulting from the former combustion cycle, have to form a proper mixture. In addition, a certain level of turbulence is required at the time of ignition to perform an accelerated flame propagation and thereby a highly efficient combustion. Therefore, it is highly importance to collect detailed information on the incylinder flow field and its temporal development during the combustion cycle. PIV has proven to be a helpful tool in order to analyze the air-flow in the cylinder. 13

14 Advantages of PIV over other Techniques such as 1. Streak Photography 2. PTV ( Particle Tracing Techniques) Advantages: 1. The identification of individual particle image, not necessary in PIV interrogation 2. PIV allows measurement of instantaneous velocity on a fine, regular measurement grid without significant interpolation. Limitations of PIV : 1. The technique has only technological limitations to achieve a temporal resolution due to the illumination source ( lasers ) and the recording media ( CCD) frequencies which are available today. Two-Colour Particle Image Velocimetry Analysis of the Effects of Inlet Port Deactivation on the Velocity Flow Field in a Fired Liquid Fuelled Spark Ignition Engine M J Haste, C P Garner*, A K Agarwal** N A Halliwell Department of Mechanical Engineering Loughborough University* Loughborough, Leicestershire, England LE11 3TU Indian Institute of Technology Kanpur **, India 14

15 Introduction Fluid motion within IC engine fundamentally affects engine performance and emissions. To analyze and optimize complex coupled processes inside and between automotive components and structure such as the reduction of a vehicle s interior or outer acoustic noise, including brake noise, and the combustion analysis for diesel and gasoline engines to further reduce fuel consumption and pollution. Deeper insight in modern engine combustion concepts such as flow generation, fuel injection and spray formation, atomization and mixing, ignition and combustion, and formation and reduction of pollutants. The need for an non-intrusive measurement system. Laser Assisted Diagnostics is an important tool for such measurements. Engine Manufacturers are developing more fuel efficient, more refined and which produce lower amount of pollutants. Engine in-cylinder fluid motion is known to fundamentally affect the combustion process. It is important to understand combustion phenomenon under different operating conditions such as valve deactivation, port injection and variable injection timing. Objective PIV Is used to investigate the in-cylinder fluid motions and its interaction with propagating flame in a production geometry pentroof multi-valve optical SI engine, fired using liquid fuel. This allows mapping of the flame position and study of the fluid motion ahead of flame front. Two color PIV is used to obtain full field instantaneous velocity data over planer regions within the combustion chamber with a spatial resolution of less than 1.5 mm. Si oil seed burn in the flame front hence it is possible to distinguish the burnt and unburned region of the inc-cylinder flow. The flow structures distribution were obtained with both open and closed inlet valve injection timing under normal running and with a single inlet port deactivated. 15

16 Optical Engine Bore mm 80 Stroke mm 89 Swept Volume cc 447 cc Compression ratio (nominal) 10 : 1 Inlet valve peak lift Exhaust valve peak lift 70 BBDC 70 ABDC Engine Speed 1000 rpm Salient Features Single Cylinder Optical Engine Pent-roof combustion Chamber Production grade, four stroke, four valve per cylinder, Rover K series Fused Silica Barrel Extended Piston incorporating Fused Silica Piston Crown Window. Port injected with iso-octane and skip fired Schematic of Experimental Facility Average diameter of Si oil droplets seed: 1.4μm Twin Oscillator, twin amplifier Nd:YAG laser, frequency doubled to generate green light at 532 nm. (Pulse duration 10µs) Large Scale Motion Cycle to Cycle Variation Small Scale Motion Flame Convection Flame Geometry 16

17 Illustration of 3-D Motion Inferred from Planer Data Measurement Conditions CAD Normal Running Conditions One Inlet Port Deactivated Condition 1000 RPM Ignition timing 25 BTDC Start of Injection 10 ATDC Data acquisition in Horizontal Plane Horizontal light sheet located 2 mm above the piston at TDC Camera imaging off the 45 mirror and through the piston window Vertical Plane Vertical light sheet falling on 45 mirror and through the piston window Camera imaging in horizontal plane close to piston at TDC 17

