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.

A two-dimensional finite volume method (FVM) based on structured grids is developed to solve the axisymmetric radiative heat transfer with absorbing, emitting and anisotropically scattering media. The present scheme is not derived by discretizing axisymmetric radiative transfer equation (RTE), but by taking the axisymmetric limit of general 3D radiation FVM, which can save the computation workloads greatly and avoid the singularity of the axisymmetric RTE when r→0. Because of sharing same philosophy and meshes, it has very strong potentials of being coupled with popular computational fluid dynamics (CFD) solvers. This method is also fully validated by three benchmark cases: the cylindrical enclosure, truncated conical enclosure, and rocket nozzle, and performs excellently both in prediction accuracy and geometric flexibility.

The effective utilization of sub-bituminous coals should be extended as an energy source for power generation because of their low cost and long minable years. However, sub-bituminous coal may undergo spontaneous combustion owing to high reactivity with oxygen in low-temperature air compared with bituminous coal. Therefore, the objective of this study is to elucidate the low-temperature oxidation behaviors of sub-bituminous coal. In particular, transition from oxygen chemisorption to low-temperature oxidation of sub-bituminous coal has been experimentally examined in detail. Isothermal gravimetric analyses are conducted for several bituminous and sub-bituminous coal samples at temperatures varying between 356 K and 476 K in oxygen-enriched air. The chemical compositions of emission gas are continuously monitored under low-temperature oxidation conditions. The changes in the chemical structure of the coals are analyzed using Fourier transform infrared (FTIR) spectrometry. The experimental results show that the mass of all tested coals, including those of bituminous and sub-bituminous coals, increases at temperatures below 426 K as a result of oxygen chemisorption. At 476 K, the mass of sub-bituminous coal increases slightly with low H₂ and CO₂ emissions at the beginning and subsequently decreases with high CO and CO₂ emissions. This mass change indicates that the transition from oxygen chemisorption to low-temperature oxidation occurs during this period. In addition, the FTIR spectra of sub-bituminous coals show the existence of carbonyl groups even in raw coal samples. These carbonyl groups are considered to contribute to the formation of CO₂ and H₂ from the beginning of reactions, triggering the transition from oxygen chemisorption to low-temperature oxidation.

The possibility of oxygen-MILD combustion without preheating of oxidants is assessed numerically and experimentally in this paper. For the formation of oxygen-MILD combustion, jet impingement method was employed to enhance the internal recirculation flow effectively and also compensate the drawback which the momentum of oxygen becomes smaller than that of air combustion. And effects of impingement and reactant jet velocity were analyzed to investigate the extent of internal recirculation necessary for formation of oxygen-MILD combustion. Results show that as the velocity of oxygen increases, the stronger internal recirculation ratio (K_V) makes the oxygen jet more diluted leading to reduce the maximum temperature and high-temperature zone inside the combustion field. It is found that MILD combustion having a uniform temperature distribution can be formed at low oxygen velocity with increasing impingement angle. The Damköhler number at the oxygen velocity of 120 m/s approaches to 1 as the impingement angle increases and MILD condition of D_a < 1 and Re_t < 10,000 was satisfied at the oxygen velocity of 220 m/s. Diffusion jet flame to MILD combustion flame was observed experimentally as the impingement angle increases at the oxygen flow velocity of 120 m/s. But at higher V_oxy = 220 m/s, the effect of impingement angle is not critical, unlike V_oxy = 120 m/s. The experimental results clearly support various numerical results which can provide valuable data for designing the oxygen MILD based the furnace.

The thermal, physical and morphological characteristics of dielectric insulating oil - based nanofluids were investigated. The nanofluids were produced by the two-step method, homogeneously dispersing AlN and TiO₂ nanoparticles within insulating mineral oil (MO). The filler fraction of the nanofluids were 0.01, 0.10 and 0.50wt.%. Thermo-physical properties such as thermal conductivity and viscosity were measured. A comparison of two produced nanofluids: non-modified surface nanoparticles, and modified nanoparticles with oleic acid (OA) is presented and shown to improve stability while preserving thermal properties.

In this paper, a possibility of the oxygen-syngas MILD combustion was assessed using the CFD analysis by applying the JHC (Jet-in Hot Co-flow) combustion system. The effect of the various syngas composition ratios was analyzed, and heating value of synthesis gas was controlled by ratio of CO and H₂. The Ansys Fluent, commercial CFD code with RANS (Reynolds Averaged Navier-Stokes), modified k-ε equations and EDC (Eddy Dissipation Concept) model using 2D-axisymmetric computational domain was used in order to analyse oxygen-syngas MILD combustion. It was confirmed that the oxygen-syngas combustion can be formed in the MILD combustion under conditions of the oxygen concentration of 3 ~ 9 %, which is similar condition to the air-fuel MILD combustion region in the Adelaide and Delft flame. As the proportion of H₂ in the composition of the synthesis gas is decreased, the region of the MILD combustion with D_a < 1 is found to become wider due to the decrease in the Arrhenius reaction rate. At higher oxygen concentration in the co-flow, the region with D_a <1 known as MILD regime tends to be decreased.

