Effects of slot-jet length on the cooling performance of hybrid microchannel/slot-jet module

https://doi.org/10.1016/j.ijheatmasstransfer.2015.05.108Get rights and content

Highlights

  • Two kinds of hybrid microchannel/jet impingement module are designed.

  • The kω SST turbulent model is used to study the performance of hybrid modules.

  • The effects of slot-jet length are analyzed and the optimal length is obtained.

  • Different local Nusselt number distributions are observed for the two hybrid modules.

Abstract

In this work, a three-dimensional numerical model based on the kω SST turbulent model is developed to investigate the single-phase cooling performance of hybrid microchannel/slot-jet module at a constant pumping power of 0.05 W. Two different hybrid modules are analyzed, and the difference between them lies in whether there exist plate fins beneath the slot jet. The temperature uniformity of cooled object and global thermal resistance are evaluated for the two hybrid modules at various slot-jet lengths. The results show that local Nusselt number distribution exhibits a bell shape for the hybrid module with plate fin, while it has a double-peak shape for the hybrid module without plate fin. When with plate fin, a larger slot-jet length of 7800 μm yields the best cooling performance. Oppositely, when without plate fin, a smaller length of 606 μm is beneficial to achieve the better cooling performance. The optimal hybrid module with plate fin has the thermal resistance of 0.105 K W−1 and the bottom wall temperature gradient of 0.27 °C, which are lower than those of the hybrid module without plate fin. Furthermore, the cooling performance of hybrid module can be further improved by optimization of geometric parameters of the heat sink. When the optimal channel number, channel aspect ratio, and width ratio of channel-to-pitch are adopted respectively, the thermal resistance of the hybrid module with plate fin can be reduced by 26.38% to 27.78% as compared with the worst geometry, while the bottom wall temperature gradient can be reduced by 86.79% to 87.46%.

Introduction

Thermal management of electronic devices is one of the important issues for electronics packaging. The rapid advances in micro-electronics technology lead to an increasing heat flux that needs to be removed from the chip surfaces. There is also a desire to improve the temperature uniformity of the cooled object, because the thermal stress caused by large temperature gradient can damage reliability of the devices. Recently several schemes have been proposed to enhance single-phase high heat flux cooling. Among these schemes, microchannel heat sink and jet impingement are regarded as two promising candidates for the cooling of high-performance microprocessors, laser diode arrays, radars, X-ray anodes, solar photovoltaic concentrators, laser and microwave directed energy weapons, and hybrid-vehicle power electronics [1].

Both the microchannel heat sink and jet impingement provide very high heat removal capability, but they still present some serious drawbacks [2], [3]. The microchannel heat sink can yield heat removal rates comparable to the jet impingement using far smaller coolant flow rates and more compact structure, however, it also causes a high temperature rise along the flow path of coolant and needs a high pressure drop [1], [4]. Using a dielectric liquid, the jet impingement produces a very high heat transfer coefficient in the impingement region [5]. The abrupt reduction in cooling effectiveness away from the impingement region can yield large temperature variations along the surface of the cooled object [6]. Therefore the jet impingement requires jet arrays to ensure a relatively good level of temperature uniformity. There are two disadvantages for the jet array. First, after the cool fluid impinges on the cooled surface and dissipates the heat from the surface, the fluid temperature increase inevitably. However, it is difficult to remove the heated fluid from the impinging region due to dense jet arrangement which inevitably causes a reduced heat transfer capacity [7]. Second, the interaction between adjacent jets also leads to a significant reduction in the local heat transfer coefficient.

