Heat Flux Transfer Research Articles

Reconstruction of the heat transfer coefficients and heat fluxes in heat conduction problems

This paper studies the reconstruction of the spatial-dependent-heat transfer coefficient γ(x) and- heat flux q(x), using the Levenberg–Marquardt (L–M) connected with the surrogate functional method and the modified conjugate gradient method (MCGM) (problem I). In addition to studying the reconstruction of γ(x) and space–time-dependent heat flux q(x,t) using the MCGM (problem II). The numerical results obtained by the L–M and MCGM demonstrate the efficiency, accuracy, and robustness of the proposed methods. The obtained results are compared with those obtained from an inverse approach connected with COMSOL Multiphysics under the same conditions. Finally, a conclusion and some remarks are presented.

Establishing regularities in the propagation of phase transformation front during timber thermal modification

The creation of environmentally friendly protective materials for building structures made of wood could make it possible to influence the processes of stability and the physical-chemical properties at the thermal modification of hornbeam wood over a certain time. That necessitates studying the conditions for investigating phase transformations when the timber is exposed to high temperature, as well as establishing the mechanism of hornbeam wood thermal modification. Given this, a mathematical model of the phase transformation process during the transfer of heat flux to a sample was built. Based on the derived dependences, it was established that when hornbeam wood is exposed to temperature treatment, it undergoes endothermic phase transformations characterized by the heat absorption and change in the color of hornbeam wood. In particular, at a temperature of 200 °C, the temperature in the wood decreases by 5 % due to the chemical changes in the structure of cell wall components (lignin, cellulose, and hemicellulose). It was found that the process of thermal modification is accompanied by the decomposition of hemicellulose and the amorphous part of cellulose, a decrease in moisture absorption, as well as a decrease in the volume of substances that are a medium for the development of fungi. In addition, lignin and the resulting pseudo lignin undergo a process of polymerization and redistribution throughout the cell volume. At the same time, they give the cell walls higher density, hardness, increase hydrophobicity (water repellency), thereby reducing the ability to absorb moisture and swell. It was established that the most effective parameter of phase transformations is the temperature and aging duration. The results of moisture absorption have been given; it has been found that over 6 hours of modified timber exposure, its moisture absorption decreases by more than 10 times, which allows its application at facilities with high humidity

Performance Characteristics of the Metal Foam Wick Prepared by Magnetron Sputtering Method

The magnetron sputtering has been used to prepare metal foam wicks for heat pipes by coating the surface of copper foam with zinc oxide (ZnO) nanoparticles. The main properties, including microscopic appearance, contact angle, and capillary performance, as well as the boiling heat transfer performance of the metal foam wicks, are characterized and tested. The results indicate that, compared with the smooth copper, the low PPI (the number of pores per inch) copper foams, for example, 30 or 70 PPI, can directly enhance the boiling heat transfer performance, whereas the 110 PPI copper foam is not conducive. In addition, the effect of sputtering time was conducted. After 12 h ours of magnetron sputtering, the heat and mass transfer performance of the 30 PPI copper foam wick improves significantly. Its wick surface changes from hydrophobic to hydrophilic, the maximum wall superheat reduces by 11.11 K, and the critical heat flux and maximum heat transfer coefficient increase by 29.35% and 80.14%, respectively.

Threaded-field-line Model for the Low Solar Corona Powered by the Alfvén Wave Turbulence

Abstract We present an updated global model of the solar corona, including the transition region. We simulate the realistic three-dimensional (3D) magnetic field using the data from the photospheric magnetic field measurements and assume the magnetohydrodynamic (MHD) Alfvén wave turbulence and its nonlinear dissipation to be the only source for heating the coronal plasma and driving the solar wind. In closed-field regions, the dissipation efficiency in a balanced turbulence is enhanced. In the coronal holes, we account for a reflection of the outward-propagating waves, which is accompanied by the generation of weaker counterpropagating waves. The nonlinear cascade rate degrades in strongly imbalanced turbulence, thus resulting in colder coronal holes. The distinctive feature of the presented model is the description of the low corona as almost-steady-state low-beta plasma motion and heat flux transfer along the magnetic field lines. We trace the magnetic field lines through each grid point of the lower boundary of the global corona model, chosen at some heliocentric distance, R = R b ∼ 1.1R ⊙, well above the transition region. One can readily solve the plasma parameters along the magnetic field line from 1D equations for the plasma motion and heat transport together with the Alfvén wave propagation, which adequately describe the physics within the heliocentric distance range R ⊙ < R < R b , in the low solar corona. By interfacing this threaded-field-line model with the full MHD global corona model at r = R b , we find the global solution and achieve a faster-than-real-time performance of the model on ∼200 cores.

