【作者】 宗露香；

【导师】 徐进良；

【作者基本信息】 华北电力大学 ， 可再生能源与清洁能源， 2014， 博士

【摘要】 随着集功能型与紧凑型为一体的微电子设备不断发展,微电子器件冷却技术发展和应用已成为设备维持其优良功能性及稳定性的主要制约问题。延续至今,微通道沸腾换热成为高热流密度芯片冷却热管理问题极具潜力的选择。本研究以丙酮为工质,对微通道内流动沸腾不稳定进行实验研究,对微通道内种子汽泡触发沸腾和抑制流动沸腾不稳定进行了模型构建及数值模拟,以期对微尺度沸腾换热领域进一步研究提供理论性指导。主要创新性工作如下：微通道沸腾不稳定性影响因素实验研究。以硅基并联三角形微通道为实验段,在热沉背面设计并加工三种长度(L1=4.5mm,L2=6.25mm,L3=8.0mm)加热膜。研究不同加热膜长度(L)和不同质量流量(G)条件下,微通道进出口压降(△P)和热沉背面加热膜中心点平均温度(TWC)随热流密度(q)变化规律。发现压降和加热膜温度随热流密度变化曲线包含单相液体对流传热区域和汽液两相流动沸腾不稳定区域,随热流密度增加,单相区域压降略微降低,加热膜温度线性升高；两相区域压降急剧增大,加热膜温度指数式上升；研究热流密度,质量流率和加热膜长度对压降,加热膜温度,温度标准偏差(Stdev),及传热系数(hwc)影响,发现高热流密度/低质量流率/长尺寸加热膜条件下,沸腾不稳定更容易被触发,系统压降增大,加热膜温度波动周期及振幅增大,标准温度偏差下降,传热系数波动周期及振幅增大,传热恶化。种子汽泡抑制微通道沸腾不稳定性实验研究。在每条微通道入口集成Pt薄膜微加热元,作为触发微汽泡(充当微通道中核化种源,称为“种子汽泡”)的发生器。研究不同触发频率(f)种子汽泡对沸腾不稳定性现象,发现随着触发频率增大,压降和传热系数不稳定波动逐渐消失,触发频率为1000Hz时,传热基本达到最佳,意味着对应质量流率下饱和触发频率fsat=1000Hz,分析本课题所设计工况发现相同Bo数范围内具有相同的饱和触发频率；研究不同触发频率下压降和加热膜温度随热流密度变化规律。发现种子汽泡在单相液体单相液体区域不起作用,两相区域,随着触发频率增加,压降和加热膜温度随热流密度变化曲线曲率下降,流动沸腾不稳定逐步得到抑制；研究种子汽泡触发频率抑制流动沸腾不稳定性效果。发现随着触发频率增加,压降和加热膜温度随时间不稳定波动周期和振幅逐渐减低,温度标准偏差增大,传热系数波动逐渐消失,换热得到强化。触发高频种子汽泡后,微通道内代表性流型由单相和环/雾状流周期性交替转换为稳定弹状流。种子汽泡强化微圆管蒸发传热数值模拟。在不考虑微圆管内自核化前提下,发展了微圆管内种子汽泡强化蒸发传热汽-液两相模型,汽-液界面蒸发蒸发过程采用本课题组开发的一种精确简单相变模型结合VOF(Volume of Fluid)方法进行描述,采用动态自适应网格技术结合预先固定分区并行计算方法精确捕捉汽泡行为；研究不同种子汽泡触发频率(f)下,微圆管壁面过热度(△Tw,x)和主流过热度(△Tb,x)随时间演化规律,以及流型和传热两者之间关系。发现了触发频率是影响传热关键因素,低频下,种子汽泡作用有限,壁面和主流过热度随时间呈明显的毫秒级(-ms)波动,高频下,微圆管内代表性流型为准稳态拉长汽泡流,壁面和主流过热度在时空分布上呈现较好温度均匀性。该模型还验证了实验中触发频率存在饱和频率的结论,发现了高频种子汽泡触发下微圆管内高效稳定传热机制由薄液膜蒸发主导,成功揭示了有限实验手段所不能揭示的种子汽泡强化微圆管传热微观(-ms)机理。种子汽泡抑制微圆管沸腾不稳定数值模拟。考虑微圆管内自核化效应,基于本课题组相关实验结论及一系列简化和假设,建立了中等Bo数下微圆管内关联核化准则(△TW)、核化频率(fn)、核化密度(Nn),以及种子汽泡触发频率(f)的种子汽泡抑制汽-液两相自沸腾不稳定模型；研究不同质量流率下,自沸腾时壁面过热度(△Tw,x)、主流过热度(△TB,x)、压降(△p)、传热系强化倍率((Nux-Nufull)/Nufull)随时间的演化规律。发现壁面和主流过热度,压降、传热系强化倍率随时间呈明显的同步周期性波动,且随着质量流率增大,核化频率增大,核化汽泡起始点越接近微圆管入口；研究不同质量流率条件下,不同触发频率种子汽泡对微圆管内流动及传热的影响。发现高频种子汽泡触发下,壁面和主流过热度,压降、传热系强化倍率大幅波动消失,时空分布呈现良好的均匀性,换热得到强化。研究还发现相同Bo数条件下,随着质量流率增大,饱和触发频率fsat不变；相同饱和触发频率下,随着质量流率增大,壁面残余过热度△Tw,resudial和主流过热度△Tb,resudial增大,但后者达到维持稳定传热所需最小过热度(Tb,x-Tsat)high f seed bubbles后变化并不明显。更多还原

【Abstract】 With the continuing increase in the functionally and compactness of microelectronics, the demand for high heat flux dissipation from the high performance microprocessors increases. So the development and application of the cooling technique becomes the primary task to maintain the excellent performance and stability of the microelectronic device. Recently, microscale two-phase boiling heat transfer provides an alternative and potential way for thermal management of high-density commercial and defense electronic devices. To provide a guide to design and optimize the two-phase boiling heat transfer in microchannels, in this thesis, the acetone is used as the working fluid, and the experimental study is performed to study the mechanism of boiling flow instability in microchannels. At the same time, the seed bubble-triggered evaporation heat transfer and the seed bubble-depressed boiling instability models are developed and numerically studied. The main contents are as follows:Effect of running parameters on flow boiling instability in microchannels. The parallel triangle silicon microchannel heat sink is designed as the experiemntal section, and three different lengths (L1=4.5mm, L2=6.25mm, L3=8.0mm) for the main heater on the back of the heat sink are used. The