【作者】 王亚青；

【导师】 夏维东； 刘明侯；

【作者基本信息】 中国科学技术大学 ， 热能工程， 2010， 博士

【摘要】 未来电子设备体积小、集成度高和功率密度高的发展趋势已使散热问题成为影响其发展的重要瓶颈。作为先进散热技术之一的喷雾冷却有着散热功率高、冷却均匀、无沸腾滞后效应、介质需求量小等特点,备受研究者关注。然而现有的研究,因实验条件的差异所得结论亦有所不同。对喷雾冷却的认识相对缺乏,且关注热点主要集中沸腾区,无沸腾区换热很少涉及。鉴于此,本文重点对喷雾冷却中无沸腾区的换热特性进行理论以及实验研究。以水为冷却介质,采用半实心旋流式机械雾化喷嘴、实心机械雾化的喷嘴,对影响无沸腾换热特性的因素、强化换热方法等进行了较为系统的研究。实验方面主要研究工作：1、喷嘴雾化特性：运用相位多普勒粒子分析仪(PDPA)对实心、半实心喷嘴雾化特性进行测试分析。结果表明：液滴径向、轴向速度、粒径分布不均匀,半实心喷嘴非均匀性更为明显；半实心喷嘴雾化中心区有一旋流导致的回流区,对应于换热面有一明显的滞止区。回流区内液滴颗粒数密度小,速度、粒径均较边界处小。2、研究喷嘴雾化特性、壁面粗糙度对换热特性的影响。结果表明,液滴速度、粒径、数量通量、介质质量通量共同影响着无沸腾区换热。结合理论分析与实验结果,研究发现：1)液滴速度、数量通量对换热有着积极的影响,因其增加会增加液膜扰动和运动速度；2)壁面温度对换热特性有着积极的影响,壁面温度越高液膜蒸发越快,换热增强；3)由于滞止区的存在,壁面温度分布不均匀,在平均温度为90℃时,局部区出现沸腾；4)粗糙度越大,换热性能越强。此外,本文对雾化特性(液滴粒径、速度、质量通量)影响进行了较为系统的分析,提出质量通量为决定无沸腾区换热特性的最主要因素。3、系统地研究了倾斜喷射时,喷嘴倾斜角度、喷射高度对喷雾换热特性的影响。通过对半实心旋流式机械雾化喷嘴倾斜喷射散热特性的研究,发现：1)当倾斜喷射底面椭圆长轴与换热面相切时换热性能最佳,此时的喷嘴高度为最佳喷射高度；其次为底面长轴与换热面内接和外接。2)倾斜角度在0-49°范围内变化时,倾斜角度越大对滞止区冲刷作用越强,换热增加。结合不同高度喷嘴雾化特性,对上述结论进行理论分析,给出倾斜喷射的角度限制条件和最佳高度计算关系式。4、对刻有微槽道冷却面的换热特性进行了系统的理论和实验分析。采用实心机械雾化喷嘴,质量通量为44—53Kg／m2s时,研究结果表明刻有微槽道结构的散热面可明显增强换热性能。对于槽道高度较小(0.1mm)的工况,面积增加比例超过了热流增加比例。槽道高度较大时(0.2mm、0.4mm),热流增加比例远小于面积增加比例。结合喷嘴雾化特性和理论分析发现当槽道高度较小时,其不仅增加了换热面积,更有利于液膜的扩散,减小液膜厚度,换热系数增加；本文的实验工况下,高度为0.2mm微结构槽道换热最佳,其次为0.1mm和0.4mm微槽。5、研究了喷雾冷却中无沸腾区换热关系式。对本文的实验结果进行无量纲化处理,发现影响无沸腾区换热的主要因素为包括反映质量通量的Re数,表征粒径、液滴速度综合效应的Weber数及本文提出的与壁面温度相关的无量纲温度ξ。对实心喷嘴高流量(240<Re<527)、半实心喷嘴小流量(116.2<Re<289.2),给出了无量纲关系式。与文献中已有的关系式比较,发现本文经验关系式能更好反映无沸腾区换热的实际状况,误差更小,运用范围更广。理论研究方面：重点考察了沸腾区中“二次成核”对换热特性的影响。基于喷雾冷却换热原理和气泡-液滴动力学基本知识,对有气泡产生的换热表面进行数值研究,研究成核范围系数β成核系数φ变化对换热的影响,提出了反映换热特性的最佳成核范围系数β和成核系数φ。研究结果表明,随着成核系数φ增大,换热性能变好。当φ>6时,换热性能随着成核系数的增强不明显；成核范围系数β=8为极值位置,增大或者减小都使得换热性能减弱。通过φ=6,β=3,5,8,10的各种情况下的换热量计算,并与Cho,Ponzel提出的经验公式所计算结果进行比较,发现在β=8,10的时,本文计算结果与经验公式结果符合很好,由此可以得出φ,β的最佳值分别为6,8.更多还原

