Integral Heat Transfer Research Articles

An approach for thermal performance assessment and optimization design of energy diaphragm walls (EDWs) with seasonal thermal load via numerical modeling

The energy diaphragm wall (EDW) is a promising emerging technology for shallow geothermal energy utilization of existing buildings. However, analyzing the heat transfer characteristics of EDWs is a prerequisite and essential guarantee for rational heat transfer design and optimization. Generally, a single field or model test cannot fully consider and analyze the potential factors affecting the heat transfer characteristics. Therefore, this work uses numerical analysis methods to establish a 3D heat transfer numerical model of EDW suitable for the climatic and geological conditions and heat transfer requirements in Chongqing areas, China. The influencing parameters are divided into design, operating, and environmental aspects. A comprehensive analysis method has been developed that considers a range of indicators related to the temperature difference between the inlet and outlet, heat transfer efficiency, total heat transfer, and average daily heat transfer. The developed method also evaluates the significance of parameter effects based on transient and integrated heat transfer performance. The results show that the heat transfer fluid flow rate, operation mode, groundwater flow, excavation ratio and wall depth have the greatest impact on the thermal performance of the EDW, with a normalized influence degree NETotal of 1.0, 0.94, 0.80 and 0.77, respectively. Recommendations on the thermal conductivity of the concrete and pipe, pipe configuration, etc., and prospects for further research are also provided for the optimal design and thermal simulation optimization of energy diaphragm wall systems.

Numerical analysis of enhanced heat transfer and nanofluid flow mechanisms in fan groove and pyramid truss microchannels

A combined groove and truss structure is designed for rectangular microchannel heat sinks with 4 % Al2O3 nanofluid as the working fluid to address the heat dissipation requirements of high heat flux density electronic devices. The effects of fan shape, elliptical, waterdrop, rectangular, trapezoidal and triangular grooves on the heat transfer characteristics and mechanical properties of microchannels are investigated. The fan-shaped groove microchannels with the best overall heat transfer performance and excellent mechanical properties. The stress of the fan-shaped grooved truss microchannel is reduced by 76.41 % compared to the smooth microchannel. Three structural parameters were investigated, including the length of the truss in the spreading direction (Lx), the ratio of truss flow downstream length to upstream length (e) and truss rod diameter (d). The performance of the microchannels is reflected by the integrated heat transfer factor and the field coordination number. The individual structural parameters were analysed in a single-factor comparison. The microchannels exhibited the best hydrothermal performance in the Reynolds number range of 500 ∼ 1300 at Lx = 0.8 mm, e = 3, d = 0.2 mm. When the Reynolds number is 900, the microchannel with the optimal parameter combination exhibits a remarkable enhancement of 234 % in the Nusselt number and an 80 % increase in the integrated heat transfer factor compared to the rectangular microchannel.

Analysis of APB in vertical wells with evaporite layers: A 1D axisymmetric multilayer thermomechanical model

Temperature increases and creep in saline rocks can cause annular pressure buildup (APB), presenting a significant challenge in oil well operations, particularly in deep and ultra-deep pre-salt fields. This paper proposes a one-dimensional axisymmetric multilayer thermomechanical model that accounts for the influence of evaporite layers and thermomechanical effects on APB. We developed a finite element model that integrates heat transfer and thermomechanical interactions, utilizing a double deformation mechanism as a constitutive model to represent the viscous behavior of the rock formation. Additionally, this model enables the iterative calculation of fluid properties based on temperature and pressure conditions. To validate the model, we conducted case studies using widely adopted commercial finite element software. Its consistent results provide a deeper understanding of the pressure increases caused by the influence of saline rock and thermal effects, offering valuable insights into the safe and efficient operation of oil wells using a model with reduced computational complexity.

