Interparticle Heat Transfer Research Articles

Flameless venting characteristics and design model of micro-nano PMMA dust explosion

The effects of Fe–Ni foam thickness and dust size on flame propagation, reduced overpressure, and venting efficiency in 30 μm and 150 nm polymethyl methacrylate (PMMA) dust explosions were investigated. The flame quenching mechanism was revealed by coupling the heat transfer and flow of the dust flame within the Fe–Ni foam. It is found that micron dust is more prone to quenching than nano dust, and the differences in the quenching process of micro-nano dust are explained by combining the dust combustion kinetics and the theory of inter-particle heat transfer. The pressure venting mechanism of micro-nano dust was analysed under different thicknesses of Fe–Ni foam, and the thermal expansion effect of the flame passing through the Fe–Ni foam and its flow resistance significantly increased the reduced explosion overpressure. The flow continuity of the micro-nano dust was classified by means of the Knudsen number (Kn), which elucidates the effect of dust size differences on the pressure evolution. A prediction model for the maximum reduced explosion overpressure is proposed based on the explosion pressure venting of micro-nano dust. Changes in vented pressure have a direct impact on the efficiency of flameless venting. Nano-dust has an efficiency of more than 20 % lower than that of micron dust.

Minimum oxygen supply rate for smouldering propagation: Effect of fuel bulk density and particle size

Smouldering is a low-temperature, flameless, and persistent combustion process driven by heterogeneous oxidations. Oxygen supply is a key parameter of smouldering and is sensitive to fuel density and particle size, but our understanding is still limited. Herein, we explore the oxygen threshold for smouldering propagation under upward internal airflow velocities up to 5 mm/s. Pine needles with different bulk densities (55–120 kg/m3) and wood samples with different particle sizes (1–50 mm) are tested. We found that the minimum airflow velocity for sustaining smouldering propagation increases with the decrease of the bulk density or the increase of the particle size. By increasing the airflow velocity, the smouldering front first propagates unidirectionally (opposed) and then bidirectionally (opposed + forward). Nevertheless, when the pore size is large (the fuel particle size is large or the fuel bulk density is small), bidirectional propagation always occurs, because the oxygen can leak through the opposed smouldering front. A simplified thermochemical analysis is proposed to reveal the influence of interparticle heat transfer on the minimum oxygen supply rate of smouldering propagation. This work advances the fundamental understanding of smouldering on solid fuel particles and their smouldering fire risks and helps optimize the efficiency and safety of smouldering processes.

Molecular dynamics study on vibration-mode matching in surfactant-mediated thermal transport at solid–liquid interfaces

Surfactants have attracted attention as a means of enhancing thermal transport across solid–liquid interfaces. In the present study, non-equilibrium molecular dynamics simulation was used to study the effect of surfactants on interfacial thermal transport at solid–liquid interfaces, from the viewpoint of vibration-mode matching. The solid atom, surfactant molecule, and solvent molecule were all represented by a single atom. The vibrational characteristics of surfactant molecules were altered by changing surfactant mass msrf, surfactant concentration csrf, and the interaction strength between solid atoms and surfactant molecules, εsld–srf. For given values of csrf and εsld–srf, the interfacial thermal resistance (ITR) between the solid and surfactant solution exhibited a minimum as a function of msrf. This minimum was found to result from the mutual interference of interparticle heat transfer among atoms in the solid surface layer, and surfactant and solvent molecules in the first and second adsorption liquid layers. The amount of interparticle heat transfer was only partly correlated with the traditionally used overlap of vibrational density of states and with the matching of the characteristic frequencies associated with the spring constant of potential of mean force, proposed here. From this result, we conclude that ITR at solid–liquid interfaces can be minimized by optimizing the vibrational characteristics of surfactant molecules, but the theory of vibration-mode matching should be refined in order to fully identify the condition under which the best vibrational matching occurs between solid, surfactant, and solvent.

