Temperature measurements of the gas-phase during surrogate diesel injection using two-color toluene LIF

Planar laser-induced fluorescence (LIF) of toluene has been applied in an optical engine and a high-pressure cell, to determine temperatures of fuel sprays and in-cylinder vapors. The method relies on a redshift of the toluene LIF emission spectrum with increasing temperature. Toluene fluorescence is recorded simultaneously in two disjunct wavelength bands by a two-camera setup. After calibration, the pixel-by-pixel LIF signal ratio is a proxy for the local temperature. A detailed measurement procedure is presented to minimize measurement inaccuracies and to improve precision. n-Heptane is used as the base fuel and 10 % of toluene is added as a tracer. The toluene LIF method is capable of measuring temperatures up to 700 K; above that the signal becomes too weak. The precision of the spray temperature measurements is 4 % and the spatial resolution 1.3 mm. We pay particular attention to the construction of the calibration curve that is required to translate LIF signal ratios into temperature, and to possible limitations in the portability of this curve between different setups. The engine results are compared to those obtained in a constant-volume high-pressure cell, and the fuel spray results obtained in the high-pressure cell are also compared to LES simulations. We find that the hot ambient gas entrained by the head vortex gives rise to a hot zone on the spray axis.     

Quantitative laser-induced fluorescence measurements of nitric oxide in a heavy-duty Diesel engine

We present quantitative, in-cylinder, UV-laser-induced fluorescence measurements of nitric oxide in a heavy-duty Diesel engine. Processing of the raw fluorescence signals includes a detailed correction, based on additional measurements, for the effect of laser beam and fluorescence attenuation, and for the pressure and temperature dependence of the fluorescence efficiency, based on numerical modelling. These corrections are largest early in the stroke, when quenching rates are high and UV transmission is low. Together, they vary over more than three orders of magnitude during the combustion stroke. Fully quantitative results are realised by an overall calibration using independent concentration measurements in the exhaust gas. The data provide evidence of NO formation during both the premixed and mixing-controlled combustion phases.     

In-cylinder gas temperature and water concentration measurements in HCCI engines using a multiplexed-wavelength diode-laser system: Sensor development and initial demonstration

A wavelength-multiplexed, fiber-optic-based, line-of-sight, diode-laser absorption sensor is developed for crank-angle-resolved measurements of temperature and water concentration in a homogeneous-charge-compression-ignition (HCCI) engine. An initial demonstration of its use on two optical HCCI engines at Sandia National Laboratories is reported. The measurements encompassed both motored- and fired-engine operation for temperatures between 300 and 1700 K and pressures between 1 and 55 bar. A spectroscopic line selection process identifies the most appropriate water absorption linepair for thermometry under these conditions. Key solutions to suppress crank-angle-dependent noise in the transmitted laser signals are reported, including careful spectroscopic design and optical engineering to accommodate beam-steering, engine vibration and polarization-related interference. Data obtained through this sensor can provide critical engine characteristics such as combustion efficiency, peak combustion temperature, and autoignition temperature. The flexibility of the wavelength-multiplexed architecture allows the straightforward addition of other wavelengths to potentially enable the simultaneous measurement of other important engine parameters such as temperature non-uniformity, and fuel, CO, and CO₂ concentrations.     

Simultaneous flow field and fuel concentration imaging at 4.8 kHz in an operating engine

High-speed particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) techniques are combined to acquire flow field and fuel concentration in a spray-guided spark-ignited direct-injection (SG-SIDI) engine under motored and fired operation. This is a crucial step to enable studies that seek correlations between marginal engine operation (misfires or partial burns) and local, instantaneous mixture and flow conditions. Correlated flow and fuel data are extracted from a 4 mm×4 mm sub-region directly downstream the spark plug to characterize the in-cylinder conditions next to the spark plug during the spray and ignition event. Values of equivalence ratio, velocity magnitude, shear strain rate, and vorticity all increase during the spray event and decrease an order of magnitude during the duration of the spark event.     