The effect of heat flux on enhancement heat transfer mechanism in copper metal foam composite paraffin wax during melting process: Experimental and simulation research

To investigate the effect of heat flux on the enhanced heat transfer in phase change materials (PCMs) composed of copper metal foam (CMF) and paraffin wax (PX), a semi-cylindrical visualized heat storage device with a varied heat flux of 7.3 kW/m2, 8.5 kW/m2, 9.7 kW/m2, and 10.9 kW/m2 was established in this paper. Moreover, a 3-D mathematical model of composite phase change materials (CPCMs) was established using hybrid grid to study the temperature distribution, imbalance effect, flow, heat transfer mechanisms, and heat storage performance during the melting process. The results showed that the melting time of the CPCMs decreased as the heat flux increased. Compared with a heat flux of 7.3 kW/m2, the melting time was shortened by 10.05 %, 20.71 %, and 27.21 %, respectively. In addition, the Rayleigh number (Ra) increased with the increase in heat flux, and the amplitudes of Ra were 1.45×108,1.54×108,1.61×108, and 1.74×108, respectively, which indicated that the higher the heat flux, the larger the amplitudes. Moreover, the heat transfer mechanism during the melting process was dominated by conduction; however, with the increase in heat flux, the maximum flow velocity of the liquid paraffin increased, which caused the proportion of natural convection to increase from 27.80 % to 31.30 %. Hence, the integrated heat transfer coefficient of CPCMs increased from 5.69 W/(m·K) to 5.81 W/(m·K). However, the temperature imbalance effect of the CPCMs was exacerbated, as evidenced by the increased maximum temperature difference in the vertical direction of the center of the CPCMs from 24.7 K to 35.6 K. Besides, the heat storage performance was enhanced. Compared with a heat flux of 7.3 kW/m2, the heat storage capacities of the CPCMs increased by 4.21%, 9.46%, and 12.66 % with increasing heat flux, and the heat storage rates of the CPCMs increased by 15.3 %, 34.9 %, and 52.3 %, respectively. Furthermore, the relative error of the overall melting time was 3.4 %, indicating that the model provided reasonable predictions.

Performance investigation of highway bridge pier columns under the sequential effects of fire followed by vehicle collision and subsequent air blast: A numerical investigation

Historically, it has been demonstrated that bridges may be vulnerable to fire, and in many circumstances, resulting damage might not be apparent, and bridges could maintain acceptable levels of serviceability. In the absence of proven assessment tools and given the limited research that addresses bridge fire, research that better understands response and strives to improve highway bridge resiliency to fire is needed. Extending the work carried out during an earlier research stage, the present study focused on investigating performance of bridge pier columns that survive fire under coupled vehicle collision and air blast. Numerical models of single reinforced concrete columns supported by a pile foundation system and surrounded by air and soil volumes were created using LS-DYNA. As explicit solvers such as those available in LS-DYNA are infrequently used for fire analysis, an indirect two-step approach that integrated heat transfer and structural analyses was developed and validated against published fire-induced impact and blast test results. A parametric study that examined the effects of various fire exposure conditions and column diameters was completed. Performance was comprehensively assessed based on various structural response parameters, which included failure modes, lateral displacement, residual axial capacities, and shear demand-to-capacity ratios. Column damage was then categorized into six levels to qualitatively assess column performance under the aforementioned multi-hazards. The developed modeling approach was shown to be viable, and results indicated that larger column diameters could potentially remain in service in their final damage states after being repaired for fire durations of less than 120 min.

Solutions of the theory of thermoelasticity and thermal conductivity in the cylindrical coordinate system for axisymmetric temperature

The paper uses the system of Navier equations in the stationary case. A cylindrical coordinate system is considered, when the temperature does not depend on the angular variable. A partial solution of the system of Navier equations, which does not contain elastic displacements, is called a purely temperature solution. It was established that for purely temperature solutions the sum of normal stresses is zero and the volume deformation is equal T e  = 3 . An analytical expression of purely temperature displacements and stresses in the cylindrical coordinate system in the axisymmetric case was found. The solution of the boundary value problem of thermal conductivity, when the cylinder is heated on one end, cooled by liquid on the other with known heat losses on the side surface, is proposed. The solution of the boundary value problem of thermal conductivity for such a cylinder is given in the form of the sum of the basic temperature, which describes the heat balance, and the perturbed temperature. The basic temperature has a polynomial form and integrally satisfies the boundary conditions. The perturbed temperature has an exponential decrease with distance from the heated end and does not carry out integral heat transfer. The found dependencies were used and a new solution to the heat conduction equation was written in a cylindrical coordinate system. Simple formulas for expressing temperature changes have been obtained. A new temperature solution to the system of thermoelasticity equations in a cylindrical coordinate system has been written, when the temperature does not depend on the angular variable.

