The Characteristics of Pressure Drop and Heat Transfer of Coils Used in Solar Collectors

Helically coiled tubes have good performance in heat transfer exchangers, and have been paid more attention to in several fields of new energy and sustainable energy. With the application in solar energy collections, the characteristics of pressure drop and heat transfer coefficient need to be studied clearly. For this, experiments and simulations of coils for flow and heat transfer in solar collector have been conducted in this paper. The effect of regularity of coils parameters of tube diameter, coil radius and coil pitches etc. on pressure losses and heat transfer coefficients were observed and discussed, respectively. It was found that under the experimental conditions of lower mass flow rate, the flow resistance pressure drop of coiled tube is larger than those of the other pipes, especially when the Re number is more than 15000 under the high mass flow rate. And the flow resistance pressure drop of bigger plate coils is smaller, and the larger the mass flow rate is, the greater the difference will be. For heat transfer characteristics, the heat transfer coefficient of coiled tubes showed a trend of increase with increasing the curvature of the spiral, and the length to diameter ratio on heat transfer coefficient of the influence for plane spiral tube. At the same time, experimental and simulation results found that length to diameter ratio of coiled tube has little effect under the experimental conditions and the effect of helical pitch on heat transfer coefficient is the least among the above factors.