加料装置是流化床的重要组成部分，起到料封和调节进料速率的作用，是流化床连续稳定运行的关键。加料装置一般由立管和料阀组成，U阀是一种常见的非机械阀，具有结构简单、运行可靠、易于调节等优点，在工业中得到广泛应用。许多研究者对U阀的运行特性进行了详细的考察。U阀的结构特征、物料特性、操作风速等均对U阀运行有重要影响。作为气体驱动的非机械阀，阀体出口气压（背压）对其运行特性有显著影响，然而目前没有人对此进行过系统研究，对背压的影响规律缺乏认识。本文建立了可调节背压的立管-U阀实验平台，通过实验研究了从0 kPa（表压）到稳定操作上限范围内，背压对U阀排料特性的影响规律。主要结论如下：(1)随背压增大，立管内和供料室底部颗粒间作用力减小，U阀排料速率明显减小，进气速率对排料速率的调控能力变弱，背压大于7 kPa时，背压对排料速率的影响尤为显著。(2)对于本实验装置，背压小于7 kPa时，孔口高度在0.09~0.11 m时U阀可获得良好的排料特性；背压在7~13 kPa时，孔口高度在0.13~0.17 m，可以使U阀获得良好的排料特性。若装置背压波动较大，则应适当减小孔口高度，以提高排料速率的稳定性。(3)建立了U阀排料速率与背压、进气速率和孔口高度之间的经验方程，计算值与实验值的平均相对误差约为20%。(4)采用内置导气管的圆锥形立管，有效缓解了高负压差下立管物料架拱的发生。(5)使用30 μm、75 μm、206 μm玻璃珠实验发现，颗粒粒径越大，U阀启动气速越大，获得相同的排料速率所需进气速率越大。使用0.75 μm的α-Al2O3颗粒进行排料试验，由于颗粒黏性大，供料室底部有周期性的颗粒架拱和塌落发生，造成U阀的排料呈现周期性，每一周期持续时间约为30 s。

The solid feeding/discharging system, consisting of a standpipe and a discharging valve, is the key component of a fluidized bed to achieve continuous and stable operation, which also functions as gas sealing and solid flow rate regulating. The U-valve (Loop Seal) is a commonly used non-mechanical valve for its simple structure, reliability and easy adjustment.The U-valve has been investigated by many researchers in the past several decades. The operation property of a U-valve is affected by various factors such as structural parameters, powder properties, the aeration rate, the back pressure, etc. The back pressure has a significant effect on the operation property of a U-valve. However, the back pressure effect has seldom been investigated in literature.In order to investigate the back pressure effect of the U-valve, a setup with adjustable back pressure was established. The main conclusions are as follows.(1) Experimental results reveal that the solid discharge rate, as well as the effect of aeration rate on the solid discharge rate decreases with increasing the back pressure, e.g., the back pressure effect is quite significant above 7 kPa. According to force analysis, the interaction force of particles at the bottom of the supply chamber, as well as in the standpipe decreases at elevated back pressure.(2) The orifice height should have a suitable range to obtain a stable discharge rate for this setup, i.e., the orifice height should be in the range of 0.09~0.11 m for the back pressure less than 7 kPa, and 0.13~0.17 m for the back pressure of 7~13 kPa. In the case of high back pressure fluctuation, a more stable operation can be achieved with a smaller orifice height.(3) An empirical equation was established to predict the solid discharge rate under different back pressure, aeration rates, and orifice heights. The prediction accuracy of the equation is ~20%. The equation can provide the guidance for the design of U-valves.(4) A conical standpipe with an inside tube to direct the aeration gas out of the standpipe has been developed to solve the arching problem frequently accured at high back pressures.(5) Glass beads of three sizes (30 μm, 75 μm, 206 μm) have been used to investigate the effect of particle size on solid discharge rate of the U-valve, and the results reveal that as particle size is increased, start-up gas velocity of the U-valve increases, as well as gas velocity needed to achieve a given solid discharge rate. When α-Al2O3 with a mean diameter of 0.75 μm used as feedstock, a periodic arching and ollapsing of particles happens at the bottom of supply chamber, which results in an intermittent discharge of the U-valve.