K.H. Luo

Compressible mixing layer growth rate and turbulence characteristics

Direct numerical simulation databases have been used to study the effect of compressibility on mixing layers. The simulations cover convective Mach numbers from 0.2 to 1.2 and all contain a fully resolved turbulent energy cascade to small spatial scales. Statistical information is extracted from the databases to determine reasons for the reduced growth rate that is observed as the convective Mach number is increased. It is found that the dilatational contribution to dissipation is negligible even when eddy shocklets are observed in the flow. Also pressure-dilatation is not found to be significant. Using an accurate relation between the momentum thickness growth rate and the production of turbulence kinetic energy together with integrated equations for the Reynolds stress tensor it is shown that reduced pressure fluctuations are responsible for the changes in growth rate via the pressure–strain term. A deterministic model for the required pressure fluctuations is given based on the structure of variable-density vortices and the assumption that the limiting eddies are sonic. Simple anisotropy considerations are used to close the averaged equations. Good agreement with turbulence statistics obtained from the simulations is found.

The pyrolytic degradation of wood-derived lignin from pulping process

Lignin is a key component in the biomass with a complex polymeric structure of the phenyl-C(3) alkyl units. The kraft lignin from the wood pulping process is tested in TG-FTIR and Py-GC-MS. The samples are pyrolyzed in TGA coupled with FTIR from 30 to 900 degrees C at the heating rate of 20 and 40K/min. The evolution of phenolic compounds in the initial pyrolysis stage of lignin is determined by FTIR, while the second stage is mainly attributed to the production of the low molecular weight species. A bench-scale fast pyrolysis unit is employed to investigate the effect of temperature on the product yield and composition. It is found that the guaiacol-type and syringol-type compounds as the primary products of lignin pyrolysis are predominant in bio-oil, acting as the significant precursors for the formation of the derivatives such as the phenol-, cresol- and catechol-types. A series of free-radical chain-reactions, concerning the cracking of different side-chain structures and the methoxy groups on aromatic ring, are proposed to demonstrate the formation pathways for the typical compounds in bio-oil by closely relating lignin structure to the pyrolytic mechanisms. The methoxy group (-OCH(3)) is suggested to work as an important source for the formation of the small volatile species (CO, CO(2) and CH(4)) through the relevant free radical coupling reactions.

Lattice Boltzmann methods for multiphase flow and phase-change heat transfer

Over the past few decades, tremendous progress has been made in the development of particle-based discrete simulation methods versus the conventional continuum-based methods. In particular, the lattice Boltzmann (LB) method has evolved from a theoretical novelty to a ubiquitous, versatile and powerful computational methodology for both fundamental research and engineering applications. It is a kinetic-based mesoscopic approach that bridges the microscales and macroscales, which offers distinctive advantages in simulation fidelity and computational efficiency. Applications of the LB method have been found in a wide range of disciplines including physics, chemistry, materials, biomedicine and various branches of engineering. The present work provides a comprehensive review of the LB method for thermofluids and energy applications, focusing on multiphase flows, thermal flows and thermal multiphase flows with phase change. The review first covers the theoretical framework of the LB method, revealing the existing inconsistencies and defects as well as common features of multiphase and thermal LB models. Recent developments in improving the thermodynamic and hydrodynamic consistency, reducing the spurious currents, enhancing the numerical stability, etc., are highlighted. These efforts have put the LB method on a firmer theoretical foundation with enhanced LB models that can achieve larger liquid-gas density ratio, higher Reynolds number and flexible surface tension. Examples of applications are provided in fuel cells and batteries, droplet collision, boiling heat transfer and evaporation, and energy storage. Finally, further developments and future prospect of the LB method are outlined for thermofluids and energy applications.

