Considering that shot noise is Poisson distribution and the envelope of background optical noise is Rayleigh distribution, the Gaussian Approximation method is firstly used to evaluate the performance of Satellites Laser Communications precisely. The Moment Generating Function expression of considering shot noise, background optical noise and hot noise is firstly derived. The system performance of On Off Key modulation of traditional method is compared with GA method in direct detection system. Results indicate that the performance of Satellites Laser Communications by GA is different from traditional method.
The plume of solid rocket motor is a high velocity flow with high temperature. Temperature distribution in the
plume is of great interest for analyzing the compatibility of rocket weapon system. The high temperature
exhausted flow field would cause damage on certain equipment and loading vehicles. An instantaneous
temperature field with sharp step is established by the exhausted flow field of rocket motor. The increasing rate
of the step depends on the flow velocity at cross section of nozzle exit. To perform an accurate measurement of
temperature inside the flow field, a thermocouple must be sturdy enough to endure the flow impingement. In
the meantime, the thermocouple must have a short time constant to trace the temperature fluctuation in flow
field and a small size to avoid disturbing the flow field severely. The dynamic performance of the
thermocouples used in exhausted flow temperature measurement must be evaluated before the experiment. The
thermocouple which can be used in measuring the temperature distribution in rocket plume was presented in
this paper. A NAMNAC<sup>(R)</sup> self-renew-erode thermocouples with a nominal time constant of 10 microseconds
was used as a reference in a dynamic calibration test for this kind of thermocouple. The thermocouple could
trace the temperature increase in the exhausted flow perfectly. This kind of thermocouples was used in several
real tests of rocket motors, such as the temperature in free exhausted flow field of a stationary rocket motor test,
the stagnate temperature in a shock flow field during the launching of a rocket, and the temperature in a launch
As the deformation in nozzle will cause thrust miss aligned and other problems which would degrade the performance
of rocket severely, deformation detection will help to correct the ballistic parameters for control and guide of the
rocket. Prior or real time detection is important for calculate the ballistic parameters. Rocket noise in active period
during rocket launch has close relationship with core length in the exhausted jet flow. The core length depends on the
nozzle structure and the combustion situation inside chamber. If the parameters inside chamber are fixed, the core
length could be determined by nozzle structure. Thus the jet noise would reflect the change in nozzle structure.
Experiments with cold jet flow were conducted to explore the relationship between nozzle deformation and the
spectrum pattern of jet noise. Three groups of nozzles with different expansion ratio, length of expand segment, and
throat structure were used in the experiment. The spectrums of jet noise for each nozzle under different chamber
pressure were obtained as a reference to detect the deformation in nozzles. Then the nozzle was deformed artificially
and the jet noise was analyzed by joint time-frequency analysis (JTFA) method. Several JTFA algorithms are used to
process the noise data. The joint time-frequency distribution pattern reflects the change in nozzle structure.
An experiment was conducted to study the noise characteristics in the boilover phenomena. The boilover occurs in the combustion of a liquid fuel floating on water. It will cause a sharp increase in burning rate and external radiation. Explosive burning of the fuel would cause potential safety consequence. Combustion noise accompanies the
development of fire and displays different characteristics in typical period. These characteristics can be used to predict the start time of boilover. The acoustic signal in boilover procedure during the combustion of heptanes-water mixture is obtained in a set of experiments. Joint time-frequency analysis (JTFA) method is applied in the treatment of noise data. Several JTFA algorithms were used in the evaluation. These algorithms include Gabor, adaptive spectrogram, cone shape distribution, choi-williams distribution, Wigner-Ville Distribution, and Short Time Fourier Transform with different windows such as rectangular, Blackman, Hamming and Hanning. Time-frequency distribution patterns of the combustion noise are obtained, and they are compared with others from jet flow and small plastic bubble blow up.
A research was conducted to determine the functions of a set of nozzle pairs. The aeroacoustical performance of these pairs can be used to analyze the deformation of structure and change of jet condition. The jet noise signal was measured by a microphone placed in the radiation field of jet flow. In addition to some traditional methods used for analyzing noise both in time and frequency domain, Gabor spectrogram is adopted to obtain the joint time-frequency pattern of the jet noise under different jet conditions from nozzles with different structures. The jet noise from three nozzle pairs worked under two types of working conditions is treated by Gabor spectrogram. One condition is both nozzles in the nozzle pair keep their structure at a fixed chamber pressure, while another condition is one of these two nozzles' throat size decreases during the jet procedure under a fixed chamber pressure. Gabor spectrograms with different orders for the jet noise under the second condition are obtained and compared. Then a rational order is selected in analyzing the jet noise. Results are presented in this paper. The Gabor spectrogram patterns of these two conditions are with marked difference. The noise keeps its frequency peak during the whole jet procedure in the first condition. But there is a frequency peak shift in the second condition at a certain size of throat. The distribution of frequency peak along with the decrease of throat presents two states. This would be helpful for nozzle structure recognition.
