Heat Transfer Calculations Research Articles

Numerical estimation of effective thermal conductivity of reconstructed 2D structures of cement-based building materials

Cement-based building materials are fundamental for constructing modern buildings, and accurate determination of their thermal conductivity is crucial for calculating building energy consumption. In this study, random growth theory is used to reconstruct the two-dimensional pore structures of cement-based building materials in different forms. Furthermore, heat transfer simulations and calculations of effective thermal conductivity based on the finite element method are conducted for these pore structures. The effect of pore structure parameters on the effective thermal conductivity of cement-based building materials is investigated by this method. The results indicate that porosity, pore size, and the degree of pore regularity can all influence the effective thermal conductivity of cement-based building materials. At a porosity of 0.3, the fluctuations in the effective thermal conductivity caused by variations in pore sizes and degrees of pore regularity are most drastic.

Computational heat transfer analysis of liquid food in a heat exchanger with varying stirrer settings in a nonclassical continuum framework

Rapid heat diffusion and stirring of food in channels and tanks for more desirable and safely regulated food delivery is of particular interest in food industry. In the present article, we investigate heat diffusion in a heat exchanger with varying stirrer settings in a nonclassical framework. Interestingly, the liquid food flows in all previous investigations are modeled using the classical continuous paradigm. Therefore, the purpose of this article is to offer a mathematical model that uses a nonclassical continuum framework to mimic the behavior of liquid food flows. This is influenced by the fact that during the stirring process, food particle microrotations affect the heat diffusion phenomenon, necessitating consideration of this in the modeling aspect of such studies. This non-Newtonian physical process is modeled based on the well-established theory of Cosserat continuum. The mathematical model presented here is taking into consideration a newly established constitutive relation by the author for the strain-based viscosity models in non-Newtonian mechanics. The applied constitutive relation incorporates the microrotational effects into the current rheology of rate-dependent viscosity model as a result of the kinematics at the macroscopic level. It offers a more practical method for examining the heat diffusion process in a heat exchanger. To the best of the author’s knowledge, this approach is first of its kind in analyzing heat diffusion process within non-Newtonian mechanics. The presented strain-based viscosity model is described in the form of PDEs along with prescribed boundary conditions. An in-house code is developed based on finite element method. The code is implemented using FreeFEM++. Liquid food flow in a heating process is analyzed numerically for heat transfer characteristics with varying Cosserat number and other physical aspects. Simulations are performed in a channel connecting a tank with different stirrer settings. Computed results are analyzed in a limiting case for accuracy and a strong agreement is achieved with the available solutions from the literature. Some interesting features of the liquid food flow and heat transfer are presented and discussed. It is observed that when liquid food particles rotate faster, the temperature difference between the walls also increases. The temperature difference between the top and bottom walls of the liquid food tank increases with faster speed of stirrers. Rise in the Cosserat number results in a dispersed vortices throughout the liquid food tank. The temperature differential between the liquid food tank’s top and bottom walls rises as a result of an increase in the microrotational motions of the food particles. Moreover, the microrotational motion of the liquid food particles rises with an increase in Cosserat number, enabling effective particle mixing.

Study on coupled heat transfer model and enhanced heat transfer law of liquid metals and supercritical fluids

ABSTRACT The supercritical carbon dioxide Brayton cycle system is being considered for application in the power conversion system of advanced liquid metal fast reactors. One important research topic is the coupled heat transfer characteristics and enhanced heat transfer laws between liquid metals and supercritical fluids. However, the traditional Reynolds analogy hypothesis with a constant turbulent Prandtl number is difficult to satisfy the numerical conjugate heat transfer research of liquid metals and supercritical fluids. Therefore, based on the open-source computational fluid dynamics program OpenFOAM, a numerical method suitable for the coupled heat transfer calculation of liquid metals and supercritical fluids was developed, which can achieve precise conjugate heat transfer solutions by applying different turbulence models and heat flux models to liquid metals and supercritical fluids. Subsequently, based on this method, the coupled heat transfer behavior of liquid metals and supercritical fluids in the straight channel of a typically printed circuit heat exchanger was studied. The focus was investigating the conjugate heat transfer characteristics of liquid heavy metal lead-bismuth eutectic and liquid metal sodium with supercritical carbon dioxide. Some sensitive results, including the inlet temperature and Reynolds number of liquid metals and supercritical fluids, were qualitatively and quantitatively analyzed.

