Producing iron ore pellets in grate-kiln plants involves several steps in order to obtain the final product. During the induration, the pellets are fired inside a rotary kiln; a large rotating oven, with two channels through which secondary process air enters. The secondary air channels are divided by the so-called back plate, causing an area of reduced velocity to form as the air flows past it. Current plants use coal to produce a flame inside the rotary kiln which heats the pellets. The coal flame is long and stable, and gives an even temperature profile inside the rotary kiln, something that is important for the quality of the pellets. As industries are transitioning towards more environmentally friendly processes and solutions, there is a desire to eliminate the use of fossil fuels such as coal. One alternative fuel of interest is hydrogen, since it can be produced and used without CO2 emissions. The issue with replacing the coal with hydrogen directly in the rotary kiln is that mixing with the surrounding secondary process air, necessary for combustion, occurs too quickly. This results in a flame that is short and intense, negatively impacting the pellet quality. Hence, different methods of injecting the hydrogen into the rotary kiln need to be investigated.

This thesis investigates the use of a so-called coaxial jet to control the mixing of fuel and secondary air. The coaxial jet consists of a central round jet mounted in the back plate, through which hydrogen is issued. Surrounding the inner jet is an annular outer jet, issuing either hydrogen or a different fluid. By altering the parameters of the coaxial jet, mixing of the hydrogen fuel and the surrounding secondary air can be controlled, making it possible to tune the resulting flame. To investigate the effect of different parameter values, Computational Fluid Dynamics (CFD) simulations are used. As a first step a simplified, axisymmetric model of the rotary kiln is modeled in 2D. This approach reduces the computational effort, enabling large amounts of parameter values to be evaluated. The so-called momentum flow ratio of the outer jet to the inner jet, Mjet, is used to compare different configurations of the coaxial jet. By increasing or decreasing the outer jet velocity, the value of Mjet is increased or decreased, respectively.

Two scenarios for the coaxial jet are considered; the first is using hydrogen through both the inner and the outer jet, and the second is using air instead of hydrogen through the outer jet. In Paper A, it is found that when using hydrogen through both the inner and outer jet, the potential core can be extended by decreasing the value of Mjet. This is achieved by decreasing the velocity of the outer jet, and thus providing a more gradual change in velocity between the high velocity inner jet, and the relatively slow secondary air. As a result the shear is reduced and the mixing is delayed. Among the values considered, Mjet = 0.25 produces the longest jet core and least mixing. This configuration, however, experiences recirculation and accumulation of hydrogen in a wake behind the back plate. If the outer jet fluid is substituted for air (Paper B) a different trend is seen, in which an increase of Mjet is beneficial for the jet length. The increased outer jet velocity, and thus momentum, enables the inner jet to be protected over a greater distance. Mjet = 2, the highest considered value, provides the longest hydrogen jet potential core and the most contained jet close to the jet exit. This is an indication that mixing with the secondary air is delayed. Further downstream, on the other hand, the increased shear that comes with higher Mjet -values contributes to increased mixing and spread.

Lastly, a three-dimensional model is simulated in Paper C, offering insights into a more complete view of the flow field inside the rotary kiln. Three-dimensional as well as transient characteristics of the flow are scrutinized, showing that there are effects not captured by the simplified steady-state 2D model. Unsteadiness along the jet core boundary, as well as a difference in the prediction of the flow behind the back plate, contribute to a difference in the jet evolution between the 3D model and the 2D model. The main conclusion from this investigation is that the simplified axisymmetric model of the rotary kiln fails to capture important features of the flow field, making it unsuitable to use for predicting accurate details about the coaxial jet evolution. The findings from this thesis will be used as a basis for the continued work, which includes investigating the influence of additional parameters on the mixing and flow field, as well as conducting experiments to enable validation of the numerical simulations.