A numerical simulation of the D.C. continuous casting process including nucleate boiling heat transfer

The steady-state thermal problem associated with the direct-chill continuous casting of A6063 aluminum cylindrical ingots is solved using the numerical finite element technique. Excellent correlation is demonstrated between the numerical model and experimental data from ingots cast at two different speeds. By application of the model, effective heat transfer coefficients are calculated as a function of vertical position on the outside surface of the ingot. It is shown that direct application of these coefficients to the modeling of different casting situations will produce substantial errors in the region in which heat transfer is by nucleate boiling. Using theories of nucleate boiling with forced convection and film cooling, a method is developed to calculate the external boundary conditions in the submold region of the ingot, thus making it possible for the first time to define explicitly all of the thermal boundary conditions associated with this casting configuration. These theories are incorporated into the numerical model, and a subsequent simulation shows excellent agreement with experimental data from a third ingot.     

Formation of silicon reentrant cavity heat sinks using anisotropicetching and direct wafer bonding

A novel silicon reentrant cavity heat sink for enhanced liquid cooling of silicon multichip substances has been fabricated using a two-step anisotropic etching process followed by silicon direct wafer bonding. Cavity mouth openings ranging from 8 to 500 μm have been batch fabricated with the two-step process. The reentrant cavities suppress the temperature overshoot normally associated with the transition between the free convection and nucleate boiling regimes of liquid immersion cooling. Nucleate boiling has been observed to occur at heater fluxes below 2 W/cm² for both increasing and decreasing heat flux conditions. Specific thermal contact resistances (heater fluid) of less than 0.6 K-cm²/W have been measured in Freon-22, R-113, and FC-72     

Experimental and numerical study of 3D stacked dies under forced air cooling and water immersion cooling

3D stacked die structure is a promising architecture to realize small feature size and enhance electronic performance. However, thermal performance in 3D stacked die has aroused extensive attention for its high density integration. In this paper, a stacked dummy die structure integrated with polyimide heater inside is presented to investigate the thermal behavior of 3D stacked dies. One-dimensional thermal resistance network is built and calculated to analyze thermal resistance distribution of the stacked dies. Under natural convection, the thermal resistance of convective heat transfer greatly influences total thermal resistance and limits heat dissipation ability of stacked dies. To significantly reduce the thermal resistance of convective heat transfer, forced air cooling and water immersion cooling have been applied in the stacked die structure. Experiment and numerical simulation have been conducted in this work. In the experiment, forced air cooling and water immersion cooling systems are set up to cool down the stacked die structure. The temperature dependence of the stacked die structure is obtained by thermocouples. The measured thermal resistances between junction and ambient environment of the stacked die structure decrease to 7.6 °C/W under forced air cooling and to 0.6 °C/W under water immersion cooling, respectively. Then heat dissipation abilities of forced convection cooling for the stacked die structure are analyzed. Simulation models are built for experimental validation and further thermal analysis. Temperature influences on the internal structure of the stacked dies with different power map are discussed. The simulation results can well capture the experimental results with 5.8% variation under forced air cooling and with 7.4% variation under water immersion cooling when total power of 3 W is applied.     

高功率板条激光介质的纵向强制对流换热技术

液体强制对流换热因具有较高的可靠性和性能稳定性而被广泛使用于高功率板条激光介质介质的制冷,但沿流场方向产生的温度梯度会显著改变激光介质的热应力状态而带来不良影响。提出了基于冷却流场与目标温度匹配控制思路的双大面侧泵激光介质纵向强制对流冷却方案（Longitudinal forced convection）,利用非定常边界条件的流?固耦合有限元仿真方法对比了全腔浸泡对流冷却（Cavity forced convection）、微通道传导冷却技术方案（Micro-channel conduction）,针对入口流量、流场状态、流道壁面条件等因素进行了详细研究。在30 L/min入口流量下,该方案热交换区域固液界面平均对流换热系数达104 W·m?2·K?1量级,且均匀分布。此外,通过改变壁面粗糙程度能够获得更高的对流换热系数。根据设计结果研制了一套板条激光放大器,实验监测点的温度结果与模拟仿真预测结果相吻合,冷却性能达到预期。     

