2. Thesis and Dissertations

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Subject: search.filters.subject.Solar air heater

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    Enhancement of Heat Transfer in Solar Air Heater Using Porous Media
    (National Institute of Technology Karnataka, Surathkal., 2024) Rawal, Diganjit Shashikant; N., Gnanasekaran
    The solar air heater (SAH) is very much useful to dry the vegetables, fruits etc. It works on the basis of solar radiation available at the respective location. So, it saves the electricity i.e. reduces the dependency on the fossil fuel. SAH is easy to operate, simple in design. It has less maintenance. The limitations of the SAH are low thermal efficiency due to handling of large volume of air. Also, the air has low thermal capacity. Hence, this problem can be solved by using addition of different metal porous media of different porosities and pores per inch (PPI) inside the empty channel SAH. Initially the empty channel rectangular domain single pass solar air heater (SPSAH) is designed analytically for 0.03 to 0.05 kg/s mass flow rates. The same has been validated with 2D geometry numerical study in ANSYS fluent software to observe the accuracy and performed the comparative study of thermal performance of SPSAH. Then, the discrete arrangement at equal distance of copper metal foam having thickness of 22, 44 and 88 mm of thicknesses with 10 (Φ=0.8769), 20 (Φ=0.8567), and 30 (Φ=0.92) PPI has used to test the thermohydraulic performance parameter in order to enhance the heat transfer in SPSAH. The numerical study shows that the 22 mm thick metal foam is 5.02 % and 16.61 % higher THPP than 44 and 88 mm thick metal foam. The 3D geometry further developed by Rosseland radiation model with solar ray tracing method has been used to account for solar radiation. Renormalization group (RNG) k-epsilon enhanced wall function with local thermal equilibrium (LTE) model has been considered to obtain the heat transfer characteristics in numerical study. Aluminium wire mesh samples with 3 (Φ=0.894), 9 (Φ=0.812), and 18 (Φ=0.917) PPI has been used for numerical and experimental study. The configurations has been combined together to form graded wire mesh (GWM) including 3-9-18, 9-18-3 and 18-3-9 of 5 mm thickness for each wire mesh. The THPP of 9 18-3 PPI wire mesh are 13.04 % and 11.92 % higher than the other two cases. Later, 25 % of 9-18-3 GWM has been considered at four different locations, i.e. 0, 0.5, 1 and 1.5 m away from the inlet and analysed best location for efficient heat transfer. 1.5 m away from the inlet is the best location among the different locations. The experiment results of GWM at 1.5 m away from the inlet demonstrated 20.91 % and 23.32 % increase in thermal efficiency compared to empty channel for the 0.027 and 0.058 kg/s mass flow rates respectively. Further investigations involve a comparative study of transverse arrangement of three layers of 9 (Φ=0.812), and 18 (Φ=0.917) PPI one over the another and same quantity of wire mesh of 50% is folded in the same direction. So, these combinations are (9-18, 18-9) T, (9-18, 18-9) L to obtain GWM in transverse and longitudinal direction, respectively. THPP of 9-18 L has an average of 2.45 % higher than all other combinations mentioned above. The thermal efficiency of 9-18 L has an average of 63.45 % higher than the empty channel for 0.05 and 0.08 mass flow rates. The GWM arrangement reduce the time of drying by 26.78 % and 55 % for tomato and onion respectively compared to empty channel SPSAH. The assessed simple payback period is 0.89 year which is shorter than lifespan of the SPSAH which is profitable. The present study was also focused on the solar panel battery operated fan and all the measuring instruments operated by battery which reduces the dependency on the external electric supply.
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    Computational and Experimental Study of Solar Air Heater With Various Duct Cross-Sections and Artificial Roughness
    (National Institute of Technology Karnataka, Surathkal, 2022) K, Nidhul; Yadav, Ajay Kumar; S, Anish
    Thermo-hydraulic performance and exergetic efficiency of solar air heater (SAH) with various duct cross-sections and artificial roughness have been investigated using numerical and experimental methodology. The RNG k- model with enhanced wall treatment is employed to study the turbulent flow behavior. Validation of the CFD results for smooth and artificially roughened SAH (triangular duct and duct with semi- cylindrical sidewalls) with theoretical correlations and experimental data indicates reasonable accuracy. In triangular duct SAH, the performance of inclined ribs and V-ribs have been studied e/D) and pitch (P/e). It is observed that V-ribs in triangular duct provides a maximum thermo- hydraulic performance parameter (THPP) of 2.01 with a 23% enhancement in exergetic efficiency compared to smooth SAH. Further, the performance of triangular duct SAH with inclined ribs in an indirect type solar dryer is studied. Dryer with ribbed triangular duct SAH exhibits a 60.4% and 55% reduction in moisture ratio for food samples robusta and nendran, respectively, for the same drying time compared to a dryer with a ribbed rectangular duct SAH. In addition, the design enhances the drying characteristics with 93.3% increase in average diffusivity coefficient for banana food samples. CFD analysis of SAH design with semi-cylindrical sidewalls and continuous W-baffles provides THPP in the range of 1.70 to 2.27. Maximum enhancement in thermal and exergetic efficiency is obtained as 40.7% and 95.4%, respectively, relative to conventional SAH at Re = 5000. Based on the optimum results obtained from CFD, an experimental setup for SAH with semi-cylindrical sidewalls and multiple discrete inclined baffles is fabricated. The experimental results indicate that THPP is further enhanced for discrete inclined baffles with the gap at the trailing apex, with a peak value of 2.69. This design has higher collector efficiency (55 to 70%) compared to ribbed rectangular SAH design exhibiting 30 to 55%. Further, the design exhibits higher exergetic efficiency owing to lower exergy losses and higher collector efficiency. Maximum exergetic efficiency of 2.2% is obtained at lower Re, higher than that obtained for rectangular duct SAH with a similar kind of artificial roughness. In addition, at low Re, this SAH design has a higher coefficient of performance (COP) than conventional SAH designs. Hence, a SAH design having lower number of sharp corners and artificial roughness capable of generating multiple secondary flow can enhance the heat transfer rate with higher thermo-hydraulic performance.