Convective Heat Exchange Research Articles

Numerical study using a double grid to model the effect of the grooves of a vertical wall in a heated cavity on the natural convection of a nanofluid

In this work we study numerically the natural-convection of a carbon-based nanofluid (water + C60) in a differentially heated square cavity. One of the vertical walls of the cavity is grooved and maintained at a high temperature. The purpose of this work is to compare the effect of the macrostructural aspect of the grooves and the effect of the addition of fullerene nanoparticles (C60) to pure water on the heat exchange by natural-convection in this type of geometry. To better quantify the convective heat exchange numerically near the grooves we used a refined grid with two non-conforming blocks at the hot wall level. The governing equations were discretized by the finite volume method using a power law scheme which offers a good stability characteristic in this type of flow. A numerical code was designed and produced in this context to use numerical simulation as an investigative tool. The results are represented in the form of streamlines and isothermal fields. The variation in the mean Nusselt number of the cold wall to the right of the cavity is calculated as a function of the volume fractions of the nanoparticles (0 ? ? ? 0.06) for different numbers and sizes of the grooves and for different Rayleigh numbers (103 ? Ra ? 106).

Computational-Analysis of the Non-Isothermal Dynamics of the Gravity-Driven Flow of Viscoelastic-Fluid-Based Nanofluids Down an Inclined Plane

The paper explores the gravity-driven flow of the thin film of a viscoelastic-fluid-based nanofluids (VFBN) along an inclined plane under non-isothermal conditions and subjected to convective cooling at the free-surface. The Newton’s law of cooling is used to model the convective heat-exchange with the ambient at the free-surface. The Giesekus viscoelastic constitutive model, with appropriate modifications to account for non-isothermal effects, is employed to describe the polymeric effects. The unsteady and coupled non-linear partial differential equations (PDEs) describing the model problem are obtained and solved via efficient semi-implicit numerical schemes based on finite difference methods (FDM) implemented in Matlab. The response of the VFBN velocity, temperature, thermal-conductivity and polymeric-stresses to variations in the volume-fraction of embedded nanoparticles is investigated. It is shown that these quantities all increase as the nanoparticle volume-fraction becomes higher.

Numerical simulation of selective laser melting by the SPH method

Abstract. Currently, additive manufacturing technologies develop actively. This requires creation of computational methods to describe physical processes occurring at the time of manufacturing. One of the methods used for the production of metal powder parts is the method of selective laser melting. This paper presents an SPH-based numerical technique for modeling the process of powder sintering under the influence of a laser beam. The flow of liquid formed as a result of melting is described by the Navier-Stokes equations. Pressure forces, viscous effects and surface forces at the interface are included in the force balance. The thermal state is determined from the energy conservation law, which takes into account thermal processes, volumetric absorption of laser radiation energy, convective heat exchange with the external environment and radiation. Phase transitions between solid and liquid phases are described in the framework of the generalized formulation of the Stefan problem. The calculation method is verified on tests specific to the class of problems under consideration. A comparison is made with the analytical solution, as well as with solutions obtained by other modifications of the SPH method, and with experimental data.

Cr2O3 Doping Effect on Silica Glass Cooling Rate

In this paper we present the results on the effect of the SiO2 melt cooling rate on the morphology of its surface. The method of atomic force microscopy explores a surface roughness of the silica glass cooled at different rates after its heating up to more than 2100 °C. The study compares three modes of cooling glass rods with a diameter of 14 mm: convective heat exchange with the surrounding air, quenching with dropping the sample into a vessel with water, and radiation of silica glass doped with 0.1 wt% Cr2O3. The surface roughness of silica glass in the second and third modes of cooling was significantly less than in the conditions of convective heat exchange. Doping the silica glass with Cr2O3 increases the heat exchange coefficient by 2 times and decreases the surface roughness. The results of the research can contribute to improving the mechanical and optical properties of silica glass light guides.

