Intensity Of Convective Heat Transfer Research Articles

Study the Convective Heat Transfer Intensification by Using Nanotechnology: A Review

Study the Convective Heat Transfer Intensification by Using Nanotechnology: A Review

Synergistic impact of horizontal fin installation height and nanoparticle volume fraction to enhance charging performance of phase change materials (PCMs)

This study numerically analyses the synergistic influence of nanoparticle and orthogonal fin on heat transfer performance of phase change materials using enthalpy-porosity method. The natural convection heat transfer and heat conduction during the melting process are coordinated by varying nanoparticle volume fraction and installation height of fin. Results illustrate that the optimal installation height of horizontal fin and nanoparticle volume fraction is 8.9 mm and 1.0 vol%, respectively. The corresponding total melting time is 1024 s and average energy charging rate is 484.5 W. Compared with the optimum conditions without nanoparticle (φ = 0.0 vol% and d = 7.0 mm), total charging time is reduced by 29.8% and average charging rate is increased by 51.1%. The equivalent Nusselt number demonstrates that the addition of high volume fraction nanoparticle decreases the convective heat transfer intensity. The low volume fraction nanoparticle enhances PCM thermal conductivity and has an insignificant effect on natural convective heat transfer. The Prandtl number demonstrates that when the volume fraction is 1.0 vol%, the effect of increasing thermal conductivity on strengthening heat conduction is greater than the negative effect of decreasing natural convection heat transfer caused by viscosity. Effective design guidelines to improve thermal charging performance of PCM-based LHTES devices are finally provided.

Numerical Simulations on Combustion and Heat Transfer in Porous Medium with Different Gyroid Triply Periodic Minimal Surfaces Structures

ABSTRACT To investigate the influence of the porous medium with Triply Periodic Minimal Surfaces (TPMS) structures on lower concentration methane combustion. This study analyzed the flow field and heat transfer in the porous medium with three types, i.e. an axial gradual period type, a conical type and a uniform type, through the pore-scale simulation (PSS) approach. In cases of this study, the flashback equivalence ratio of the conical PMB is about 12.5% lower than that of the gradient PMB at the same inlet flow velocity, and the blow-off inlet flow velocity of the conical PMB is about 30% higher than that of the gradient PMB at the same equivalence ratio. Moreover, the flame shape is closely related to the local solid structure and flow velocity. The solid surfaces radiation heat transfer occurred mainly in regions with high solid temperature gradients. Additionally, the absolute value of the radiant heat flux is about 1/4 to 1/5 of the total heat flux, which reflects the proportion of thermal convection in the heat transfer on the solid surface. Convective heat transfer between solid and fluid occurred mainly in the areas of heat recuperation known as preheat zone, and higher inlet flow rates significantly increase the upstream convective heat transfer intensity.

Radiative–Convective Heating of the Surface of the Martian Descent Vehicle MSL with Taking into Account the Turbulent Nature of Flow

The three-dimensional computer model based on the Reynolds-averaged Navier–Stokes equations together with the Baldwin–Lomax and Prandtl algebraic models of turbulent mixing is used to calculate radiative–convective heat transfer on the surface of the MSL descent space vehicle. The intensification of convective heat transfer on the leeward side of the frontal aerodynamic shield and the superiority of the radiative heat flux density over the convective one on the rear surface are demonstrated. The calculations are performed using the physically and chemically nonequilibrium gas model. The results are compared with the calculations based on other computational models and the flight data on the heat load to the descent vehicle obtained during MSL descent in the dense layers of the Martian atmosphere.

Steam-side heat transfer analysis in spirally corrugated tube for grate furnace superheater

In the case of low Reynolds number, the structure of the spirally corrugated tube will make the solid particles in the fluid be affected by the centrifugal force, and be more evenly distributed in the fluid to achieve anti-fouling effect. Therefore, it can be used in heat exchangers with serious fouling problems to improve heat exchange efficiency. In this paper, the first stage low-temperature superheater in the grate furnace is selected as application equipment. The local resistance coefficient, local Nusselt number and local heat flux are analyzed, the thermal-hydraulic characteristics of different sample points are evaluated. Combinatorial method of central composite design and Latin hypercube design is used to select four representative sample points, and effect of the corrugation pitch ratio and depth ratio on the convective heat transfer intensity, fluid resistance and fluid work consumption of the steam side is studied. The research results show that as the corrugation depth increases, the local heat transfer coefficient in the circumferential direction increases, and more heat is transferred from the flue gas to the steam, but the pump power consumed in the flow process has also increased. Due to the existence of spiral structure, the local heat flux density of the wall is symmetrically distributed in the circumferential direction, which represents that the temperature gradient increases, and the heat flow is improved.

