The effect of biomass size and aspect ratio on intra-particle tar decomposition has been investigated both numerically and experimentally to achieve a high rate of intra-particle tar decomposition. In one experiment, wood cylinders with a diameter of 8 mm and lengths of 2, 5, and 9 mm were pyrolyzed in an infrared reactor in an argon environment. The final reactor temperature was 973 K and the heating rate was 30 K/s. To make a calculation, a two-dimensional, unsteady state, single particle model was used, and the same convective and radiative heat fluxes were given to the top and side surface of all wood cylinders. Both calculation and experiment showed that tar yield when the length of the biomass was increased and the diameter was kept constant. The calculation showed that, first, tar was formed in the wood cylinder, and then it moved outwards during decomposition. To find an effective aspect ratio of the wood cylinder for further tar decomposition, calculations were also performed in which the aspect ratio (D/L) varied from 0.4 to 6.9 and the wood volume was fixed. As a result, a low aspect ratio was suited for intra-particle tar decomposition because of the difference in the thermal conductivity along the grain and radial directions, although there is an optimum aspect ratio because of the change of residence time. It is well known that the thermal conductivity of unpyrolyzed wood in the radial direction is much lower than that along the grain. By decreasing the aspect ratio, the ratio of the side surface area to total surface area increases. This means that more heat entered from side surface, and low thermal conductivity in the radial direction caused a temperature gradient in the cylinder. When the intra-particle temperature gradient was large, primary tar, which has been formed in the biomass with a relatively low temperature, passed through the side surface layer at high temperatures, enough to advance intra-particle tar decomposition before the tar was released. This resulted in the enhancement of intra-particle tar decomposition.
Responses of the premixed methane/air mixture flames under equivalence ratio oscillations were numerically investigated assuming axi-symmetric stagnation flow fields. The flame motion was numerically investigated at three different oscillation frequencies (10, 20 and 50 Hz) and with three oscillation cases namely: lean case, rich case and lean rich crossover case. Methane/air mixture with equivalence ratio oscillation was issued from the burner exit with 1.0 m/s uniform velocity profile. The effects of frequency and amplitude of the equivalence ratio oscillation were discussed. The amplitude of the equivalence ratio oscillation is attenuated between the burner exit and the upstream edge of the preheat zone. The attenuation is much significant for higher frequency. The amplitude of the flame temperature oscillation attenuates following the attenuation of the equivalence ratio oscillation. The flame location makes the closed cycle around the flame location of correspond equivalence ratio in the steady state condition. The formation of the cycle can be explained by the back support effect. It was further demonstrated that, the back support effect influences the dynamic response of the flame location, in that, the direction of the cycles of the dynamic response in the lean case and the rich case are different. Furthermore, the time variation of the flame location plays a significant effect to the flame displacement speed.
The Soret effect is a phenomenon in which a temperature gradient gives rise to a concentration gradient. In this study, we attempted a simple technique for the separation of hydrogen from a gas mixture by utilizing the Soret effect. A gas mixture of hydrogen and carbon dioxide was introduced into a metal mini tube in which a temperature gradient was established in the direction normal to the flow. The experiment demonstrated that hydrogen was actually separated from the gas mixture, although the separation was very small with the present apparatus. From results of the experiments and computational fluid dynamics (CFD) analysis, we believe that the layer of high hydrogen concentration that moved toward the heating wall was very thin. It would be necessary to extract the high concentration of hydrogen flowing near the wall to increase the separation efficiency.
The structure and thermal properties of molybdenum nanoparticles are investigated by molecular dynamics simulation. Specifically, the solidification of molybdenum nanoparticles from liquid droplets at various cooling rates is performed to discuss the variety of nanoparticle structures with 54 to 16000 atoms. Bcc single-crystalline and glassy nanoparticles are formed at cooling rates on the order of 1010 and 1013 K/s, respectively, for all sizes of nanoparticle except for the smallest cluster, Mo54. In addition, a polycrystalline structure is formed in nanoparticles with 2000 atoms or more at intermediate cooling rates on the order of 1012 K/s. On the other hand, a fivefold rotationally symmetric Mo54 cluster is formed at low cooling rates. The solidification point decreases with increasing cooling rate for all nanoparticles considered. At a constant cooling rate, the depression of the solidification point is proportional to the inverse of the cube root of the number of atoms in the nanoparticle even for the highly symmetric cluster consisting of only 54 atoms.