Simulasi Thermal-Struktur Blade Stage Satu Turbin Geothermal

Condensing type geothermal turbine is a mechanical device that used to convert thermal energy into mechanical energy, then after converted to electrical energy in the generator by the rotation of the rotor’s magnetic field. Development of blade material for geothermal turbine application which using local content is crucial due to the blade’s turbine are 100% imported. So, efforts to produce blade material with high local content are very strategic to support national energy independence in the future. The Agency for the Assessment and Application of Technology (BPPT) started from 2020 until 2023 will focus on the development of the blade material for geothermal turbine application. As a starting point of the research, modelling and virtual testing of the stage-1 blade turbine were performed. In this study, the model is done in CATIAV5 software. Solid model is then imported to ANSYS workbench 15.0. Thermal-Structural analysis of turbine blade using the Stainless steel-based alloy with the ANSYS workbench 15.0 was carried out. In the thermal analysis, the model is given temperature, convective heat flux transfer coefficient and atmosphere gas temperature and then is solved for the heat flux distribution over the blade surface. The obtained heat flux distribution is taken as the thermal load into the Static structural analysis. In the static structural, the obtained model is given structural load, namely centrifugal force. The model is then conducted virtual testing in order to obtain stresses and deflections. The results showed that the model of stage one blade turbine is safe when operating at the determined loads.

Numerical Study of Thermal Properties of a Silica Aerogel Material

AbstractSilica Aerogel is material with a special structure consisting of interconnected solid particles of SiO2 forming the skeleton that enclose nanopores filled with confined air and occupying more than 90% of the volume. It is characterized by a thermal conductivity that can reach lower values than any other material. Our aim was to explain the causes of the super-insulation of this material with the use of a calculating method of conductive heat transfer flux in an aerogel structure and determine the equivalent thermal conductivity. For this purpose, numerical specific software was developed to generate random structure of silica aerogel with predefined concentration of solid particles and properties of both skeleton and confined air. Calculation of the conductivity at any point in the confined gas domain shows variable values as a function of the pore size and the location of the point in the pore. A new developed numerical method was used to calculate the equivalent thermal conductivity of the whole structure. Concentration of solid particles has proved not to be the only parameter in which determine the thermal conductivity of silica aerogel and the influence of tortuosity has been demonstrated. A correlation linking thermal conductivity to both concentration of solid particles and tortuosity of the material was suggested.

Application of a MEMS heat flux sensor to heat transfer research on an impinging diesel jet

To clarify the mechanisms of heat transfer on an engine wall is an important challenge because the heat loss on the wall is one of the dominant factors that limit the thermal efficiency. A thin-film resistance type heat flux sensor fabricated with Micro-Electro-Mechanical Systems (MEMS) technologies was applied to research on the heat transfer of an impinging diesel jet. The MEMS sensor had a high sensitivity and multiple measurement points with a scale comparable to the turbulence of in-cylinder flow for the investigation of local instantaneous heat transfer characteristics in engines. Measurements of wall temperature and heat flux were conducted in a constant volume chamber under the reference conditions of Engine Combustion Network (ENC) Spray A with variations in injection pressure. As a result, the maximum heat flux and heat transfer coefficient reached about 5 MW/m2 and 3 MW/(m2 K), respectively. The results were compared to those obtained with a commercially available thermocouple, and both sensors produced similar trends. In addition, fluid motion near the wall was estimated from the heat flux fluctuations using a cross-correlation analysis. The relationship between heat transfer and flow was investigated in a dimensionless number fashion, and it was compared to a semi-theoretical prediction that was based on a steady state laminar flow heat transfer model.

