Numerical investigation of mist flow regime in a vertical tube

• Mist flow regime in a heated vertical tube is investigated numerically.
• Discrete phase model is adopted to simulate mist flow regime.
• The impacts of vapor quality and droplets diameter on heat transfer rate are investigated.
• Thermodynamic equilibrium is investigated in different conditions in mist flow regime.

In this research, the mist flow regime, consisting dispersed water droplets in vapor flow, is simulated numerically in a vertical tube; using a discrete phase model (DPM). In this method, in addition to solving transport equations for continuous phase, a discrete phase is simulated in a Lagrangian approach and the coupling between phases is modeled through interaction terms in the transport equations. The aims of this research are to investigate thermal equilibrium and evaluate the heat transfer coefficient of mist flow regime. The results show, when thermodynamic non-equilibrium takes happen, the rate of heat transfer from the vapor to the droplets is too slow that their presence is ignored, therefore by increasing the water mass flow rate in this case, due to reduction of vapor mass flow rate, the heat transfer coefficient decreases. But when complete thermodynamic equilibrium condition is established, the rate of heat transfer from the vapor to the droplets is too fast that the vapor temperature remains at the saturation temperature until all the droplets have been evaporated and by increasing the water mass flow rate, the heat transfer coefficient will increase. In order to simulate mist flow regime in thermal equilibrium and non-equilibrium conditions, water droplets with two different diameters are injected into steam flow. The numerical results of heat transfer coefficient and wall temperature in four different vapor qualities are investigated in each state, which show good agreement with experimental data and correlations.