An isothermal reactor in which reaction solutions can be controlled at constant temperature under constant microwave irradiation was developed. This is useful for investigating microwave effects on chemical reactions that are not observed under conventional heating conditions. We devised a structure in which a heat-transfer medium with a low dielectric loss factor, which hardly absorbs any microwaves, flowed outside a spiral reaction tube and designed the basic structure of the reactor using electromagnetic simulation to optimize the energy absorption rate. The conditions for increasing the temperature controlling ability of the reactor were also investigated theoretically and experimentally by taking into consideration the influences of three elements: the velocity of the internal fluid, the material for the tube, and the velocity of the external fluid. The velocity of the external fluid had the greatest influence on temperature controlling ability and the material for the tube had the least influence under the experimental conditions. The overall heat transfer coefficient was about 3.9×102 W/(m2·K) when water flowed through the quartz reaction tube at 7.1 mm/s and the external fluid flowed outside the tube at 44 mm/s. We also tested and confirmed that the temperature of water used as internal fluid could be controlled to within ±1.5 K at 309.3 K when microwaves at 26 W were irradiated into the reactor, whereas the temperature of water was over 373 K and boiled without the heat-transfer medium flowing outside the reaction tube using a conventional method of microwave heating. In addition, we investigated microwave effects on Suzuki-Miyaura coupling reaction using the developed isothermal reactor and we confirmed that the temperatures were controlled well in the reactor. The yields obtained by microwave heating were almost the same as that obtained by oil-bath heating.
The purpose of this study is to clarify the fundamental and general features of N2O formation during the combustion of pulverized biomass under low temperature. First, the effect of various important factors, i.e., combustion temperature, volatilization process (i.e., either slow or rapid dispersion), and nitrogen content in biomass on N2O formation were investigated by theoretical analysis. The analysis of the effect of combustion temperature on the formation of nitrous oxide showed that N2O emission level increases with the decrease in combustion temperature, and both N2O and NO levels are strongly dependent on the combustion temperature. In other words, there is a trade-off relationship between the formation of NO and that of N2O. The analysis of the effect of the slow/rapid volatilization process on the formation of nitrous oxide showed that the conversion ratio of biomass-N to N2O increases with the decrease in the dispersion of volatile matter per unit time; it means that biomass-N is effectively converted to N2O during slow volatilization. Further, the gasification reactions between CO2, O2, and C occur simultaneously on the surface of biomass particles during combustion. With respect to the effect of nitrogen content in biomass, the N2O emission level increases with the increase in N-content of the biomass, while the NO emission level remains constant during low-temperature combustion.
This paper described the effect of the diesel cetane number on exhaust emissions characteristics according to various additives. In addition, the emission characteristics of test fuels blended with three additives (GTL, biodiesel and additive for improving CN) were analyzed and the potential for uses of these additives were evaluated in this study. To achieve this purpose, the test diesel vehicle with a two-thousand cubic centimeter displacement was used to analyze the emission characteristics according to the CN. Also, the NEDC (New European Driving Cycle) was applied as the test mode which is widely used as the test method for environmental certification of diesel vehicles. To analyze the characteristics of HAPs, the VOCs and PAHs were analyzed from the BTEX and the particulate matter, respectively. The analysis results revealed that the CO emissions show the largest reduction rate while the NOx+THC emissions are reduced at a low as the CN got higher. In the NEDC mode, the PM emissions in the EUDC mode were found to be at a lower level than those in the UDC mode. As for the VOCs and PAHs characteristics, the VOCs of the CN 58 show the lowest amounts. Also, the PAHs of diesel blended with GTL show the highest level, followed by those of diesel blended with biodiesel and diesel blended with cetane additive.