Tunable mid-IR laser absorption sensor for time-resolved hydrocarbon fuel measurements

A wavelength-tunable mid-infrared (mid-IR) laser is used to make time-resolved absorption measurements of methyl-cyclohexane (MCH) and n-dodecane vapor concentration, demonstrating the use of this novel laser source for sensing hydrocarbon fuels. Two sensitive and species-specific diagnostic strategies are investigated: (1) direct absorption at a fixed wavelength, and (2) dual-wavelength differential absorption with two rapidly-alternating wavelengths. The tunable laser light is produced using difference frequency generation by combining two near-infrared diode lasers in a periodically poled lithium niobate crystal, providing a continuous-wave (cw), room temperature mid-IR source with the low intensity noise, and rapid wavelength tunability typical of telecommunications diode lasers. Direct absorption measurements of MCH with a wavelength of 3413.7 nm demonstrate fast time response (1 μs) and low noise in cell (300-675 K) and shock tube (650-1450 K) experiments. The detection limits of MCH range from 0.5 ppm-m at 300 K to 11 ppm-m at 1440 K (pressure = 101 kPa). Next, time-division multiplexing is used to alternately generate two mid-IR wavelengths at 20 kHz, enabling the use of dual-wavelength differential absorption to eliminate interference absorption. Measurements of MCH concentration are first made in a cell, with varying amounts of n-heptane interference absorption. Accurate values of MCH concentration are obtained for n-heptane/MCH ratios as high as 15, demonstrating the utility of this sensor for species-specific hydrocarbon detection in systems with interfering absorption. Finally, time-resolved n-dodecane vapor concentration measurements are made in a shock-heated evaporating aerosol. The dual-wavelength differential absorption diagnostic is sensitive only to the vapor concentration, rejecting droplet extinction. These measurements illustrate the power of the differential absorption strategy for sensitive vapor-phase detection in the presence of particle scattering. The tunability of this new source will allow these concepts to be extended to other hydrocarbon fuels.     

Calibration of imaging laser-induced fluorescence measurements in highly absorbing flames

It has been described earlier that imaging measurements of laser-induced fluorescence (LIF) in flames can be calibrated to number densities with an integrated absorption measurement provided the integrated absorption is small. In this paper a method is presented that extends the technique to flames with substantial absorption, improves the number density determination and allows the experimental parameters to be chosen more freely. The method is based on an iterative computer procedure that reconstructs the 1-D spatially resolved absorption profile from laser measurements of the 1-D spatially resolved LIF and the integrated absorption of the laser beam. The technique is experimentally demonstrated by measurements of OH number densities in atmospheric flames. It is potentially a single-pulse method. Other applications of the iterative procedure are mentioned.     

Influence of the in-cylinder flow on cycle-to-cycle variations in lean combustion DISI engines measured by high-speed scanning-PIV

In this study, the influence of the three-dimensional (3D) in-cylinder flow on engine's cycle-to-cycle variations (CCV) in a spray-guided direct-injection spark-ignition engine is investigated. The engine is operated at homogeneous lean air–fuel mixture which enhances the sensitivity to CCV due to reduced laminar flame speed. To compensate this, intake velocity is increased by a tumble-flap (TF) in the intake-port. To address the 3D-nature of the temporal evolution of the instantaneous in-cylinder flow for different TF-positions, time-resolved scanning particle image velocimetry (Scanning-PIV) is applied to the engine. The required scan-frequency is provided by an acousto-optical-deflector (AOD) to measure the flow field quasi-simultaneously in the central tumble-plane and both mid-valve-planes. The three planes are 18?mm displaced from each other to capture the variability of the large-scale tumble vortex. The in-cylinder flow measurements are combined with combustion analyses by the in-cylinder pressure-trace and the detection of the location of ignition through the evaluation of the luminous spark-plasma. A correlation-map analysis is conducted to identify coherent flow features responsible for CCV of the combustion parameters. This reveals a strong dependency of the spark position to variations of an upward directed flow pointing onto the spark plug. The variations of the upward flow are due to strong CCV of the bended tumble-axis position. An increased tumble motion caused by the TF results in favorable flow conditions by stabilizing the tumble-axis in the middle of the cylinder which decreases the CCV of the spark position significantly. Further correlation analysis including the combustion process exhibits that flow-structures moving the spark and early flame kernel towards the cylinder center reduces the crank angle of 5% heat release and the combustion duration considerably.     