On the Estimates of Convective Heat Transfer Intensification

On the Estimates of Convective Heat Transfer Intensification

Numerical study on the effect of thermal diffusivity ratio in phase change heat transfer of crude-oil using lattice Boltzmann method

When the long distance crude-oil pipe-line is stopped, the crude-oil will solidify gradually with the decrease of the temperature in the pipe-line. The solid-liquid thermal diffusivity can reflect the ability of heat diffusion in the phase change heat transfer process of crude-oil stop-transport. Based on the mathematical model of oil phase transition heat transfer established by enthalpy method, the lattice Boltzmann method is used to solve the governing equations of oil phase transition heat transfer. According to the simulation results, the phase change heat transfer process of crude-oil is divided into three-stages, and the mechanism of the phase change heat transfer process of crude-oil is studied. The results show that the influence of solid-liquid thermal diffusion ratio increase gradually with time. When Fourier number ?0.043, Nusselt number and convective heat transfer intensity of mobile phase interface slightly decrease with the increase of solid-liquid thermal diffusivity ratio. The research results of this paper can provide a reference for controlling the stoppage time of pipe-line.

Оценки равновесной температуры на поверхности сферического затупления спускаемого орбитального аппарата

The descent of a stabilized capsule from a low near-earth orbit along a ballistic trajectory is considered. The descent of a stabilized capsule from a low Earth orbit along a ballistic trajectory is considered. A reliable preliminary estimate of the intensity of the thermal impact of the air flow, which enters the surface of the capsule, is the equilibrium temperature of this surface. The value of the equilibrium temperature follows from the condition of equality of the density of the total heat flux brought to the surface and the density of the heat flux, removed due to the own radiation of this surface. Based on the peculiarities of the heat exchange process on the surface of the spherical blunt capsule three sections of the trajectory are distinguished. At free-molecular flowing of the blunt, at the section of descent the intensity of heat exchange is assumed to be proportional to the energy of the molecule flow. At the section corresponding to the continuum flow regime, the intensity of convective and radiative heat transfer is estimated by empirical dependences obtained by processing the calculated and experimental results. When combining laminar and turbulent flow modes on the attenuation surface, the highest of two local values of convective heat flux density is taken. At the intermediate section of the descent, the Knudsen number is taken as an argument determining the intensity of convective heat transfer, and empirical dependences are used to estimate the radiation component. A quantitative analysis of the change in the time of descent of the maximum value of the equilibrium temperature on the blunted surface for a ball capsule and a capsule with a ball segment was carried out. The results of such analysis are the purpose of the present work and allow to identify the most heat-stressed areas of the descent trajectory on the blunt surface at the preliminary stage of the design and to choose the acceptable materials of the thermal protection coating

Experimental Study on Gas-Solid Heat Transfer Characteristics for the Vertical Waste Heat Recovery Using the Inverse Problem Method

To establish an accurate model to optimize the vertical cooling process of the sinter, the inverse problem method is used to calculate the gas-solid heat transfer coefficient based on the gas outlet temperature, which is fitted into the correlation. The research indicates that the increase in the gas velocity is beneficial to the enhancement of the gas-solid heat transfer. With the gas velocity u g increasing from 0.8 m·s-1 to 1.6 m·s-1, the heat transfer coefficient h v increases by about twice. But this effect will weaken with the increase in the particle size. Besides, the reduction of the particle size is conducive to improving the convective heat transfer intensity between the gas and solid. With the particle size decreasing, this enhancement effect is progressively evident. At u g of 0.8 m·s-1, the increasing extent of h v is 1142.25 W·m-3·K-1 with the particle size decreasing from 20~25 mm to 15~20 mm, while that is 3152.65 W·m-3·K-1 with the particle size decreasing from 15~20 mm to 10~15 mm. In addition, the variation of the measured value of the Nusselt number with the Reynolds number has the same trend as predicted values obtained by other works. However, there is a considerable deviation in the value. Among them, the minimum value of the mean relative error is 26.81%. It is proved that the previous empirical correlations are no longer applicable, while the predicted value of this work is in good agreement with the measured value with the mean deviation of only 7.61%. Therefore, the modified correlation can accurately predict the gas-solid heat transfer characteristics in the sinter bed, which lays a foundation for the numerical design and optimization of the new process.

