紧凑型喷雾冷却系统强化换热的研究

发布时间：2017-12-27 10:36

本文关键词：紧凑型喷雾冷却系统强化换热的研究　出处：《中国科学技术大学》2017年博士论文　论文类型：学位论文

【摘要】：电子元器件的功率密度逐年增长,随之而来的热量堆积和温度增高必然会影响电子设备的稳定性、可靠性和使用寿命。因此,如何在有限的空间中实现高效散热已经成为高性能、高集成电子器件发展的瓶颈。喷雾冷却是一种新兴、高效的散热方式,相较于传统热控方法,具有散热能力强、控温响应快、接触热阻小、流量需求少和过热度低等优点,在电子元器件热控方面具有广阔的应用前景。本文重点研究实用化的紧凑型喷雾冷却系统,进而分析喷雾冷却在紧凑系统中的换热特性和影响参数,并探索适用于紧凑喷雾冷却系统的高效、稳定的换热强化方法。首先,本文对紧凑型喷雾冷却系统的核心部件——雾化喷嘴的紧凑化和阵列化进行了研究。针对紧凑型喷雾冷却系统的特点,提出了一种新型紧凑雾化喷嘴的设计方案,并对该新型喷嘴的雾化特性展开了实验研究,分析喷嘴的喷雾锥结构、优化喷雾场液滴分布等参数。研究结果表明,该喷嘴可以满足紧凑型喷雾冷却系统对喷雾特性的要求。以该新型喷嘴为基础,根据不同应用系统与使用环境的特点,研制出相应的紧凑型喷嘴阵列,包括适用于较大面积热源的板式多喷嘴阵列、适用于大热流的环状多喷嘴阵列和适用于分散热源的线性多喷嘴阵列。该部分研究为紧凑腔体与集成系统的研究提供基础。利用板式多喷嘴阵列,建立了针对计算机芯片的紧凑型喷雾冷却系统,并对其换热特性进行了系统研究。该系统采用板式多喷嘴阵列构成厚26mm的紧凑型喷雾腔体,以实现冷却30mm×30mm方形模拟芯片的发热表面。研究结果表明,利用该系统可以在喷雾冷却单相阶段实现高于90W/cm2的散热能力,被冷却表面的温度不均匀性随着表面热流的升高而升高,提高体积流量对散热热流提高显著,而喷雾高度存在最佳范围。利用环状多喷嘴阵列,建立了模拟反应堆中子靶的超高功率喷雾冷却系统。该系统采用环状多喷嘴阵列组成的大型扁薄腔体,冷却内径450mm、外径470mm的环状热源。通过优化多喷嘴阵列排布和点热源面分散方式,在发热表面低于140℃的条件下,成功实现了 80kW的大功率热源的冷却,满足大热流中子靶的冷却要求。该研究结果也表明,紧凑型喷雾冷却系统不仅可以用于电子元器件等功率相对较低的场合的散热,也可以实现高功率、大面积热源的散热。在紧凑喷嘴和紧凑腔体研究的基础上,建立了针对插件式集成机箱的紧凑式喷雾冷却系统,并对该系统的换热特性和换热强化展开了系统研究。在该系统中,利用实验方法研究流量、喷雾倾斜角和表面结构等因素对HFE7100、PF5060这两种工质的换热及其强化特性的影响。实验结果表明,PF5060和HFE7100作为工质获得的最大热流分别为79.1W/cm2和81.6W/cm2;流量显著影响换热,而温度不平衡性主要与流量和流阻相关;喷雾瞬态冷却响应迅速,倾斜角影响响应的时间;脉冲加热时,温度平衡迅速,平衡时间受表面热流影响;采用合理的倾斜角能够提高系统的换热性能,实验条件下,采用75°倾斜角可以获得最佳的散热效果,其原因在于在该倾斜角下,喷雾冷却的工质利用率和流体管理都得以改善;采用强化表面同样能提高散热热流,实验条件下,直肋表面强化特性优于混合肋和方针肋表面,其原因在于强化表面能够增加接触面积、延长接触时间、改善液膜流动。最后,为了分析该系统的实用性和适用性,本文提出针对喷雾冷却系统的综合评估方法。为了进一步探索紧凑空间中高效、稳定的换热强化方法,本文在完成喷雾冷却系统紧凑化的基础上,研究了高醇类表面活性剂对喷雾冷却换热的影响。本文选用正庚醇(1-heptanol)、正辛醇(1-octanol)、异辛醇(isooctanol)和正癸醇(n-decanol)等四种高醇类表面活性剂,研究结果表明,表面活性剂对喷雾冷却瞬态和稳态换热都具有明显的增强作用。在瞬态换热中,可以使表面温度在极短时间内下降得更加明显,并达到更高的瞬态热流;在稳态换热中,表面活性剂对两相阶段的换热强化更为明显。研究同时发现,表面活性剂对喷雾冷却换热的强化存在最佳的浓度,正庚醇与正辛醇在质量浓度为0.3‰时强化效果最好,而异辛醇和正癸醇的最佳强化浓度分别为0.5‰和0.1‰。由于高醇类表面活性剂相对于传统的离子型添加剂具有添加量少、无腐蚀、不结晶、不易堵塞等优点,该研究结果对喷雾冷却的实用化和换热强化具有重要意义。最后从工质物性方面研究了高醇类表面活性剂对喷雾特性的影响,并进一步分析了表面活性剂对喷雾冷却换热的强化机理。利用实验方法研究了表面活性剂的浓度与饱和蒸汽压、表面张力、接触角和动力粘度等各项物性之间的关系,得到相应关系式;同时分析了表面活性剂对喷雾锥结构和液滴参数的影响。研究结果表明,表面活性剂降低了溶液蒸汽压,但是低温下变化不明显;表面张力受表面活性剂浓度影响明显,符合特劳贝准则,而接触角的变化趋势与之相似;动力粘度随活性剂种类和浓度变化均不明显。同时通过PDA测试发现,表面活性剂对液滴数目影响明显,包括喷雾锥的卷吸区扩大、主流区外移;对喷雾主流区张角没有明显影响;液滴直径在中心区域减小,在边缘区域局部增大;加入活性剂后液滴速度的变化不明显。结合物性关系式,实验得出了预测液滴直径和液滴速度的关联式,并根据该关联式分析了表面活性剂在喷雾冷却换热中强化的机理。
[Abstract]:The power density of electronic components increases year by year, and the consequent heat accumulation and temperature increase will inevitably affect the stability, reliability and service life of electronic devices. Therefore, how to achieve efficient heat dissipation in limited space has become a bottleneck for the development of high performance and high integrated electronic devices. Spray cooling is a new and efficient way of heat dissipation. Compared with the traditional thermal control method, spray cooling has the advantages of strong heat dissipation, fast response to temperature control, little contact heat resistance, less flow demand and low superheat. It has broad application prospects in the field of electronic components thermal control. This paper focuses on the research of a practical compact spray cooling