Optical loss measurements in femtosecond laser written waveguides in glass

The optical loss is an important parameter for waveguides used in integrated optics. We measured the optical loss in waveguides written in silicate glass slides with high repetition-rate (MHz) femtosecond laser pulses. The average transmission loss of straight waveguides is about 0.3 dB/mm at a wavelength of 633 nm and 0.05 dB/mm at a wavelength of 1.55 μm. The loss is not polarization dependent and the waveguides allow a minimum bending radius of 36 mm without additional loss. The average numerical aperture of the waveguides is 0.065 at a wavelength of 633 nm and 0.045 at a wavelength of 1.55 μm. In straight waveguides more than 90% of the transmission loss is due to scattering.     

An iterative calculation to derive extinction-to-backscatter ratio based on lidar measurements

The aerosol optical thickness (AOT) is an important parameter for understanding the radiative impact of aerosols. AOT based on lidar measurements is often limited by its finite detection range. In this paper, we have reported a method of fitting and iterative calculation to derive the extinction profile of background aerosols from 0 to 30 km at 532 nm, which is virtually the AOT of the entire atmosphere. The mean extinction derived from this method at the ground level tallies with visibility measurement and it is also consistent with the sun-photometer data, within experimental error. These data have been further treated to study the dust cases. For most of the cases, transmission losses were determined to estimate the extinction as well as lidar ratio. The result of the analysis shows that for background aerosols, a mean lidar ratio of 47±15 sr was found. For dust layers, a mean lidar ratio of 44±19 sr and an optical thickness of 0.53±0.49 were determined at 532 nm.     

Temperature measurement of air convection using a Schlieren system

In this work, we have developed a procedure for full-field measurement of temperature of a fluid flow by using the schlieren technique. The basic idea is to relate the intensity level of each pixel in a schlieren image to the corresponding knife-edge position measured at the exit focal plane of the system. The method is applied in the measurement of temperature fields of the air convection caused by a heated rectangular metal plate (7.3 cm×12 cm). Our tests are carried out at plate temperatures of 50 °C and 80 °C. To validate the proposed method, the schlieren temperature results are compared to those obtained by a thermocouple. Thermocouple data are obtained along two mutually perpendicular directions (one direction along the optical axis, z-direction, and other direction along the x-axis, which is perpendicular to the optical axis) at points located on a 9×9 grid with a variable spacing. The thermocouple measurements were integrated along the z-axis in order to be compared with the measurements obtained by the schlieren technique. The results from the two methods show good agreement between them.     

Optical measurements of silicon wafer temperature

An optical technique for precise, non-contact, and real time measurement of silicon wafer temperature that uses the polarized reflectivity ratio R_p/R_s is described. The proposed method is based on temperature dependence of the optical functions of silicon. Expected strong temperature sensitivity is obtained near band gap. Simultaneous monitoring of temperature and oxide layer thickness is discussed using measurements at four wavelength 365 nm, 405 nm, 546 nm, and 820 nm.     

On-ground simulation of optical links for free-space laser communications

A method of on-ground simulation of optical links for free-space communications by Fourier-transform, image magnification and wavefront sampling with limited apertures was proposed. An optical simulator for free-space laser links was designed, which has the range of equivalent links distances from 160,000 km to 25 km. It can be used for the evaluation of communication performances of optical links in space, particularly for measuring the bit error rate under a constant transmission distance or for testing the equivalent transmission distance under a constant bit error rate. System analysis indicates that the wavefront aberrations of Fourier-transform lens have the main influence on the measurement results. But these effects can be corrected only if the RMS value of wavefront aberrations of Fourier-transform lens on the work aperture is known. Simulation experiment further shows that this method will be feasible by providing excellent optical devices and fine calibration accuracy and by overcoming the environmental disturbances.     

