检索结果-南通市图书馆

Flow boiling heat transfer in copper foam fin microchannels with different fin widths using R134a

在线全文

学校读者我要写书评

暂无评论

Science China(Technological Sciences) 2023年第11期66卷 3245-3258页

作者： GAO WuHuan FU Kai xu xianghua LIANG XinGang School of Aerospace Engineering Key Laboratory for Thermal Science and Power Engineering of Ministry of EducationTsinghua UniversityBeijing 100084China Beijing Institute of Astronautical Systems Engineering Beijing 100076China

Heat sinks of copper foam fin microchannels are developed to deal with cooling challenges.The heat sinks consist of fins made of copper foam and channels.The channels are 0.5 mm in width and 1 mm in height,and the fins are 0.5 and 2.0 mm in width.Flow boiling experiments are conducted using R134a at subcooled and saturated inlet conditions.The heat flux is between 22 and 172 W/cm^(2),and the mass flux ranges from 264 to 1213 kg/(m^(2)s).The influence of the quality,the heat flux,and the mass flow rate on the heat transfer coefficient is obtained.It is found that wider fin raises the heat transfer coefficient.A correlation is developed based on heat transfer mechanisms,and it predicts the experimental result with a 12%mean absolute error.Compared with a solid fin microchannels heat sink,the heat transfer coefficient of the copper foam fin microchannels is higher(up to 60%)when the heat flux is lower than 100 W/cm^(2).The copper foam fin microchannels may enhance the heat transfer coefficient and reduce the pressure drop at the same time.

关键词： microchannel copper foam flow boiling heat transfer correlation

Theoretical analyses of the performance of a concentrating photovoltaic/thermal solar system with a mathematical and physical model, entropy generation minimization and entransy theory

在线全文

学校读者我要写书评

暂无评论

Science China(Technological Sciences) 2018年第6期61卷 843-852页

作者： CHENG xueTao xu xianghua LIANG XinGang Key Laboratory for Thermal Science and Power Engineering of Ministry of Education School of Aerospace Tsinghua University Beijing 100084 China Yushan Administrative Center Yushan District Maanshan 243000 China

In this paper, the performance of a concentrating photovoltaic/thermal solar system is numerically analyzed with a mathematical and physical model. The variations of the electrical efficiency and the thermal efficiency with the operation parameters are calculated. It is found that the electrical efficiency increases at first and then decreases with increasing concentration ratio of the sunlight, while the thermal efficiency acts in an opposite manner. When the velocity of the cooling water increases, the electrical efficiency increases. Considering the solar system, the surface of the sun, the atmosphere and the environment, we can get a coupled energy system, which is analyzed with the entropy generation minimization and the entransy theory. This is the first time that the entransy theory is used to analyze photovoltaic/thermal solar system. When the concentration ratio is fixed, it is found that both the minimum entropy generation rate and the maximum entransy loss rate lead to the maximum electrical output power,while both the minimum entropy generation numbers and the maximum entransy loss coefficient lead to the maximum electrical efficiency. When the concentrated sunlight is not fixed, it is shown that neither smaller entropy generation rate nor larger entransy loss rate corresponds to larger electrical output power. Smaller entropy generation numbers do not result in larger electrical efficiency, either. However, larger entransy loss coefficient still corresponds to larger electrical efficiency.

关键词：熵产生率太阳系最小化光电数学表演模型物理

Simplification method of thermal-fluid network with circulation reflux based on matrix operation

在线全文

学校读者我要写书评

暂无评论

Science China(Technological Sciences) 2020年第7期63卷 1202-1211页

作者： HAN RuBing xu xianghua LIANG XinGang Key Laboratory for Thermal Science and Power Engineering of Ministry of Education Department of Engineering MechanicsTsinghua UniversityBeijing 100084China

Modeling and simulation of thermal-fluid systems are very important in industrial numerical simulation and play key roles in their design and control. In this paper, the modeling and simplification method of one-dimensional thermal-fluid network with variable-property are presented, including matrix representation of the network, simplification algorithm for series/parallel connection based on matrix operation and generation of flow equations based on system topology. This simplification is suitable for the simulation of thermal-fluid systems with arbitrary topological structure. The method to treat reflux during iteration is proposed. The outstanding features of the simplification algorithm are the significant reduction in the thermal-fluid network and therefore the number of the related governing equations, as well as the computation burden. The example in this paper shows that the number of the governing equations for flow is reduced by about 45% and the calculation time of flow calculation is reduced by an average of 32% after the simplification.

