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With the application of oxyfuel combustion technology, the status of radiative heat transfer in oxyfuel flames is prominent, and accurate description of it is essential. This study simulates the turbulence radiation interaction (TRI) in a swirling gas-fired furnace to understand the thermal radiation behavior under oxycombustion conditions. Employing the non-gray weighted sum of gray gases model calculates the radiative properties of the participating media and the TRI is modeled. The effects of TRI on the radiative heat transfer in the furnace under different oxycombustion conditions and swirl numbers are analyzed. The results show that the effects of thermal radiation on temperature fields are critical in the gas-fired furnace, especially for oxycombustion condition with high oxygen concentration, and the same is true for TRI effect. With the increase of O2 concentration in O2/CO2 oxidizer, the radiative heat flux on the chamber wall, the radiative heat flux contribution to total heat flux, and the net radiative heat loss are increased. Increasing swirl number can weaken the TRI effect. It is observed that with 35% O2 in oxidizer, the radiation contribution to heat flux along the entire wall and local increase of radiative heat flux due to TRI effect are almost than 45% and 18%, respectively.

Rocket exhaust plume treated as a significant radiation source has been widely used in space-based detection. In this paper, infrared radiation signatures of plumes were studied in the view of the phenomenon. The reacting plume flows were calculated by an axisymmetric computational fluid dynamics (CFD) solver. Radiative properties of gases were evaluated with the statistical narrow-band (SNB) model by relying on the NASA-3080 database. The line-of-sight (LOS) based on the single-line-group (SLG) approximation was employed for radiative transfer computations. The numerical model was validated against reference data. Based on this model, radiation characteristics of Atlas-II liquid rocket plumes were computed and analyzed in aspects of afterburning, atmospheric attenuation, spectral bands and observation angle. Results show that afterburning has a significant improvement in the wavelengths of 1.5–6.0 μm. Also, the atmospheric attenuation greatly decreases the plume radiance at low altitudes but it can be ignored above 40 km. Radiance is strongly dependent on the spectral bands. The altitude-varying spectral dependency can contribute to the rocket altitude estimation. The effect of the observation angle on the radiance distributions is varying with the spectra band and the flight altitude. These results are helpful for the design of the rocket motors and the application of the infrared detecting system.

Radiative heat transfer plays an important role in combustion systems. Good compromise between the accuracy and computational requirements of the weighted sum of gray gases (WSGG) model makes it widely used in CFD simulations. In this paper, the accuracy of various WSGG models is assessed by the comparison with the results of radiative heat source and heat flux calculated by line by line (LBL) approach in a realistic non-isothermal and non-homogeneous flame using decoupled calculations. Then these models are implemented to CFD code to investigate the radiative heat transfer for a 600 kW furnace using coupled calculations. The experimental data from the furnace has been used to evaluate the computational applicability and reliability of WSGG models. Results show that the non-gray formulation of WSGG model is more accurate than gray modelling approach, compared with LBL results. The non-gray WSGG models of Krishnamoorthy et al. (2013), Bordbar et al. (2014) and Shan et al. (2018) provide the minimal overall errors compared with the measured temperature fields. The results demonstrate that using the non-gray approach instead of gray approach for global CFD simulation improves the simulation accuracy of thermal radiation transfer. The computing time of non-gray models is larger, approximately 3–4 times that of gray WSGG models.

In this study, a novel elliptical jet-stabilized combustor is developed; the combustion and emission characteristics are numerically investigated and compared with those of a circular jet-stabilized combustor. Five elliptical combustors with different semi-major/semi-minor axis ratios (1.00, 1.27, 1.56, 1.89, and 2.25) are considered, and an optimization method for adjustable stabilizer jet air supply is proposed. An Eulerian-Lagrangian approach is used for solving the liquid fuel spray. Employing the realizable k-e turbulence model and the finite-rate/eddy-dissipation model simulates the turbulent combustion. The discrete ordinate method is used to model thermal radiation, and the thermal and prompt NO formations are to be predictive of NO emissions. The numerical models are validated against available experimental data. The results show that the semi-major/semi-minor axis ratio significantly affects the performance of elliptical jet-stabilized combustors. To improve the temperature uniformity and to reduce NO emissions, the proposed optimization method is successfully applied. Compared with the circular combustor, the optimized elliptical combustor achieves strong mixing performance, resulting in improved combustion and emission performance. By adjusting the major/minor axis jet mass flow rate ratio (ma/mb = 0.6) in the elliptical combustor with semi-major/semi-minor axis ratio of 1.56, NO emissions are reduced by 50% compared with the circular combustor.

