The effect of circumferentially non-uniform heat flux on flow boiling heat transfer in a horizontal tube
The effect of circumferentially non-uniform heat flux on flow boiling heat transfer in a horizontal tube
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周向非均匀热通量对水平管内流动沸腾传热的影响
DOI：
10.1016/j.ijheatmasstransfer.2021.122428
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发表时间：
2022
影响因子：
5.2
通讯作者：
J. Dirker;Hannalie Scheepers;J. Meyer
中科院分区：
文献类型：
--
作者：
J. Dirker;Hannalie Scheepers;J. Meyer
流动沸腾是几种热力循环中一种重要的能量传递机制。在直接蒸汽发电集中太阳能发电应用中，流动沸腾发生在热流不均匀的条件下，例如在主要从下方加热的水平管中。这会影响局部和平均换热系数。本实验测定了不同周向非均匀热流密度下饱和流动沸腾的换热特性。为了证明热通量不均匀的影响，在内径为8.5 mm的管内进行了R245fa的试验，质量通量分别为S 200℃和S 300℃，饱和温度分别为35℃和40℃，蒸汽质量在0.2到0.7之间，平均热流密度约为6.8 kW/m2。考虑了8种非均匀热流密度分布，包括从上方、侧面和下方加热的变化，并与均匀热流密度分布的结果进行了比较。在所有情况下，都保持相同或可比的总热效率。结果表明，无论质量流量、饱和温度或蒸汽质量如何，均匀热流密度条件下的内壁换热系数都最高。与均匀热流密度情况相比，非均匀热流密度条件下的平均换热显著降低，且在较高的水汽质量条件下变得更差。根据不同的热流情况，换热性能降低了大约45%，对于从下加热的情况，观察到了大约25%的降低。基于反问题模型求解的周向局部换热系数，得出了外加热流密度分布值得关注和研究的结论。总压降不受热流条件的影响。
Flow boiling is an important energy transfer mechanism in several thermodynamic power cycles. In direct steam generation concentrated solar power applications, flow boiling occurs under non-uniform heat flux conditions, such as in horizontal tubes which are predominantly heated from below. This can influence the local and average heat transfer coefficient. In this experimental study the heat transfer characteristics of saturated flow boiling under different circumferentially non-uniform heat fluxes were determined. To demonstrate the influence of a non-uniform heat flux, tests were conducted on R245fa in an 8.5 mm inner diameter tube at a mass fluxes of 200 kg/m²s and 300 kg/m²s, at saturation temperatures of 35 °C and 40 °C, for vapour qualities between 0.2 and 0.7, and for an averaged heat flux of approximately 6.8 kW/m2. Eight non-uniform heat flux distributions, including variations of heating from above, from the side and below, were considered and their results were compared against those of a uniform heat flux distribution. In all cases, the same or comparable total heat rate was maintained. It was found that uniform heat flux conditions exhibited the highest inner wall heat transfer coefficients, irrespective of the mass flux, saturation temperature or vapour quality under consideration. Compared to the uniform heat flux cases, non-uniform heat flux conditions exhibited a significantly reduced average heat transfer which became worse at higher vapour qualities. Depending on the heat flux scenario, heat transfer performances reduced by up to approximately 45%, and for conditions where heating was applied from below, a reduction of approximately 25% was observed. Based on the circumferential local heat transfer coefficients, that were solved via an inverse problem model, it was concluded that the applied heat flux distribution needs more attention and research. The overall pressure drop was not affected by the heat flux condition.