Forced Convection Phase-Change Heat Transfer with StreamwiseCurvature and Two-Phase Approach Flow

本项目主要是从小区域强制对流沸腾换热到槽道流动的实验研究。结果的应用在于必须冷却小的、高热流密度区域的设备的设计：用于光刻和医学的X射线源、微波速调管、激光、微电子和光学设备、燃气轮机、聚变反应堆仪器和第一壁、高速飞机前表面和材料加工中的淬火。理论或科学价值在于描述热边界层和气泡边界层厚度相对于通道尺寸较小时的沸腾行为。需要记录的影响包括：加热长度、体积速度、过冷度、压力、流体类型、加热壁的流向曲率和接近加热面的流动的空泡率。该计划首先完成了一项正在进行的关于壁凹曲率对过冷流动沸腾特性的影响的研究。在这项研究中，所有的空洞都被限制在离加热表面非常近的地方。下一个阶段也是研究的主要部分是靠近加热区的流动是处于热力学平衡的汽液两相混合物。在这项研究中，将研究接近流含气率和两相流流型在直线通道和凹形弯曲通道内流动的影响。在弯曲流动中形成的横流压力梯度，在凹面曲率下，空穴被迅速地输送到加热壁面之外。目前正在研究这种效应，用于接近的流动是过冷液体的情况。延续项目的特点是，当入口流动是两相混合物时，将会看到更大的好处，因为在直流和曲流之间，从核态沸腾(DNB)出发的点的热流将大幅增加。一旦进行了表征和描述，就可以在设计上述工程系统的冷却方案时加以利用。

This project is predominantly an experimental study of forced convection boiling heat transfer from small regions to a channel flow. Application of the results lie in the design of equipment where small, high-heat-flux regions must be cooled: x-ray sources for lithography and medicine, microwave klystrons, lasers, microelectronic and optical devices, gas turbines, fusion reactor instrumentation and first walls, high-speed aircraft leading surfaces and quenching in materials processing. Theoretical or scientific value lies in the description of the boiling behavior in situations where the thermal boundary layer and bubble boundary layer thicknesses are small relative to the channel size. Effects to be documented are: heating length, bulk velocity, subcooling, pressure, fluid type, streamwise curvature of the heated wall and the void fraction of the flow approaching the heated surface. The program begins by completing an ongoing study on the effect of wall concave curvature on subcooled flow boiling characteristics. In this study, all the voids are constrained to very near the heated surface. The next phase and the main portion of the study is one where the flow approaching the heated region is a two-phase mixture of vapor and liquid in thermodynamic equilibrium. In this study, the effect of approach flow void fraction and two-phase flow regime will be investigated in flow within straight channels and within concave-curved channels. With concave curvature, voids are rapidly transported away from the heated wall by the cross-stream pressure gradient established in the curved flow. This effect is presently being investigated for a situation where the approaching flow is a subcooled liquid. The continuation project will characterize the stronger benefit that will be seen when the entry flow is a two-phase mixture in that there will be a substantial increase, between straight flow and curved flow, of the heat flux at the point of departure from nucleate boiling (DNB). Once characterized and described, this can be capitalized upon in the design of cooling schemes for the above engineering systems.