Modelling of heavy fuel oil spray combustion using continuous thermodynamics
Creators: name: Garaniya, VB
Publisher: name: University Of Tasmania
Year: 2023

description: Commercial liquid petroleum fuels are complex mixtures of various hydrocarbons. In multicomponent fuel modelling, these liquid fuels are represented typically with two components or up to ten discrete components. Even with ten components, there are limitations on the representation of real commercial fuels such as heavy fuel oil (HFO), which contains large numbers of hydrocarbons with a wide range of molecular weights and dissimilar structures. Continuous thermodynamics and pyrolysis chemical kinetics are used to model the behaviour of HFO in diesel spray combustion. The evaporation model is developed using the principle of continuous thermodynamics rather than discrete component modelling, to accurately cover the entire range of composition. Continuous thermodynamics reduces the computational simulation load compared to conventional discrete thermodynamics, without degrading the quality of prediction of the complex behaviours of such multicomponent fuels. In continuous thermodynamics modelling, liquid mixture compositions are simply represented by probability density functions (PDF). In the present study, HFO is represented by four fuel fractions: n-paraffins, aromatics, naphthenes and heavy residue. Each of these fractions is assigned a separate distribution function. In the evaporation model, both low-pressure and high-pressure formulations for the calculation of vapour-liquid equilibrium (VLE) at the droplet surface are provided. The formulations for high-pressure VLE are developed for a semicontinuous mixture and a generic approach to the equation of state (EOS) is used. Therefore, depending on the mixture compositions (continuous or semicontinuous) these formulations can be applied with any EOS. It is identified that in the high-pressure model, interaction coefficients between the liquid-liquid components and between the liquid component and air plays an important role during evaporation. Interaction coefficients help to improve the evaporation rate of heavy molecules. HFO is primarily composed of high molecular weight hydrocarbons which cannot evaporate but are pyrolised at high temperatures. Pyrolysis produces volatile gases and polymers through thermal cracking and polymerisation respectively. Baert's pyrolysis model based on chemical kinetics for thermal cracking and polymerisation rate is developed. Results of this pyrolysis model show that polymer formation within a droplet is dependent on droplet heating rate and composition. Moreover, it is observed in Baert's pyrolysis model results that the process of polymerisation starts prior to the thermal cracking. This order of thermal cracking and polymerisation is contradictory to the experimental evidence. Subsequently, Baert's pyrolysis model parameters are modified. Results of the modified pyrolysis model did not show any significant dependency of polymer formation on droplet heating rate and in addition it showed thermal cracking beginning earlier than the polymerisation. The low and high-pressure evaporation models along with the modified pyrolysis model are applied to a single HFO droplet in a high pressure environment, showing good agreement with experimental results obtained by other researchers. A comparison of the low-pressure model with the high-pressure model for 100 micron and 30 micron droplets at high pressure and temperature show that evaporation of the volatile hydrocarbons (nparaffins, aromatics, naphthenes) from HFO occurs at a faster rate for the high-pressure model. However, this faster evaporation does not significantly affect the droplet lifetime because modelled HFO contains only 30% volatile hydrocarbons (cutter stock) by mass. Therefore, droplet lifetime is found to be similar for both models. Thus in sprays where droplets are generally small, the VLE calculation can be obtained with sufficient accuracy by the low-pressure model avoiding the use of the complex high-pressure EOS model. The low-pressure evaporation and modified pyrolysis models along with a heterogeneous liquid phase soot burnout model are implemented via subroutines in a diesel spray simulation in the CFD package StarCD. This simulation is applied to two different constant volume spray combustion chambers which are used to examine the combustion characteristics of HFO. The models are tested for two representative fuel samples; one with the good combustion quality and the other poor. Good qualitative agreement is shown between the computer simulations and the published experimental data. A sample of HFO is characterised in the laboratory using chemical characterisation procedures such as; sequential elution solvent chromatography (SESC), gaschromatography (GC), mass spectrometry (MS) and elemental analysis, to obtain the composition and mean molecular weights of HFO fractions required for continuous thermodynamics modelling. A CFD simulation of the characterised HFO is performed using the developed low-pressure evaporation and modified pyrolysis models.