Rapid Heating Conditions Research Articles

Role of Structural Stresses in the Thermodestruction of Supercoiled Cellulose Macromolecules after Nitration

Thermogravimetry is used to study the thermodestruction of nitrocellulose (NC) with various nitrogen contents at various heating rates. At high degrees of nitration and high heating rates of the sample, the reaction occurs in an explosion mode with a threshold of its weight loss depending on the temperature. To explain this behavior, it is assumed that the nitration of cellulose gives rise to structural stresses, which weaken the covalent bonds in it by ∼37 kJ/mol (at a nitrogen content of ∼13%). This process apparently involves two different mechanisms of weight loss during heating: (a) conventional thermal destruction of NC macromolecules through the rupture of covalent bonds (with k0 = 1013 s−1, E = 150.2 kJ/mol, and n = 1) at heating rates of up to 10 K/min and nitrogen content in NC of up to 9%; (b) Zhurkov’s thermofluctuational mechanism of the destruction of strained macromolecules, characterized by a sharp (threshold) dependence of the weight loss on the heating rate, which is operative at heating rates above ∼4 K/min and high (>13%) nitrogen contents and at 20 K/min and a low (∼9%) nitrogen content. Under conditions of rapid heating, ∼10–20 K/min, the work done by stressed states to overcome the potential barrier to the rupture of covalent bonds causes an increase in the decomposition rate by a factor of 2000. The observed threshold pattern of weight loss during the thermodestruction of NC explains the long-known critical dependence of the properties of NC used to manufacture propellants on small changes in the nitrogen content.

Oxidation Reactivity of Char Produced in a Pilot-Scale Blowpipe: Effect of the Heating Rate during Pyrolysis

In this study, oxidation reactivity of char produced in a pilot-scale blowpipe was investigated with a focus on the difference in the heating rate during pyrolysis. In addition to the blowpipe, thermogravimetry and a Curie-point pyrolyzer were employed for preparing the char sample produced at different heating rate conditions. The changes of morphological and crystalline structures as a result of the heating rate were evaluated. Both the morphological and crystalline structures of the char differed according to the heating rate during pyrolysis. For the char produced in a blowpipe, the melting of the char was observed and the specific surface area of the char drastically increased in comparison to that of the char produced in the other reactors. On the other hand, the crystalline structure of the char also developed in the rapid heating condition. According to the study results, the positive effect of the expanded specific surface area of the char was stronger than the negative effect by the ordered crys...

Local out-of-plane deformation of CFRP ablator subjected to rapid heating

This study investigates local out-of-plane deformation of a carbon fiber reinforced phenolic polymer ablator subjected to very rapid heating. Local out-of-plane deformation was measured using a three-dimensional digital image correlation technique at high temperatures. This was achieved by attaching high temperature resistant random patterns on the specimen surface using a ceramic bond. Additionally, blue filters intended for cutting strong infrared radiation from the specimen were also used. This study then discusses the mechanisms of the local out-of-plane deformation under rapid heating conditions in terms of carbonization, pyrolysis gas occurrence, gas pressure storage, and interlaminar debonding due to gas pressure.

Using ignition of coal dust produced by different types of mechanical treatment under conditions of rapid heating

Experiments aimed at studying the ignition of coal dust obtained by coal disintegration in high-energy mills are performed in a tubular furnace. Effective kinetic constants of ignition of coal dust ground in a vibrational–centrifugal mill and in a disintegration mill under conditions of rapid heating are determined for the first time. It is shown that the volatile release rate depends on the method of coal grinding.

Microstructural refinement in spark plasma sintering 3Y-TZP nanoceramics

A commercial 3Y-TZP nanopowder was consolidated by spark plasma sintering (SPS) techniques. By special die-upset designs, each possible influence from the electric field, uniaxial pressure and heating rate was peeled to identify its contribution. Besides density and grain growth, the evolution of pore structure was consulted to clarify the relationships between microstructural development and densification kinetics. The results showed that neither electric current nor fast heating had no decisive contributions while external force was a necessity for the microstructural refinement. The authors proposed that the essentially underlying mechanism was the intensive particle rearrangement, which involves no grain growth but particle close-packing through grain rotation and sliding. The full advantages of this mechanism can be taken in rapid heating conditions, which combined with the application of higher pressures, make the SPS family of techniques to have advancement in the preparations of nanoceramics over their conventional counterparts characterized by slow heating features.