Study of the enhanced heat transfer characteristics of wavy-walled tube heat exchangers under pulsating flow fields

Heat exchangers have a very wide range of applications in many industrial fields, so the rational design and the performance are to improve the heat exchanger energy efficiency and reduce the production cost of an important means. In this paper, the heat transfer mechanism of pulsation is revealed by simulating and analyzing the effects of three pulsation parameters: volumetric flow rate, pulsation frequency, and pulsation amplitude on the flow and heat transfer of a wavy-walled tube heat exchanger, with the study focusing on the instability behavior that affects the heat transfer mechanism. The results reveal that the combined heat transfer performance of the wavy-walled tube heat exchanger is about 8.2% higher than that of the straight-walled tube heat exchanger. The flow field of the heat exchanger is more fully developed under the pulsating flow field, and its performance evaluation coefficients (PECs) are all greater than 1. It is also found that with the increase in the volume flow rate Qv, the heat transfer enhancement coefficient and PEC first increase and then decrease, and reach the maximum value near Qv = 2.0 m3/h; with the increase in the amplitude A, the vortex and heat transfer enhancement coefficient inside the heat exchanger increase, but the integrated heat transfer performance of the heat exchanger is gradually weakened; with the increase in the frequency fT, the heat transfer enhancement coefficient and PEC first increase gradually, but the increase gradually decreases and levels off in the later stages, reaching a maximum value near fT = 0.5 Hz. Meanwhile, the field coefficients of the wavy-walled tube heat exchanger were analyzed and found to be much smaller than 1, with an order of magnitude of 10−4, and the results also showed that the field coefficients were inversely proportional to the volume flow rate and directly proportional to the amplitude of pulsation and that the field coefficients showed a tendency of increasing and then decreasing with the change of pulsation frequency and reached a maximum value near fT = 0.5 Hz. This work provides an important reference for current manufacturing industries to optimize heat exchanger sizing and develop efficient thermal management systems.

Effect of pulsation parameters on the spatial and temporal variation of flow and heat transfer characteristics in liquid metal cross flow the in-line tube bundle

Liquid metal has a broad application prospect in tube bundle heat exchangers because of its excellent thermal conductivity. However, conventional in-line tube bundle arrangements often exhibit limited secondary flow, so new methods are urgently needed to improve heat transfer. The topic of this paper is to combine two methods, liquid metal cross flow tube bundle and pulsating flow, to improve the heat transfer performance in-line tube bundles. Firstly, the applicability of the model was validated through experimental data from existing literature, encompassing overall flow dynamics, heat transfer performance, and circumferential pressure distribution around the tubes. Subsequently, numerical simulations are conducted involving liquid metal cross flow in-line tube bundles at varying pulsating frequencies, amplitudes, and Reynolds numbers. Circumferential pressure distributions and temperature profiles for individual tubes were analyzed and compared. The amplitude of reduction in thermal resistance compared to the no-oscillation condition increases gradually from a minimum of 8 % in the first row to a maximum of 40 % in the third row, while the reduction in the rear row of tubes is similar to that of the third row. Remarkably, a reversal in the trend of thermal resistance for the first three rows of tubes under pulsating flow were observed, contrasting conventional flow except for the case with an amplitude of 0.1. By analyzing the local transient flow field distribution near single tube and the corresponding circumferential heat transfer performance, the influence of the vortex evolution characteristics on the heat transfer performance at different phases of the pulsating velocity is revealed, which transient Nu is up to three times that of flow with no pulsating. Finally, a series of coherence analyses reveal that as frequency rises, turbulence exhibits a richer spectrum of small-scale vortex structures, intricately linked to enhanced energy cascade efficiency. Amplitude augmentation intensifies velocity fluctuations, particularly at high amplitudes, resulting in substantial energy peak amplification. The integrated heat transfer performance PEC factor varies in the range of 1.25 to 1.51 for all the conditions simulated in this paper, which indicates that the liquid metal coupled pulsating flow approach can significantly increase the comprehensive performance of the tube bundle heat exchangers. These findings hold significant promise for advancing heat exchanger design and enhancing thermal efficiency, with implications for various engineering applications.