Composite Spreader for Cooling Computer Chip With Non-Uniform Heat Dissipation

The performance of a composite spreader, with a 0.4 mm thick top layer of porous graphite (PG), for enhanced cooling with nucleate boiling of FC-72 dielectric liquid, and a 1.6 mm copper (Cu) substrate, for achieving better cooling of underlying 10 X 10 mm computer chip, with a non-uniform surface heat flux, is investigated. This spreader takes an advantage of the enhanced nucleate boiling heat transfer of FC-72 dielectric liquid on PG and the good heat spreading by Cu. The dissipated thermal power by the chip has a cosine-like distribution with a peak-to-average heat flux, Phi_max, which varied up to 2.467. The spreader surface area, the total thermal power dissipated by the chip, removed from the surface of the spreader, and the total thermal resistance are calculated and compared with those of PG and Cu spreaders of same thickness, 2.0 mm. With Phi_max = 2.467, 39.48 W and 72.0 W can be removed from the surface of composite spreaders cooled with saturation and 30 K subcooled boiling, compared to 43.0 and 65.3 W for Cu spreaders. The calculated surface areas and total thermal resistances of the composite spreaders, 6.82 cm² and 4.90 cm² and 0.284 and 0.68degC/W, are smaller than for Cu spreaders, 12.26 cm² and 11.92 cm², and 0.51 and 0.83degC/W. In addition, the calculated chip maximum surface temperatures of 62.37degC and 72.2degC, are lower than with Cu spreaders (72.67degC and 76.30degC).     

电子管水冷阳极散热能力的设计计算

给出了电子管水冷阳极耗散能力的计算方法和提高水冷阳极耗散能力的措施。指出通过阳极的特殊设计可以把冷却水的对流换热状态过渡到高效率的沸腾换热状态，大大提高了阳极允许功率耗散密度。     

Flow Boiling in a Micro-Channel Coated With Carbon Nanotubes

This study examines the heat transfer enhancement attributes of carbon nanotubes (CNTs) applied to the bottom wall of a shallow rectangular micro-channel. Using deionized water as working fluid, experiments were performed with both a bare copper bottom wall and a CNT-coated copper wall. Boiling curves were generated for both walls, aided by high-speed video analysis of interfacial features. CNT arrays promoted earlier, abundant and intense bubble nucleation at low mass velocities, consistent with findings from previous pool boiling studies. However, high mass velocities compromised or eliminated altogether any enhancement in the nucleate boiling region. The enhancement achieved at low mass velocities appears to be the result of deep, near-zero-angle cavities formed by the mesh of CNT arrays. On the other hand, high mass velocities tend to fold the CNTs upon the wall, greatly reducing the depth of the CNT-mesh-induced cavities, and compromising the effectiveness of CNTs at capturing embryos and sustaining the bubble nucleation process. CHF enhancement was also achieved mostly at low mass velocities. It is postulated CNT arrays enhance CHF by increasing the heat transfer area as well as by serving as very high conductivity fins that penetrate into the cooler, bulk liquid flow and take advantage of the liquid subcooling away from the wall. While these mechanisms are prevalent at low velocities, they are both weakened, especially the fin effect, at high mass velocities because of the folding of CNT arrays upon the wall.     

Ebullition characteristics of an isolated surface microstructure for immersion cooled heat sinks

An understanding of the fundamentals of the boiling mechanism is essential if phase-change liquid immersion cooling is to emerge as a cooling option for the next generation of data servers and other high performance electronics. The present work is an experimental study on the effect of cavity mouth size on the nucleation characteristics of a single isolated micro-pyramidal reentrant cavity. Cavity mouth size has long been known to be a primary variable determining the ebullition characteristics of microscopic structures present on a pool boiling heat sink surface. Isolated pyramidal shaped cavities with square mouth sizes of 7, 19, and 25/spl mu/m etched in polished silicon using an anisotropic etch were evaluated in this study. Serpentine thin film heaters (6.9mm /spl times/ 6.9mm) deposited on a Borofloat glass substrate and anodically bonded to the silicon cavity section served as heat sources. All experiments were conducted at atmospheric pressure in dielectric fluids, HFE-7100 and FC-72, popular in thermal management applications. High speed photography (up to 600frames/s) was used to record and quantify the effect of heat flux on bubble departure frequency and departure diameter under subcooled and saturated conditions. The bubble departure diameter increased with an increase in the cavity mouth size. Frequency and bubble departure size, both decreased with increased subcooling.     

高功率LED热管理方法研究最新进展

LED芯片结温的高低直接影响其出光效率、工作寿命和可靠性。在分析系统各个环节热阻的基础上,详细评述了高功率LED产品从芯片到系统级的热管理研究新动向,包括:自然对流冷却,采用压电风扇、电离方法所进行的强迫空气对流冷却,采用水、液态金属作为冷却工质的液冷方法,采用热管实现的相变冷却,采用热电片进行的固态冷却方案以及利用热电片对余热进行回收利用的热管热回收方案和液体金属冷却方法。并在上述基础上提出了发展更高功率密度LED热管理方法的关键科学问题。     

Investigation of Artificial Forced Cooling in the Bridgman Crystal Growth of Cadmium Zinc Telluride