Unsteady Heat Transfer Analysis of an Impinging Jet

Unsteady heat transfer caused by a confined impinging jet is studied using direct numerical simulation (DNS). The time-dependent compressible Navier-Stokes equations are solved using high-order numerical schemes together with high-fidelity numerical boundary conditions. A sixth-order compact finite difference scheme is employed for spatial discretization while a third-order explicit Runge-Kutta method is adopted for temporal integration. Extensive spatial and temporal resolution tests have been performed to ensure accurate numerical solutions. The simulations cover several Reynolds numbers and two nozzle-to-plate distances. The instantaneous flow fields and heat transfer distributions are found to be highly unsteady and oscillatory in nature, even at relatively low Reynolds numbers. The fluctuation of the stagnation or impingement Nusselt number, for example, can be as high as 20 percent of the time-mean value. The correlation between the vortex structures and the unsteady heat transfer is carefully examined. It is shown that the fluctuations in the stagnation heat transfer are mainly caused by impingement of the primary vortices originating from the jet nozzle exit. The quasi-periodic nature of the generation of the primary vortices due to the Kelvin-Helmholtz instability is behind the nearly periodic fluctuation in impingement heat transfer, although more chaotic and non-linear fluctuations are observed with increasing Reynolds numbers. The Nusselt number distribution away from the impingement point, on the other hand, is influenced by the secondary vortices which arise due to the interaction between the primary vortices and the wall jets. The unsteady vortex separation from the wall in the higher Reynolds number cases leads to a local minimum and a secondary maximum in the Nusselt number distribution. These are due to the changes in the thermal layer thickness accompanying the unsteady flow structures.

Lattice Boltzmann modeling of multiphase flows at large density ratio with an improved pseudopotential model

Owing to its conceptual simplicity and computational efficiency, the pseudopotential multiphase lattice Boltzmann (LB) model has attracted significant attention since its emergence. In this work, we aim to extend the pseudopotential LB model to simulate multiphase flows at large density ratio and relatively high Reynolds number. First, based on our recent work [Q. Li, K. H. Luo, and X. J. Li, Phys. Rev. E 86, 016709 (2012)], an improved forcing scheme is proposed for the multiple-relaxation-time pseudopotential LB model in order to achieve thermodynamic consistency and large density ratio in the model. Next, through investigating the effects of the parameter a in the Carnahan-Starling equation of state, we find that the interface thickness is approximately proportional to 1/√a. Using a smaller a will lead to a wider interface thickness, which can reduce the spurious currents and enhance the numerical stability of the pseudopotential model at large density ratio. Furthermore, it is found that a lower liquid viscosity can be gained in the pseudopotential model by increasing the kinematic viscosity ratio between the vapor and liquid phases. The improved pseudopotential LB model is numerically validated via the simulations of stationary droplet and droplet oscillation. Using the improved model as well as the above treatments, numerical simulations of droplet splashing on a thin liquid film are conducted at a density ratio in excess of 500 with Reynolds numbers ranging from 40 to 1000. The dynamics of droplet splashing is correctly reproduced and the predicted spread radius is found to obey the power law reported in the literature.

Forcing scheme in pseudopotential lattice Boltzmann model for multiphase flows

The pseudopotential lattice Boltzmann (LB) model is a widely used multiphase model in the LB community. In this model, an interaction force, which is usually implemented via a forcing scheme, is employed to mimic the molecular interactions that cause phase segregation. The forcing scheme is therefore expected to play an important role in the pseudoepotential LB model. In this paper, we aim to address some key issues about forcing schemes in the pseudopotential LB model. First, theoretical and numerical analyses will be made for Shan-Chens forcing scheme [Shan and Chen, Phys. Rev. E 47, 1815 (1993)] and the exact-difference-method forcing scheme [Kupershtokh et al., Comput. Math. Appl. 58, 965 (2009)]. The nature of these two schemes and their recovered macroscopic equations will be shown. Second, through a theoretical analysis, we will reveal the physics behind the phenomenon that different forcing schemes exhibit different performances in the pseudopotential LB model. Moreover, based on the analysis, we will present an improved forcing scheme and numerically demonstrate that the improved scheme can be treated as an alternative approach to achieving thermodynamic consistency in the pseudopotential LB model.

Numerical study of momentum and heat transfer in unsteady impinging jets

Direct numerical simulations of an unsteady impinging jet are performed to study momentum and heat transfer characteristics. The unsteady compressible Navier–Stokes equations are solved using a high-order finite difference method with non-reflecting boundary conditions. It is found that the impingement heat transfer is very unsteady and the unsteadiness is caused by the primary vortices emanating from the jet nozzle. These primary vortices dominate the impinging jet flow as they approach the wall. Detailed analysis of the instantaneous flow and temperature fields is performed, showing that the location of primary vortices significantly affects the stagnation Nusselt number. Spatio-temporal behaviour of the heat transfer is analysed, with instantaneous Cf and Nu variations showing the correlation between the local heat transfer and the flow field. Near the secondary vortices, the breakdown of the Reynolds analogy is observed.

