High-temperature Air Flows Research Articles

Numerical simulation of sound attenuation in an acoustically lined duct in high-temperature air flows

Noise reduction structures are important for the vibration and noise reduction design of aerospace engines. The design of noise-reducing structures often needs to be quickly evaluated via numerical simulations. Hence, the simulation results of the corresponding system are very important for guiding the design of noise-reducing structures. High temperature is one of the key environmental factors that need to be considered when evaluating the sound attenuation process via numerical methods. In this study, numerical simulations of acoustic wave propagation on an acoustic liner structure considering air temperature variations are carried out by using compressible Navier–Stokes equations and the ideal gas equation of state. The results showed that the effect of temperature on sound attenuation under grazing flow conditions is complex. Moreover, an increase in temperature will reduce the transmission loss of the acoustic liner in the grazing flow at high air speed.

Stochastic determination of thermal reaction rate coefficients for air plasmas.

This work deals with the stochastic inference of gas-phase chemical reaction rates in high temperature air flows from plasma wind tunnel experimental data. First, a Bayesian approach is developed to include not only measurements but also additional information related to how the experiment is performed. To cope with the resulting computationally demanding likelihood, we use the Morris screening method to find the reactions that influence the solution to the stochastic inverse problem from a mechanism comprising 21 different reactions for an air mixture with seven species: O2, N2, NO, NO+, O, N, e-. A set of six reactions, mainly involving nitrogen dissociation and exchange, are the ones identified to impact the solution the most. As such, they are assumed to be uncertain and estimated along with the boundary conditions of the experiment and the catalytic recombination parameters of the materials involved in the testing. The remaining 15 reactions are set to their nominal values. The posterior distribution is then propagated through the proposed boundary layer model to produce the posterior predictive distributions of the temperature and mass fraction profiles along the boundary layer stagnation line. It is identified that NO concentrations have the largest increase in uncertainty levels compared to cases where the inference problem is carried out for fixed chemical model parameter values. This allows us to inform a new experimental campaign targeting the reduction of uncertainties affecting the chemical models.

Evaporative and Wicking Functionalities at Hot Airflows of Laser Nano-/Microstructured Ti-6Al-4V Material.

A novel multifunctional material with efficient wicking and evaporative functionalities was fabricated using hierarchical surface nano-/microstructuring by femtosecond laser micromachining. The created material exhibits excellent multifunctional performance. Our experiments in a wind tunnel demonstrate its good wicking and evaporative functionalities under the conditions of high-temperature airflows. An important finding of this work is the significantly enhanced evaporation rate of the created material compared with the free water surface. The obtained results provide a platform for the practical implementation of Maisotsenko-cycle cooling technologies for substantially increasing efficiency in power generation, thermal management, and other evaporation-based technologies. The developed multifunctional material demonstrates long-lasting wicking and evaporative functionalities that are resistant to degradation under high-temperature airflows, indicating its suitability for practical applications.

Development of a Stark shift measurement technique using excited-state oxygen atoms to determine electron number density in shock heated O2/Ar above 10 000 K

We present measurements of electron number density, n e, in partially ionized gases containing electronically excited atomic oxygen, at nominal translational temperatures between 10 100–11 200 K. The measurements were based on the Stark shift of the O() to O() transition at 926 nm. The current work scanned the laser across the transition and utilized the Stark shift of the absorbance for robust determination of n e. The measurements were conducted in a shock-heated 1% O2/Ar mixture, with a measurement time resolution of 20 μs. A line-center shift of −0.06 cm−1 was observed within around 500 μs of the test time after the mixture was instantaneously heated to 11 200 K by the reflected shock wave, which corresponded to an electron number density increase from 0 to 2 × 1021 m−3 due to collisional ionization. The measurement method and resulting data may be of interest in future studies of high-temperature air flows.

