Prediction of Excess Air Factor in Automatic Feed Coal Burners by Processing of Flame Images

In this study, the relationship between the visual information gathered from the flame images and the excess air factor 2 in coal burners is investigated. In conventional coal burners the excess air factor 2. can be obtained using very expensive air measurement instruments. The proposed method to predict ） for a specific time in the coal burners consists of three distinct and consecutive stages; a） online flame images acquisition using a CCD camera, b） extrac- tion meaningful information （flame intensity and bright- ness）from flame images, and c） learning these information （image features） with ANNs and estimate 2. Six different feature extraction methods have been used： CDF of Blue Channel, Co-Occurrence Matrix, L-Frobenius Norms, Radiant Energy Signal （RES）, PCA and Wavelet. When compared prediction results, it has seen that the use of co- occurrence matrix with ANNs has the best performance （RMSE = 0.07） in terms of accuracy. The results show that the proposed predicting system using flame images can be preferred instead of using expensive devices to measure excess air factor in during combustion.

Effects of air damping on quality factors of different probes in tapping mode atomic force microscopy

The AFM probe in tapping mode is a continuous process of energy dissipation,from moving away from to intermittent contact with the sample surfaces.At present,studies regarding the energy dissipation mechanism of this continuous process have only been reported sporadically,and there are no systematic explanations or experimental verifications of the energy dissipation mechanism in each stage of the continuous process.The quality factors can be used to characterize the energy dissipation in TM-AFM systems.In this study,the vibration model of the microcantilever beam was established,coupling the vibration and damping effects of the microcantilever beam.The quality factor of the vibrating microcantilever beam under damping was derived,and the air viscous damping when the probe is away from the sample and the air squeeze film damping when the probe is close to the sample were calculated.In addition,the mechanism of the damping effects of different shapes of probes at different tip–sample distances was analyzed.The accuracy of the theoretical simplified model was verified using both experimental and simulation methods.A clearer understanding of the kinetic characteristics and damping mechanism of the TM-AFM was achieved by examining the air damping dissipation mechanism of AFM probes in the tapping mode,which was very important for improving both the quality factor and the imaging quality of the TM-AFM system.This study’s research findings also provided theoretical references and experimental methods for the future study of the energy dissipation mechanism of micro-nano-electromechanical systems.

PAHs pollution from traffic sources in air of Hangzhou, China: Trend and influencing factors

PAHs pollution in air of arterial roads was investigated from October 1998 to October 2001 in Hangzhou, China. The results showed that Σ10 PAHs was 13—36 μg/m3, among which, BaP, a strong carcinogenic kind ranged from 0.034 μg/m3 to 0.12 μg/m3. PAHs pollutions in four seasons were winter>autumn>spring-summer. The annual averages of ΣPAHs concentration were 25 μg/m3 for 1999, 28 μg/m3 for 2000, and 29 μg/m3 for 2001, respectively. Leaded gasoline was banned in December 1998 in Hangzhou, thus comparative measurements with PAHs in leaded and lead-free gasoline powered motor exhausts made it certain that the use of lead-free gasoline leaded to a heavier PAHs pollution in roadside air from December, 1998, in China, and ΣPAHs in air samples after the lead-banning were more than twice of that in samples before the action. For the large contribution of vehicle discharge to air pollution in roadside, further research was performed to suggest the factors influencing PAHs distribution in vehicle exhaust in order to control air pollution effectively. Compared to gasoline engines, emissions from diesel engines were less toxic, although they might produce more PAHs. Of the same vehicular and oil type, automobiles of longer mileages produced more toxic PAHs. PAHs distributions in the vehicular exhausts were related to the oil type. Large difference was found in the abundance of 3-, 5- and 6-ring PAHs between exhausts from gasoline and diesel oil engines. Diesel oil engines produced relative lighter PAHs such as NAPH, ACEN, FLUOR, while gasoline engines emitted heavier kinds such as BkF, IN and BP. The automobile produced more PAHs with the increase of mileage especially FLUR, PY, BaP, BP. Some significant ratios for traffic source in Hangzhou such as PHEN/AN, FLUR/PY, IN/BP were 0.50—4.3, 0.58—7.4, 0.51—1.5, respectively. A source fingerprint for vehicle exhausts of a mixture of vehicle and oil types in the city district for light-duty vehicle was the abundance of BaA, followed by NAPH, BP, IN. 4-ring PAHs such as FLUR, PY, BaA and CHRY were the most predominant kinds followed by 6-ring PAHs(BP, IN).

