Based on the data from the Cost-benefit Data of Farm Produce and the China Agricultural Yearbook, this paper aims to examine the spatial and temporal change characteristics of total grain production and its affecting ...Based on the data from the Cost-benefit Data of Farm Produce and the China Agricultural Yearbook, this paper aims to examine the spatial and temporal change characteristics of total grain production and its affecting factors. The results show that： 1） During 1980 to 2007, total grain production increased from 3.20 ~ 108 t to 5.02 x 108 t in China, with annual increasing rate of 1.68%. From the regional disparities, most of the regions present increasing trend of total grain production except for several regions with higher level of economic development; 2） Grain sown area decreased from 1.17 × 108 ha in 1980 to 1.06 x 108 ha in 2007, which has negative effect on total grain production; 3） The increase of grain yield per unit area caused by land use intensity changes contributed to the increase of total grain yield greatly. However, as the land use intensity showed that farmers pay more attention to labor-saving input but not yield-increasing input, the less enthusiasm of farmers in grain production may become an important constraint on fu- ture grain production increase in China; 4） Based on the results, this paper proposed different land management poli- cies in different regions, for example, the government should protect cultivated land, promote large scale production. As to the less developed regions, the government should pay more attention to agricultural subsidies to promote farm- ers＇ enthusiasm in grain production.展开更多
运用对数平均迪氏指数(logarithmic mean weight division index,LMDI)分解方法,首次建立了我国有色金属产品进出口贸易的指数分解模型,从多个角度系统地分析了2004年1月2009年1月,我国有色金属产品特别是铜和铝产品进出口贸易及其三种...运用对数平均迪氏指数(logarithmic mean weight division index,LMDI)分解方法,首次建立了我国有色金属产品进出口贸易的指数分解模型,从多个角度系统地分析了2004年1月2009年1月,我国有色金属产品特别是铜和铝产品进出口贸易及其三种主要影响因素(总量因素、结构因素及价格因素)的贡献程度和变化趋势.研究表明:有色金属进口额的主要影响因素在2006年前后发生显著变化.2006年之后,随着有色金属价格的不断攀升,价格因素成为了影响中国有色金属进口额波动的主要因素.同时有色金属出口额的主要影响因素在2006年前后并未出现明显变化.在国际有色金属价格大幅攀升时,我国有色金属出口额并未同步增长.展开更多
The effects of boundary layer bleed on the scramjet thrust are studied in the present paper.A theoretical model is developed to evaluate the thrust increment and influencing factors.The thrust increment resulting from...The effects of boundary layer bleed on the scramjet thrust are studied in the present paper.A theoretical model is developed to evaluate the thrust increment and influencing factors.The thrust increment resulting from the bleed is dominated by the rise in total pressure recovery and bleed mass flow rate.The bleed mass flow rate exerts stronger impact on the engine thrust than the total pressure.According to current bleed design,it is a severe challenge for the engine to enhance its total pressure to maintain the original thrust when there is no bleeding.Furthermore,the initial total pressure recovery,fuel mass addition,combustion efficiency and area ratio of engine exit to entrance can affect the contributions of the bleeding to the thrust increment.The scramjet needs a higher rise in total pressure recovery to counteract the negative effect of bleed mass loss at higher initial total pressure recovery or larger area ratio of engine exit/entrance.More heat release results in a little lower demand on the rise in total pressure recovery for maintaining the scramjet thrust.These results will aid in understanding the fundamental mechanism of bleeding on engine thrust.展开更多
基金Under the auspices of National Natural Science Foundation of China (No. 40971062)China Postdoctoral ScienceFundation (No. 20100480441)
文摘Based on the data from the Cost-benefit Data of Farm Produce and the China Agricultural Yearbook, this paper aims to examine the spatial and temporal change characteristics of total grain production and its affecting factors. The results show that： 1） During 1980 to 2007, total grain production increased from 3.20 ~ 108 t to 5.02 x 108 t in China, with annual increasing rate of 1.68%. From the regional disparities, most of the regions present increasing trend of total grain production except for several regions with higher level of economic development; 2） Grain sown area decreased from 1.17 × 108 ha in 1980 to 1.06 x 108 ha in 2007, which has negative effect on total grain production; 3） The increase of grain yield per unit area caused by land use intensity changes contributed to the increase of total grain yield greatly. However, as the land use intensity showed that farmers pay more attention to labor-saving input but not yield-increasing input, the less enthusiasm of farmers in grain production may become an important constraint on fu- ture grain production increase in China; 4） Based on the results, this paper proposed different land management poli- cies in different regions, for example, the government should protect cultivated land, promote large scale production. As to the less developed regions, the government should pay more attention to agricultural subsidies to promote farm- ers＇ enthusiasm in grain production.
文摘运用对数平均迪氏指数(logarithmic mean weight division index,LMDI)分解方法,首次建立了我国有色金属产品进出口贸易的指数分解模型,从多个角度系统地分析了2004年1月2009年1月,我国有色金属产品特别是铜和铝产品进出口贸易及其三种主要影响因素(总量因素、结构因素及价格因素)的贡献程度和变化趋势.研究表明:有色金属进口额的主要影响因素在2006年前后发生显著变化.2006年之后,随着有色金属价格的不断攀升,价格因素成为了影响中国有色金属进口额波动的主要因素.同时有色金属出口额的主要影响因素在2006年前后并未出现明显变化.在国际有色金属价格大幅攀升时,我国有色金属出口额并未同步增长.
基金supported by the National Natural Science Foundation of China(Grant Nos.90716014 and 91216115)
文摘The effects of boundary layer bleed on the scramjet thrust are studied in the present paper.A theoretical model is developed to evaluate the thrust increment and influencing factors.The thrust increment resulting from the bleed is dominated by the rise in total pressure recovery and bleed mass flow rate.The bleed mass flow rate exerts stronger impact on the engine thrust than the total pressure.According to current bleed design,it is a severe challenge for the engine to enhance its total pressure to maintain the original thrust when there is no bleeding.Furthermore,the initial total pressure recovery,fuel mass addition,combustion efficiency and area ratio of engine exit to entrance can affect the contributions of the bleeding to the thrust increment.The scramjet needs a higher rise in total pressure recovery to counteract the negative effect of bleed mass loss at higher initial total pressure recovery or larger area ratio of engine exit/entrance.More heat release results in a little lower demand on the rise in total pressure recovery for maintaining the scramjet thrust.These results will aid in understanding the fundamental mechanism of bleeding on engine thrust.
