Impingement Cooling by Multiple Asymmetric Orifice Jets

This study presents impingement cooling from a flat plate by multiple asymmetric jets. Such jets are discharged through blunt-edge inline orifice holes with a thickness-to-diameter ratio of t/Dj = 0.5 and a jet-to-jet spacing of T/Dj = 4.0, at the Reynolds number of 20,000. Firstly, fluidic features are established both in free exit and with impingement, at varying short target spacing (e.g., H/Dj = 4.0). Secondly, thermal characteristics of the jet impingement are elucidated. Results demonstrate that, due to a skewed incidence of the coolant stream upstream of concave orifice holes, the resulting multiple orifice jets are asymmetric and skewed relative to the orifice axis. These results mimic multiple fluidically inclined jets. However, asymmetric entrainment that takes place causes faster mixing with the surrounding fluid at rest as well as faster decay of momentum. This shows more effective cooling from a flat plate for the relatively short H/Dj range than conventional symmetric orifice and nozzle jets.

Impingement Heat Transfer Innovations and Enhancements: a Discussion on Selected Geometrical Features

Impingement heat transfer is considered as one of the most effective cooling technologies that yields in high localized convective heat transfer coefficient. This paper studies different configurational parameters involved in jet impingement cooling such as, exit orifice shape, crossflow regulation, target surface modification, spent air reuse, impingement channel modification, jet pulsation, and other techniques to understand what are critical and how these heat transfer enhancement concepts work. These enhancement factors have been explored in detail by many researchers, including standard parameters such as normalized distance between adjacent jets and jet-to-target spacing, and those known benefits are not repeated here. The aim of this paper is to stimulate the current scientific knowledge of this efficient cooling technique and instill some thoughts for future innovations. New orifice shapes are becoming feasible due to 3D printing technologies. However, the orifice studies show that it is hard to beat a sharp-edged round orifice. Any attempt to streamline the hole shape indicated a drop in the Nusselt number. Reduction in crossflow has been attempted with channel modifications. Use of high porosity conductive foam in the impingement space has shown marked improvement in heat transfer performance. A list of possible research topics based on this discussion are provided in conclusion.

Array Jet Impingement On High Porosity Thin Metal Foams: Effect of Foam Height, Pore-Density and Spent Air Crossflow Scheme On Flow Distribution and Heat Transfer

An experimental investigation was carried out to study heat transfer and fluid flow in high porosity (93%) thin metal foams subjected to array jet impingement, under maximum and intermediate crossflow exit schemes. Separate effects of pore-density and jet-to-target spacing (z/d) have been studied. To this end, for a fixed pore-density of 40PPI foams, three different jet-to-target spacing (z/d=1, 2, 6) were investigated, and for a fixed z/d of 6, three different pore-density of 5, 20 and 40PPI were investigated. The jet diameter-based Reynolds number was varied between 3,000-12,000. Experiments were carried out to characterize local flow distribution and Nusselt numbers for different jet impingement configurations. The heat transfer results were obtained through steady-state experiments. Local flow measurements show that, as z/d decreases, the mass flux distributions are increasingly skewed with higher mass flow rates near the exits. Heat transfer enhancement has been calculated and the optimum foam configuration has been deduced from the pumping power. It was observed that Nusselt number increases with increasing pore density at a fixed z/d and reduces with increase in z/d at constant pore density. Intermediate crossflow had higher heat transfer than maximum crossflow with significantly lower pumping power. Under a constant pumping power condition, z/d = 2, 40ppi foam provided an average enhancement of 35% over the corresponding baseline configuration for intermediate crossflow scheme and was found to be the most optimum configuration.

Analysis of heat transfer and fluid flow of a slot jet impinging on a confined concave surface with various curvature and small jet to target spacing

Purpose This study aims to focus on the surface curvature, jet to target spacing and jet Reynolds number effects on the heat transfer and fluid flow characteristics of a slot jet impinging on a confined concave target surface at constant jet to target spacing. Design/methodology/approach Numerical simulations are used in this research. Jet to target spacing, H/B is varying from 1.0 to 2.2, B is the slot width. The jet Reynolds number, Rej, varies from 8,000 to 40,000, and the surface curvature, R2/B, varies from 4 to 20. Results of the target surface heat transfer, flow parameters and fluid flow in the concave channel are performed. Findings It is found that an obvious backflow occurs near the upper wall. Both the local and averaged Nusselt numbers considered in the defined region respond positively to the Rej. The surface curvature plays a positive role in increasing the averaged Nusselt number for smaller surface curvature (4-15) but affects little as the surface curvature is large enough (> 15). The thermal performance is larger for smaller surface curvature and changes little as the surface curvature is larger than 15. The jet to target spacing shows a negative effect in heat transfer enhancement and thermal performance. Originality/value The surface curvature effects are conducted by verifying the concave surface with constant jet size. The flow characteristics are first obtained for the confined impingement cases. Then confined and unconfined slot jet impingements are compared. An ineffective point for surface curvature effects on heat transfer and thermal performance is obtained.

Effect of Nozzle-to-Target Spacing on Fin Effectiveness and Convective Heat Transfer Coefficient for Array Jet Impingement Onto Novel Micro-Roughness Structures

With recent advancements in the field of additive manufacturing, the design domain for development of complicated cooling configurations has significantly expanded. The motivation of the present study is to develop high-performance impingement cooling designs catered towards application’s requiring high rates of heat removal, e.g. gas turbine blade leading edge and double-wall cooling, air-cooled electronic devices etc. Jet impingement is a popular cooling technique which results in high convective heat rates. In the present study, jet impingement is combined with strategic roughening of the target surface, such that a combined effect of impingement-based and curved-surface area based enhancement in heat transfer coefficient could be achieved. Traditionally, for surface roughening, cylindrical and cubic elements are used. We have demonstrated, through our steady-state experiments, a novel “concentric” shaped roughness element design which has resulted in about 20–60% higher effectiveness compared to smooth target jet impingement, for jet-to-target spacing of one jet diameter. The cubic shaped roughened target yielded about 20% to 40% enhancement in effectiveness, and the cylindrical shaped roughened target yielded 10% to 30% enhancement. Through the plenum pressure measurements, it was found that the addition of the micro-roughness elements does not result in a discernable increment in pressure losses, compared to the standard impingement on the smooth target surface. Hence, the demonstrated configuration with the highest heat transfer coefficient also resulted in the highest thermal hydraulic performance.

A Numerical Study of an Impingement Array Inside a Three Dimensional Turbine Vane

A simplified impingement high pressure turbine vane is modeled and solved via Fluent. A relatively flat section of the vane is fitted with 15 0.51mm diameter impingement holes — 5 rows of 3 jets. Results are then compared to known experimental data. Two different turbulence models are used to study this preliminary configuration: K-omega SST and the RNG k-epsilon model. The jet exit Reynolds numbers, cross flow velocity, and the average and local heat transfer distribution are analyzed with varying Reynolds numbers and jet to target spacing. It is observed that the static pressure decreases across the vane with the cross flow velocity increasing towards the trailing edge exit, thereby uniformly increasing the jet exit velocity at each row. Forced convection is seen in the downstream rows in-between span-wise jets due to high cross flow velocities. All numerical results were capable of replicating the higher heat transfer obtained with a higher Reynolds number, and conversely, a lower heat transfer with an increase in jet to target spacing. In its entirety, validating against all correlations, the RNG model obtained an average deviation of 15.7%, while the K-omega SST yielded only 7.8%.