An Investigation of the Physical Mechanism of Heat Transfer Augmentation in Stagnating Flows Subject to Freestream Turbulence

Abstract

Experiments have been performed in a water tunnel facility to examine the physical mechanism of heat transfer augmentation by freestream turbulence in classical Hiemenz flow. A unique experimental approach to studying the problem is developed and demonstrated herein. Time-Resolved Digital Particle Image Velocimetry (TRDPIV) and a new variety of thin film heat flux sensor called the Heat Flux Array (HFA) are used simultaneously to measure the spatio-temporal influence of coherent structures on the heat transfer coefficient as they approach and interact with the stagnation region. Velocity measurements of grid generated freestream turbulence are first performed to quantify the turbulence intensity, integral length scale, and isotropy of the flow. Laminar flow and heat transfer at low levels of freestream turbulence (Tux ≅ 0.5–1.0%) are then examined to provide baseline flow characteristics and heat transfer coefficient. Similar experiments using the turbulence grid are then performed to examine the effects of turbulence with mean turbulence intensity, Tux ≅ 5.5%, and integral length scale, Λx ∼ 3.25 cm. At a mean Reynolds number of ReD = 21,000 an average increase in the mean heat transfer coefficient of 43% above the laminar level was observed. To better understand the mechanism of this augmentation, flow structures in the stagnation region are identified using a coherent structure identification scheme and tracked in time using a customized tracking algorithm. Tracking these structures reveals a complex flow field in the vicinity of the stagnation region. Filaments of vorticity from the freestream are amplified near the plate surface leading to the formation of counterrotating vortex pairs and single sweeping vortex structures. By comparing the transient heat flux measurements with the tracked vortex structures it is clear that heat transfer augmentation is due primarily to amplification of stream-wise vorticity and subsequent vortex formation near the surface. The vortex strength, length scale, and distance from the stagnation plate are key parameters affecting augmentation. Finally, a mechanistic model is examined which captures the physical interaction near the wall. Model results agree well with measured heat transfer augmentation.