Junhui Gong

Bio: Junhui Gong is an academic researcher from Nanjing Tech University. The author has contributed to research in topics: Ignition system & Heat flux. The author has an hindex of 12, co-authored 52 publications receiving 514 citations. Previous affiliations of Junhui Gong include University of Maryland, College Park & University of Science and Technology of China.

Gasification experiments for pyrolysis model parameterization and validation

Abstract: A new experiment has been designed within the framework of a standard cone calorimeter to enable inert atmosphere, radiation-driven gasification of coupon-sized solid samples under thoroughly controlled, near-one-dimensional heating conditions. Sample mass loss (or burning) rate and surface temperature were measured simultaneously and recorded as a function of time. The temperature data were obtained by focusing a calibrated infrared camera on the non-radiated sample surface. These data were subsequently employed to compute thermal conductivity of the gasifying material. The thermal conductivity information was combined with the kinetics and thermodynamics of the thermal decomposition and broadband radiation absorption data to construct pyrolysis models for a set of widely used, non-charring thermoplastics including poly(methyl methacrylate), high-impact polystyrene and poly(oxymethylene). The resulting models were employed to predict the measured burning rate histories at 20–70 kW m −2 of external radiant heat flux. These predictions were found to be, on average, within 10% of the experimental values.

Experimental Study of Thermal Runaway Process of 18650 Lithium-Ion Battery.

Abstract: This study addresses the effects of the SOC (State of Charge) and the charging–discharging process on the thermal runaway of 18650 lithium-ion batteries. A series of experiments were conducted on an electric heating and testing apparatus. The experimental results indicate that 6 W is the critical heating power for 40% SOC. With a 20 W constant heating rate, the thermal runaway initial temperature of the lithium-ion battery decreases with the increasing SOC. The final thermal runaway temperature increases with the SOC when the SOC is lower than 80%. However, a contrary conclusion was obtained when the SOC was higher than 80%. Significant mass loss, accompanied by an intense exothermic reaction, took place under a higher SOC. The critical charging current, beyond which the thermal runaway occurs, was found to be 2.6 A. The thermal runaway initial temperature decreases with the increasing charging current, while the intensity of the exothermic reaction varies inversely. Mass ejection of gas and electrolytes exists during thermal runaway when the charging current is higher than 10.4 A, below which only a large amount of gas is released. The thermal runaway initial temperature of discharging is higher than that of non-discharging.

Development of pyrolysis models for charring polymers

Abstract: Controlled atmosphere, radiation-driven gasification experiments were conducted on a series of synthetic polymers including poly(acrylonitrile butadiene styrene), poly(ethylene terephthalate), poly(methyl methacrylate)-poly(vinyl chloride) alloy (Kydex) and polyetherimide. Mass loss rate and non-radiated surface temperature of coupon-sized material samples were measured simultaneously and recorded as a function of time. These temperature data were combined with the results of broadband radiation absorption measurements and previously conducted thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) to characterize the transport of thermal energy inside the gasifying materials through inverse modeling. Subsequently, complete pyrolysis models, based on the kinetics and thermodynamics of the thermal decomposition derived from the TGA and DSC experiments, were formulated and employed to predict the mass loss rate histories obtained at 30–90 kW m −2 of external radiant heat flux simulating fire exposure. Satisfactory predictions were obtained for all materials with the exception of polyetherimide, which highly intumescent behavior introduced large uncertainties in the gasification conditions.

Influences of low atmospheric pressure on downward flame spread over thick PMMA slabs at different altitudes

Abstract: This study addresses the influences of low atmospheric pressure on heat and mass transfer process of downward flame spread over thick PMMA (polymethyl methacrylate) slabs in quiescent air. Series of experiments were conducted at three altitudes: Hefei (1.0 atm), Xining (0.77 atm) and Lhasa (0.67 atm). Burning rate (mass loss rate), flame spread rate and flame height were investigated in this paper. From experimental results in reduced pressure, burning rates are correlated by the expression: m ˙ ∝ P 1.8 . At lower pressure, it was found that flames go quenching below a critical Damkohler number, which is caused by chemical kinetic change and reduction of total heat feedback from flame to solid fuel. For finite width samples, the flame spread rate derived from experiments increased with the sample thickness, which is different from the previous conclusions based on an assumption of infinite width. Power-law progressions of flame spread rate and flame height to pressure were produced, and linear relationships between the index and thickness of samples were obtained.

