A comparison of the numerical predictions of the supersonic combustion of Hydrogen using the S-A and SST k – ω models

Abstract: In this numerical study, supersonic combustion of kerosene in three model combustor configurations is investigated. To this end, 3-D, compressible, turbulent, nonreacting, and reacting flow calculations with a single step chemistry model have been carried out. For the nonreacting flow calculations, the droplet diameter distribution at different axial locations, variation of the Sauter mean diameter, and the mixing efficiency for three injection pressures are presented and discussed. In addition, the effect of turbulent dispersion on the mixing efficiency is studied using a stochastic model in conjunction with the two-equation shear stress transport k-ω turbulence model. For the reacting flow calculations, contours of heat release and axial velocity at several axial locations are used to identify regions of heat release inside the combustor. Combustion efficiency predicted by the present results is compared with earlier predictions for all the combustor models. Furthermore, the predicted variation of static pressure along the combustor top wall is compared with experimental data reported in the literature. Calculations show that the penetration and spreading of the fuel increases with an increase in the injection pressure. Predicted values of the combustion efficiency are more realistic when the spray model is used for modelling the injection of the fuel. The importance of the mixing process, especially for a liquid fuel such as kerosene, on the prediction of heat release is discussed in detail.

Abstract: The present work numerically investigated the effect of variation in the inlet Mach number and stagnation temperature on the mixing of fuel with the oxidizer and the subsequent stabilization of a flame in a combustor at supersonic conditions. Dimensions of the studied combustor were taken from literature. It had a 10° wedge located at the top wall of the combustor. The combustor was modeled and analyzed using ANSYS FLUENT software. Three-dimensional, compressible, reacting flow calculations with a detailed chemistry model were performed. Turbulence was modeled using SST k-ω model. Necessary grid refinement was done to capture the incident oblique shock formed at the 10° wedge. Hydrogen was injected through the fuel inlet port. The computations were performed for Mach numbers of 2.0, 2.5 and 3.0 at the combustor inlet for a combustion inlet stagnation temperature of 1500 K. Later, the combustor inlet Mach number was kept constant at 2.5 and the combustor inlet stagnation temperature was varied as follows: 1500 K, 1700 K, and 1900 K. The results indicated that as the combustor inlet Mach number increased, the location of incidence of the oblique shock shifted to the downstream of the fuel inlet and it resulted in the better mixing of the fuel with cross flow stream of air and led to better degree of combustion of hydrogen. The contours of mole fraction of OH radical and hydrogen also corroborated the improvement in the mixing of fuel with the cross flow air and the subsequent flame stabilization at higher Mach numbers. The flow pattern, mixing of fuel with air and flame stabilization was not affected significantly till 1700 K whereas for 1900 K, combustion of hydrogen was more uniform.

Abstract: Achieving mixing and combustion augmentation of air and rocket exhaust gases become a crucial issue in RBCC ejector mode, in which the mixer is an effective way to improve mixing efficiency. In current study, the three-dimensional coupled implicit Reynolds average Navier-Stokes (RANS) equations, SST k - ω turbulent model and Eddy-Dissipation Concept (EDC) reaction model are employed to investigate the flow fields with different mixer configurations, namely the lobe, rectangle, triangle, and pentagon. The simulated pressure of the prototype has the same trend with the experimental results. The different grid scales bring only a subtle difference for the simulation results, which prove the validity of the calculation results. Simultaneously, the vortex structure, mixing efficiency, combustion efficiency and total pressure loss are investigated. The results show that the lobed mixer and rectangle mixer retain the best mixing combustion augmentation characteristics, and the triangle is the worst. Besides, with the number of lobes decreases, the larger scale vortex structure is induced downstream the mixers. It improves the combustion performance by enhancing the penetration depth. Finally, all mixers only generate the total pressure loss downstream instead of the current location, revealing that mixers have less impact on the friction loss of the inflow.

Abstract: Subsea is a term that refers to drilling and processing of gas and oil in underwater locations. One example of a subsea technology is a wet gas compressor which is used to compress fluids that consists of multiple phases. By compressing the wet gas the recovery of unprocessed streams can be increased and the investment cost reduced. The Norwegian company OneSubsea has designed and manufactured a wet gas compressor, first of its kind, and is developing the next generation of the compressor with assistance from the technical consultancy company AF. At AF’s department for technical analysis in Gothenburg simulations of the compressor with pure gas flow are performed. To compliment these simulations a separate project is performed to evaluate the effects of a flow that is multiphase. Therefore the aim of this project is to study the effect of different droplet sizes on a gas flow around a compressor blade in a wet gas compressor. Multiphase flow, consisting of natural gas and oil droplets, around one blade in the first step of the wet gas compressor is considered. Computational fluid dynamic simulations of one way coupled multiphase flow are solved using the conservation equations of mass and momentum, Lagrangian particle tracking and the k −! SST turbulence model. The range of the droplet size and volume fraction evaluated are 1-200 μm and 1-2%, respectively. Several different studies were performed. The results are characterised by flow properties outputted just after the blade, at the start of the next blade row, and with visualisations of the particle tracks around the blade. The main study, the Base case, consisted of 22 different case studies where the droplet size was held constant for each case, but varied within the size range between the cases. A coefficient of restitution (COR) was used to model the droplet wall interaction and the results showed that the droplets have an effect on the outflow from the first compressor step. The droplets decrease the average velocity angle at the axial clearance for all droplet sizes. The decrease is low, at a relatively constant value, for droplet sizes up to around 80 μm. For droplets larger than 80 μm, velocity angle decreases with increasing droplet size. By studying the particle tracks around the blade the droplet flow could be divided into three characteristic regions, according to the importance of wall interaction and effect of gas flow on the droplet. v After analysing the results from the Base case the importance of wall interaction was studied further. Simulations showed that the majority of the droplets are colliding with the wall. A sensitivity study for the COR was performed which showed that the droplet flow is independent of COR for droplet sizes up to 50 μm, almost independent up to 100 μm, and strongly dependent for the rest of the size range. A case study where the droplets were trapped at the wall was performed, but the reliability of these results are questionable since the data is based on a small fraction of droplets that pass the blade. For the final wall interaction study a liquid wall film at the blade was modelled. According to the theory this should be the most realistic way to model a droplet wall interaction. Due to lack of time this case study could not be fully completed and only an idea of the result is presented. The result shows that a thin film will cover the blade which has an effect on the particle tracks. The conclusion from this project is that the droplets will effect the flow around the blade by decreasing the average velocity angle for the flow entering the next blade row; the magnitude of the effect is increasing with increased droplet size. The droplet wall interaction is important for the particle tracks, thus it is recommended to further evaluate this aspect.

Abstract: In this paper, a new asymmetric strut-based fuel injection is proposed and investigated for enhanced fuel-air mixing and combustion in supersonic combustors. The strut configuration is re-designed ...