Effects of tube diameter and submergence ratio on bubble pattern and performance of air-lift pump

Air-lift pump can pump liquid and sediments in it by using air-induced lift force. This working principle can be applied to direct carbon fuel cell (DCFC) to carry or transport high-temperature molten fuel or carbonate in the DCFC system. For this application, pumping performance and essential hydrodynamic features of air-lift pump with high submergence ratio are investigated experimentally. In this study, a model system of air-lift pump is designed and made for this purpose, where air is supplied from the bottom of the tube, flows up with a shape of bubble formed, entrains water inside the tube, and discharges it. Flow rate of water discharged from the top is measured with two variable parameters of tube diameter and submergence ratio. Bubble patterns formed in the tube are observed with various combinations of the two parameters and show four distinct patterns depending on air flow rate. A theoretical model is employed to predict discharge flow rate and it is comparable with the experimental results in slug-flow regime. Cracking flow rate, at which discharge of water is initiated, increases as tube diameter increases and submergence ratio decreases. The flow-rate ratio of water to air, i.e., pumping effectiveness, provides a feasible operating range of air-lift pump in terms of the two parameters.