This article deals with a numerical study of entropy analysis in mixed convection MHD flow of nanofluid over a non-linear stretching sheet taking into account the effects of viscous dissipation and variable magnetic field. The nanofluid is made of such nano particles as SiO2 with pure water as a base fluid. To analyze the problem, at first the boundary layer equations are transformed into non-linear ordinary equations using a similarity transformation. The resultant equations are then solved numerically using the Keller-Box scheme based on the implicit finite-difference method. The effects of different non-dimensional governing parameters such as magnetic parameter, nanoparticles volume fraction, Nusselt, Richardson, Eckert, Hartman, Brinkman, Reynolds and entropy generation numbers are investigated in details. The results indicate that increasing the nano particles to the base fluids causes the reduction in shear forces and a decrease in stretching sheet heat transfer coefficient. Also, decreasing the magnetic parameter and increasing the Eckert number result in improves heat transfer rate. Furthermore, the surface acts as a strong source of irreversibility due to the higher entropy generation number near the surface.
Heat transfer and fluid flow in a single-rib mounting channel were investigated by directly solving Navier-Stokes and energy equations. The flow and thermal fields were considered to be fully developed at the inlet of the channel, and the simulation was made for spatial advancement of turbulent heat transfer. The Reynolds number based on the friction velocity at the inlet and the channel half width was 150. The Prandtl number was 0.71. The budgets for turbulent heat fluxes and temperature variance at various sections were presented and were investigated, which would be useful for testing and developing turbulence models. Near a circular vortex in front of the rib, pressure diffusion terms made an important contribution. Remarkable production terms were generated near a reattachment point. Production and dissipation terms were not dominant in front of and above the rib, and a time scale ratio exceeded 2.0 in the region.
The performance of high purity hydrogen production from methanol for a compact steam reformer with a hydrogen purification membrane was investigated experimentally. A 77 wt.% Pd/23 wt.% Ag membrane with 25µm thickness and CuO/ZnO/ Al2O3 catalyst were used. Heating was performed by a Bunsen type burner using City Gas 13A. The methanol reforming and purification of H2 were investigated at different reference catalyst zone temperatures (589-689K), pressures at the retentate side (0.2-0.5MPa), steam to methanol(S/C) ratios (0.8-1.6) and reactant flow rates (1.7 ×10-4 to 4.4×10-4 mol/s). The results show that at high reference temperature, high pressure and certain points of the reactant flow rate, the maximum hydrogen permeation rate is obtained when the S/C ratio is around 1. The modified Sieverts’ equation which considers the decrease in H2 concentration at the membrane surface, was proposed. The experimental result was lower than the permeation rate estimated by the modified Sieverts’ equation, which is probably caused by the adsorption of non-H2 species during permeation. It is further demonstrated that the modified Sieverts’ equation is able to estimate a more reasonable hydrogen permeation rate in comparison to the estimation by the ordinary Sieverts’ equation. In addition, it is shown that the compact methanol steam reformer with a Pd/Ag membrane is able to produce high purity hydrogen with very low CO concentration, which fulfills the Polymer Electrolyte Fuel Cell (PEFC) requirement (<10ppm).
Numerical simulation with the Forchheimer flow model and local thermal non-equilibrium model for porous region is performed on forced convective heat transfer in a tube partially filled with metallic foams. Flow and heat transfer of fluid in the hollow region and those of fluid in the porous region are conjugated together via the coupling conditions at porous-fluid interface. A heat flow model is proposed with special numerical treatments employed for non-equilibrium conjugated heat transfer in foam-fluid system. Velocity and temperature profiles in the flow direction are obtained and validated with analytical results. Effects of porosity, pore density, dimensionless interfacial radius and fluid-to-solid thermal conductivity ratio on flow characteristics and thermal performance are examined. Accordingly, the entrance effect is analyzed through the numerical simulation in terms of both flow and heat transfer. The present tube exhibits more excellent heat transfer performance at the expense of moderate pressure drop compared with the tube without porous material. The numerical work is not only developed for forced convection in metal-foam partially filled tube, but can also be extended to similar problem with porous-fluid interface for other porous media with significant thermal non-equilibrium effect.