A DEM modeling of biomass fast pyrolysis in a double auger reactor

Thermochemical conversion of biomass via fast pyrolysis is a proven pathway to product low-carbon crude bio-oils. In this research, an extended discrete element method (DEM) is proposed for simulating biomass fast pyrolysis reacting granular flows in a double auger reactor, in which particle hydrodynamics and interparticle heat transfer processes are involved and coupled with chemical reactions in solid particles. An adaptive time step algorithm is proposed to achieve a stable coupling between the integration of reaction ordinary differential equations and the DEM solver, and the algorithm is proven computationally efficient. A multi-component fast pyrolysis kinetics is adopted and its modeling accuracy is assessed by carrying out simulations of benchmark biomass pyrolysis experiments and comparing the prediction results with experimental data. The predicted product yields of bio-oil, char and non-condensable gas from the simulation of the biomass fast pyrolysis in the auger reactor are in satisfactory agreement with experimental measurements. The decomposition rates of biomass components in the reactor are revealed from the simulation and the pyrolysis number Py is calculated from the decomposition rate of biomass and the heat transfer coefficient. The Py number illustrates that the biomass fast pyrolysis process is limited by the heat transfer process at particle size of 2 mm.

The Role of Heat Transfer Limitations in Polymer Pyrolysis at the Microscale

Thermal degradation of synthetic and natural polymers is an important process in many fields of engineering such as fire safety, thermal recycling, and biomass power generation. The kinetics of thermal degradation is usually studied by thermogravimetric analysis (TGA), which is based on measuring the mass loss rate of a microscale sample and the temperature of the surrounding fluid during controlled heating. The literature is rich in TGA measurements, which are commonly assumed to be governed solely by kinetics. Heat and mass transfer effects, however, can occur when the sample is insufficiently small in size. Only a few studies quantify a threshold for this maximum size. These thresholds vary due to different input parameters and formulations of the used model. Here, we aim to systematically analyse the role of heat transfer in TGA experiments, quantify the uncertainty of current models, and provide a novel threshold for maximum sample size. We focused on the natural polymer cellulose, a surrogate for biomass, and split the problem into heat transfer within the sample (intraparticle) and between the sample and the fluid (interparticle). Using dimensional analysis we derived two upper bound thresholds for the initial sample mass as a function of heating rate above these thresholds heat transfer effects are significant. One threshold is calculated based on interparticle heat transfer and depends on flow conditions, material and fluid properties. The other is calculated based on intraparticle heat transfer and only depends on material properties. Both thresholds were validated with experiments and previous studies from the literature. Comparing both thresholds shows that the maximum sample mass in a TGA is always limited by interparticle heat transfer. These results enable the selection of appropriate samples masses and heating condition in TGA experiments, which in turn will lead to a better understanding of the pyrolysis chemistry of polymers.

Micro-heterogeneous regimes for gasless combustion of composite materials

ABSTRACTReactive Ni/Al composite particles with different internal microstructures were fabricated by ball milling (BM). The propagation of gasless combustion waves through the compacted composite particle media was investigated using high-speed microscope video recording (HSMVR), with a resolution of 10 μm/pixel and 21.25 μs/frame. The microstructural combustion-wave characteristics, including hesitation time, propagation step size, instantaneous velocity, intraparticle reaction time, and average combustion-wave velocity, were studied as functions of measured internal microstructural parameters. The micro-heterogeneous relay-race combustion mechanism prevails across the investigated conditions. Decreasing the metal layer thicknesses in the composite particles leads to significant decrease in hesitation time, while only weakly affecting the instantaneous velocity. Characteristic times of hesitation and thermal relaxation defined two combustion front propagation regimes limited by interparticle heat transfer and by chemical reaction kinetics. Understanding the existence of these two discrete regimes allows us to effectively control the combustion parameters in this high-energy-density system.

