Radial Temperature Distribution Research Articles

Start-up of waxy crude oils in pipelines

The transport of waxy crude oils in aggressive environments, usually at low temperatures, is very common. Whenever a shutdown occurs and the oil rests at temperatures below the gelation point (GT), wax crystals appear, forming an interlocked structure, with a gel character. The start-up of waxy crude oils after a shutdown is a great problem concerning flow assurance. In fact, a minimal pressure, or equivalent minimal wall shear stress, is required to restart the flow. Usually, the minimal wall shear stress is assumed to be the rheometer yield stress, which is reasonable, but not necessarily true. There are different possible causes for the discrepancies between the rheometer’s data and that measured in a pipeline start-up approach. For a faithful comparison between them, we must consider: 1) the thermal history experienced by the oil, mainly the initial cooling temperature; 2) the radial temperature distribution in the pipeline; 3) the oil shrinkage during the cooling process and 4) the different mechanisms of gel’s rupture (adhesive and cohesive failures). In this work, we use a pneumatic system in which a piston is pressurized step by step to displace the viscoplastic material. By doing so, we record the minimal shear stress necessary to overcome the cohesive forces near the wall. The preliminary tests were carried out with Carbopol solutions before conducting the main tests with a waxy crude oil in a range of initial cooling temperatures. The values of critical stresses in the start-up approach fitted quite well the rheometer yield stress data.

Numerical and Experimental Investigation on an Effusion-Cooled Lean Burn Aeronautical Combustor: Aerothermal Field and Emissions

Lean burn combustion is increasing its popularity in the aeronautical framework due to its potential in reducing drastically pollutant emissions (NOx and soot in particular). Its implementation, however, involves significant issues related to the increased amount of air dedicated to the combustion process, demanding the redesign of injection and cooling systems. Also, the conditions at the combustor exit are a concern, as high turbulence, residual swirl, and the impossibility to adjust the temperature profile with dilution holes determine a harsher environment for nozzle guide vanes. This work describes the final stages of the design of an aeronautical effusion-cooled lean burn combustor. Full annular tests were carried out to measure temperature profiles and emissions (CO and NOx) at the combustor exit. Different operating conditions of the ICAO cycle were tested, considering Idle, Cruise, Approach, and Take-off. Scale-adaptive simulations with the flamelet generated manifold (FGM) combustion model were performed to extend the validation of the employed computational fluid dynamics (CFD) methodology and to reproduce the experimental data in terms of radial temperature distribution factor (RTDF)/overall temperature distribution factor (OTDF) profiles as well as emission indexes (EIs). The satisfactory agreement paved the way to an exploitation of the methodology to provide a deeper understanding of the flow physics within the combustion chamber, highlighting the impact of the different operating conditions on flame, spray evolution, and pollutant formation.

Experimental and Numerical Comparison of Small-scale Gaseous Fire Whirls

Fire whirls can occur during urban fires, especially in intense fires in combustible building structures, and more often in forest or wildland fires. They are a special swirling diffusion flame characterized by significant enhancement in burning rates, flame heights and flame temperatures, along with a strong whirling motion of the flame. This whirling motion can pick up large firebrands and scatter them afar leading to spot fires. Many researchers have published experimental work on small- and medium-scale pool fire whirls and gaseous fuel fire whirls using split cylinders and various fixed-frame apparatus to investigate axial and tangential velocity profiles, axial and radial temperature distribution, burning rates, and flame heights. Likewise, several researchers have attempted to predict the experimental results of fire whirls using different modelling approaches and simulation software.In this paper, experiments were undertaken to study the dynamics of propane gas fire whirls in a small-scale, square-based, fixed-frame apparatus. Measurements of flame height and temperature profiles (both axial centerline and radial) were made for a low initial momentum burner of 76.2 mm internal diameter. The burner was operated at a volumetric flow rate of 6 dm3/min, which gave a heat release rate of 9.12 kW. Simulations using Fire Dynamics Simulator (FDS 6.6.0) and ANSYS Fluent 17.1 were performed to compare with the experimental measurements. Four separate mesh refinements were employed and four different sub-grid-scale (SGS) turbulence models were tested with FDS. The Deardorff, Wall-Adapting Local Eddy-viscosity (WALE), and dynamic Smagorinsky models, formed stable fire whirls for the two largest mesh refinements.The temperature profiles were overpredicted at the core of the flame with FDS and underpredicted with Fluent. The FDS simulation prematurely predicts the peak temperature for the axial centreline profile, whereas with Fluent the axial temperature profile matches the general trend of the experimental measurements.The visible flame height, determined through image processing, was approximately 0.88 ± 0.06 m, which corresponds to a measured temperature of ∼500°C. The 500°C temperature contour was used as a rough approximation of the flame height in the numerical simulations. It was found that with Fluent the 500°C contour grew until the fire whirl stabilized and reached the top of the hood at 1.6 m, clearly overpredicting the flame height. The height estimates based on the predicted 500°C contours show a strong dependence on the mesh resolution. This is primarily due to increased instability resulting in more mixing and spreading of the temperature for the coarser mesh size. However, the simulated flame heights show less dependence on the SGS turbulence models.