Retrospective analysis of a multi-stage experiment on developing high-performance insulation panels to sustain jet impingement at high temperatures

Over the years, numerous insulation panels have been developed to reduce heat transfer and meet the growing requirements imposed by industries. For a panel, the topology of the core can be delicately designed while novel and advanced materials are put into use. However, in the shipbuilding industry, the development of high-performance insulation panels using low-cost or cost-effective materials is still in great demand. A four-stage experimental study was conducted to develop high-temperature-proof panels without major thermal bridges caused by fastening methods for ships. A specimen chamber matching setup was used to test 36 panels based on nine designs with three commonly found core materials, i.e., polycrystalline filaments (PF), silica aerogel (SA), and aluminum silicate, under an impinging jet at temperatures ranging from 500 to 585 °C for 30 min. Besides analyzing overall heat transfer coefficient (U), heat flux (q), and Fourier number (Fo) for all panels, a method was proposed to estimate thermal diffusivity at testing temperatures (α) based on the Levenberg–Marquardt algorithm and an analytical model of heat conduction. Major conclusions include the following: First, thermal properties varied among the eligible panels: U: 0.4–1.43 W·m−2·°C−1; q: 100–318 W/m2; α: 6.0 × 10–8–1.3 × 10–6 m2/s; and Fo: 0.07–0.35. Second, because the improvement of thermal performance by adding an air interlayer was not significant, traditional panel designs with internal insulation were preferable. Third, by balancing thermal performance and economic and practical considerations, panels with SA or PF core and fastened by in-core battens were recommended for large-scale industrial applications, and it would be more cost-effective to use PF core.

Pool boiling enhancement using hierarchically structured ZnO nanowires grown via electrospraying and chemical bath deposition

This study entailed the fabrication of hierarchically structured ZnO nanowires via electrospraying and chemical bath deposition for pool boiling applications. Nanoscale ZnO seeds were patterned on a metal substrate by electrospraying, after which the seeds were grown into ZnO nanowires via chemical bath deposition. Next, the effect of the patterned ZnO nanowires on the pool boiling performance was investigated. In addition, the optimal nanowire pattern that yielded the highest critical heat flux (CHF) and effective heat transfer coefficient (heff) was identified. The numerous nanoscale cavities that existed among the ZnO nanowires acted as nucleation sites, thereby facilitating an efficient boiling process. The hierarchical structure of the ZnO nanowires increased the CHF by 40% compared with that of the non-coated, bare surface. Furthermore, the cooling effect increased owing to the ZnO nanowires; this in turn decreased the superheat and increased heff. In addition, the ZnO nanowires exhibited surface wettability owing to their hierarchical structure. The optimal combination of a bare and hydrophobic surface and a hydrophilic surface covered with ZnO nanowires yielded the highest CHF and heff. Moreover, the hydrophilic and hydrophobic surfaces promoted capillary pressure and rapid bubble departure, respectively, and their combination yielded the optimal pool boiling condition. Bubble formation and dynamics were observed using a CCD camera, and the patterned ZnO nanowires were characterized via scanning electron microscopy, optical profilometry, and optical microscopy. Moreover, the theoretically predicted heat transfer was found to be consistent with the experimental data.

Heat flux and heat transfer coefficient model analysis during turning of al (7068) alloy

Variation of temperature during turning is the major cause of tool wear and that is one of the responsible factor for reducing tool life due to excessive tool wear. Presented work is an experimental investigation of heat flux and heat transfer coefficient due to increasing temperature cause of variation of cutting speed feed rate and depth of cut, all experiments are performed based on the design of experiment followed by response surface methodology and mathematical relations are established in between dependent and independent variables. Experimental work is performed in dry conditions by selecting Aluminium alloy (7068) as a work piece with Titanium Nitride coated carbide tool bits. The comparison results show high agreement between the physical cutting experiments with an analytical model that are also drawn with the help of MATLAB (simulink). The aim of presented work is to develop mathematical model of generated temperature on tool work interface at different input parameters during machining of turning operation. Utility of the whole study is beneficial for an aerodynamic in which the application of this alloy plays a vital role.