Analysis on the Relationship between Air Pollution Status and Meteorological Factors in Zhumadian City

Based on monitoring data of air quality and corresponding meteorological observation data in Zhumadian City during 2013-2015,temporalspatial distribution characteristics and influence factors of air pollution were analyzed. The results showed that >Grade II of pollution occupied a certain proportion in Zhumadian City throughout the year,and annual pollution probabilities at three stations were 11%,11% and 6%; serious pollution occurred for six times at three stations,and they all occurred in autumn and winter; pollution probabilities at three stations in autumn and winter were 18%,17% and 12%,and pollution situation in autumn and winter was more serious than that in spring and summer,and seasonal sequence of pollution frequency from low to high was spring,summer,autumn and winter. Seen from three monitoring stations,there was little difference between new and old industrial zones. Since Branch II of China Meheco Topfond Pharma Co.,Ltd. which represented largescale pollution source took pollution prevention measures,pollution was relatively light,but serious pollution was easy to appear. Seen from temporal change of pollutant concentration,monthly distribution characteristics of three kinds of air pollutants( SO_2,NO_2 and PM_(10)) showed typical one-peak-one valley pattern,and peak occurred during December-January,while valley was during July-August. Due to straw burning,monthly change curve of PM_(10) concentration had two peaks in June and October. Dekad change characteristics of three kinds of air pollutants also showed one-peak-one-valley pattern,and peak occurred from middle dekad of December to middle dekad of January,while valley was from early dekad of July to last dekad of August. The concentration correlation among three kinds of pollutants was the most significant at station 3.Additionally,concentration correlation was significant in autumn and winter,but was relatively weaker in spring and summer. The correlation between pollutant concentration and meteorological factors in the same period was the most significant in autumn,followed by winter. Negative correlation between horizontal visibility and pollutant concentration was extremely significant in winter. There was positive correlation between air pressure and pollutant concentration in summer and autumn,while the correlation was unobvious in winter and spring. There was negative correlation between air temperature and pollutant concentration,which was the most significant in autumn. Negative correlation between relative humidity and pollutant concentration was significant in summer and autumn. Total cloud cover showed significantly positive correlation with pollutant concentration in winter,but the correlation was unobvious in other seasons. Average wind velocity and NO_2 concentration maintained significantly negative correlation in whole year,and there was significantly negative correlation between average wind velocity and concentrations of other two kinds pollutants in winter,but the correlation was worse in other seasons.

The Impact of Meteorological Factors on the Short-term Cumulative Effect of Air Particulate in Tianjin

In order to reveal the rapid increase mechanism of particulate concentration in short time,a notion of short-term cumulative effect of air particulate is defined as the significant increase of pollutant concentration in a short time under the condition of breeze,stable weather and constant emission caused by human being's activities. Subsequently,an index of short-term cumulative effect is established with air diffusive equation,and then the macro-scale meteorological situation and micro-scale factors of forming the short-term cumulative effect are discussed with the observation data. The macro-scale meteorological variables contain upper-level weather situations and surface weather situations. The micro-scale factors mainly include the boundary-layer height and boundary-layer stability. The analyses show that boundary-layer factors and weather variables have a significant influence on the short-term cumulative effect. The notion of short-term cumulative effect will play an important role in interpreting the severe pollution weather.

Impact of Ambient Air Pollution and Meteorological Factors on Hospital Admission to the Cardiovascular System Disease on Ulaanbaatar City of Mongolia

