The behavior of falling film over horizontal has significant impacts on the efficiency of heat and mass transfer for the horizontal tube evaporators and absorbers especially for intermittent droplet model at low spray density, which has lower heat and mass transfer resistance. Thus it is critical to illuminate transient and intermittent flow mechanism of droplet dynamic behavior and film thickness evolution regulation over horizontal tube falling film at low spray density. A two dimension multi-phase flow mode for superhydrophilic surface was established to capture the microscopic flow mechanism. The results indicated that droplet generation and growth stages accounted for more than half of the droplet life cycle and an obvious droplet retraction and pulsation were first observed during droplet generation due to surface tension and local additional pressure. Further, the whole life cycle transient and intermittent film thickness map of droplet mode was depicted. And found that the liquid film thickness transient evolution regulation of droplet mode could be split into two zones: impact zone and stable zone. For impact zone, the film thickness was rapidly decreased with the spreading of the liquid film from the millimeter level to 200 $\mathrm{\mu }$m within 0.2 s. While for stable zone, film thickness almost maintained stable over a long period time until the next droplet impacted. Because of the intermittent of droplet generation and movement, the change of film thickness of droplet mode with spray Reynolds was achieved by increasing impact frequency of droplet impact instead of the volume of a single droplet.

水平降膜的行为对水平管蒸发器和吸收器的传热传质效率有显着影响，特别是对于低喷雾密度下的间歇液滴模型，其传热传质阻力较低。因此，阐明低喷雾密度下水平管降膜上液滴动态行为和膜厚演变规律的瞬态和间歇流动机制至关重要。建立了超亲水表面的二维多相流动模式来捕捉微观流动机制。结果表明，液滴生成和生长阶段占液滴生命周期的一半以上，并且在液滴生成过程中，由于表面张力和局部附加压力，首先观察到明显的液滴收缩和脉动。此外，描绘了液滴模式的整个生命周期瞬态和间歇膜厚度图。并发现液滴模式的液膜厚度瞬态演化规律可分为两个区域：撞击区和稳定区。对于冲击区，随着液膜从毫米级扩展到 200 并发现液滴模式的液膜厚度瞬态演化规律可分为两个区域：撞击区和稳定区。对于冲击区，随着液膜从毫米级扩展到 200 并发现液滴模式的液膜厚度瞬态演化规律可分为两个区域：撞击区和稳定区。对于冲击区，随着液膜从毫米级扩展到 200$\mathrm{\mu }$米在 0.2 秒内。而对于稳定区，薄膜厚度在很长一段时间内几乎保持稳定，直到下一个液滴撞击。由于液滴产生和运动的间歇性，通过增加液滴撞击的撞击频率而不是单个液滴的体积来实现喷雾雷诺兹液滴模式的膜厚度变化。