Influence of Ventilation Flow Rate and Gap Distance on the Radiative Heat Transfer in Aircraft Avionics Bays

The feasibility of the future more-electric, hybrid-electric, and all-electric aircraft configurations will depend on a good understanding of thermal aspects early in the design. However, thermal analysis of aircraft equipment bays is typically performed at later design stages to validate if the design meets the minimal certification requirements rather than to optimize the cooling strategy. The presented work aims to provide new insight into thermal aspects in typical aircraft equipment bays. In particular, system thermal interactions, such as radiation, play a more significant role in tightly packaged bays, such as avionics bays. This paper investigates the influence of radiation on the overall system heat dissipation in two representative avionics bays. Using Computational Fluid Dynamics (CFD) simulation, combined with an analytical approach, the authors analyze the impact of several parameters, such as varying mass flow rates and distances between adjacent systems, on their thermal interaction. The results suggest that the radiative effects must be considered when the gap distance between the systems is larger than 0.1 m, the flow rate between two systems is not strong enough to have high convective heat exchanges, when the systems of interest are hidden by other systems from the ventilation sources, and when the system’s internal heat dissipation is significant. Overall, this paper’s results will contribute enhance conceptual design methods, such as the previously developed Thermal Risk Analysis, and help optimize thermal management strategies for future aircraft.

Optimization of the distance between the vertical plates in the convective air heat exchanger

This paper examines the influence of the distance between vertical plates on the intensity of free convective heat transfer along with the optimization of this distance. Experimental tests were carried out for one model channel of such an heat exchanger with widths s=0.045, 0.085 and 0.18 m. This channel, open at the top and sides, was formed by two isothermal symmetrically heated parallel vertical plates of dimensions H=0.5 m and B=0.25 m. The influence of the heating surface temperatures tw=30, 40, 50, 55, 60 and 70 °C on the convective temperature fields and velocity generated inside the channels was investigated. Directly measured temperature fields, as well as velocity fields measured indirectly using the NRP, enabled the thermodynamic parameters of the heat exchanger to be determined. Based on the temperature gradient distribution ∂Tw∂x on the wall, its average value was determined for each of the plates and for the entire channel, after which the heat flux Qw transferred from the plates was calculated. The heat flux transferred with the air Q and the efficiency η=Q/Qw of heat transfer in the channel were determined using the balance method based on the average temperatures and air velocities at the inlet t̄bott and ūin and at the outlet t̄top and ūout of the channel obtained from the temperature and velocity fields. A grid placed vertically in the channel, halfway across the panel width and perpendicular to the heating surfaces was used to detect the temperature field in air. The image and matrix of these temperatures were determined using a thermal imaging camera. The numerical reconstruction procedure (NRP) was used to determine the velocity field.For technical reasons and lack of time it was not possible to test more channels. Since three values of the width s were insufficient to determine sopt, it was decided to use numerical research. For this purpose, numerical results were compared with those obtained experimentally for the channels already tested. As the compliance verification was successful, further studies of the channels were continued numerically. As a result of optimizing the heat exchange intensification, optimal widths of a single channel sopt=0.035 m and n channels s0=0.0123 m, included in the heat exchanger, were obtained.

THERMAL PROCESSES OF ASYNCHRONOUS FREQUENCY-CONTROLLED MOTOR TAKING INTO ACCOUNT TRACTION LOADS

In the article proposes a method for estimating the thermal state of an asynchronous traction motor. A block diagram of a thermal model of an asynchronous traction motor is presented in which the stator and rotor are modeled as concentric rotating cylinders. The convection heat exchange between two rotating cylinders is represented by modeling an air gap with an effective thermal conductivity that takes into account both thermal conductivity and convection.

The 3D Printing Potential for Heat Flow Optimization: Influence of Block Geometries on Heat Transfer Processes

The building envelope is a crucial element in the regulation of thermal energy in the indoor environment, from which comfortable living inevitably depends. Designing a low-dispersion envelope represents a fundamental strategy to minimize the energy demand and HVAC systems’ consumption. To this end, the need to select suitable insulation has become increasingly important, and the search for new solutions is constantly evolving. This justifies the great interest in the study of energy-efficient and sustainable insulation materials that are able to provide the low thermal transmittance values of multilayer components. To date, 3D printing has experienced a growing popularity for the research of alternative building materials (e.g., concrete). Conversely, it still appears to be very uncommon for the research of purely energy-efficient solutions. The aim of this work is to compare the thermal performance of three 3D-printed PLA (polylactic acid) blocks, characterized by different internal geometries and air cavities: (i) a multi-row structure; (ii) a square structure; (iii) a honeycomb structure. The study was conducted theoretically, with two-dimensional heat transfer modeling, and experimentally, by means of a heat flow meter and infrared thermography. The results showed that the configurations of the 3D-printed blocks reduced the flow of heat exchange. In addition, as the complexity of the blocks’ internal structure increased, a heat flow reduction could be observed. In particular, the honeycomb structure showed a better behavior than the other two blocks did, with an experimental transmittance value that was equal to 1.22 ± 0.04 W/m2K. This behavior, which was mainly due to an attenuation of convective and radiative internal heat exchanges, suggests that the 3D printing has great potential in this field.

