Heat transfer and fluid mechanics over surface indentations: state-of-the-art

Part I

A.A.Khalatov

Institute of Engineering Thermophysics, National Academy of Sciences

Kiev, Ukraine

halatov@n-t.org

Over the last years, an increased interest was shown to the surface indentations of different shape. It is because of their unique flow features, such as lower pressure losses, the best thermal-hydraulic performance, unsteady fluctuations and over multiple indentations. The paper represents the comprehensive review of heat transfer and hydrodynamics over surfaces, structured with single and multiple indentations (dimples). This includes classification of single indentations, unsteady flow phenomena, heat transfer and pressure losses over single and multiple indentations, the effects of surface curvature, flow compressibility, pressure gradient, and dimple rim shape. The experimental data regarding in-tube flow and cross flow of dimpled tubes, numerical simulations and thermo-hydraulic performance of dimpled configurations are analyzed. The results of American studies are considered in detail, a few examples of industrial applications are given.

Preface

Over the last ten years, very significant progress was made in the study of heat transfer and hydrodynamics over surface indentations (dimples). Three-dimensional surface indentations (dimples) demonstrate some extraordinary flow features, including surprisingly low-pressure drop, equivalent growth of heat transfer and pressure drop, and the bulk flow fluctuations after indentations. In many case the Reynolds analogy factor (RAF) exceeds the unity factor that is unachievable for all other heat augmentation techniques. The "gang" of vortices over multiple indentations is not a chaotic cluster of vortices, but rather the "self-organized" vortex totality coordinating mutually in-space behavior. These physical properties are already used in various industrial applications, but potentially attractive in many other aerothermal vortex technologies. This work is based on the fundamental data obtained in Russia, Ukraine, USA and provides the detailed analysis of the heat transfer and hydrodynamics over surfaces structured with single and multiple indentations. Unlike the early surveys, detailed review of the U.S. publications is given. The new experimental data on heat transfer, flow phenomena, and bulk flow oscillations obtained by the author jointly with colleagues from the Institute of Engineering Thermophysics (Ukraine), Russian Academy of Sciences, Cardiff University (United Kingdom), U.S. Air Force Academy (Colorado Springs), and University of Utah (Salt Lake City, USA) is also included in this paper. The author greatly acknowledges the international cooperation in the field and a good opportunity to provide joint experimental program.

1. Introduction

Over the last few years the intensive research effort was focused on studies of heat transfer and hydrodynamics over surfaces with indentations (dimples). The early discoveries of low drag penalties for golf balls made in have demonstrated the effectiveness of shallow surface concavities (dimples) to cause early boundary layer transition and the pressure losses without the drag penalties associated with sand roughness. Unlike round balls with either smooth or sand roughened surfaces, the dimpled ball drag curve remains almost constant at the supercritical Reynolds numbers. Due to reductions in separation zone the drag coefficient of a dimpled ball at Re<60,000 is substantially lower than that over a sanded or smooth ball. These results indicate the dimples have a more beneficial effect on the drag reduction and laminar-turbulent transition than traditional sand roughness.

These important issues resonated in a few Russian research programs some twenty five years ago with primary scientific results reaching the worldwide engineering community only after the former Soviet Union disintegration. The initial studies focused on a single dimple and multiple dimples on a flat plate have shown the fundamental potential of this technique for heat transfer augmentation as they produce substantial heat transfer augmentation rates with pressure drop factors, which are smaller than all other types of heat transfer augmenters.

The Russian researchers were the first who revealed the in-dimple "side-to-side" fluctuating vortex structures bursting periodically out of a deep dimple (h/D=0.5) at high Reynolds numbers (Re_D>100,000). The analytical solutions and flow visualizations have disclosed the "tornado-like" nature of this vortex with substantial in-vortex energy concentration. Further experiments showed the multiple dimples on a flat plate provide the high heat transfer augmentation rates (factor of 2.1 to 2.3) accompanied by an approximately equivalent pressure drop factor. This is due to a specific vortex nature and vortex pattern ("gang" of vortices) not protruding vortices into the freestream flow and reducing the friction losses over the "vortex grid".