Pulse Combustor Research Articles

Intelligent aerodynamic identification method based on wavelet transform and deep learning

Abstract Wind tunnel force testing is particularly crucial for the aerodynamic characteristics of hypersonic vehicles, and the intelligent and high-precision aerodynamic identification technology serves as the key supporting technology. Due to the short effective test time of the pulse combustion wind tunnel, the inertial force vibration generated by the force measurement system is difficult to be attenuated completely at the start of the wind tunnel. At the same time, there are many interference signals in the output signal during the test, which makes it difficult to achieve high-precision aerodynamic load identification. Therefore, in order to eliminate the influence of interference signals on aerodynamic identification accuracy and obtain accurate aerodynamic loads, this paper introduces an intelligent aerodynamic identification method based on wavelet transform and deep learning. Firstly, meyer wavelet is selected as the wavelet basis, and signal processing is performed on the output signal through wavelet transform to eliminate interference signals and improve signal quality. Then, through RNN-GRU network model to identify the load. The reliability of the method is verified by the rod balance test platform. The results show that the method can effectively reduce the influence of interference signal, and the relative error between the identified load and the real load is about 2%. At the same time, the identification results of the method are compared with those of the mean value method, which further verifies the accuracy of the proposed method.

Research on the Design and Bidirectional Work Process of Metal Diaphragms in Small Double-Pulse Solid Rocket Motors

According to the requirements of the small double-pulse solid rocket motor, a compartmentalized isolation device has been designed. This device consists of a metal diaphragm and a support frame. An experimental study and numerical simulation were used to verify the bidirectional working process of the metal diaphragm during operation of the double-pulse motor. The results show that the pressure-bearing capacity of the metal diaphragm meets the requirements under the working conditions of pulse I without affecting pulse II, because the metal diaphragm can provide insulation and flame retardancy. The metal diaphragm can be cracked in the direction of the preset V-groove in a relatively short time under the working conditions of pulse II, which allows the gas to flow to the first pulse combustion chamber normally. This indicates that the metal diaphragm can meet the requirements of bidirectional working process in dual-pulse motors.

Influence of fluidic shock control on the combustion flow field of a dual-mode scramjet

To improve the resistance capability of a dual-mode scramjet isolator, fluidic shock control devices with different control parameters were designed. First, the effects of the control parameters on the position distribution, morphological structure, evolution process of the shock train in the isolator and the resistance capability of the isolator were numerically simulated. Then, cold- and hot-state tests of the scramjet through a direct-connected pulse combustion wind tunnel were carried out at a simulated flight Mach number of 4 to obtain the ignition and combustion process in the scramjet combustor under fluidic shock control through measurements by high-speed schlieren and pressure transducers. Finally, the influence of fluidic shock control on supersonic combustion was analysed by comparing the flow field structure and combustion process with and without a control device at different equivalent ratios and under different fuel injection schemes. The results showed that the fluidic shock control parameters with a great influence on the resistance capability of the isolator were the angle and width of the suction slot and the angle of the injection slot and that the resistance capability of the isolator could be effectively improved by reasonably designing the control parameters. Under fluidic shock control, the morphological structure and positional distribution of the shock train showed an obvious hysteresis evolution process near the injection slot and the suction slot in the isolator with increasing and decreasing back pressures, and this hysteresis effect could further improve the combustion performance of the combustor.

TEMPERATURE FACTOR EFFECT ON PULSATING HYDROGEN COMBUSTION IN A HIGH-SPEED AIR FLOW

The results of a numerical study of the combustion of hydrogen jets injected into a highspeed air flow before a sudden channel expansion are presented. It is shown that under constant input conditions, a pulsating combustion mode is realized in the channel, which is maintained when the wall temperature increases from 300 to 900 K. An increase in the temperature factor leads to a decrease in the oscillation frequency and amplitude.

Air-breathing rotating detonation engine supplied with liquid kerosene: propulsive performance and combustion stability

Experimental results are presented for a rotating detonation engine supplied with liquid kerosene and preheated air without liquid or gaseous additions to the propellant mixture. Various combustion modes for the generic combustor geometry design were observed—from deflagration, through pulsed combustion and high-frequency instabilities, to stable detonation propagation. Attention was paid to detonation stability (if present), its characteristics, and the propulsive performance of the combustor with a focus on specific thrust and pressure gain through thrust and outlet total pressure measurement. These parameters measured for the observed modes were compared. The stability of the detonation combustion proved not to be critical to achieve high performance of the combustion chamber. For example, high performance was achieved for combustion modes with high-frequency instabilities.