Background: Mongolia is situated in northern Central Asia. Landlocked between China and Russia, it is a vast expanse of high attitude grassland steppe, desert, and mountain covering an area of 1,565,000 square kilometers. Air pollution is an increasingly series problem in Mongolia. Materials and Methods: This is a time-series cross over study. All health and air pollution data of 2008-2017 was used for this survey. Results: The mean level of SO2 during the cold season was 35.22 μg/m3 and during the warm season it was 4.65 μg/m3. 24 hours PM10 concentration, during the cold season daily average concentration was 226.77 μg/m3. The 8 hours average daily carbon monoxide concentration (1352.85 μg/m3 [95% CI: 1313.07 - 1396.15]) was high during the cold season, ozone concentration (39.10 μg/m3 [95% CI: 37.95 - 40.35]) was high during the warm season. Air quality depends on metrological parameters. All correlation was statistically significant during the whole year and cold season. In total, 288,832 people get admitted to the hospital due to cardiovascular system disease in Ulaanbaatar during the year of 2008-2017. In general, hospitalization is increasing year by year. Significant associations were found for SO2 with hypertensive diseases (I10 - I15), ischemic heart diseases (I20 - I25), cerebrovascular diseases (I60 - I69), diseases of pulmonary circulation and other forms of heart (I00 - I09, I26 - I52) in all lags. For NO2 was less associated with Ischemic heart diseases (I20 - I25) and diseases of pulmonary circulation and other forms of heart (I00 - I09, I26 - I52). For both PM10 and PM2.5, every disease had observed significant RR in lag 0 - 3. Significant associations were found for air pollutants such as PM10, PM2.5, CO, SO2, and O3 in all lags had a statistically significant association with cold season’s cardiovascular system disease admission. As expected during the warm season significant association was found only lag 1 with PM2.5 and lags (0, 1) CO and O3. Conclusion: As expected this study demonstrated significant correlations between cardiovascular morbidity with PM2.5, PM10, NO2, SO2, CO, O3, and some meteorological parameters.

Relationship of meteorological factors and air pollutants with medical care utilization for gastroesophageal reflux disease in urban area

BACKGROUND Gastroesophageal reflux disease(GERD)is a highly prevalent disease of the upper gastrointestinal tract,and it is associated with environmental and lifestyle habits.Due to an increasing interest in the environment,several groups are studying the effects of meteorological factors and air pollutants(MFAPs)on disease development.AIM To identify MFAPs effect on GERD-related medical utilization.METHODS Data on GERD-related medical utilization from 2002 to 2017 were obtained from the National Health Insurance Service of Korea,while those on MFAPs were obtained from eight metropolitan areas and merged.In total,20071900 instances of GERD-related medical utilizations were identified,and 200000 MFAPs were randomly selected from the eight metropolitan areas.Data were analyzed using a multivariable generalized additive Poisson regression model to control for time trends,seasonality,and day of the week.RESULTS Five MFAPs were selected for the prediction model.GERD-related medical utilization increased with the levels of particulate matter with a diameter≤2.5μm(PM2.5)and carbon monoxide(CO).S-shaped and inverted U-shaped changes were observed in average temperature and air pollutants,respectively.The time lag of each variable was significant around nine days after exposure.CONCLUSION Using five MFAPs,the final model significantly predicted GERD-related medical utilization.In particular,PM2.5 and CO were identified as risk or aggravating factors for GERD.

Factor Analysis on Air Quality of Key Environmental Protection Cities

Statistical analysis was performed on 678 data of 6 key indices on air pollution in 113 key cities on basis of China Statistical Yearbook in 2015.Besides,factor analysis was then performed with SPSS17.0 to make overall assessment on air quality in different cities.The results showed that air pollution is quite severe in varying degrees in different cities and the research is expected to provide references for air pollution control in key environment protection cities.

Effect of Two Kinds of Similarity Factors on Principal Component Analysis Fault Detection in Air Conditioning Systems

Screening similar historical fault-free candidate data would greatly affect the effectiveness of fault detection results based on principal component analysis(PCA).In order to find out the candidate data,this study compares unweighted and weighted similarity factors(SFs),which measure the similarity of the principal component subspace corresponding to the first k main components of two datasets.The fault detection employs the principal component subspace corresponding to the current measured data and the historical fault-free data.From the historical fault-free database,the load parameters are employed to locate the candidate data similar to the current operating data.Fault detection method for air conditioning systems is based on principal component.The results show that the weighted principal component SF can improve the effects of the fault-free detection and the fault detection.Compared with the unweighted SF,the average fault-free detection rate of the weighted SF is 17.33%higher than that of the unweighted,and the average fault detection rate is 7.51%higher than unweighted.

