In combustion systems operated at low-calorific conditions, physical boundary conditions for numerical modeling have to be chosen carefully. In particular, heat-loss mechanisms have to be taken into consideration, especially when they are expected to be in the order of magnitude of combustion power output itself.
Therefore, in the presented paper, different modeling strategies for heat loss are taken into account and results are evaluated against experimental data in order to subsequently complete experimental findings with information on flow field and combustion. The studies are carried out on a low-power, laboratory scale micro gas turbine combustor, operated with off-gas from a solid oxide fuel cell (SOFC).
Several operation conditions are investigated with different formulations of boundary conditions for heat loss, whereas radiation effects are neglected due to low combustion temperatures. Numerical simulations of the application case are carried out with the commercial ANSYS Fluent CFD tool. Investigated modeling variants of heat loss are threefold. Those are isothermal temperatures, heat flux boundaries, where losses are estimated from preceding simplified simulations, and fully coupled modeling.
Averaged flame chemiluminescence measurements serve as validation data for the numerical results. Recommendations for the stable and reliable simulation of fluid-solid coupling are given for RANS based solution strategies. Generality of applicability is tested by applying the established numerical procedures to several base-load and part-load operation conditions. After validation, flow field and combustion properties of the operation points are discussed in view of heat loss influences based on numerical findings, which support the limited experimental data. Finally, advantages and drawbacks of the different approaches are discussed and overall respective heat loss is balanced and compared to overall thermal power of the combustion system.