GPU-DEM-based heat transfer model for an HTGR pebble bed

Based on the discrete element method (DEM) and GPU parallel computing, a particle heat transfer model is developed to simulate the heat transfer in a pebble bed of the high-temperature gas-cooled reactor (HTGR). The model is implemented based on a previously developed GPU-DEM program by our team and uses the mesh-based neighbor searching algorithm for the heat transfer calculation. This model couples the conduction and radiative heat transfer between the pebbles and incorporates neural networks and empirical fittings to calculate the radiation view factors, which can improve computational efficiency. The effective thermal conductivity of different models and experimental data are used to verify the accuracy of the model, and the influence of different radiation heat transfer models on the results is also compared. The results show that the effective thermal conductivity derived from the current model is comparable to the classical models at different temperatures, and the numerical simulation results based on the current model are in good agreement with the corresponding experimental data. Additionally, the model achieves a single-core speedup ratio of 126–395 times with GPU acceleration, significantly enhancing computational efficiency. In conclusion, the current model has been effectively verified for accuracy and computational efficiency, and it demonstrates great potential in dealing with large-scale pebble flow and heat transfer challenges in HTGRs.

Heat transfer in the melting of ice slurry during flow in a vertical slit channel

The article concerns the heat transfer process of ethanol ice slurry during downward and upward flow in a vertical slit channel (3x35.8x700mm) under constant heat flux density conditions. Experimental studies were conducted for three initial concentrations of ethanol (Xvai = 10.5 %, 13.2 %, 15.8 %) and mass fractions of ice of 0 ≤ xs ≤ 30 %. For upward flow, heat transfer coefficients were 6–14 % higher than for downward flow. For laminar flow, a greater effect of the melting process was observed, and for turbulent flow, the convection process had a greater effect on heat transfer coefficients. For laminar flow, the heat transfer coefficients of the ice slurry can be 200 % higher than the corresponding values for the carrier fluid. In turbulent flow, the corresponding increase in heat transfer coefficients did not exceed 15 %. For generalised non-Newtonian flow of ice slurry, criterion relationships were proposed for calculating heat transfer coefficients, taking into account the effect of the concentration of the carrier fluid, mass fractions of ice, its melting process, as well as the nature and direction of the flow.

Numerical Calculation of Fluid Heat Transfer in Rotor of Large Air-cooled Generator Based on Global Ventilation Network Model

Numerical Calculation of Fluid Heat Transfer in Rotor of Large Air-cooled Generator Based on Global Ventilation Network Model

Preface

The 2023 UIT (Italian Union of Thermo-Fluid Dynamics) International Conference, hereafter referred as 40th UIT 2023, was organized by the Department of Engineering at the University of Perugia (Italy), in collaboration with the UIT, on June 26-28 2023, at Palazzo Bernabei, Assisi.The annual UIT Conference was held in Assisi for the first time. The scope of the Conference covered a range of topics in computational fluid-dynamics and heat transfer; thermophysical properties; heat and mass transfer for sustainable energy systems; experimental techniques for heat and mass transfer; multiphase fluid-dynamics; natural, forced and mixed convection.The 40th UIT Heat Transfer Conference 2023 program scheduled three keynote lectures by international recognised scientists: Ibrahim Dincer from Ontario Tech. University (Oshawa, Ontario) about the role of thermodynamics in integrated energy systems; Gary Neil Coleman from NASA Langley Research Center (Hampton, USA) about numerical studies of turbulent supersonic plane-channel flows; and Sauro Filippeschi from University of Pisa (Italy) about wickless two phase heat transfer devices. A total of 97 papers were submitted to the 40th UIT Heat Transfer Conference 2023, 75 of which presented by the Authors in oral sessions and the remaining 22 in one poster session.About 150 researchers participated to the 40th UIT 2023. The Conference was a useful occasion to stimulate discussion, further understanding about heat transfer and related phenomena, present the state-of-the-art of some topics, discuss emerging trends, and promote collaborations.The Organizing Committee hopes that the event results constituted a significant contribution to the knowledge in the fields of thermos-fluid dynamics and heat transfer.A special thanks to UIT, all the people who contributed to the success of the event, the International Advisory Committee, the Local Organizing Committee, and to all the Conference attendees.With Kind RegardsFranco Cotana, Università di Perugia, ItalyFederico Rossi, Università di Perugia, ItalyCinzia Buratti, Università di Perugia, ItalyList of Committees are available in this pdf.

