A Photoacoustic-Based Measurement System for Dual Detection of NO2 and CO2 in Combustion Exhaust Gases

In this article, a low-cost, low-complexity photoacoustic (PA) sensing system for the simultaneous detection of CO2 and NO2 in exhaust gas is presented. The proposed system is designed as part of a continuous emissions monitoring system (CEMS) for gas turbine emissions. The system exploits the amplification of the PA signal provided by an acoustic ring resonator, which is characterized by a simple and robust structure and is suitable for in- field measurements. The dual gas detection is obtained by exploiting two measurement principles; the first one, dedicated to the detection of NO2, which is present in the target mixture in the ppm range, is the classical PA effect. In fact, the optical source is a light-emitting diode (LED) with a center wavelength of 405 nm matched on an adsorption peak of NO2. This allows for deriving the NO2 concentration measurement directly from the amplitude of the PA signal. The other mechanism is used to measure the concentration of one of the major components of the exhaust gas, with a concentration in the range of some percentage. The quantity of CO2 is sensed, exploiting its effect on the sound speed, and consequently on the resonance frequency of the resonator. To measure the CO2 concentration, the system automatically tracks the acoustic resonance shift. The detection of the two gases is realized simultaneously by a unique sensor with real-time measurements. A laboratory characterization of the proposed systems showed its feasibility. Experimental results show the possibility to detect NO2 with a resolution lower than 1 ppm, whereas CO2 resolution is about 0.2%

Frequency-Resolved High-Frequency Broadband Measurement of Acoustic Longitudinal Waves by Laser-Based Excitation and Detection

Optoacoustics is a metrology widely used for material characterisation. In this study, a measurement setup for the selective determination of the frequency-resolved phase velocities and attenuations of longitudinal waves over a wide frequency range (3–55 MHz) is presented. The ultrasonic waves in this setup were excited by a pulsed laser within an absorption layer in the thermoelastic regime and directed through a layer of water onto a sample. The acoustic waves were detected using a self-built adaptive interferometer with a photorefractive crystal. The instrument transmits compression waves only, is low-contact, non-destructive, and has a sample-independent excitation. The limitations of the approach were studied both by simulation and experiments to determine how the frequency range and precision can be improved. It was shown that measurements are possible for all investigated materials (silicon, silicone, aluminium, and water) and that the relative error for the phase velocity is less than 0.2%

On Dispersion Compensation for GAW-Based Structural Health Monitoring

Guided acoustic waves (GAW) have proven to be a useful tool for structural health monitoring (SHM). However, the dispersive nature of commonly used Lamb waves compromises the spatial resolution making it difficult to detect small or weakly reflective defects. Here we demonstrate an approach that can compensate for the dispersive effects, allowing advanced algorithms to be used with significantly higher signal-to-noise ratio and spatial resolution. In this paper, the sign coherence factor (SCF) extension of the total focusing method (TFM) algorithm is used. The effectiveness is examined by numerical simulation and experimentally demonstrated by detecting weakly reflective layers with a highly dispersive A0 mode on an aluminum plate, which are not detectable without compensating for the dispersion effects

Simulating copolymeric nanoparticle assembly in the co-solvent method: How mixing rates control final particle sizes and morphologies

The self-assembly of copolymeric vesicles and micelles in micromixers is studied by External Potential Dynamics (EPD) simulations – a dynamic density functional approach that explicitly accounts for the polymer architecture both at the level of thermodynamics and dynamics. Specifically, we focus on the co-solvent method, where nanoparticle precipitation is triggered by mixing a poor co-solvent into a homogeneous copolymer solution in a micromixer. Experimentally, it has been reported that the flow rate in the micromixers influences the size of the resulting particles as well as their morphology: At small flow rates, vesicles dominate; with increasing flow rate, more and more micelles form, and the size of the particles decreases. Our simulation model is based on the assumption that the flow rate mainly sets the rate of mixing of solvent and co-solvent. The simulations reproduce the experimental observations at an almost quantitative level and provide insight into the underlying physical mechanisms: First, they confirm an earlier conjecture according to which the size control takes place in the earliest stage of the particle self-assembly, during the spinodal decomposition of polymers and solvent. Second, they reveal a crossover between different morphological regimes as a function of mixing rate. Hence they demonstrate that varying the mixing rate in a co-solvent setup is an effective way to control two key properties of drug delivery systems, their mean size and their morphology

Systematic classification of micromixers

This work is a systematic classification of passive micromixers according to their performance. The basis is the introduction of an efficiency measure which relates both mixing quality and pressure drop. This measure is then applied to data from several experimental characterizations. It is found that micromixers can be classified into two groups: inertial micromixers with a strong increase in performance above a critical flow rate and mixers based on lamination with a continuous change of the efficiency with growing Reynolds number. This classification is achieved by analysis of the mixing performance rather than describing the mixing principle

Dynamic model for investigation of instabilities in microchannel evaporators

Microchannel evaporators are widely used for both cooling applications and for vapor generation in process engineering. They allow high heat transfer rates within minimal space. However, the massive changes of the fluids properties during evaporation can trigger different types of instabilities. These mostly undesired effects are caused by complex interactions of heat conduction, heat transfer and fluid flow within the evaporator. A comprehensive dynamic model is developed to obtain a deeper understanding of the effects occurring her. Experimental validations show that the model is capable of correctly predicting the pressure drop behavior of practical microchannel evaporators

Ultrasound Measurement Technique for Validation of Cryogenic Flows

An ultrasound sensor system based on the transmission-mode approach is developed to enable the monitoring and sensing of cryogenic liquids and gases—especially gaseous bubbles and gas-liquid interfaces in liquid nitrogen (LN2). Common sensors do not meet requirements of cryogenic and microgravity-environments. Therefore, a special encapsulation design for the optimization of the electrical connection and the mechanical coupling of the ultrasound sensors is needed. The ultrasound system is qualified in LN2 and is able to measure bubbles (size and location) and fill levels with a high spatial resolution in a submillimetre range and a sampling rate of more than 500 Hz