摘要
热力蒸汽压缩器是一种以蒸汽为工质的气体喷射器。近年来带有热力蒸汽压缩器的多效蒸发海水淡化(TVC-MED, Multi-effect Distillation with Thermal Vapor Compression)系统的应用愈加广泛,喷嘴带有调节锥的可调式热力蒸汽压缩器也较多地应用于该系统中。TVC作为系统中的重要设备,其热力性能的计算方法研究将有较大意义。过去的研究工作多数集中讨论喷射器在制冷循环当中的性能,多数以制冷剂为工质,且关于可调式热力蒸汽压缩器的研究较少。本文以将水蒸气作为工质的气体喷射器为研究对象,分析热力蒸汽压缩器在MED系统工况下的热力性能特点。
本文基于气体动力学函数方法,建立了包括圆柱形混合室的固定结构喷射器、锥形混合室的固定结构喷射器、喷嘴带有调节锥的可调式喷射器的设计性能、变工况性能一维计算模型。将本文的计算结果与一些实验数据、数值模拟数据、喷射器生产厂商提供的数据对比,对比结果较好地验证了本文所采用计算模型的可靠性、准确性。对比中采用的实验数据、数值模拟数据、喷射器生产厂商的数据都参照了TVC-MED系统工况,因此上述对比结果可以表明本文的计算模型在TVC-MED系统工况下的适用性。
在证明模型的适用性的基础上,本文计算了在TVC-MED系统设计工况下的TVC喷射系数ω等参数,分析了TVC性能在此种工况下的特点,对比了圆柱形混合室的喷射器与锥形混合室的喷射器在喷射系数与压缩比关系ω-Pd以及出口压缩蒸汽过热度等方面的不同点。同时,本文计算了给定结构的喷射器在变工况下的特性曲线,计算模型中考虑了圆柱形混合室喷射器可能出现的三种极限状态、锥形混合室喷射器可能出现的两种极限状态以及喷射器亚极限状态的性能,这使模型能够计算较大范围的TVC-MED系统工况下的TVC性能参数。
特别地,在建立可调式喷射器的计算模型当中,本文假设喷嘴调节锥的加入并没有给喷嘴的效率带来影响,忽略了调节锥的加入所增加的摩擦损失。在计算过程中,经验系数为常数,忽略了喷嘴调节锥位置、膨胀比的变化对经验系数的影响。将该模型的计算结果与国外某喷射器厂商的数据做对比,结果表明两者平均偏差为4.5394%,相对偏差的浮动范围为-7.34%—21.35%。在此基础上本文计算了可调式喷射器的变结构、变工况性能曲线,分析了影响经验系数的因素以及可调式喷射器在TVC-MED系统中的变工况调节特性。
Thermal vapour compressor (TVC) belongs to gas-ejectors using steam as the working fluid. It has been widely used recently to improve the energy utilization ratio and increase the gained output ratio (GOR) of Multi-effect Distillation (MED) System. And because a fixed structure TVC has only one optimum operating point, a kind of adjustable TVCs have been applied to fit for variable operating conditions. As an important element of MED, the performance of TVC should be well considered, but more researchers before have paid their attention to the ejector using in refrigeration system with the refrigerant as working fluid.
In this paper, mathematical models, based on gas-dynamic theory, a kind of semi-empirical calculation method of ejector, including calculation of the design point and variable operating condition of ejector with ether cylindrical mixing tube or converging mixing tube have been presented. The results of the models have been compared with the data from the experimental results, the computational results and the data from the producing company of TVC to validate the models. And the results show reasonably good agreement with the latter ones. Furthermore, three kinds of the critical modes of operating condition of the ejector with cylindrical mixing tube and both two kinds of the critical modes of the ejector with converging mixing tube were considered in these models, therefore the models could predict the ejector performance in a wide operating condition range of TVC-MED.
As an important proportion, the 1-D mathematical model of the adjustable ejector has been developed. The effects of the needle moving in the nozzle have been considered to be changes of consumption of the motive steam and the loss effect of the needle has been neglected. The empirical coefficient P is considered to be constant. The calculation results show much close to the data from the ejector producing company. As a result, the model is verified by the comparison of the average deviation of 4.5394%, and the deviation ranging from-7.34% to 21.35%. Furthermore the performance curves are calculated to analyze the performance of the adjustable ejector in the MED system and an analyses of the factors which effect the value ofμare also presented.
引文
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