Regulation mechanism of bionic wireless power device thermal management system based on nanofluids and magnetic field

For further optimal power conservation and dissipation performance of wireless power devices, this paper presented a cooling system that couples a wave bionic surface structure with composite magnetic nanofluids. Heat transfer mechanisms near the wall of the channel under magnetic field excitation have been developed. Effect of changes in the near-wall thermal boundary layer on the integral heat transfer was analyzed, and a comprehensive evaluation of the model performance was given. The outcomes revealed that as the flow approaches the crest, the difference in thermal boundary layer thickness for different flow Reynolds numbers becomes small, and in the flow region in the second half of the wave crest, the thermal boundary depth is no longer regular at different Reynolds numbers, which is due to the “detached flow”. “Detached flow” effectively breaks the thermal boundary enhancing the heat transfer performance. Temperatures excited by magnetic forces show disordered “jumps”, and the heat transfer characteristics are substantially improved by 41.59 % under magnetic field. Comprehensive evaluation index under magnetic field excitation is up to 1.54 for wave bionic surfaces with a frequency of 20 rad/s.

Multiphase hybrid model and thermal insulation simulation of elytra-mimetic ceramic fiber aerogel

In our previous work, the coaxial electrostatic spinning method was applied to construct nanofiber/SiBCN ceramic aerogel with an elytra-mimetic structure. The material presented excellent mechanical, insulation, oxidation resistance, and other properties. Based on preliminary experiments and literature reports, the integrated heat transfer behavior of nanofiber/SiBCN composite ceramic aerogels in high-temperature environments was affected by several factors, such as coaxial fiber size properties, fiber particle content, interactions, and spatial distribution. Moreover, the above factors overlapped and interacted with each other, and it was hard to generalize a clear law of action through traditional experiments and characterization. Therefore, the paper constructed a representative volume unit (RVE) model based on the complex spatial distribution characteristics of composite aerogels, using fractal theory, Rosseland equation theory, and multiphase model theory. This paper investigated the fiber and particle content, the size characteristics of the coaxial fibers, and the overall heat transfer process in high-temperature environments using the finite element method. The finite element simulated the effective thermal conductivity of multiphase aerogel composites under multiple parameters, such as fiber occupancy, fiber core-shell diameter ratio, temperature, and heat transfer mode. We obtained the property parameters of the complex composition of the material through fractal theory. The model was combined with finite elements to study the fiber particle content, the size characteristics of coaxial fibers, and the integrated heat transfer process in high-temperature environments. The fibers improved the absorption and reflection of thermal radiation with increasing fiber mass fraction, leading to improved ability of ceramic fiber aerogel to extinguish light. The fibers had a larger area to absorb and reflect thermal radiation. Based on the excellent extinction ability of SiC fibers, the composite ceramic aerogel had improved thermal performance at a specifically fiber-to-shell diameter ratio through a comprehensive multifactor simulation analysis.

Heat transfer and pressure drop characteristic research of sine wavy flying-wing fins

In recent years, heat transfer enhancement of heat exchange equipment has attracted more and more attention. In this paper, the heat transfer and pressure drop characteristics of sine wavy flying-wing fins are studied by numerical method. The objective is to improve the integrated heat transfer and pressure drop performance of sine wavy flying-wing fins. The degrees of freedom of fin sizes include fin pitch to fin height ratio fp/fh, fin height to fin wavelength ratio fh/W, fin amplitude to fin pitch ratio 2A/fp and fin inclined angle α. The results show that among the calculated 17 flying-wing fins, the optimal values of fp/fh, fh/W, 2A/fp, and α are 0.5, 0.4, 1.9 and 70° respectively. The optimized SWFWF simulation model is established, and the average JF factor is 1.307, which is about 10.9% higher than that of Fin 05 (JF = 1.18). Multiple linear regression is used to obtain the correlations of flow and heat transfer characteristics of flying-wing fins. The average deviation of the correlations for j and f are 0.85% and 4.9% respectively. The correlations can be used for the design and optimization of sine wavy flying-wing fins.

