非均匀水流量对热力膨胀阀控制稳定性的影响

为研究热力膨胀阀(TEV)的控制失稳机理,采用TEX2型外平衡式TEV进行实验,在7组非均匀水流量率的情况下,实现TEV控制从稳定到振荡的动态变化.结果表明:当支路2的水流量F2从36.9%递减至27.0%,TEV流量控制特性稳定,蒸发器换热量仅衰减了3.4%;当F2从27.0%降至9.1%,总出口的制冷剂状态由过热变为带液,其出口温度由稳定变波动,且波动幅度持续增大,此时TEV的流量控制失稳,蒸发器换热量衰减了14.1%.支路1过热蒸汽的显热不足以提供支路2带液量完全蒸发所需的潜热,混合后总出口的过热度小于热力膨胀阀的MSS曲线要求的最小过热度,进而导致热力膨胀阀的持续振荡.实验结果可为多流路制冷系统中换热器的设计提供参考.

Effects of water mal-distribution on thermostatic-expansion-valve stability

To explore the hunting behavior of the thermostatic-expansion-valve (TEV), dynamic characteristics of the TEV (TEX2 externally balanced type) control-loop ranged from stability to vibration were studied under the selected 7 sets of water mal-distribution experimentally. Results showed that as F₂ dropped from 36.9% to 27.0%, the TEV control-loop was stable and the overall evaporator capacity decreased by only 3.4%. As F₂ dropped from 27.0% to 9.1%, evaporator overall outlet temperature changed from stability to vibration. Its vibration amplitude increased gradually with the phase transformation from vapor to mist, indicating that hunting behavior of TEV control-loop occurred, and the overall evaporator capacity decreased by 14.1%. The superheated refrigerant gas from Circuit 1 outlet only provided sensible heat and couldn''t evaporate the liquid refrigerant out of Circuit 2 into superheated steam. Consequently, refrigerant in the evaporator overall outlet was of a smaller superheat than the necessary minimal stable signal (MSS) of the TEV, even at two-phase state, and would inevitably give rise to the hunting of the TEV -controlled system according to the MSS theory. These experimental results might provide references for the design of the heat exchanger of multi-circuit refrigerating system.