【作者】 周峰；

【导师】 马国远；

【作者基本信息】 北京工业大学 ， 热能工程， 2011， 博士

【摘要】 在总的能源消耗中,建筑用能和通信耗能所占比重不容忽视。通信机房作为特殊的建筑,全年均需要制冷,而电子设备舱中密集的电子元件的散热负荷也很巨大,但上述环境对温度、湿度、洁净度等方面的要求比较苛刻。作为一种高效的传热元件,两相闭式热虹吸管能够充分利用室外的自然冷源,降低空调能耗,避免室外空气中的湿气、灰尘和腐蚀性气体等的进入,是节能降温的最佳选择。但在0-60℃温度区间内两相闭式热虹吸管的传热效率、工质选配以及与环境的匹配特性等关键问题还有待进一步的研究。针对上述情况,本文在室温范围内着重进行了以下几方面的研究工作:(1)对不同工质及不同配比混合物的热物性、工质准则数、携带传热极限和沸腾传热极限等进行了理论计算和试验研究,提出了热虹吸管工质选取和多元混合物配制的指导原则和方法。结果表明,优先选取工质准则数M?较大的工质,15种优选工质中R152a、R290、R32和R410A性能较佳;出于毒性、可燃性等考虑,可配置二元或三元混合工质,基于R32的混合物在允许范围内尽量提高其比重,二元混合物以90/10配比为宜,三元混合物R32/R134a/R245fa以40/30/30为宜,R32/R290/R245fa以30/40/30为宜,R32比重为最小比重。建议在单一工质中加入一定比例其他成分,以满足实际要求,也可以在不同温区内不同管排中充注不同配比的工质。(2)建立两相闭式热虹吸管及其换热器的理论模型,从理论和试验两方面研究其传热过程、流动特性、压力变化等。结果表明,为获得较好的传热性能,对热虹吸管,冷热段温差不宜超过10℃;蒸发段长度与总管长之比应避免在0.2-0.5区间;长径比不宜超过35;热流密度不宜超过5 kW/m2。对热虹吸管换热器,冷热段温差对其传热性能的影响大于工作温度的影响,建议选择较低的室外环境温度、较高的冷热段温差和较低的迎面风速。理论和试验结果的变化趋势一致,误差在15%以内。(3)在两相闭式热虹吸管工质中添加不同浓度的单壁碳纳米管(SWNTs)和分散剂聚乙二醇(PEG)400,研究其传热性能。分析得到,PEG 400对其传热性能影响在3%以内,可以忽略;SWNTs能够改善传热性能40%以上,浓度以不超过0.005 wt%为宜,工作温度在30℃以下效果更佳,且压力增幅也较小。因此,建议工作温度在30℃以下时在基准工质中添加不超过0.005 wt%浓度的SWNTs,以获得较优的热工性能。(4)对传统的分离式热虹吸管换热器进行改进,提出一种带气泵的分离式热虹吸管换热系统,并研制出了样机。结果表明,其效率、换热量和能效比(EER)分别提高了47.8%、130%和29%,表明气泵能够有效改善系统性能。(5)将通信机房统筹为一个整体,对全国五个气候区内典型城市的通信机房进行模拟,从理论和试验方面系统量化地探讨了不同地区围护结构散热、空调及热虹吸管换热器能耗负荷特性,提出了联动运行机制。结果表明,在通行的6种围护结构中,全年空调总能耗最低的围护结构,哈尔滨地区为砖混370 mm墙体,北京地区为砖混240 mm墙体,上海、昆明和广州三个地区为砖混180 mm墙体;围护结构传热量约占通信设备总散热量的19.5%;空调每24小时耗能为3-4 kW h,室内设定温度平均提高1℃,空调能耗降低约3.6%,建议将室内温度上限提至27℃;联动机制以热虹吸管换热机组为主,空调为辅,优先考虑热虹吸管换热机组工作。使用热虹吸管换热器后能耗仅为原空调能耗的41%,全年最高节能率达到56%。(6)研究了电子设备舱中不同工质热虹吸管均热器的热工匹配特性。结果表明,舱内外温差低于20℃时,舱内气流最高温度不超过65℃;不同工况下,舱内外温差均在11-17℃范围内,表明热虹吸管均热器非常适合舱内外温差为15℃左右的散热场合。R152a和R22性能优于R12和R134a,且R152a的工质准则数随温度的变化斜率较小,平均水平较高,具有更为宽泛的适用温区。本文的研究工作为提高两相闭式热虹吸管的传热性能提供了参考,尤其是在工质选配以及单壁碳纳米管强化传热等方面提供了重要的理论依据。同时,在自然冷源利用新领域——通信机房和电子设备舱的空调用能方面作了相应的匹配研究。另外,对传统分离式热虹吸管进行了改进,提出了新的结构形式,解决了实际运行中出现的问题。在满足散热需求、节能降耗的同时,也考虑到了工质替代的环保安全性,为通信行业节能减排提供了坚实的技术支持。从目前的工程应用来看,节能效果显著。更多还原