Heat transfer and irreversibility evaluation of non-Newtonian nanofluid density-driven convection within a hexagonal-shaped domain influenced by an inclined magnetic field

Non-Newtonian flows are a relatively recent aspect of industrial heat transfer and fluid mechanics. The development of new products, the use of conventional fluids under special conditions, the interaction of heat exchange phenomena with viscosity variations, and the search for new manufacturing processes all encourage us to evaluate industrial problems involving fluid flows that do not obey Newton's classical law. The present study deals with the modelling and the numerical simulation of natural convective flow, heat exchange, and entropy generation features within a hexagonal-shaped domain influenced by an inclined magnetic field and filled with a non-Newtonian shear-thickening fluid charged with Alumina nanoparticles. Three variant configurations of three cooling channels positioned inside the domain are considered. The equations system resulting from the mathematical modelling of the physical problem has been discretized via the finite element method. The spatial distribution of apparent viscosity was portrayed in this study. The findings represented that by growing the Power-law index from 1.2 to 1.8 as well as the magnetic field's power from 0 to 80, the mean Nusselt would descend 27.5% and 12.1%, respectively. Moreover, it was recognized that Config.2 might present the highest mean Nu among other cases examined in this research.

Numerical investigations on overfire air design for improved boiler operation and lower NOx emission in commercial wall-firing coal power plants

Air staging using overfire air (OFA) is a common practice in a pulverized coal combustion for reduction of NOx emission without deteriorating the combustion efficiency. In opposed wall-firing boilers, a simple single-level arrangement of OFA nozzles aligned with swirl burners is typically employed. In this study, an ideal OFA design was developed using computational fluid dynamics for efficient combustion, heat transfer, and emission reduction in a 595-MWe industrial-scale boiler. Using the modeling methodology validated using boiler design data, various OFA designs were evaluated for detailed impact on the boiler performance including char burnout, NOx emission, furnace exit gas temperature (FEGT), and heat absorption including convective heat exchangers. Comparing with single-level arrangements at various heights or two-level arrangements with an increased number of nozzles, the most favorable results were achieved for a two-level staggered arrangement with wide spacings between nozzles and a doubled flow ratio in the upper level. Stronger upper-level jets penetrated deeper into the central region, whereas weaker lower-level jets covered the near-wall region. This enhanced mixing with hot gas from the burner zone resulted in a uniform flow and temperature distributions in the upper furnace. In addition to the reduction of NOx emission, a detailed analysis of the results revealed various benefits of an ideal OFA design that have not been identified in existing studies, including i) reduced ash deposition potential in the tube bundles because of the lower FEGT, ii) reduced attemperating water spray requirement for steam because of the increased heat absorption in the evaporator, and iii) alleviated tube metal overheating because of a lower peak heat flux in the platen superheater. The proposed OFA design can be applied to other large-scale wall-firing boilers.

Influence of Thermal Sensitivity of Materials on the Thermal Stressed State of a Three-Layer Hollow Cylinder Under the Conditions of Convective Heat Exchange

Influence of Thermal Sensitivity of Materials on the Thermal Stressed State of a Three-Layer Hollow Cylinder Under the Conditions of Convective Heat Exchange

Investigation of convective heat transfer at the facade with balconies for a multi-story building