New Aspects of the Pulse Combustion Process

Pulse combustion is an attractive yet still little-known form of combustion that can be successfully used in many industrial applications. Experimental studies show that the course of the combustion process in the valveless pulse combustion chamber is conditioned by the process of creating a well-mixed fuel–air mixture inside the chamber. In the paper, numerical calculations were carried out for selected operating conditions of the pulse chamber and compared with experimental results. This allowed for a better understanding and interpretation of the course of the pulsating combustion process itself. The role and importance of the rate of changes in the volume of the combustible mixture zone in the process of improving the efficiency of the combustion process were determined, and the reasons for changes in the pulsation frequency of the combustion process were also explained.

Innovative vs classical methods for drying heterotrophic Chlorella vulgaris: Impact on protein quality and sensory properties

Drying is a necessary step in the microalgae production chain to reduce microbial load and oxidative degradation of the end product. Depending on the differences in applied temperature and treatment time, the process of drying can have a substantial impact on protein quality and aroma, important characteristics determining the incorporation potential in food products. In this study, we compared the drying of heterotrophic Chorella vulgaris with both innovative (agitated thin film drying (ATFD), pulse combustion drying (PCD) and solar drying (SolD)) and commonly used drying techniques (spray drying (SprD) and freeze drying (FD)). To evaluate the impact on protein quality, we evaluated techno-functional properties, in vitro digestibility (INFOGEST) as well as protein denaturation using differential scanning calorimetry (DSC). A sensory analysis was performed by a trained expert panel, combined with headspace solid-phase microextraction (HS-SPME) - gas chromatography-mass spectrometry (GC–MS) to determine volatile organic compounds (VOCs). ATFD was found to increase techno-functional properties such as gelling, water holding and solubility as well as in vitro protein digestibility. These observations could be related to induced cell disruption and protein denaturation by ATFD. Sensory analysis indicated an increased earthy off-flavor after ATFD. Interestingly, the high-temperature PCD led to an increase in cacao odor while low-temperature FD resulted in lower flavor, odors and VOCs. These results demonstrate that protein quality and sensorial properties of C. vulgaris can be steered through the type of drying, which could help in the selection of application-specific drying methods. Overall, this work could promote the incorporation of microalgal single cell proteins in different innovative food products.

Experimental Investigation of a Pulsation Reactor via Optical Methods

Material treatment in pulsation reactors (PRs) offers the potential to synthesize powdery products with desirable properties, such as nano-sized particles and high specific surface areas, on an industrial scale. These exceptional material characteristics arise from specific process parameters within PRs, characterized by the periodically varying conditions and the resulting enhanced heat and mass transfer between the medium and the particulate material. Understanding flame behavior and the re-ignition mechanism is crucial to controlling the efficiency and stability of the pulse combustion process. In order to accomplish this objective, an investigation was conducted into flame behavior within the combustion chamber of a Helmholtz-type pulsation reactor. The study was focused on primarily analyzing the flame propagation process and examining flame velocity throughout the operational cycle of the reactor. Two optical methods—natural flame luminosity (NFL) and particle image velocimetry (PIV)—were applied in related experiments. An analysis of the NFL measurement data revealed a correlation between the intensity of light emitted by the pulsed flame and the air-fuel equivalence ratio (range from 0.89 to 2.08 in this study). It is observed that a lower air-fuel equivalence ratio leads to higher flame luminosity in the PR. In addition, in order to study the parameters related to system stability and energy transfer efficiency, this study also focuses on the local velocity field measurement method and an example of a fluid flow result in a combustion chamber by using a phase-locked PIV measurement system upgraded from a classic PIV system. The presented results herein contribute to the characterization of flame propagation within a pulsation reactor, as well as in pulsatile flows over one working cycle in a broader context, with flow velocity in the center of the combustion chamber ranging from 1.5 m/s to 5 m/s. Furthermore, this study offers insights into the applicable experimental methodologies for investigating the intricate interplay between flames and flows within combustion processes.

Aerovalved Pulse Combustor for Enhancing Efficiency and Sustainability of Fossil Energy Conversion

Abstract The paper actualizes earlier research in developing a pulse combustion technique for enhancing the efficacy of utility and industrial boilers and furnaces. Some unpublished results of the experimental investigation of self-sustained pulsating combustion of a gas fuel in an aerovalved pulse combustor (PC) are presented. Relationships have been established between all important design and operating parameters and the combustion characteristics. It was demonstrated that a well-designed pulse combustor can operate efficiently in a stable self-pumping regime in a wide range of operating conditions with the loading from 20% to 100% of the maximum power. While a PC can operate autonomously as a gas burner, the present focus is on their use for mitigating slug and ash deposits on heating surfaces of coal- or biomass fired power and industrial boilers, thus providing an alternative to the proven detonation pulse (DPC) or other techniques for on-line cleaning of heating surfaces during the operation of power and industrial boilers.

