Huahua Xiao

Bio: Huahua Xiao is an academic researcher from University of Science and Technology of China. The author has contributed to research in topics: Combustion & Premixed flame. The author has an hindex of 24, co-authored 55 publications receiving 1613 citations. Previous affiliations of Huahua Xiao include University of Maryland, College Park & Chinese Ministry of Public Security.

Thermal degradation characteristics of rigid polyurethane foam and the volatile products analysis with TG-FTIR-MS

Abstract: Thermal degradation characteristics of rigid polyurethane (PUR) foam in both air and nitrogen gaseous environments were studied using thermogravimetry and differential scanning calorimetry (TG-DSC) hyphenated techniques. And in situ Fourier Transform Infrared (FTIR) was employed to investigate the characteristic functional groups of the decomposition residues at different temperatures. It is found that the thermal degradation of PUR material in air and N 2 present a three-stage and a two-stage process, respectively. And the degradation reaction rate of PUR in air is accelerated significantly due to the presence of oxygen. The thermal degradation mechanism of PUR under non-oxidizing gaseous environment was evaluated using a TGA instrument coupled with Fourier Transform Infrared and mass spectrometer (TG-FTIR-MS). HCFC-141b served as blowing agent is detected at the initial stage. The urethane bond groups of PUR start to break up into isocyanates segments and polyols segments from about 200 °C. With an increase of temperature, the polyols decompose into some kinds of aliphatic ether alcohol. In the temperature range of 350–500 °C, the dominant volatile products are primary amines, secondary amines, vinyl ethers and CO 2 .

Experimental and numerical investigation of premixed flame propagation with distorted tulip shape in a closed duct

Abstract: High-speed schlieren photography, pressure records and large eddy simulation (LES) model are used to study the shape changes, dynamics of premixed flame propagation and pressure build up in a closed duct. The study provides further understanding of the interaction between flame front, pressure wave and combustion-generated flow, especially when the flame acquires a ‘‘distorted tulip’’ shape. The Ulster multi-phenomena LES premixed combustion model is applied to gain an insight into the phenomenon of ‘‘distorted tulip’’ flame and explain the experimental observations. The model accounts for the effects of flow turbulence, turbulence generated by flame front itself, selective diffusion, and transient pressure and temperature on the turbulent burning velocity. The schlieren images show that the flame exhibits a salient ‘‘distorted tulip’’ shape with two secondary cusps superimposed onto the two original tulip lips. This curious flame shape appears after a well-pronounced classical tulip flame is formed. The dynamics of ‘‘distorted tulip’’ flame observed in the experiment is well reproduced by LES. The numerical simulations show that large-scale vortices are generated in the burnt gas after the formation of a classical tulip flame. The vortices remain in the proximity of the flame front and modify the flow field around the flame front. As a result, the flame front in the original cusp and near the sidewalls propagates faster than that close to the centre of the original tulip lips. The discrepancy in the flame propagation rate finally leads to the formation of the ‘‘distorted tulip’’ flame. The LES model validated previously against large-scale hydrogen/air deflagrations is successfully applied in this study to reproduce the dynamics of flame propagation and pressure build up in the small-scale duct. It is confirmed that grid resolution has an influence to a certain extent on the simulated combustion dynamics after the flame inversion.

Formation and evolution of distorted tulip flames

Abstract: The development and evolution of tulip and distorted tulip flames in closed channels were simulated by solving the fully compressible reactive Navier–Stokes equations using a high-order numerical method and a single-step Arrhenius model for the reactions and energy release in a stoichiometric mixture of hydrogen and air. Important features of the simulations include (1) the development and propagation of acoustic waves and their effects on flame evolution, (2) the formation and collapse of flame cusps, both at the flame front and near the sidewalls, and the effects of cusp collapse on flame propagation, and (3) the appearance of adverse pressure gradients at the onset of the tulip or a distorted tulip flame, which result in reverse flow in the unburned gas. The simulations highlight the coupling between pressure waves, adverse pressure gradients, boundary layers, and the propagating flame front. Whereas the formation of the tulip flame can be attributed to several effects (such as pressure waves, vortex motion and Landau–Darrieus instabilities), the onset of the distorted tulip flame is strongly influenced by the Rayleigh–Taylor instability.

Experimental study on the behaviors and shape changes of premixed hydrogen–air flames propagating in horizontal duct

Abstract: The behaviors and shape changes of premixed hydrogen–air flames at various equivalence ratios propagating in half-open and closed horizontal ducts are experimentally investigated using high-speed schlieren imaging and pressure sensors. The study shows that the premixed hydrogen–air flame undergoes more complex shape changes and exhibits more distinct characteristics than that of other gaseous fuels. One of the outstanding findings is that obvious distortion happens to tulip flame after its full formation when equivalence ratio ranges from 0.84 to 4.22 in the closed duct. The salient tulip flame distortions are specially scrutinized and distinguished from the classical tulip collapse and disappearance. The dynamics of distorting tulip flame is different from that of classical tulip flame. The normal tulip flame can be reproduced after the first distortion followed by another distortion. The initiation of flame shape changes coincides with the deceleration both of pressure rise and flame front speed for flames with tulip distortions. And the formation and dynamics of tulip/distorting tulip flames depend on the mixture composition.