【Abstract】 In the total energy consumption, the energy consumption of buildings and telecommunication centers is considerable. As a special building, a telecommunication center needs to be cooled for a whole year. And the heat dissipation for a sealed electric cabin with dense electronic components is huge. But the required environmental conditions, such as temperature, humidity and cleanliness, are very strict to make sure the devices work well. As an effective heat transfer unit and a good cooling technique, a two-phase closed thermosyphon is able to make full use of outdoor ambient energy, reduces the air conditioning energy consumption, and isolates the outdoor humidity, dust or acid gas. However, the key points of a two-phase closed thermosyphon, like heat transfer efficiency, working fluid match and environmental characteristic, still need to be studied further in the room temperature from 0 to 60 degrees. According to the above situations, the following jobs were done:(1) The numerical simulation and experimental investigation of the thermal property, Merit Number, entrainment heat transfer limit and boiling heat transfer limit were carried out for different working fluids and its compositions with different mass fractions. It is pointed that, the working fluids with bigger Merit Number values are priorities. The working fluids, R152a, R290, R32 and R410A, show better performances among the fifteen working fluids. Due to the toxicity and flammability of the working fluid, compositions with two or three ingredients are feasible. For R32-base compositions, the mass fraction of R32 should be higher, and is 90% for compositions with two ingredients. For the R32/R134a/R245fa composition, the mass fractions should be 40%, 30% and 30%, respectively. For the R32/R290/R245fa composition, the mass fractions should be 30%, 40% and 30%, respectively. It is suggested that some other ingredient should be added into the base working fluid to meet the actual needs. And some working fluids with different proportions can be charged for different pipe rows and different temperature zones.(2) The simulation heat transfer models of a two-phase closed thermosyphon and a heat exchanger were built up. The heat transfer process, flow characteristics and pressure variations were studied theoretically and experimentally. It is found that, for a thermosyphon, the temperature difference of indoor and outdoor should be less than 10 degrees for a better heat transfer performance. And the length ratio between evaporation section and a thermosyphon should be under 0.2 or over 0.5. The ratio between evaporation section length and inner diameter should be not more than 35, and the heat flux should be less than 5 W/m2. For a thermosyphon heat exchanger, the effect of indoor and outdoor temperature difference is over that of working temperature. It is recommended that lower outdoor ambient temperature, lower facing air velocity and higher temperature difference will be better. The changing rules of theoretical and experimental results keep the same, and the error is not over 15%.(3) The single wall carbon nano tubes (SWNTs) were applied in a thermosyphon base working fluid to enhance the heat transfer with some surfactant polyethylene glycol (PEG) with molar mass 400 and the heat transfer characteristics were studied. It is found that, the improvement of PEG 400 for a thermosyphon is less than 3%, which can be ignored. The improvement of SWNTs is over 40%, which is higher with lower pressure when under the 30 degrees working temperature. And the suitable mass fraction is not more than 0.005%. So the SWNTs with not over 0.005% mass fraction are suggested to be added into a base fluid for a better thermal performance under the 30 degrees working temperature.(4) A new separated thermosyphon heat exchanging system with a gas pump was proposed to improve a conventional separated thermosyphon heat exchanger and a prototype was developed. It is found that, the efficiency, heat transfer rate and energy efficiency ratio (EER) increase by 47.8%, 130% and 29%, respectively. It shows that the gas pump can improve the system performance effectively.(5) The simulations of a telecommunication center for national typical cities in five climatic zones were carried out. The numerical and experimental researches of the building envelope heat dissipation, the heat load and energy consumption of an air conditioner and a thermosyphon heat exchanger were completed under different working conditions and different building envelopes. The quantitive results were obtained. And a union operation mode was presented for an air conditioner-thermosyphon heat exchanger system. It is found that, the building envelope with the lowest yearly air conditioning energy consumption among the six type building envelopes is 370 mm brick wall in Harbin, 240 mm brick wall in Beijing, 180 mm brick wall in Shanghai, Kunming and Guangzhou. The heat dissipation through the building envelope takes about 19.5% of the total communication equipment heat load. And the air conditioner energy consumption is 3 to 4 kW h for 24 hours. The air conditioning energy consumption dereases by about 3.6% as the indoor set temperature increases by 1 degree. And the indoor set temperature is recommended to be 27 degrees. In the union operation mode, the thermosyphon heat exchanger is prior to the air conditioner. The energy consumption decreases to 41% of the air conditioning energy consumption when a thermosyphon heat exchanger works. And the whole year energy saving ratio reaches to 56%.(6) The thermal performances of a thermosyphon spreader with different working fluids in a sealed electric cabin were studied experimentally. It is found that, the highest temperature in the electic cabin is not more than 65 degrees when the indoor and outdoor temperature difference is less than 20 degrees. The indoor and outdoor temperature difference remains in the range of 11 to 17 degrees, which indicates that it is very suitable to the situation with 15 degrees temperature difference. The working fluids, R152a and R22, show better performance than the other fluids, R12 and R134a. Further, the Merit Number variation of the working fluid R152a is smooth, the average is higher, and the optimum temperature zone is wider.The above investigations supply a theoretical reference for improving the heat transfer of a two-phase closed thermosyphon, especially for the working fluid and its compositions matching and single wall carbon nano tubes (SWNTs) heat transfer enhancement. A thermosyphon heat exchanger is matched in a new ambient energy utilization field, the air conditioning energy consumption of a telecommunication center and a sealed electric cabin. In addition, the proposal of a new separated thermosyphon heat exchanger with a gas pump solves the problems of a conventional separated thermosyphon heat exchanger in engeering applications. In generally, the reseaching jobs consider the environmental protection of working fluid alternatives while meeting the heat dissipation and reduction of energy consumption. And it also supports the energy saving and emission reduction of the telecommunication industry. The energy saving is remarkable in present engineering projects.更多还原