A uranium (VI) complex: Synthesis, structural and thermal kinetic analysis

A new complex [UO2(2,6–DNP)2phen] (1) (2,6–DNP = 2,6–dinitrophenol, phen = 1,10–phenanthroline) was synthesized, and identified by elemental analysis, IR, Powder XRD and single crystal X-ray crystallography. Crystal structure provides the abundant information's about the bonding and geometry around the U(VI) metal center. The thermal decomposition was studied by TG–DSC, and the kinetics of thermolysis was investigated by applying model fitting as well as isoconversional methods. Explosion delay measurement (De) was also evaluated to determine the response of this complex under the condition of rapid heating.

Effects of processing conditions on biocrude yields from fast hydrothermal liquefaction of microalgae

This study investigated the effects of algae species, reaction time, and reactor loading on the biocrude yield from fast hydrothermal liquefaction (HTL) of microalgae. Fast HTL reaction times were always less than 2min and employed rapid heating and nonisothermal conditions. The highest biocrude yield obtained was 67±5wt.% (dry basis). With all other process variables fixed, increasing the reaction time in a 600°C sand bath by 15s increments led to a rapid increase in biocrude yield between 15 and 45s. At longer times, the biocrude yield decreased. Low reactor loadings generally gave higher biocrude yields than did higher loadings. The low reactor loadings may facilitate biocrude production by facilitating cell rupture and/or increasing the effective concentration of algal cells in the hot, compressed water in the reactor.

Using Ignition of Coal Dust Produced by Different Types of Mechanical Treatment under Conditions of Rapid Heating

Using Ignition of Coal Dust Produced by Different Types of Mechanical Treatment under Conditions of Rapid Heating

高炉下部におけるプラスチック粒子の移動挙動

Injection of plastics into blast furnaces as an alternative reducing agent has been carried out for the purpose of mitigation of carbon dioxide emissions. Several studies on the pyrolysis of plastic particles have been reported, however flow behavior of plastic particles or unburnt char in actual blast furnace is not clear. In this study, hotmodel experiments and thermogravimetry analysis were conducted for the modeling of gasification behavior of plastic particles, then flow behavior of plastic particle and unburnt char in a blast furnace was calculated by numerical simulation. According to the observation results of gasification experiment, volatile matter of plastics seems to be evolved at the surface of the particles. Regarding reaction of char derived from plastics, thermogravimetry analysis showed that rate of gasification of unburnt char depended on the rate of heating. Gasification rate in the case of rapid heating condition tended to increase. As the results of numerical simulations, initial diameter of plastics and char diameter determined the flow behavior in the blast furnace. When relatively small plastic was injected or diameter of unburnt char was small, unburnt char went upward along with gas flow and most of it might be consumed at the cohesive zone. On the other hand, when relatively coarse plastics was injected and diameter of unburnt char was relatively coarse, it was suggested that unburnt char accumulated around the deadman. Therefore, it is considered that fine plastics injection is desirable for the stable operation of blast furnace.

Constitution of Drop-Tube-Generated Coal Chars from Vitrinite- and Inertinite-Rich South African Coals

The structural transformations of two Permian-aged South African coals, one vitrinite-rich [91.8% dry mineral matter free (dmmf)] and one inertinite-rich (87.7% dmmf), and their resultant char morphologies were compared in this study. With these two maceral-rich coals, the opportunity presented itself to compare the degree of thermoplasticity during coal-to-char formations for the macerals without the need to use maceral separation techniques. The thermoplasticity is affected by its petrographic composition and, consequently, influences the combustion behavior. Pyrolysis chars were generated from wet-screened coal (200 × 400 mesh), under rapid-heating conditions (104–105 °C/s) in a drop-tube reactor, to closely resemble chars generated in pulverized combustion conditions. The chemical and physical structures of the chars were characterized through a range of different analytical techniques, including scanning electron microscopy, X-ray diffraction (XRD), small-angle X-ray scattering (SAXS), nitrogen adsor...