Models validation and code profiling in state-to-state simulations of shock heated air flows

The objective of this study is to examine various combinations of vibration-chemical coupling models in high-temperature air and oxygen flows behind shock waves. Simulations are carried out in the frame of the state-to-state approach for detailed vibrational and chemical kinetics in the Euler approximation. Great variety of state-resolved models for the rate coefficients of vibrational energy transitions and chemical reactions are assessed and validated against experimental data; among them are first-order perturbation (SSH) and forced harmonic oscillator (FHO) models; well-known Marrone–Treanor dissociation model with parameters either obtained empirically or by fitting quasiclassical trajectory calculations; new models for state-specific Zeldovich reactions. To validate the models, the results of numerical flow simulations are compared with experimental investigations in a wide range of initial mixture compositions and shock wave velocities. Among numerous model variants, a few models providing the best agreement with the experimental data on the vibrational temperature in oxygen flows and measured NO radiation in air flows are recommended for different sets of free stream conditions. The impact of NO excited states produced in Zeldovich reactions is evaluated; it is shown that neglecting the vibrational excitation of NO molecules may lead to significantly underpredicted maximum of NO number density. In the framework of the code performance study, Matlab and Fortran implementations were compared, the most significant bottlenecks procedures were identified by profiling analysis, and remarkable speed-up was achieved by code optimisation.

Numerical investigation of heat transfer characteristics of high-speed and high-temperature air cooled open-cycle reactor

The nuclear aircraft is one important type in the application of near space. The high-speed and high-temperature air flows through the open-cycle reactor of the aircraft engine differs from typical reactors. The heat transfer characteristics of high-speed air flow have great effects on the reactor performance. Therefore, the simulation of the air flow in the reactor coolant channel is conducted. Firstly, the numerical methods are validated against experiments. Subsequently, the effects of the distribution of the heat flux, inlet velocity, temperature and pressure on the heat transfer performance of the coolant channel are investigated. The results reveal that the heat transfer correlations evaluated at average bulk temperature are more appropriate compared to that evaluated at the average film or wall temperature. The heat flux influences the heat transfer coefficient indirectly due to the varied air properties. The higher inlet air temperature means the larger thermal conductivity, which leads to the higher heat transfer coefficient. The heat transfer coefficient of air under high-pressure conditions can be equivalent to that of water under the forced circulation conditions. The research can lay basis for the analysis of the flow and heat transfer characteristics of the air flow in the open-cycle reactor.

Impact of Residential Building Layouts on Microclimate in a High Temperature and High Humidity Region

Microclimatic condition is a fundamental indicator for evaluating outdoor space livability and vitality. Research has shown that poorly designed building layouts can lead to discomfort; however, the mechanisms influencing outdoor microclimate based on residential building layout are unclear for high temperature and high humidity regions. This study explores the relationship between residential building layouts and the outdoor wind and thermal environment at the pedestrian level in Wuhan, a city renowned for high temperatures and high humidity. Six typical residential building layouts were simulated, using the ENVI-met numerical model, to determine the spatial distribution of wind speed and air temperature. The Universal Thermal Climate Index was adopted as a comprehensive index with which to assess spatial and diurnal variations in microclimates surrounding each building layout. Results showed that parallel building layouts formed a ventilation corridor that increased wind speeds by approximately 0.3 m/s. A staggered building layout, in line with the prevailing wind direction, facilitated airflow in the ventilation corridor and further increased wind speeds. Windward buildings blocked high-temperature airflows and reduced air temperatures by approximately 1 ℃ in parallel layouts, and 1.4 ℃ in enclosed layouts. However, the cooling effect of windward buildings on high-temperature airflow was weaker than the warming effect caused by the wind shadow effect and direct sunlight. Additionally, the performance of the thermal comfort of the enclosed type layout was significantly better, for most of the day, than the parallel type layout.

A theoretical and computational study of the vibration excitation on the transition criteria of shock wave reflections

In this paper, we study the vibration excitation on the reflection of shock waves in hypersonic flows by using analytical and computational approaches. First, a theoretical approach is established to solve the shock relations which are further applied to develop the shock polar analytical method for high-temperature air. Then, a comparative investigation using calorically perfect gas model and thermally perfect gas model considering vibration excitation indicates an obvious change to the overall profile of the shock polar. The post-shock pressure increases within the strong branch of the shock polar while decreases within the weak branch due to vibration excitation of air molecules. A more notable phenomenon is the increase in the maximum deflection angle of the shock polar which can significantly influence the detachment criterion of shock reflection transition in high-temperature air flows. The shock polar analysis of shock reflection shows that the vibration excitation result in an obvious increase to the detachment criterion while a slight increase to the von Neumann criterion. A series of computations are conducted to confirm the above analytical findings on the shock reflection considering the vibration excitation. A slight difference of transition criterion between the theory and computations is found to be caused by the existence of the expansion fan which is an inherent flow structure. The proposed shock polar analytical method is proved to be an effective but simple approach for the study of shock wave reflections in hypersonic flows.

