Numerical Calculations Of Heat Transfer Research Articles

Evaporation of a heated saline marble: Interplay of interfacial and thermal effects

A multistage evaporation of continuously heated liquid marbles, coated with polytetrafluoroethylene colloidal particles was investigated experimentally and theoretically. Liquid marbles initially contained an almost saturated aqueous NaCl solution. Numerical heat transfer calculations and analytical evaluations of initial evaporation showed that the equilibrium marble temperature and saturated salt concentration are reached relatively rapidly compared to the “long” marble evaporation time. This means that a typical long period of marble evaporation is determined by the low permeability of the salt crust formed beneath the marble colloidal shell. A theoretical estimation of the total evaporation time of marble water based on Darcy's law for the porous crust and the marble shell is suggested. A comparison with experimental data allowed obtaining an approximate value of the very low permeability of this bilayer barrier to marble evaporation. The proposed phenomenological model of the saline marble evaporation provides insight into why marble evaporation is so slow.

An effect of a snow cover on solar heating and melting of lake or sea ice

Solar radiative heating and melting of lake and sea ice is a geophysical problem that has attracted the attention of researchers for many years. This problem is important in connection with the current global change of the climate. Physical and computational models of the process are suggested in the paper. Analytical solutions for the transfer of solar radiation in light-scattering snow cover and ice are combined with numerical calculations of heat transfer in a multilayer system. The thermal boundary conditions take into account convective heat losses to the ambient air and radiative cooling in the mid-infrared window of transparency of the cloudless atmosphere. The study begins with an anomalous spring melting of ice on the large high-mountain lakes of Tibet. It was found that a thick ice layer not covered with snow starts to melt at the ice-water interface due to volumetric solar heating of ice. The results of the calculations are in good agreement with the field observations. The computational analysis showed a dramatic change in the process when the ice is covered with snow. A qualitative change in the physical picture of the process occurs when the snow cover thickness increases to 20–30 cm. In this case, the snow melting precedes ice melting and water ponds are formed on the ice surface. This is typical for the Arctic Sea in polar summer. Known experimental data are used to estimate the melting of sea ice under the melt pond. Positive or negative feedback related to the specific optical and thermal properties of snow, ice, and water are discussed.

Studies on Influence of Chromium Layer on Inner Surface of Steel Tube on Heat Transfer

This paper presents the results of numerical calculations of heat transfer in the steel wall of a tube with different chromium layer thicknesses on the inner surface for a single and a sequence of heat impulses. In modeling the thermal phenomenon, the tube material was assumed to be homogeneous, and the inner surface of the tube had a protective chromium layer. The calculations were performed for temperature-dependent parameters of the 30HN2MFA steel and chromium. The tube was divided into 30 zones. The time varying the heat transfer coefficient and the heat flux density on the inner surface of the tube in each zone were calculated. The numerical simulations of heat transfer in the tube wall were performed using the FEM implemented in COMSOL Multiphysics software. On account of the high thermal conductivity of chromium, after the first and subsequent heat impulses, as the thickness of the chrome layer increases, the highest peak temperature drops, while the temperature inside the steel wall of the tube increases. After the first heat impulse, the temperature of the wall at a depth 0.5 mm below the inner surface is twice as high for a tube with a 200 µm thick chrome coating compared to a tube without this coating. This paper showed that, after seven heat impulses, the temperature of the steel tube substrate does not exceed 750 °C, at which the shrinkage of the material occurs, causing cracks in the chrome coating.

Definition and Determination of Fin Substitution Factors Accelerating Thermal Simulations

The effort of numerical heat transfer calculations increases with the complexity and size of the domains and surfaces under consideration. When calculating heat transfers on finned arrays, one way to reduce this effort is to substitute the fins. Therefore, this work defines the fin substitution factor by considering that a smooth surface behaves thermally sufficiently similar to a specific finned array. A process for determining the case-specific most accurate analytical computation path for fin substitution factors is also defined. The performance of the process and the resulting solution is demonstrated using the example of vertical rectangular finned arrays under natural convective heat transfer with a constant fin base temperature and air as the surrounding fluid. The heat flows determined in solid-state simulations of flat plates considering fin substitution factors deviated by an average of 6.2% from the heat flows resulting from detailed CFD simulations of the corresponding finned arrays.