【Abstract】 Developmental Trend of future electronic systems is smaller size, higher integration value, high heat flux and how to remove the so amount of heat flux is a great challenge for this domain. Spray cooling is an advanced control cooling technology increasing interest for electronic cooling and other high flux heat removal technique, which is characterized by small fluid inventory, high heat transfer coefficient and no hysteresis of boiling.However, the current studies which contradict each other because of the experimental condition mainly focused on boiling regime and the conclusion was only apporiate for high temperature. The non-boiling regime’s spray cooling performances, as an important part of the spray cooling process, has been investigated by very few researchers.In view of this, the primary purpose of this paper is to develop comprehensive design tools vital to the implementation of spray cooling heat transfer performance using semi-solid swirl nozzles and solid pressure-atomizing nozzles with a large ranged mass flux.The main experimental studies:1. Spray characteristics:The spray parameters, mainly the droplet diameter, velocity and the spray pattern, were measured by a Dantec PDPA (Phase Doppler Particle Analyzer).It showed that distribution of the streamwise mean velocity and Sauter Mean Diameter (SMD) of droplet were not uniform and this tendencies were more obvious to semi-solid swir pressure-atomizing nozzles.There was a stagnation zone at the center of the heated surface which may be attributed to the effect of the recirculating zone resulting from the strong rotational flow. In the recirculating zone the SMD and mean velocity are minimum, while at outer side of the spray zone, they are comparatively large.2. Studying the spray characterics, surface roughness influencing the heat transfer performance of the spray cooling in the non-boiling regime.The results showed that droplet velocity, droplet diameter, droplet number flux and mass flux all affected the heat transfer. Combining theoretical analysis with experimental results, we concluded that:1) heat flux was increased with the increasing of droplet velocity and droplet number flux; 2) Flim evaporation is very important to heat transfer in non-boiling regime of spray cooling. As test surface temperature increased, film evaporation increased as well and heat transfer performance enhanced; 3) The surface temperature unevenly distributed in the non-boiling regimen using semi-soild swir nozzle.4) Compared with smooth wall, the rough wall has better heat transfer performance and cooling efficiency in non-boiling region.Besides those, the effect of spray characteristics on heat transfer was studied thoroughly and the conclusion that mass flux was the main factor to affect the heat transfer was proposed.3. Experiments were performed to study the effects of spray inclination angle (the angle between the normal of the square test surface and the axis of symmetry of the spray), the mass flux as well as the surface temperature on the heat transfer performance in non-boiling regime by using water sprays. Experiments revealed that there is an optimal orifice-to-surface distance where heat transfer performance is best. And this occurred when the major axis of the elliptical spray impact area is just intersecting the square test surface at inclined sprays. Knowing the inclination angle and spray cone angle as well as the test size, the optimum orifice-to-surface distance can be easily determined. Too small orifice-to-surface distances would result in only a small fraction of the test surface impacted by the spray, while too large a distance causes a substantial fraction of the spray liquid falling wastefully outside the test surface. Both extremes made heat transfer performance decrease. The heat transfer performance and the cooling efficiencies increased with increasing inclination angle from 0°to 49°. Despite the increased heat transfer, it takes longer time at larger inclination angles for the test surface to reach a steady state between power increments.4. Experiments were conducted to study the effects of enhanced surfaces on heat transfer performance during water spray cooling in non-boiling regime. The surface enhancement is straight fins. The structures were machined on the top surface of heated copper blocks with a cross-sectional area of 10 mm×10 mm. The spray was performed using solid pressure-atomizing nozzles with a mass flux of 44-53 Kg/ (m2·s). It is found that the heat transfer is obviously enhanced for straight fin surfaces relative to the flat surface. However, the increment decreases as fin height increases. For flat surface and enhanced surfaces with a fin height of 0.1 mm and 0.2 mm, as mass flux increases, heat flux increases as well. However, for finned surface with a height of 0.4 mm, heat flux is not sensitive to coolant mass flux. Changed film thickness and the form of water/surface interaction due to enhanced surface structure (different fin height) are the main reasons for changing of local heat transfer coefficient. Straight fin surface not only increased the area of heat transfer, but also provided a driving force for liquid spreading and enhanced heat transfer. The optimum heat transfer performance in the experiments is the enhanced surface with fins in 0.2mm height, and the surface with fins in 0.1mm and 0.4mm height next.5. Studying the heat flux corrlection in the non-boiling regime.The former datas was normalized in terms of non-dimensional groups.It showed that the main dimensional number influencing the heat transfer were Re Number,Weber Number and non-dimensional temperatureξ. Furthermore, Generalized correlations were developed for local Nusselt number as a function of the spray Reynolds number (116.2<Re<289.2, semi-solid swirl nozzles; 240<Re<527, solid pressure-atomizing nozzles) and the non-dimensional temperatureξin non-boiling regime of spray cooling.Compared to the former, the corrlection refered in the paper has smaller error and a wide usage range.The main theoretical studiesBased on the fundamental principle of spray cooling and bubble-droplet dynamics, a numerical method was developed to study the heat transfer characteristics of heated surface which have the bubble generated with a focus on "secondary nucleation" and the influence of "secondary nuclei range" coefficient(β)and secondary nucleation coefficient (φ) in the whole spray cooling. The results indicated that increasing the secondary nucleation (φ) could result in the heat flux increased, whenφis 6, the heat flux in not increasing obviously. The point whichβis 8 is extreme point, increasing and reducing theβcould weaken the effect of heat transfer. In the case ofφ=6,β=3,5,8,10,comparing the calculating results to the results computed from the correlation provided by Cho,Ponzel,we conclude that the simulation fits the correlation well whenβis 8 and 10.So the primeφ,βare 6 and 8.更多还原