Pulsed Method of Rapid Cooling of Thermally Massive Bodies

We present the data on pulsed rapid intense cooling of thermally massive bodies. The process of rapid pulsed heating consists of alternating periods of intense heating and holding in which the main part of heat is supplied to the metal by convection and guarantees high rates of heating of the metal just in the period of heating. These cooling conditions exert a strong influence on the radial distribution of temperature inside the metal in the course of cooling.

Numerical and Experimental Studies of a Wet Multidisc Clutch on Temperature and Stress Fields Excited by the Concentrated Load

The outer circlip constraint is a typical way for a wet multidisc clutch to limit the axial displacement of friction components. The pressure transmission mechanism in a clutch, excited by the concentrated reactive force of the outer circlip, is revealed by a simplified pressure calculation model. Moreover, a finite element model is constructed to investigate the contact pressure distributions on friction surfaces. Thermal analysis to determine the radial temperature distributions on friction surfaces is performed numerically. The computational results indicate that the concentrated reactive force contributes to the dissimilar distributions of the contact pressure along the radial and axial directions. The radial contact pressure considerably affects the temperature fields on friction surfaces, which is verified effectively by bench tests under the creeping condition. Both simulation and experimental results demonstrate that the outer circlip is identified to be one of the main reasons for the expansion of the radial temperature difference. In order to smooth the contact pressure in the radial direction, three new designs for a circlip are proposed, including an inner circlip, medial circlip, and both inner and outer circlips. Considering the utility and feasibility of the four cases, the design with both the inner and outer circlips is the most appropriate one.

Radial distributions of power and isotopic concentrations in candidate accident tolerant fuel U3Si2 and UO2/U3Si2 fuel pins with FeCrAl cladding

Monte Carlo simulations show similarity on radial distributions of power and isotopic concentrations at any effective full power depletion time among five kinds of fuel-cladding combinations with the same cycle length, including the normal UO2-zircaloy combination, the candidate Accident Tolerant Fuel (ATF) UO2/U3Si2-FeCrAl combination, and three kinds of candidate ATF U3Si2-FeCrAl combinations. An analytical formula f(x,s) including the fuel exposure (s) and the relative radial (x) is proposed to describe the radial properties for all five kinds of fuel-cladding combinations. f(x,s) has the form of the second order polynomial term of s with the exponential type of coefficients depending on x. It is shown that the suggested function f(x,s) gives a nice description on the simulation data with rather small deviations and can immediately provide radial distribution of power, burnup, and isotopic concentrations of 235U, 238U, 239Pu, and 241Pu at any fuel exposure and relative radius. It is useful to discuss the fuel temperature through the present analytical formula. The realistic radial power distribution gives flatter radial temperature distribution compared with the uniform power distribution. Because of the different thermal conductivities of fuels and claddings and the different thicknesses of claddings, the present discussed five kinds of fuel-cladding combinations have different radial temperature distributions, although their radial power distributions are quite similar. The present work provides an analytical formula to describe the radial properties of the ATF which is expected to be helpful for further neutronic and multi-physics coupling studies.

Modeling sterilization value and nutrient degradation in the thermal processing of liquid foods under diffusive laminar flow with associations of tubular heat exchangers

AbstractThermal processing of a liquid food under diffusive laminar flow was modeled coupling fluid flow, heat transfer, diffusion, and inactivation/degradation kinetics. Model considers associated countercurrent tubular exchangers for heating and cooling and the holding tube. Axial and radial distributions of temperature and concentration in the food product (non‐Newtonian power‐law flow) were accounted and parameters for effective diffusion of heat and mass were used to model enhanced transfer from corrugate tubes or coils. Heat exchanged with environment was included. A case study of 40.2 °Brix blackberry juice pasteurization is presented, and results discussed. Influence of effective diffusivities, inlet heating media temperature and product flow rate were analyzed, and it was possible to minimize anthocyanin degradation while meeting food safety requirements (5‐log reduction on yeasts) considering the contributions from the whole process.Practical applicationsMathematical modeling of a chemical process based on first principles, also called physical or phenomenological models, is a powerful tool to couple fluid flow, heat transfer, mass diffusion, and reaction kinetics, thus creating a virtual prototype of the process that is useful for design, analysis, control, and optimization. The model presented considers important features that are often neglected in process design, but without unnecessarily increasing complexity and computational requirements, such as in a CFD model (computational fluid dynamics) that requires a detailed representation of the geometry and specialized software and hardware for simulation. Lower computational times allow the use of this model to solve optimization or predictive control problems with a reliable representation of transport phenomena and reaction kinetics.

