Abstract
In the present decade, research regarding solar thermal air heaters (SAHs) has noticed a continuous progression in thermo-hydraulic performance augmentation approaches. There now exists a wide variety of thermo-hydraulic performance augmentation approaches and researchers have designated various structures. Nevertheless, there seems to be no generalization to any of the approaches employed. The present numerical investigation reports on the thermo-hydraulic characteristics and thermal performance for flow through a varied length (full, medium, half, and short length) dimple solar air heater (SAH) tube. The study highlights recent developments on enhanced tubes to augment heat transfer in SAH. The influence of different length ratio, dimple height ratio (H), and pitch ratio (s) on thermo-hydraulic characteristics have been investigated in the Reynolds number (Re) range from 5000 to 25,000. Air is used as the working fluid. The commercial software ANSYS Fluent is used for simulation. The shear stress transport (SST) model is used as the turbulence model. Thermal energy transport coefficient is increased in the full-length dimple tube (FLDT), compared to the medium-length dimple tube (MLDT), half-length dimple tube (HLDT) and short-length dimple tube (SLDT). Similarly, the pitch ratio (s) has more influence on Nusselt number (Nu) compared to the dimple height ratio (H). The friction factor decreases with an increase in pitch ratio. Nu increases and f decreases with increasing Re for all combinations of H and s. Low s and higher H yields high enhancement of HT and PD. Integration of artificial roughness on the tube increases the values of Nu and f by 5.12 times and 77.23 times for H = 0.07, s = 1.0 at Re value of 5000 and 25,000, respectively, in regard to the plain tube. For all the tested cases, the thermo-hydraulic performances (η) are greater than unity.
Highlights
The last few decades have marked the acceptance of solar air heater (SAH) applications, which have flagged the way and highlighted the importance of solar energy use
One of the notable merits of solar air heating is the lack of requirement for heat exchange from one working fluid to another working fluid, i.e., the air itself serves as the working fluid without any corrosion or sealing issues
The effects of dimples height ratio, pitch ratio, and length of the dimpled tube on thermal-hydraulic performance are discussed in a detailed manner
Summary
The last few decades have marked the acceptance of solar air heater (SAH) applications, which have flagged the way and highlighted the importance of solar energy use. Many investigators have talked, established, and contributed towards understanding and strengthening the fundamental aspects associated with solar air and water heating applications. Air heating is a common practice for the utilization of solar energy. Heat transfer augmentation techniques can be classified into two groups: (i) active methods, (ii) passive methods [1,2,3]. There have been many laboratory experiments on heat augmentation techniques that utilize twisted tape as a passive technique [4,5,6,7,8,9,10,11].
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