Quantum Langevin equation approach to electromagnetic energy transfer between dielectric bodies in an inhomogeneous environment

Near-field and resonance effects have a strong influence on nanoscale electromagnetic energy transfer, and detailed understanding of these effects is required for the design of new, optimized nano-optical devices. We provide a comprehensive microscopic view of electromagnetic energy transfer phenomena by introducing quantum Langevin heat baths as local noise sources in the equations of motion for the thermally fluctuating electric dipoles forming dielectric bodies. The theory is, in a sense, the microscopic generalization of the well-known fluctuational electrodynamics theory and thereby provides an alternative and conceptually simple way to calculate the local emission and absorption rates from the local Langevin bath currents. We apply the model to study energy transfer between silicon carbide nanoparticles located in a microcavity formed of two mirrors and next to a surface supporting propagating surface modes. The results show that the heat current between dipoles placed in a cavity oscillates as a function of their position and separation and can be enhanced by several orders of magnitude as compared to the free-space heat current with a similar interparticle distance. The predicted enhancement can be viewed as a many-body generalization of the well-known cavity Purcell effect. Similar effects are also observed in the interparticle heat transfer between dipoles located next to a surface of a polar material supporting surface phonon polaritons.

Inter-particle heat transfer in a riser of gas–solid turbulent flows

Effects of the particle–particle heat transfer in a gas–solid turbulent flow in a riser were evaluated. An Eulerian/Lagrangian four-way interaction formulation including the particle collisions in conjunction with the k − τ and the k θ − τ θ model equations were used in the numerical simulation. Inter-particles and particle–wall interactions were accounted for with an inelastic collision model, where the restitution coefficient was evaluated for each collision. The special case when the flow initially contains two groups of hot and cold particles was treated in details. Particular attention was given to the nature of heat transfer to particles due to inter-particle interactions. The results showed that the effect of particle–particle heat transfer was more significant for smaller sizes, lower flow Reynolds numbers, and for higher loading ratios. Solid thermal properties, however, did not have a noticeable effect on the inter-particle heat transfer. The simulation results indicates that although the heat transferred to each group of hot and cold particles was significant, the mean values of gas and particle temperatures and suspension heat transfer was insensitive to the inter-particle heat transfer.

Modelling of interactive populations of disperse systems

The paper presents a general mathematical model for disperse systems in which direct interactions between particles are taken into consideration. The model is formulated in terms of transition measures, introduced on the basis of conditional Markov processes. The population balance equation, describing the behaviour of interactive populations, is developed in a general form of continuous and discontinuous terms. Moment equations are presented and analysed for the axial dispersion model with interparticle exchange processes of heat and mass. The applicability of the model is illustrated by applying it for describing a process of heating particles by gas with interparticle heat transfer, and a mass exchange process between fluid particles.

Population balance model for particle-to-particle heat transfer in gas–solid systems

A mathematical model is presented for particle-to-particle heat transfer in gas–solid systems. In developing the model, simple kinetic equations are assumed to describe the inter-particle heat transfer process, and the particle–particle interactions are considered stochastic. A stochastic approach is used to derive the population balance equation describing the variation of the density function of temperature distribution of the particulate phase. The moment equations and numerical solution of the partial integro-differential equation of the population balance model are derived and used to analyse the behaviour of a batch gas–solid system. The results indicate that the population balance approach, used for developing the model describing the inter-particle contact heat conduction during collision capable to take into account a number of parameters affecting the process, can be applied also for predicting heat transfer in gas–solid processing systems.

Enhancement of liquid phase adsorption column performance by means of oscillatory flow: an experimental study

Oscillations have been applied to the bulk fluid flowing upwards through a column packed with spherical beads of 3A zeolite that is used to dry an ethanol solution containing about 3.2 wt.% water. For a given bulk flow rate, concentration and temperature of the feed, the application of oscillations leads to a delay in the breakthrough time and a sharper breakthrough curve than for the case when there is no oscillation. Use of oscillatory flow, therefore, leads to an increased bed capacity up to the point of breakthrough, a lower length of unused bed and a reduced mass transfer zone length (MTZL). Improvements in all these parameters of up to 20% have been found for oscillations with frequencies up to 2.4 Hz and amplitudes up to 0.001 m. The best improvements were found for the highest frequencies and amplitudes that could be achieved with the apparatus. In the development of the process it is believed that a further 20–30% improvement could realistically be achieved. However, practical limitations to the upper bounds of these two oscillatory parameters arise from the strength of the adsorbent particles and the risk of movement of particles within the bed. The main reason for the improvement in the process, when oscillations are applied, is the increase in interparticle transport. The increase in interparticle heat transfer is potentially as relevant as that in interparticle mass transfer. The results obtained for this model chemical system show that process improvements through the use of oscillatory flow can be obtained with well-rounded particles and it is speculated that the method could be applicable more generally to all liquid phase adsorption processes.

