Thermo-hydrodynamic modeling of flow boiling through the horizontal tube using Eulerian two-fluid modeling approach

Abstract

The flow boiling process is common in many industrial applications as well as direct steam generation (DSG) in the receiver of parabolic trough solar collector (PTSC) and linear Fresnel reflector (LFR) systems. The objective of the present work is to investigate the flow boiling heat transfer through the horizontal tube numerically for the various potential applications in the DSG process in the solar collectors. The thermo-hydrodynamic study of flow boiling through the horizontal tube is presented through the two-fluid model (TFM) approach using computational fluid dynamics (CFD) software, ANSYS Fluent 19.0. Three dimensional (3-D), steady-state numerical simulations are performed by applying the Eulerian multiphase critical heat flux (CHF) boiling model. The flow conditions considered are similar as in the DSG in the solar collectors. The simulations are performed for 12 m length of the horizontal stainless-steel tube having inner and outer diameter 70 mm and 50 mm respectively with the mass flow rates ranging from 0.3 kg/s to 0.6 kg/s, operating pressures 30 to 100 bar, and wall heat flux 17.74 kW/m2. The variations in the pressure drop, vapor volume fraction, circumferential wall temperature, contours of vapor volume fraction, velocities have been predicted under the considered operating conditions. The pressure drop in the tube for the various operating conditions considered is in the range of 115 Pa to 426 Pa and the pressure drop is observed higher at the lower operating pressure. The pressure drops are observed as 222.4 Pa, 145 Pa, and 115.2 Pa respectively for the operating pressure of 30 bar, 60 bar, and 100 bar at the mass flow rate (MFR) of 0.3 kg/s. The large thermal gradient in the tube wall is observed in the circumferential direction at the liquid-vapor interface. The minimum and maximum value of ∆T (∆T = Tmax – Tmin) around the circumference are observed as 28.3 K and 77.1 K respectively for the given operating conditions. The circumferential temperature difference decreases with an increase in the operating pressure. The value of ∆T around the circumference is observed for the MFR of 0.3 kg/s as 77.1 K, 58.1 K, and 45.7 K respectively for operating pressure of 30 bar, 60 bar, and 100 bar. The contours of tube wall temperature at various axial positions have been plotted. The vapor volume fractions for the MFR of 0.3 kg/s are 0.6, 0.51, and 0.45 respectively for operating pressures of 30 bar, 60 bar, and 100 bar. The vapor volume fraction (VVF) decreases at the outlet of the tube as the MFR or operating pressure increases. The mixture velocity and the relative velocity between the phases have been predicted. The present study concluded that higher operating pressure is more suitable in terms of pressure loss and the thermal gradient. The presented numerical model is useful for the thermal performance analysis of DSG in the PTSC as well as the LFR system and other industrial applications having flow boiling through the horizontal tubes.