滴头最大流量偏差率计算方法及影响因素评价

首先分析了滴头水力特征曲线方程中流量系数分布规律,当流量系数服从正态分布时,流量系数在其平均值的±3倍标准差之间变化的概率为9973％(近似于100％),因此流量系数最小值为平均值减3倍标准差,最大值为流量系数平均值加3倍标准差,在此基础上提出了综合考虑水力偏差、制造偏差和微地形偏差的综合流量偏差率计算公式;然后以制造偏差系数、流态指数、压力差、平均工作水头及田面局部高差作为影响因子进行流量偏差率及毛管造价敏感性分析．结果表明:对流量偏差率影响程度由大到小的顺序为滴头制造偏差系数、滴头流态指数、压力偏差．当制造偏差系数大于004时,毛管造价急剧增大,在设计中应尽可能选择制造偏差系数小、流态指数小的滴头.对于常规滴灌系统,当滴头工作水头大于10 m时,田面局部高差对流量偏差率影响可忽略．

Calculation method of the maximum flow variation and evaluation of factors for micro irrigation engineering

The distribution characteristic of emitters′ flow coefficient is analyzed. When flow coefficient complies with normal distribution, the probability of flow coefficient fluctuating at ±3 times standard deviations is 9973％ (approximating 100％). Thus, the minimum flow coefficient is average flow coefficient minus 3 times standard deviation and the maximum flow coefficient is average flow coefficient plus 3 times standard deviation. Based on the above analysis, a comprehensive flow variation rate calculation formula, taking hydraulic variation, manufacture variation and micro terrain variation into consideration, is presented. Then, with emitter manufacturing variation coefficient, flow exponent, pressure difference and average field roughness height as influential factors, flow variation rate and lateral cost sensitivity were analyzed. The results show that emitter manufacturing variation coefficient is the most influential factor, followed by flow exponent and pressure difference. The lateral cost increases dramatically when manufacturing variation coefficient is larger than 004. In designing, emitters with minimal manufacture variation and minimal flow exponent should be elected. For normal drip irrigation systems, when the emitter′s water drop is larger than 10m, the effects of local height differences in fields on flow variation can be ignored.