This case study models the boiling process in riser tubes using Flownex, coupled with 3D CFD to calculate accurate, non-uniform heat fluxes through side walls for combustion simulations. The combined approach improves boundary conditions and system behavior by simulating liquid phase change, enhancing the accuracy of both CFD and fluid network models.
CHALLENGE
The objective of this study is to model the boiling process in the riser tubes with Flownex and calculate the heat fluxes through the side walls, as needed for accurate CFD models of the combustion process.
BENEFITS
This case study demonstrates the capability of Flownex to do simulations where Flownex and a 3D CFD code are coupled.
SOLUTION
The results illustrate the advantages of using the combined approach to model coupled problems in situations where previous (uncoupled) assumptions in simulations proved to be rather inaccurate. In this instance, the ability to model the liquid phase change in the pipes enabled the calculation of more realistic, non-uniform wall heat fluxes for the CFD model.
By linking the two different methods, improved boundary values and system behaviour could thus be obtained for both the CFD model and fluid network model.
INTRODUCTION
This case study demonstrates the capability of Flownex to do simulations where Flownex and a 3D CFD code are coupled.
SYSTEM DESCRIPTION
The illustration in Figure 1 shows the boiler configuration considered in this example. Inlet mass flow and pressure for the air, fuel and water inlets are known and the combustion is modelled with CFD.
OBJECTIVE OF SIMULATION
The objective of this study is to model the boiling process in the riser tubes with Flownex and calculate the heat fluxes through the side walls, as needed for accurate CFD models of the combustion process.
FLOWNEX MODEL
The combined CFD and Flownex models of the system are shown in Figure 2 and Figure 3. The riser tubes are modelled in Flownex using pipe elements with external heat sources and boundary flow conditions on inlet and outlet nodes. Two pipe sections represent front and back riser tubes that combine before entering the boiler superheater (modelled using the heat exchanger element). The gas flow from the boiler outlet is linked to the inlet conditions of the primary side of the superheater.
DESCRIPTION OF SIMULATION
The fixed inlet temperature and pressure are specified on the riser tubes and the required steam mass flow is specified at the outlet of the heat exchanger.
The combustion process and gas flow through the boiler is simulated with CFD. Heat flux through the side walls is linked to Flownex, which calculates the steam quality in the riser tubes. The pipe temperatures are linked back to the CFD model as boundary conditions.
The process of transferring data is repeated until convergence is reached for both the Flownex network model and the detail CFD model.
RESULTS
The temperature distribution in the boiler is shown in Figure 4. Results from the Flownex model for the element temperatures and heat fluxes are shown in Figure 5 and the steam quality for the different nodes is shown in Figure 6. The steam quality is seen to increase due to the higher temperatures at the top of the boiler and results in superheated steam in the topmost riser tubes.
CONCLUSION
The results illustrate the advantages of using the combined approach to model coupled problems in situations where previous (uncoupled) assumptions in simulations proved to be rather inaccurate. In this instance, the ability to model the liquid phase change in the pipes enabled the calculation of more realistic, non- uniform wall heat fluxes for the CFD model.
By linking the two different methods, improved boundary values and system behaviour could thus be obtained for both the CFD model and fluid network model.