As a common building appendage, balconies can significantly alter the flow pattern near the building façade, thus greatly affecting the convective heat transfer coefficient (CHTC) of façades. In the present study, computational fluid dynamics (CFD) simulations are performed with the 3D steady Reynolds-averaged Navier-Stokes (RANS) SST k-ω model to evaluate the convective heat transfer at the building façade with balconies. This model is systematically validated by a reduced-scale wind tunnel experiment and then utilized to conduct simulations with high-resolution grids. The effects of the balcony height (height of balcony parapet walls), depth, and length on the forced convective heat exchange at the façade and balcony surfaces are analyzed in detail. The results show that with the presence of balconies, the surface-averaged CHTC (CHTCavg) is reduced by about 17.5% at the windward facade, while it is reduced by 35.2% at the leeward façade. Furthermore, when the height of the balcony (Hp) varies from 0.5 to 1.5 m, CHTCavg decreases by up to 39%, 48.8%, and 50% on the leeward facade, the inner surfaces of windward and leeward balconies, respectively. However, balcony depth and length have relatively non-significant effects on the CHTCavg of building facades and balconies' surfaces. Finally, new correlations are established to describe the average CHTC along the façade and balconies’ surfaces. The difference in CHTCavg obtained from the correlations and the simulations is less than 6%. The findings of the present study would facilitate the calculation of the cooling and heating loads of buildings with balconies.

Experimental research on enhanced heat transfer of double-pipe exchanger with audible acoustic field

Acoustic reinforced heat transfer is an active reinforced heat transfer method that has the advantages of having no contacts and being high energy and adjustable. To explore the convective heat transfer characteristics under the audible acoustic field, a convective heat exchange experimental platform was built; the platform was a double-pipe heat exchanger. Comparative experimental research on heat transfer of hot flue gas both with acoustic field and without acoustic field was carried out. The results verified that the heat exchanger with an acoustic field had better heat transfer effect. The effects that parameters (such as sound pressure level, acoustic frequency, and the inlet temperature of the hot flue gas) had on the enhanced heat exchange effect were systematically studied. The influence law of the acoustic field on the convective heat transfer capacity of heat exchanger was obtained. The results show that, with an acoustic field, the temperature of the hot flue gas at the outlet of the heat exchanger decreases significantly, whereas the heat transfer power, convective heat transfer coefficient, and the Nusselt number increased significantly. Within the selected range of experimental parameters, 0.6 kHz is the optimal acoustic frequency for enhanced heat transfer. With an increase in the sound pressure level, the reinforcement efficiency increased. At the same time, increasing the inlet temperature of the hot flue gas helped to enhance the capacity of the sound wave to strengthen the heat transfer. When the temperature of the inlet hot flue gas was 59 °C, the sound pressure level was 145 dB, the acoustic frequency was 0.6 kHz, and the maximum reinforcement efficiency was 76.22%.

Deforming devices with a power drive made of a shape memory material Design solutions, calculation and design procedure

The article presents engineering solutions developed to create deforming devices with a power drive made of a shape memory material. As an example, the paper considers designs of a press, a stamp press made using new designs of multi-link power drives. A method is proposed for engineering a universal multi-link power drive with power elements made of a thermally thin shape memory material. The paper provides the analysis of thermal processes in power elements of various shapes, geometric dimensions and using different methods of their heating (current transmission heating, convective and radiant heat exchange) to determine the efficiency of the engineered devices. Processing and operational properties of thermally thin power elements of a multi-link power drive are investigated. To determine their qualitative and quantitative indicators, a measuring bench was created with such functions as recording the amperage, temperature change, displacement, and developed forces on a single time scale. A relationship between the heating rate, rate of operating force development and return deformation force was found. A line of universal power drives with a developed deformation force of 500–10000 N and a displacement of 1.0–8.0 mm was created based on the calculations performed with the results of their testing and use in existing models of deforming devices presented.

Wellbore Temperature and Pressure Calculation of Offshore Gas Well Based on Gas–Liquid Separated Flow Model

