Experimental investigation of the annular flow caused by convective boiling in a heated annular channel

An experimental investigation was performed on the convective boiling heat transfer of the annular flow in concentric and eccentric annuli with a central heating rod in an unheated tube. Advanced measurement techniques including laser-induced fluorescence (LIF) and confocal chromatic sensor (CCS) were applied to acquire the dynamics and instabilities of the liquid thin film on the tube. The boundary conditions were as follows: heating rod heat flux from 167 kW/m² to 201 kW/m² and mass flow rate from 58 g/s to 155 g/s. The annuli which the flow occurred has a hydraulic diameter of 15.5 mm. As indicated previously by the research on isothermal annular flow in bare tubes, the vapor superficial velocity is the primary factor that influences the liquid film dynamics including the base film thickness and wave amplitude. The liquid film thickness in the eccentric geometry was observed to be constant for liquid superficial velocities from 0.15 m/s to 0.34 m/s and vapor superficial velocities from 6.5 m/s to 13.2 m/s. The heat transfer coefficient ranged from 2.734 kW/m²∙K to 4.279 kW/m²∙K for the concentric geometry and 2.063 kW/m²∙K to 3.096 kW/m²∙K for the eccentric geometry. An increasing trend of the heat transfer coefficient was observed as the liquid superficial velocity increased, whereas the reverse trend was observed for the vapor superficial velocity. When the measured heat transfer coefficient was exceptionally low (< ~2.0 kW/m²∙K), the boiling condition was assumed to be the dry out. An extended range of boundary conditions reported in this study (from the annular flow to the dry out regime) can provide useful data sets for validating computer codes. In particular, the detailed information on liquid film thickness, wave characteristics, and heat transfer coefficient provided in the present paper can help develop annular flow boiling models in computational fluid dynamics.