Analysis of pulse combustion processes and thermodynamic cycles in pulse combustors

This study explores the dynamic behavior of pulsating combustors during stable operation, examining pressure characteristics, flame dynamics, and thermodynamic cycles. Through analysis of pressure signal derivatives and high-speed photography, it unveils a distinctive counterclockwise thermodynamic cycle in these combustors, diverging from prior research. Crucial points in the combustion process, including zero-pressure points, valve timings, and flame ignition dynamics, were successfully identified. The research introduces a novel clockwise thermal cycle diagram based on ignition events and valve operations, providing real-time insights into combustion processes. Key findings include the identification of critical combustion process points, immediate flame ignition upon valve opening, and the introduction of an innovative thermal cycle diagram. The study's comprehensive approach, integrating pressure analysis, flame dynamics, and derivative curves, offers profound insights into the dynamics of pulsating combustors, furthering our understanding and providing avenues for optimization. Our comprehensive approach enhances the understanding of pulsating combustor dynamics and provides valuable insights for system improvement. The unconventional thermodynamic cycle underscores the novelty of our work. These findings are pivotal for enhancing combustion efficiency and reducing emissions, emphasizing the significance of further research and development in this field.

Intelligent reconstruction algorithm of hydrogen-fueled scramjet combustor flow based on knowledge distillation model compression

The real-time perception of the combustion flow of a scramjet can help to quickly evaluate the working state and provide a new opportunity for the intelligent reconstruction of the data-driven combustion flow field. However, the working process of supersonic combustors is extremely short, usually of the order of milliseconds, and the inference speed is slow due to the large number of parameters of traditional deep learning flow field reconstruction models within a very short time. Currently, knowledge distillation is known as an effective model compression method. In this paper, a neural network model based on symmetric structure cascade (NNSSC1) is proposed. By means of symmetric structure cascade, features of different levels are spliced and fused, which can effectively improve the reconstruction performance of the model. Secondly, to accelerate the application of the neural network model in the flight test, a student model with a simple structure is constructed. By distillation learning, the NNSSC model is used as the teacher model, combining the high performance of the teacher model and the high efficiency of the student model, and the combustion flow field is reconstructed with high speed and high precision based on the supersonic combustor wall pressure. In a direct connected supersonic pulse combustion wind tunnel with an inflowing Mach number of 2.5, the model was trained and tested on a dataset constructed in a hydrogen fuel scramjet experiment with different equivalent ratios. This method achieves high-precision and efficient reconstruction of complex flow fields in the combustor based on sparse wall pressure, providing more reliable data for accurately evaluating the flow and combustion status of the combustor of a scramjet.

Intelligent flow field reconstruction based on proper orthogonal decomposition dimensionality reduction and improved multi-branch convolution fusion

The rapid and accurate reconstruction of the supersonic combustor flow field is of great significance for sensing and predicting the combustion state. Existing deep learning methods pay less attention to the convergence speed of flow field reconstruction, which results in longer training and prediction times for the models. This study proposes a method for reconstructing the flow field in supersonic combustor by combining a reduced-order model based on proper orthogonal decomposition (POD) with a multi-branch convolutional neural network. This method first analyzes the effectiveness of POD reconstruction. Then, based on the wall pressure data of the supersonic engine combustor, it performs flow field image reconstruction. Finally, through error calculation and gradient updating with low-resolution principal component flow field shadow images obtained from the POD algorithm, the high-precision and efficient prediction of flow field images is achieved. Different equivalence ratio hydrogen fuel combustion experiments were conducted in a pulsed combustion wind tunnel with an incoming flow Mach number of 2.5. The learning model was trained and tested using the dataset obtained from these experiments. Numerous experiments demonstrated that the model can effectively reconstruct the wave structures of complex flow fields. Multiple evaluation indicators indicated that the reconstructed flow field of the combustor shows good agreement with that obtained from ground wind tunnel testing. Furthermore, after introducing the POD dimensionality reduction model, the training time was reduced by 32.03%, effectively improving the training time complexity of the model.