Modeling of Vibration-Vibration Energy Transfer in the DSMC Method

Modeling of vibration-vibration (V-V) energy transfer in collisions of diatomic anharmonic oscillators is discussed, applicable for the direct simulation Monte Carlo method. An attempt is made to take into account the fast vibrational relaxation of nitrogen through V-V exchange with oxygen by increasing the vibration-translation relaxation rate by a constant factor. The results have shown that such an approach still significantly overpredicts the available relaxation times. Then, a new approach to account for V-V exchanges is proposed, which incorporates the calculation of near-resonant transitions according to a temperature-dependent probability calibrated with available experimental data. That approach was found to provide quantitative agreement with shock tube measurements of vibrational relaxation in mixtures. While targeting primarily high-temperature air flows, the approach is applicable to any flow where the VV energy transfer needs to be modeled. It provides the benefits of numerical efficiency, simplicity of implementation, and acceptable physical accuracy.

An Experimental Study of Combustion and Emissions of Wheat Straw Pellets in High-Temperature Air Flows

ABSTRACTThe present work is performed to investigate the combustion and emission behavior of wheat straw pellets of different sizes and shapes in a high temperature air flow. Prior to the combustion experiments, thermogravimetric analysis of the pellet was carried out to understand the thermo-chemical characteristics of the wheat straw pellet. A sequential method was used to determine the kinetic parameters. A combustor using preheated air is used to investigate the effect of different parameters, such as pellet size, starting temperature, and primary air flow velocity on combustion and emission characteristics of biomass pellets. The surface temperatures as well as the time-dependent mass loss rates are recorded to study the ignition behavior of pellets. The results showed that the increase in starting temperature, and air flow velocity and decrease in pellet size leads to a reduction in combustion time and increase in the total combustion rate. The results also showed that the concentration of CO formed increases as the heating time increases and the formation of CO2 increased with increasing the starting air temperature and air flow velocity inside the combustion chamber. It was found that the combustion efficiency ranged from 99% to 100% and the combustion efficiency reached 100% in some experiments due to complete combustion.

Performance of constant exhaust ventilation for removal of transient high-temperature contaminated airflows and ventilation-performance comparison between two local exhaust hoods

Transient high-temperature contaminated airflows generated during the pouring of high-temperature materials are widespread in Chinese industrial plants. In this study, the performance of both the local exhaust and the general exhaust with a constant flow rate to remove these transient contaminants was experimentally investigated, and the ventilation performance was compared between two local exhaust hoods. This study focused on the dispersion and removal characteristics of contaminant concentrations under the conditions of high-temperature airflows. The results showed that the cumulative contaminant concentration could be divided into four time slots and that there was a time lag between the cumulative concentration of the local and general exhaust. Additionally, the fitting formula of time-varying concentration decay was discussed to highlight the advantages of the designed hood performance. Finally, the dynamic evaluation index (TCRRt) for evaluating exhaust hoods on removing transient contaminants was proposed, and the connections between the proposed index and the traditional capture efficiency were illustrated. The minimum TCRRt for the improved exhaust hood was invariably greater than that for the existing hood for each hood flow rate. This study demonstrates that increasing the containment volume of the exhaust hood can help to remove transient high-temperature contaminants in industrial plants.

A high temperature seeding technique for particle image velocimetry

Non-intrusive measurements of gas velocities via particle image velocimetry (PIV) or laser Doppler velocimetry (LDV) requires entraining particles into the flow field. There are many techniques and materials available for seeding gas phase flows. However, when the flow temperatures exceed 200 °C, the available options for seed materials becomes limited. In high temperature applications refractory seed materials are required. The established technique for seeding flows with metal oxide powders is via fluidized beds by themselves or in combination with cyclone separators. These systems are fraught with problems which limit their ability to provide consistent, uniform flow seeding. In this work, we describe a technique for reliably introducing metal oxide particles into high temperature flows. The employment of pH stabilization techniques typically used to obtain stable dispersions in ceramic processing can provide a source of seed material for high temperature air flows. By pH stabilizing submicron alumina particles in ethanol, a stable dispersion is obtained which when atomized, produces a high quality aerosol. Commercial grade alumina is used with a moderate size distribution. The technique is not limited to alumina/ethanol and is also demonstrated with an alumina/H2O system. Other ceramic powders in other polar solvents can also be used once their point of zero charge (pHpzc) of the powder in the solvent has been determined. We present an example of the pH stabilized dispersions applied to a very challenging high temperature supersonic flow and a particle dynamics analysis across a shock.

