Abstract:The device performance of cross-corrugated double-pass solar air heaters under external recycle conditions was investigated experimentally and theoretically, and solved numerically using the Newton method. Comparisons were made and represented graphically among three different configurations of the single-pass, flat-plate double-pass and cross-corrugated double-pass devices. Air flowing simultaneously over the wavelike corrugated absorbing plate and in-between both wavelike cross-corrugated absorbing and transverse bottom plates was conducted under double-pass operations. A considerable heat-transfer efficiency enhancement is obtained employing such a recycling double pass device with welding cross-corrugated absorbing plates, instead of using the flat-plate device. An effective thermal performance was achieved because the heat transfer area is doubled and the turbulent intensity is enhanced as well. The power consumption increment owing to the reduction in the cross-sectional area was taken into account associated with the heat-transfer efficiency enhancement for comparisons in determining the optimal design on an economic consideration for the recycling cross-corrugated double-pass device.

Abstract:The influence of recycling on double-pass solar air collectors with welding of the V-corrugated absorber has been studied experimentally and theoretically. Welding the V-corrugated absorber and the recycle-effect concept to the solar air collector was proposed to strengthen the convective heat-transfer coefficient due to turbulence promotion. Both the recycle effect and the V-corrugated absorber can effectively enhance the heat transfer efficiency compared to various designs such as single-pass, flat-plate double-pass, and double-pass wire mesh packed devices. Recycling operations and welding the V-corrugated absorber could enhance the collector efficiency by increasing the recycle ratio, incident solar radiations, and air mass flow rates. The most efficient and economical operating conditions were found at R ≈ 0.5, with relatively small hydraulic dissipated energy compensation. It was found that the turbulence intensity increase from welding the V-corrugated absorber into the solar air collector channel could compensate for the power consumption increase, when considering economic feasibility.

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The effect of external-recycle operations on the thermal performance of double-pass cross-corrugated solar air heaters (SAH) under different operating conditions was investigated experimentally and theoretically. Additionally, the simultaneous ordinary equations were solved analytically for each proposed configuration. Four recycling types are introduced for improving the solar thermal performance with different external recycle processes, which are expected to enhance the heat transfer coefficient with a convective turbulent flow between the air and the absorber in the present study. Using recycling double-pass operations, two processes were conducted sequentially: air first flowed over the sinusoidal corrugated absorber plate and then flowed back later over the transverse sinusoidal corrugated bottom plate. Improved device performance was achieved due to the doubled heat transfer area over and under the corrugated absorber plate, from which both the sinusoidal cross-corrugated absorber plate and bottom plate enhanced turbulent intensity. Theoretical predictions and experimental results both indicated that the recycle ratio increased with the SAH thermal efficiency for all proposed designs. The results show a higher heat transfer efficiency for the proposed four configurations using wavelike corrugated plates compared to those conducted in single-pass and flat-plate absorber plates with up to a maximum 133% (from 0.301 to 0.703) increment. The optimal device performance was examined for all external-recycle configurations under the same working dimensions and operational conditions. The best configuration for optimal thermal performance was the device that lengthened the air flow pathway and increased the air velocity within the collector; thus, a higher heat transfer rate was accomplished. The evaluation of increments in the power consumption and of the heat-transfer efficiency enhancement together determined the optimal design based on an economic consideration across various configurations of cross-corrugated double-pass devices.