High-enthalpy Flow Research Articles

Temperature measurement in the PMMA k-phase during combustion in a high enthalpy flowunder the electrostatic field influence

The purpose of present work is to clarify the mechanisms of the electrostatic field influence on the polymers (PMMA) combustion in high enthalpy flows. For this, the PMMA temperature profiles during combustion in a hybrid rocket engine studies were carried out. Based on the results obtained the assumption about the intensification of the substance entrainment from the polymer surface combusting in a high-enthalpy flow under the electrostatic field influence is confirmed.

On the Formation and Accumulation of Solid Carbon Particles in High-Enthalpy Flows Mimicking Re-Entry in the Titan Atmosphere

The problem relating to the formation of solid particles enabled by hypersonic re-entry in methane-containing atmospheres (such as that of Titan) has been tackled in the framework of a combined experimental–numerical approach implemented via a three-level analysis hierarchy. First experimental tests have been conducted using a wind tunnel driven by an industrial arc-heated facility operating with nitrogen as working gas (the SPES, i.e., the Small Planetary Entry Simulator). The formation of solid phases as a result of the complex chemical reactions established in such conditions has been detected and quantitatively measured with high accuracy. In a second stage of the study, insights into the related formation process have been obtained by using multispecies models relying on the NASA CEA code and the Direct Simulation Monte Carlo (DSMC) method. Through this approach the range of flow enthalpies in which carbonaceous deposits can be formed has been identified, obtaining good agreement with the experimental findings. Finally, the deposited substance has been analyzed by means of a set of complementary diagnostic techniques, i.e., SEM, spectroscopy (Raman, FTIR, UV–visible absorption and fluorescence), GC–MS and TGA. It has been found that carbon produced by the interaction of the simulated Titan atmosphere with a solid probe at very high temperatures can be separated into two chemically different fractions, which also include “tholins”.

Revisiting slug calorimeters for hypersonics

This paper describes the first steps toward developing a highly-accurate multi-layered slug calorimeter for estimating the surface heat flux in high-enthalpy flows associated with hypersonic ground test campaigns. Slug geometry, material choices, test conditions, manufacturing method, mathematical modeling and instrumentation selection must be integrated into the design process in order to achieve the desired accuracy outcome. Integration is often overlooked in lieu of simplified data reduction equations that relate in-depth temperature measurements to the desired surface heat flux. Simplified models are often physics deficient and can lead to misleading interpretations. This paper illustrates the need for inclusive modeling in arriving at the data reduction equation for the heat flux gauge. Several important results are highlighted in this paper including the: (a) identification of data reduction model discrepancies; (b) significance of Volterra integral formulations for algorithm development; (c) implementation of a parameter-free preconditioner operated on data reduction model for low-pass filtering; (d) development of the approximation thread for uncertainty propagation; (e) achievement of stability through the future-time method; and, (f) extraction of the optimal regularization parameter through phase-plane and cross-correlation concepts for estimating the “best” heat flux.

Mathematical Modeling of Active Thermionic Heat Protection in a High Enthalpy Flow past a Shell

A mathematical model of the process of unsteady heat transfer of a thermionic heat-protection system in convective heating has been formulated and investigated. The effects of evaporation (emission) of electrons from the surface of the emitter on the drop in the temperature of a composite thermionic heat-protection shell have been demonstrated. The effects of some types of heat-transfer agents in a composite shell on the heat-transfer regimes in a body have been investigated. Qualitative agreement of the computation results with the existing data has been achieved.

Performance analysis of a rotating detonation combustor based on stagnation pressure measurements