Ignition Behavior of Bio-Coke (Highly Densified Biomass Fuel) in High-Temperature Air Flows

The ignition behavior of a newly developed biomass briquette, Bio-coke (BIC), is investigated. The fuel has unique features such as economical advantages for its versatility of biomass resources, high volumetric calorific value because of its high density (1300 kg/m3; twice or more than that of ordinary wood pellets) and high mechanical strength. The ignition characteristics of cylindrical BIC blocks (48 mm in diameter and 85 mm in length), important when using the fuel in actual combustion furnaces, are investigated in high temperature air flows (473-873 K, 550-750 NL/min.). In the experiments, preheated air is blown onto the bottom surface of BIC cylinders and the ignition behavior of the bottom surface is observed monitoring the surface temperature as well as the time dependent mass loss rates. The results show two ignition modes; (1) solid surface ignition preceding gas-phase ignition in high air temperature conditions (T≥598K), and (2) gas-phase ignition accompanied by simultaneous surface ignition occurring at relatively low air temperature conditions. The appearance of each mode depends on the preheated air supply condition in terms of the air temperature, flow velocity, and moisture content of the fuel. The rate of evolution of volatile gases is closely correlated with the temperature distribution inside the BIC briquette which depends on the heating rate, implying that variations in the temperature distribution inside the fuel could be one reason for the appearance of the observed ignition modes. It is suggested that the temperature distribution inside the fuel has to be taken into account in the control of the ignition behavior of BIC briquettes.

Seeding of high temperature air flow

Seeding of high temperature air flows is problematic because common seeding materials can ignite and may pose a threat for human health in case of inhalation. Sodium chloride is proposed as a seeding material to avoid these problems. The generation of crystals using an ultrasonic atomizer is described and the measured particle size distribution is presented. The described method has the additional advantage that the particle size can easily be adjusted. This allows the operator to comply with the experimental requirements in terms of resolvable frequencies. The described method has been successfully applied to LDA measurements in a 500°C air flow.

Computation of high-temperature equilibrium airflows using discontinuous Galerkin finite element method

In this paper, the discontinuous Galerkin (DG) finite element method is applied to simulate high-temperature equilibrium airflows. Two approximate Riemann solvers are used to compute the advective flux. A second-order TVD Runge–Kutta time integration scheme is employed to march the solution in time. The extension to equilibrium chemistry is implemented by using the effective γ approach. The effective γ is obtained through a polynomial curve fit. With this approach, the computation of the species composition of the equilibrium air can be completely separated from the main solver. Several numerical examples are provided to demonstrate the effectiveness and accuracy of the DG method in chemically reacting airflows. Copyright © 2005 John Wiley & Sons, Ltd.

A Study of a Method for Temperature Measurement of High Temperature Air Flows by O2-LIPF.

We propose a new method for measurement of temperature in high temperature air using LIF(laser-induced fluorescence)of O2 molecules excited by a broad band ArF excimer laser. In this method, temperature can be measured by using temperature dependence of ratios between fluorescence intensities integrated over two spectral regions including spectral peaks. The theoretical broadband spectra of O2 molecules are calculated and compared with the experimental ones of static hot air(300K-1100K)inside an electric furnace. Temperatures measured by this method agree with those measured by a thermocouple within about ± 100K at a temperature below 1000K.

A Study of a Method for Temperature Measurement of High Temperature Air Flows by O2-LIPF. Temperature measurement using a broadband laser.

We propose a new method for the measurement of temperature in high temperature air using the LIPF (laser-induced predissociative fluorescence) of O2 molecules excited by a broadband ArF excimer laser. In this method, temperature is measured using the temperature dependence of the ratios between the fluorescence intensities integrated in two spectral regions including peaks. The theoretical broadband spectra of O2 molecules are calculated and compared with the experimental spectra of static hot air (300∼1100K) inside an electric furnace. Temperatures measured using this method agree with those measured by a thermocouple within about ±100 K at temperatures below 1000K.