The effects of artificial forced cooling on the solid–liquid interface and on solute segregation were investigated by modeling the vertical Bridgman method for the single-crystal growth of CdZnTe, taking into consideration effects such as increasing the axial outward heat flux from the crucible bottom, the radial outward heat flux from the crucible wall, and the carbon film thickness on the crucible inner wall. Axial artificially forced cooling noticeably increases convection and the temperature gradient in the melt next to the solid–liquid interface, and substantially reduces interface concavity at the initial solidification stage. Interface concavity increases a little when the solidification proceeds further, however. Axial artificially forced cooling reduces radial solute segregation of the initial segment of the grown crystal and slightly increases the solute iso-concentration segment. Radial artificially forced cooling enhances melt convection substantially, affects solid–liquid interface concavity only slightly, and hardly affects solute segregation in the grown crystal. Doubling the carbon film thickness weakens convection of the melt in front of the interface, substantially increases interface concavity, and hardly affects solute segregation in the grown crystal.     

Spatial and Temporal Resolution of Conjugate Conduction-Convection Thermal Resistance

A transient, 3-D solution to the heat conduction equation with a small square heat source on an adiabatic surface and Newtonian convection on the opposite side was obtained using Green's functions. The geometry conservatively models conduction spreading resistance encountered by small, concentrated heat sources such as light-emitting diodes and integrated circuits in general, mounted to larger substrates such as the base of a heat sink experiencing Newtonian convection. The solution is presented for a range of nondimensional parameters. Superposition techniques can also be used to extend the applicability of the current solution to the temperature prediction of arbitrary heat flux patterns in certain cases. This technique only holds for applications where the heat transfer coefficient is not a function of temperature, such as thermal management strategies designed to rely on forced convection with air.     

Nature‐Inspired Boiling Enhancement by Novel Nanostructured Macroporous Surfaces

World energy crisis has triggered more attention to energy saving and energy conversion systems. Enhanced surfaces for boiling are among the applications of great interest since they can improve the energy efficiency of heat pumping equipment (i.e., air conditioners, heat pumps, refrigeration machines). Methods that are used to make the state‐of‐the‐art enhanced surfaces are often based on complicated mechanical machine tools, are quite material‐consuming and give limited enhancement of the boiling heat transfer. Here, we present a new approach to fabricate enhanced surfaces by using a simple electrodeposition method with in‐situ grown dynamic gas bubble templates. As a result, a well‐ordered 3D macro‐porous metallic surface layer with nanostructured porosity is obtained. Since the structure is built based on the dynamic bubbles, it is perfect for the bubble generation applications such as nucleate boiling. At heat flux of 1 W cm^‐2, the heat transfer coefficient is enhanced over 17 times compared to a plain reference surface. It's estimated that such an effective boiling surface would improve the energy efficiency of many heat pumping machines with 10–30%. The extraordinary boiling performance is explained based on the structure characteristics.     

Flow Boiling Heat Transfer to a Dielectric Coolant in a Microchannel Heat Sink

This paper presents an experimental study of flow boiling heat transfer in a microchannel heat sink. The dielectric fluid Fluorinert FC-77 is used as the boiling liquid after it is fully degassed. The experiments were performed at three flow rates ranging from 30-50ml/min. The heat transfer coefficients, as well as the critical heat flux (CHF), were found to increase with flow rate. Wall temperature measurements at three locations (near the inlet, near the exit, and in the middle of heat sink) reveal that wall dryout first occurs near the exit of the microchannels. The ratio of heat transfer rate under CHF conditions to the limiting evaporation rate was found to decrease with increasing flow rate, asymptotically approaching unity. Predictions from a number of correlations for nucleate boiling heat transfer in the literature are compared against the experimental results to identify those that provide a good match. The results of this work provide guidelines for the thermal design of microchannel heat sinks in two-phase flow     

表面加工多晶硅薄膜热扩散系数的在线提取

提出了一种在线提取多晶硅薄膜热扩散系数的简单测量方法,测试结构可随表面器件工艺加工制作,无需附加工艺。通过分析两个长度相同、宽度与厚度相同梁的瞬态冷却特性,获得其冷却时的温度衰减时间常数,便可以提取出表面加工多晶硅薄膜的热扩散系数。给出了瞬态冷却热电分析模型,综合考虑了梁冷却过程中各种散热因素即对流、辐射以及向衬底传热的影响。实验测得的该表面加工多晶硅薄膜热扩散系数是0.165 cm2-s 1(方块电阻是116.25Ω/sq)。该方法能够实现多晶硅薄膜热扩散系数的在线测试。     

Nonuniform temperature distribution in electronic devices cooled byflow in parallel microchannels

