The optimum position of water heat transfer coils downstream of a radial swirler in a 20kw heater

Abstract

A 76mm outlet diameter radial swirler with 8mm vane depth was investigated in a 140mm combustor diameter condensing 20 kW ultra-low NOx boiler. The aim was to show that small turbulent flames could achieve compact ultra-low NOx water heating. Low NOx was achieved using lean well mixed low flame temperature combustion with a 0.7 equivalence ratio (Ø). Thermal NOx formation was also minimised by cooling the flame downstream of the swirler outlet. A water cooled heat transfer coil was traversed into the flame to determine how close to the swirler exit the heat transfer could occur, without a major increase in the combustion inefficiency. This was shown to be 70mm from the radial swirler throat outlet. Rapid fuel and air mixing was achieved using fuel injection through the wall of the 76mm swirler outlet throat, assisted by a 41mm diameter outlet orifice at the exit of the 76mm internal diameter wall fuel injector. This created swirling flow with higher axial velocities and a more concentrated high turbulence region downstream of the orifice outlet. A 4 mb burner pressure loss was used, which is typical of domestic forced draught combustion systems. The air inlet temperature was 400K, which is typical of reverse air flow cooled combustion chambers at domestic water heater conditions. The strong swirling flow interaction with the heat exchanger coil give an 89% thermal efficiency with the front of the coil 70mm from the swirler outlet. The emissions measurements showed that the combustion inefficiency was below 0.1%, the CO/CO2 ratio <0.001 and the NOx emissions were 5ppm at 0% oxygen with the heat exchanger at 70mm from the radial swirler outlet. This design easily met the 2018 EU legislation for eco-design of domestic water heaters.