Clinical relevance of Chinese syndrome factors and thromboelastography levels in patients with phlegm-heat and fu-organ excess syndrome

Objective:Syndrome differentiation is a unique part of traditional Chinese medicine(TCM).Syndrome factors play an important role in the diagnosis and treatment of TCM syndromes.Thromboelastography(TEG)intuitively reflects the blood status of patients with acute ischemic stroke(AIS)and is important in the treatment and prognosis of AIS.To identify the relationship between TCM syndrome factors and TEG in AIS patients and standardize TCM syndrome differentiation and treatment objectives,we designed a prospective cohort study of 103 AIS patients.Methods:We used the diagnostic criteria for AIS in the Chinese Guideline for Diagnosis and Management of Acute Ischemic Stroke 2010.Diagnosis of phlegm-heat and fu-organ excess syndrome was based on the TCM Scale for the Syndrome of Phlegm-heat and fu-organ Excess.The ischemic Stroke TCM Syndrome Factor Diagnostic Scale was used to identify and diagnose syndrome factors.General information,scores of syndrome factors and values of TEG parameters of all enrolled patients were recorded.Results:There were significant differences in internal fire and phlegm-dampness scores between patients with and without phlegm-heat and fu-organ excess syndrome(P<.05).In patients with phlegm-heat and fu-organ excess syndrome,internal fire was negatively correlated with TEG parameters R and K(P<.05)and positively correlated with alpha Angle and coagulation index(P<.01).There were no significant correlations between the two syndrome factors and MA(P Z.058)and LY30(P>.05)or between both syndrome factors and TEG parameters in patients without phlegm-heat and fu-organ excess syndrome.Conclusion:The syndrome factor internal fire is a potential predictor of increased platelet function and fibrinogen activity in AIS patients with phlegm-heat and fu-organ excess,and a potentially important predictor of blood hypercoagulability in TCM.

Air flow control based on optimal oxygen excess ratio in fuel cells for vehicles

Air flow control is one of the most important control methods for maintaining the stability and reliability of a fuel cell system, which can avoid oxygen starvation or oxygen saturation. The oxygen excess ratio （OER） is often used to indicate the air flow condition. Based on a fuel cell system model for vehicles, OER performance was analyzed for different stack currents and temperatures in this paper, and the results show that the optimal OER was affected weakly by the stack temperature. In order to ensure the system working in optimal OER, a control scheme that includes an optimal OER regulator and a fuzzy control was proposed. According to the stack current, a reference value of air flow rate was obtained with the optimal OER regulator and then the air compressor motor voltage was controlled with the fuzzy controller to adjust the air flow rate provided by the air compressor. Simulation results show that the control method has good dynamic and static characteristics.

Meteorological factors, ambient air pollution, and daily hospital admissions for depressive disorder in Harbin: A time-series study

BACKGROUND The literature has discussed the relationship between environmental factors and depressive disorders;however,the results are inconsistent in different studies and regions,as are the interaction effects between environmental factors.We hypo-thesized that meteorological factors and ambient air pollution individually affect and interact to affect depressive disorder morbidity.AIM To investigate the effects of meteorological factors and air pollution on depressive disorders,including their lagged effects and interactions.METHODS The samples were obtained from a class 3 hospital in Harbin,China.Daily hos-pital admission data for depressive disorders from January 1,2015 to December 31,2022 were obtained.Meteorological and air pollution data were also collected during the same period.Generalized additive models with quasi-Poisson regre-ssion were used for time-series modeling to measure the non-linear and delayed effects of environmental factors.We further incorporated each pair of environ-mental factors into a bivariate response surface model to examine the interaction effects on hospital admissions for depressive disorders.RESULTS Data for 2922 d were included in the study,with no missing values.The total number of depressive admissions was 83905.Medium to high correlations existed between environmental factors.Air temperature(AT)and wind speed(WS)significantly affected the number of admissions for depression.An extremely low temperature(-29.0℃)at lag 0 caused a 53%[relative risk(RR)=1.53,95%confidence interval(CI):1.23-1.89]increase in daily hospital admissions relative to the median temperature.Extremely low WSs(0.4 m/s)at lag 7 increased the number of admissions by 58%(RR=1.58,95%CI:1.07-2.31).In contrast,atmospheric pressure and relative humidity had smaller effects.Among the six air pollutants considered in the time-series model,nitrogen dioxide(NO_(2))was the only pollutant that showed significant effects over non-cumulative,cumulative,immediate,and lagged conditions.The cumulative effect of NO_(2) at lag 7 was 0.47%(RR=1.0047,95%CI:1.0024-1.0071).Interaction effects were found between AT and the five air pollutants,atmospheric temperature and the four air pollutants,WS and sulfur dioxide.CONCLUSION Meteorological factors and the air pollutant NO_(2) affect daily hospital admissions for depressive disorders,and interactions exist between meteorological factors and ambient air pollution.