Air separation and N2 purification with Ba0.15Sr0.85FeO3-δ via a two-step thermochemical process

Thermochemical air separation to produce high-purity N2 was demonstrated in a vertical tube reactor via a two-step reduction–oxidation cycle with an A-site substituted perovskite Ba0.15Sr0.85FeO3–δ (BSF1585). BSF1585 particles were synthesized and characterized in terms of their chemical, morphological, and thermophysical properties. A thermodynamic cycle model and sensitivity analysis using computational heat and mass transfer models of the reactor were used to select the system operating parameters for a concentrating solar thermal-driven process. Thermal reduction up to 800 °C in air and temperature-swing air separation from 800 °C to minimum temperatures between 400 and 600 °C were performed in the reactor containing a 35 g packed bed of BSF1585. The reactor was characterized for dispersion, and air separation was characterized via mass spectrometry. Gas measurements indicated that the reactor produced N2 with O2 impurity concentrations as low as 0.02 % for > 30 min of operation. A parametric study of air flow rates suggested that differences in observed and thermodynamically predicted O2 impurities were due to imperfect gas transport in the bed. Temperature swing reduction/oxidation cycling experiments between 800 and 400 °C in air were conducted with no statistically significant degradation in N2 purity over 50 cycles.

Coupled simulation on flue gas and steam deviation of final reheater in a 600 MW tangentially fired boiler with reversed separated overfire air under different loads

Flue gas and steam deviation of the final reheater is an inevitable problem in the tangentially fired boiler due to the remaining gas spinning at the furnace exit. An in-house one-dimensional process simulation code coupled with comprehensive three-dimensional combustion simulation was adopted to accurate investigate the characteristics of the flue gas and steam maldistribution of the final reheater in a 600 MW supercritical tangentially fired boiler. Firstly, the combustion simulation result was validated by the in-house fire-side heat transfer calculation data, then the effect of boiler loads and reversed separated overfire air (SOFA) deflection angles on the gas temperature deviation, velocity deviation and outlet steam temperature of reheater were conducted. The results showed that the deviation of flue gas temperature and velocity increases as the boiler load decreases. The reversed SOFA deflection angle plays a crucial role in improving the gas distribution under low loads, and the deviation of flue gas temperature and velocity is the smallest when the angle is -18? under 100% boiler maximum continuous rating (BMCR), 75% and 50% turbine heat acceptance (THA), but it is the smallest when the angle is -9? under 35% BMCR load. Moreover, the maximum outlet steam temperature difference between parallel heating surfaces of the final reheater increases from 84.68 K to 106.48 K as the boiler load decreases from 100% BMCR to 75% THA, and it is mainly affected by the flue gas temperature deviation rather than the mass flow rate maldistribution when the deflection angle changes from -9? to -18?.

Detailed modeling of the radiative feedback in the large eddy simulation of a lab-scale heptane pool fire

This article proposes a detailed modeling of gas/soot radiation for large eddy simulation (LES) of sooting pool fires with a focus on the radiative heat feedback through the fuel vapor dome. The numerical model combines a non-adiabatic steady laminar flamelet (SLF) model, a two-equation acetylene/benzene-based soot production model, the Rank-Correlated Full-Spectrum K (RCFSK) and a presumed filtered density function (FDF) approach to model the subgrid-scale (SGS) interactions between turbulence, chemistry, soot production and radiation. This LES model is applied to simulate 15 cm heptane pool fire. The reliability of the RCSFK and the acetylene/benzene soot model for heptane pool fire scenarios is investigated through a priori analysis. First, decoupled radiative heat transfer calculations along lines of sight, representative of heptane pool fires and covering a wide range of optical thicknesses, demonstrate that the RCFSK computes the absorption of the fuel vapor dome with a similar accuracy as the narrow-band correlated-k (NBCK) model. Second, simulations of laminar coflow diffusion flame are used to extend the acetylene/benzene-based soot production model, previously validated for C1-C3 hydrocarbons, to n-heptane flames. LES of the 15 cm heptane pool fire reproduces with fidelity the flame structure in terms of mean and fluctuating temperatures and mean soot volume fraction (SVF). Simulations with and without considering heptane vapor radiation evidences its effects on the radiative outputs and the heat feedback. The radiative contribution of heptane vapor noticeably enhances the radiative loss to the surroundings and the peak of axial radiative flux. On the hand, the heat feedback to the pool surface is predicted within the experimental uncertainty when this contribution is taken into account with the radiative component being reduced by more than 10 %.

