Yan Zhao

Simultaneous measurements of multiple flow parameters for scramjet characterization using tunable diode-laser sensors

This paper reports the simultaneous measurements of multiple flow parameters in a scramjet facility operating at a nominal Mach number of 2.5 using a sensing system based on tunable diode-laser absorption spectroscopy (TDLAS). The TDLAS system measures velocity, temperature, and water vapor partial pressure at three different locations of the scramjet: the inlet, the combustion region near the flame stabilization cavity, and the exit of the combustor. These measurements enable the determination of the variation of the Mach number and the combustion mode in the scramjet engine, which are critical for evaluating the combustion efficiency and optimizing engine performance. The results obtained in this work clearly demonstrated the applicability of TDLAS sensors in harsh and high-speed environments. The TDLAS system, due to its unique virtues, is expected to play an important role in the development of scramjet engines.

Multidimensional Monte Carlo model for two-photon laser-induced fluorescence and amplified spontaneous emission

Abstract This paper describes the development of a multidimensional model based on the Monte Carlo (MC) method for the modeling of laser-induced fluorescence (LIF) and amplified spontaneous emission (ASE) signals involved in multi-photon processes. Multi-photon LIF finds applications in a broad range of topics; however, the interpretation of the LIF signal is plagued by the nonlinear effects caused by the ASE. Past work focused on developing one-dimensional (1D) models. Therefore, this work developed an MC method to solve the governing equations of ASE and LIF in multidimension. The results were validated using existing 1D data, both experimental and modeling. The results suggest that past 1D models cause noticeable error in the ASE signal even when the measurement volume has a large aspect ratio. We expect this work to facilitate the ongoing research of multi-photon LIF, and to stimulate new experiments that can provide data to validate the model in 2D.

Demonstration of a New Laser Diagnostic Based on Photodissociation Spectroscopy for Imaging Mixture Fraction in a Non-premixed Jet Flame

The study of turbulent combustion calls for new diagnostics that can measure multidimensional mixture fraction under a wide range of flame conditions. A laser diagnostic technique based on photodissociation spectroscopy (PDS) is proposed to address this need. This paper describes the concept of the PDS-based diagnostic, reports its experimental demonstration in a non-premixed jet flame, and assesses its performance and applicable range. This new technique is centered around the creative use of photodissociation (PD) for flow visualization. A carefully chosen PD precursor is seeded into the flow of interest to measure mixture fraction. The precursor is chosen such that (1) both the precursor itself and the products formed from the precursor (if it reacts) can be completely and rapidly photodissociated; thus, the concentration of one of the photofragments forms a conserved scalar and can be used to infer the mixture fraction, and (2) the target photofragment offers friendly spectroscopic properties (e.g., strong laser-induced fluorescence signals and/or simple signal interpretation) so multidimensional imaging can be readily obtained. Molecular iodine (I2) was identified as a precursor satisfying both requirements and was seeded into a carbon monoxide (CO)–air jet flame for single-shot two-dimensional imaging of mixture fraction. This demonstration illustrates the potential of the PDS-based technique to overcome the limitations of existing techniques and to provide multidimensional measurements of mixture fraction in a variety of reactive flows.

Assessment of a Novel Flow Visualization Technique Using Photodissociation Spectroscopy

The study of complicated flows continuously calls for new nonintrusive flow diagnostics. A novel flow visualization technique based on photodissociation spectroscopy (PDS) is described, demonstrated, and assessed in this paper. This technique is centered around the creative use of photodissociation (PD). A PD precursor is seeded in the flow of interest, either passive or reactive. A laser pulse is then generated to completely and rapidly photodissociate both the precursor and the products formed from the precursor (if it reacts) into photofragments. A target photofragment is then imaged to obtain multidimensional information about the flow. An analytical methodology was developed to assess the feasibility of the PDS-based technique. This analytical method was applied to the case where molecular iodine was used as an example PD precursor, and the results were validated by experimental data. Both the analytical and experimental findings provided a promising outlook for this new technique as a practical flow visualization technique. With a properly chosen PD precursor, the PDS-based technique provides an attractive alternative for imaging several critical flow properties, including the mixture fraction and temperature field. This technique shares some key advantages with established techniques, e.g., a high spatial and temporal resolution comparable to the planar laser-induced fluorescence (PLIF) technique. Meanwhile, this technique offers several unique advantages to overcome the limitations of existing techniques, including enhancing the signal level and simplifying the interpretation of the signal.

Method to correct the distortion caused by amplified stimulated emission as motivated by LIF-based flow diagnostics

Amplified stimulated emission (ASE) represents a significant issue in two-photon laser-induced fluorescence (TPLIF). The ASE effects are nonlinear and nonlocal, i.e., the ASE effects distort the LIF signal nonlinearly, and the distortion at one location depends on conditions at other locations. In this sense, the ASE effects pose a greater challenge to quantitative TPLIF than quenching and ionization. This work therefore seeks a method to correct such distortion. The method uses two LIF measurements, one with low signal-to-noise ratio (SNR) and negligible ASE distortion and another with high SNR but significant distortion, to generate a faithful measurement with high SNR. Extensive simulations were performed to evaluate the performance of this method for practical applications.

Multidimensional Monte Carlo Model for Amplified Spontaneous Emission

A new model based on the Monte Carlo method has been developed to analyze amplified spontaneous emission in multidimensional. This paper introduces the model, its validation, and implication for diagnostics with high laser irradiance.

Biomedical OpticsInformation Content of Spectral Depolarization in Scattering Measurements

A method is developed to analyze the information content of spectral depolarization in scattering measurements. Applications of this method to the detection of airborne biological agents and energetic nanoparticles will be discussed. Article not available.