Flow and heat transfer of hydrocarbon fuel at supercritical pressure in additive manufacturing channel

Experimental investigations of n-decane flow behavior and heat transfer performance at supercritical pressure about AM (additive manufacturing) and smooth channels are presented in this article. The effect of changing system pressure (3 MPa, 5 MPa and 7 MPa) and heating power (550 W, 600 W and 650 W) have been considered with a Reynolds number range of 3000–6000. The inner diameter of the two channels is 2 mm, and the wall thickness is 0.5 mm. AM channels have a mean roughness of 11 μm, while smooth channels’ mean roughness of 3 μm. Additionally, it is discovered from the consequences that flow behavior and heat transfer performance are greatly affected by the rough surface of AM channels. For flow behavior, the friction factor of AM channels is 2.8–3.5 times more than that of smooth channels. AM channels will show a decrease in pressure loss with higher system pressure in contrast to smooth channels. Besides, the pressure loss of AM channels will hardly be affected by changing heating power. For heat transfer performance, the Nu number of AM channels is 2.7–3.3 times more than that of smooth channels. Moreover, the wall temperature of AM channels is hardly affected by changes in system pressure. Furthermore, changing the heating power has a negligible effect on the two channels' wall temperatures. Eventually, the integrated heat transfer coefficient of AM channels is 1.98–2.32, indicating that AM channels exhibit a prominent reinforcement of heat transfer.

CALCULATION OF HEAT TRANSFER INTENSITY OF GAS FUEL COMBUSTION PRODUCTS

Link for citation: Mrakin A.N., Afanaseva O.V., Kuleshov O.Yu. Calculation of heat transfer intensity of gas fuel combustion products. Bulletin of the Tomsk Polytechnic University. Geo Аssets Engineering, 2023, vol. 334, no. 5, рр.109-115. The relevance of the research is determined by the modern trend in the field of thermal power engineering and heat engineering for the transition from traditional gaseous fuel (methane) to the use of hydrogen, methane-hydrogen mixtures, as well as thermochemical conversion gases. Switching to new non-design fuel is justified by considerations of reducing the negative impact on the environment and increasing the thermal efficiency of fuel combustion plants. In this case, the use of fuels with a composition different from the design one will affect the heat transfer processes. The main aim: carrying out a comparative analysis of indicators of the intensity of radiant and convective heat transfer of combustion products of non-design fuels, such as hydrogen, methane-hydrogen mixture and thermochemical conversion gases. As an assumption in the formulation of the problem and objectives of the study, the constancy of the heat release power in the apparatus due to changes in the amount of fuel burned was taken. Objects: heat exchange surface of a fire-tube hot water boiler. Methods: carrying out numerical calculation using traditional approaches to determine the indicators of the intensity of heat transfer in the system «combustion products – metal wall of the pipe of thermal power plants». We also used the relations tested earlier by other authors to calculate the thermophysical parameters of gas mixtures. Results. According to the results of the performed comparative calculations, we can conclude that the transition from the use of conventional fuel (natural gas/methane) to its thermochemical conversion gases under the considered conditions has almost no effect on the integral heat transfer performance. To a greater extent, this transition is caused by changes in the intensity of heat transfer for products of combustion of hydrogen and methane-hydrogen mixture, which will affect the operation of thermal power and heat technological installations. At the same time, it is necessary to conduct additional research on the combustion kinetics of thermochemical methane conversion gases, their thermophysical properties, etc., because the hardware design, type of the catalyst used and operating parameters of the process will affect the composition of obtained synthesis gas.

Assessment of high order regularized hybrid lattice Boltzmann scheme for turbulent thermal convection

In this paper, fluid behavior and the second order statistics of developed turbulent natural convection is numerically predicted by the hybrid lattice Boltzmann method. In order to simulate the high Rayleigh number flow, the high order regularized lattice Boltzmann scheme combined with the finite difference solution of energy equation is used. An in-house code was carefully validated against the reference data computed by different numerical macroscopic and mesoscopic techniques. Simulations were performed for cavity aspect ratio 1 ≤ Ar ≤ 4 and Rayleigh number 1010 ≤ Ra ≤ 1011. It was found that two-dimensional (2D) models overpredicted turbulent statistics in comparison with three-dimensional (3D) models. Moreover, less stratified thermal fields are observed under 2D simulations. However, the mean Nusselt numbers are well agreed both for 2D and 3D models. The proposed hybrid model with uniform grid spacing reproduced integral heat transfer characteristics with acceptable accuracy for engineering applications. In particular, the relative error in the mean Nusselt number is within 5% when comparing the results with conventional fully resolved Navier-Stokes formulation.

