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

Part II

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 survey 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.

This survey has 13 thematic sections, and it is presented in two parts (Part I; Part II). In first issue of this Journal ("Actual problems of aviation and aerospace systems: processes, models, experiment", No.1(19), v.10, 2005) it was published Part I of survey, presenting 6 first sections and the references list (in English version of article). In this issue it is presented Part II with next sections (7¸13) and the references list (in Russian version of article).

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 here, in this Part II. 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.

Surface indentations (dimples) demonstrate the unique heat transfer and fluid flow features potentially attractive in many industrial applications. The high heat transfer rate (factor of 3.0 at h/D=0.3) and simple production technology is combined with record thermal-hydraulic performance unachievable for all other heat augmentation techniques. Recent advanced applications include internal cooling of gas turbine blades and combustion lines, flow separation control in the low-pressure aeroengine gas turbines, improved combustion processes, oscillating film cooling technique, and some others. The physical properties of surface indentations could be employed in many other aerothermal technologies for mechanical and combustion engineering, chemical processing, energy and power systems. The future fundamental studies should be addressed to the following primary directions.

ћ        Unsteady and steady hydrodynamics and heat transfer in single and multiple dimples of different shape and depth, including symmetrical and asymmetrical indentations. Combustion processes in indentations.

ћ        Unsteady and steady hydrodynamics and heat transfer beyond limited row (one to three) of indentations of different shape. Unsteady vortex structures inside indentations of different shape and beyond them, instant and average flow fluctuations.

ћ        Laminar-turbulent flow transition inside indentations of different shape and beyond them, effect of different factors.

ћ        Improved turbulent models of unsteady heat transfer and hydrodynamics, computer simulation of vortex hydrodynamics.