Large eddy simulations of high-heat-flux supercritical CO2 convection in microchannels: Mixed convection and non-uniform heating
Large eddy simulations of high-heat-flux supercritical CO2 convection in microchannels: Mixed convection and non-uniform heating
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DOI：
10.1016/j.ijheatmasstransfer.2019.118710
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发表时间：
2019-12-01
影响因子：
5.2
通讯作者：
Rattner, Alexander S.
中科院分区：
文献类型：
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作者：
Nabil, Mahdi;Rattner, Alexander S.
近临界点超临界流体对流由于具有较高的流体导热系数和比热，是新兴的高通量热管理需求的有前途的解决方案。超临界对流已经广泛研究了大直径均匀加热通道（4.1 < D < 22.7 mm, 0.05 < q " < 330 W cm(-2)）。然而，高通量致密超临界传热设备的传热过程信息有限，无法指导其工程设计，这些设备通常具有不均匀的热分布。为了满足这一需求，采用大涡模拟（LES）来研究微通道（750 μ m x 737 μ m截面）中的超临界CO2对流。首先，用已发表的实验数据对仿真方法进行了验证。然后对减压P-r = 1.1，质量通量G = 100 - 1000 kg m(-2) s(-1) (Re = 3000 - 35,700)，平均壁面热流通量q " = 24 - 62 W cm(-2)，体流温度T-bulk = 291 - 317 K（假临界范围内外）的水平等温加热通道进行了研究。结果用于评估已发表的超临界对流相关性对微通道换热器的适用性。发现所有可用的超临界相关性都低估了高热流（q " = 58 - 62瓦厘米（-2））下的传热系数。这些情况下的底部到顶部壁面传热系数（HTC）比率被用来确定显著混合对流效应开始的新准则。在低质量通量(G = 100 kg m（-2) s(-1)）下，发现HTC比率超过2.5倍。模拟结果表明，在这些条件下，在低质量通量和高质量通量条件下，热通量的增加导致热通量降低，但在中等质量通量值下，热通量增加。最后，通过一个实例对微尺度超临界对流中共轭传热效应的影响进行了评估，并指出了传统建模方法的局限性。本研究量化了高热流通量微通道伪临界条件下对流相关性的准确性，为预测微通道超临界流动中混合对流的发生提供了新的准则，并论证了微通道超临界换热器中共轭换热效应的影响。(C) 2019 Elsevier Ltd.版权所有。
Near-critical-point supercritical fluid convection is a promising solution for emerging high-flux thermal management needs because of the high fluid thermal conductivities and specific heats. Supercritical convection has been extensively studied for large diameter channels with uniform heating (4.1 < D < 22.7 mm, 0.05 < q '' < 330 W cm(-2)). However, limited information is available on transport processes to guide engineering of high-flux compact supercritical heat transfer equipment, which often have non-uniform heating distributions. To address this need, large eddy simulations (LES) are employed to study supercritical CO2 convection in microchannels (750 mu m x 737 mu m cross-section). First, the simulation approach is validated with published experimental data. Studies are then conducted for horizontal isothermal heated channels at reduced pressure P-r = 1.1, mass fluxes G = 100 - 1000 kg m(-2) s(-1) (Re = 3000 - 35,700), average wall heat fluxes q '' = 24 - 62 W cm(-2), and bulk flow temperatures T-bulk = 291 - 317 K (inside and outside pseudocritical range). Results are used to assess the applicability of published supercritical convection correlations for microchannel heat exchangers. All available supercritical correlations are found to under-predict heat transfer coefficients at high heat fluxes (q '' = 58 - 62 W cm(-2)). The bottom-to-top wall heat transfer coefficient (HTC) ratios from these cases are used to determine a new criterion for the onset of significant mixed convection effects. At low mass fluxes (G = 100 kg m(-2) s(-1)), this HTC ratio is found to exceed 2.5x. Simulations indicate that, at these conditions, increased heat fluxes lead to reduced HTCs for low and high mass fluxes, but increased HTCs at intermediate mass flux values., Finally, an illustrative case is evaluated to assess the impact of conjugate heat transfer effects in microscale supercritical convection, and highlight limitations of conventional modeling approaches. This study quantifies the accuracy of convection correlations for high-heat-flux microchannel pseudocritical conditions, provides new criteria for predicting the onset of mixed convection in microchannel supercritical flows, and demonstrates the impact of conjugate heat transfer effects in microchannel supercritical heat exchangers. (C) 2019 Elsevier Ltd. All rights reserved.