Experimental research and optimization of a thermoelectric generator excited by pulsed combustion mode under limited heat dissipation for combined heat and power supply

In this paper, a scheme of pulse combustion mode with pulse input of gas is proposed to solve the problems of low output electrical performance and energy conversion efficiency of combustion-based thermoelectric systems under limited heat dissipation conditions, which is based on experiments. The characteristics of the thermoelectric system show that pulse combustion mode can increase the instantaneous temperature difference, so as to achieve the excitation effect on the output power. The relationships among output power, system efficiency, excitation intensity and pulse input parameters, and the geometry of thermoelectric modules are obtained. The influence of pulse input parameters on the excitation intensity is interactive, and top dead center of pulse inlet power is the parameter that has the greatest influence on the performance. Compared to the constant combustion mode, the pulse combustion mode can increase the output power by up to 28.3%. A maximum output power of 7.15 W with air-cooled heat dissipation and a maximum system efficiency of 3.26% were experimentally obtained, which is a 30% increase in efficiency over current air-cooled heat dissipation thermoelectric systems. Therefore, this paper can provide solutions for more practical power supply, provide effective and feasible guidance for the study of thermoelectric systems, and provide useful insights into the electrical properties of thermoelectric systems.

Flame reconstruction of hydrogen fueled-scramjet combustor based on multi-source information fusion

Using various types of data to extract different features and reconstruct flame spontaneous image, the rapid acquisition of flame propagation data and automatic identification of the combustion state in a scramjet combustion chamber in confined space can be promoted. A reconstruction model based on multi-source information fusion is proposed that can effectively reconstruct a flame self-luminous image based on the combustor wall pressure and schlieren image data. A dense convolutional neural network is used to construct and reconstruct the model and train the network based on the experimental datasets. The ground pulse combustion wind tunnel test is conducted under the condition of hydrogen fuel self-ignition using the variation rules of different injection pressures. The evolution process of the combustor wall pressure, schlieren image, and flame self-luminescence image is obtained over time and a dataset is constructed. The reconstruction accuracy of the flame spontaneous luminescence image using the schlieren image and the fusion of the schlieren image and wall pressure under different injection pressure is comprehensively analyzed. Moreover, both qualitative and quantitative results show that the prediction accuracy of flame spontaneous illumination based on multi-source information fusion well matches the experimental result, and the flame boundary, flame head region, flame area, and burning intensity are all well reconstructed. The average peak signal to noise ratio reached 31.57 dB, average structural similarity was 0.939, average mean square error was 0.979, correlation coefficient was 0.0013, and intersection over union reached 0.876.

Enhancing ammonia combustion with minimum hydrogen blended in presence of self-excited intermittent pulsating oscillations

In this work, we propose and test a partial premixed fuel injection design of NH3−H2−O2 with double ring-shaped inlets to enhance ammonia combustion in an open-ended combustor by generating and sustaining pulsating combustion oscillations. Emphasis is being placed on determining the minimum amount of hydrogen being blended with ammonia in the presence of such self-excited pulsating oscillations. With the numerical model validated by comparing with experimental and theoretical data, we identify and systemically investigate three key thermodynamic parameters. They are shown to strongly affect the thermal, combustion, and emission performances. These parameters include the following: (1) total fuel mass flow rate ṁf; (2) mass fraction of hydrogen ω̇H2; and (3) the temperature TH of a heat exchanger implemented downstream of the combustor. It is interesting to observe that intermittent pulsating oscillations are sustained by such ammonia–hydrogen combustion. Furthermore, comparison is conducted between the present results and those with the classical single ring-shaped fuel inlet under the same flow and operating conditions. It is found that the exothermic heat of the proposed double-ring inlets is increased by 98.7% on average. The frequency of such intermittent oscillations is shown to increase with the decreased NH3 proportion. When pure hydrogen is supplied and passing through the outer ring inlet, the combustion limit can be greatly expanded, even if the inlet mass fraction of hydrogen is very small. The minimum hydrogen blended with ammonia is shown to be 0.1% to achieve a sustainable combustion and large-amplitude oscillations. The NO emission is found to be decreased, and H2O is shown to increase. The present study open ups an approach to enhance ammonia combustion by improving its flammability limit with the minimal hydrogen blended.

