Computer simulation of liquid fuel jet injection into heated atmosphere of combustion chamber, mixture formation, ignition and combustion need adequate modeling of evaporation, which is extremely ...important for the curved surfaces in the presence of strong heat and mass diffusion fluxes. Combustion of most widely spread hydrocarbon fuels takes place in a gas-phase regime. Thus, evaporation of fuel from the surface of droplets turns to be one of the limiting factors of the process as well. The problems of fuel droplets atomization, evaporation being the key factors for heterogeneous reacting mixtures, the non-equilibrium effects in droplets atomization and phase transitions will be taken into account in describing thermal and mechanical interaction of droplets with streaming flows. In the present paper processes of non-equilibrium evaporation of small droplets will be discussed. As it was shown before, accounting for non-equilibrium effects in evaporation for many types of widely used liquids is crucial for droplet diameters less than 100
μm, while the surface tension effects essentially manifest only for droplets below 0.1
μm. Investigating the behavior of individual droplets in a heated air flow allowed to distinguish two scenarios for droplet heating and evaporation. Small droplets undergo successively heating, then cooling due to heat losses for evaporation, and then rapid heating till the end of their lifetime. Larger droplets could directly be heated up to a critical temperature and then evaporate rapidly. Droplet atomization interferes the heating, evaporation and combustion scenario. The scenario of fuel spray injection and self-ignition in a heated air inside combustion chamber has three characteristic stages. At first stage of jet injection droplets evaporate very rapidly thus cooling the gas at injection point, the liquid jet is very short and changes for a vapor jet. At second stage liquid jet is becoming longer, because evaporation rate decreases due to decrease of temperature. But combustion of fuel vapor begins which brings to increase of heat flux to droplets and accelerates evaporation. The length of the liquid jet decreases again and remains constant slightly oscillating.
► Mathematical model and simulator for sprays evaporation and ignition in combustion chamber. ► Non-equilibrium effects increase droplet lifetime as compared with equilibrium models. ► Scenario of fuel spray ignition in a heated combustion chamber has three characteristic stages. ► At first stage of jet injection droplets evaporate very rapidly thus cooling the gas at injection point. ► At second stage liquid jet becomes longer, evaporation rate decreasing in cool gas. ► Finally jet shortens as combustion of fuel vapor increases heat flux and accelerates evaporation.
The paper presents the results of developing of physical and mathematical model making it possible to take into account the effect of droplets non-uniformity in space and size distribution on ...ignition conditions for fuel sprays. The influence of condensed phase volume fraction on ignition and combustion of sprays was studied, physical and mathematical models for multi-phase flows, mixture formation and combustion of liquid fuels based on solving Navier–Stokes equations for gas phase accounting for thermal and mechanical interaction with poly-dispersed droplets array. The problems of particulate phase dynamics are regarded accounting for the interaction with gas phase atomization, evaporation and combustion.
It was shown that depending on droplet size distribution and aerosol cloud density different flow scenarios were possible.
Several ignition zones could be formed behind incident shock wave depending on mixture properties and initiation parameters. The possibility of numerical simulation permitting variation of definite parameters only made it possible to explain this fact.
► Droplets non-uniformity in space and size effect on fuel sprays ignition was simulated numerically. ► Several ignition zones could be formed behind incident shock wave depending on spray properties. ► Increase of cloud density and decrease of droplet size create more favorable ignition conditions. ► Shock wave attenuation in combustible spray is studied.
Paper presents the results of numerical studies of hybrid rocket engines operating cycle including unsteady-state transition stage. A mathematical model is developed accounting for the peculiarities ...of diffusion combustion of fuel in the flow of oxidant, which is composed of oxygen-nitrogen mixture. Three dimensional unsteady-state simulations of chemically reacting gas mixture above thermochemically destructing surface are performed. The results show that the diffusion combustion brings to strongly non-uniform fuel mass regression rate in the flow direction. Diffusive deceleration of chemical reaction brings to the decrease of fuel regression rate in the longitudinal direction.
ATmega series AVR microcontrollers (e.g., ATmega8A or ATmega16A) and ARM microcontrollers with a Cortex M3 core (e.g., STM32F103С8T6) are among the most popular controllers used in solving ...engineering problems related to servicing various fire and security alarm sensors and devices for metering household and industrial power consumption. At the same time, to transfer the data via a twisted pair or AC voltage network, it needs to be encoded using Manchester 1 or Manchester 2 code, and said AVR and ARM microcontroller units (MCU) contain no hardware encoder and decoder for the Manchester code.
The study aimed at solving the engineering problem of implementing the codec (encoder-decoder) of the Manchester code based on the hardware interfaces of low-performance microcontrollers.
The direct implementation of the encoding algorithm with the Manchester code using a purely software-driven approach significantly decreases the information transfer rate. At the same time, among the hardware interfaces of the MCU, there is always a serial universal asynchronous data transceiver (UART). However, the UART data transfer protocol is not generally suitable for Manchester encoding and, therefore, needs to be software adapted.