18 Normal Operating Conditions Normal Running 20 CAD BTDC 18

19 Normal Running 10 CAD BTDC Normal Running TDC 19

20 Normal Running 10 CAD ATDC Normal Running 10 CAD ATDC 20

21 Inlet Port Deactivated Condition Port Deactivated Condition 20 CAD BTDC 21

22 Port Deactivated Condition 10 CAD BTDC Port Deactivated Condition TDC 22

23 Port Deactivated Condition 10 CAD ATDC Port Deactivated Condition 20 CAD ATDC 23

24 Tumble and Skewed Tumble Motion Valve Jet Flow for: (1) Normal Running and (2) Valve Deactivated Conditions Conclusions Under normal running conditions some tumble flow remains after TDC, however, the bulk flow is highly three dimensional and exhibits a torroidal vortex-like structure. With a single inlet port deactivated, the bulk flow shows significant differences to normal running bulk flow structure and exhibits characteristics more like axial swirl and a three dimensional helical structure. Significant cyclic variations in large scale structure are observed, and are greatest under normal 4-valve running conditions. Fuel injection timing was not found to significantly affect the large scale flow structure ahead of the flame around TDC. Scattering 24

25 Scattering Principle: When light interacts with matter different scattering processes can happen simultaneously or exclusively depending on chemical and physical properties of the scatterer. Therefore scattered light contains information about the material, it's size and environmental conditions like temperature. Mie imaging: elastic scattering; same wavelength as the incident light; intensity is proportional to the size of the scattering particles; for particles which are large compared to the wavelength of the incident light. Rayleigh imaging: elastic scattering; same wavelength as the incident light; intensity is proportional to the intensity of incident light, a material-dependant constant and the number density of particles; for particles are small compared to the wavelength of the incident light. Raman imaging: inelastic scattering; shows a spectral response that is shifted from the laser line and characteristic for the Raman active molecules; do not suffer from collision quenching. Applications Mie Scattering : particle analysis (size, shape, distribution) flow analysis (velocity information PIV) spray analysis (particle size distribution and spray geometry) general imaging tasks Rayleigh Scattering: combustion processes pollutant formation total gas density temperature fields Raman Scattering: majority species concentrations space and time-resolved mixture fractions local temperature 25

26 Self Emission Spectroscopy Principle: During combustion many species get excited due to the high temperatures involved, as their electrons come back to the ground state they emit light which can be resolved to give the fingerprint spectra studying which the type and concentration of the species can be determined The two-colour method relies on the measurement of the radiation intensity from soot particles which are generated during combustion. The radiation intensity can then be measured at two wavelengths. Applications Flame temperature, flame location & stability Spatial Soot concentration excited species distribution like OH*, CH*, C2* on-set of ignition Laser Induced Fluorescence (LIF) 26

27 LASER Induced Fluorescence (LIF) Principle: the species of interest is excited using a LASER light of specific wavelength. the electrons move to a higher energy level, emits light at some characteristic wavelengths on returning, this is the fingerprint of the species. the emission spectrum is specific for the molecule. the incident light needs to match the energy levels of the observed molecule. LIF signal is quite strong and can be filtered from the incident laser wavelength. LIF signal is proportional to the volume of a liquid droplet, leads to a direct measurement of the droplet size. LIF and Scattering Spectra Laser Induced Fluorescence (LIF) Molecules/atoms are excited to higher energy states. Intensity of fluorescence is a function of species concentration (number density), and the gas temperature and pressure. Fluorescence is linearly related to number density. Spectral absorption regions are discrete. Fluorescence occurs at wavelengths laser wavelength. A chemiluminescence detector (CLD) is used to control the intake NO concentration during calibration measurements and for additional exhaust gas NO X concentration measurements for the different operating conditions. Selective detection of NO is possible even in inhomogeneous combustion environments like in direct injecting gasoline and diesel engines. This technique allows the effective suppression of interfering LIF signals due to hot oxygen and partially burned hydrocarbons. With this technique, influence of laser beam attenuation is minimized. The LIF images represent the NO concentration present in the plane defined by the position of the laser beam whereas the exhaust gas measurements represent averaged concentrations after homogeneously mixing the burned gases during the expansion and exhaust stroke. 27

28 LASER Induced Fluorescence (LIF) Applications of LIF: OH, NO, O 2, C n H m, H 2 and H 2 O Pollutant formation rot./vib. Temperature spray injection liquid/gas transition velocity fields droplet size LASER Induced Fluorescence (LIF) PLIF: Sheet of LASER light is used to excite; 2-D imaging LIPF: to avoid quenching short lived quantum states are excited and these 'predissociative' states are so fast that no collisions occur during their lifetime Tracer-LIF: a medium is seeded with proper tracer material to make it visible or to observe its mixing with other medium 28