Evaluation of the Gas Turbine Inlet Temperature with Relation to the Excess Air

This paper shows the effect of excess air on combustion gas temperature at turbine inlet, and how it determines power and thermal efficiency of a gas turbine at different pressure ratios and excess air. In such a way an analytic Equation that allows calculating the turbine inlet temperature as a function of excess air, pressure ratio and relative humidity is given. Humidity Impact on excess air calculation is also analyzed and presented. Likewise it is demonstrated that dry air calculations determine a higher level for calculations that can be performed on wet air.

Effects of Excess Air Coefficients on Combustion Characteristics of N-butanol by a Constant Volume Combustion Bomb

The effects of excess air coefficients on the combustion characteristics have been experimentally investigated by means of a constant volume combustion bomb.N-butanol was tested as the research fuel at different air-fuel equivalence ratios.Through the discussion of the combustion pressure,the combustion temperature,accumulated heat release,ignition delay and combustion duration,the effects of the excess air coefficient on combustion characteristics is clarified.Experimental results show that near the theoretical air-fuel ratio,the combustion rate is the fastest accompanying with shorter combustion duration while the combustion pressure and temperature reach the maximum.With increase or decrease of the excess air coefficient the combustion pressure,the temperature and the heat release reduce.Simultaneously,the combustion timing is deferred and the combustion duration becomes longer.

Research Progress on the Causes and Influencing Factors of Air Pollution in China

At the present stage,China is facing a large number of new risks and challenges in the safety environment,all kinds of environmental pollution problems are widespread,and the atmospheric environmental problems are particularly prominent.According to the discussion and research of a large number of scholars at home and abroad,a series of factors such as the development of urbanization,economic growth and changes in industrial structure,human consumption,and a large number of large-scale enterprises with high energy consumption are the important reasons for the aggravation of air pollution in our country.Starting with the socio-economic factors closely related to human activities,this paper establishes Sustainability Evaluation using Indicators(SEI),and explores the current situation of the research on the causes of air pollution in China through literature research and summary methods.It not only provides a scientific basis for the reasonable formulation of policies and strategies,but also makes it more convenient for the government to carry out accurate governance on this basis,which is of great significance to the construction of a beautiful China.

Factor Analysis on Industrial International Competitiveness： An Empirical Study on China Air Transport Industry

After China＇s entry into WTO, our industries will be confronted with fiercer competition. How to strengthen their international competitiveness to seek for survival and development has become a very urgent problem. This paper first simply discusses the meaning and theo^es of industrial international competitiveness, then chooses proper indexes according to some principles, explains the factor analysis method of industrial international competitiveness, and finally makes empirical analysis on China air transport industry.

Block Incremental Dense Tucker Decomposition with Application to Spatial and Temporal Analysis of Air Quality Data

How can we efficiently store and mine dynamically generated dense tensors for modeling the behavior of multidimensional dynamic data?Much of the multidimensional dynamic data in the real world is generated in the form of time-growing tensors.For example,air quality tensor data consists of multiple sensory values gathered from wide locations for a long time.Such data,accumulated over time,is redundant and consumes a lot ofmemory in its raw form.We need a way to efficiently store dynamically generated tensor data that increase over time and to model their behavior on demand between arbitrary time blocks.To this end,we propose a Block IncrementalDense Tucker Decomposition(BID-Tucker)method for efficient storage and on-demand modeling ofmultidimensional spatiotemporal data.Assuming that tensors come in unit blocks where only the time domain changes,our proposed BID-Tucker first slices the blocks into matrices and decomposes them via singular value decomposition(SVD).The SVDs of the time×space sliced matrices are stored instead of the raw tensor blocks to save space.When modeling from data is required at particular time blocks,the SVDs of corresponding time blocks are retrieved and incremented to be used for Tucker decomposition.The factor matrices and core tensor of the decomposed results can then be used for further data analysis.We compared our proposed BID-Tucker with D-Tucker,which our method extends,and vanilla Tucker decomposition.We show that our BID-Tucker is faster than both D-Tucker and vanilla Tucker decomposition and uses less memory for storage with a comparable reconstruction error.We applied our proposed BID-Tucker to model the spatial and temporal trends of air quality data collected in South Korea from 2018 to 2022.We were able to model the spatial and temporal air quality trends.We were also able to verify unusual events,such as chronic ozone alerts and large fire events.