THERMAL CHARACTERISTICS OF A HEAT EXCHANGER TUBE FITTED WITH DIFFERENT PERIPHERALLY-CUT TWISTED TAPE INSERTS IN LAMINAR FLOW

Experimental analyses of the alternation of clockwise and counterclockwise twisted tape (AT tape), and the AT tape in conjunction with three types of edge notches (V-cut, semicircle-cut, and rectangular-cut) on heat transfer and laminar flow friction characteristics in a uniform heat flux, circular copper tube using water as working fluid have been investigated in this paper. The investigations of plain tube were also carried out for comparison. The experiments covered a range of Reynolds numbers 500 ≤ Re ≤ 1800. The obtained results reveal that all of the present twisted tapes utilized in the experiment can significantly increase the Nusselt number and friction factor. The Nusselt number increases with increase in the Reynolds number while the friction factor shows an opposite trend for all cases. For a fixed Reynolds number, the Nusselt number and friction factor both increase with the reduction of edge notch area. Over the range investigated, the maximum value of performance evaluation criterion (PEC) achieved by the AT tape insert reached 1.54 at the Reynolds number equal to 1800. Finally, the empirical correlations for the Nusselt number, friction factor, and PEC were also determined. The predicted Nusselt number, friction factor, and PEC were, respectively, within ± 6.1%, ± 4.1%, and ± 3.3% deviation compared to the experimental data.

Теплосъём и термостабилизация высокотемпературных поверхностей диспергированным потоком теплоносителя

The effectiveness of stabilizing the surface temperature by a dispersed coolant flow is experimentally studied on a bench simulating energy intensive elements of thermonuclear installations A test section in which the maximum heat flux density can be obtained when being subjected to high-frequency heating was developed, manufactured, and assembled. The test section was heated using a VCh-60AV HF generator with a frequency of not lower than 30 kHz. A hydraulic nozzle with a conical insert was used as the dispersing device. Techniques for carrying out an experiment on studying a stationary heat transfer regime and for calculating thermophysical quantities were developed. The experimental data were obtained in the stationary heat transfer regime with the following range of coolant operating parameters: water pressure equal to 0.38 MPa, water mass flow rate equal to 5.35 ml/s, and induction heating power equal to 6--19 kW. Based on the data obtained, the removed heat flux density and the heat transfer coefficients were calculated for each stationary heat transfer regime. The dependences of the heat transfer coefficient on the removed heat flux density and of the removed heat flux density on the temperature difference have been obtained. High values of heat transfer coefficients and heat flux density at a relatively low coolant flow rate were achieved in the experiments.

Experimental investigation of pool boiling heat transfer performance of refrigerant R-134a on differently roughened copper surfaces

Pool boiling heat transfer of refrigerant R-134a at saturation temperature of 5 °C was investigated experimentally on copper. The effect of surface roughness was studied at various average roughness (Ra) values ranging from 0.130 to 0.274 µm. All experimental samples were vertically oriented, and experiments were carried out at varying heat flux ranges between 10 and 70 kW/m2. The investigations are carried out to calculate the boiling heat transfer coefficient (h), wall superheat (ΔT) and heat flux (q) for different roughness of heat transfer surfaces. The obtained results revealed that the Cu surface with Ra = 0.274 µm showed outstanding heat transfer performance compared to other Cu surfaces. The overall boiling performances enhanced with an increase in roughness value of the heat transfer surface owing to the increased number of cavities.

Mass transfer and Cattaneo-Christov heat flux for a chemically reacting nanofluid in a porous medium between two rotary disks

This paper examines the transport of a chemically reacting nanofluid in a porous medium between two rotary disks with Cattaneo-Christov?s heat flux. The non-linear ordinary differential system formed under Vonn Karman transformation of a non-linear partial differential system is solved via a shooting method with MATLAB bvp4c. The nanofluid thermodynamics profiles with variation in physical properties of thermal relaxation time, thermal radiation, porosity, and chemical reaction are observed. Axial, radial, and tangential velocities are found to be increasing functions of porous medium. A decrease in the fluid temperature is perceived as thermal radiation and thermal relaxation increase since more heat can be transported to neighboring surroundings. The concentration is enhanced with intensified Cattaneo-Christov?s thermal relaxation but it oscillates with reacting chemicals. The rotary disks bound the oscillating nanofluid from downward to up-ward directions and vice versa. The axial velocity represents the change in force due to porosity and radial stretching of the disks.