29 Case Studies of LIF Optically Accessible DI Gasoline Engine Schematic of Laser-Based Imaging Setup LIF imaging measurements based on in-cylinder-formed formaldehyde and 3- pentanone as a fuel tracer under controlled auto-igniting (CAI) conditions. Fuel consisting of 50% n-heptane and 50% iso-octane is used to ensure stable auto-ignition while having the reduced compression ratio and temperature typical of most optically accessible engines. Case Studies of LIF NO distribution in a DI diesel engine with PLN and CR injection System: Pump-line-nozzle (PLN) system: it consists of a cam driven pump, a short injection line and an injection nozzle. The injection pressure increases from a low level after start of injection. Common-rail (CR) system: A constant rail pressure is provided. Two different detection systems for recording 2-D images and spectroscopic data 29

30 Case Studies of LIF LIF to visualize the flash boiling effects on the development of GDI engine sprays: LIF is used for spray characterization because of its possibility to differentiate between the liquid and the vapor phase. LIF can be combined with a long distance microscope so it is possible to analyze the spray propagation and evaporation directly at the nozzle orifice and the area nearby. Flash boiling effect causes a rapid spray breakup into a mixture of vapor and small droplets. Planer LIF 30

31 What is Combustion PLIF? Combustion PLIF tells us how the fuel is burning, by showing the location of key species like OH, CH, NO, band CHO. Soot (C2) OH CO 2 + H 2 O + N 2 OH CH CHO C 3 H 8 + O 2 + N 2 What is Combustion PLIF? We work with molecules, NOT particles Species absorbs laser light, emits fluorescence Fluorescence light is collected and analyzed We examine the intensity of this light, and spatial variation Where does it exist in space? It is possible to relate the image intensity to temperature or concentration Propane Flame: Where is the OH? 31

32 How does PLIF work? A sheet of laser light illuminates a plane A target species within the plane of the sheet absorbs light at the wavelength of the laser, exciting the species to a higher energy state The high energy state decays to a lower energy state, emitting a photon The emitted photons are collected on a CCD array The digital image is interpreted Measurements with PLIF Temperature or concentration Heat transfer Mass transfer Mixing ph Possible but not common Species measurement Used to monitor chemical reaction intermediates Combustion, flame, and engine studies Pressure Requires calculations based on known temperature, concentration 32

33 PLIF System Components Illumination Subsystem - Laser (Mixture of Dye Laser & Nd:YAG 266/532 nm Lasers), Beam delivery, Light optics. Wavelength Pulse energy Repetition rate Image Capture Subsystem CCD Camera, Intensified CCD Camera. Capture the Fluorescence image and record them. Analysis Subsystem Calculates and Displays a two-dimensional scalar field from the fluorescence image field. Planer LIF A laser source, usually pulsed and tunable in wavelength, is used to form a thin light sheet. If the laser wavelength is resonant with an optical transition of a species, a fraction of the incident light will be absorbed. Absorbed photons may subsequently be reemitted with a modified spectral distribution. The emitted light, known as fluorescence, is collected and imaged onto a solid-state array camera. The light detected by a camera depends on the concentration of the interrogated species within the corresponding measurement volume and the local flow field conditions. This technique offers excellent temporal resolution (order of ns) and yields information along a thin (0.2 mm and better) 2-D plane. PLIF Imaging of Self-Ignition Centers in SI Engine Plan view into the combustion chamber, showing the self-ignition regions Knock intensity is related to pressure traces. The pressure recorded during knocking operation is non-uniform throughout the cylinder. Analysis of such traces does not yield any spatial information about the selfignition process. Thus, optical techniques (e.g. PLIF) are used to obtain spatially and temporally resolved information. 33

34 Experimental Engine Optical Setup 3D view of Cylinder Head Equipped with Optical Access Setup for PLIF imaging in the combustion chamber PLIF Measurements in HCCI Engine Scania D12 single cyl engine Bowditch type Scania D12 engine Optical setup for measurements in the Scania D12 engine 34

35 Previously, it was impossible to decide about growth of structure. Also, measurements did not reveal if new ignition kernels appeared during the combustion event. High speed imaging system reveal distributed gradual consumption of fuel or reaction fronts that spread. The PLIF sequences shows a well-distributed gradual decay of fuel concentration during the first stage of combustion. During the later parts of the combustion process, the fuel concentration images present much more structure, with distinct edges between islands of unburned fuel and products. Intensity histograms reveals that the transition from fuel to products in the HCCI engine is a gradual process. The engine configuration, laser sheet orientation and air/fuel ratio do not influence the general results. Laser Induced Incandescence (LII) 35

