Enhancement of Heat Transfer in Solar Air Heater Using Porous Media

Abstract

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.