The Influence of linear Heating on Free Convection in a Cylindrical Enclosure

The current study aims to numerically investigate free convection airflow within a horizontal cylinder with a linearly heated side wall. The computation of heat transfer and fluid flow structure has been carried out using the finite element software COMSOL Multiphysics. The influence of the heat source position on fluid flow and heat transfer is inspected. Special attention is paid to the effect of Rayleigh number and the heater position on energy efficiency within the cavity. The results indicate that the best heat transfer performance is achieved for low Rayleigh numbers and when the active wall is centered in the vicinity of 90°.

Recent Advances in Computational Convective Heat Transfer Study in a Sub-Channel for Nuclear Power Reactor and Future Directions

A nuclear power reactor's primary use is to generate thermal energy, which in turn produces electricity. The primary heat source is a nuclear fission event occurring inside the fuel rod. The convection heat is transmitted through the coolant by the heat energy generated at the fuel rod wall boundary. Better heat transfer is produced in the flow area by turbulence and irregularity. As a result, turbulent flow heat transfer may present a significant challenge when predicting and assessing the thermal performance of nuclear power reactors. Computational techniques in convective heat transfer have become indispensable for solving challenging issues in the fields of science and engineering thanks to the development of current sophisticated numerical methods and high-performance computer hardware. The development of novel computational techniques and models for complicated transport and multi-physical phenomena is constantly in demand throughout applicable disciplines. This chapter's objective is to provide some recent developments in computational techniques for convective heat transfer, taking into account research interests in the community of mass and heat transfer, and to showcase relevant applications in nuclear power plant engineering domains including future directions. This study describes the most recent advancements in nuclear reactor convective heat transfer research utilizing the computational fluid dynamics (CFD) method, particularly at Ansys Fluent. This work examines the convective heat transfer and fluid dynamics fluid dynamics for turbulent flows across three rod bundle sub-channels that are typical of those employed in the PWR-based VVER type reactor. In this paper, CFD analysis is carried out using the software tool Ansys Fluent. Temperature distribution profile, velocity profile, pressure drop, and turbulence properties were investigated in this study. Boundary conditions i.e. temperature, velocity, pressure, heat flux, and heat generation rate were applied in the sub-channel domain. The main obstacles and bright spots for the CFD methods in nuclear reactor engineering are discussed, which helps to further its further uses. We intend to research a full-length fuel bundle model for VVER-1200 in the future to gather specific fluid characteristic data and use the findings to analyze safety and operate nuclear power facilities in Bangladesh. This paper presents a thorough analysis of the sub-channel thermal hydraulic codes used in nuclear reactor core analysis. This review discusses several facets of previous experimental, analytical, and computational work on rod bundles and identifies potential future directions based on those earlier studies.

New gas radiation model of high accuracy based on the principle of weighted sum of gray gases

The standard WSGG (Weighted Sum of Gray Gases) model is very fast and simple but can lead to relatively high discrepancies in the computation of the radiative heat transfer. A new model of high accuracy relying on the principle of the WSGG model is proposed. Contrary to the standard WSGG model, the pressure absorption coefficient depends here weakly on the temperature while the weighting factor is not predefined by a mathematical function. Mathematical properties on the model parameters are obtained and we show that all the model parameters can be determined from only one of them. This last is reconstructed with an efficient inverse algorithm from the total radiative heat source data computed with the LBL method using HITEMP 2010. We propose an efficient method to find good initial guesses (of the model parameters) leading to the best accuracy of the model. It is shown, on 17 selected 1D cases representative of the combustion of CO2H2O mixtures, that the maximum relative errors on the radiative heat source for the new model (based on 6 gray gases) does not exceed 3.5% (for these 17 cases) whereas for the standard WSGG model, these errors vary up to 14.5% (five cases have errors higher than 10.0% and nine cases have errors between 5.0% and 10.0%). The accuracy of the total radiative heat flux is also greatly enhanced with the new model. We also show that the present model is robust and can be used in 3D geometry.

