The full-spectrum k-distribution (FSK) method and Generalized Multiparticle Mie-solution (GMM) method are applied to study the influences of soot aggregation on the radiative heat transfer in homogeneous gas-soot mixtures. The radiative transfer equation is solved by the finite volume method. The effect of soot aggregates with different fractal dimensions, monomer radiuses and the number of monomers on distributions of radiative heat flux and radiative heat source in the gas-soot mixture are studied, and the results show that increasing the fractal dimension, monomer radiuses, or decreasing the number of monomers will result in larger deviation between local radiative heat flux and radiative heat source of gas-soot mixture with soot aggregates and those with single soot particles. As a whole, the soot aggregation has a significant impact on the radiative heat transfer in the homogeneous gas-soot mixtures, and the effect of soot aggregation should better be considered as much as possible to obtain more accurate calculated results.
Convective heat transfer enhancement in a round tube mounted with a centrally perforated twisted tape (CP-TT) was numerically investigated. Influences of space of cut ratio (s/w = 0.5, 0.7 and 0.9) and twist ratio (y/w = 2.0, 3.0 and 4.0) under laminar and turbulent flow regimes on heat transfer characteristics were determined. Numerical encompassed Reynolds numbers (Re ) from 400 to 2000 for laminar flow and 5000 to 15,000 for turbulent flow. At a given Reynolds number, the tubes with centrally perforated twisted tape (CP-TT) inserts offer higher heat transfer rate than those the plain tube alone. Heat transfer enhancement in a round tube equipped with centrally perforated twisted tape (CP-TT) is strongly dependent on twist ratio (y/w ) and space of cut ratio (s/w ). The results also found that the heat transfer rate (Nu ) and friction factor (f ) increase as twist ratio (y/w ) and space of cut ratio (s/w ) decreases. The thermal enhancement factor (TEF ) increases as space of cut ratio (s/w ) and twist ratio (y/w ) decreases in laminar flow regime while the opposite trend is observed in the turbulent flow regime. Over the studied range, the tube equipped with centrally perforated twisted tape (CP-TT) with s/w = 0.5 and y/w = 2.0 gives the maximum thermal enhancement factor (TEF ) of 8.92 for laminar flow at Re = 2000. In turbulent flow at Re = 5000, the centrally perforated twisted tape (CP-TT) with s/w = 0.9 and y/w = 3.0 yields the maximum thermal enhancement factor (TEF ) of 1.33. In addition, the flow structure, temperature field and local Nusselt number of heat exchanger tubes equipped with centrally perforated twisted tape (CP-TT) are also reported for the clarification of heat transfer and flow topology mechanisms.
The fuel oil atomization is closely related to the combustion performance of the fuel in the internal combustion engine. However, during the oil atomization, the fast evaporating and moving oil droplets interact strongly with the surrounding gas, posing challenges towards the precise simulation of the combustion process. To precisely capture the interactions between fuel droplets and the surrounding gas, a new multi-droplets motion and evaporation model is deduced by introducing the local parameters surrounding the fuel droplets. The proposed model is validated comparing the simulation results against the experimental data of the containment spray process. Subsequently, the present model is adopted to simulate the fuel spray process in the typical constant volume bomb. The parameters evolution is presented during the course of the spray such as the gas temperature, droplet temperature, evaporating rate, velocity, radii variation, vapor concentration and air-fuel ratio. The analysis of the spray performance is conducted and the superiority of the new model is also discussed. The simulation analysis reveals that the spray droplets interact strongly with the surrounding gas; the developed droplet motion and evaporation model is advantageous in the simulation of the dense multi-droplets spray process.