36 LASER Induced Incandescence Principle: intense laser light sheet is used to illuminate and slice the (reactive) particle flow at user defined locations. the particles within the light sheet are heated up to the carbon evaporation temperature (> 4000K). the resultant incandescence (blackbody emission) of the heated particles is detected with a fast shutter camera synchronized to the laser pulse. appropriate filtering and time-gating of the LII emission assure accurate soot volume fraction measurements. Mechanism: It involves heating the particles using an intense laser pulse to their sublimation temperature. A soot particle can absorb energy from the beam, which causes the particle s temperature to increase. At the same time, the soot can loose energy. If the energy absorption rate is sufficiently high, the temperature will rise to levels, where significant incandescence and vaporization can occur. These thermal radiation, when collected after an appropriate time delay is found to be directly proportional to the local mass concentration under specific controlled conditions. LASER Induced Incandescence A technique used for exhaust emission measurements in engines. Planar imaging of soot distributions in steady flames and diesel sprays. LII gives a direct measure of the soot volume fraction (elemental carbon only). It is insensitive to other species. However, sensitivity is limited only by the size of the measurement volume. Neither cooling nor dilution are required, and measurements can be made either in situ or by continuous sampling through an external optical cell. The LII technique is capable of real-time measurements during transient vehicle operation for optimizing soot emissions performance. Also, this technique can be executed with other laser-based techniques to obtain particle size and number density information. Ensemble-averaging for many engine cycles can be used to reconstruct cycleresolved transient behavior. Sectional top view of optical cell for LII measurements 36

37 Laser Holography Sectional top view of optical cell for LII measurements Principle of Holography Method An object beam and a reference beam are required during recording. In-line method: The object beam also serves as the reference beam. It is used for droplet measurement. However, it is difficult to obtain a clear image in an area where ambient density or droplet density is high. Off-axis method: The reference beam and object beam take different optical path. Interference fringes on the holographic plate are recorded by adjusting the difference of the optical path of object beam and reference beam. Reconstruction beam is incident on the holographic plate to reproduce the spray image in the space. Enlarged photograph is taken using CCD camera and diameter of each droplet is measured. Then the 3-Dimensional structure of the droplet is obtained. 37

38 Experimental Setup Principal of Holography Optical Recording System Optical Reconstruction System Laser Holography To measure atomization: Laser holography method, Direct recording method, The PDPA method, and the Fraunhofer diffraction method. The holography method is a 3-D measuring method, which utilizes the interference of light. The Phase Doppler Particle Anemometry (PDPA) method utilizes the Doppler signal of the droplets. The Fraunhofer diffraction method obtains the distribution of the droplet diameter from the distribution of diffracted light. Measuring Methods of Spray Droplets Laser holography method can record the shape of each droplet in the entire spray area and the spatial distribution with one recording. Drawbacks: Droplet measurement in high-density fields, long analyzing period. 38

39 Laser Doppler Velocimetry (LDV) Major Components of LDV System Laser light source Light separation optics Light transmitting optics/ Light collecting optics Photo-detectors Signal processing electronics External data input devices Computer Software Traversing system Experimental Setup of LDV for Diesel Spray Breakup Length Seed particles 39

40 Laser Doppler Velocimetry (LDV) Data at a single point. Offers flexibility. It works in air or water. As micron-sized particles entrained in a fluid pass through the intersection of two laser beams, the scattered light received from the particles fluctuates in intensity. The frequency of this fluctuation is equivalent to the Doppler shift between the incident and scattered light, and is thus proportional to the component of particle velocity. The velocity direction can be fixed if one of the laser beams has a frequency slightly different from that of the other. The frequency is measured using digital computers or photon correlators or spectral analyzers. Applications: Measurements of rotor tip vortices using three-component Laser Doppler Velocimetry. LDV measurements in a boundary layer. Survey of a wake field. Measurements in a shock tube flow. Measurement of Diesel Spray Breakup Length Diesel spray characteristics by laser Doppler signals: Spray tip penetration and spray breakup length are simply obtained by measuring the delay time of Doppler signals from injection start to spray tip arrival at each measuring point. Spray breakup length is estimated by measuring the standard deviation of the delay time of Doppler signals, which indicates dispersion of the time from injection start to Doppler signal rising. Comparison of Various Methods of Flow Measurements Thermal Anemometry LDV Invasive method of measuring 1, 2, or 3 components of velocity using a heated wire or film sensor PIV Non-invasive method of measuring 1, 2, or 3 components of velocity using a laser technique Particle Diagnostics Non-invasive method of measuring 2 and 3 components of velocity in a plane using a double-pulsed laser Non-invasive method of measuring Particle, droplet, or bubble size using laser techniques 40

41 41