Thermal resistance capacity model for the cold release characteristics of cemented paste backfill with phase change materials

A new method based on thermal resistance and capacity (RC) network model for fast predicting cold release characteristics of the cemented paste backfill with phase change materials is developed. The simplified heat transfer calculation model was established, and temperature variation was calculated by MATLAB. The RC model and program were validated by experimental results and numerical heat transfer simulations. Based on the verified RC model, the influence law of the initial ice-water ratio and slurry concentration were analyzed. With the increase of initial ice-water ratio and the decrease of slurry concentration, the cooling ability was enhanced and the cold release process was extended. Furthermore, the cemented paste backfill with ice and paraffin were compared. The cooling release effect of slurry containing ice grains was better than that of slurry containing paraffin throughout the melting process. Compared with the numerical heat transfer simulation method, RC model can effectively improve the calculation efficiency under the premise of satisfying the accuracy. This study is of great significance in providing a timely forecast of the cold release characteristics of the cemented paste backfill with phase change materials in engineering design and practice.

АВТОМАТИЗАЦИЯ ТЕПЛОВОГО РАСЧЁТА РЕАКТОРА ДЛЯ ПРОИЗВОДСТВА СЕРОСОДЕРЖАЩЕГО СОРБЕНТА ПРИ ОХЛАЖДЕНИИ РЕАКЦИОННОЙ СМЕСИ

The problem of automating the calculation of a reactor for the synthesis of a sulfur-containing sorbent used for wastewater treatment from heavy metal compounds during cooling of the reaction mixture is considered. The initial values for the automated calculation of a reactor with a propeller agitator have been determined, the choice of which is based on the results of calculations of the physical properties of ingredients and a review of the designs of mixing devices. A mathematical algorithm for calculating heat transfer during cooling of the reaction mixture is given, as well as the interface of a computer program written in C# with the results of automated calculations

Metastable liquid immiscibility in the 2018–2021 Fani Maoré lavas as a mechanism for volcanic nanolite formation

Nanoscale liquid immiscibility is observed in the 2018–2021 Fani Maoré submarine lavas (Comoros archipelago). Heat transfer calculations, Raman spectroscopy, scanning and transmission electron microscopy reveal that in contrast to thin (500 µm) outer rims of homogeneous glassy lava (rapidly quenched upon eruption, >1000 °C s−1), widespread liquid immiscibility is observed in thick (1 cm) inner lava rims (moderately quenched, 1–1000 °C s−1), which exhibit a nanoscale coexistence of Si- and Al-rich vs. Ca-, Fe-, and Ti-rich melt phases. In this zone, rapid nanolite crystallization contrasts with the classical crystallization process inferred for the slower cooled ( < 1 °C s−1) lava interiors. The occurrence of such metastable liquid immiscibility at eruptive conditions controls physicochemical characteristics of nanolites and residual melt compositions. This mechanism represents a common yet frequently unobserved feature in volcanic products, with the potential for major impacts on syn-eruptive magma degassing and rheology, and thus on eruptive dynamics.

Direct disposal concepts of spent mixed oxide fuel from light water reactors based on heat transfer calculations

ABSTRACT Direct disposal concepts of mixed oxide spent fuel (MOXSF) consumed in light water reactors were investigated via heat transfer calculation. The temperature of the buffer material surrounding the waste is the most stringent limitation on the direct disposal of MOXSF. Therefore, the effects on the maximum temperature of the buffer material were examined by changing the occupied area, cooling term of MOXSF, disposal depth, and buffer material thickness referring to the direct disposal concepts of uranium spent fuel. Moreover, some possible disposal concepts for MOXSF were obtained based on the examination. The examination results showed that it is necessary to change the cooling term and disposal depth in addition to the change of the occupied area. The obtained disposal concepts with a combination of these changes needed the occupied area per unit waste of the MOXSF was three to five times that of uranium spent fuel.