关键词： thermal-fluid system network simplification incidence matrix multiple pumps and multiple circulation refluxes

Experimental study on spray cooling under reduced pressures

在线全文

学校读者我要写书评

暂无评论

Science China(Technological Sciences) 2019年第2期62卷 349-355页

作者： PENG Can xu xianghua LI Ye Ming LI YuLong LIANG XinGang Key Laboratory for Thermal Science and Power Engineering of Ministry of Education School of Aerospace Engineering Tsinghua University School of Energy and Power Engineering Beihang University

Spay cooling is a complicated flow and heat transfer process affected by multi-factors among which the environmental pressure is extremely important. However the influence of pressure is not investigated sufficiently, especially the reduced pressure. In the present study, spray cooling under low initial environmental partial pressures and vapor partial pressures with R21 are investigated with a closed spray and condensation system. To study the influence of initial environmental partial pressure, different amounts of nitrogen are inflated into the vacuum flash chamber, while the vapor partial pressure is kept constant. To study the influence of vapor partial pressure, a cascade refrigerator is used to condense the vapor with different condensation temperatures so that the vapor partial pressure can be maintained or adjusted, while the initial environmental partial pressure is kept constant.The experimental results show that the spray cooling power increases monotonically with the increasing spray flow rate in the experimental range, while the cooling efficiency decreases with the increasing spray flow rate. The spray cooling power and cooling efficiency vary with the initial environmental partial pressure or the vapor partial pressure non-monotonously, which indicates there is an optimal pressure for the heat transfer performance. Besides, the mechanism of the non-monotonous variation trend is discussed based on the key aspects including flash evaporation, convection and boiling. Especially, the boiling heat transfer curve is applied to explain the trend.

关键词： spray cooling flash evaporation reduced pressure phase change

圆管瞬态换热的附加热容处理方法及热容影响分析

在线全文

学校读者我要写书评

暂无评论

工程热物理学报 2024年第1期45卷 246-254页

作者：林远方杨世宇徐向华梁新刚清华大学航天航空学院热科学与动力工程教育部重点实验室北京100084

管道的附加部件导致难以使用一维模型准确描述管内流体的瞬态流动换热,本文提出将其等效为温度不同于管道的部件,按照集总参数法处理附加部件与管道的换热。本文进行温度阶跃和余弦变化工况下圆管的瞬态换热特性实验研究,并采用附加热... 详细信息

管道的附加部件导致难以使用一维模型准确描述管内流体的瞬态流动换热,本文提出将其等效为温度不同于管道的部件,按照集总参数法处理附加部件与管道的换热。本文进行温度阶跃和余弦变化工况下圆管的瞬态换热特性实验研究,并采用附加热容法仿真计算出口温度响应,结果表明:附加热容法能够显著提升计算精度,各工况下仿真结果与实验数据的平均相对误差均不超过2%。同时,本文定义平均迟滞时间量化固体热容对流体温度变化的影响。

关键词：阶跃响应余弦响应附加热容法平均迟滞时间非线性拟合

太湖常见3种沉水植物附着生物的生物量及潜在反硝化速率

在线全文

学校读者我要写书评

暂无评论

湖泊科学 2024年第1期36卷 77-87页

作者：任天一徐向华宋玉芝郭婧南京信息工程大学应用气象学院南京210044 中国气象局生态系统碳源汇开放重点实验室南京210044 南京信息工程大学固碳减排与全球变化研究中心南京210044