Properties of an accretion disc with a power-law stress–pressure relationship

Recent numerical simulations of magnetized accretion discs show that the radial-azimuthal component of the stress tensor due to the magnetorotational instability (MRI) is well represented by a power-law function of the gas pressure rather than a linear relation which has been used in most of the accretion disc studies. The exponent of this power-law function which depends on the net flux of the imposed magnetic field is reported in the range between zero and unity. However, the physical consequences of this power-law stress-pressure relation within the framework of the standard disc model have not been explored so far. In this study, the structure of an accretion disc with a power-law stress-pressure relation is studied using analytical solutions in the steady-state and time-dependent cases. The derived solutions are applicable to different accreting systems, and as an illustrative example, we explore structure of protoplanetary discs using these solutions. We show that the slopes of the radial surface density and temperature distributions become steeper with decreasing the stress exponent. However, if the disc opacity is dominated by icy grains and value of the stress exponent is less than about $0.5$, the surface density and temperature profiles become so steep that make them unreliable. We also obtain analytical solutions for the protoplanetary discs which are irradiated by the host star. Using these solutions, we find that the effect of the irradiation becomes more significant with decreasing the stress exponent.

Wellbore temperature distribution during circulation stage when well-kick occurs in a continuous formation from the bottom-hole

In the exploration and development of deep reservoir resources, the occurrence of well kick will have a very important impact on wellbore temperature distribution during circulation. This study aims to investigate the wellbore temperature distribution during circulation when well kick occurs in a continuous formation from the bottom-hole. A transient temperature model was established by the first law of thermodynamics. The finite volume method was used to discretize the model and the under-relaxation iterative method was used for numerical calculation. The validity of the model has been verified by the previous typical models and actual measured temperatures. The calculation results showed that the highest temperature of annulus fluid was at the bottom-hole and annulus fluid temperature increased as the formation high-temperature fluid continuously enters the annulus when well kick occurred. The occurrence of well kick led to a significant increase in outlet fluid temperature and bottom-hole fluid temperature. The occurrence of well kick changed the formation radial temperature distribution and the temperature difference distribution between the annulus fluid and the drilling fluid inside the drill string. The flow rate of the well kick and the location of the well kick occurred also had an important impact on wellbore temperature distribution.

Polytypism in Hexagonal Tungsten Trioxide: Insights from Ab Initio Molecular Dynamics Simulations

Temperature-dependent microstructural evolution of hexagonal WO3 (h-WO3) polytypes is explored via ab initio molecular dynamics calculations within the density-functional theory framework. We present simulated finite temperature radial distribution function and X-ray diffraction patterns to reinterpret recent experimental pair distribution function analysis. This work clearly demonstrates that after a more careful analysis of the finite temperature structural properties of h-WO3, an intermediate H1-like structure is predicted at higher temperatures, while the more stable H4 polytype (and not the experimentally suggested H2 polytype) is obtained nearer ambient temperatures. This is further corroborated by our electronic structure analysis which shows that the electronic band gap energy of the ambient temperature H4-like structure agrees much better with the experimentally reported band gap energies.

Relation of heat and mass transfer in wax diffusion in an emulsion of water and waxy crude oil under static condition

Wax diffusion characteristics are of great importance to the wax deposition prediction in crude oil pipelines. The wax diffusion process of water in oil emulsion (w/o emulsion) is more complicated on account of the influence from dispersed water phase. This study aims to reveal the relation of heat and mass transfer in wax diffusion with the effect of dispersed water phase. Via wax diffusion experiments of w/o emulsion on cold finger apparatus, the variations of the average wax diffusion rate with water fraction, temperature difference and diffusion period are obtained under static condition. A method to calculate the equivalent thermal conductivity of wax deposit is proposed according to emulsion property and deposit composition. Based on this, the radial temperature fields inside crude oil and emulsion after different diffusion periods are simulated. The results provide the evidence that the existence of water phase is able to influence wax diffusion by altering the radial temperature distribution. In addition, by comparing the variations of radial temperature field and wax diffusion rate, the close interaction effect between them is clarified. This study will offer useful information to the mechanisms study and the prediction of wax deposition in oil-water multiphase transportation pipelines.