Numerical Study of Liquid Metal Turbulent Heat Transfer in Cross-Flow Tube Banks

Heavy liquid metals (HLM) are attractive coolants for nuclear fission and fusion applications due to their excellent thermal properties. In these reactors, a high coolant flow rate must be processed in compact heat exchangers, and as such, it may be convenient to have the HLM flowing on the shell side of a helical coil steam generator. Technical knowledge about HLM turbulent heat transfer in cross-flow tube bundles is rather limited, and this paper aims to investigate the suitability of Reynolds Average Navier–Stokes (RANS) models for the simulation of this problem. Staggered and in-line finite tube bundles are considered for compact (a=1.25), medium (a=1.45), and wide (a=1.65) pitch ratios. The lead bismuth eutectic alloy with Pr=2.21×10−2 is considered as the working fluid. A 2D computational domain is used relying on the k−ω Shear Stress Transport (SST) for the turbulent momentum flux and the Prt concept for the turbulent heat flux prediction. The effect of uniform and spatially varying Prt assumptions has been investigated. For the in-line bundle, unsteady k−ω SST/Prt=0.85 has been found to significantly underpredict the integral heat transfer with regard to theory, featuring a good to acceptable agreement for wide bundles and Pe≥1150. For the staggered tube bank, steady k−ω SST and a spatially varying Prt has been the best modeling strategy featuring a good to excellent agreement for medium and wide bundles. A poor agreement for compact bundles has been observed for all the models considered.

Interdependency of Core Temperature and Glabrous Skin Blood Flow in Human Thermoregulation Function: A Pilot Study.

Human thermoregulation is governed by a complex, nonlinear feedback control system. The system consists of thermoreceptors, a controller, and effector mechanisms for heat exchange that coordinate to maintain a central core temperature. A principal route for heat flow between the core and the environment is via convective circulation of blood to arteriovenous anastomoses located in glabrous skin of the hands and feet. This paper presents new human experimental data for thermoregulatory control behavior along with a coupled, detailed control system model specific to the interdependent actions of core temperature and glabrous skin blood flow (GSBF) under defined transient environmental thermal stress. The model was tuned by a nonlinear least-squared curve fitting algorithm to optimally fit the experimental data. Transient GSBF in the model is influenced by core temperature, nonglabrous skin temperature, and the application of selective thermal stimulation. The core temperature in the model is influenced by integrated heat transfer across the nonglabrous body surface and GSBF. Thus, there is a strong cross-coupling between GSBF and core temperature in thermoregulatory function. Both variables include a projection term in the model based on the average rates of their change. Six subjects each completed two thermal protocols to generate data to which the common model was fit. The model coefficients were unique to each of the twelve data sets but produced an excellent agreement between the model and experimental data for the individual trials. The strong match between the model and data confirms the mathematical structure of the control algorithm.

Numerical study of the integrated heat transfer of a condensation-free radiant cooling panel covered with multiple interlayer infrared membranes

Radiant cooling panels have been widely used because of their energy efficiency, high thermal comfort, and ability to improve indoor air quality. However, low cooling capacity and condensation become the key problems that limit the application of radiant cooling panels, especially in hot and humid areas. Aiming at improving the cooling capacity and reducing condensation risk, this study proposed a new condensation-free radiant cooling panel structure with multiple air layers. A two-dimensional heat transfer model of the radiant cooling panel with multiple air layers was established. Numerical methods were used to investigate the effects of the number and thickness of air layers on the cooling capacity and temperature uniformity of the radiant cooling panel. The number and thickness of the air layers were also optimized to maximize the cooling capacity and condensation resistance. The results showed that the optimum number and thickness of air layers were 2 and 10 mm, respectively. In an environment of 28 °C and a relative humidity of 50%, the cooling capacity and radiant heat flux of the optimized radiant cooling panel were 169.42 W/m2 and 138.69 W/m2, respectively. The results were improved by 25.7% and 30.5% compared to a panel with a single air layer, respectively. This study demonstrated a high-performance radiant cooling structure with multiple air layers having improved cooling capacity and reduced condensation risk and will be beneficial to radiant cooling panel applications, especially in hot and humid regions.

