Bubble dissolution in horizontal turbulent bubbly flow in domestic central heating system

Abstract

In a domestic central heating system, the phenomenon of microbubble nucleation and detachment on the surface of a boiler heat exchanger finds its origins in the high surface temperature of the wall and consequential localised super saturation conditions. If the surrounding bulk fluid is at under-saturated conditions, then after exiting the boiler, the occurrence is followed by bubbly flow and bubble dissolution. A comprehensive understanding of the fundamentals of bubble dissolution in such a domestic wet central heating system is essential for an enhanced deaeration technique that would consequently improve system performance. In this paper, the bubble dissolution rate along a horizontal pipe was investigated experimentally at different operating conditions in a purpose built test rig of a standard domestic central heating system. A high speed camera was used to measure the bubble size at different depths of focal plane using two square sectioned sight glasses at two stations, spaced 2.2m apart. A dynamic model for bubble dissolution in horizontal bubbly flow has been developed and compared with experimental data. The effects of several important operating and structural parameters such as saturation ratio, velocity, temperature, pressure of the bulk liquid flow, initial bubble size and pipe inside diameter on the bubble dissolution were thus examined using the model. This model provides a useful tool for understanding bubble behaviours in central heating systems and optimising the system efficiency.