Lattice Boltzmann modeling of boiling heat transfer: The boiling curve and the effects of wettability

A hybrid thermal lattice Boltzmann (LB) model is presented to simulate thermal multiphase flows with phase change based on an improved pseudopotential LB approach (Li et al., 2013). The present model does not suffer from the spurious term caused by the forcing-term effect, which was encountered in some previous thermal LB models for liquid–vapor phase change. Using the model, the liquid–vapor boiling process is simulated. The boiling curve together with the three boiling stages (nucleate boiling, transition boiling, and film boiling) is numerically reproduced in the LB community for the first time. The numerical results show that the basic features and the fundamental characteristics of boiling heat transfer are well captured, such as the severe fluctuation of transient heat flux in the transition boiling and the feature that the maximum heat transfer coefficient lies at a lower wall superheat than that of the maximum heat flux. Furthermore, the effects of the heating surface wettability on boiling heat transfer are investigated. It is found that an increase in contact angle promotes the onset of boiling but reduces the critical heat flux, and makes the boiling process enter into the film boiling regime at a lower wall superheat, which is consistent with the findings from experimental studies.

Large-eddy simulation of a turbulent forced plume

Abstract This paper reports on an application of large-eddy simulation (LES) to a spatially-developing round turbulent buoyant jet. The numerical method used is based on a low-Mach-number version of the governing equations for compressible flow which can account for density variations. The second-order centre-difference scheme is used for spatial discretization and an Adams–Bashforth scheme for temporal discretization. Comparisons are made between LES results, experimental measurements and plume theory for the forced plume under moderate Reynolds number and good agreement has been achieved. It is found that the plume spreading and the centerline maximum mean velocity strongly depend on the forcing conditions imposed on the inflow plane. The helical mode of instability leads to a larger spreading rate as compared to an axisymmetric mode. The enhanced entrainment is directly related to the strong turbulent momentum and energy transports between the plume and surrounding fluid induced by vortex dynamics. The entrainment ratio is about 0.09 and falls into the range of experimentally determined values. Budgets of the mean momentum and energy equations are analyzed. It is found that the radial turbulent transport nearly balances the streamwise convection and the buoyancy force in the axial momentum equation. Also, the radial turbulent stress is balanced by the streamwise convection in the energy equation. The energy-spectrum for the axial velocity fluctuations shows a −5/3 power law of the Kolmogorov decay, while the power spectrum for the temperature fluctuations shows both −5/3 and −3 power laws in the inertial-convective and inertial-diffusive ranges, respectively.

Combustion effects on turbulence in a partially premixed supersonic diffusion flame

Abstract Effects of chemical heat release on turbulence in a partially premixed diffusion flame are investigated using direct numerical simulation (DNS). The full three-dimensional time-dependent compressible Navier-Stokes equations are employed to simulate the coupling between supersonic turbulence and heat release from a one-step chemical reaction governed by the Arrhenius kinetics. Four reacting cases with increasing heat release and one nonreacting case have been studied. Combustion is found to produce strong coupling among fluctuations in velocity, pressure, density, and other thermochemical quantities. In the Reynolds stress budget, the pressure–strain term becomes dominant as the heat release increases. Despite its wave behavior and its alternate roles as a source and a sink, the pressure–strain serves to promote energy transfer from the streamwise to the transverse and spanwise directions, and from the diagonal to the off-diagonal components of the Reynolds stresses, thus reducing anisotropy. Moreover, the pressure–strain, which in the case of the turbulent kinetic energy budget can be split up into a mean pressure work and a pressure–dilatation, helps to convert chemical energy from combustion into turbulence energy, leading to the phenomenon of “combustion-generated turbulence.” Both the solenoidal dissipation and the dilatational dissipation in particular increase as the heat release increases, but their effects are secondary to those of the pressure–strain within the main combustion period. The mixing layer growth rate can be predicted directly through the integrated Reynolds stress generation, with the accuracy of prediction depending on the heat release rate and the Reynolds number. A simple mechanism for the interactions of combustion and turbulence is proposed, while the modeling difficulties involved are also outlined. Finally, the differences and similarities of heat release effects and compressibility effects are clarified.