Теплообмен в охлаждаемых камерах сгорания теплогенераторов малой мощности

Introduction. The study of heat exchange in cooled combustion chambers and the influence of various factors on its intensity is a complex and relevant task. The analysis of theoretical and experimental data on heat exchange in combustion chambers of various plants has proven that current methods fail to take into account features of heat exchange in furnaces, having small geometric dimensions and, therefore, they cannot be used for their thermal calculation. Materials and methods. The author presents the experiments and the results of their generalization performed using the criterial equation. Generalization results are of great importance for analyzing heat transfer processes and the thermal analysis of cooled combustion chambers of small capacity heat generators. The contribution of radiative and convective components to complex heat exchange processes in combustion chambers of low-capacity boilers is evaluated using this generalized dependence. The author has identified qualitative and quantitative dependence of integral radiative and convective heat exchange on the main factors of operation of small combustion chambers. Results. The author has obtained generalizing criterial dependence that makes it possible to evaluate the contribution of radiative and convective components to complex heat exchange in combustion chambers of low-capacity boilers. Conclusions. The generalized criterial dependence, obtained by the author, can be used to identify the qualitative and quantitative dependence of integral radiative and convective heat transfer on the main geometric, physical and performance factors of operation of small capacity furnace chambers. The assessment of reliability of the experimental data, obtained by the author, has shown that the value of the limiting mean square error of determining the value of integral heat transfer Kh.t will be 3.24 %. For all experiments, the deviation of the calculated data from the experimental results with a 95 % probability does not go beyond the confidence interval of ±9.52 %.

Modeling of heat transfer and hydraulic resistance in short and long straight round pipes with semicircular turbulators depending on geometric and operating parameters

Objective. The aim of the study was to determine the dependence of the distributions of average integral hydraulic resistances and convective heat transfer in turbulent flow in pipes with small (short channel) and large (long channel) sequences of semicircular periodic protrusions based on numerical solutions of the Reynolds equation systems, closed using Menter's shear stress transfer models, and energy equations on a multi-scale intersecting structured grid.Method. The calculation technique based on finite volume solutions of the Reynolds equation, closed using the Menter shear stress transfer model and the energy equation on a multi-scale intersecting structured grid, made it possible, with acceptable errors, to calculate the average coefficients of hydraulic resistance and heat transfer in a pipe with different quantities annular semicircular ledges.Result. Analytical comparisons were made of the calculated ratios for relative heat transfer and hydraulic resistance on the number of protrusions in channels with different values of relative heights of turbulators h/D, relative steps between turbulators t/D, different values of the Reynolds criteria Re, with other equivalent parameters, which showed that in in which case the qualitative deviations of the above characteristics are carried out monotonously, and in which case they are accompanied by extrema or inflections, and also showed cases of qualitative changes in the calculated characteristics. With the transition from 30 protrusions to 50, as a rule, only quantitative differences occur for the relative parameters of hydraulic resistance and heat transfer, and their qualitative changes are insignificant; with a further transition from 50 protrusions to 100, their quantitative changes also become insignificant.Conclusion. The nature of the patterns of distributions of the mean integral characteristics of flows and heat transfer for a channel with protrusions of various numbers must be taken into account for a short channel. An analysis of the calculated information obtained showed that when moving from a short channel with turbulators to a long one, most often, there is an increase in relative heat transfer and a decrease in relative hydraulic resistance, which justifies the advantage, from the point of view of intensification, of heat transfer by turbulence of the flow of the last channels in relation to the first channels.

Dynamic modeling of a water tube boiler

AbstractThe present work aims to develop an integrated heat transfer and hydrodynamic model for the study of boiler dynamics in fluctuating load conditions. A transient heat transfer model has been developed to estimate water side heat transfer and it is directly integrated with the hydrodynamic model. Two independent natural circulation circuits for furnace water wall and convection tube bank have been considered due to differences in heat transfer, dryness fraction, and void fraction. The transient heat transfer model calculates the heat transfer and water wall temperature of the various sections to calculate total gas side heat transfer, gas temperature profile, and waterside heat transfer in the furnace water wall and convection tube bank. Mass momentum and energy conservation equations are generated for both natural circulation circuits using the estimated value of waterside heat transfer of the transient heat transfer model. These equations are integrated with mass and energy conservation equations for the drum below water level and drum above water level to generate a system of equations to estimate drum pressure and water level fluctuation. This model has been extended for boiler feedback control systems, including the load management and the water level control system.