Premixed flame propagation in hydrogen explosions

Abstract: Hydrogen as an energy carrier is a very promising alternative fuel in the future. Accidental hydrogen explosions remain one of the major concerns in hydrogen energy utilization and process industries. This paper summarizes recent experimental and numerical efforts towards understanding combustion wave propagation in hydrogen explosions, including flame instabilities, flame acceleration, deflagrations, and deflagration-to-detonation transition (DDT). The fundamental problems involve understanding physical mechanisms that significantly influence the dynamic flame behavior in hydrogen explosions, such as combustion/hydrodynamic instabilities, vortex motion, pressure waves and flow turbulence. Advances achieved over recent years in new experimental observations, theoretical models and numerical simulations are discussed. Future research is required to quantitatively understand flame instabilities, turbulence properties and DDT in hydrogen explosions and improve reliability of theoretical and numerical predictions for hydrogen safety applications.

New insights into large eddy simulation

Abstract: Fluid dynamic turbulence is one of the most challenging computational physics problems because of the extremely wide range of time and space scales involved, the strong nonlinearity of the governing equations, and the many practical and important applications. While most linear fluid instabilities are well understood, the nonlinear interactions among them makes even the relatively simple limit of homogeneous isotropic turbulence difficult to treat physically, mathematically, and computationally. Turbulence is modeled computationally by a two-stage bootstrap process. The first stage, direct numerical simulation, attempts to resolve the relevant physical time and space scales but its application is limited to diffusive flows with a relatively small Reynolds number (Re). Using direct numerical simulation to provide a database, in turn, allows calibration of phenomenological turbulence models for engineering applications. Large eddy simulation incorporates a form of turbulence modeling applicable when the large-scale flows of interest are intrinsically time dependent, thus throwing common statistical models into question. A promising approach to large eddy simulation involves the use of high-resolution monotone computational fluid dynamics algorithms such as flux-corrected transport or the piecewise parabolic method which have intrinsic subgrid turbulence models coupled naturally to the resolved scales in the computed flow. The physical considerations underlying and evidence supporting this monotone integrated large eddy simulation approach are discussed.

Thermochemical conversion of lignin to functional materials: a review and future directions

Abstract: Lignin valorization is considered an important part of the modern biorefinery scheme. The unique structure and composition of lignin may offer many effective routes to produce several bulk chemicals and functional materials. Thermochemical conversion of lignin to synthesize value-added functional materials has recently attracted a lot of attention. In this review, we have presented currently available approaches and strategies for the thermochemical conversion of lignin to functional carbon materials. The transformation behavior and mechanism of lignin during the thermochemical process (e.g., pyrolysis and hydrothermal carbonization) are illuminated. The characteristics (structure and surface chemistry) of lignin-based functional carbon materials are summarized systematically. The advances in the functionalization of lignin-based carbon materials (surface functionality tuning and porosity tailoring) and the applications of lignin-based functional carbon materials in the fields of catalysis, energy storage, and pollutant removal are reviewed. Perspectives on how lignin-based functional materials would develop and, especially, in which fields the use of these functionalized materials could be expanded are discussed. This review clearly shows that a rational design of the functionalized lignin-based materials will lead to a rich family of hybrid functional carbon materials with various applications toward a green and sustainable future.

Application of Hydrides in Hydrogen Storage and Compression: Achievements, Outlook and Perspectives

Abstract: Metal hydrides are known as a potential efficient, low-risk option for high-density hydrogen storage since the late 1970s. In this paper, the present status and the future perspectives of the use of metal hydrides for hydrogen storage are discussed. Since the early 1990s, interstitial metal hydrides are known as base materials for Ni – metal hydride rechargeable batteries. For hydrogen storage, metal hydride systems have been developed in the 2010s [1] for use in emergency or backup power units, i. e. for stationary applications. With the development and completion of the first submarines of the U212 A series by HDW (now Thyssen Krupp Marine Systems) in 2003 and its export class U214 in 2004, the use of metal hydrides for hydrogen storage in mobile applications has been established, with new application fields coming into focus. In the last decades, a huge number of new intermetallic and partially covalent hydrogen absorbing compounds has been identified and partly more, partly less extensively characterized. In addition, based on the thermodynamic properties of metal hydrides, this class of materials gives the opportunity to develop a new hydrogen compression technology. They allow the direct conversion from thermal energy into the compression of hydrogen gas without the need of any moving parts. Such compressors have been developed and are nowadays commercially available for pressures up to 200 bar. Metal hydride based compressors for higher pressures are under development. Moreover, storage systems consisting of the combination of metal hydrides and high-pressure vessels have been proposed as a realistic solution for on-board hydrogen storage on fuel cell vehicles. In the frame of the “Hydrogen Storage Systems for Mobile and Stationary Applications” Group in the International Energy Agency (IEA) Hydrogen Task 32 “Hydrogen-based energy storage”, different compounds have been and will be scaled-up in the near future and tested in the range of 500 g to several hundred kg for use in hydrogen storage applications.

Fates of Chemical Elements in Biomass during Its Pyrolysis

Abstract: Biomass is increasingly perceived as a renewable resource rather than as an organic solid waste today, as it can be converted to various chemicals, biofuels, and solid biochar using modern processes. In the past few years, pyrolysis has attracted growing interest as a promising versatile platform to convert biomass into valuable resources. However, an efficient and selective conversion process is still difficult to be realized due to the complex nature of biomass, which usually makes the products complicated. Furthermore, various contaminants and inorganic elements (e.g., heavy metals, nitrogen, phosphorus, sulfur, and chlorine) embodied in biomass may be transferred into pyrolysis products or released into the environment, arousing environmental pollution concerns. Understanding their behaviors in biomass pyrolysis is essential to optimizing the pyrolysis process for efficient resource recovery and less environmental pollution. However, there is no comprehensive review so far about the fates of chemical ...