Influence of heat flux and Reynolds number on the entropy generation for different types of nanofluids in a hexagon microchannel heat sink

Based on the Second Law of Thermodynamics, the entropy generation is studied for laminar forced convection flow of different nanoparticles(Al_2 O_3, CuO and SiO_2) mixed with water through a hexagon microchannel heat sink(HMCHS). The effects of different heat fluxes and Reynolds numbers on the entropy generation for different nanofluids, volume fractions and nanoparticles diameter are investigated. The heat flux is in the range of 125 to 500 kW·m~(-2) and the Reynolds numbers vary between 200 and 1500. The thermal, frictional and total entropy generations are calculated by integrating the volumetric rate components over the entire HMCHS. The results clearly show that the rise in the heat flux leads to an increase in the thermal entropy generation for nanofluids and pure water but they don't have any influence on the frictional entropy generation. Moreover, when the Reynolds number increases, the frictional entropy generation increases while the thermal entropy generation decreases. The results revealed that at low heat fluxes and high Reynolds numbers, pure water gives the lowest entropy generation, while at high heat flux the nanofluid has to be used in order to lower the overall irreversibility.     

微通道临界热通量的基础理论与提升技术研究进展

临界热通量（CHF）是微通道流动沸腾换热的限制参数之一,当热通量大于CHF时,换热性能急剧恶化,换热设备易发生烧毁与故障,因此CHF对于微通道换热的安全运行具有重要影响。微通道换热是目前电子冷却的主流技术,然而近年来电子设备热通量不断提高,CHF已成为限制微通道应用的关键参数之一。针对微通道CHF的研究进展,详细阐述微通道CHF的形成机理,分析工况参数和通道尺寸对微通道CHF的影响机制,总结微通道CHF的预测模型,详述微通道CHF提升的各类技术方法与原理,探讨学术界观点差异和今后研究方向。该综述为微通道在高热通量条件下安全可靠运行提供了研究借鉴。     

Effects of the porous medium and water-silver biological nanofluid on the performance of a newly designed heat sink by using first and second laws of thermodynamics

The aim of this numerical investigation is to evaluate the laminar forced convection of biologically synthesized water-silver nanofluid through a heat sink (HS) filled with porous foam (PHS) using first and second laws of thermodynamics. The impacts of inlet velocity (V = 0.5–3 m·s⁻¹) and volume fraction of nanofluid (φ = 0–1%) on the performance metrics of HS are assessed and the outcomes are compared with those of the non-porous HS (NHS). The outcomes revealed that for both the PHS and NHS, the increase of V causes an intensification in convection coefficient, pumping power, and entropy generation due to fluid friction, while the maximum CPU temperature, thermal resistance, and entropy generation due to the heat transfer reduces by boosting V. Also, it was found that the augmentation of V results in intensification in convection coefficient, pumping power, overall hydrothermal performance, and frictional entropy generation, while the opposite is true for maximum CPU temperature, thermal resistance, and thermal entropy generation. Furthermore, it was reported that, except for φ = 0.5%, the overall hydrothermal performance of NHS is better than that of PHS, while PHS has better second-law performance than NHS in all the studied cases. Also, it can be concluded that the best hydrothermal performance for PHS belongs to φ = 1% and V = 0.5 m·s⁻¹, while for NHS, these values are 1% and 2 m·s⁻¹.     

Impact of nanoparticle shape on thermohydraulic performance of a nanofluid in an enhanced microchannel heat sink for utilization in cooling of electronic components