Electrical Conductivity of Cabbage and Daikon Radish as Affected by Electrical Voltage, Frequency, Salt Concentration and Temperature

AbstractShredded cabbage (50% v/v) and Daikon radish cubes (57% v/v) were mixed with different salt solutions (0.15, 0.5, 1, 1.5 and 1.85%), poured into a Teflon‐coated static Ohmic cell and heated from 30 to 70°C at different alternating current voltages (65, 80, 100, 120 or 135 V) and frequencies (60, 2,070, 5,030, 7,990 or 10,000 Hz). Voltage, current, time and temperature were measured to calculate electrical conductivities at different temperatures. For the modeling part, 750 g of a blended crude puree (particle size < 0.5 mm) was used to fill the Ohmic cell. Daikon radish gave the highest value for electrical conductivity of 1.07 S/m at 30°C and 1.85%, 100 V and 5,030 Hz while cabbage gave a value of 0.81 S/m under the same conditions. For cabbage, the electrical conductivity values increased with increasing frequency at higher voltage, but decreased at low voltage levels. An opposite trend was observed for Daikon radish. Modeling indicated that electrical conductivity increased quadratically with temperature, salt concentration and electrical voltage. Response surface models revealed that linear, cross products, as well as quadratic effects were significant with R2 > 0.98. The Maxwell–Eucken model, which describes solid particles dispersed in continuous liquid, showed good fit for the electrical conductivity data.Practical ApplicationsOhmic heating can be an alternative to conventional heat processing of food, reducing treatment time and improving quality. Ohmic heating has been generally recognized to provide uniform and rapid heating conditions. The electrical conductivity (EC) of food components is the key property in the Ohmic heating process and is dependent on many product and system‐dependent properties, especially the salt content. It is also the primary property needed for modeling the Ohmic heating effects of a system or product. This manuscript details the gathered data on electrical conductivity of cabbage and radish, and an appropriate modeling approach to describe their dependence on product and system properties. The study generates new data EC for the vegetables and how the EC is influenced by the two vegetables that differ significantly in structure.

Evaluation of Mechanical Properties of the Rocket Thrust Chamber Material under Flight Conditions

Thrust chamber of rocket engines often operate under conditions of rapid heating environments with temperatures approaching the melting points of the materials involved. Therefore, for the optimum design of rocket engines, it is necessary to obtain the properties of thrust chamber materials under actual operating conditions. The heating rates and loading rates in conventional (laboratory) tensile stress-strain tests, which are intended to evaluate the high temperature tensile properties of the materials, are usually very low compared to that actually encountered in rocket engines. The heating rates in conventional stress-strain tests are of the order of 0.4K per second to 0.5K per second only, whereas the combustion and aerodynamic heating rates in rockets and re-entry vehicles will usually exceed 100K per second. In this context, a very important beginning has been made to experimentally determine the high temperature tensile properties of KC20WN (a cobalt based superalloy) used in earth storable liquid rocket engine thrust chamber under conditions of rapid rates of heating which actually exists during flight. These investigations have shown that the elevated temperature strength of KC20WN depends upon the heating rate (or heating time) and can be considerably higher for rapid-heating conditions than for conventional heating conditions.

A refined methodology for evaluation of heat transfer coefficients in canned particulate fluids under rapid heating conditions

The conventional methodology was refined for calculating the heat transfer coefficients (U and hfp) under rapid heating conditions. The conventional methodology of hfp evaluation became unreliable and/or non-reproducible under rapid heating conditions such as those encountered during reciprocation thermal processing. This was due to fluctuations in gathered transient temperatures during rapid reciprocating motion and due to very short equilibration times (EQT) (heat-up time to 1°C below operating temperature).In this method, U and hfp were calculated employing temperature profiles predicted by using an average retort temperature (after the come-up period) and heat penetration parameters, instead of real data points from experimental temperature profile as used in conventional methodology. The method was validated for various reciprocation frequencies (0–3Hz) and amplitudes (0–20cm). Results revealed that the refined methodology was robust and produced reliable and consistent values of U and hfp, under both slow and rapid heating conditions. Also, the heat transfer coefficient values, using the developed method, consistently matched the results and trends observed with conventional methods. A parameter sensitivity analysis was also conducted and the effects of perturbations of different parameters employed in the refined methodology were reported.