Numerical investigation of non-uniform temperature fields for proppant and fluid phases in supercritical CO2 fracturing

The non-uniform temperature distribution in supercritical CO2 (Sc-CO2) fracturing influences the density, viscosity, and volume expansion or shrinkage rate of Sc-CO2, impacting proppant migration. This study presents a coupled computational fluid dynamics-discrete element method and heat transfer model to examine the effects of proppant bed shape and the heat transfers of proppant-wall, proppant-fluid, and fluid-wall on the fluid and proppant temperature fields. The Sc-CO2 volume expansion is assessed under various temperature conditions by evaluating the volume-averaged Sc-CO2 density. Several factors, including proppant size, shape, thermal conductivity, concentration, temperature difference, and injection velocity, are carefully analyzed to elucidate their impacts. The findings elucidate the existence of four distinct zones in the fluid temperature field. Each zone exhibits different magnitudes of temperature change under diverse conditions and undergoes dynamic transformations with the development of the proppant bed. The fluid-wall heat transfer and the fluid temperatures in Zones C and D are significantly subject to the fluid injection velocity (governing the heating duration), the temperature difference between fluid and formation (impacting the magnitude of heat flux), and the proppant bed shape (controlling the effective heating area). Additionally, the proppant-wall and proppant-fluid heat transfers determine the temperatures of both the proppant bed and the fluid within Zone B, showing a strong correlation with proppant thermal conductivity, proppant size, injection velocity, and temperature difference. The proposed coupled model provides valuable insights into the temperature distributions and flow behaviors of temperature-dependent fracturing fluids and proppants.

Biological H2(g) Production and Modelling with Computational Fluid Dynamics (CFD)

In this study, bio-hydrogen gas [bio-H2(g)] production and modeling with a three-phase computational fluid dynamics (CFD) model, heat and mass transfer of bio-hydrogen production, reaction kinetics, and fluid dynamics; It was investigated by dark fermentation process in an anaerobic continuous plug flow reactor (ACPFR). The three-phase CFD model was used to determine the bio-H2(g) production in an ACPFR. The effect of different operating parameters, increasing hydrolic retention times (HRTs) (1, 2, 4, 8, and 12 days), different pH values (4.0, 5.0, 6.0, 7.0, and 8.0), and increasing feed rate as organic loading rates (OLRs) (0.5, 1.0, 2.0, 4.0, 8.0 and 10.0 g COD/l.d) on the bio-H2(g) production rates were operated in municipal sludge wastes (MSW) with Thermoanaerobacterium thermosaccharolyticum SP-H2 methane bacteria during dark fermentation for bio-H2(g) production. The effect of HRT, pH, and feed rate on the bioH2(g) efficiencies and H2(g) production rates were examined in the simulation stage. Production of volatile fatty acids (VFAs) namely, acetic acids, butyric acids, and propionic acids were important points influencing the bio-H2(g) production yields. The artificial neural network (ANN) model substrate inhibition on bio-H2(g) production to the methane (CH4) bacteria was also investigated. The reaction kinetics model used Thermotoga neapolitana microorganisms with the Andrews model of substrate inhibition. Furthermore, the ANN model was well-fitted to the experimental data to simulate the bio-H2(g) production from chemical oxygen demand (COD).

Experimental investigation of the convective heat transfer coefficients at different angular orientations inside the void in a cold flow pebble bed reactor using a non-invasive heat transfer pebble probe

This study focuses on investigating the effect of the surface location of the pebble inside the bed, by varying the angular orientation of the heat transfer pebble probe and the velocity of the gas on the convective heat transfer coefficients in a cold-flow pebble bed reactor. An advanced non-invasive technique consisting of a cartridge heated copper pebble, a micro-foil sensor and a thermocouple probe was used to obtain accurate measurements of the local heat transfer coefficients from three surface locations inside the void. The effect of the wall on the local convective heat transfer coefficients was highlighted due to the low aspect ratio of the bed (bed diameter to pebble diameter (5cm)) used in this work. The local measurements showed, for the first time ever, that the surface location inside the void substantially impacts the heat transfer coefficients, with higher heat transfer coefficients at the center of the void and bottom of the void. Furthermore, the measured overall heat transfer coefficients showed good agreements with the predictions of the correlations of [52,21] and [14]. A second order polynomial model with an R2=0.9807 and an average absolute relative error (AARE) equal to 9.08%, was developed for the prediction of the local heat transfer coefficient within the design and operation conditions of this work. The accurate local heat transfer data collected in this work is valuable as benchmark data for the validation of computational fluid dynamics (CFD) simulations and heat transfer calculations.