To examine the effect of atmospheric trace species on the infrared thermal radiation of a supersonic dome, a series of radiation characteristics with different geometry sizes at near-ground altitudes were investigated numerically. The conjugate heat transfer method was applied to build the heat transfer model of the optical dome. Three major radiating species of H2O, CO2, and CO were taken into account in the shock layer. A line-by-line (LBL) method was used for evaluating the radiative properties of species. A line-of-sight (LOS) approach was applied to solve the radiative transfer equation (RTE). The simulated and measured results of the dome were also proposed to validate the numerical method. The effects of the dome geometry size, the dome material and the time-varying altitude on the infrared radiation noise were studied in detail. The results show that the altitude-varying radiation intensity along the LOS is related to the ambient density and velocity. The variation of the dome radius is proportional to the total radiation received on the dome surface. It is observed that the maximum radiation intensity along the LOS does not occur in the normal direction of the receiving point, but it is determined by both the flow field parameters and the path length. Also, the radiation increment corresponding to different dome sizes approximately obeys a Gaussian distribution related to the product of density and velocity.

As the core component of the micro thermophotovoltaic (MTPV) system, the micro combustor with a high and uniform wall temperature distribution is beneficial to improve the energy conversion efficiency. In this paper, a micro tube combustor with converging-diverging channel is proposed and the thermal performance is numerically investigated, compared with that of the micro combustor with cylindrical channel. The effects of inlet velocity of H2/air mixture, dimensionless position and diameter of throat, and solid material on the thermal performance are widely analyzed. Results show that the outer wall temperature and emitter efficiency of the micro combustor with converging-diverging channel are higher than that of the micro combustor with cylindrical channel, and the converging-diverging channel has more uniform temperature distribution. The converging-diverging micro combustor with dimensionless throat position l = 0.375 and dimensionless throat diameter β = 0.4 is more suitable for the application of MTPV system. When H2/air inlet velocity is 11 m/s and H2/air equivalence ratio is 1.0, the mean wall temperature is increased by 82.39 K and the emitter efficiency is increased by 6.59%, while the normalized temperature standard deviation is reduced by 65.85%. Additionally, the use of SiC as wall material can improve the thermal performance of the micro combustor. It is worth noting that this work will offer us significant guidelines for the optimized work of micro tube combustor.

The growing physical (PHY) layer capabilities of Wi-Fi have made it possible to use Wi-Fi signals for both communication and human sensing. Wi-Fi channel state information (CSI) in PHY layer can be obtained from commodity Wi-Fi devices. As CSI can detect the minute environment changes that alter signal propagation, it is thus capable of capturing the subtle human activities to provide cost-effective and easy-to-use human sensing. However, the limited bandwidth of each individual Wi-Fi channel fundamentally constrains the resolution of signals, resulting in poor performance of human sensing. In this paper, we present WiRIM, a resolution improving mechanism for Wi-Fi based human sensing. We design a channel switching and aggregation algorithm to extend the effective bandwidth of commodity Wi-Fi signals and improve the performance and efficiency of human sensing applications. With aggregated CSI, WiRIM constructs feature images which contain rich frequency, temporal and spatial characteristics, and then uses a deep learning method to process the extracted features. We propose a cross-location human activity recognition (CLHAR) scenario as a case study. The CLHAR scenario requires a high enough resolution of the Wi-Fi signals to accurately recognize different activities under the interference of tiny changes in human location. The experiments demonstrate the generality and effectiveness of the proposed mechanism.