Biomass and RDF Gasification Using Ballistic Heating TGA Analysis

Rapid heating rates (≥700 °C min−1) were utilized to gasify Clean Wood and two refuse derived fuel (RDF) samples using thermogravimetric analysis coupled to gas chromatography (TGA/GC). Reaction atmospheres included Air, 5 % O2/95 % Ar, 10 % O2/90 % Ar, 100 % Ar and steam and were used to produce gas evolution profiles for hydrocarbons ranging from H2 to C4H10. While expected results were obtained using Air reaction atmospheres, some interesting results were observed using steam and Ar. Different concentration profiles and production rates of C2H6 compared to C2H4 and C2H2 enabled some understanding of the reaction sequence occurring during gasification under rapid heating conditions. Kinetic analysis showed pre-exponential factors of 8.00 × 1027 s−1 K1/2 for the Clean Wood sample, 2.02 × 1029 s−1 K1/2for sample RDF C (industrial solid waste basis) and 3.71 × 1023 s−1 K1/2 for sample A (municipal solid waste basis). Furthermore the apparent activation energy was determined to be 22 kJ mol−1 for Clean Wood, 71 kJ mol−1 for RDF C, and 185 kJ mol−1) for A indicating that the Clean Wood is slightly more reactive than RDF C and more reactive than RDF A.

Effect of Carbonaceous Deposit on Bed Particles During Rapid Pyrolysis on Gasification Rate of Biomass Char in Fluidized Bed

Gasification characteristics were evaluated for various biomass samples such as softwoods and hardwoods. They were pyrolyzed at 1000 °C in a small scale fluidized bed under rapid heating condition with N2 flow. After keeping 10 min at the temperature, the produced char was gasified with 25% CO2 at 1000 °C. Char gasification characteristic was investigated by monitoring CO producing rate and the effects of woody biomass species on gasification rate and its time variation were examined. The conversion rate, dX/dt was plotted against the value of conversion. As a result, the following characteristic curves were observed mainly for softwoods; at nearly conversion X = 0.05, high reactivity peak was found, CO production was decreased first rapidly and then slowly until X = 0.5, and then almost followed volume reaction behavior. In order to elucidate the cause of this phenomenon, the high temperature char with bed material of alumina in the bed was cooled down under the N2 flow to the room temperature and the char and alumina particles were separated by sieving. The alumina particles became black which suggests existence of some carbonaceous materials. Then each sample was introduced into the fluidized bed and separately gasified in the same bed. The gasified gas analyses showed that gasification rapidly proceeded for alumina particles only at the first stage of its gasification which explained the peak of gas evolution at low conversion of the continuous in situ pyrolysis/gasification experiments. In case of hardwoods, the amount of carbonaceous materials remaining in bed materials was quite small and it does not affect the results of the in situ pyrolysis/gasification experiments. Gasification difference of the char separately collected depended on their own properties rather than the common properties in their group.

In situ imaging of ultra-fast loss of nanostructure in nanoparticle aggregates

The word “nanoparticle” nominally elicits a vision of an isolated sphere; however, the vast bulk of nanoparticulate material exists in an aggregated state. This can have significant implications for applications such as combustion, catalysis, and optical excitation, where particles are exposed to high temperature and rapid heating conditions. In such environments, particles become susceptible to morphological changes which can reduce surface area, often to the detriment of functionality. Here, we report on thermally-induced coalescence which can occur in aluminum nanoparticle aggregates subjected to rapid heating (106–1011 K/s). Using dynamic transmission electron microscopy, we observed morphological changes in nanoparticle aggregates occurring in as little as a few nanoseconds after the onset of heating. The time-resolved probes reveal that the morphological changes initiate within 15 ns and are completed in less than 50 ns. The morphological changes were found to have a threshold temperature of about 1300 ± 50 K, as determined by millisecond-scale experiments with a calibrated heating stage. The temperature distribution of aggregates during laser heating was modeled with various simulation approaches. The results indicate that, under rapid heating conditions, coalescence occurs at an intermediate temperature between the melting points of aluminum and the aluminum oxide shell, and proceeds rapidly once this threshold temperature is reached.