system, and then analyzes the heat transfer characteristics and influencing parameters of spray cooling in compact system, and explores an effective and stable heat transfer enhancement method suitable for compact spray cooling system. First, the compactness and array of atomization nozzles, the core components of the compact spray cooling system, are studied in this paper. Aiming at the characteristics of the compact spray cooling system, a new compact atomizing nozzle design is put forward, and the atomization characteristics of the new nozzle are studied experimentally. The spray cone structure and the droplet distribution of spray nozzle are analyzed. The results show that the nozzle can meet the requirements for the spray characteristics of the compact spray cooling system. Based on the new type of nozzle, according to the different characteristics of the application system and the use of the environment, developed a compact array of nozzles corresponding, including plate for large area heat source multi nozzle array, suitable for high heat flux annular multi nozzle array and is suitable for the heat supply line of multi nozzle array. This part of the study provides a basis for the research of compact cavity and integrated system. A compact spray cooling system for computer chips is established by using a plate type multi nozzle array, and the heat transfer characteristics of the system are systematically studied. The system uses a plate type multi nozzle array to form a compact spray cavity with thick 26mm, in order to achieve the heating surface of the cooling 30mm x 30mm square analog chip. The results show that the system can realize the cooling capacity is higher than that of 90W/cm2 in spray cooling by cooling the surface of the single stage, the nonuniformity of temperature increases with the increase of surface heat flux, volume flow on heat transfer is significantly improved, and the best range of spray height. A super high power spray cooling system for the neutron target of a simulated reactor is established by using a circular multi nozzle array. A large flat and thin cavity composed of annular multi nozzle array is used to cool the annular heat source with the inner diameter of 450mm and the outer diameter of 470mm. By optimizing the layout of multi nozzle array and dispersing the point heat source surface, the cooling of the high-power heat source of 80kW is successfully realized under the condition that the heating surface is less than 140 degrees, so that the cooling requirement of the large heat flux neutron target can be satisfied. The research results also show that compact spray cooling system can not only be used for heat dissipation of relatively low power such as electronic components, but also can achieve high power and large area heat dissipation. Based on the research of compact nozzle and compact cavity, a compact spray cooling system for plug-in integrated chassis is established, and the heat transfer characteristics and heat transfer enhancement of the system are systematically studied. In this system, the