In this paper, a high-capacity reversible data hiding scheme in encrypted image with separable capability is proposed. In order to mask image contents, original image is first encrypted in a blockwise manner based on stream cipher with the encryption key. By the pre-processing of run-length coding (RLC) incorporating with Huffman coding, all encrypted blocks are categorized into two sets according to their encoded lengths. Then, the data hider conducts data embedding for the two types of blocks through different mechanisms and the data-hiding key. For the blocks with shorter encoded lengths, their pixel differences are compressed by RLC; for the blocks with longer encoded lengths, matrix compression is conducted on LSB layers. Thus, a large amount of spare room is vacated to embed length bits, compressed bits, hash bits, location-map bits and additional bits. On the receiver side, through a progressive decryption process, a directly-decrypted image similar with the original image can be obtained with the encryption key; through bit parsing, the embedded additional bits can be correctly extracted with the data-hiding key. With both the two keys, besides direct decryption and data extraction, the blocks of the first type can be recovered by RLC decoding, and the blocks of the second type can be recovered with the assist of coding length judgment and hash authentication. Experimental results demonstrate that, our scheme can generally achieve better performances, including higher embedding rate and better visual quality of the direct-decrypted image and the final recovered image, than some of the state-of-the-art schemes.

This work investigates the infrared radiation characteristics of high-temperature hypersonic flows with two reaction control system (RCS) plumes in the glide phase. Computational procedures are performed with thermal and chemical non-equilibrium fluid mechanics, gas-solid interaction, radiation physical parameter and transfer calculations. Hypersonic flows are simulated by solving two-temperature Navier-Stokes (N-S) equations with the finite volume technique. A 7-species, 6-reaction air-chemistry scheme and an 8-species, 10-reaction CO/H2 plume reaction system are utilized. The line-by-line (LBL) method is used to evaluate radiative properties of high-temperature gases, and the radiation transfer equation (RTE) is integrated with the line of sight (LOS) method. Species formation, radiation properties of radiating species and radiation transfer calculations are verified against experimental and reference data. Three trajectory points of an HTV-2 type vehicle (at altitudes of 30, 50 and 70 km) are selected to analyze the radiation characteristics. Spectral intensities within the wavelengths of 2–12 μm at different observation angles are calculated. Computational results indicate that species NO mainly forms in the shock layer and its mole fraction reaches 10−2 order of magnitude. The high-temperature surfaces are near the head and leading edge of the vehicle, and surface emissions can be equivalent to a constant-temperature grey body. The spectral intensity of gases without RCS plumes is several orders of magnitude lower than surface emissions. Comparing the spectra with and without RCS plumes, it is shown that RCS plumes do change the spectral structure and increase the spectral intensity in the 2.7-μm and 4.3-μm bands. The integrated irradiances show that the instinct radiation intensity is closely related to the spectral band and the observation angle. In the 4.3-μm band, the presence RCS plumes has a significant contribution to the radiation.

Predicting thermal radiation for turbulent combustion highlights the significance of turbulence radiation interactions (TRI). Thermal radiation behaviors of methane/hydrogen flames under elevated pressures are investigated numerically using the developed TRI module integrated into CFD codes. The updated non-gray weighted sum of gray gases model is used to calculate the radiative properties of participating media. TRI effects have been analyzed with 0%–50% volumetric fraction of hydrogen in the methane/hydrogen blended fuels under 1–5 atm working pressures. Employing the radiation model considering TRI achieves closer predicted consistency to the experimental data. Only thermal radiation makes the flame temperature dropped about 60–140 K, while the predicted radiative source term calculated with TRI is higher than that without TRI, which results in a colder flame (approximately 13–60 K lower). The impact of TRI on the radiation behavior is enhanced in hydrogen-enriched high-pressure flame as the predicted radiation heat flux and radiative source term are increased above 25% than that without TRI. On account of TRI effect, the net radiative heat loss increases almost 50% at elevated pressure. The strong radiation of participating media in methane/hydrogen flames under elevated pressures emphasizes the importance of TRI effect on accurate predictions of thermal radiation and NO emission.

In this paper, we propose an idea of using asymmetric dielectric resonators to increase the number of perfect absorption bands. We break the symmetry of the dielectric resonators by reducing the length of one of the two silicon split strips. A quad-band MPA is achieved in the near-infrared band based on the asymmetric-dielectric–dielectric–metal (ASDDM) structure, which can excite two extra electric quadrupole (EQ) modes comparing with the symmetry structure exciting only two dipole modes. Operated as a refractive index sensor, those four narrow absorption bands have the figure of merit (FOM) as high as 338.6, 98.5, 38, and 60.6, respectively. This preliminary analysis shows that the resonators asymmetric method is feasible for designing multiple-band MPAs. To obtain more absorption bands, we propose another idea of stacking the dielectric resonators on metal resonators other than only stacking metal resonators. A single-band MPA is first designed based on the metal–dielectric–metal (MDM) structure. Next, we stack the asymmetric dielectric resonators of the quad-band MPA on the metal resonators of the single-band MPA. The absorption spectrum shows that five perfect absorption bands are achieved in the near-infrared band, which preliminary proves the feasibility of the modified resonators stacked method for designing multiple-band MPAs.

