Convective dropwise condensation out of humid air inside a horizontal channel – Experimental investigation of the condensate heat transfer resistance
Convective dropwise condensation out of humid air inside a horizontal channel – Experimental investigation of the condensate heat transfer resistance
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DOI：
10.1016/j.ijheatmasstransfer.2018.08.015
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发表时间：
2018-12
影响因子：
5.2
通讯作者：
F. Eimann;Shaofei Zheng;C. Philipp;T. Fieback;U. Gross
中科院分区：
文献类型：
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作者：
F. Eimann;Shaofei Zheng;C. Philipp;T. Fieback;U. Gross
本文详细介绍了一种用于研究垂直聚合物表面（9× 6 mm，长×高）滴状冷凝的新装置。液滴从湿空气中液化，湿空气流过矩形通道（12× 32 mm，W× H）并通过传感器的过冷表面。后一个是齐平安装到一个垂直通道壁超过流体动力学进入长度为550毫米确定垂直热通量通过测量温度差。此外，通过红外相机通过相对的通道壁观察液滴覆盖的表面。可变的实验参数包括湿空气的温度、含水量和流速，以及传感器的表面温度。实验研究的范围是研究在基底表面上形成的冷凝液滴的平均传热阻力。该过程是基于热通量的测定和评价的基板表面和液体/气体界面之间的温差通过热成像，它们两者同步进行。实验液滴的传热系数，h d，作为一个平均值进行评估，它认为通过液滴的热传导和由于内部对流的传热增强。参数变化涵盖了7900和21400之间的雷诺数，56%和95%之间的相对湿度和30° C和46° C之间的空气温度，适用于广泛的冷却温度范围。后者影响液滴表面温度，并与本体的水含量一起，决定了传质的驱动力。h d随雷诺数和传质驱动力的增大而增大。然而，增加后者超过一定的值会导致大量的不可冷凝气体在液体/气体界面处的积累，抑制蒸汽质量通量，并导致不变的液滴传热系数。对实验数据进行了关联，为今后的液/气界面传热实验研究提供了依据。
A new facility feasible for the investigation of dropwise condensation at a vertical polymer surface (9× 6 mm, L× H) is described in detail. The droplets are liquefied out of humid air, which flows through a rectangular channel (12× 32 mm, W× H) and passes the sub-cooled surface of a sensor. The latter one is flush mounted into one of the vertical channel walls beyond a hydrodynamic entry length of 550 mm determining perpendicular heat fluxes by measuring temperature differences. Furthermore the droplet covered surface is observed through the opposite channel wall by an infrared camera. The variable experimental parameters include temperature, water content and flow velocity of the humid air, as well as the surface temperature of the sensor. The scope of the experimental study is to investigate the mean heat transfer resistance of the condensate droplets which form on the substrate surface. The procedure is based on the determination of heat fluxes and the evaluation of the temperature differences between the substrate surface and the liquid/gas interface by means of thermography, both of them carried out synchronously. The experimental droplet heat transfer coefficient, h d, is evaluated as an average value and it considers both heat conduction through the droplets and the augmentation of heat transfer due to inner convection. The parameter variation covers Reynolds numbers between 7900 and 21400, relative humidities between 56% and 95% and air temperatures between 30° C and 46° C for a broad range of cooling temperatures. The latter one influences the droplet surface temperature and together with the water content of the bulk, it determines the driving force for mass transfer. h d was found to increase with the Reynolds number and the driving force for mass transfer. However, increasing the latter one beyond a certain value causes massive accumulation of noncondensable gases at the liquid/gas interface inhibiting the vapour mass flux and leading to invariant droplet heat transfer coefficients. The experimental data is correlated providing the basis for the experimental investigation of the heat transfer at the liquid/gas interface which will be done in future.