Entropy Analysis of the Thermal Convection of Nanosuspension within a Chamber with a Heat-Conducting Solid Fin

Heat transport augmentation in closed chambers can be achieved using nanofluids and extended heat transfer surfaces. This research is devoted to the computational analysis of natural convection energy transport and entropy emission within a closed region, with isothermal vertical borders and a heat-conducting solid fin placed on the hot border. Horizontal walls were assumed to be adiabatic. Control relations written using non-primitive variables with experimentally based correlations for nanofluid properties were computed by the finite difference technique. The impacts of the fin size, fin position, and nanoadditive concentration on energy transfer performance and entropy production were studied. It was found that location of the long fin near the bottom wall allowed for the intensification of convective heat transfer within the chamber. Moreover, this position was characterized by high entropy generation. Therefore, the minimization of the entropy generation can define the optimal location of the heat-conducting fin using the obtained results. An addition of nanoparticles reduced the heat transfer strength and minimized the entropy generation.

Simulation of convective heat transfer of a bank of pipes when using turbulators with cutouts

The article discusses the mathematical modeling of the intensification of convective heat transfer of a smooth tube bundle. Turbulators with cutouts were used as heat transfer intensifiers. The mathematical model of the heat exchanger consisted of a section of a rectangular channel with a turbulator device. To solve the differential equation, the finite difference method was used, approximated by difference equations. On the basis of the calculations, for the purpose of choosing the size of the experimental object, the profiles of the velocity distribution before and after the turbulizer were obtained. Keywords: m

Modeling Method for Thermal Field of Turbulent Cooling Dry-Type On-Board Traction Transformer in EMUs

Adopting dry-type on-board traction transformer (OBTT) is one of the main ways to reduce the weight of electric traction systems, and its thermal characteristics are crucial for the safe operation of high-speed electric multiple units (EMUs). However, the existing thermal modeling methods either cannot realize the fast calculation of thermal characteristics of dry-type OBTT, or cannot accurately obtain the turbulence heat-transfer intensity when using the high-speed train-induced wind cooling. This article proposes an efficient method for dry-type OBTT thermal characteristics calculation. First, the thermal network of the winding area is proposed considering the thermal developing region of the air duct and the heat flow in the insulation paper between conductors. Then, the effects of key parameters on convective heat-transfer intensity are obtained by computational fluid dynamics (CFD) parametric sweeps, and then a novel local Nusselt number correlation for accurately calculating the turbulent convective thermal resistance is proposed. Finally, validation is performed with an experimental setup. The experimental results show that the computationally efficient thermal network model proposed in this article can quickly obtain the winding temperature distribution and the location of the hot-spot temperature. Moreover, the thermal analysis of a 6.3 MVA dry-type OBTT was conducted using the proposed modeling method.

INFLUENCE OF CONTAMINATION OF LOW-TEMPERATURE HEATING SURFACES OF BOILER ON THE INTENSITY OF CONVECTIVE HEAT EXCHANGE DURING THE BURNING OF NON-DESIGN FUEL

The study of the influence of the degree of contamination of low-temperature heating surfaces on the intensity of convective heat transfer during the burning of non-design fuel based on computer modeling has been carried out. The results of the analysis showed that the amount of deposits varies depending on the temperature of the heat carriers in the air heater. A new method for calculating the thermophysical parameters of the heat transfer process has been developed on the basis of the ANSYS software package, which makes it possible to obtain a complete description of complex heat transfer processes in the boiler air heater. In the developed computational three-dimensional thermophysical model of a recuperative tubular air heater, there is no tube wall, the created thermal resistance of the wall is taken into account by introducing the coefficient of effective thermal conductivity of the flue gas flow.