Many studies have attempted to reduce these unfavorable effects and at the same time to enhance the heat transfer associated with the two cooling technologies [8], [9], [10], [11], [12], [13]. Recently, a hybrid cooling scheme with the combination of microchannel heat sink and jet impingement technologies (referred to as hybrid microchannel/jet-impingement module) was proposed which is expected to both achieve a more uniform temperature distribution on the surface of the cooled object and reduce global thermal resistance of the cooling system [14], [15], [16], [17], [18], [19]. Jang et al. [14] for the first time evaluated the cooling performance of the hybrid microchannel/jet-impingement module experimentally. Their results indicated that the thermal resistance of the hybrid module is only 6.1 K W−1 with about 48.5% reduction as compared with the conventional microchannel heat sink with a parallel flow at a constant pumping power of 0.072 W. Moreover, the pressure drop of the hybrid module is decreased by about 90.5% and the temperature difference across the base surface is decreased by about 87.6%. Subsequently, Jang and Kim [15] suggested two correlations for the thermal resistance and the pressure drop across the hybrid module. The correlations are compared with their experimental results, and both are shown to match with experimental results to within ±10%. Sung and Mudawar [16], [17] investigated the cooling performance of a hybrid microchannel/slot-jet module both experimentally and numerically. They adopted the so-called standard kε turbulent model to analyze the flow and heat transfer of coolant. Their numerical predictions showed that lower surface temperatures can be achieved by decreasing the jet width and microchannel height, and the hybrid module can maintain surface temperature gradients below 2 °C for heat fluxes up to 50 W cm−2. Barrau et al. [18] conducted an experimental study on hybrid microchannel/jet-impingement cooling scheme. Their experiments confirmed that the hybrid scheme has the favorable capacity to improve the temperature uniformity of the cooled object. Barrau et al. [19] also used the kω SST turbulent model to carry out a parametric analysis of longitudinal distribution and channel height. However, the effect of jet size was not discussed in their work.

The hybrid microchannel/jet-impingement cooling module has been shown to significantly improve the temperature uniformity of the cooled object; however, the relevant studies are still very insufficient up to now. Especially, the temperature uniformity, pressure drop, and thermal resistance for the hybrid module can be further improved by optimizing the jet and channel sizes. In this work, a three-dimensional numerical model based on the kω SST turbulent model is developed to investigate the cooling performance of two different hybrid cooling modules at a constant pumping power. The slot-jet length is first optimized for both the modules and the corresponding cooling performances are compared. Subsequently, effects of three geometric parameters of microchannels, including channel number N, channel aspect ratio α, and width ratio of channel-to-pitch β are analyzed to search for the optimal design of the hybrid module.

Section snippets

Geometry for hybrid microchannel/slot-jet module

The schematics of two kinds of hybrid microchannel/slot-jet cooling modules are illustrated in Fig. 1, Fig. 2. The coolant impinges on the microchannel heat sink through a slot jet (along the negative y-axis). The slot jet is characterized by its length and width of Ljet and Wjet. The heat sink has a dimension of Lx × Ly × Lz and is composed of N channels and N + 1 fins. The heights of the channel and fin are Hc, while the widths of the channel and fin are Wc and Wf, respectively. The thickness is δ1

Hybrid microchannel/slot-jet module model

Barrau et al. [19] and Menter [21] have demonstrated that the kω SST model is very effective for predicting the performance of the hybrid cooling module, and hence the present work adopts the same kω SST turbulent model. The following assumptions are made: (1) steady state; (2) single phase and turbulent flow; (3) constant fluid and solid properties; (4) negligible gravitational force, radiation heat transfer, and contact resistance on the interfaces between the hybrid module and the cooled

Results and discussion

The heat sink is made of silicon with ks = 148 W m−1 K−1. Pure water is the coolant with kf = 0.613 W m−1 K−1, μ = 0.000855 kg m−1 s−1, ρ = 997 kg m−3, cp = 4179 J kg−1 K−1 and Tin = 290 K. A uniform heat flux of qw = 100 W cm−2 is applied to the heat sink bottom wall. The hybrid module operates at a constant pumping power of Ω = 0.05 W.

The performance of hybrid module can be evaluated by global thermal resistance, local Nusselt number, and temperature gradient along the bottom wall of the hybrid module. The global thermal

Conclusion

In this study, single-phase cooling performances for two kinds of hybrid microchannel/slot-jet modules are numerically investigated by a three-dimensional kω SST turbulent model. The temperature uniformity of cooled object and the global thermal resistance are evaluated for the two hybrid modules at various slot-jet lengths with a constant pumping power of 0.05 W. The key findings from the study are as follows:

  • (1)

    For the hybrid module with plate fin, the jet hydraulic diameter ranges from 113 to

Conflict of interest

None declared.

Acknowledgments

This study was partially supported by the National Natural Science Foundation of China (No. 51276060), the 111 Project (No. B12034), Program for New Century Excellent Talents in University (No. NCET-11-0635), and the Fundamental Research Funds for the Central Universities (No. 13ZX13).

References (25)

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