Fine water mist with droplet diameter less than 20 microns is less studied in water mist family used as firefighting agent. Experiments were conducted to study the radiant heat blocking function of fine water mist in suppressing pool fire. The fine water mist was generated by a jet atomizer. The jet flow of mist was with low momentum when it exerted on the pool fire. The interaction of this kind of fine water mist with pool fire is quite different from those mists with large droplet size and momentum. A group of ethyl-alcohol and gasoline pool fires with different power is used in the research. Fine water mist was exerted on the pool fire from different distances. Radiant heat flux was measured before and after the mist application. The experiment results show that because of its small droplet size and low momentum, the fine water mist could not even penetrate the buoyant plum zone of relatively larger scale pool fire. No water mist could reach the surface of the flame and result in no radiant heat blocking function would realize in the flame zone. This function is valid in low power pool fire and this limitation is obtained from the research for the studied pool fire.
Fire fluctuation is caused by internal physic-chemical changing mechanism and variations of external environment. The study of fire fluctuation will help to predict developing tendency of fire and to understand the structure and stability of flame. Several parameters can represent the fluctuation of fire, such as velocity, temperature and optical scattering. The transparency of fire changes with flow state in fire plume. Thus, light attenuation-time history of fire is corresponding to fire fluctuation. A technology utilizing laser-based photoelectric pairs as sensors to measure fire fluctuation in intermittent and smoke zone of a pool fire is described here. In this technique, joint time frequency analysis (JTFA) algorithm is used to determine the frequency change of fire fluctuation during combustion procedure. Ethyl-alcohol and gasoline fire were studied with this technique and the results are presented here. Some limitations of this method are also briefly discussed.
Environments have great influence on the development and propagation of flame during fire disaster. When the fire is under the exertion of air flow, the structure of flame is changed by the air flow. The effect results of air flow depend on its flow parameters, such as velocity and flux. The influence of air flow on fire is one of the important parts in fire dynamics research. Experiment study was conducted to study the structure change of a pool fire under air flows with fixed scale but different velocities. Flame temperature was measured by fine wire thermocouples in different positions of fire. Heat flux gauge was used to measure the change of heat flux. A pair of photoelectric probe measured the fluctuation of flame. The velocity of air flow was measured and all other test data were processed by wavelet transforms. Low frequency components corresponding to stable part of fire and high frequency component with corresponding to unstable part of fire are separated by wavelet analysis. The stable threshold that disappears under a certain velocity of air flow is illustrated by the data process. These results are also compared with those obtained from short time Fourier transforms.
The near-field rocket noise involving initial shock wave is harmful to human and launching equipment. The rocket noise and shock wave are inherent flow phenomena in exhausted flow field of rocket engine in its launching stage. Experiments were conducted to research the characteristics of this special flow. Pressure was measured by piezoresistive pressure transducers with high nature frequency. The shockwave data combined with rocket noise was obtained in the experiment. This paper describes several different analytical methods in study of the pressure-time history data obtained in these experiments. In time domain, several parameters such as peak overpressure, positive duration and waveform coefficient are determined from traditional methods. The frequency band of initial shock wave can be obtained by using of these parameters. But this result is quite different from result obtained frequency domain analysis method. This is attributed to the abnormal waveform of the initial shock wave. A further analysis is performed by wavelet transform. Wavelet analysis confirms the frequency domain characteristic of initial shock wave and near-field rocket noise. Several aspects of near-field and far-field rocket noise are compared on the basis of above analysis.
Fine water mist can be used to restrain thermal hazard in some special situations, such as information center and telephone exchange center. A set of experiment was conducted to study the interaction of fine water mist with hot objects of different shape. An adjuster was designed to control the surface temperature of them avoiding over-heated. These objects were chosen to simulate the shape of cable and box used in electric equipment. Change of thermal field around these hot objects was measured before and after exertion of fine water mist. The flow procedure of fine water mist on the objects was displayed by an optical system. Vaporization height of fine water mist above different hot object was also measured. The restrain efficiency of fine water mist with different flux but same droplet size is compared. The experiment results serve as reference to engineering application of fine water mist to restrain thermal hazard of electric apparatus.
Water mist is used as an efficient extinguishing media instead of banned halogen-based fire extinguishing agent. Experiment was conducted to study the interaction of fine water mist and a small-scale fire. This fine water mist serves as candidate agent of fire protection system for electronic equipment fire. The characteristics of the fine water mist was obtained by three-dimensional laser Doppler velocimetry (LDV) / adaptive phase velocimetry (APV). Two kinds of fine water mist with equal mean droplet size, different velocity and caliber of mist jet were used to suppress a small-scale fire. A temperature measurement system based on micro-thermocouples was used to record temperature variation of the fire during exertion of fine water mist. A laser Moire deflectometry system displayed the flame pattern. Serial photographs of fire suppression process were present in this paper. The result shows fine water mist offers an enhanced cooling performance on the surrounding of fire flame.