Compared with land wells, the production environment and reservoir depth of offshore oil and gas wells are more complex and shallower. Further, HPHT production fluid there will produce strong temperature and pressure disturbance that affects the wellbore, which easily generates wellbore safety problems, such as wellhead growth and leakage caused by the incompatible deformation of casing and cement sheath. Therefore, obtaining an accurate wellbore temperature and pressure field is the key to implementing a wellbore safety assessment. Based on the gas–liquid two-phase separated method, this paper established an improved calculation model of wellbore temperature and pressure field for offshore HPHT wells. This model also takes into account the heat transfer environment characteristics of “formation-seawater-air” and the influence of well structure. Compared with the measured data of the case well, the error of temperature and pressure calculation results of the improved model are only 0.87% and 2.46%. Further, its calculation accuracy is greatly improved compared to that of the traditional gas–liquid homogeneous flow calculation model. Based on this model, the influencing factors of wellbore temperature and pressure in offshore gas wells are analyzed. The results show that forced convection heat exchange between seawater–air and wellbore is stronger than that between wellbore and formation. Reducing the gas–liquid ratio of the product can effectively reduce wellbore temperature and increase wellbore pressure. The gas production has a significant impact on the wellbore temperature. When the gas production rises from 10 × 104⋅m3/d to 60 × 104⋅m3/d, the wellhead temperature rises from 63 °C to 99 °C. However, due to the mutual influence of friction pressure drop and hydrostatic pressure drop, wellbore pressure increases first and then decreases with the increase in gas production. The improved model can provide a more accurate estimate of the time to reach the rated wellhead temperature. Meanwhile, this model displays accurate theoretical support for the rational formulation of the production plan after the well opening, so as to avoid excessive restrictions on the initial production rate.

Quantifying Temperature Effects in Large-Scale PEM Water Electrolysis Stacks

Introduction Large-scale water electrolysis is needed for scaling up the production of hydrogen as a renewable energy carrier. The required size and number of cells of the PEMWE stack exceed laboratory-scale settings, which leads to additional effects that could impact the operation and performance [1]. The interaction of large, neighbouring cells with each other are expected to lead to significant temperature differences within the stack of cells, which cause deviations in the contribution to the loss mechanisms and uneven stresses or loading of the members. Methods This contribution presents a dynamic PEMWE-stack model, which is formulated in two dimensions: through the stack and along the channel. Due to the high electric conductivity of the bipolar plates the cell voltage of each cell is independent of the cell/channel flow coordinate. The model is based on mass and enthalpy balances of the fluids on the anodic and cathodic side. Conductive heat transport along the cell and through the stack is considered as well as convective transport by the fluids. Time-dependency of mass and enthalpy balances is considered to capture unsteady operation of the stack. In addition, a temperature-dependent polarisation model for determining the cell voltage is included in the model. The polarisation model includes the contributions of activation overpotentials, mass transport losses and ionic membrane and contact resistances. The cells on both edges of the stack are modelled with dedicated boundary conditions to capture the physical arrangement and convective heat exchange with the environment. Water flow and electrical settings are prescribed as external boundary condition. The model is validated with experimental data from industrial stacks. Results The temperature profile inside the stack is found unevenly distributed in both spatial directions between the cells with colder boundary and warmer inner cells. The colder outer cells operate at a higher cell voltage in an electrical series connection. For the inner cells, a significant temperature gradient is found in flow direction. As the polarisation loss mechanisms depend on temperature, the share of the losses is unevenly distributed in both coordinates. This leads to higher current densities at the parts of a cell that experience elevated temperatures (Fig. 1). These mechanisms depend on the macroscopic design of the cell, boundary and operating conditions. Load variations induce a dynamic stack response in temperature, cell voltage and the fluids until a steady state is reached. At the same time local temperature and current density variation, can be analyzed as performance drivers, including degradation. Both aspects can be used for design and optimization of industrial scale PEMWE stacks.

Thermodynamic Characteristics Study with Pyrolysis Steam Coupled Multi-Stage Condensers

A four-stage condensers in series system was adopted to solve the problems of insufficient condensation of high-temperature pyrolysis steam and difficulty in the classification and recovery of pyrolysis oil, where the internal fluids conducted countercurrent convection heat exchange. A steady-state physical and mathematical model of a single condenser was established to clarify the discipline of heat transfer between the internal fluids. Meanwhile, the model of pyrolysis steam coupled multi-stage condensers was proposed with the help of the model compound firstly. A numerical simulation was carried out and the results showed that when the number of condensers in series was four, the heat transfer process of the system reached saturation, and the heat exchange of the cold and hot fluids was completely realized, and it was of little significance to continue to connect more condensers in series for the condensation of pyrolysis steam. To quickly condense the hot fluid, the key was to increase the mass flow rate of the cold fluid in the first-stage condenser. Compared with the experimental values, the calculated values of hot fluid outlet temperature were not higher than 10%, indicating that the model was highly reliable.