A calibration method for stereo vision sensor with large FOV based on 1D targets

Large FOV (field of view) stereo vision sensor is of great importance in the measurement of large free-form surface. Before using it, the intrinsic and structure parameters of cameras should be calibrated. Traditional methods are mainly based on planar or 3D targets, which are usually expensive and difficult to manufacture especially for large dimension ones. Compared to that the method proposed in this paper is based on 1D (one dimensional) targets, which are easy to operate and with high efficiency. First two 1D targets with multiple feature points are placed randomly, and the cameras acquire multiple images of the targets from different angles of view. With the fixed angle between vectors defined by the two 1D targets we can establish the objective function with intrinsic parameters, which can be later solved by the optimization method. Then the stereo vision sensor with two calibrated cameras is set up, which acquire multiple images of another 1D target with two feature points in unrestrained motion. The initial values of the structure parameters are estimated by the linear method for the known distance between two feature points on the 1D target, while the optimal ones and intrinsic parameters of the stereo vision sensor are estimated with non-linear optimization method by establishing the minimizing function involving all the parameters. The experimental results show that the measurement precision of the stereo vision sensor is 0.046 mm with the working distance of about 3500 mm and the measurement scale of about 4000 mm×3000 mm. The method in this paper is proved suitable for calibration of stereo vision sensor of large-scale measurement field for its easy operation and high efficiency.     

Characterization of evaporated selenide semiconductor alloy on p-type substrates

The properties of chalcogenides that are most important for applications as infrared transmitting materials are reported and their mutual relationships are given. Si₅Se₈₀In₁₅ films were produced by means of thermal evaporation. The refractive index and the optical energy gap were determined by transmission measurements. Parameters considered in this study are density, molar volume, transition temperatures, electrical properties, infrared transmission, extinction coefficient and refractive index. This composition has no extrinsic and intrinsic absorption between 14 and 20 μm and the value of absorption coefficient is estimated lower than 10⁻³ cm⁻¹ at 10.6 μm. This glass is also suitable for infrared optical elements. A p-n junction is observed due to evaporated thin film of alloy on p-type Ge substrate.     

A review of optical measurements at the aerosol and cloud chamber AIDA

This paper provides a survey of recent studies on the optical properties of aerosol and cloud particles that have been conducted at the AIDA facility of Forschungszentrum Karlsruhe (Aerosol Interactions and Dynamics in the Atmosphere). Reflecting the broad accessible temperature range of the AIDA chamber which extends from ambient temperature down to 183 K, the investigations feature a broad diversity of research topics, such as the wavelength-dependence of the specific absorption cross sections of soot and mineral dust aerosols at room temperature, depolarization and infrared extinction measurements of ice crystal clouds generated at temperatures below 235 K, and the optical properties of polar stratospheric cloud constituents whose formation was studied in chamber experiments at temperatures well below 200 K. After reviewing the AIDA research activity of the past decade and introducing the optical instrumentation of the AIDA facility, this paper presents illustrative examples of ongoing and already published work on optical measurements of soot aerosols, mineral dust particles, and ice crystal clouds.     

Experimental and numerical investigation of temperature fluctuations in the near-wall region of an optical reciprocating engine

Accurate prediction of in-cylinder heat transfer processes within internal combustion engines (ICEs) requires a comprehensive understanding of the boundary layer effects in the near-wall region (NWR). This study investigates near-wall temperature fluctuations of an optical reciprocating engine using a combined approach of planar laser-induced fluorescence (PLIF) thermometry and numerical conjugate heat transfer modeling. Single-line excitation of toluene and subsequent one-color emission detection is employed for PLIF thermometry, while large-eddy simulations (LES) using commercial CFD software (CONVERGE v2.4.18) is utilized for modeling. The PLIF signal is calibrated to predicted in-cylinder temperatures from a GT-POWER simulation, and precision uncertainty of temperature is found to be ±1.5 K within the calibration region. Near-wall temperature fluctuations are determined about the multi-cycle mean, and the development of thermal stratification is captured in the NWR under motored and fired conditions during the compression stroke. Regions of the largest cycle-to-cycle temperature fluctuations are identified closer to the in-cylinder head surface indicating the unsteadiness of the thermal boundary layer. Analysis includes an assessment of cyclic variability of near-wall temperature fluctuation, and the effects of compression on temperature fluctuations. Additionally, thermal stratification is found to be similar under motored and fired conditions before ignition timing. Lastly, spatial correlation analysis of temperature fluctuations is performed in the wall-normal direction, and it reveals higher correlations under fired conditions. Spatial correlations experience an initial drop outside of the buffer layer in the NWR, and the location of the drop is well captured in the simulations. Analysis of fluctuating temperatures needs to be extended to fluctuations about the spatial average temperature which directly affects the spatial thermal gradients relevant to engine heat transfer.     