Transfer learning: A new aerodynamic force identification network based on adaptive EMD and soft thresholding in hypersonic wind tunnel

The aerodynamic test in the pulse combustion wind tunnel is very important for the design, evaluation and optimization of aerodynamic characteristics of the hypersonic aircraft. The test accuracy even affects the success or failure of hypersonic aircraft development. In the aerodynamic test of pulse combustion wind tunnel, the aerodynamic signal is disturbed by the inertial force signal, which seriously affects the test accuracy of aerodynamic force. Aiming at the above problems, this paper innovatively proposes an aerodynamic intelligent identification method, that is the transfer learning network based on adaptive Empirical Modal Decomposition (EMD) and Soft Thresholding (TLN-AE&ST). Compared with the existing aerodynamic intelligent identification model based on deep learning technology, this study introduces the transfer learning idea into the aerodynamic intelligent identification model for the first time. The TLN-AE&ST effectively alleviates the problem of scarcity of training samples for intelligent models due to the high cost of wind tunnel tests, and provides a new idea for further implementation of deep learning technology in the field of wind tunnel aerodynamic testing. And this study designed residual attention block with soft threshold and dense block with adaptive EMD in TLN-AE&ST model. Residual attention block with soft threshold module can more effectively suppress the influence of instrument noise signal on model training effect. Dense block with adaptive EMD makes the deep learning model no longer a black box to a certain extent, and has certain physical significance. Finally, a series of wind tunnel tests were carried out in the Φ = 2.4 m pulse combustion wind tunnel of China Aerodynamic Research and Development Center to verify the effectiveness of TLN-AE&ST.

In situ measurement of combustion wave propagation during combustion synthesis of α‐Si3N4 powder

Abstractα‐Si3N4 powder was prepared by combustion synthesis method. The propagation characteristics of combustion wave were investigated by thermocouple temperature measurement and “resistance—combustion wave displacement response device.” The results show that the combustion reaction takes place in a narrow area, and there is a maximum temperature gradient of 180°C/mm in the combustion front. The “resistance—combustion wave displacement response device” was innovatively used to realize the in situ measurement of combustion wave propagation process. The test results showed the pulse combustion mode in which the combustion front took 10 s as a cycle, and the quantitative data of Si–N2 discrete combustion characteristics were obtained for the first time.

An experimental study of NOx emissions from a Helmholtz resonator

A detailed experimental investigation of NOx emissions was conducted in order to get more precise combustion data for a Helmholtz resonator valveless pulse combustor. A laboratory-scale model of the combustor was designed and operated with a non-premixed input of air and propane. The effects of fuel flow rate, temperature, and pressure amplitude on NOx emissions through a variety of experimental tests were studied. The tests were carried out for three tailpipe lengths, two inlet pipe lengths, and two combustion chamber volumes. The results revealed that the variations in operational and design parameters have a significant impact on NOx emissions. NOx emissions reach a maximum value near the stoichiometric condition as temperature and pressure, which are related to the fuel flow rate, reach their maximum values. Experiments show inverse relationships between CO and NOx emissions. Furthermore, the findings show that NOx reduction increases with increasing tailpipe and inlet pipe lengths, as well as small volume combustion chambers.

Flame development prediction of supersonic combustion flow based on lightweight cascaded convolutional neural network

The flame development prediction of a scramjet combustor forecasts the combustion state and provides valuable information for active flow control. Experiments were performed on a hydrogen-fueled scramjet at different equivalence ratios in a ground pulse combustion wind tunnel with a Mach-2.5 incoming flow. Five image datasets of the flame evolution process were constructed at different predicted periods. The memory fusion cascade network (MFCN) was developed to predict flame images after a certain span using flame image sequences of the previous periods. A complete evaluation system was constructed to compare and analyze the performances of MFCN, Kongs, and ResNet16 models in multi- and long-span conditions. Experimental results show that MFCN achieves a maximum increase of 46.16% of the peak signal-to-noise ratio index, 69.14% of the structural correlation coefficient index, and 5.72% of the correlation coefficient index in the independent test set. Moreover, the volume of the model only reaches the KB level, which has the characteristics of being lightweight. MFCN outperforms other methods in terms of the prediction accuracy and maintains stable prediction results during multi- and long-span tasks.

A new approach to the phenomenon of pulsed combustion

Despite the multiple use and testing of pulse combustion devices, there is a great deal ofuncertainty about the very processes taking place in the pulsating combustion chamber. Theresearch program carried out by the authors of this study focused on the parameters influencing periodic combustion in a valveless pulsating combustion chamber (pulse jet engine). The obtained results surprisingly showed that it is possible to greatly influence theoperation of the pulsating combustion chamber, which does not necessarily have tofunction as an acoustic device. It has been found that the appropriate fuel supply allows tochange the operating frequency, the amplitude of the pressure in the combustion chamber as well as the amplitude of the thrust.