Measurement of bubble detachment diameters from the surface of the boiler heat exchanger in a domestic central heating system

Wet central heating systems are used throughout the world and the domestic variants are known to account for 16% of the carbon dioxide emissions in the UK. It is an industry wide fact that significant quantities of air leak into the system’s closed loop circuit, mostly due to thermal cycling during routine use. The water in the system results in high dissolved gas saturation levels thus resulting in microbubble nucleation and detachment at the boiler wall. An in depth knowledge into the behaviour of microbubbles in such systems could result in better deaeration systems thus improving the overall system efficiency. In this study, we will report a measurement of the bubble detachment sizes in this system at different operating conditions, these being, the system flow rate, pressure, heating load and saturation ratio. The Winterton models for zero and finite contact angles for the prediction of the bubble detachment diameters in supersaturated solutions were used as a comparison with the test results. The model for the prediction of bubble detachment diameters at finite contact angles in round pipes has yielded good predictions. A new correlation has been proposed so as to include the effects of pressure and heat flux on the bubble characteristics which were not represented through the Winterton model.

► We investigate the expected bubble sizes in wet domestic central heating systems.
► Microbubbles are formed due to gas supersaturation conditions.
► We have investigated the bubble detachment size with the system parameters.
► The fluid velocity has the greatest effect on the bubble detachment size.
► We correlated the force balance model by Winterton (1972) to fit our results.