Numerical calculations of heat transfer during FC-72 flow in minichannels performed with ANSYS software

The main aim of the paper was to apply numerical calculations in ANSYS software to solve problems related to steady-state convective heat transfer during FC-72 flow in a minichannel heat sink. A mathematical model of fluid flow in a minichannel heat sink was proposed. Data from research conducted on the experimental stand with asymmetrically heated test section were selected. The element cooled by fluid flowing in minichannels was a thin foil. The distribution of temperature on the outer heated foil surface was measured using infrared thermography. Thermocouples and pressure meters were installed at the inlet and outlet of the minichannels. A CAD model was prepared with respect to the geometry of the test section. The mesh was created in ANSYS Mesher. A suitable model of turbulences was selected and materials temperature dependencies were considered. The iterative process of creating the model and comparing the results obtained by the CFD calculation was used to capture the fluid behavior. The velocity magnitudes and temperature distributions were presented as the main results of numerical calculations.

Thermal analysis of a Gramme‐ring‐winding high‐speed permanent‐magnet motor for pulsed alternator using CFD

Due to the merit of very-short end turns, Gramme-ring windings are widely used in high-speed permanent-magnet motors (HSPMMs). This study investigated the thermal issues of a 300 kW, 12,000 rpm Gramme-ring-winding HPSMM with housing water jacket and forced air cooling, based on the computational fluid dynamics (CFD) and numerical heat-transfer calculations. The reasons that cause the overheat of the prototype in the experimental test are thoroughly analysed. The high wind resistance of air gap as the ventilation duct and the large interface gap between the stator lamination and housing water jacket is thought to be the main reasons. Then improvement of the ventilation structures is studied to decrease the wind resistance. Besides, the modification of water channels is also investigated to avoid the influence of a large interface gap on the water cooling performance. Finally, a new cooling structure, combining the slot ventilation ducts and inside water channels, is proposed for the HSPMM with Gramme-ring windings. With the new cooling structure, the temperature rise of the motor is decreased, and the power density can be increased.

Machine-Thermal Coupling Stresses Analysis of the Fin-Type Structural Thermoelectric Generator

The design structure and heat-transfer mechanism of a thermoelectric generator (TEG) determine its body temperature state. Thermal stress and thermal deformation generated by the temperature variation directly affect the stress state of thermoelectric modules (TEMs). Therefore, the rated temperature and pressing force of TEMs are important parameters in TEG design. Here, the relationships between structural of a fin-type TEG (FTEG) and these parameters are studied by modeling and “machine-thermal” coupling simulation. An indirect calculation method is adopted in the coupling simulation. First, numerical heat transfer calculations of a three-dimensional FTEG model are conducted according to an orthogonal simulation table. The influences of structural parameters for heat transfer in the channel and outer fin temperature distribution are analyzed. The optimal structural parameters are obtained and used to simulate temperature field of the outer fins. Second, taking the thermal calculation results as the initial condition, the thermal-solid coupling calculation is adopted. The thermal stresses of outer fin, mechanical force of spring-angle pressing mechanism, and clamping force on a TEM are analyzed. The simulation results show that the heat transfer area of the inner fin and the physical parameters of the metal materials are the keys to determining the FTEG temperature field. The pressing mechanism’s mechanical force can be reduced by reducing the outer fin angle. In addition, a corrugated cooling water pipe, which has cooling and spring functionality, is conducive to establishing an adaptable clamping force to avoid the TEMs being crushed by the thermal stresses in the body.

Tailoring weld geometry during keyhole mode laser welding using a genetic algorithm and a heat transfer model

Tailoring of weld attributes based on scientific principles remains an important goal in welding research. The current generation of unidirectional laser keyhole models cannot determine sets of welding variables that can lead to a particular weld attribute such as specific weld geometry. Here we show how a computational heat transfer model of keyhole mode laser welding can be restructured for systematic tailoring of weld attributes based on scientific principles. Furthermore, the model presented here can calculate multiple sets of laser welding variables, i.e. laser power, welding speed and beam defocus, with each set leading to the same weld pool geometry. Many sets of welding variables were obtained via a global search using a real number-based genetic algorithm, which was combined with a numerical heat transfer model of keyhole laser welding. The reliability of the numerical heat transfer calculations was significantly improved by optimizing values of the uncertain input parameters from a limited volume of experimental data. The computational procedure was applied to the keyhole mode laser welding of the 5182 Al–Mg alloy to calculate various sets of welding variables to achieve a specified weld geometry. The calculated welding parameter sets showed wide variations of the values of welding parameters, but each set resulted in a similar fusion zone geometry. The effectiveness of the computational procedure was examined by comparing the computed weld geometry for each set of welding parameters with the corresponding experimental geometry. The results provide hope that systematic tailoring of weld attributes via multiple pathways, each representing alternative welding parameter sets, is attainable based on scientific principles.

