Effects of vertical coil springs on the performance of solar air heaters: Experimental study

Using extended surfaces or turbulators on the absorber plate (AP) is one of the great ways to enhance the thermal performance of solar air heaters (SAHs). Although a large number of studies have been conducted in this field, new geometries are still being proposed to improve the tradeoff between heat transfer enhancement and cor-responding blowing power penalty. Coil springs are new offerings as turbulators, and the scientific literature currently lacks a parametric study to address the benefits of using such extended surfaces. To cover this gap, an experimental analysis of the AP equipped with vertical coil springs is carried out. The effects of different geometrical factors (coil height, coil pitch, coil diameter, and wire diameter) at three different levels are investigated for Reynolds number ranging from 5867 to 23471. One of the most important findings of this research is that, unlike most other turbulators, the heat transfer enhancement obtained using CSs is often greater than the resulting pressure drop. Therefore, the overall performance index is found to be always higher than unity, reaching a maximum value of 2.98 for the case with a coil diameter of 10 mm at Re = 5867. It is worth mentioning that upon varying the coil height from 3 mm to 9 mm, the highest Nusselt number and best overall performance are achieved at 6 mm. It is found that the overall performance of SAH declines by increasing both the coil pitch and the coil diameter. Comprehensive correlations are developed considering the effects of all geometrical parameters. The results predicted by the correlations match well with the experimental data with average errors within +/- 10%.