In order to adapt the UART protocol to the conditions of Manchester encoding, the 8N1 mode is selected at the hardware level, which means 8 data bits, 1 stop and 1 start bit. The software implementation of the encoder consists in the fact that out of the total possible volume of 256 code combinations, 16 are selected that satisfy the Manchester 2 code, while the start and stop conditions for data bytes transmitted in a row also satisfy the structure of the Manchester code.
To maximize the efficiency of the program code, all code words are stored in the memory of the MCU in such a way that their serial number corresponds to the decimal interpretation of 4 bits of the input data. The decoding of the received data is also performed as efficiently as possible using the decoding table.
The implementation of the software and hardware codec of the Manchester code significantly expands the scope of popular AVR and ARM microcontrollers and reduces the cost of the finished product by excluding any additional units that perform the function of encoding and decoding the Manchester code.
Anti-interference pulsed laser ranging system Sadchenko, A. V.; Kushnirenko, O. A.; Koshelev, E. K.
Tekhnologii͡a︡ i konstruirovanie v ėlektronnoĭ apparature,
08/2020
1-2
Journal Article
Recenzirano
Odprti dostop
Pulsed laser rangefinders prove to be cost-effective and practical devices when used at distances of several tens of kilometers due to their compactness, portability and energy efficiency. However, ...the measurement accuracy is significantly reduced by the presence of pulsed interference affecting the input of the optical receiver both during the sensing period and when the reflected signal is being received. Using the algorithms with the accumulation and subsequent processing of the results of several successive measurements reduces the speed of decision-making and does not guarantee the convergence of the results to the real value of the distance.
The paper proposes a structural diagram of a laser rangefinder with the ability to detect pulsed interference in the range interval and correct errors that occur in the structure of the signal reflected from the target. The basis of the rangefinder circuit is a logical consistent filter, the structure of which contains multipliers (multiplication operations).
The following requirements were formulated for the structure of the probe signal:
— the first element should always be set to +1 to synchronize the receiver decider;
— the weight of the coding sequence is equal to half its length;
— the length of the coding sequence is even.
Based on the requirements for coding sequences, the optimal structures of binary probing signals of length 8 were found, providing the best corrective ability.
Comparison of the correlation properties of the found sequences and the sequences that are constructed using the Walsh functions showed the advantage of the optimal sequences by the criterion of the minimum level of the ACF side lobes.
The simulation of the rangefinder under pulsed noise conditions has shown that the logical filter is advisable to use for those cases when the duration of the obstacle does not exceed 1/3 of the duration of the probing signal.
The paper considers typical designs of surveillance devices for acousto-optic leakage channels. To assess the quality of the channel, it is proposed to use the criterion of the minimum permissible ...power of the information signal at the input.
To analyze the processes that occur while the probe signal propagates to the pick-up point and back to the surveillance device, the authors propose a detailed model of the information leakage channel with the indication of the main transformations of the intercepted signal. The paper presents the main criteria for the correct reception of an optical signal and for the extraction of acoustic information from it. A number of assumptions are formulated regarding the practical implementation of the optoelectric converter of the receiving device, and a simplified equivalent functional model of the optical information transmission channel is developed.
The authors also propose a method for calculating the voltage at the load of the photodiode depending on the distance to the vibrating surface, power and the coefficient of directional action of the transmitter emitter. The numerical dependences of the voltage at the load of the photodiode on the distance are obtained for specific numerical values of the load resistance, supply voltage, directional coefficient of the emitter and receiver, as well as the geometric dimensions of the information interception zone. A simplified technique is developed that allows determining the signal level at the input of the sensitive element of a photodetector under idealized conditions, if the minimum permissible optical signal power condition is met and there is no background illumination of the photodetector of the surveillance device.
An optical signal is usually converted into an electrical one by using photosensitive matrices with a large number of discrete elements based on charge-coupled device (CCD) technology or CMOS ...technology.
One of the disadvantages of CCD and CMOS technologies is the impulse conversion noise that appears on digitized images, impairing visual perception and significantly reducing the likelihood of correct identification in pattern recognition tasks. Traditionally, impulse noise is removed from images using median filters with a fixed aperture within each iteration of full-format processing. However, such filters reduce the sharpness of the reconstructed image at high noise levels or insufficiently suppress the interference under the same noise conditions. These setbacks call for a need to develop an adaptive median filtering algorithm, which would produce a reconstructed image as a joint result of processing with median filters with different apertures.
The essence of this algorithm is to select image areas with different noise levels and process these areas with filters with different apertures. As an objective criterion for assessing the efficiency of the proposed filtering algorithm, the authors used the criterion of the maximum correlation coefficient between noise-free and non-noisy images at various values of the noise variance. The mathematical modeling performed in this study allowed finding that with an increase in the impulse noise variance, the gain of the adaptive median filtering algorithm increases exponentially, in comparison with the algorithms using the filters with a fixed aperture value.
The proposed algorithm can be used for pre-preprocessing images intended for recognition by machine vision systems, scanning text, and improving subjective image characteristics, such as sharpness and contrast.
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