This study explores the effect of differing inlet and outlet boundary conditions on the operation and performance of a rotating detonation combustor (RDC) over an annulus mass flux range of 50 to 210 kgs−1m−2 and equivalence ratios of 0.7, 1.0, and 1.3. The RDC is equipped with either a uniform outlet restriction or with a set of nozzle guide vanes to simulate turbine integration. Stagnation pressure data from Kiel probes placed in the high-enthalpy exhaust flow are presented for the operational envelope. The RDC’s operation is categorized into different modes distinguished by the number of co- and counter-rotating combustion waves in the annulus. With increasing mass flux, a typical progression proceeding from a pair of counter-rotating waves, to a single detonation wave, and then further to multiple co-rotating waves is observed with wave speeds reaching up to 84% of the CJ velocity. It is shown that a choking condition at the outlet throat correlates with the transition from two counter-rotating waves to a single wave detonation regime. The channel Mach number is then calculated from measured pressure ratios and is shown to agree with area ratio-based estimates. The pressure gain of the RDC is expressed as the stagnation pressure change from the air plenum to the outlet throat, and it is shown that the specific operating mode of the device – in conjunction with the chosen injector and outlet area ratios – can significantly decrease the pressure gain performance in some cases, while not significantly affecting it in others. While no positive pressure gain was achieved in the experiments, the presented experimental data compare well with numerical results of similar boundary conditions and underline the importance of minimizing injector pressure loss while applying outlet restrictions to the combustor. The data also suggest that specific geometric combinations may lead to adverse modes such as longitudinally pulsing combustion, resulting in a reduction in the measured pressure gain relative to numerical results. This observation occurs more often for geometric combinations which are the most promising for exhibiting positive pressure gain, and suppressing these modes will be an important topic to achieving this goal. It is further shown that transition regions exist between modes of one and multiple co-rotating waves, and that wave multiplication may be necessary to unlock further increases in the pressure gain.

Experimental Study on Plasma Ignited Single-base Propellant using SEM and EDS

To explore the reaction mechanism in the plasma-propellant interaction, plasma ignition and conventional ignition experiments are carried out on a single-base propellant. The samples after ignition are analyzed by scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS). Results show that the plasma jet brought a variety of high enthalpy flows into the propellant and deposited them on the surface of the propellant. These high enthalpy particles reacted with the propellant on the surface rapidly and produced voids and cracks different from the surface melting mode in the conventional ignition process. These voids provided microchannels for ignition and heat transmission. The area of ignition and combustion increased accordingly.

Experiments on High-Temperature Hypersonic Fluid–Structure Interaction with Plastic Deformation

This study provides experimental results on hypersonic fluid structure interaction (FSI) with focus on high thermal loads and plastic deformation designed for validation of coupled simulations. We conducted experiments with thin panels in a high enthalpy flow environment in the arc-heated wind tunnel L3K. Time-resolved surface deformation and temperature fields were measured. The challenging digital image correlation (DIC) measurements through the flow field were validated by comparison to internal laser triangulation measurements. The resulting data set allows a detailed analysis of the coupling between thermal loads and structural deformation. We obtained severe localized heating and resulting deformations of more than 12 times the panel thickness. In addition to single exposure measurements, the effects of repeated load cycles were investigated.

Investigation of dual ignition for a detonation-driven shock tunnel in forward driving mode

A detonation-driven shock tunnel is useful as a ground test facility for hypersonic flow research. The forward detonation driving mode is usually used to achieve high-enthalpy flows due to its strong driving capability. Unfortunately, the strong detonation wave front results in diaphragm fragments that disturb the test flow and scratch the nozzle or test models. In this study, a dual ignition system was developed to burst a metal diaphragm without fragmentation in the forward driving mode. A series of experiments were conducted to validate the proposed technique. The influences of the delay time setting on the test conditions were investigated in detail. Numerical simulations were also conducted to obtain a better understanding of the wave processes in the shock tube. The results showed that the dual ignition system solved the diaphragm issues in the forward driving mode. The test time was shortened due to the additional ignition close to the primary diaphragm; the smaller the delay time, the shorter the effective test time. However, a small amount of time loss is considered worthwhile because the severe diaphragm problems have been solved.

Variety of gas-dynamic turbulent configurations at internal flow in channels of structurally complex bodies

AnnotationFor the study of turbulent configurations in complex channels, it is proposed to use the experimental method of mass entrainment in combination with theoretical analysis and mathematical description of the results obtained. The paper presents the results of numerous experiments conducted on high-enthalpy gas flows in nozzles of complex shape. Images were obtained that visualize the picture of the torsion-wave dynamic movement of gas flows, the presence of stable structured turbulent flow configurations, and the abrupt transition from one form of flow stability to another.

Assessment of Thermochemistry Modeling for Hypersonic Flow over a Double Cone

The influence of different assumptions for thermochemistry modeling in hypersonic flow over a double-cone geometry is investigated. A computational fluid dynamics analysis is used to study the double cone in three different thermochemical cases, nonequilibrium flow, equilibrium flow, and frozen flow, for four different mixtures of nitrogen and oxygen. Specific areas of interest include the thermochemistry model effects on the flowfield and surface properties. The resulting aerodynamic loads are compared with experiments and indicate that thermochemistry modeling assumptions play a significant role in determining surface properties. It is also shown that heat loading is more sensitive to thermochemical modeling than drag and suggests that an accurate measurement of surface heat transfer is of particular interest. Careful analysis also reveals that high-enthalpy and pure oxygen flows are particularly sensitive to the thermochemistry model assumed. Consistent overprediction or underprediction of pressure drag and heat load by all three chemistry models for most of the cases considered indicates a fundamental difference between the actual experiments and the simulations, thus limiting the usefulness of the double-cone data for validation of thermochemistry models.