This paper discusses the scaling laws of the flow structures around geometrically similar fire whirls, an insight that will be necessary when applying laboratory-scale data to real-scale fire whirls in urban or wildland fires. A fixed-frame-type fire-whirl generator is used to form geometrically similar fire whirls of three different scales. A particle image velocimetry (PIV) technique is used to measure the tangential (rotational) and the radial velocity components around the fire whirls. Scaling analysis is then conducted to derive two pi numbers, namely, the Reynolds number and the Froude number. Measured tangential velocity distributions are nearly uniform in the vertical direction when the height from the floor is greater than ~1 cm, where the velocity profiles are found to be well correlated using the Froude number, suggesting that viscosity plays only a minor role. Near the floor, on the other hand, the magnitude of tangential velocity is reduced, leading to an enhanced radial inflow toward the flame, a phenomenon similar to the Ekman layer. This inflow layer pushes the flame toward the fuel surface, increasing the heat input from the flame to the fuel and hence the burning rate. It is found that the Froude number can also correlate the radial velocity distributions near the floor.

To evaluate the energy efficiency of the energy conversion and transfer, and the dig energy-saving potential during coal-fired power generation, the energy-loss mechanism was determined through a comparative analysis of thermal and exergy equilibria. Energy loss is first identified as exergy destruction, which is then gradually collected and finally discharged in the form of waste heat through terminals such as a condenser. Furthermore, the exergy-destruction distribution represents the theoretical energy-saving potential, which occurs mainly in the boiler. Accordingly, an in-depth study was conducted on the technical approaches to reduce the combustion- and heat transfer-induced exergy-destruction and other exergy-destruction in the boiler. These technical approaches include elevating hot-air temperature, oxy combustion, reducing excess air coefficient, and increasing the main steam/reheated steam parameters. Results show that technical solutions of elevating the hot-air temperature, increasing the air oxygen content, elevating the main steam/reheated steam temperature, and increasing the main steam pressure can reduce the net coal-consumption rate of a coal-fired power unit by 6–9, 6–10, 11, and 5– 12 g/kW·h, respectively. These conclusions indicate the direction of the development of energy-saving technologies for coal-fired power generation.

To improve cooling effectiveness of film-cooling hole, bending of hole was investigated and optimized using surrogate modelling based on three-dimensional Reynolds-averaged Navier-Stokes analysis. The turbulence was modeled using a shear stress transport model. The spatially-averaged film-cooling effectiveness was employed as the objective function, and the injection angles of the lower and upper cylindrical parts and height of the bending point were selected as design variables for optimization. The Kriging model was used to approximate the objective function. 27 design points were selected by the Latin hypercube sampling, where objective function values were calculated to construct the surrogate models. The results at a blowing ratio of 0.5 showed that the optimum bended film-cooling hole improved the spatially-averaged film-cooling effectiveness by 39.9 % and 78.0 % compared to a reference bended hole and a cylindrical hole, respectively.

In this study, the properties of the flat-plate film-cooling with transverse and arched trenches are investigated. The conjugate temperature field and thermal stress field were both predicted by the multi-field coupling method. In addition, the transverse trench configuration was investigated and compared as the benchmark case. The results show that the blockage effect formed by the arched trench is helpful to improve the coolant lateral coverage and thus the cooling performance. Meanwhile, the thermal stress concentration may be stronger due to the higher temperature gradient for the arched trench configuration. The distance between the downstream trench edges and the holes is of great importance to the cooling performance. The closer this distance, the more conductive to the blockage effect and the lateral coverage, which is helpful to improve the cooling performance. The distance between the upstream trench edges and the holes shows less impact on the cooling performance. Compared with the non-trench film cooling, the trenched models can form higher temperature gradient, so the thermal expansion near the film hole is more different from that in other region, which tend to form stronger stress concentration near the film hole. Compared with the transverse trench, the arched trench can deliver the coolant jet from the center line to the lateral side, which is helpful to form laterally coverage in the downstream region, especially under the higher blowing ratio. So the arched trench can form lower temperature gradient and relative uniform thermal expansion, which is helpful to decrease the stress concentration around the cooling hole.

Information and communication technology (ICT) cooling devices increasingly need to be able to be installed in high-density packaged equipments, especially these exposed to high-temperature environments such as 1U-height servers and packet transport systems (PTS) for telecommunication networks. Therefore, in this study, we have developed a low-height and cost-effective aluminum thermosyphon with a boiling surface that has a porous structure by using a micro-curl skived fin and fluorine-based refrigerant (HFE7000) usable at over 50°C. This research aimed to evaluate the cooling performance and reliability of this aluminum thermosyphon when exposed to temperatures over 50°C and to determine an operating limit temperature. Thus, we carried out cooling performance tests of our thermosyphon supplied with hot air up to 100°C and high-temperature aging tests to examine its corrosiveness in an environment containing aluminum and HFE7000. From these examinations, we concluded the following. (1) The cooling performance at a heat input of 100 W was quite stable with the intake air up to 100°C without an excessive temperature rise of the boiling surface. (2) Until the intake air was up to 60°C, in which the plastic deformation of the boiling part did not occur, the thermal resistance of the boiling surface decreased as the increase of the saturated vapor pressure, and the minimum wall superheat of the boiling surface was 1.6 K and the maximum boiling-heat-transfer coefficient was 57 kW/(m²K). (3) The generation rate of fluorine ions increased as the temperature rose due to the promotion of the hydrolysis reaction between HFE7000 and dissolved water. (4) Under an actual use condition of the thermosyphon, fluorine ions due to the hydrolysis reaction of HFE7000 were barely generated, and aluminum corrosion due to fluorine adsorbed on the aluminum surface barely occurred either. (5) The operating limit temperature of this aluminum thermosyphon was 60°C from strength constraints.