Abstract

Flat-plate solar collectors are used in solar water heating systems to transfer solar energy to water in the form of heat. The research on solar water heating systems is mainly focused on the design and optimization of the overall system with little attention given to study the thermofluid behaviour inside the collector. The collector is a very crucial part of the system as it is where the actual energy conversion talks place. In the present research, the thermofluid behaviour within the tube of a flat-plate solar collector for a forced circulation solar water heating system is experimentally investigated. Particle Image Velocimetry (PIV) was used to measure the velocity field within the collector tube, while thermocouples were used for temperature measurements. The experiments were performed for six flow rates under one unheated and two heated conditions. The mean velocity profiles for heated and unheated conditions show that the flow behaviour within the collector tube is significantly modified by the heating. With the heat addition, the location of the maximum velocity is in the lower half of the tube, and the magnitudes of mean and turbulent velocities increased with the heat flux. The temperature data indicated the formation of stably stratified layer of water in the upper region of the tube, which reduces the heat transfer rate. The heat transfer analysis showed that free convection is the dominant mode of heat transfer within the flat-plate solar collector.