97/02961 Heat transfer and fluid flow studies in the collector tubes of a closed-loop natural circulation solar water heater

The water-in-glass evacuated collectors are made up of parallel circular tubes. They are installed with some inclination angle to the horizontal. The thermal performance of water-in-glass evacuated tube solar water heater heavily depends on weather conditions. The analysis of the sensitivity of the model parameter and weather conditions on heat transfer process is extremely important to install a solar water heater system in order to achieve its maximum efficiency. The evaluation of the sensitivity of the system parameters is done by considering one parameter after another while keeping the remaining fixed. Further to the analysis of the heat transfer process, the average heat transfer coefficient and the average natural circulation flow rate are calculated. The fluid flow is assumed to be unsteady, two-dimensional, laminar and incompressible. The heat and fluid flow are analyzed using the Navier-Stokes equations and temperature equation for an incompressible fluid, subject to density variation with temperature. The discretization of the governing equations is done by Finite Volume Method (FVM). The Open FOAM computational fluid dynamic software with PISO-SIMPLE algorithm is used for the simulation. The results show that the heat transfer process is improved when there is a moderate level inclination angle. Further, it is found that when the ratio of tube length to diameter is high, the heat transfer process is improved. The solar radiation input highly affects the performance of a solar water heater. The cold-water inlet temperature does not directly affect the buoyancy induced flow, but it influences the temperature gain. The angle of the solar rays vary within the daytime, however it does not affect the performance of the solar water heater since an evacuated-tube has a circular absorbing surface, it passively tracks the sun throughout the day. These results recommend using moderate level tube inclination angle and high ratio to improve the performance of a solar water heater.

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Parametric optimization of a stamped longitudinal vortex generator inside a circular tube of a solar water heater at low Reynolds numbers

SN Applied Sciences ◽

10.1007/s42452-021-04723-0 ◽

2021 ◽

Vol 3 (8) ◽

Author(s):

Felipe A. S. Silva ◽

Luis Júnior ◽

José Silva ◽

Sandilya Kambampati ◽

Leandro Salviano

Keyword(s):

Heat Transfer ◽

Reynolds Number ◽

Pressure Drop ◽

Reynolds Numbers ◽

Geometric Parameters ◽

Vortex Generators ◽

Longitudinal Vortex ◽

Solar Water Heater ◽

Water Heater ◽

Solar Water

AbstractSolar Water Heater (SWH) has low efficiency and the performance of this type of device needs to be improved to provide useful and ecological sources of energy. The passive techniques of augmentation heat transfer are an effective strategy to increase the convective heat transfer coefficient without external equipment. In this way, recent investigations have been done to study the potential applications of different inserts including wire coils, vortex generators, and twisted tapes for several solar thermal applications. However, few researchers have investigated inserts in SWH which is useful in many sectors where the working fluid operates at moderate temperatures. The longitudinal vortex generators (LVG) have been applied to promote heat transfer enhancement with a low/moderate pressure drop penalty. Therefore, the present work investigated optimal geometric parameters of LVG to enhance the heat transfer for a SWH at low Reynolds number and laminar flow, using a 3D periodical numerical simulation based on the Finite Volume Method coupled to the Genetic Algorithm optimization method (NSGA-II). The LVG was stamped over a flat plate inserted inside a smooth tube operating under a typical residential application corresponding to Reynolds numbers of 300, 600, and 900. The geometric parameters of LGV were submitted to the optimization procedure which can find traditional LVG such as rectangular-winglet and delta-winglet or a mix of them. The results showed that the application of LGVs to enhance heat transfer is an effective passive technique. The different optimal shapes of the LVG for all Reynolds numbers evaluated improved more than 50% of heat transfer. The highest augmentation heat transfer of 62% is found for the Reynolds number 900. However, the best thermo-hydraulic efficiency value is found for the Reynolds number of 600 in which the heat transfer intensification represents 55% of the pressure drop penalty.

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