Reverberation time measurements in non-diffuse acoustic field by the modal reverberation time

The increasing presence of low frequency sources and the lack of acoustic standard measurement procedures make the extension of reverberation time measurements to frequencies below 100 Hz necessary. In typical ordinary rooms with volumes between 30 m³ and 200 m³ the sound field is non-diffuse at such low frequencies, entailing inhomogeneities in space and frequency domains. Presence of standing waves is also the main cause of bad quality of listening in terms of clarity and rumble effects. Since standard measurements according to ISO 3382 fail to achieve accurate and precise values in third octave bands due to non-linear decays caused by room modes, a new approach based on reverberation time measurements of single resonant frequencies (the modal reverberation time) has been introduced. From background theory, due to the intrinsic relation between modal decays and half bandwidth of resonant frequencies, two measurement methods have been proposed together with proper measurement procedures: a direct method based on interrupted source signal method, and an indirect method based on half bandwidth measurements. With microphones placed at corners of rectangular rooms in order to detect all modes and maximize SNRs, different source signals were tested. Anti-resonant sine waves and sweep signal turned out to be the most suitable for direct and indirect measurement methods respectively. From spatial measurements in an empty rectangular test room, comparison between direct and indirect methods showed good and significant agreements. This is the first experimental validation of the relation between resonant half bandwidth and modal reverberation time. Furthermore, comparisons between means and standard deviations of modal reverberation times and standard reverberation times in third octave bands confirm the inadequacy of standard procedure to get accurate and precise values at low frequencies with respect to the modal approach. Modal reverberation time measurements applied to furnished ordinary rooms confirm previous results in the limit of modal sound field: for highly damped modes due to furniture or acoustic treatment, the indirect method is not applicable due to strong suppression of modes and the consequent deviation of the acoustic field from a non-diffuse condition to a damped modal condition, while standard reverberation times align with direct method values. In the future, further investigations will be necessary in different rooms to improve uncertainty evaluation.     

Optical limiting behavior of Sudan III dye doped polymer

The optical limiting performance of Sudan III dye doped into ethylene propylene diene polymethylene polymer (EPDM) is investigated using 532 nm, 10 ns pulses from a frequency-doubled Nd-YAG laser. The optical limiting behavior is investigated by transmission measurement through the sample at different concentrations. Our results show that the optical limiting efficiency is concentration dependent.     

Low loss optical channel waveguides for the infrared range using niobium based hybrid sol-gel material

In this work, we report the fabrication of single-mode Nb₂O₅ based hybrid sol-gel channel waveguides. Nb₂O₅ based hybrid sol-gel material has been deposited by spin-coating on silicon substrate and channel waveguides have been fabricated by a UV direct laser writing process. Optical guided modes have been observed to confirm single-mode conditions and optical propagation loss measurements have been performed using the cut-back technique. Optical propagation losses were measured to be 0.8 dB/cm and 2.4 dB/cm at 1.31 μm and 1.55 μm respectively. These experimental results demonstrate low loss optical waveguiding within the infrared range and are very promising in view of material choice for the development of integrated optical devices for telecommunication.     

Adaptive upstream optical power adjustment depending on required power budget in PON access

According to the present passive optical network (PON) standard, the fiber transmission lengths are from 500 m to 20 km between the optical line terminal (OLT) and different optical network units (ONUs). It will result in difference power losses (ΔP_loss) from 4 to 5 dB. Hence, we propose to adjust adaptively the output optical power of the upstream laser diode (LD) depending on the different fiber lengths. With the different fiber transmission lengths, we can properly adjust the bias current and modulation index of upstream LD for energy-saving. We characterize and analyze experimentally the relationship of output optical power and modulation amplitude Vamp under different fiber transmissions in PON access. Moreover, due to the adaptive power control of upstream signal, the optical upstream equalization also can be retrieved with power variation of 1.1 dB in this experiment.