Transient Heat Conduction in a Planar Slab With Convection and Radiation Effects

Abstract This article presents new semi-analytical solutions for transient heat conduction in a slab, which exchanges heat at the surface with its surroundings via convection, radiation, or simultaneous convection–radiation mechanisms. The concept of thermal penetration depth together with the integral method of heat balances form the basis of the formulation. Explicit expressions are derived that eliminate the need for numerical integration of the ordinary differential equations (ODEs) obtained from space integration of the heat equation. Verification of the temperature relations with a convection boundary condition is performed using an exact solution. In the case of radiation and combined convection–radiation boundary conditions, the accuracy of the explicit solutions is assessed against a numerical model. In all three cases, the new relations predict the surface temperature with high precision. For the radiation boundary condition, the computed midpoint temperature is also in excellent agreement with the numerical solution. In the case of convection heat exchange at the surface, the temperature at internal locations has close agreement with the exact solution for Biot numbers up to Bi = 1. The model prediction deviates from the exact solution in the interior positions for higher values of Bi with the highest deviation occurring at the center. For example, at Bi = 3, the highest relative error is 7.5%. Likewise, for the case of combined convection–radiation boundary conditions, the predicted midpoint temperature is found to closely follow the numerical solution at early stages of the process only. The predictability of the explicit solution of the convection-only boundary condition is more accurate than other approximate solutions at a Fourier number less than 0.2. Additional relations are presented for determination of the total heat transfer as a function of the Fourier and Biot numbers.

On the application of the Fourier method to solve the problem of correction of thermographic images

The work is devoted to the construction of computational algorithms implementing the method of correction of thermographic images. The correction is carried out on the basis of solving some ill-posed mixed problem for the Laplace equation in a cylindrical region of rectangular cross-section. This problem corresponds to the problem of the analytical continuation of the stationary temperature distribution as a harmonic function from the surface of the object under study towards the heat sources. The cylindrical region is bounded by an arbitrary surface and plane. On an arbitrary surface, a temperature distribution is measured (and thus is known). It is called a thermogram and reproduces an image of the internal heat-generating structure. On this surface, which is the boundary of the object under study, convective heat exchange with the external environment of a given temperature takes place, which is described by Newton’s law. This is the third boundary condition, which together with the first boundary condition corresponds to the Cauchy conditions - the boundary values of the desired function and its normal derivative. The problem is ill-posed. In this paper, using the Tikhonov regularization method, an approximate solution of the problem was obtained, stable with respect to the error in the Cauchy data, and which can be used to build effective computational algorithms. The paper considers algorithms that can significantly reduce the amount of calculations.

Математична модель тепломасообміну в каналі з нанорідиною при його нерівномірному нагріві концентрованим тепловим потоком

This work is aimed at determining the expediency of using a nanofliud (a special suspension with nanoparticles) as a heat transfer agent for a parabolic trough solar plant. Adding nanoparticles to a base heat transfer agent intensifies convective heat exchange inside the channel, thus increasing the total heat efficiency of the receiver system. A refined nonlinear 3D mathematical model was developed to study heat-and-mass transfer in the receiver system of a parabolic trough solar plant that consist of a concentrator and a tube heat receiver with a nanofluid. In the mathematical model, the values of the nonuniform heat flux on the tube heat receiver surface are found by approximating numerical data obtained by the Monte Carlo method. This simplifies the classical coupled deterministic-statistical mathematical model and allows one to obtain a purely deterministic model solved by the finite volume method. The model also accounts for the thermal conductivity of the heat receiver wall, the actual ambient conditions, and the heat loss from the heat receiver surface. A numerical algorithm was developed to conduct numerical parametric studies on determining the temperature fields of Syltherm800/Al2O3 nanofluid heat transfer agent. This nanofluid is prepared from the traditional heat transfer agent of parabolic trough solar plants – Syltherm800 silicone oil – by adding aluminum oxide nanoparticles thereto. The numerical studies were conducted both for pure Syltherm800 oil and for Syltherm800/Al2O3 nanofluid with an Al2O3 nanoparticle concentration of 3, 5, and 8 per cent. This study is the first to find that the use of a nanofluid as a heat transfer agent for a parabolic trough solar plant produces a positive effect only in the case of the laminar flow of a nanofluid heat transfer agent with a high nanoparticle concentration. A verification of the obtained numerical data showed that they are in satisfactory agreement with experimental ones.