机械过冷跨临界CO2热泵供暖系统性能分析

采用辅助的蒸气压缩循环进行过冷,可改善传统跨临界CO_2热泵系统用于冬季供暖性能。本文通过构建机械过冷跨临界CO_2热泵系统的热力模型,分析了机械过冷跨临界CO_2热泵系统供暖工况下的运行特性,结果表明:机械过冷CO_2热泵系统存在最大COP,对应最优排气压力和过冷度,标准工况下比常规CO_2系统能效提高15.9%。该系统可有效解决常规CO_2热泵回水温度过高导致COP迅速衰减的问题,当回水温度由40℃升至50℃时,常规系统COP下降16.9%,而机械过冷热泵系统COP仅下降8.4%。通过改进可有效降低CO_2压缩机的排气压力和温度,且供水温度越低排气压力降低效果越显著。机械过冷循环工质的选取会影响系统整体性能,选取的11种过冷循环工质中能效最高的为R717,最低的为R1234yf。在低环境温度工况下性能的提升更加明显,通过配置小型常规工质蒸气压缩循环即可实现CO_2热泵系统性能显著改进,经济性优势明显。     

基于四氢呋喃的相变蓄冷材料的制备及其性能研究

为满足北部湾海洋运输果蔬冷链保鲜的特定需求,研发一种适宜的相变蓄冷材料显得尤为重要。由于四氢呋喃（THF）相较于其他水合物所需严苛的低温高压生成条件不同,其水合物能够在常压环境下生成,因此被视为一种适宜的运输蓄冷介质。以浓度99%的THF（10.446 g）和去离子水（30 g）作为核心基液,以氯化钠（10 g）作为温度调节剂,选用3种不同质量分数（3%、6%、9%）的聚乙烯醇作为增稠剂,研制出了一种高效且性能稳定的相变蓄冷材料（PCM）。利用差示扫描量热法（DSC）探究了THF与氯化钠（NaCl）混合水合物的蓄冷性能,同时进行了不同质量分数增稠剂的对比实验。结果表明：浓度为99%的THF和去离子水作为核心基液,构成了PCM材料的主体框架,保障其基本热物理特性的稳定;引入的氯化钠作为温度调节剂,通过调控其比例可优化PCM材料的相变温度范围,以更好地满足特定应用场景的需求;聚乙烯醇作为增稠剂,不仅使PCM相变温度得到改善,而且还增强了PCM的稳定性、有效地抑制了相分离,确保了材料在固液相变过程中的稳定,从而提高了PCM蓄冷性能,当聚乙烯醇质量分数为6%时有助于维持材料在相变过程中的均匀性。为解决材料过冷度问题,引入了质量分数为1.4%的壬酸成核剂,并进行了梯度实验。结果表明,壬酸的加入能显著降低相变材料的过冷度（近乎降至零）,从而使PCM材料表现出理想的相变特性,相变温度（Onset温度）稳定位于6.3—8.0℃的区间内,适用于低温储能领域。同时,PCM材料的相变潜热值为115 J·g^-1左右,表明其具有较高的能量存储密度,能够有效地吸收和释放热量。通过合理的药剂配比与优化,制备出了一种性能较好、过冷度极低的THF基共晶体相变材料,为蓄冷技术的发展提供了有力的支持。     

Vapour phase motion in cryogenic systems containing superheated and subcooled liquids

The development of vent pipelines, and venting storage tanks for cryogenic liquids requires the knowledge of the law of motion as well as regularities of vapour content variation in the liquid and heat dissipation by the vapour phase. This is a theoretical study of the effect of superheating (subcooling) of the liquid, relative acceleration and reduced pressure upon the size and velocity of noninteracting vapour bubbles, moving in the liquid, and upon their resistance and heat transfer coefficients.     

