2013 1(12)

NUMERICAL SIMULATION OF TORNADO-LIKE HEAT ENHANCEMENT IN THE DIMPLED NARROW CHANNEL AT WATER FLOW

Isaev S.A.¹, Leontiev A.I.², Gultsova M.E.¹, Chudnovskiy Ya.P.³

¹ St. Petersburg State University of Civil Aviation, Saint-Petersburg, Russia
² Bauman University, Moscow, Russia
³ Gas Technology Institute (GTI), Des Plaines, USA

Introduction

There are many passive techniques of heat transfer enhancement ranging from surface (2D) to volumetric (3D) vortex generators, however only a few of them are capable to provide a reliable increase in a heat transfer rate overrunning the increase in pressure losses. One of such successful techniques is the profiling of a heat transfer surface with the regulated arrangement of 3D cavities (dimples). The authors explored that the deviation of the dimple geometry from the spherical shape affects the flow structure and thermal and hydraulic performance of the dimpled wall. Detailed numerical simulation of fluid flow and heat transfer has been performed in the narrow channel with the 2.5 × 0.33 cross section normalized by the equivalent diameter of the dimple footprint at the constant Reynolds number Re=10 000 and the constant heat flux through the dimpled wall. The oval dimple geometry was varied by changing the aspect ratio of the dimple footprint from 1 to 4.5 keeping the same footprint area.

Purpose

In the course of the numerical study, the optimal geometry, the arrangement and the orientation of oval dimples on the heated surface to achieve the superior thermal and hydraulic performance over the spherical cavities are established. Numerical results of local and integral heat transfer characteristics enhanced with the visual representation of the generated vortices clearly illustrated the flow restructuring and an increase in the thermal and hydraulic performance.

Method

Since 2003 Menter’s shear stress model involving the modification with regard to the flow curvature effect has been used in combination with the original equations solved using multi-block computational technologies.

Results

1. The longitudinal extending of the oval hole (oriented 45º to the flow direction) while maintaining the footprint area and the depth (in a fraction of equivalent spherical dimple diameter) leads to faster increase heat transfer from dimpled portion of the channel, and monotonic increase in thermal efficiency is accompanied by non-increasing of hydraulic losses.

2. It was explored an certain restructuring of vortex turbulent flow in the oval dimple at the simultaneous relative elongation, broadening and deepening of the cavity associated with the removal of the separation flow at high elongations, the reversal of vertical component of speed and a significant increase (up to 80% of the weight-average value) the speed of the secondary flow. It was noted that the level of eddy viscosity in large-scale vortex is decreased with oval dimple elongation.

3. Remarkable that flow restructuring in the oval hole is accompanied by transformation of the coupled vortex structure in the separation zone into mono-vortical structure in the same separation zone followed by the formation and development of the spiral vortex within the dimple.

© SPIC "Kappa", LLC 2009-2013