Spatially resolved temperature measurement in the carbon dioxide arc under different gas pressures.

Carbon dioxide (CO2) is a promising alternative to sulfur hexafluoride for high-voltage circuit breaker applications. It is important to have a detailed understanding of CO2 arc properties. In this paper, radial temperature distribution of the free burning direct current arc in pure CO2 was investigated. Optical emission spectrometry was applied under different pressures (0.5atm, 1atm, and 1.5atm) and at different axial positions (1mm, 2mm, 3mm above the cathode). Assuming local thermodynamic equilibrium, the Fowler-Milne method was adopted for O I 715.67nm and O I 777.19nm in the periphery of the arc, and the single-line method was adopted for C II 657.81nm near the center of the arc. Radial temperature profiles obtained by these two methods were combined at the position where normal temperature was assigned. The results indicate that near the center of the arc, higher pressure would lead to lower temperature; as the distance from the cathode to the position measured increases, the maximum temperature in the arc center would decrease. In addition, the temperature would decrease more sharply toward the periphery if the central temperature of the arc is higher.

Investigation of the Edge Plasma Parameters and Measurements of the Plasma Longitudinal Rotation Velocity by a Mach Probe in a Lithium Experiment on the T-11M Tokamak

The edge plasma parameters were measured by means of a Mach probe in a lithium experiment on the T-11M tokamak. The angular and radial distributions of the ion saturation current, along with the radial distribution of the electron temperature, were obtained in different modes of tokamak operation. The radial distributions of the electron temperature and ion saturation current in the main operating mode (L-mode) revealed a peak in the scrape-off-layer of the vertical limiter (lithium emitter), which can indicate the formation of a magnetic island in this region. The measured plasma flow velocity along the magnetic field was found to be close to one-half of the ion sound velocity for Li+ ions.

Effect of imposed circulation on temperature and velocity in general fire whirl: An experimental investigation

This paper presents experimental effort to reveal the independent effect of imposed circulation on the evolutions of thermal and velocity fields in general fire whirl. Experiments were performed by using a small-scale rotating screen facility with finely controlled imposed circulation (Γ) under fixed heat release rate of Q˙ = 7.5 kW. It was found that the centerline excess temperature and axial velocity varied consistently with the flame shape. The buoyant regime and circulation-controlled regime were distinguished in specific ranges of Γ. For the buoyant fire whirl, the centerline axial velocity increased by a 1/2 power law of height in the entire continuous flame. For the circulation-controlled fire whirl, the centerline axial velocity depended on height by a 1/3 power law at lower levels and differed slightly with height in the rest continuous flame. At a certain level, the variations of centerline excess temperature and axial velocity with Γ varied noticeably in three consecutive axial regions. In the bottom region, the behavior of centerline axial velocity was closely related to the occurrence of a special jump in flame size and the drop of its height with increasing Γ. In the upper region, the centerline axial velocity decreased first, then increased and decreased, and finally increased quickly again with Γ. It was found that the imposed circulations at the interval boundaries increased steadily with height. The centerline axial velocity increased a little faster than the tangential velocity with Γ in the circulation-controlled regime. The radial distributions of excess temperature and axial velocity varied significantly with Γ and height, and unimodal profile, plateau-type profile and hump-type profile were found. The radial profiles of tangential velocity had self-similarity within the mean flame height. The Richardson number increased with Γ and height in the buoyant fire whirl and exhibited some decrease in the circulation-controlled fire whirl.

Analisis Termal-hidrolik Reaktor Cepat Berpendingin Gas (Gas Cooled Fast Reactor) Menggunakan Metode Runge Kutta

The Research of Gas Cooled Fast Reactor (GCFR) thermal-hydraulics analysis has been done. This reseacrh aim to solve fuel rod heat conduction equation by Runge Kutta method and to get thermal-hydraulics parameters such as coolant axial temperature distribution, pressure drops, convection heat transfer coefficient, and fuel rod radial temperature distribution. Heat transfer of the reactors was assumted steady state (time independent) then obtained coolant inlet tempertaure about 450 o C, outlet temperature about 474,752 o C and convection heat transfer coefficient about 2,5903 W/cm 2 o C. Pressure drop by friction was 0,16968 bar, pressure drop by form was 0,31292 bar, pressure drop by gravity was 0,20580 bar and total pressure drop was 0,68838 bar. While centerline fuel obtained the maximum temperature of fuel rod about 2720,33812 o C and the lowest fuel rod temperature at cladding surface about 488,8205 o C.