Interaction between conduction and radiation during the phase-change process within multidimensional participating media was numerically investigated. The solutions were based on the reproducing kernel particle method (RKPM). Test cases were analyzed and compared to results from analytical solutions and other studies to examine the performance of RKPM. Comparisons indicate that the RKPM is stable and accurate. The effect of thermo-optics effect on the phase-change process was carried out with the refractive index increases/decreases linearly with the temperature. The solidification process involving radiation was studied in a 2D enclosure. The effects of different parameters, i.e., extinction coefficients, scattering albedos, conduction-radiation parameters, and latent heat, on the temperature profiles were investigated, and the results show the parameters have significant effects on the solidification process. In addition, the solidification process during convection-radiation cooling was analyzed in a cuboid enclosure with the effect of various phase transition temperature ranges.

In this paper, we propose two schemes for visible-watermark removal and reversible image recovery. In the first scheme, we consider the scenario for the image generated by a specific visible (not completely reversible) watermarking algorithm Chen et al. (2017). A run-length coding based method is utilized to compress the difference between the preliminary recovered image and original image. After embedding the difference information invisibly and reversibly, the final embedded image can be exactly recovered to its original version after visible-watermark removal, which avoids the problem of overflow and underflow in Chen et al. (2017). In the second scheme, the scenario of visible-watermark removal for the image generated by any visible watermarking algorithms (no matter the sender and the receiver know the algorithms or not) is considered. The scheme can perfectly remove the embedded visible watermark and can also exactly recover original image with the assist of image inpainting technique. In addition, for both two proposed schemes, the invalid user without the knowledge of secret key cannot achieve reversible recovery for original image. Experimental results demonstrate the effectiveness and superiority of our schemes.

The polarization-controllable structure color has been theoretically designed by utilizing metamaterial resonators with polarization-dependent multiband absorption peaks. Our designed nanostructure, consisting of the periodic circle and ellipse resonators, shows above 80% reflectivity from 502 to 553 nm (the green structure color) for TE-polarized incident white light, but above 80% reflectivity from 610 to 750 nm (the red structure color) for TM-polarized incident white light. Therefore, the structure colors gradually change from green to red when the polarization angles increasing from 0° (TE) to 90° (TM). The polarization-controllable structure colors come from the polarization-dependent multiband absorption peaks. When the polarization changes from TE to TM, the absorption peaks shift from 454, 624, and 714 nm to 454, 498, and 550 nm. The physical mechanism is explained by the electromagnetic fields distribution and the equivalent LC circuit model. It can keep the structure color for a wide incident angle.

The interaction between the coaxial air microjet structure and the swirling flow is investigated numerically based on the nonpremixed bluff-body stabilized swirling SM1 flame. The effects of microjet velocity and swirl number on flame combustion characteristics are analyzed. The modified standard k–e model and the realizable k–e model are considered for the turbulent model using FLUENT software. The flamelet model is used with the GRI 2.11 detailed kinetic mechanism. In addition, the discrete ordinate model and the weighted-sum-of-gray-gases model are used together to deal with the radiative heat transfer in the combustion process. Results of the modified standard k–e model are in better agreement with the experimental data than the realizable k–e model. Results show that the inner flame generates due to the existence of a microjet configuration. High microjet velocity (such as 50 m/s) can reduce the soot formation and keep the inner flame high temperature region away from the microjet nozzle. The structu...