Numerical study on the first stage of the high-pressure casing in an industrial synthesis gas (syngas) compressor is presented here. Detailed flow field comparisons are made between impeller/stage models. The stage model is composed of impeller, vaneless diffuser, bend and return channel, while the impeller model is composed only with impeller and vaneless diffuser. Compared to the results from stage model, the impeller model results indicate that the predicted aerodynamic performance is higher, and operating range is wider in both stall and choke side. Under the same inlet volume flow rate, the blade pressure coefficients, Mach number and flow angle in the blade passage for both models are nearly the same, suggesting that the flow field data in the rotating impeller is to some degree credible for stage performance prediction. However, as the impeller model neglects the matching effect with the downstream stationary parts, there needs some correction for stage working range with stable operation. Besides, the internal flow fields of stage using air and syngas mediums are compared respectively. Results indicate that the aerodynamic performance and operation range are different for both mediums because of different density and gas constant. For the flow field of the whole stage, large discrepancy occurs in the leading edge of the return channel under the same inlet volume flow rate. It suggests that the existing air model stage couldn't be directly used for the syngas compressor and needs redesigning.
In this study, tar formation and steam-reforming mechanisms are discussed by separating the tars into heavy, middle, and light tars. Cellulose was heated in a drop-tube furnace under an Ar or Ar/steam atmosphere. After the tars were passed through the furnace for thermal cracking and polymerization, they were trapped by filters set at different temperatures (573, 393, and 273 K), and were respectively defined as heavy, middle, and light tars. Incondensable volatiles and gaseous products were measured using gas chromatography with thermal conductivity (GC-TCD), and flame ionization (GC-FID) detectors. The middle and light tars obtained under an Ar atmosphere were first characterized using time-of-flight mass spectrometry (TOF-MS). The analysis showed that the middle tar did not contain any low-boiling-point light tar components, while the light tar did contain them. It was also found that complex species in the tars were separated to a certain degree by changing the trap temperature. Moreover, the formation of heavy tar was quite different from that of the light tar. With increasing temperature, the formation of heavy tar was inhibited, while that of the light tar was enhanced during pyrolysis. The steam-reforming characteristics of these tars were also different. The heavy tar was barely reformed at a low temperature of 873 K, even with a long residence time, while the middle tar was well reformed by steam. While it was difficult to describe the tar formation and steam-reforming characteristics when the tar was considered as a single condensable matter, the tar formation and steam-reforming characteristics were clarified by separating the tars. This study shows that, to prevent tar emissions, the formation of heavy tar, which barely reacts with steam, should be inhibited during pyrolysis by controlling the heating.
Interfacial thermal transport of multi-walled carbon nanotubes (MWNTs) is investigated by using bulk pellet specimens. Steady-state conduction method gives thermal conductivity of 1 to 4 W/mK for the pellets with mass density from 0.2 to 0.35 g/cm3. This low thermal conductivity is due to the thermal boundary conductance between the nanotubes. Computational analysis is conducted for the pellet modeled as a random network of spherocylinders (SCs) and calculated dependency of thermal conductivity on pellet density shows good agreement with experimental data when we treat non-uniform SCs. By comparing the experimental and computational results, the thermal boundary conductance between two MWNTs can be taken as 1.5×10-8 W/K. This result agrees well with the reported data obtained by individual measurement, which suggests this simple method is applicable to probe the interfacial thermal phenomena of nanomaterials. An improved scaling law, k ∝ ρ2.14, for thermal conductivity of MWNTs aggregations is also proposed and discussed.
