Experimental investigation of gas bubble diameter distribution in a domestic heat pump water heating system

A significant amount of national energy in the UK has been consumed in the residential sector mainly for space heating and domestic hot water production. Currently, gas boilers are mostly applied in the domestic water heating which contribute significantly to excessive CO2emissions and consumption of primary energy resources. The issues can be solved by popularizing heat pump systems that are attributable to higher performance efficiency than those of traditional gas boilers. Even so, the heat pump system performance can be further enhanced and improved if the dissolved gases in its hot water circuit can be efficiently discharged. To achieve this target, the bubble behaviors in the heat pump water heating system need to be extensively investigated. Based on a purposely built test rig of an air-water heat pump system, the effects of various heat pump hot water side parameters on gas microbubble diameters were measured and analyzed through different experimental conditions. The operational parameters include water flow rate, water flow pressure, system heating load and water flow dissolved gas saturation ratio. It is found from the measurements that the hot water flow rate is the dominate parameter which affects the average bubble diameter and bubble number distribution at different bubble sizes. The research outcomes can significantly contribute to the understanding of gas bubble behaviors at domestic heat pump water heating systems and thus the efficient way for the discharging of the associated dissolved gases.