Purpose H2O, CO2 and CO are three main species in combustion systems which have high volume fractions. In addition, soot has strong absorption in the infrared band. Thus, H2O, CO2, CO and soot may take important roles in radiative heat transfer. To provide calculations with high accuracy, all of the participating media should be considered non-gray media. Thus, the purpose of this paper is to study the effect of non-gray participating gases and soot on radiative heat transfer in an inhomogeneous and non-isothermal system. Design/methodology/approach To solve the radiative heat transfer, the fluid flow as well as the pressure, temperature and species distributions were first computed by FLUENT. The radiative properties of the participating media are calculated by the Statistical Narrow Band correlated K-distribution (SNBCK), which is based on the database of EM2C. The calculation of soot properties is based on the Mie scattering theory and Rayleigh theory. The radiative heat transfer is calculated by the discrete ordinate method (DOM). Findings Using SNBCK to calculate the radiative properties and DOM to calculate the radiative heat transfer, the influence of H2O, CO2, CO and soot on radiation heat flux to the wall in combustion system was studied. The results show that the global contribution of CO to the radiation heat flux on the wall in the kerosene furnace was about 2 per cent, but that it can reach up to 15 per cent in a solid fuel gasifier. The global contribution of soot to the radiation heat flux on the wall was 32 per cent. However, the scattering of soot has a tiny influence on radiation heat flux to the wall. Originality/value This is the first time H2O, CO2, CO and the scattering of soot were all considered simultaneously to study the radiation heat flux in combustion systems.

Improving the flame stability and thermal behavior of the micro-combustor (MC) are major challenges in microscale combustion. In this paper, the micro combustions of an H2/air premixed flame in a swirl MC with various channel diameters (Din = 2, 3, 4 mm) were analyzed based on an established three-dimensional numerical model. The effects of hydrogen mass flow rate, thermal conductivity of walls, and the preferential transport of species were investigated. The results indicated that the flame type was characterized by the presence of two recirculation zones. The flame was anchored by the recirculation zones, and the anchoring location of the flame root was the starting position of the recirculation zones. The recirculation zones had a larger distribution of local equivalence ratio, especially in the proximity of the flame root, indicating the formation of a radical pool. The combustion efficiency increased with an increasing Din due to the longer residence time of the reactants. Furthermore, the MC with Din = 2 mm obtained the highest outer wall temperature distribution. However, the MC with Din = 4 mm had a better uniformity of outer wall temperature and large emitter efficiency due to the larger radiation surface. An increase in thermal conductivity boosts the thermal performance of combustion efficiency, emitter efficiency, and wall temperature uniformity. But there is a critical point of thermal conductivity that can increase the thermal performance. The above results can offer us significant guidance for designing MC with high thermal performance.

1-D cavity modes in 1-D resonators, such as strips, have been used to design multiband metamaterial perfect absorbers. As for 2-D resonators, such as squares and crosses, most studies still focus on exciting the 1-D cavity modes. In this paper, a symmetry-breaking idea is proposed for 2-D cavity mode excitation. An asymmetric sword-shaped notched square resonator is proposed to excite a new 2-D (1, 1) cavity mode, in addition to the 1-D (1, 0) and (3, 0) cavity modes. Thus, a triple-band MPA was successfully produced with average absorptivity of 98.8% in the infrared range. The 2-D cavity mode provides better performance as a biosensor than the 1-D cavity modes do. To obtain more absorption bands, a six-band metamaterial perfect absorber with average absorptivity of 97.2% was successfully produced by combining two sword-shaped notched square resonators with different sizes. A nine-band metamaterial perfect absorber was successfully produced with average absorptivity of 94.5% by combining three sword-shaped notched square resonators with different sizes.

In conventional dual porosity models, the interactions between matrix and fractures are normally characterized through two equilibrium systems within the same REV (representative elementary volume). This pseudo-steady approach cannot capture the true impact of the non-equilibrium period within each system since it ignores the true transient nature of the fracture-matrix interaction. In this study, a conventional dual porosity model is extended to include the impact of equilibration time lag between matrix and fractures caused by their contrasting properties. To incorporate this important mechanism, the matrix REV is divided into two sub-REVs by using the MINC concept (Multiple Interacting Continua). The time lag effect is defined as a function of the difference between the strain in the matrix REV and that in the sub-matrix REV, and incorporated into a coal permeability model. Consequently, the coal permeability evolves also from initial to final equilibrium. Conventional dual porosity/permeability models represent two end points (initial and final equilibrium) while this new permeability model represents the evolution of coal permeability between these two end points. The model is verified against experimental observations of the evolution of coal permeability under a constant effective stress that extend for more than 80 days. If effective stress remains unchanged, conventional permeability models predict no permeability changes while our new model predicts that coal permeability evolves as a function of time from initial to ultimate equilibrium. Our results suggest that the impact of matrix strain variations on the evolution of coal permeability is significant and should not be ignored.

