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首页> 外文期刊>Experimental Thermal and Fluid Science: International Journal of Experimental Heat Transfer, Thermodynamics, and Fluid Mechanics >Thermo- and fluid dynamics characterization of spray cooling with pulsed sprays
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Thermo- and fluid dynamics characterization of spray cooling with pulsed sprays

机译：脉冲喷雾喷雾冷却的热力学和流体动力学特性

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A fundamental study is reported here on the spray impingement of pulsed sprays on heated surfaces. The experiments consider simultaneous measurements of surface heat flux and droplet characteristics performed with a Phase Doppler Anemometer (velocity, size and flux) prior to impact, to provide a better insight into the interaction between thermal and fluid dynamic effects during the period of injection. The experimental conditions are relevant for engines at steady rotational speeds between 1800 rpm and 3600 rpm, which are of interest for hybrid configurations, where the IC engine is set to operate at maximum efficiency. The analysis addresses the effects of injection conditions (e.g., duration, frequency and pressure) on the thermodynamic behaviour of the surface. It is observed that the heat flux decreases when the pressure of injection increases, due to dynamic variations of the film induced by interaction with impacting droplets in the film evaporation regime. Results further suggest that, for the range of injection conditions found in real engines, the time variation of the heat transfer during injection depends more on the liquid mass flux than on droplet size and axial velocity. However, when the engine load increases, the mechanism by which heat is removed from the surface varies from thermally controlled to mass diffusion controlled, due to saturation of the atmosphere with gasoline vapour. The time resolved measurements are processed to quantify the critical points of the boiling curves, e.g., the critical heat flux (CHF) at the Nukiyama temperature and the minimum heat flux (MHF) at the Leidenfrost temperature. The dynamic characteristics of those curves are used as an approach to describe the heat transfer mechanisms in pulsed spray systems. It is suggested that multiple droplet interaction alters the thermal behaviour of the target in the sense that both, CHF and MHF, increase in regions of large droplet concentration and when the frequency of injection increases. However, load variations associated with times of injection in the range 5-10 ms, do not produce siginificant alterations. Analysis shows the likely importance of transient mechanisms occurring at the solid-liquid and liquid-vapour interface in describing the interaction of pulsed sprays with hot surfaces. (c) 2005 Elsevier Inc. All rights reserved.
机译：在此报道了关于脉冲喷雾在加热表面上的喷雾冲击的基础研究。实验考虑了在撞击前用相多普勒风速计同时测量表面热通量和液滴特性（速度，大小和通量），以便更好地了解注入期间热力和流体动力效应之间的相互作用。实验条件与发动机在1800 rpm和3600 rpm之间的稳定转速相关，这对于混合动力配置至关重要，在混合动力配置中，IC发动机设置为以最高效率运行。该分析解决了注入条件（例如持续时间，频率和压力）对表面热力学行为的影响。可以看到，当注射压力增加时，热通量减少，这是由于在薄膜蒸发状态下与撞击液滴相互作用而引起的薄膜动态变化。结果进一步表明，对于实际发动机中发现的喷射条件范围而言，喷射过程中传热的时间变化更多地取决于液体质量通量，而不是液滴尺寸和轴向速度。然而，当发动机负荷增加时，由于大气被汽油蒸气饱和，从表面除去热量的机理从热控制变为质量扩散控制。处理时间分辨的测量结果以量化沸腾曲线的临界点，例如Nukiyama温度下的临界热通量（CHF）和Leidenfrost温度下的最小热通量（MHF）。这些曲线的动态特性被用作描述脉冲喷涂系统中传热机理的一种方法。建议多液滴相互作用会改变靶标的热行为，即在大液滴浓度区域以及注入频率增加时，CHF和MHF都会增加。但是，与注入时间相关的5-10 ms范围内的负载变化不会产生明显的变化。分析表明，在描述脉冲喷雾与热表面的相互作用时，固-液和液-气界面处发生瞬变机制的重要性。 （c）2005 Elsevier Inc.保留所有权利。

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