A numerical study of thermal degradation of polymers: Surface and in-depth absorption

Abstract: A one-dimension numerical model is developed in this study to investigate numerically the absorption effect, including surface and in-depth absorption, on thermal degradation process that occurs in polymer gasification. Surface and in-depth absorption hypotheses, most commonly used in literatures and their effect on the simulation results are discussed. Three polymers are selected to conduct the simulation: Poly(methyl Methacrylate) (clear PMMA), High Impact Polystyrene (HIPS) and Poly(acrylonitrile Butadiene Styrene) (ABS). The availability of the developed model is verified by the published experimental data. The result indicates that large temperature gradient exists in the heat penetration layer for surface absorption, and in-depth absorption leads to a relatively uniform temperature distribution in this layer. Top surface temperature increases with increasing heat flux (HF) and higher value is observed for surface absorption than that of in-depth absorption. The model overestimates the mass loss rate of ABS at low HF due to the negligence of existence of a thin char layer generated on the surface of sample which cracks immediately after generation under high HF. Good agreement between the numerical and experimental results of polymers suggests that both assumptions are acceptable in modeling bench scale tests.

The fluid mechanics of natural ventilation

Abstract: Natural ventilation of buildings is the flow generated by temperature differences and by the wind. The governing feature of this flow is the exchange between an interior space and the external ambient. Although the wind may often appear to be the dominant driving mechanism, in many circumstances temperature variations play a controlling feature on the ventilation since the directional buoyancy force has a large influence on the flow patterns within the space and on the nature of the exchange with the outside. Two forms of ventilation are discussed: mixing ventilation, in which the interior is at an approximately uniform temperature, and displacement ventilation, where there is strong internal stratification. The dynamics of these buoyancy-driven flows are considered, and the effects of wind on them are examined. The aim behind this work is to give designers rules and intuition on how air moves within a building; the research reveals a fascinating branch of fluid mechanics.

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Polyurethane Foams: Past, Present, and Future

Abstract: Polymeric foams can be found virtually everywhere due to their advantageous properties compared with counterparts materials. Possibly the most important class of polymeric foams are polyurethane foams (PUFs), as their low density and thermal conductivity combined with their interesting mechanical properties make them excellent thermal and sound insulators, as well as structural and comfort materials. Despite the broad range of applications, the production of PUFs is still highly petroleum-dependent, so this industry must adapt to ever more strict regulations and rigorous consumers. In that sense, the well-established raw materials and process technologies can face a turning point in the near future, due to the need of using renewable raw materials and new process technologies, such as three-dimensional (3D) printing. In this work, the fundamental aspects of the production of PUFs are reviewed, the new challenges that the PUFs industry are expected to confront regarding process methodologies in the near future are outlined, and some alternatives are also presented. Then, the strategies for the improvement of PUFs sustainability, including recycling, and the enhancement of their properties are discussed.

Pyrolytic behavior of waste extruded polystyrene and rigid polyurethane by multi kinetics methods and Py-GC/MS

Abstract: The utilization of polymer wastes for volatile fuel production has been considered as a sustainable and environmental-friendly approach for achieving better waste management, pollution protection, and renewable energy security. Polymer pyrolysis, as an ideal method for polymer waste converted into storable fuel, was explored thoroughly in this study from pyrolysis kinetics to evolved gas analysis. Two typical building-used polymer wastes, extruded polystyrene (XPS) and rigid polyurethane (RPU), were selected to conduct a series of thermogravimetry (TG) experiments. Then commonly-used isoconversional methods were employed to calculate the kinetic parameters of the pyrolysis during the whole conversion. Kinetic models of XPS and RPU thermal degradations were identified from nineteen reaction models by Coats-Redfern and masterplots methods. Then accommodation function was employed to adjust the theoretical model for reconstruction. Considering the complexities of RPU component and degradation process, Py-GC/MS was used to identify the volatile product component at 250, 340, and 460 °C, respectively. Results showed that there are large parts of volatile alcohols and ethers escaped during RPU pyrolysis process. The results of this study have implications concerning kinetic triplet determination method and escaped gas analysis during polymer waste pyrolysis process.

Synthesis of a novel graphene conjugated covalent organic framework nanohybrid for enhancing the flame retardancy and mechanical properties of epoxy resins through synergistic effect

Abstract: A novel graphene conjugated covalent organic framework (AGO@COF) nanohybrid was synthesized by solvothermal method and used to enhance the flame retardancy and mechanical performances of epoxy resins (EP) through synergistic effect firstly. It is deduced from thermogravimetric analysis and cone calorimeter results that the obtained AGO@COF nanohybrid improves the flame retardancy of EP significantly. The time to peak heat release rate of EP increases by 17 s, the peak heat release rate and total heat release of EP decrease by 43.6 and 24.3% due to incorporation of 2 wt% AGO@COF. It is referred from thermogravimetric analysis-fourier transform infrared spectrometry results that the release of toxic gases and combustible volatiles during pyrolysis is also suppressed. As for mechanical performance of EP nanocomposites, the storage modulus of EP/2 wt% AGO@COF (34.66 GPa) in the glassy state increases by 23.8% compared with the neat epoxy (28.00 GPa). The possible mechanism for enhanced flame retardant and mechanical performances is proposed according to the test results. This work has opened up a new application for AGO@COF nanohybrid in the field of flame-retardant polymers.