• Energy Research

A lean-premixed swirling combustor with synthesis gases is studied in non-reacting and reacting cases using large-eddy simulation. Code validation and grid dependence test are performed to validate the models and mesh resolution. With the introduction of unmixedness and correlation coefficients in the non-reacting cases, the influence of Reynolds number on recirculation, vorticity breakdown and mixing is studied. In reacting cases, instant and time-averaged scalar fields are examined to study the flame dynamics for a varied fuel composition and operating conditions. The results reveal the effects of hydrogen concentration, Reynolds number, equivalence ratio and pressure on the combustion processes. Conclusions for syngas combustion operation from this work are expected to provide useful information for gas turbine combustor design.

Abstract Large-eddy simulation is conducted to study the turbulent combustion processes of hydrogen and syngas flames. The subgrid scale momentum transport is performed with a one-equation model using the subgrid scale turbulent kinetic energy, while the linear-eddy model is employed to represent the scalar transport. Reduced chemical kinetics is used to describe the flame chemistry with the extended Zeldovich mechanism to account for the thermal formation of NOx. Results show the effects of the hydrogen content and of the Reynolds number on the characteristics of the flames. Higher hydrogen content contributes to increase the heat released after combustion leading to higher temperature peaks and thicker shear layers. The presence of CO in the fuel stream affects the flame dynamics with the development of a more vortical and wrinkled flow field. The vorticity field and turbulence/chemistry interactions are also discussed. Scalar profiles and comparison against experimental data are presented where a reasonable agreement is observed.

The partitioning behaviours of CO2 with three kinds of common impurities, i.e., N2, CH4 and H2S, in the formation brine are investigated by numerical simulations. The results indicate that the effects of N2, CH4 or the mixture of N2 and CH4 at the same concentrations are generally similar. The leading gas front is usually made up of less soluble impurities, such as N2, CH4 or the mixture of N2 and CH4, while more soluble species such as H2S has dissolved preferentially in the formation brine. The separations between different gas species increase as the gas displacement front migrates forwards and contacts more of the aqueous phase. Compared with the partitioning results of the 98% CO2 and 2% H2S mixture, the results indicate that the inclusion of less soluble N2 and/or CH4 results in an earlier gas breakthrough and a longer delay between the breakthrough times of CO2 and H2S. The early breakthrough of the gas phase is mainly because that the addition of N2 and/or CH4 lowers the viscosity of the gas phase, resulting in a higher gas velocity than that of the CO2–H2S mixture. Meanwhile, the mobility ratio is higher and the gas mixture contacts the formation brine over a larger area, giving rise to more efficient stripping of the more soluble gas species like H2S and thus larger separations. In the meantime, with the same total concentrations of impurities (12%), when 2% H2S is contained in the CO2 streams, gas phase flows slower and thus the breakthrough time is later. Furthermore, the effects on the partitioning phenomenon are weaker with decreasing concentrations of N2 and/or CH4 (from 10% to 2%) with fixed concentrations of other impurity like H2S (2%). The migration distances and the separations between different gas species change linearly with time on the whole, as confirmed by a simulation in a longer model.

An investigation of the local flame extinction of H2/CO oxy-syngas and syngas-air nonpremixed jet flames was carried out using three-dimensional direct numerical simulations (DNS) with detailed chemistry by using flamelet generated manifold chemistry (FGM). The work has two main objectives: identify the influence of the Reynolds number on the oxy-syngas flame structure, and to clarify the local flame extinction of oxy-syngas and syngas-air flames at a higher Reynolds number. Two oxy-syngas flames at Reynolds numbers 3000 and 6000 and one syngas-air flame at Reynolds number 6000 were simulated. The scattered data, probability density function distributions and fully burning probability provide the local flame characteristics of oxy-syngas and syngas-air nonpremixed jet flames. It is found that the H2/CO oxy-syngas flame burns well compared to the syngas-air flame and the high Reynolds number causes more flow straining, resulting in higher scalar dissipation rates which lead to lower temperatures and eventually local flame extinction. The oxy-syngas flames burns more vigorously than the syngas-air flame with the same adiabatic flame temperature of approximately 2400 K. Keywords : DNS; Oxy-syngas flame; Syngas-air flame; Probability density functions; Fully burning probability