Analisis Termal-hidrolik Reaktor Air Bertekanan (Pressurized Water Reactor) Menggunakan Metode LU Faktorisasi

The Research about Pressurized Water Reactors (PWR) thermal-hydraulics analysis has been done. This reseacrh aim to solve fuel rod heat conduction equation by LU Factorization method and to get thermal-hydraulics parameters such as coolant axial temperature distribution, pressure drops, convection heat transfer coefficient, and fuel rod radial temperature distribution. Heat transfer of the reactors was assumted steady state (time independent) then obtained coolant inlet tempertaure about 300 o C, outlet temperature about 326,6 o C and convection heat transfer coefficient about 1,43 W/cm 2 o C. Pressure drop by friction was 0,423 bar, pressure drop by form was 0,51 bar, pressure drop by gravity was 0,25 bar and total pressure drop was 1,183 bar. While centerline fuel obtained the maximum temperature of fuel rod about 1.589,2 o C and the lowest fuel rod temperature at cladding surface about 360,9953 o C

Development of the arc plasma torch operation mathematical model for spheroidization of fine-dispersed powders

Brief review of the fine-dispersed powders production technologies for additive laser technologies is presented in the paper. Non-stationary mathematical model of DC plasma torch operation and stationary model of the reactor operation for powder spheroidization have been developed. Air is used as the plasma-forming gas. The influence of the transporting flow and compressing gases on the characteristics of the plasma flow is analyzed. Verification of the non-stationary mathematical model of DC plasma torch operation is based on experimental studies. Radial temperature distributions obtained from the results of numerical simulation and spectral diagnostics of the plasma torch PN-V1 are presented. The results of the experimental study describing plasma flow behavior for given parameters of the fine-dispersed powders spheroidization process are presented in the paper.

Hydrothermal hydrolysis pretreatment of microalgae slurries in a continuous reactor under subcritical conditions for large–scale application

Biofuel production from microalgae biomass via fermentation is promising as a renewable technology. Hydrothermal hydrolysis pretreatment of microalgae slurry prior to fermentation is an efficient method to enhance the biogas production. Herein, we report, for the first time, the hydrothermal hydrolysis pretreatment of microalgae slurries in a continuous reactor under subcritical conditions (temperature <200 °C, pressure ∼2 MPa). The results show that the yield of carbohydrates from microalgae slurries first increased and then decreased with increasing average outlet temperature, which reached a maximum value at 160 °C for 10 min. This phenomenon was caused by the combined effects of the release of organics and biochemical reactions. Notably, the radial temperature distribution of microalgae slurries, influenced by the mass fraction and the flow rate during the heating process, had effects on the yield of organics (carbohydrates, proteins) in aqueous solution. This study provides useful information for the operating conditions in large-scale applications.

Investigation of wax deposition and effective diffusion coefficient in water-in-oil emulsion system

As pipeline transportation is widely used in the petroleum industry, the problem of wax deposition is a severe threat to the safety of oil and gas transportation. In addition, the mechanism of wax deposition is very complicated due to the presence of water phase. This paper tries to clarify the effects of water fraction, temperature difference and experimental period on the wax deposition process in water-in-oil emulsion system by a series of static cold finger experiments. The experimental results reveal that the average diffusion rates decrease with increased water fraction, longer experimental period and reduced temperature difference. Furthermore, on the basis of wax deposition experiments in cold finger apparatus and radial temperature distribution simulations via Fluent, the influence of water phase on heat transfer occurring in the wax molecular diffusion process is revealed, and relationship between mass transfer and heat transfer is investigated. Additionally, the effective diffusion coefficient of wax molecules is calculated on the basis of experimental and simulation results. The calculated effective diffusion coefficients using this approach are significantly lower than the calculated results from conventional methods. This explains the remarkable disparity with previous works due to underestimating the influence of dispersed water.

Analysis of the influence of pellet-to-cladding gap on the transient heat transfer in nuclear fuel rods via the integral transform technique

This work analyzes the transient conduction heat transfer with variable source term in a nuclear reactor fuel rod. Three models are presented to determine the transient temperature distribution across the fuel rod. The results obtained by each modeling are compared in order to evaluate the influence of the gap, as well as the cladding on the thermal problem. The main contribution of this paper is the third modeling, where a lumped analysis that encompasses the cladding material is performed, while considering convective heat transfer in the gap region. For this modeling, the integral transform technique is employed to solve the problem and the resulting ordinary differential equations system is evaluated numerically. Special attention is given to variations in the critical time as a function of Biot number, since this is an important engineering parameter to infer the beginning of the cladding melt down process. The results showed that the gap does not have a major influence in the cladding temperature profile; however, it has great influence on fuel radial temperature distribution.