Neural network model of heat and fluid flow in gas metal arc fillet welding based on genetic algorithm and conjugate gradient optimisation

Although numerical calculations of heat transfer and fluid flow can provide detailed insights into welding processes and welded materials, these calculations are complex and unsuitable in situations where rapid calculations are needed. A recourse is to train and validate a neural network, using results from a well tested heat and fluid flow model to significantly expedite calculations and ensure that the computed results conform to the basic laws of conservation of mass, momentum and energy. Seven feedforward neural networks were developed for gas metal arc (GMA) fillet welding, one each for predicting penetration, leg length, throat, weld pool length, cooling time between 800°C and 500°C, maximum velocity and peak temperature in the weld pool. Each model considered 22 inputs that included all the welding variables, such as current, voltage, welding speed, wire radius, wire feed rate, arc efficiency, arc radius, power distribution, and material properties such as thermal conductivity, specific heat and temperature coefficient of surface tension. The weights in the neural network models were calculated using the conjugate gradient (CG) method and by a hybrid optimisation scheme involving the CG method and a genetic algorithm (GA). The neural network produced by the hybrid optimisation model produced better results than the networks based on the CG method with various sets of randomised initial weights. The CG method alone was unable to find the best optimal weights for achieving low errors. The hybrid optimisation scheme helped in finding optimal weights through a global search, as evidenced by good agreement between all the outputs from the neural networks and the corresponding results from the heat and fluid flow model.

FREE-CONVECTIVE HEAT TRANSFER FROM CYLINDRICAL SURFACES WITH TRANSVERSE SECTIONAL FINNING

A mathematical model has been developed and numerical calculations of heat transfer have been performed for a horizontal heated cylindrical base with transverse sectional finning under the conditions of free-convective flow of an external heat-transfer agent. The effect of concurrent flows from lower fins on the heat transfer of upper fins has been studied. Local thermal characteristics of individual sectional fins on different portions of the cylindrical base and total heat fluxes removed by a finned surface have been determined. Results of the calculation have been compared to experimental data and satisfactory agreement of them has been obtained.

Heat conduction through a rarefied gas between two rotating cylinders at small temperature difference

Numerical calculations of heat transfer between two coaxial rotating cylinders at a small temperature difference are carried out over wide ranges of the Knudsen number and the angular velocity. The calculations have been performed based on the S-model of the Boltzmann equation by the discrete velocity method. It has been confirmed that in a rotating gas a radial temperature gradient causes both radial and tangential heat fluxes. Also, it has been found that the radial heat flux is affected by the rotation.

Broad Beam Ultrahigh Current Density Ion Implantation

ABSTRACTIn order to achieve low cost rapid surface modification of mechanical components a broad beam (10 cm. dia.), ultrahigh current density (up to 1500 μA/cm2) ion implanter has been developed at Colorado State University. The characteristics of ultrahigh current density ion implantation are compared to those of conventional ion implantation and the relative advantages of the ultrahigh current density implantation are identified. The temperature rise that typical implanted surfaces experience during implantation at various current densities are measured and compared with numerical heat transfer calculations. It is shown that even though the temperature rise can be high at ultrahigh current density implantation conditions, the times at temperature are sufficiently short (typically, 11 sec.) so that no significant tempering, annealing or other diffusion based transformations in the bulk material (i.e. beneath the implanted layer) are induced. The microstructure and hardness beneath the implanted layer are compared before and after ultrahigh current density ion implantation. The results on 1018 martensitic steel, 440C martensitic steel, 52100 martensitic steel, 304 stainless steel, Ti-6A1–4V and 2024-T3 aluminum are presented. The substantial advantage of much deeper (fourfold) ion penetration observed with ultrahigh current density implantation is identified. It is suggested that this increased depth of penetration is due to the phenomenon of radiation enhanced diffusion.

Transient reflectivity behavior of pure aluminum at 10.6 μm

Time-resolved measurements of specular reflectivity of polished pure aluminum surfaces subjected to intense 10.6-μm pulsed-laser radiation in vacuum are reported. A sharp decrease in reflectivity to an anomalously low value (35%) was observed midway in the pulse followed by nearly complete recovery of full reflectivity near the end of the pulse. The peak-power-density threshold for this phenomenon was found to agree with the intrinsic surface melt threshold as determined from numerical heat-transfer calculations and microscopic examination of the samples.