Coarse-grained modeling of thermochemical nonequilibrium using the multigroup maximum entropy quadratic formulation.

This work addresses the construction of a reduced-order model based on a multigroup maximum entropy formulation for application to high-enthalpy nonequilibrium flows. The method seeks a piecewise quadratic representation of the internal energy-state populations by lumping internal energy levels into groups and by applying the maximum entropy principle in conjunction with the method of moments. The use of higher-order polynomials allows for an accurate representation of the logarithm of the distribution of the low-lying energy states, while preserving an accurate description of the linear portions of the logarithm of the distribution function that characterize the intermediate- and high-energy states. A comparison of the quadratic and the linear reconstructions clearly demonstrates how the higher-order reconstruction provides a more accurate representation of the internal population distribution function at a modest increase in the computational cost. Numerical simulations carried out under conditions relevant to hypersonic flight reveal that the proposed model is able to capture the dynamics of the nonequilibrium distribution function using as few as three groups, thereby reducing the computational costs for simulations of nonequilibrium flows.

Investigation of Convective Heat Transfer Simula-tion Accuracy in Subsonic High-Enthalpy Air Flows in HF-Plasmatron

Investigation of Convective Heat Transfer Simula-tion Accuracy in Subsonic High-Enthalpy Air Flows in HF-Plasmatron

Numerical Simulation of an Arc Jet Test Section

High-fidelity modeling tools are essential for accurate predictions of planetary entry. The same tools can also be used to interpret ground test data. A hypersonic aerothermodynamic computational fluid dynamic code is used to compute the flowfield of a high-enthalpy flow facility, and the results are compared to published experimental data. A test case is selected that consists of a bulk enthalpy flow that discharges into a vacuum chamber at 0.228 kPa. The simulation parameters are compared to radial and axial velocity profiles. Surface pressure and heat flux profiles are analyzed, and excellent agreement is found with the experimental data. A parametric study is carried out in order to better assess the effects of the initial and boundary conditions.

Forward and backward uncertainty quantification with active subspaces: Application to hypersonic flows around a cylinder

We perform a Bayesian calibration of the freestream velocity and density starting from measurements of the pressure and heat flux at the stagnation point of a hypersonic high-enthalpy flow around a cylinder. The objective is to explore the possibility of using stagnation heat flux measurements, together with pressure measurements, to rebuild freestream conditions since such measurements are available for recent space missions but not exploited for freestream characterization. First, we formulate an algorithm of mesh adaptation, enabling accurate numerical solutions in an automatic way for a given set of inputs. Secondly, active subspaces are used to find a low-dimensional dependence structures in the input-to-output map of the forward numerical solver. Then, surrogate models on the active variables are used to accelerate the forward uncertainty propagation by Monte Carlo sampling and the Markov Chain Monte Carlo sampling of the posterior distribution for Bayesian inversion. A preliminary sensitivity analysis with sparse Polynomial Dimensional Decomposition is performed on the chemical model of the air mixture, to determine the most influential uncertain chemical parameters in the forward problem. Then, the forward and backward methodologies are applied to the simulation of a hypersonic flow around a cylinder, in conditions for which experimental data are available, revealing new insights towards the potential exploitation of heat flux data for freestream rebuilding.

Structure and properties of YBCO before and after the short-term exposure of the plasma flow

The results on the structure and electrical resistance of the porous YBa2Cu3O7–δ nanoceramics before and after plasma treatment are shown. It is shown that after exposure to high-enthalpy argon plasma flow, the structural, stoichiometrical and superconducting properties in the volume of this ceramics change slightly. An amorphous layer forms on the surface, the mechanical strength of the inter-granular bonds of which becomes higher than for the material in the bulk.

The operating mode of the water-cooled MHD accelerator of SMGDU with a magnetic field that changed along the channel

The work is aimed at solving the problem of creating in laboratory conditions hypersonic high-enthalpy gas flows that parameters are kept constant for 5-10 seconds. For this purpose the test section of the MHD channel with water-cooled electrodes and side walls was under investigation at the set up with a magnetohydrodynamic accelerator in TsAGI. A new series of runs was performed in which the MHD-acceleration stage was successively increased to 5 seconds. It was determined that the electrical and gas-dynamic characteristics of the MHD accelerator did not depend on the duration of the run. It was first demonstrated that the use of a magnetic field special shaped along the axis of the channel made it possible to avoid catastrophic damage to the side walls of the channel at the entrance and exit of the MHD accelerator, which increased the service life of the installation.