demand curves of pressure drops (ΔP) and temperatures at the central point of the main heater (TWC) versus heat fluxes are examined for different mass fluxes and heater lengths. It is found that, both the single-phase liquid region and two-phase flow region are included in the plots. And in the single-phase liquid flow, the pressure drops slightly decrease and the main heater temperatures linearly increase with the heat flux increasing. And in the two-phase flow region, the pressure drops rapidly increase and the main heater temperatures exponentially increase with the heat flux increasing. The effects of heat fluxes(q), heat lengths(L), and mass fluxes (G) on pressure drops, main heater temperatures, the standard temperature deviations (Stdev), and the heat transfer coefficients (hWC) are experimently studied. The results show that, for the higher heat flux/lower mass flux/longer main heater length, the boiling instability is earlily triggered, the standard temperature deviations are decreased, the pressure drops are increased, also the cycle lengths and oscillation amplitudes of main heater temperatures and heat transfer coefficients are increased.Experimental study of seed bubble-depressed flow boiling instability in microchannels. Seed bubbles can be considered as a type of intelligent and controllable’cavity’bubbles to control the flow boiling instability, which were producted on a set of microheaters upstream of microchannels driven by the pulse voltage signal. The effect of seed bubble frequencies (f) on boiling flow instability is experimentally studied. It is found that, the oscillation amplitudes of the pressure drops and main heater temperatures become smaller with the seed bubble frequency increasing. And the heat transfer enhancement attains the maximum degree for f=1000Hz, inferring that a saturation frequency of1000Hz for the corresponding mass flux. It is noted that for the middle boiling number of1.5×1e-03-3.21e-03there is only one saturation frequency. The demand curves of pressure drops (ΔP) and temperatures at the central point of the main heater (TWC) versus heat fluxes are investigated under different seed bubble triggering frequencies. It is found that, the seed bubble has no effect for the flow and heat transfer in the single-phase liquid flow. In the two-phase flow region, the slope of the pressure drops and the main heater temperatures decreases with the heat flux increasing, inferring to the progressive control of the boiling flow instabilities. The effects of seed bubble frequencies(f), on pressure drops, main heater temperatures, the standard temperature deviations, and the heat transfer coefficients are experimently studied. The results show that, with the seed bubble frequency increasing, the cycle lengths and oscillation amplitudes of pressure drops and main heater temperatures and heat transfer coefficient are decreased, the standard temperature deviations is increased. Naturally the heat transfer enhancement is induced. With the assistant of high frequency seed bubbles, the typical flow patterns in microchannesl can be converted from the alternating single-phase liquid flow and annular/mist flow to the quasi-stable slug flow.Numerical study of seed bubble-enhanced evaporation heat transfer in microchannels. Selfboiling in microchannels was