Investigation on Jet Mixing Rate Based on DNS of Excitation Jets

et al. 2014

JFST

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] IntroductionImpinging jet is widely used for cooling of industrial applications such as electric device, blades of gas turbine, hot steel and so on, since it possesses high performance of heat transfer rate and is easier to implement into various system. Their characteristics are reviewed by a few notable literatures (Kataoka, 1990 ;Viskanta, 1993). So far, a few or several ten thousand Reynolds number, defined with inlet nozzle diameter, are mainly targeted in laboratory research. However, according to the downsizing of electronic equipments, several one hundred of Re numbers are going to be dealt with (Lee, et al., 2008). Impinging jets are subdivided into three regions, such as free jet region, stagnation region and wall jet region (Viskanta,1993). Their performance of heat transfer on the impinging wall is depending on the Re number, the shape of nozzle, the number of nozzle, the distance between the nozzle and the impinging wall and so on (Viskanta,1993). In particular, although a single impinging jet produces high heat transfer rate around an impinging position on an impinging wall, however the heat transfer performance decays with increasing the distance from the impinging position. Thus in order to overcome the shortcoming of single impinging jet (SIJ), i.e., the occurrence of both inhomogeneous heat distribution on the wall and the narrow heating area, multiple impinging jets (MIJ) are generally introduced in industrial applications. Thus far it is well-known that each jet of MIJ interacts with each other, producing the complex flow field (Geers, Hanjalić, and Tummers, 2006;Weigand and Spring, 2011). In addition, the heat transfer performance is significantly influenced with their interaction. Therefore to realize an optimum heat transfer performance, the influence of geometrical arrangement of jets, i.e., the separation length between jets, the arrangement pattern, should be investigated.Recently significant advances of computer power lead to the realization of fluid phenomena including miniature vortices through the direct numerical simulation (DNS). Under the state of the art, we found the effectiveness of the active control of a round jet for the mixing enhancement (Tsujimoto, et al., 2008). Also DNS of impinging jets are conducted so far (Tsubokura, et al., 2003;Hattori and Nagano, 2004) and highly resolved LES ( large eddy simulation ) (Hadžiabdić, and Hanjalić, 2007) has demonstrated the flow structure and the statistical properties of heat transfer. Hence at present it is capable of investigating systematically the effect of control parameter. AbstractIn order to elucidate the heat and flow characteristics of multiple impinging jet (MIJ), we conduct the DNS (direct numerical simulation) of four round impinging jet arranged at an inflow of flow field. To realize the high accurate computation, the discretization in space is performed with hybrid scheme in which sine or cosine series and 6th order compact scheme are used. For the surrounding boundary condition, in order to reduce the artificial n...

Section: Evaluation Of Numerical Accuracysupporting

confidence: 89%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 89%

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DNS analysis of multiple impinging jets

Koide

et al. 2014

JFST

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“…From figure 9, the characteristics of SIJ remain even in the MIJ; high heat transfer performance near the impinging point of each jet and the heat transfer performance promptly decays away from jet itself. The heat transfer performance between the impinging jets decreases by the influence of interference as mentioned in previous literatures [13,15]. In addition, high heat transfer area of the central jet distributes as a circle, but that of the outer jet is elliptical.…”

Section: Heat Transfer Characteristicsmentioning

confidence: 73%

“…Recently significant advances of computer power lead to the realization of fluid phenomena including miniature vortices through the DNS. Under the state of the art, we found the effectiveness of the active control of a round jet for the mixing enhancement [13]. Also DNS of impinging jets and highly resolved LES (large eddy simulation) [5] have demonstrated the flow structure and the statistical properties of heat transfer.…”

Section: Introductionmentioning

confidence: 83%

DNS of flow and heat transfer characteristics of multiple impinging jets

Suzuki

The Proceedings of Mechanical Engineering Congress, Japan

et al. 2018

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Multiple impinging jet (MIJ) is widely used for cooling of industrial applications such as electric device, blades of gas turbine, hot steel and so on, since it possesses high performance of heat transfer rate and is easier to implement into various systems. Thus, in order to improve the heat transfer performance, it is indispensable to elucidate the detail of flow phenomenon. In the present study, in order to analyze the flow and heat transfer characteristics formed by the interaction of MIJ, we conduct the DNS (direct numerical simulation) of 13 and 19 round impinging jets and investigate the influence of interference between jets. From the instantaneous velocity distribution, it is observed that each jet impinges the impingement wall and diffuses in the radial direction in both multiple jets. From the time-averaged velocity and Nusselt number distribution, it is confirmed that flow phenomena such as up-wash flow due to the interaction between jets are generated and that the Nusselt number shows a high value at the impinging points and decreases toward the outside of the calculation region. In addition the heat transfer performance decreases in the interaction region between the jets.

Flow and heat transfer characteristics of blooming jets impinging upon wall using DNS

Echigo

et al. 2020

JFST

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A single impinging jet exhibits high heat transfer performance around an impingement point on a wall. However, the heat transfer performance deteriorates as it moves away from the impingement point. Consequently, multiple impinging jets are commonly introduced to overcome the shortcomings of a single jet: inhomogeneous heat distribution on the wall and a narrow heating area. However, inhomogeneous heat transfers still occur. Therefore, a new jet control is required to improve the uniformity of heat transfer. Meanwhile, blooming jets are produced by appropriate combinations of axial and helical excitations at the nozzle exit. Using appropriately selected excitations, a jet can split into two separate jets (bifurcating jet) or spread into a shower of toroidal vortex rings. Blooming jets exhibit good performances of mixing and diffusion, suggesting possible applications in flow control. However, studies regarding the heat transfer performance of blooming jets are non-existent. In this study, we conducted direct numerical simulations of blooming jets impinging upon a wall and investigated their flow characteristics and heat transfer performances. As control parameters, the impingement distance (the distance from the nozzle to the wall) and frequency ratio (the axial excitation frequency to the helical frequency) are varied. The vortex structures and velocity magnitude reveals flow modulations due to blooming control. With the time-averaged local Nusselt number, the heat transfer performance of the blooming jets is evaluated quantitatively. Compared with uncontrolled jets, the uniformity of heat transfer of blooming jets is better, suggesting their potential application for leveling the heat transfer of impinging jets.