Development of water heater using tubular flame - Heat transfer characteristics on the coiled tube and the inserted tube heat exchangers

The purpose of this study is to develop the compact auxiliary water heater using tubular flame for home CHP (combined heat and power). The tubular-flame water-heater consists of the tubular flame burner, the coiled tube heat exchanger and the inserted tube installed at the center of the coiled tube as an additional heat exchanger. The tubular flame is formed in a rotating flow field by injecting the air-fuel mixture tangentially into the cylindrical chamber. The swirling flow of the combustion gas flows through the gap between the coiled tube and inserted tube. The high heat transfer performance can be achieved on these heat exchangers without the expanded heat transfer surface due to the swirling flow. Thus, the compact water heater can be constructed by using the tubular flame burner. At the first stage of the development, the maximum thermal efficiency achieves 81 % at the combustion rate of 15 kW. The heat transfer characteristics on the coiled tube and the inserted tube is investigated under the combustion and non-combustion conditions. Heat transfer coefficient on the coiled tube has about 5.6 times higher than that under the no-swirling flow condition at the same flow rate. Nusselt number on the coiled tube heat exchanger and the inserted tube can be estimated by Dhir's correlation equation for the swirling flow, which was provided as a function of the ratio between tangential momentum and axial momentum. The heat transfer characteristics on not only the coiled tube but the inserted tube was dominated by the swirling convection flow.