Local hierarchical non-negative tensor factorization and its application in machinery fault diagnosis

Aiming at the slow convergence and low accuracy problems of the traditional non-negative tensor factorization, a local hierarchical non-negative tensor factorization method is proposed by applying the local objective function theory to non- negative tensor factorization and combining the three semi-non- negative matrix factorization（NMF） model. The effectiveness of the method is verified by the facial feature extraction experiment. Through the decomposition of a series of an air compressor＇s vibration signals composed in the form of a bispectrum by this new method, the basis images representing the fault features and corresponding weight matrices are obtained. Then the relationships between characteristics and faults are analyzed and the fault types are classified by importing the weight matrices into the BP neural network. Experimental results show that the accuracy of fault diagnosis is improved by this new method compared with other feature extraction methods.

缺血性脑卒中痰热腑实证大鼠模型的建立与评价

背景:中医药在治疗缺血性脑卒中痰热腑实证中具有丰富的经验和独特的优势,为深入挖掘中医药对该病的治疗靶点和治疗机制,建立稳定、可靠的缺血性脑卒中痰热腑实证病证结合动物模型是关键。目的:探讨缺血性脑卒中痰热腑实证大鼠模型的建立方法及评价体系。方法:采用随机数字表法将60只SD大鼠随机分为4组:空白对照组(n=12)、缺血性脑卒中组(n=18)给予普通饲料喂养25 d,病证结合组(n=18)、痰热腑实证组(n=12)给予高脂饲料喂养25 d;第26天,空白对照组、缺血性脑卒中组灌胃给予生理盐水,病证结合组、痰热腑实证组灌胃给予自体粪便混悬液,连续灌胃3 d(灌胃期间继续给予高脂饲料);灌胃结束后,缺血性脑卒中组、病证结合组采用线栓法建立缺血性脑卒中模型,记录各组大鼠饮食量、饮水量、体质量、体温、粪便性状、鼻分泌物、喉中痰鸣、舌象等表征变化。造模结束后24 h,进行神经功能缺损评分、舌象表征、血脂水平检测、脑组织及颈动脉形态学观察以及血清胃动素和生长抑素水平检测。结果与结论:(1)与空白对照组比较,病证结合组大鼠呼吸急促、精神萎靡、烦躁、行动迟缓,鼻周可见大量分泌物、可闻及厚重的喉中痰鸣,饮食量、饮水量下降,体质量、体温及舌下脉络评分升高,粪便粒数、Bristol评分及粪便含水率降低;血清总胆固醇、三酰甘油、低密度脂蛋白胆固醇及生长抑素水平升高,胃动素水平降低,神经功能缺损评分升高,颈动脉发生显著病理改变,脑组织显示明显缺血形态学改变。缺血性脑卒中组仅能造成缺血性脑组织病理改变,不能复制出痰热腑实证的证候特征。痰热腑实证组能复制出典型的中医证候特征,但不能构建出缺血性脑卒中的脑组织病理改变。(2)结果表明,高脂诱导结合线栓法及自体粪便灌胃的多因素复合造模方法,可建立缺血性脑卒中痰热腑实证病证结合动物模型。

自身免疫调节因子(AIRE)的研究进展

自身免疫调节因子(autoimmune regulator,AIRE)是一种具有转录活化潜能的DNA结合蛋白。由于AIRE基因的突变可导致自身免疫病APECED(autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy,APECED),又称自身免疫性多腺体综合征I(autoimmune polyg-landular syndrome type I, APS I)。因此,这一基因在正常生理状态下很可能对维持自身免疫耐受、控制自身免疫起着重要作用。对自身免疫耐受产生机制的揭示将为自身免疫病、超敏反应、移植排斥及肿瘤的治疗提供新的策略。本文对AIRE的基因鉴定、分子结构和生化功能、亚细胞定位、组织分布及其在自身耐受产生中的作用作一综述性介绍。