Central Arctic weather forecasting: Confronting theECMWF IFSwith observations from the Arctic Ocean 2018 expedition

AbstractForecasts with the European Centre for Medium‐Range Weather Forecasts' numerical weather prediction model are evaluated using an extensive set of observations from the Arctic Ocean 2018 expedition on the Swedish icebreakerOden. The atmospheric model (Cy45r1) is similar to that used for the ERA5 reanalysis (Cy41r2). The evaluation covers 1 month, with the icebreaker moored to drifting sea ice near the North Pole; a total of 125 forecasts issued four times per day were used. Standard surface observations and 6‐hourly soundings were assimilated to ensure that the initial model error is small. Model errors can be divided into two groups. First, variables related to dynamics feature errors that grow with forecast length; error spread also grows with time. Initial errors are small, facilitating a robust evaluation of the second group; thermodynamic variables. These feature fast error growth for 6–12 hr, after which errors saturates; error spread is roughly constant. Both surface and near‐surface air temperatures are too warm in the model. During the summer both are typically above zero in spite of the ongoing melt; however, the warm bias increases as the surface freezes. The warm bias is due to a too warm atmosphere; errors in surface sensible heat flux transfer additional heat from the atmosphere to the surface. The lower troposphere temperature error has a distinct vertical structure: a substantial warm bias in the lowest few 100 m and a large cold bias around 1 km; this structure features a significant diurnal cycle and is tightly coupled to errors in the modelled clouds. Clouds appear too often and in a too deep layer of the lower atmosphere; the lowest clouds essentially never break up. The largest error in cloud presence is aligned with the largest cold bias at around 1 km.

Flow Regime Transition in the Post-CHF Flow Regimes under Subcooled and Low-quality Conditions

Post critical heat flux (post-CHF) heat transfer may occur during loss of coolant accidents (LOCAs) in water-cooled nuclear reactors when makeup water quenches the uncovered portion of fuel rods in the reactor cores. Post-CHF flow regimes include inverted annular film boiling (IAFB), dispersed flow film boiling (DFFB), and a transition regime between these two, i.e., inverted slug film boiling (ISFB) for low mass fluxes or agitated inverted annular film boiling (AIAFB) for high mass fluxes. Flow regime transition is important for modeling heat transfer characteristics in the post-CHF flow regimes. Breakup of the liquid core in the IAFB regime represents the flow regime transition from the IAFB to ISFB/AIAFB regimes. This paper first presents a literature review of previous experimental and theoretical studies of the mechanisms of breakup of the liquid core in the IAFB regime and flow regime transition in the post-CHF regimes. To physically explain the breakup of the liquid core in the IAFB regime, the Weber number is calculated from the experimental data obtained from steady-state post-CHF experiments at subcooled and low-quality conditions and a critical Weber number is obtained to describe the flow regime transition from the IAFB to ISFB/AIAFB regimes. Parametric effects of the inlet subcooling, mass flux, and system pressure on the critical Weber number are then studied, based on which a new correlation for the critical Weber number is proposed to predict the flow regime transition from the IAFB to ISFB/AIAFB regimes. Due to the different flow patterns in the transition regime, i.e., ISFB and AIAFB respectively for low and high mass fluxes, the heat transfer characteristics show different trends. The indication of the flow regime transition from the IAFB to ISFB/AIAFB regimes by the critical Weber number criterion matches well with the trend of the wall heat transfer coefficient.

