Thermal and Hydrodynamic Phenomena in the Stagnation Zone—Impact of the Inlet Turbulence Characteristics on the Numerical Analyses

Abstract

One of the methods of heat transfer enhancement is utilization of the turbulent impinging jets, which were recently applied, for example, in the heat exchangers. Their positive impact on the heat transfer performance was proven, but many questions related to the origin of this impact are still unanswered. In general, the wall-jet interaction and the near-wall turbulence are supposed to be its main reason, but their accurate numerical analysis is still very challenging. The authors’ aim was to construct the numerical model which can represent the real phenomena with good or very good accuracy. Starting with an analysis of single jet and obtaining the agreement with experimental data, it will be possible to extend the model towards the whole minijets heat exchanger. The OpenFOAM software, Bracknell, UK was used for that purpose, with our own implementation of the ζ-f turbulence model. The most difficult area to model is the stagnation region, where the thermal effects are the most intensive and, at the same time, strongly affected by the conditions in the pipe/nozzle/orifice of various size (conventional, mini, micro), from which the jet is injected. In the following article, summary of authors’ findings, regarding significance of the velocity profile and turbulence intensity at the jet place of discharge are presented. In addition, qualitative analysis of the heat transfer enhancement is included, in relation to the inlet conditions. In the stagnation point, Nusselt number differences reached the 10%, while, in general, its discrepancy in relation to inlet conditions was up to 23%.

Curved Surface Minijet Impingement Phenomena Analysed with ζ-f Turbulence Model

“…where D is the jet issuing part diameter, m and ν is the kinematic viscosity, m 2 /s. As the inlet diameter size might strongly impact the results of the heat transfer at the impinged surface, which was proven in [42][43][44], it was kept constant throughout the research and equal to 0.01 m. This information is very often missed in published sources, which can rise misunderstandings. Moreover without it, particular results should not be used as the data validation.…”

“…It is important to note, that such intensity was relatively high, and strongly impacted the results in the stagnation zone. As shown in [22,23,44], assumed inlet conditions (both regarding velocity profile and turbulence parameters) affect significantly the heat transfer rates at the impinged surface. Such information very often is not presented in the literature, which precludes exact representation of boundary conditions, because behind the statement fully-developed profile many possible configurations can be hidden.…”

“…Based on the conclusions from [39,40,44] the ζ-f model was chosen to be used in the following paper, after being successfully implemented in the OpenFOAM software. The governing equation for the model including the turbulence kinetic energy is presented in Equation ( 6) and the governing equation for its dissipation is shown in Equation ( 7):…”

Computational and experimental evaluation of heat transfer intensity in channels of complex configuration for gas flow with different levels of turbulence

“…He et al [8] propose to improve heat transfer in order to increase the efficiency of cooling of turbine parts and assemblies, based on numerical simulation. The turbulence level of flows is of great importance for the heat exchange intensity in various heat exchangers [10][11][12]. For example, Gu et al [10] fine-tunes the design of cooling fins for a heat exchanger taking into account gas-dynamic effects based on experimental studies.…”

“…For example, Gu et al [10] fine-tunes the design of cooling fins for a heat exchanger taking into account gas-dynamic effects based on experimental studies. Kura et al [11] improves the efficiency of heat exchangers through the use of turbulent counter jets. It should be noted that there are other technical applications in which the gasdynamic parameters of flows have a strong influence on the functioning of machines and installations: the flow of nanofluids in heat exchangers [13], the combustion of fuel-air mixtures in combustion chambers [14], and the flow of gases in gas exchange systems of piston engines [15].…”