influence of flow rate, spray angle and surface structure on heat transfer and enhancement characteristics of HFE7100 and PF5060 two kinds of working fluids is studied by experimental method. The experimental results show that the maximum heat flux of PF5060 and HFE7100 as working fluids obtained were 79.1W/cm2 and 81.6W/cm2; traffic significantly affect the heat and temperature imbalance and main flow and flow resistance; spray cooling transient response speed, inclination angle response time; pulse heating, temperature balance rapidly, balance time the effect of surface heat flux; reasonable tilt angle can improve the heat transfer performance of the system, under the experimental conditions, using the 75 degree angle can obtain the best cooling effect. The reason lies in the tilt angle, the utilization rate of working fluid and fluid management of spray cooling can be improved; the reinforcement can also increase the heat transfer surface under the experimental conditions, straight rib surface strengthening rib and rib characteristics are better than the mixed policy surface, the reason is that the surface strengthening can increase the contact area and prolonging contact time, improve the film flow. Finally, in order to analyze the practicability and applicability of the system, a comprehensive evaluation method for spray cooling system is proposed in this paper. In order to further explore the efficient and stable heat transfer enhancement method in compact space, based on the completion of the compact spray cooling system, the effect of high alcohol surfactant on spray cooling heat transfer is studied. This paper selects n-heptanol (1-heptanol) and octanol (1-octanol), 2-ethyl-hexanol (isooctanol) and n-decanol (n-decanol) and other four kinds of high alcohol surfactants, the results show that the surfactant to the spray cooling transient and steady-state heat transfer has made obvious enhancement. In the transient heat transfer, the surface temperature will decrease more obviously in a very short time and achieve higher transient heat flow. In steady state heat transfer, the heat transfer enhancement of the surfactant on the two-phase phase is more obvious. The study also found that the surfactants on spray cooling heat transfer enhancement of the existence of the best concentration, n-heptanol and n-octanol at the concentration of 0.3 per 1000 strengthening effect is the best, the best strengthening concentration varies of octanol and decanol was 0.5 per thousand and 0.1 per thousand. Due to the advantages of high alcohol surfactants, such as less amount, no corrosion, no crystallization and no blockage, compared with the traditional ionic additives, the research results are of great significance for the practical application and heat transfer enhancement of spray cooling. Finally, the effect of high alcohols surfactants on the spray characteristics was studied, and the surface active agent was further analyzed.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TK124;TK172

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