Wall plays an important role in building fire. It can be used to block the fire and prevent its propagation. It restricts free air entrainment, heat transfer and radiation from the wall side of fire. In other cases the wall will accelerate fire to spread to ceiling. Experiment research was carried out to study the influence of wall on the shape of flame and temperature distribution. An array of thermocouple is applied to study the temperature distribution along the wall. A large caliber laser Moiré Deflectometry was used to display the flow procedure during the interaction of wall and flame, especially the changing of buoyant plum along the wall. Five cases, in which the plain wall was settled at 0, 5, 10, -5, and -10 degree with the axis of flame respectively, were studied. The distance between wall and flame was also changed in these five cases. The inclination effect of wall exerting on flame was clearly displayed. Influence of wall on the flame persistent zone height is calculated from experiment data in each case.
A set of experiment was conducted to study the initial shock wave, which is generated in early launching stage and harmful to human and equipment. Profile of pressure-time history consists of shock wave and rocket noise, which are the two inherent flow features of rocket exhaust flow. This makes it difficult to calculate the typical parameters such as peak overpressure, positive duration and waveform coefficient. Thus the intensity of shock wave is hard to determine by traditional methods. Wavelet threshold de-noise is used in this paper to detect shock wave profile from noise. Daubechies and Symlets wavelet families are compared in threshold treating of shock wave data. Wavelet threshold plus a priori knowledge makes the initial shock wave well detected from rocket noise. Three characteristic parameters of shock wave are determined and compared in the study. The results show that low order wavelet with small support width can keep singularity of shock wave better.This helps to understand the performence of initial shock wave of rocket jet in engineering application.
An improved laser Moire deflectometry system is designed for fire safety science research. It has larger scope than conventional system. Relatively larger scale experimental simulation can be carried out with this system. It was used to study the flow procedure of ceiling jet and propagation of fire on solid combustible material. Research of ceiling jet illustrates flow procedures of ceiling jet of different ceiling constructions. The procedure of hot buoyant plum propagating across barrier was clearly displayed. It provides reference to ceiling construction design and dynamic performances evaluation of fire detect system. Detection time lag is one of three distinct time lags that contribute the effectiveness of fire detection systems and fire mitigation strategies. Two sets of photographs are present corresponding to ceiling jet with different cross girders.
Rocket noise is an inherent behavior of rocket exhaust. Static experiment of rocket engine with synthetic propellant was conducted in rocket engine static-state experiment laboratory to study the properties of the noise. The near-field rocket noise has high intensity and is rich in shocks that are perceived as crackle. Wavelet transformation was used to analyze near-field solid rocket noise measured by piezo-resistive pressure transducers in experiment. Mexican Hat wavelet was used to detect the sharp edge of shock waveform in near-field noise. Large magnitude and short duration shock waveforms are detected. Discrete Daubechies10 wavelet is used to decompose the noise data. Features of near-field noise are analyzed in each scale. The conception of scale energy spectrum based on wavelet coefficients is also introduced to study the noise data. The energy distribution of near-field noise in scale domain is illustrated in this paper. Scale concentration of near-field energy is identified by the scale energy spectrum. The distinct difference between near-field and far-field rocket noise is that the energy of near-field noise concentrates in a much higher frequency band than that of far-field noise. Form the analysis a tendency can be perceived that the energy peak moves from small scale (high frequency) to large scale (low frequency) slightly with the test position goes far away from the exit plane. Wavelet transformation is proved useful in analysis of these noise signals which rich in shock waveform.
When transient temperature is measured by thermocouples, the result is largely influenced by thermocouple's dynamic performance. This problem is encountered in measuring flame temperature fluctuation by using of fine wire thermocouple. Because the heat transfer coefficient mostly depends upon the gas temperature and flow speed, the time constant of fine wire thermocouple is not equal in different temperature ranges. The time constant calibrated in low temperature is not valid in this kind of measurement. Typically frequency of fluctuation in flame is less than 7KHz. The fine wire thermocouple often has to be compensated to meet this working frequency band. The compensation range is determined by its time constant in high temperature. High temperature step source with sharp rise is needed in dynamic calibrating this kind of fine wire thermocouples. A dynamic calibration system based on a set of rocket engine and different propellant is introduced to study the dynamic performance of the thermocouples. This set of rocket engine is designed to have same nozzle exhaust Mach number. Different engines with different propellant provide different high temperature step sources. Experiment is conducted in rocket engine static state experiment laboratory. The position of thermocouple is determined according to numerical simulation results. Dynamic modeling and compensation methods are introduced to process the calibration results.