Experimental study on the correlation of subcooled boiling flow in horizontal tubes

Subcooled boiling is the most effective form of heat exchange in the water jacket of the cylinder head. Chen?s model is the most widely used correlation for predicting boiling heat transfer, but the selection of the correlation for the nucleate boiling is controversial. The work of this paper is to simulate the heat transfer process in the water jacket of the cylinder head with a horizontal rectangular channel that is heated on one side. Using the coolant flow velocity, inlet temperature and system pressure as variables, the heat flux and heat transfer coefficient were obtained. The results show that the increase of the coolant flow velocity can effectively promote the convection heat transfer, and the change of inlet temperature and system pressure will affect the occurrence of nucleate boiling. However, the Chen?s model predictions does not fit well with the experimental data. Four nucleate boiling correlations were selected to replace Chen?s model nucleate boiling correlation. The correlation proposed by Pioro coincides best with the experimental data. The mean error after correction is 18.2%.

Recommendation for prediction of pool boiling heat transfer of various liquids on microstructured surfaces

Recommendations for prediction heat transfer coefficients and critical heat flux according to available in the literature experimental data of heat transfer and critical heat flux for boiling different liquids on microstructured surfaces made by deformed cutting method are obtained. In the work, on available in the literature sources of experimental data on heat transfer and critical heat flux at boiling of various liquids on the microstructured surfaces made by the deforming cutting method, recommendations for prediction of heat transfer coefficients and critical heat fluxes are received. Microstructured surfaces allow to intensify the heat transfer is 1.1 to 6 times. Due to the variable wettability of microstructured surfaces elements, critical heat flux increase before 4 times. The proposed criteria equations allow predicting heat transfer coefficients with an error of 30%, and critical heat fluxes with an error of 30-35%. In order to improve the accuracy of forecasting, the possibility of using an artificial neural network model for generalizing heat transfer coefficients is shown. Forecasting using an artificial neural network model allows us to determine the heat transfer coefficients with an error of ±20%.The equations are of interest for designing cooling systems for microelectronic devices, heat and mass transfer devices, boiling zones of heat pipes and thermosyphons, etc.

Mathematical Modelling Of Extraction Of The Underground Fluids: Application To Peristaltic Transportation Through A Vertical Conduit Occupied With Porous Material

This paper gives a theoretical idea for extracting the underground fluids through peristalsis. Due to the similarity in aforesaid problem, peristaltic transportation of a viscous Newtonian fluid over a vertical porous conduit is studied by considering the impact of heat transfer. Presuming the long-wavelength approximation, explicit solutions are found as asymptotic expansions with reference to free convection and porosity parameters. Mathematical expressions for coefficient of heat transfer, temperature and mean flux are derived. It is experiential that for few precise values of dissimilar parameters under contemplation, the coefficient of heat transfer increases significantly as Grashof number and Eckert number increases. This narrates to optimization of heat transfer in some processes. Further, it has been observed that mean flow enhances with amplitude ratio, porosity in addition to pressure drop. This authorizes auxiliary research on the impacts of peristalsis on the flow features in vertical channels.

Exploration of types of ventilated air chambers to improve thermal efficiency of bricks in fired clay

The products and construction systems for facades regulate energy transfer in the architectural envelopes. Therefore, this paper developed an exploration of types of ventilated air chambers to improve thermal efficiency of brick for masonry walls in fired clay, taking into account their role in buildings. The methodology involves three fundamental stages: design, simulation and analysis of results. The design stage proposes 3 brick typologies with ventilated air chamber. The second stage simulates the behavior of the heat transfer and heat fluxes of the models in extreme climatic conditions of San Jose de Cucuta, Colombia, in September in Ansys software through the finite element method. Finally, the analysis of results studies the relationship between the incorporation of the ventilated chamber, shape of the product and heat transfer of proposed models, in comparison with the multi-perforated brick. The results indicate that thermal benefit varies between 2.52 °C and 3.64 °C in the interior surfaces of proposed bricks. Moreover, incorporation of ventilated air chamber mitigates heat transfer and reduces energy concentration in the interior sides of new bricks design. In conclusion, innovation in brick design proves that new forms proposition generates added values such as thermal benefits related to comfort. This aspect is important to consider because it establishes a transformative perspective for brick industry through an innovative market that is committed to sustainability of architectural envelopes in order to reduce energy consumption of buildings.