A systemic and validated model was developed to predict ultraviolet spectra features from the shock layer of near-space hypersonic vehicles in the “solar blind” band region. Computational procedures were performed with 7-species thermal non-equilibrium fluid mechanics, finite rate chemistry, and radiation calculations. The thermal non-equilibrium flow field was calculated with a two-temperature model by the finite volume technique and verified against the bow-shock ultra-violet (BSUV) flight experiments. The absorption coefficient of the mixture gases was evaluated with a line-by-line method and validated through laboratory shock tube measurements. Using the line of sight (LOS) method, radiation was calculated from three BSUV flights at altitudes of 38, 53.5 and 71 km. The investigation focused on the level and structure of ultraviolet spectra radiated from a NO band system in wavelengths of 200–400 nm. Results predicted by the current model show qualitative spatial agreement with the measured data. At a velocity of 3.5 km/s (about Mach 11), the peak absolute intensity at an altitude of 38 km is two orders of magnitude higher than that at 53.5 km. Under the same flight conditions, the spectra structures have quite a similar distribution at different viewing angles. The present computational model performs well in the prediction of the ultraviolet spectra emitted from the shock layer and will contribute to the investigation and analysis of radiative features of hypersonic vehicles in near space.

A tournament is a directed graph obtained by assigning a direction for each edge in an undirected complete graph. A digraphDis cycle complementary if there exist two vertex disjoint cyclesCandC′such thatV(D)=V(C)∪V(C′). LetDbe a locally almost regularc-partite tournament withc≥3and|γ(D)|≤3such that all partite sets have the same cardinalityr, and letC3be a3-cycle ofD. In this paper, we prove that ifD-V(C3)has no cycle factor, thenDcontains a pair of disjoint cycles of length3and|V(D)|-3, unlessDis isomorphic toT7,D4,2,D4,2⁎, orD3,2.

A swirl micro-combustor which has strong capability to stabilize the flame has been developed and is promising to be used in micro thermophotovoltaic power generators in this work. Heat recirculation plays a requisite role in flame stability and thermal performance of micro combustors. Thus, the effects of three parameters (center inlet radius w1, swirler width w2, and step height w3) that determine the size and location of the recirculation zones in a premixed hydrogen/air flame are investigated numerically. The results show that inner recirculation zone and corner recirculation zone generated by a swirler are critical for stabilizing the flame. The parameter w1 has an insignificant influence on the thermal performance. Enlarging the inner recirculation zone by decreasing w2 provokes enhanced heat transfer, and increases the temperature level and temperature non-uniformity of combustor walls. The size of corner recirculation zone is increased by increasing w3, which raises the outer wall temperature uniformity. Based on the parametric analysis, the newly designed micro combustor possesses a high wall temperature level. Additionally, the energy output increment is between 9.1% and 14.2% at hydrogen mass flow rate of 0.0006 g/s when stoichiometric hydrogen/air ratio ranges from 0.6 to 1.4.

Pulse gas fracturing (PGF) is a method of injecting high-pressure gas into coal cyclically to stimulate the coalbed methane reservoir. However, the evolution of coal pore structure during pulse gas fracturing is still unclear, especially when the injection pressure is lower than the breakdown pressure of coal. In this paper, 11 groups of PGF experiments with the injection pressure amplitude no more than 8 MPa were carried out on coal. Furthermore, the pore size distribution and the crack morphology of coal samples before and after PGF were observed. The experimental results show that pulse gas fracturing with relatively lower injection pressure increases the total pore volume and the permeability exponentially. Moreover, with the increase of pulsation frequency or of injection pressure amplitude, the volume of macropores and its proportion in total pore volume rise significantly, and the increasing rate gradually grows. However, the mesopore volume and its proportion in the total pore volume go up with the increase of pulsation frequency, but remain basically unchanged with the rise of injection pressure amplitude. Besides, the volume of micropores and transition pores slightly fluctuates. In addition, PGF hardly generate wide main fractures in the coal body, and instead mainly causes the tip of pores to propagate and induces substantial micro-cracks. The accumulation of irreversible deformation such as plastic deformation of pores and propagation of micro-cracks is the fundamental cause why the total pore volume and the permeability of coal grow continuously. The research results lay a foundation for understanding the fracturing effect of PGF.

