In this thesis three different combustion systems, equipped with either a single or multiple ?ameless combustion burner(s), are discussed. All these setups were investigated both experimentally and numerically, i.e., using Computational Fluid Dynamics (CFD) simulations. Flameless combustion is a combustion technology capable of accomplishing the combination of high energy ef?ciency (by preheating of the combustion air) and low emissions, especially nitrogen oxides (NOx ). These high combustion air preheat temperatures normally account for increased thermal formation of NOx , however, in ?ameless combustion, by delayed mixing of the fuel and oxidizer and high internal ?ue gas recirculation, the rates of these reactions are decreased. Nitrogen oxide plays a key role in acid rain formation and the generation of photochemical smog. The ?rst setup that has been investigated is a furnace equipped with two regenerative ?ameless combustion burner pairs, with a thermal power of 100 kWth each, located at the laboratories of Kungliga Tekniska H¨ gskolan (KTH) in Stockholm, Sweden (Chapter 4). The objective of this study is to investigate the performance of the furnace operating in two different ?ring modes, parallel and staggered. The furnace performance is de?ned as the energy ef?ciency and the NO emissions. Experimental results show that for parallel ?ring mode both the ef?ciency was higher and the NO emissions were lower compared to staggered ?ring mode. With the use of the CFD simulations, it was shown that in parallel mode the radiative heat transfer was higher due to formation of a larger zone with gases with improved radiative properties and that higher velocities along the cooling tubes, due to lower momentum destruction, led to higher convective heat transfer. Both of these heat transfer methods contributed to the higher energy ef?ciency in parallel ?ring mode. Additionally, the lower formation of NO emissions in parallel ?ring mode was due to the fact that the low-momentum fuel jets merged slower with the high-momentum combustion air jets, resulting in more internal ?ue gas recirculation and a less intense combustion zone. Moreover, it was found that NOx was formed via the thermal and N2O intermediate pathways. No prompt NO was formed, while the reburning pathway resulted in a reduction of the total NO emissions. The second setup is a 300 kWth furnace equipped with three pairs of regenerative ?ameless combustion burners, located at Delft University of Technology (DUT) in the Netherlands (Chapter 5). An experimental parametric study was performed, varying the positions of the burners in the furnace (the burner con?guration), the ?ring mode (parallel and staggered), the excess air ratio and the cycle time, with the objective to optimize the furnace performance. Since similar trends in the furnace performance, as for the furnace at KTH, comparing parallel and staggered ?ring mode, were observed, staggered mode was exempted from further analysis. Additionally, one of the ?ve investigated burner con?gurations has also been exempted due to a signi?cant lower energy ef?ciency compared to the other con?gurations. The experimental results show that the burner positioning and the cycle time had a signi?cant in?uence on the temperature inside the regenerators, and thus on the preheat temperature of the combustion air. This temperature turned out to be important regarding the CO emissions. Furthermore, it was found that comparing different cases ?ring in ?ameless mode, an improved temperature uniformity in the furnace was not re?ected by a higher energy ef?ciency. Finally, a horizontal setup of the ?ring burners (the three ?ring burners positioned in a horizontal row) improved the energy ef?ciency at similar temperature uniformities. Steady CFD simulations have been performed for this furnace for four different burner con?gurations ?ring in parallel mode (Chapter 6). During the careful selection of the set of physical models to be used, it was found that, due to relatively low Reynolds numbers in the cooling air ?ow in the annulus of the cooling tubes, predictions of the heat extraction rates of these cooling tubes were improved by treating the ?ow in the cooling tubes as laminar. Furthermore, the applied error tolerance of the ISAT procedure was insuf?cient for accurate species concentration predictions, however, based on analysis of the main species concentrations in the ?ue gas, this inaccuracy did not in?uence the overall predictions. It was possible to explain the most important results of the experimental study using the CFD simulations. In the ?rst place, it was found that a recirculation zone between the upper ?ring burners and the stack in two con?gurations resulted in a smaller fraction of the ?ue gases leaving the furnace via the stack compared to the other two con?gurations. Thus, a larger fraction of the ?ue gas left the furnace via the regenerating burners, which resulted in higher preheat temperatures of the combustion air. Secondly, the experimentally observed differences in the temperature uniformity between the four con?gurations could be explained by the presence of less or more pronounced recirculation zones, the latter leading to higher temperature uniformities in the furnace. Finally, it was con?rmed that the jets of the burners showed similar merging behaviour for different burner con?gurations, leading to similar NO emissions, a trend that was also observed in the experiments. The third setup is a prototype ?ameless combustion gas turbine combustor (Chapter 7). The combustor was ?red with various Low Calori?c Value (LCV) gases. The in?uence of several parameters (the fuel composition, the outlet temperature and the inlet nozzle diameter) on the CO and NO emissions has been investigated. In the ?rst place, it was shown that this prototype ?ameless combustion gas turbine combustor could be operated in ?ameless mode ?ring the LCV gases. Moreover, for both pollutants ultra-low emissions (single-digit) have been achieved. In the CFD simulations, different turbulence models and chemistry mechanisms have been compared, leading to a set with models which gave the best results. Comparing the measured and predicted axial temperature pro?les in the combustor, it was concluded that the observed discrepancies were within the range of uncertainty in what are optimal values of the model constants. From NO calculations, ultra-low emission combustion was con?rmed. Also, it was found that 90% of the NO was formed via the N2 O path, and the remaining 10% via the thermal pathway. No prompt NO was formed, a trend also observed for the KTH furnace. In conclusion, important knowledge on the behaviour of furnaces equipped with multiple ?ameless combustion burners has been attained. Especially, the in?uence on the furnace performance of the ?ring con?guration of the burners and the burner positioning in the furnace will contribute to more successful (industrial) application of this combustion technology in the future. Recommendations for the installation of ?ameless combustion burners in large industrial-scale furnaces have been proposed. Finally, the shown possibility of ?ring a (prototype) gas turbine combustor with low calori?c value gases in ?ameless mode, enables the utilization of biomass derived gaseous fuels in existing equipment.