Search

Jet impingement in engineering applications is used because of the capacity to transport high levels of heat flux from a surface of interest for cooling purposes. Thus far, based on a vast database of experiments and numerical simulations, several correlations have been established for local and average heat transfer on target surfaces as functions of relevant fluid properties and geometric parameters. In addition to these correlations, significant efforts have been made to gain fundamental understanding of jet impingement in varying configurations. However, the physics governing heat transfer by jet impingement is conjectured and even unclear. Thus, this article collates and discusses recent advances in fluidic mechanisms underlying the heat transfer by submerged jet impingement on a convex surface. The fluid properties developed on a convex surface due to jet impingement with varied characteristics, including jet-to-target surface spacing, interchange their primary roles in heat transfer from/to a convex surface. Particularly, conjectures associated with relevant fluidic mechanisms that have been widely accepted, are confirmed, clarified, and corrected.

A short pin-fin array has been used to improve the internal cooling characteristics at the trailing edge of some gas turbine blade designs. In such a cooling scheme, the pin-fin array that is sandwiched by the turbine blade’s inner surfaces, experiences a uniform-like coolant stream. The local elevation of internal heat transfer especially on the end-walls (i.e., inner blade surfaces) at the trailing edge is achieved predominantly by horseshoe vortex-type secondary flows whose fluidic behavior has been well established. A modification to this cooling scheme has been made by introducing a blockage upstream, causing multiple jets to impinge on the pin-fins – the blockage jets. Previous studies on the internal cooling scheme employing the blockage jets have assumed that the end-wall flow topology is similar to that formed by the horseshoe vortex-type secondary flows due to similar local heat transfer distributions. However, there is no detailed and sufficient acknowledgement made of the lack of an approaching boundary layer. Therefore, the present study experimentally investigates local heat transfer around a single short pin-fin subjected to a fully turbulent jet impingement simulating the blockage jet impingement and demonstrates that the end-wall flow topology loosely resembles that formed by a horseshoe vortex system and is strictly different, depending on the distance between the jet exit and the pin-fin, relative to the length of the jet’s potential core.

Secondary flows and corresponding endwall heat transfer around a fully permeable (porous) short circular cylinder differ from those around an impermeable (solid) cylinder. There exists a through flow which is portion of the mainstream entering the front of the cylinder and exiting at the rear of the cylinder, and the alteration of an adverse pressure gradient along the plane of symmetry that causes three-dimensional boundary layer separation (or forms horseshoe vortices). This study experimentally demonstrates how these distinctive features vary horseshoe vortices, wake patterns, and endwall heat transfer around the porous short circular cylinder. Furthermore, the fluidic effect of the extent of the through-flow that is determined by the permeability of the porous cylinder on downstream heat transfer is discussed.

Louvered fin-and-tube heat exchanger cores, operating inside of an automotive climate control system (ACCS), are used to control temperature and humidity in a passenger cabin. The thermal performance of these cores is typically sensitive to flow features inside the core, which have been well studied for uniform flow fields as well as some non-uniform and inclined flow-fields upstream of the core. The compact geometry of an ACCS unit results in the flow-fields upstream of the core being different from those used in previous studies. When measuring the performance of a louvered fin-and-tube heater core inside a commercially manufactured ACCS unit, up to a 60% reduction in the core’s thermal performance compared to the performance when exposed to a uniform upstream flow is detected. Such a drastic performance reduction has not been reported in the literature. As such a systematic breakdown is made to elaborate how some key and specific ACCS unit features designed for the sake of compactness could play a detrimental role in reducing the overall thermal performance of a given louvered fin-and-tube heater core. By mimicking the actual ACCS features the performance deterioration in the commercial unit could be largely replicated. The individual contributing effects of the various fluidic features such as flow separation and recirculation are determined. Only 50% of the measured 60% deterioration, however, could be accounted for, suggesting that other ACCS unit properties not assessed in detail in this study may also adversely affect the core’s performance.