Condensing Heat Transfer in Steam-Water Condensing-injector

An experiment on the direct heat transfer process between supersonic steam and subcooled water jet was performed, using a steam-water condensing-injector. Photographic observation provided information on the state of flow, and establishment of a critical separate steam-water flow was confirmed. The temperature and pressure distributions along the flow were measured and the effective coefficients of condensing heat transfer were evaluated from the observed data, based on a model embodying an idealized interface between vapor and liquid. In the vicinity of the water nozzle exit, where the vapor-liquid interface was distinct, the heat transfer coefficients obtained were 14–28 (cal/°C.cm².sec), and some correlation was observed among Nusselt, Reynolds and Jakob numbers, upon adopting the velocity and the physical properties of the steam phase. The relations Nu=6.0.Re ^0.9(Pr=1.04–1.10), and Re=1.8×10⁸.Ja ^3.0, i.e., Nu=1.6×10⁸.Ja ^2.7 were derived as a rough estimation. No clear correlation could be discerned in the corresponding data obtained from observation points further downstream, where a distinct steam-water interface no longer existed. In conclusion, it is proposed that, in deriving the correlations between Nu and Re or Ja, the physical properties of the vapor and the vapor-liquid relative velocity should be adopted, on account of the strong dependence of condensing heat transfer on steam velocity and water subcooling.     

The 2nd International Topical Meeting on Neutron Radiography System Design and Characterization November 12—18, 1995, the Shonan Village Center,Hayama and the Rikkyo University,Yokosuka, Japan

Framatome-ANP has developed S-RELAP5, a RELAP5/Mod2 based thermal hydraulic code, and PANBOX, a 3D core kinetics code. By coupling both codes, a powerful neutronic and thermal hydraulic plant model was developed, which is capable of calculating extremely complex transients, particularly events bearing strongly asymmetric phenomena. The capability of the code system has been tested by recalculation of several transients that occurred in Siemens built PWRs. The most complex transient was a loss of load combined with a temporary coastdown of one main coolant pump, which is presented here. Since the measured values from the data recording system of the plant were available, the calculation could be compared to measured parameters.

The key phenomenon of the transient is a highly asymmetrical neutronic condition, which was caused by:

• —the drop of 5 control rod pairs in an asymmetric pattern upon detection of “loss of load.”

• —the coastdown of one main coolant pump, due to failure to connect to the auxiliary bus, which allowed coolant in one loop to stagnate and cool. Subsequent reactivation of that pump forced a plug of cold water into one side of the core.

The physical progress of the transient is strongly dependent on this double asymmetry; therefore, a 3-D calculation is indispensable for an accurate simulation. The calculated results are in good agreement with measurements and represent an important contribution to code validation.     

竖直圆管外含空气蒸汽冷凝传热的实验研究

通过对含空气蒸汽在竖直圆管外表面冷凝传热的实验研究,分析了空气质量分数、压力及过冷度对蒸汽冷凝换热的影响,给出了含空气蒸汽的冷凝传热过程中的实验关联式。结果表明：在空气质量分数及压力不变的条件下,壁面过冷度对冷凝传热系数的影响高于纯蒸汽冷凝过程中的Nusselt层流解;所得到的实验关联式具有更广的适用范围,且其与实验值的误差在±10%以内。     

人工神经网络在预测流动沸腾曲线中的应用

选用20世纪60年代以来的实验数据,应用人工神经网络分析入口欠热度、质量流速、压力等主要参数对沸腾曲线的影响。在整个传热区内,热流密度随入口欠热度的增加而增大;在过渡沸腾和膜态沸腾区,热流密度随质量流速的增加而增加;压力起重要的作用,除膜态沸腾区外,增加压力能强化传热。除泡核沸腾外,稳态和瞬态的流动沸腾曲线的差异很小。     

低压高过冷度下自然循环流动不稳定性实验研究

对具有长直上升段的自然循环系统,开展了流动不稳定性实验研究。同时,详细分析了低压、高入口过冷度条件下典型的流动不稳定现象。实验表明：自然循环系统的结构、流体的热边界条件会影响自然循环的运行特性及流动不稳定性类型。较高入口过冷度下,高热流密度导致系统脱离稳态后,很难重新回到稳定的两相自然循环流动状态。随着热流密度的提高,系统会经历间歇沸腾、复合动态流动不稳定性等状态。依据实验结果得到了高入口过冷度下的不稳定性边界图。在两相振荡期间,自然循环驱动压头和回路阻力的主要影响因素集中在长直上升段和加热段。加热段出口积聚的大量气泡对上、下游流体的强烈挤压作用是流量大幅振荡及逆流的主要原因。     

蓄能式商用数码多联机组的设计研究

设计了一款新型过冷式变制冷剂流量的多联系统。分析了该系统的原理、结构和设计思路,并对比了非蓄冷系统与蓄冷系统之间的制冷能力、制冷效率和过冷度之间的区别。经济性分析表明该系统的回收年限少于3年。