Combustion oscillation of a self-excited thermo-acoustic phenomenon occurs inside the gas turbine combustor. Its excessive pressure fluctuation may impair the gas turbine engine operation, and could result in hardware severe damages. Therefore, combustion oscillation is one of the problems of the gas turbine development. In this paper, acoustic liner of an acoustic damping appending device to suppress the combustion oscillation is developed and discussed. Acoustic liner consists of a perforated plate, which the acoustic analysis models have been researched well at other papers referenced in this paper, and back cavity. The accuracy of these acoustic analysis models was verified by the laboratory model tests, and then the effectiveness that acoustic liner can suppress combustion oscillation at high frequencies was verified by the actual engine operation tests. This suppression method can be designed without the detailed combustion prediction. Hence the credible design is relatively easy. As the results of the actual engine operation tests shown in this paper, the gas turbine combustor with acoustic liner can be operating robustly without worrying about high frequency oscillation, and future more contributions to the combustor development can be expected.
We perform detailed numerical simulations of natural convection in a reservoir model induced by surface heating. The transient behaviour of the flow, including streamlines, isotherms, surface velocity profiles, volumetric flow rates, heat transfer rates, and temperature averaged over the local water depth are presented at different Rayleigh numbers. At an instantaneous time within the sloping bottom region, the surface velocity initially increases and then decreases with an increase of the offshore distance. Over a certain range of Rayleigh numbers, the location of the peak surface velocity may initially move in the offshore direction and then retract with the elapse of time. The point at which the rates of horizontal conduction and convection balance also retracts towards the tip region with the elapse of time. Some insight into understanding the mechanisms that drive the flow is provided according to the detailed transient behaviour of the flow.
This paper aimed to study the feasibility of application of infrared thermography to detect osteoarthritis of the knee and to compare the distribution of skin temperature between participants with osteoarthritis and those without pathology. All tests were conducted at LACM (Laboratory of Mechanical Stresses Analysis) and the gymnasium of the University of Reims Champagne Ardennes. IR thermography was performed using an IR camera. Ten participants with knee osteoarthritis and 12 reference healthy participants without OA participated in this study. Questionnaires were also used. The participants with osteoarthritis of the knee were selected on clinical examination and a series of radiographs. The level of pain was recorded by using a simple verbal scale (0-4). Infrared thermography reveals relevant disease by highlighting asymmetrical behavior in thermal color maps of both knees. Moreover, a linear evolution of skin temperature in the knee area versus time has been found whatever the participant group is in the first stage following a given effort. Results clearly show that the temperature can be regarded as a key parameter for evaluating pain. Thermal images of the knee were taken with an infrared camera. The study shows that with the advantage of being noninvasive and easily repeatable, IRT appears to be a useful tool to detect quantifiable patterns of surface temperatures and predict the singular thermal behavior of this pathology. It also seems that this non-intrusive technique enables to detect the early clinical manifestations of knee OA.
Complete numerical simulations are performed for bubble growth in water jet impingement on a hot plate. The governing equations for the conservation of mass, momentum and energy are numerically solved in the liquid, vapor and air phases. The liquid-vapor and liquid-air interfaces are tracked by a level-set method which is modified to include the effect of phase change at the liquid-vapor interface. The level-set approach is combined with a non-equilibrium k-ε turbulence model. The effects of jet velocity, jet temperature and wall superheat on the bubble growth in water jet and the associated flow and heat transfer are quantified.
In order to reconstruct the arbitrary shaped incompressible velocity field with noises, a new data-processing fluid dynamics (DFD) based upon the seamless immersed boundary method is proposed. The velocity field with noises is reconstructed by the Helmholtz's decomposition. The performance of DFD is demonstrated first for the reconstruction of velocity with noises and erroneous vectors. Also, the seamless immersed boundary method is incorporated into the velocity reconstruction for complicated flow geometry. Some fundamental flow fields, i.e., the square cavity flows with a circular cylinder and a square cylinder, are considered. As a result, it is concluded that the present DFD based upon the seamless immersed boundary method is very versatile technique for velocity reconstruction of the arbitrary shaped incompressible velocity with noises.
