Presidential Young Investigators Award: Experimental Investigations of Convective Heat Transfer in Complex Internal Flows

本研究的重点是复杂内部流动中对流换热的实验研究。特别强调的是电子产品的强制对流冷却，其中由相邻的印刷电路板形成的内部通道促进了高度扰动的流动。复杂的内部换热的其他例子出现在气冷式换热器中，其中管阵列形成与先进燃气轮机叶片的分离流和内部冷却相互作用的通流区域，其中传热强化装置，如短销IN或周期性肋片湍流被浇铸到壁面中。所有这些内部流动都表现出难以预测的现象学特征，包括：(1)分离和再附着流动的区域，(2)包含二次运动的区域，以及(3)平均流动中的高水平湍流。本研究集中于三个典型几何的实验研究，这三个正则几何共同展示了所有这些特征。这些几何形状是：(1)二维后向台阶，或表面栅栏，(2)具有重复肋粗糙度的二维通道，以及(3)在一个或两个壁上具有三维粗糙度的二维通道。第一种配置是专门化程度最低的。在这项研究中，研究了自由流动湍流对再循环、再附着和再发展边界层区换热的影响。具体地说，测量包括湍流换热，同时还包括动量输运，以澄清非平衡壁流中现有的湍流扩散模型。尽管实际设备中的流动可能由于上游效应而受到高度“扰动”，但自由流动在分离流动中的作用在很大程度上还没有被探索过。研究的第二个主要分支是具有分离和再附区的充分发展的槽道流动的实验研究，如具有重复肋形粗糙度的二维槽道和具有三维大粗糙度的槽道。所有这些情况下的对流换热之间都存在着显著的统一关系。例如，在所有这些情况下，无因次换热系数Nusselt数都随着雷诺数的2/3次方而增加。此外，这种关系似乎既适用于某些类型的小尺度粗糙度，也适用于大尺度突起或肋骨。产生这种普遍行为的传输现象正在被研究，希望确定尺度、粘性和分子扩散以及湍流传输的作用。

The emphasis of this research is in Experimental Investigations of Convective Heat Transport in Complex Internal Flows. Particularly emphasized is forced convection cooling of electronics, wherein the internal passageways formed by adjacent printed circuit boards promote highly disturbed flow. Other examples of complex, internal heat transfer occur in gas-cooled heat exchangers where tube arrays form regions of through-flow interacting with separated flow and internal cooling of advanced gas turbine blades where heat transfer enhancement devices, such as short pin ins or periodic rib turbulators, are cast into the walls. All of these internal flows exhibit phenomenological features that make them difficult to predict including: (1) regions of separated and reattaching flow, (2) regions containing secondary motions, and (3) high levels of turbulence in the mean flow. This research focuses on experimental studies on three canonical geometries that together exhibit all of these features. These geometries are: (1) the two-dimensional rearward facing step, or surface fence, (2) the two-dimensional channel with repeated rib roughness, and (3) the two-dimensional channel with three-dimensional roughness on one or both walls. The first configuration is the least specialized. In this branch of the research, the effects of free-stream turbulence on the heat transfer in the recirculation, reattachment, and redeveloping boundary layer region are examined. Specifically, the measurements include turbulent heat transfer and, simultaneously, momentum transport, in order to clarify existing models of turbulent diffusion in non-equilibrium wall flows. The role of free-stream turbulence in reattaching separated flows is largely unexplored, even though flow in real devices may be highly "disturbed" due to upstream effects. The second major branch of the research centers on experimental investigations of fully-developed channel flow with regions of separation and reattachment as exhibited by the two-dimensional channel with repeated rib roughness and the channel with three-dimensional large roughness. There is a remarkable unifying relationship between the convective heat transfer in all of these cases. For example, the dimensionless heat transfer rate, the Nusselt number, increases as about the 2/3 power of the Reynolds number in all of these cases. Furthermore, this relationship seems to hold for both small-scale roughness, of certain types, and large-scale protrusions or ribs. The transport phenomena that produce this universal behavior are being examined with the hope of identifying the roles of scale, viscous and molecular diffusion, and turbulent transport.