Gas-phase hydrogenation of ethylbenzene over Ni.: Comparison of different laboratory fixed bed reactors

Gas phase ethylbenzene hydrogenation was studied over a supported Ni catalyst in two laboratory fixed bed reactors. Difference in deactivation behavior and reaction kinetics was attributed to existence of interparticle heat transfer in the case of reactor with bigger inner diameter.

Influence of Reaction Mixture Porosity on the Effective Kinetics of Gasless Combustion Synthesis

The effective activation energies for the self-propagating high-temperature synthesis of Ti 5 Si 3 were determined for samples with porosities ranging from 30 to 62%. A low-activation-energy regime was found for highly porous samples, while relatively high effective activation energies of 200-250 kJ/mol were measured for samples with porosity less than 50%. The same effect was also observed in two other systems, involving syntheses of TiC and MoSi 2 . The limiting role of interparticle heat transfer in propagation of the combustion front for highly porous samples is suggested as a possible explanation for the observed phenomenon.

Application of a cold flow model in testing the feasibility of an oil shale retorting process

An oil shale fluid bed process was successfully tested in a 1.5 tons/day retort. A pilot plant previously used for catalytic cracking studies was modified for this purpose. The successful conversion of the existing pilot plant to a retort and the remarkably smooth startup and operation were attributed to the concurrent construction and operation of a full-scale cold flow model to test the design of solid feeders and a unique injector/mixer. Operation of the cold flow model over the range of anticipated pilot plant operating conditions provided pressure drop and solids hold data for the mixer. The process was based on rapid heating of small oil shale particles with a hot heat carrier. Key to the process was the design of a mixer, of proprietary geometry, which effects rapid interparticle heat transfer, substantial retorting of oil shale, and rapid removal of the hydrocarbon vapors. Several tests were carried out showing that shale oil yields up to 110% of Fisher assay are feasible by using this unique process scheme. Data are presented showing the application of cold flow results in the interpretation of pilot plant data such as gas and liquid yields.

Gas-to-particle and particle-to-particle heat transfer in fluidized beds of large particles

Gas-to-particle and interparticle transfer rates were measured, at room temperature, for three different materials, in the range 1 ≤ U/ U mf ≤ 3.5 and 0.900 ≤ d p ≤ 2.250 mm by means of batch runs in which wet capillary-porous particles were dried in fluidized beds of similar dry ones. It was observed that gas-to-particle convective transfer coefficients vary only slightly with the air velocity. A “slow-bubble model” convenient for large particle fluidization, was developed to explain this finding and provided interesting data concerning the gas flow distribution in beds of large particles. The particle-to-particle heat transfer coefficients, nearly independent on the particle thermal properties, were correlated by the following equation: h pp = 206 d p −1.22 ( U/ U mf ) −0.56. A theoretical analysis of the interparticle heat transfer confirmed the observed trends and its application to our results allowed to estimate the dense phase expansion.

Particle-to-particle heat transfer in fluidized bed drying

Particle-to-emulsion and interparticle heat transfer rates were estimated in the range 1.5 ⩽ u/ u mf ⩽3.5, 0.69 ⩽ d p ⩽ 2.15 mm by drying wet refractory particles in fluidized beds of similar dry particles of the same sizes. Overall particle-to-emulsion heat transfer coefficients decrease roughly as the inverse of the particle diameter. Particle-to-particle heat transfer coefficients vary with the power-2 of the particle diameter and decrease as the fluidization velocity increases.