In this article, the thermal–hydraulic efficacy of a boehmite nanofluid with various particle shapes is evaluated inside a microchannel heat sink. The study is done for particle shapes of platelet, cylinder, blade, brick, and oblate spheroid at Reynolds numbers (Re) of 300, 800, 1300, and 1800. The particle volume fraction is assumed invariant for all of the nanoparticle shapes. The heat transfer coefficient (h), flow irregularities, pressure loss, and pumping power heighten by the elevation of the Re for all of the nanoparticle shapes. Also, the nanofluid having the platelet-shaped nanoparticles leads to the greatest h, and the nanofluid having the oblate spheroid particles has the lowest h and smallest pressure loss. In contrast, the nanofluid having the platelet-shaped nanoparticles leads to the highest pressure loss. The mean temperature of the bottom surface, thermal resistance, and temperature distribution uniformity decrease by the rise in the Reynolds number for all of the particle shapes. Also, the best distribution of the temperature and the lowest thermal resistance are observed for the suspension containing the platelet particles. Thereby, the thermal resistance of the nanofluid with the platelet particles shows a 9.5% decrement compared to that with the oblate spheroid particles at Re = 300. For all the nanoparticle shapes, the figure of merit (FoM) uplifts by elevating the Re, while the nanofluids containing the brick- and oblate spheroid-shaped nanoparticles demonstrate the highest FoM values.     

Effects of the porous medium and water-silver biological nanofluid on the performance of a newly designed heat sink by using first and second laws of thermodynamics

The aim of this numerical investigation is to evaluate the laminar forced convection of biologically synthesized water-silver nanofluid through a heat sink (HS) filled with porous foam (PHS) using first and second laws of thermodynamics. The impacts of inlet velocity (V=0.5-3 m·s^-1) and volume fraction of nanofluid (φ=0-1%) on the performance metrics of HS are assessed and the outcomes are compared with those of the non-porous HS (NHS). The outcomes revealed that for both the PHS and NHS, the increase of V causes an intensification in convection coefficient, pumping power, and entropy generation due to fluid friction, while the maximum CPU temperature, thermal resistance, and entropy generation due to the heat transfer reduces by boosting V. Also, it was found that the augmentation of V results in intensification in convection coefficient, pumping power, overall hydrothermal performance, and frictional entropy generation, while the opposite is true for maximum CPU temperature, thermal resistance, and thermal entropy generation. Furthermore, it was reported that, except for φ=0.5%, the overall hydrothermal performance of NHS is better than that of PHS, while PHS has better second-law performance than NHS in all the studied cases. Also, it can be concluded that the best hydrothermal performance for PHS belongs to φ= 1% and V=0.5 m·s^-1, while for NHS, these values are 1% and 2 m·s^-1.     

连续螺旋折流板换热器流动与传热性能及熵产分析

采用数值模拟的方法,研究了螺旋角对连续螺旋折流板换热器流动与传热性能的影响,并以熵产数为指标对换热器性能进行了基于热力学第二定律的分析评价。结果表明,相同质量流量时壳程传热系数和压降均随螺旋角的增大而降低,且后者降低的幅度大于前者。连续螺旋折流板换热器壳程横截面上切向速度分布较弓形折流板换热器更加均匀。在靠近中心假管的内层区域,同一径向位置的轴向速度随螺旋角的增大而降低,而在靠近壳体壁面的外层区域则相反。螺旋角越大,不同径向位置的换热管间的换热量分布均匀性越好。壳程质量流量相等时,换热器中传热引起的熵产占总熵产的比重随着螺旋角的增大而增加,熵产数随着螺旋角的增大而降低。     

不同倾角螺旋叶片转子综合传热性能数值模拟

对叶片倾角在30°～60°之间的7种开槽螺旋叶片转子的湍流流动与传热特性进行了数值模拟研究。数值模拟采用RNG 湍流模型,采用SIMPLE算法进行速度和压力的耦合,对管内壁面采用强化壁面处理法。选取管程入口流量为2.6 m3/h时管程中部截面的速度场、温度场和湍流强度进行了分析,结果表明,开槽螺旋叶片转子可在换热管内引起旋流,增强管内流体湍动,促进对流传热。考虑转子引起传热和阻力两方面因素,采用综合评价指标PEC对7种不同倾角转子进行比较,叶片倾角为60°的开槽螺旋叶片转子具有较好的综合传热性能。