Investigation of unsteady heat transfer in a porous medium of a discontinuous heater

This paper presents the results of determining the features of heat transfer on the example of a discontinuous heat exchanger with ball filling, used to generate a high-enthalpy flow. In the first part, we developed a one-dimensional mathematical model (an improved model of Schumann) unsteady heat transfer in a porous medium taking into account heat wall heater cooperage. An analytical solution is found in the quadratures of the system of partial differential equations. The second part of the paper is devoted to numerical modeling. The influence of empirical coefficients on heat transfer was revealed. The results of numerical simulation were compared with the results of one-dimensional mathematical model. As a result of the computational study, the correspondence and differences in thermal and hydrodynamic processes using one-dimensional differential models and numerical modeling are established. A significant influence of empirical coefficients on the qualitative and quantitative nature of the temperature distribution of the gas and the solid matrix over time was revealed. The correspondence of results between two approaches is established: the solution of differential equations and numerical modeling. On the basis of the analysis the applicability of mathematical models of different levels for solving a certain class of problems is established.

Numerical Simulation of High-Enthalpy Flows at Thermochemical Equilibrium

In the present work, different modeling approaches are applied for the numerical simulation of high-enthalpy flows under the assumption of local thermochemical equilibrium. The first approach uses Srinivasan curve fits for estimating thermodynamic and transport properties of air at high temperatures, whereas the second approach estimates the properties by computing the equilibrium composition at a given density and temperature. A two-dimensional finite volume solver has been developed and is validated for shock-wave–boundary-layer interaction over a compression corner. Comparison of the flow characteristics obtained for calorically perfect gas and equilibrium air assumptions are presented for different test cases of inviscid and viscous hypersonic flows. The effect of the presence of argon on the properties of high-enthalpy flows is also studied. The increase in argon concentration from 0 to 20% is found to result in less than 10% increase in the peak temperature in the flowfield and heat flux at the wall.

Extension of the Frozen Sonic Flow Method to Mixtures of Polyatomic Gases

A contemporary issue of crucial importance for further developments in the field of thermal protection systems and related arcjet-based testing activities calls for improvements in existing abilities to measure the centerline total enthalpy. Starting from the original assumptions of Vincenti and Kruger (Introduction to Physical Gas Dynamics, Wiley, New York, 1965) and through the elaboration of a mathematical framework relying on a specific modeling hierarchy of balance equations for the moles of different species involved, it is shown that the extension of the frozen sonic flow method (FSFM) to the case of polyatomic molecules can be made well posed. Dedicated experiments have been conducted using a reentry simulation facility and varying the mass-averaged enthalpy in the range between 5 and . In particular, three different gas mixtures have been considered (using nitrogen as a hot feeding gas and adding cold oxygen, carbon dioxide, and methane, respectively). The enthalpy ratios calculated by the FSFM, which were found to depend on the gas mixture, have been compared with the values determined using two alternate techniques, namely, 1) the heating rate method and 2) the calorimetric probe method. Given the extremely complex experimental conditions considered (high-enthalpy, low density, and supersonic reactive flows), the agreement between the theoretical and experimental results can be considered very satisfactory.

Electronic-state-resolved analysis of high-enthalpy air plasma flows.

In the present study, three different electronic state-to-state methods are proposed to analyze nonequilibrium air plasma flows behind a strong shock wave. In the first approach representing the conventional method, a two-temperature model combined with the electronic quasi-steady-state assumption is adopted. In the second and the third methods, atomic and molecular electronic master equations are coupled with a conservation equation to describe the electronic state-to-state kinetics. State-of-the-art electronic transition rates for atmospheric gas species are compiled with comparisons of existing data. A prediction of the measured nonequilibrium radiation is made for the flow conditions of recent electric-arc shock tube experiments. In a comparison with the measured spectrum, the present electronic master equation coupling methods are more accurate than the conventional approach when used to estimate the initial rising rate and peak value of the diatomic intensity and small amounts of atomic radiation when the diatomic nonequilibrium condition is dominant. Moreover, the spatial distributions of the intensity and electron number density are more accurately predicted by the present methods when the flow fields are dominated by atomic nonequilibrium.