We fabricated a novel thermal microsystem (simulating a computer chip) consisting of a heater, microchannels, inlet and outlet plena and we studied the effect of the geometry on the flow and heat transfer. The vapor-water two-phase flow patterns were observed in the parallel microchannels through a microscope and high-speed video camera. It was observed that hydraulic instabilities occur. Existence of a periodic annular flow was also observed, which consists of a symmetrically distributed liquid ring surrounding the vapor core. Along the microchannel axis, the periodic dry zone appears and develops. The thermal visualization and temperature measurements of the heated device were carried out using infrared thermography. As long as the flow was single phase liquid, the forced convection heat transfer resulted in a moderate irregularity on the heated chip. These temperature differences do not cause damage to a real electronic device. The steady-state heat transfer for different types of microchannels has been studied also at the range of heat flux where phase change of the working fluid from liquid to vapor took place. Under conditions of flow boiling in microchannels, a significant enhancement of heat transfer was established. In the case of uniform heat flux the hydraulic instabilities lead to irregularity of temperature distribution on the heated chip. In the case of nonuniform heat flux the irregularity increased drastically     

封装有相变材料的热沉结构对电子器件高温保护的传热分析

电子器件在工作过程中有可能由于突增电流而产生大量的热量,利用固液相变潜热存储系统,可以使得电子器件在一定的时间内保持相对平稳的温度,从而达到高温保护的目的。针对一种封装有相变材料的热沉结构在自然对流冷却条件下的传热特性进行了理论分析,得到了热沉、相变材料和空气的平均温度及相变材料的相变过程,并对比分析了同一热沉结构和自然对流冷却条件下,在不同热流密度下高温保护时间以及对流换热特性。     

电子设备散热技术研究

随着微电子技术的发展,使得电子器件的热流密度不断增加,这样势必对电子器件有更高的散热要求,因此有效地解决散热问题已成为电子设备必须解决的关键技术.针对现代电子设备所面临的散热问题,就自然对流散、强制风冷散热、液体冷却、热管、微槽道冷却、集成热路、热电致冷等常用的电子设备散热技术及某些前沿的研究现状、发展趋势及存在问题分别予以阐述,希望对同行能有所帮助.     

Smart pumpless loop for micro-channel electronic cooling using flat and enhanced surfaces

Two-phase cooling of a square simulated electronic device surface of 21.3 mm side was successfully carried out without the need for a pump. This smart, passive cooling system incorporates a self-enhancing and self-sustaining mechanism, wherein the system inherently enhances its cooling capacity by increasing the velocity of the two-phase mixture along the boiling surface when an increase in heat flux is sensed. Other practical attributes of this pumpless loop are small liquid inventory requirements and absence of the incipient boiling temperature drop. It is shown small surface tension and contact angle render dielectric coolants such as FC-72 ideally suited for flow in narrow gaps. These unique properties are responsible for very small bubble size, precluding any appreciable blockage of the replenishment liquid flow even in narrow gaps. Critical heat flux (CHF) was found to generally increase with decreasing boiler gap. CHF for flat, micro-channel (with 0.2 mm rectangular fins) and mini-channel (with 1.98 mm rectangular fins) surfaces was 4.5, 5.9, and 5.7 times greater than for pool boiling from a flat surface for corresponding gaps. A pressure drop model was formulated to predict coolant mass flow rate, boiling surface inlet and exit velocities, and pressure drop components throughout the loop. The model predictions illustrate the pumpless loop's self-sustaining and self-enhancing attributes, and relate CHF trends to those of the two-phase mixture acceleration along the boiling surface.     

A high-performance thermal module for computer packaging

A thermal module was designed to transfer heat efficiently from high power dissipation chips to a liquid coolant via forced convection. Turbulent and laminar flow regimes were investigated. Channel geometries for deep channels (1000 μm deep, and used for turbulent flow), and shallow channels (100 μm deep, and used for laminar flow) were optimized for high heat transfer coefficient, ease of fabrication, and better structural rigidity of the module. A 4″ x 4″ module, made out of Cu, was tested using a 4″ Si “thermal” wafer as a heat generating source as well as a temperature sensor. Wafer scale integration and high energy ion implantation were employed to obtain nine l x l cm heat sources, and temperature sensing diodes embedded within the thermal wafer. For the deep channel design, the maximum device temperature rise on the module was 18° C for a power dissipation of 42 W/chip, and a flow rate of 126 cc/sec. For the shallow channel design, the temperature rise was 19° C for a flow rate of 19 cc/sec, and a power dissipation level of 42 W/chip. With all nine chips on the thermal module powered to 42 W/chip, the maximum chip to chip temperature variations were found to be 2 and 8° C for deep and shallow channel designs, respectively.