Abstract A 3D Numerical study of mixed convection air flow in upward solar air heater with large spanwise aspect ratio (A = 10 to 40) was performed using CFD commercial code Fluent 14.5 (ANSYS). The main objective of this study is to investigate the channel height's effect (aspect ratio) on flow pattern and heat transfer in upward solar air heater in the particular case of low Re and high aspect ratio. The bottom plate (absorber) was submitted to Constant Heat Flux (CHF) in the range of 200 to 1000 W/m2 and Reynolds number was varied from 50 to 1000. Our results are in concordance with most of authors conclusions about Poiseuille–Rayleigh–Benard flows. In mixed convection, increasing heat flux enhances heat transfer unlike forced convection flows. Simulation results of flow visualizations and Nusselt number calculations have shown that depending on Ri*, the velocity and temperature distributions in SAH vary greatly with the channel's height. The obtained results were different from previous studies. Indeed, our investigation of channel's height was achieved for the same heat flux but different Grashof numbers. For low channel's heights (high aspect ratio), increasing heat flux has not a significant effect but for higher channel's heights, an augmentation of heat flux enhances buoyancy effects in the flow and causes high turbulence. Also, increasing Reynolds number in low channel's heights (high A), can enhance substantially heat transfer. For higher channel's heights (low A), increasing Reynolds number decreases Ri* and thus buoyancy forces. Heat transfer is reduced and so Nusselt number. The obtained results may be very useful for engineers in designing and testing solar collectors.

中文翻译：

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长宽比对向上太阳能空气加热器混合对流气流影响的3D数值研究

摘要 使用 CFD 商业代码 Fluent 14.5 (ANSYS) 对大展宽比（A = 10 至 40）的向上太阳能空气加热器中的混合对流气流进行了 3D 数值研究。本研究的主要目的是研究在低 Re 和高纵横比的特殊情况下，通道高度（纵横比）对向上太阳能空气加热器的流动模式和传热的影响。底板（吸收器）承受 200 到 1000 W/m2 范围内的恒定热通量 (CHF)，雷诺数从 50 到 1000 不等。我们的结果与大多数作者关于 Poiseuille-Rayleigh 的结论一致。贝纳德流动。在混合对流中，与强制对流不同，增加热通量会增强传热。流动可视化和努塞尔数计算的模拟结果表明，根据 Ri*，SAH 中的速度和温度分布随通道高度变化很大。获得的结果不同于以往的研究。事实上，我们对通道高度的研究是针对相同的热通量但不同的 Grashof 数实现的。对于低通道高度（高纵横比），增加热通量没有显着影响，但对于较高通道高度，热通量的增加会增强流动中的浮力效应并导致高湍流。此外，在低通道高度（高 A）中增加雷诺数可以显着增强热传递。对于更高的通道高度（低 A），增加雷诺数会降低 Ri*，从而降低浮力。传热减少，因此努塞尔数减少。获得的结果可能对工程师设计和测试太阳能集热器非常有用。