反硝化作用是将氮素从湖泊中彻底去除的重要途径之一,对于减轻湖泊富营养化具有重要意义。为了探究太湖沉水植物附着生物的生物量、潜在反硝化速率变化规律及其与环境因素的关系,在沉水植物生长盛期(7月),以太湖沉水植物主要分布区域东... 详细信息

反硝化作用是将氮素从湖泊中彻底去除的重要途径之一,对于减轻湖泊富营养化具有重要意义。为了探究太湖沉水植物附着生物的生物量、潜在反硝化速率变化规律及其与环境因素的关系,在沉水植物生长盛期(7月),以太湖沉水植物主要分布区域东部湖湾作为采样区域,研究了太湖常见的3种沉水植物上附着生物的生物量,并利用乙炔抑制法测定沉水植物上附着生物的潜在反硝化速率,分析了太湖沉水植物附着生物的生物量及其潜在反硝化速率的主要影响因素。结果表明,太湖常见的3种沉水植物单位体表面积附着生物的生物量(AFDM)在0.037~0.789 mg/m^(2)之间,均值为(0.389±0.261)mg/m^(2),沉水植物上附着生物的生物量存在空间差异,最大值出现在贡湖湾G1采样点((0.794±0.007)mg/m^(2)),最小值出现在胥口湾X1采样点((0.041±0.005)mg/m^(2))。太湖沉水植物单位体表面积附着生物的潜在反硝化速率在3.09~58.80μmol/(m^(2)·h)之间,均值为(24.75±5.96)μmol/(m^(2)·h)。太湖沉水植物附着生物的潜在反硝化速率存在空间差异,其中最大值出现在贡湖湾G1采样点((58.80±20.20)μmol/(m^(2)·h)),最小值出现在胥口湾X1采样点((3.09±1.79)μmol/(m^(2)·h))。相同条件下,沉水植物附着生物的生物量及潜在反硝化速率因植物种类的不同而存在差异,狐尾藻(Myriophyllum spicatum)上的附着生物的生物量及潜在反硝化速率显著高于苦草(Vallisneria natans)。相关性分析结果表明,沉水植物上附着生物的生物量与水体氮磷浓度相关。沉水植物附着生物的潜在反硝化速率与附着藻类的生物量、水体溶解有机碳及pH存在显著相关关系,这3种因子可以解释81.2%的沉水植物附着生物潜在反硝化速率变化,本研究结果可为进一步研究浅水湖泊脱氮过程及富营养化治理提供一定的理论依据。

关键词：附着生物潜在反硝化速率太湖沉水植物

多温度限制点条件下燃油热管理系统热回油特性分析

在线全文

学校读者我要写书评

暂无评论

清华大学学报（自然科学版） 2024年第5期64卷 841-851页

作者：杨世宇林远方于海育徐向华梁新刚清华大学航天航空学院热科学与动力工程教育部重点实验室北京100084

热回油是提高飞行器燃油热管理系统性能的重要途径。为研究多温度限制点条件下燃油热管理系统的热回油特性,该文基于Python语言自主开发了燃油热管理系统稳态仿真模型,并进行了实验验证。首先分析了热回油流量对各温度限制点温度的影响... 详细信息

热回油是提高飞行器燃油热管理系统性能的重要途径。为研究多温度限制点条件下燃油热管理系统的热回油特性,该文基于Python语言自主开发了燃油热管理系统稳态仿真模型,并进行了实验验证。首先分析了热回油流量对各温度限制点温度的影响规律,结果表明热回油流量的增加并不一定使得全部温度限制点的温度均下降,甚至导致了供油箱出口温度和热回油阀出口温度的升高,这说明过大或过小的热回油流量均可能引发燃油热管理系统的失效,热回油流量存在满足温度限制要求的变化区间。对于飞机热载荷加热器出口温度和燃油喷嘴出口温度,存在热回油临界流量,当热回油流量达到临界值时,进一步增加热回油会提升系统超温的风险。随后探究了热回油作用下系统的飞机热载荷极限,发现飞机热载荷极限对应于热回油流量限制区间的大小突变为0的临界状态,且此时限制区间的下界是系统的热回油临界流量。进一步研究发现,当飞机热载荷加热器出口温度和燃油喷嘴出口温度在极限状态下同时达到各自的限制值时,系统达到正常工作时的总热载荷极限。为了充分利用系统的总热载荷极限,该文提出了可以实现系统热载荷之间相互传递的中间回路并给出了潜在方案,为飞发一体化热管理提供了一种优化途径。