Radiative heating of a Mars entry capsule was evaluated along a reconstruction trajectory. Reactive flows were computed using a three-dimensional computational fluid dynamics (CFD) solver. Based on the narrow-band k-distribution (NBK) method, a new seven-point radiative property database was established relying on the line-by-line (LBL) approach. The CFD solver and the radiative model were verified by use of the experimental data. Some issues were numerically studied including flow field parameters, radiative heat fluxe (RHF) at forebody and afterbody, and contribution of radiative heating to aero-heating. The results reveal that the high RHF region is distributed in the windward side of the forebody and the leeward side of the afterbody. A comparison between the RHF and the convective heat flux (CHF) shows that the contribution of the RHF has a noticeable increase at low-altitude trajectory points. The maximum increase of the RHF reaches up to 45% of the total heat flux. In addition, the computational results without the angle-of-attack (AOA) indicate that the AOA has a less than 20% influence on the maximum RHF. These computational results provide a guide for the design of the thermal protection system (TPS) of Mars entry capsules.

Hypersonic vehicles emit strong infrared radiation signatures that can be treated as a detecting source for object identification and routine diagnosis. This paper is aimed at examining the intrinsic radiation characteristics of a Boost-Glide Vehicle (BGV) under the condition of various angles of attack (AOA). A two-temperature model considering the thermal and chemical non-equilibrium effects is coupled with Navier–Stokes equations solved by the finite volume technique. A gas-solid conjunction heat transfer model is also added into the fluid solver to simulate the surface temperature of the vehicle. Gas absorption coefficients are evaluated through Line-by-Line (LBL) method and the radiative transfer equation is solved with line of sight (LOS) algorithm. Each part of the numerical modules is validated separately. The computational results for a Hypersonic Technology Vehicle 2 (HTV-2) type vehicle show that radiances emitted from gas are at least three orders of magnitude lower than that from the surface. The presence of AOA results in different distributions in surface temperature and therefore changes infrared radiation characteristics in intensity and spectral band. Two group simulations are performed in the steady and transient states, these results indicate that the variation of AOA does have a great effect on the infrared radiance and closely relates to observation angle, spectral band, angle size, angular velocity and time history. A low AOA rate of change is beneficial to reduce the radiance in the side view, but increase the radiance in the upward view. AOA rate of change has little effect on radiation intensity-time variations in the top view.

A swirling micro combustor fueled by hydrogen/air has been proposed based on the concept of swirling flow stabilization. The effects of premixed and non-premixed combustion modes on the combustion efficiency, flame stability, and thermal performance of the micro combustor at various hydrogen/air mass flow rates and equivalence ratios are discussed and compared. The results show that premixed mode has a lower flame location than non-premixed mode, and the flame location of non-premixed mode is more susceptible to changes of hydrogen/air mass flow rate and equivalence ratio than premixed mode. When the hydrogen mass flow rate is lower, the non-premixed mode is more suitable for micro thermophotovoltaic (MTPV) system because the wall temperature is more uniform. However, the premixed mode has a higher and more uniform wall temperature distribution at high hydrogen mass flow rate. Moreover, the hydrogen/air equivalence ratio has a significant effect on the thermal performance in the swirling micro combustor, and the advantage of a high and uniform wall temperature distribution of premixed mode is more obvious at small equivalence ratio. This work provides a reference for the choice of micro combustor combustion modes under different hydrogen/air mass flow rates and equivalence ratios in the MTPV system.