ignored in this chapter, The seed bubble-triggered evaporation heat transfer model is in microchannels developed. A simple but accurate phase change model for vapor-liquid two phase flow proposed by our research team is used to describe the evaporation mass flux on the vapor-liquid interface. To reduce time consumption and avoid data transmission error at the allocation interface, the fixed grid allocation technique is proposed to successfully perform the parallel computation via a set of computer core solvers. The wall superehats (ΔTw,x) and bulk superheats(ΔTb,x) versus time under different triggering frequencies, and the relationship between the flow pattens and heat transfer are explored. It is found that the seed bubble frequency is a key parameter to influence the heat transfer performance. Low-frequency seed bubbles cause apparent spatial-time oscillations of wall and bulk superheats. High-frequency seed bubbles result in quasi-stable elongated bubbe flow, corresponding to quasi-uniform and stable wall and fluid superheats. And the evaporation of thin liquid film between the elongated bubble and the wall is responsible for the heat transfer enhancement. In addition, the experimental evidence that the saturation seed bubble beyond which no further performance improvement can be made is also verified by the numerical model, and the seed bubble-triggering evaporation heat transfer mechanism in ms-level, which is difficult to capture by the limitation of measurement technique, is successfully revealed by the present numerical study.Numerical study of seed bubble-depressed flow boiling instability in microchannels. Selfboiling in microchannels is considered in this chapter. Based on the experimental conclusions by our research team and a series of assumptions, the seed bubble-depressed flow boiling instability vapor-liquid two phase model for middle boiling number is developed. The relevant parameters for the model consist of the nucleation criterion(ΔTw), nucleation-frequency (fn), nucleation density (Nn), and seed bubble triggering frequency(/). The wall superheats (ΔTW,x), bulk superheats(ΔTb,x), pressure drops(AP) and enhanced heat transfer coefficient((Nux-Nufull)/Nufull) versus time for different mass fluxes (G) are examined. It is found that the wall superheats, bulk superheats, pressure drops and enhanced heat transfer coefficient varies synchronously with apparent oscillation amplitudes and cycle lengths. With the mass flux increasing, the nucleation frequency is increased and the first nucleate point is more close to the microtube entrance. And the effect of seed bubble frequencies on the flow boiling instability and heat transfer enhancement under different mass fluxes is also studied. It is found that, the intense oscillation of the wall superheats, bulk superheats, pressure drops and enhanced heat transfer coefficient is successfully depressed, corresponding to a nice quasi-stable spatial-time distribution. However, the saturation seed bubble frequency is not changed with the mass flux increasing. And for the same saturation frequency, the residual wall and bulk superheats increase with mass flux increasing, when the latter reaches the required minimum temperature difference to maintain the evaporation heat transfer, the ingored change will be induced.更多还原