S0420202 炭素繊維強化フェノール樹脂の昇温過程における劣化と変形([S042-02]セラミックスおよびセラミックス系複合材料(2),機械材料・材料加工部門)

The degradation and deformation of an ablator made of carbon fiber reinforced phenolic resin was observed after arc-plasma wind tunnel tests. The observation revealed the delamination was generated inside material and it caused the expansion in an out-of-plane direction. Under such a rapid heating condition, the thermal stress is induced inside material. In order to simulate the thermal stress inside material heated by the arc wind tunnel tests, temperature distributions and thermal stresses were calculated using simulation software: ABAQUAS. The thermal stress compressed in the in-plane direction by 10〜11 MPa which is close to the compressive strength of it. Consequently, the delamination observed after heating resulted from the rapid one-dimensional heating generating the thermal stress.

Effect of heating rate on ferrite recrystallization and austenite formation of cold-roll dual phase steel

Abstract An investigation was made to determine the effect of heating rate on ferrite recrystallization and austenite formation of cold rolled dual-phase (DP) steel (0.1C–0.4Si–1.6Mn). Three heating rates, 5, 50 and 500 °C/s, and different annealing temperatures ranged from 650 °C to 950 °C were applied to study their effects on the progress of recrystallliztion and austenitization. It has been found that faster heating strongly influences the overlapping extent of the ferrite recrystallization and austenite formation process, the morphology of martensite is observed to shift from network structure to a chain shape under lower temperatures. Higher annealing temperatures minimize the transformation and microstructure difference caused by a difference in heating rate. Rapid heating with high temperature could improve the strength. The microstructural evolution mechanism under rapid heating conditions was also tentatively discussed.

Non-isothermal phenomena in Mo/Si diffusion couple: Reaction kinetics and structure formation

Macrokinetic features and mechanism of Mo-Si reaction in conditions of rapid heating on a single particle level (Mo/Si diffusion couple) were studied by high-speed scanning electrothermography. The experiments were conducted in the temperature-time conditions (T = 1000–1600°C, t = 0.01–3.0 s) comparable to those of combustion synthesis of molybdenum disilicide. The rates of chemical heat release (depending on the thickness of silicon layer, δSi) were measured and the processes of phase/structure formation were explored. Over a wide range of δSi, two-stage chemical heat release was associated with occurrence of solid-solid Mo(s) + Si(s) (for T TmpSi), where TmpSi stands for the melting point of Si. Proposed was a new mechanism for siliconization of Mo in non-isothermal conditions.

가열 속도에 따른 콘크리트의 폭렬 특성 및 내부 수증기압력 평가

콘크리트의 폭렬 발생 메커니즘에 대해서는 수증기압력에 의한 파괴, 내·외부의 온도 차이에 의해 발생하는 표면 압축력에 의한 파괴, 앞선 두 가지 요인의 복합작용에 의한 파괴가 있다. 이러한 폭렬에 영향을 주는 요인은 콘크리트 자체의 재료적 특성과 관계된 내부 요인과 환경에 의한 외부 요인으로 나눌 수 있으며 폭렬 현상을 이해하기 위해서는 두 가지 요인에 대한 충분한 고려가 필요하다. 외부 환경의 요소로써 가열 속도가 다른 경우 콘크리트 내부의 수분응집 및 수증기압력의 거동이 달라질 것으로 판단된다. 따라서 이 연구에서는 30, 50, 70, 90, 110 MPa의 다양한 강도 영역의 콘크리트를 대상으로 ISO-834 표준가열곡선과 1℃/min의 가열 속도를 적용하여 가열 속도에 따른 콘크리트의 폭렬 성상 및 수증기압력, 열팽창 변형을 평가하였다. 실험 결과 콘크리트의 폭렬은 급속 가열조건에서 발생하며, 콘크리트가 고강도화될수록 폭렬에 의한 단면손실량이 증가하였다. 또한, 가열 초기에 콘크리트 표면부의 수증기압력 상승 속도 및 가열 속도에 따른 열팽창에 의한 초기압력 상쇄효과가 콘크리트의 폭렬 발생에 중요한 영향을 미치는 것으로 나타났다.