The performance parameters of thermal protection system are essential for the design and optimization of high-speed aircraft. The flight-ground conversion is a valid method to provide the effective support to the design of the thermal protection structure (TPS), because the performance data of TPS were generally obtained from wind tunnel test and should be conversed to the corresponding environment. In this paper, the similarity parameters of heat conduction and thermoelasticity equations are studied, the similarity criteria proposed, and the effectiveness of some of the similar parameters are calculated and analyzed. The research results indicated that wind tunnel test can be better designed using the proposed similarity criteria, and the data obtained from wind tunnel test can be modified more rational to accommodate the reality flight condition so as to improve the precision and the efficiency of wind tunnel experiment.
Experimental investigation of the relationship between mass transfer and turbulent drag reduction of the drag-reducing channel flow with dosed polymer solution from a wall was carried out. Planar laser induced fluorescence (PLIF) measurement was employed to investigate the mass transfer of dosed polymer solution. In addition, the polymer concentration distribution was measured directly by extracting samples from the channel flow (Sampling method). In the PLIF measurement, Reynolds number based on the channel height was set to 20000 and poly(ethylene oxide) was used as a polymer. The polymer solution with a concentration from 10 ppm to 200 ppm was dosed at 3 L/min from the whole surface of the wall. As a result, in the case of water flow, dosed dyed water was ejected from the wall and was well diffused by the strong turbulent eddy motion. In contrast, when the polymer solution was dosed from the wall, the diffusion was largely suppressed in the near-wall region and drag reduction occurred. This result indicates that turbulent diffusion was suppressed in the near-wall region and momentum transport in the wall-normal direction was also largely suppressed. Moreover, because the polymer solution could be provided continuously into the channel flow downstream of the leading edge of the blowing wall, the drag reduction rate was reduced downstream. Finally, we estimated the Sherwood number based on the mass transfer logic, and the relationship between the drag reduction and mass transfer was discussed.
Some recent results of molecular dynamics simulations of the condensation/evaporation and velocity distribution of n-dodecane (C12H26), the closest approximation to Diesel fuel, at a liquid-vapour interface in equilibrium state are briefly described. It is shown that molecules at the liquid surface need to gain relatively large translational energy to evaporate. Vapour molecules with large translational energy can easily penetrate deep into the transition layer and condense in the liquid phase. The evaporation/condensation coefficient is shown to be controlled mainly by the translational energy. The properties of the velocity distribution functions of molecules at the liquid, interface and vapour regions are summarised. It has been shown that the distribution functions of evaporated and reflected molecules for the velocity component normal to the surface deviate considerably from the Maxwellian, while the distribution function for all molecules leaving this surface (evaporated and reflected) is close to Maxwellian. The evaporation coefficient has been shown to increase with increasing molecular energy in the direction perpendicular to the surface. These properties have been recommended to be taken into account when formulating boundary conditions for kinetic modelling.
Thermal transport of coherent phonons at a few picosecond pulse heating is studied by molecular dynamics method in the presence of diffusion. The presence of the acoustic and optical phonons at the high density heating of nanostructures has been confirmed theoretically and experimentally in the absence and presence of diffusion. In the present study, coherent phonon spectral characteristics are compared for different shapes of the heating pulse, half-pulse square, Gaussian, and triangle at the time of propagation, in the Lennard-Jones (LJ) nanoribbon model for emitted train (3 to 5) of coherent phonons. In order to analyze the spectral contributions of individual phonons, in the molecular dynamic (MD) model, density of states (DOS) at propagation subregions is utilized for identification of coherent phonon spectra for the different pulse shapes and heating times in the nanoribbon sample. The MD equations can resolve wave motion for coherent phonons over sampling subregions that correspond in size (several atomic layers) to a single phonon vibration period. However, the definition of DOS does not distinguish the spectral characteristics of each of individual phonons as well as separates the diffusional spectrum from the coherent phonons one. In the presence of diffusion, spectrum of the first generated phonon is studied by varying the coherent phonon position inside of time interval that is used for the quasi-equilibrium DOSes defined over one of the central regions. The identification of diffusional frequencies for the nanoribbon permit to extract the coherent phonon frequencies characteristic for the propagation in the nanoribbon studied. In the case of propagating coherent phonons, it was shown that the frequencies of some phonons can be identified. Increase of the temporal resolution for the DOS calculation is shown to be critical for separation of the diffusion process spectrum and the one of phonons at the same area of calculation.