关键词：热管理系统温度限制热回油热载荷极限数值模拟燃油

1964—2022年河南省许昌市人间布鲁氏菌病流行特征

在线全文

学校读者我要写书评

暂无评论

现代疾病预防控制 2024年第2期35卷 91-94页

作者：徐向华许昌市疾病预防控制中心河南许昌461000

目的了解许昌市人间布鲁氏菌病流行特征,为进一步制定防控策略提供科学依据。方法运用描述流行病学对1964—2022年许昌市布病疫情年报和中国疾病预防控制信息系统报告的布病流行特征进行描述,采用Excel 2013进行数据整理,采用SPSS 27.0... 详细信息

目的了解许昌市人间布鲁氏菌病流行特征,为进一步制定防控策略提供科学依据。方法运用描述流行病学对1964—2022年许昌市布病疫情年报和中国疾病预防控制信息系统报告的布病流行特征进行描述,采用Excel 2013进行数据整理,采用SPSS 27.0软件进行数据分析。结果1964—2022年许昌市辖区9个县(市、区)均有病例发生,共报告人间布鲁氏菌病病例1450例,年发病率波动在0.00/10万~6.87/10万,平均报告发病率为0.66/10万。2006—2022年发病率呈上升趋势(χ_(趋势)^(2)=1568.424,P<0.05);报告病例中男性1049例(2.78/10万),女性386例(1.09/10万),差异有统计学意义(χ^(2)=306.319,P<0.05);年龄主要集中在30~<70岁,共1182例(82.37%),发病率以60~<70岁年龄组最高(5.31/10万),不同年龄组发病率差异有统计学意义(χ^(2)=988.686,P<0.05);职业分布以农民为主,共1213例(84.53%);4—7月份为发病高峰,占47.86%;发病率前三位县区为东城区(1.00/10万)、禹州市(0.82/10万)和襄城县(0.59/10万),差异有统计学意义(χ^(2)=98.000,P<0.05);疾控机构报告986例(68.71%),本地报告诊断病例1080例(75.26%),发病至诊断时间间隔由142.00 d缩短至25.89 d。结论2006—2022年许昌市人间布鲁氏菌病发病呈上升趋势,2022年发病数和发病率达历史新高,本地病例发现能力逐步提升,应强化卫生和畜牧部门的联防联控机制,加大防控力度。

关键词：布鲁氏菌病流行病学特征分析

Minimum description length criterion based decision tree dynamic pruning method in speech recognition

在线全文

学校读者我要写书评

暂无评论

Chinese Journal of Acoustics 2006年第2期25卷 149-158页

作者： xu xianghua HE lin School of Control Science and Engineering Shandong University Ji'nan 250061 Institute of Acoustics Chinese Academy of Sciences Beijing 100080

In phonetic decision tree based state tying, decision trees with varying leaf nodes denote models with different complexity. By studying the influence of model complexity on system performance and speaker adaptation, a decision tree dynamic pruning method based on Minimum Description Length （MDL） criterion is presented. In the method, a well-trained, large-sized phonetic decision tree is selected as an initial model set, and model complexity is computed by adding a penalty parameter which alters according to the amount of adaptation data. Largely attributed to the reasonable selection of initial models and the integration of stochastic and aptotic of MDL criterion, the proposed method gains high performance by combining with speaker adaptation.

关键词： speech recognition decision tree speaker adaptation system performance penalty parameter high performance initial model selection of