eulerian eulerian approach

Boiling process in a heated tube is commonly used in different industries such as electronic equipment cooling, power plant, and air conditioning systems. Despite the significance of thoroughly and separately analyzing of heat transfer in different two-phase flow regimes encountered in boiling process, just a few simulations have been conducted. That is because of the lack of proper understanding of the many numerical methods that are now in use and their relative efficacy under various circumstances. This leads to dispersed effort and the application of disparate numerical methods, which incurs significant computational expenses. In this study, Eulerian-Eulerian approach was used to simulate the bubbly flow, which includes vapor bubbles in the rising water flow within a vertical tube. In order to identify the optimal numerical model and the extent of application of available numerical models in the simulation of bubbly flow, volume of fluid (VOF) and Eulerian boiling model of Rensselaer Polytechnic Institute (RPI) models were compared and evaluated. Results demonstrated that while the RPI boiling model results are more appropriate for estimating the heat transfer coefficient and wall temperature in this regime, the VOF model is more effective than the RPI model at simulating the regime, bubble formation and interface between phases. Moreover, RPI model was used to examine how changes in wall heat flux and inlet mass flow rate affected effective parameters. Results revealed that in the bubbly flow regime, a 100% increase in wall heat flux relative to its original value of 5000 W/m2, resulted in a 150% increase in the outlet vapor quality, a 75% rise in temperature difference between the wall and the saturation temperature, and a 20.8% increase in the mean wall heat transfer coefficient. Furthermore, by increasing the inlet mass flow rate, the nucleate boiling zone increases and the outlet vapor quality decreases.

The hydrodynamic behaviour of air-glass beads bubbling fluidized bed reactor containing spherical glass beads is numerically studied, using OpenFoam v7 CFD software. Both Gidaspow and Syamlal-O'Brien drag models are used to calculate momentum exchange coefficients. Simulation predictions of pressure loss, bed expansion rate, and air volume fraction parameters were compared and validated using data, existing in the literature obtained experimentally and performed by other numerical softwares. Pressure loss and rate of bed expansion were calculated with relative root mean square error (RMSE) equal to 0.65 and 0.095 respectively; Syamlal-O'Brien model is considered more accurate than Gidaspow model. Hence, numerical model reliability developed on OpenFoam was also proved. The hydrodynamic aspect study of the fluidized bed reactor was then performed, to analyse the impact of inlet air velocity (U) on particles motion. It was revealed that with U increment, air and glass beads axial velocities increase in the reactor centre and decrease in the sidewalls. Thus, a greater particle bed expansion is induced and the solid particles accumulated highly on the reactor sidewalls. In general, with the increase of U, the solid volume fraction decreases from 0.63 to 0.58 observed at 0.065 m/s and 0.51 m/s, respectively.

Wall-to-bed (or wall-to-fluid) heat transfer issues in trickle bed reactors (TBR) has an important impact on operation and efficiency in this category of reactors. In this study, the hydrodynamic and thermal behavior of trickle bed reactors was simulated by means of computational fluid dynamics (CFD) technique. The multiphase behavior of trickle bed reactor was studied by the implementation of the Eulerian-Eulerian multiphase approach. Also, bed porosity effect was modeled by porosity function method. In order to study the effect of operating parameters on wall-to-bed heat transfer, the influence of catalyst particle diameter and catalytic bed porosity was investigated on wall-to-bed Nu number. The results showed that the enhancement of catalytic bed porosity from 0.36 to 0.5 decreases the Nu number about 15% due to a reduction of liquid velocity adjacent to the reactor wall. Also, the increase of particle diameter from 4 to 6 millimeter decreases wall-to-bed Nu number about 15% owing to a reduction in liquid phase volume fraction.