The objective of this study is to investigate a thermal field in the turbulent boundary layer by means of direct numerical simulation (DNS), in which the wall heating has suddenly vanished in the downstream region, i.e., the wall is heated by a constant temperature condition followed by an adiabatic condition. The DNS of spatially developing boundary layers with heat transfer using the generation of turbulent inflow data method has been conducted. In this study, two types of flow field with heat transfer are investigated via DNS. One is a turbulent boundary layer along flat plate, and the other is a turbulent boundary layer over the forward-facing step. In both cases, constant temperature wall followed by adiabatic wall condition is adopted. In particular, the turbulent heat transfer phenomena around suddenly-changing wall thermal condition are revealed. In the case of forward-facing step flow, since the adiabatic wall thermal condition is applied on the step, a peculiar phenomenon is observed in comparison with the case of flat plate. DNS results clearly show the statistics and structure of turbulent heat transfer in a constant temperature wall followed by an adiabatic wall. Also, DNS clearly shows the wall-limiting behaviour of turbulence in thermal field whose index number with reference to the distance from the wall changes due to the modification of wall thermal conditions, which may be useful for the turbulence modelling.
The freezing-thawing processes of the soil around the buried oil and gas pipelines in permafrost regions due to the effect of the pipe and atmospheric environment may bring about dangers to the pipelines as frost heave and thaw settlement occur and go on, and then the buried pipes may face huge challenges for safe operation. To analyze the thermal effect of the buried pipe on the surrounding soil, a two-dimensional computational model of the soil temperature fields was established based on the process of the heat transfer with phase change in the soil. The temperature fields and the thaw characteristics of the soil around the operating pipeline in permafrost regions were studied using numerical methods via the software FLUENT in this paper. The developments of the maximum thawed cylinders and corresponding thaw depths under the pipeline within operation life cycle were predicted and analyzed for various medium temperatures, water contents of soils, insulation layer thicknesses and imposed boundary conditions by climatic warming. In addition, the maximum thaw settlement of the soil under the pipeline in 5 typical permafrost areas along the Russia — China oil pipeline (the section in China) within operation life cycle was calculated. The medium temperatures were assumed to be constant and sinusoidal. The results indicated that the maximum thaw depths and thawed cylinders around the pipeline in permafrost regions enlarged with time elapse and the decrease in water content of the soils under the same boundary conditions. The maximum thaw depths and thawed cylinders increased with the increase of medium temperatures after the same operation time. The insulation layer weakened heat exchange between the pipeline and the surrounding soils and thus reduced the development of the thawed cylinders effectively during the early operation period. This research may provide an effective method for engineering application, and the results may provide references for predicting the thaw settlement of the soil and pipeline in permafrost regions.
We investigated the initial stage of nucleate boiling on ideally smooth surface with a molecular dynamics simulation technique. Lennard-Jones (LJ) model liquid was confined in a rectangular simulation cell, contacting with a flat smooth solid wall. The wall consists of fcc crystal of LJ-like particles. After the system was thermally equilibrated, the temperature of wall particles was raised to transfer thermal energy to the liquid. We examined two cases, the overall heating where the surface temperature is kept constant all over the area, and the partial (spot) heating where two regions of heating and cooling are placed. In both cases, when the liquid in the vicinity of the heating surface obtains sufficient energy, it thermally expands and its pressure decreases, leading to formation of bubble nuclei of atomic size. The inception time of nucleation was found to be affected by surface wettability as well as the surface temperature. When the surface is hydrophobic and the heating area is small, size oscillation of the generated bubbles was observed.