The bandwidths of thermal-tunable metamaterial perfect absorbers (MPAs) based on the phase change materials such as Ge2Sb2Te5 and VO2 are usually hundreds of nanometers at near-infrared frequency. Amorphous silicon (a-Si) provides the approach to achieve linearly thermal-tunable ultra-narrowband MPAs, if the absorption band is narrow enough. Four-nanorod-coupled a-Si resonators are proposed in this Letter. An absorption band at 1064 nm is obtained with ultra-narrow bandwidth (FWHM) only 1.4 nm by exciting the coupled magnetic dipole (CMD) mode, which exhibits great linearity to the temperature. In addition, the thermo-optical sensitivity (S=Δλ/ΔT) is about 0.08 nm/°C. The figure of merit of the thermal tunability performance FOM=S/FWHM=0.06. As a modulator, the critical temperature of absorptivity at 1064 nm is only 40°C, which is much lower than the Ge2Sb2Te5 (GST) and VO2. In addition, the modulation depth is up to 82% at near-infrared frequency.

Plasmonic resonances in metal-dielectric-metal structures have shown strong electric and magnetic fields enhancement in the sub-wavelength region, which can significantly boost the performance of plasmon-based metamaterial absorbers. Square resonators (SR) and ring resonators (RR) are both the most common blocks for designing metamaterial perfect absorbers (MPAs) by exciting fundamental electric dipole plasmonic modes. Actually, they can also excite the higher-order electric sextupole plasmonic modes but the absorptivity is very low, which are not available for the design of MPAs. In this paper, the near-field-coupling idea is introduced to enhance the absorptivity of higher-order electric sextupole modes. We propose the outer-square inner-ring coupled resonators (OSIRCR) made of the Ag and ZnS. When the coupling distance has decreased from 70 to 10 nm, the electric field intensity has increased 10.8 times from 55 to 594, which leads to a 5.4 times increase in absorptivity of higher-order electric sextupole modes of the outer-square from 17.6% to 95.3%. In addition to the higher-order electric sextupole modes, the OSIRCR can also excite the fundamental electric dipole modes of the outer-square and the inner-ring. Then a six-band polarization-independent MPA in the infrared range is designed with an average absorptivity of 91.5% utilizing two different sized OSIRCRs.

Multiband metamaterial perfect absorbers (MPAs) have promising applications in many fields like microbolometers, infrared detection, biosensing, and thermal emitters. In general, the single resonator can only excite a fundamental mode and achieve single absorption band. The multiband MPA can be achieved by combining several different sized resonators together. However, it's still challenging to design the MPA with absorption bands of more than four and average absorptivity of more than 90% due to the interaction between differently sized resonators. In this paper, three absorption bands are successfully achieved with average absorptivity up to 98.5% only utilizing single one our designed ring-strip resonator, which can simultaneously excite a fundamental electric dipole mode, a higher-order electric quadrupole mode, and a higher-order electric octopole mode. As the biosensor, the sensing performance of the higher-order modes is higher than the fundamental modes. Then we try to increase the absorption bands by combining different sized ring-strip resonators together and make the average absorptivity above 90% by optimizing the geometry parameters. A six-band MPA is achieved by combining two different sized ring-strip resonators with average absorptivity up to 98.8%, which can excite two dipole modes, two quadrupole modes, and two octopole modes. A twelve-band MPA is achieved by combining four different sized ring-strip resonators with average absorptivity up to 93.7%, which can excite four dipole modes, four quadrupole modes, and four octopole modes.

In this paper, an all-metal nanostructure is designed with a large frequency ratio (∼6) and a large bandwidth ratio (∼32), and consists of period slit-box cavities and nanodisk clusters. It is a nearly perfect absorber at 1.064 μm to achieve laser stealth, a frequency-selective emitter with low emissivity in wavelength ranges 3-5 and 8-14 μm to achieve infrared stealth, and also an emitter with near unity emissivity at 2.709 μm and 6.107 μm to compensate for the decrease of radiation heat transfer owing to the low emissivity. The absorption/emission peaks are all the Lorentzian shape, and the bandwidths, defined as full width at half-maximum, are 35, 408, and 1124 nm at 1.064, 2.709, and 6.107 μm, respectively. The electric and magnetic field distribution shows that the slit behaves like a capacitor, the box behaves like an inductance, and the nanodisk clusters can excite electric dipole resonance. Considering the solar irradiation, the nanostructure maintains middle-wavelength infrared signal reduction rates greater than 80% from 450 to 1000 K, and long-wavelength infrared signal reduction rates greater than 90% from room temperature to 1000 K. The laser and infrared stealth performances of our nanostructure at 473 K are also studied with different incident angles and polarization angles.