A simplified one-dimensional transient computational model of a prismatic lithium-ion battery cell is developed using thermal circuit approach in conjunction with the thermal model of the heat pipe. The proposed model is compared to an analytical solution based on variable separation as well as three-dimensional (3D) computational fluid dynamics (CFD) simulations. The three approaches, i.e. the 1D computational model, analytical solution, and 3D CFD simulations, yielded nearly identical results for the thermal behaviours. Therefore the 1D model is considered to be sufficient to predict the temperature distribution of lithium-ion battery thermal management using heat pipes. Moreover, a maximum temperature of 27.6 °C was predicted for the design of the heat pipe setup in a distributed configuration, while a maximum temperature of 51.5 °C was predicted when forced convection was applied to the same configuration. The higher surface contact of the heat pipes allows a better cooling management compared to forced convection cooling. Accordingly, heat pipes can be used to achieve effective thermal management of a battery pack with confined surface areas.

In this study, a swirler combining the vane swirler and the plasma swirler is designed to control the flame lift-off height. The plasma swirler is located near the rim of the injector and the vane swirler is placed upstream of the plasma swirler. The vane swirler is employed to form a divergent flow to sustain the detached flame and the plasma swirler is adopted to control the flame lift-off height. The ionic wind clinging to the inner wall of the injector tube does not penetrate into the center, leaving the main stream flow in the central region largely undisturbed. Characteristics of the flow field are analyzed from the results of laser Doppler anemometry (LDA) measurement and mechanism of the flame lift-off control by the combined vane-plasma swirler is revealed. The flame lift-off locations calculated from the LDA measurement are consistent with those from direct observation. The dielectric barrier discharge (DBD) voltage influences the height of the flame lift-off with a linear relationship observed, which means the flame lift-off height can be controlled precisely by the DBD voltage without any mechanical movement or changing the mass flow rate. The combined vane-plasma swirler has the potential to improve the fuel flexibility, increase flame stability and attenuate the injector overheating. Highlights 1. A combined vane-plasma swirler is designed to control the flame lift-off height. 2. Flame lift-off height can be adjusted by changing the voltage of the dielectric barrier discharge. 3. A linear relationship between flame lift-off height and the dielectric barrier discharge voltage is observed. 4. Flame lift-off control by the combined vane-plasma swirler is mainly associated with the aerodynamic effect. 5. The swirler can improve fuel flexibility, increase flame stability and attenuate injector overheating.

Experiments at laboratory scales have been conducted to investigate the behavior of the release of supercritical CO2 from pipelines including the rapid depressurization process and jet flow phenomena at different sizes of the leakage nozzle. The dry ice bank formed near the leakage nozzle is affected by the size of the leakage nozzle. The local Nusselt numbers at the leakage nozzle are calculated and the data indicate enhanced convective heat transfer for larger leakage holes. The mass outflow rates for different sizes of leakage holes are obtained and compared with two typical accidental gas release mathematical models. The results show that the “hole model” has a better prediction than the “modified model” for small leakage holes. The experiments provide fundamental data for the CO2 supercritical-gas multiphase flows in the leakage process, which can be used to guide the development of the leakage detection technology and risk assessment for the CO2 pipeline transportation.

A small ethanol diffusion flame exhibited interesting characteristics under a DC electric field. A numerical study has been performed to elucidate the experimental observations. The flow velocity, chemical reaction rate, species mass fraction distribution, flame deformation and temperature of the flame in the applied DC electric field were considered. The results show that the applied electric field changes the flame characteristics mainly due to the body forces acting on charged particles in the electric field. The charged particles are accelerated in the applied electric field, resulting in the flow velocity increase. The effects on the species distribution are also discussed. It was found that the applied electric field promotes the fuel/oxidizer mixing, thereby enhancing the combustion process and leading to higher flame temperature. Flame becomes shorter with applied electric field and its deformation is related to the electric field strength. The study showed that it is feasible to use an applied DC electric field to control combustion and flame in small-scale.