Finite element modeling of postmortem hyperthermia in the absence of internal heat sources

The objective of the study is to develop a finite element model (FEM) of postmortem hyperthermia in the absence of internal and external heat sources and to clarify the conditions necessary for the occurrence of this phenomenon. ELCUT 6.5 software was used. We have developed a two-dimensional FEM of the postmortem temperature field of the head under convective heat exchange with the ambient air, taking into account the intensity of convective heat transfer and thermophysical parameters of anatomical layers of this body area. The possibility of postmortem heating of the surface and subsurface tissues of the corpse in the absence of internal and external heat sources was demonstrated. It was found that the occurrence of postmortem hyperthermia requires cooling the cadaver under low convective heat transfer with a small initial temperature gradient between the body surface and the ambient environment. It is recommended to take into account the possibility of postmortem hyperthermia and the conditions necessary for it in forensic medical practice when determining the time of death (TOD).

Numerical study on Nusselt number of moving phase interface during wax melting in tube using lattice Boltzman method

Paraffin melting is widely applied to the fields of PCM energy storage, gathering and transportation pipe-line paraffin removal, etc. Natural-convection is the main heat transfer mode during paraffin melting, and Rayleigh number is an important factor affecting the change of natural-convection intensity. Nusselt number variation can reflect the influence of natural-convection on heat transfer. The conventional Nusselt number of hot wall surface reflects only the convective heat transfer intensity of the fixed wall, while it does not take into account that the phase change interface has the characteristics of moving in the phase change process. A double distribution model of paraffin phase transformation in circular tube based on lattice Boltzmann method is established in this paper. The influence of Rayleigh number on the temperature field and flow field of wax in circular tube is analyzed. The heat transfer process is reflected by Nusselt number of moving phase interface. The relation between Nusselt number of moving interface and Nusselt number of hot wall surface is also presented. The results show that the Nusselt number of moving phase interface can reflect the complex non-linear characteristics of natural-convection and describe the phase change heat transfer process of wax more accurately. Calculation formula of Nusselt number of moving phase interface and hot wall during wax phase change is proposed. Increasing Rayleigh number can quicken the melting of wax to meet the actual engineering requirements.

High‐Performance Charging Method for Cold Energy Storage Using Foam Freezing

Phase change cold energy storage can effectively stabilize the peak value of the power grid, but the cold energy charging rate of phase change material decreases seriously which will reduce the energy efficiency of the system. The cold energy charging performance can be effectively improved by foam freezing, and the foam freezing model is proposed to explore the influence factors and the advantages of this method. Foam freezing is compared with other methods based on the novel concept of cold energy charging coefficient which is carried on the analogy to heat transfer coefficient. It is found that the cold energy charging rate of foam freezing are exponential functions of the height based on e. The foam cold energy charging coefficient is affected by the superposition of convective heat transfer intensity and bubble terminal velocity. The technical path of increasing the cold energy charging coefficient by continuously reducing the bubble size is not feasible. When foam freezing at the radius of 0.6 mm, the cold energy charging coefficient is 237.1 W m−2 K−1, the water equivalent height is 18 mm, and it is the most economical. Foam freezing has advantages over the ice ball and ice‐on‐coil in cold energy charging performance.

Computational study of the efficiency of various methods of intensification of convective heat transfer

This paper presents the results of a computational study of the efficiency of various methods of heat transfer intensification in model channels containing various types of intensifiers. The following methods of intensification of convective heat transfer are considered: acoustic, the intensifiers twisted tape, wire spiral, and joint intensifier of a wire spiral and twisted tape. The study of thermal and hydraulic processes in the channels is carried out using computer modeling based on the solution of the Navier-Stokes equations averaged by Reynolds, the energy and state equations supplemented by the turbulence model. The thermal and hydraulic characteristics of various methods of heat transfer intensification are determined in the range of Reynolds numbers from 10000 to 60,000, and the efficiency of the intensification is determined based on the author's criterion. The characteristics of a smooth channel in the above-mentioned range of Reynolds numbers are considered as reference thermal and hydraulic characteristics. Comparative analysis has shown that the acoustic method of heat transfer intensification is most effective in the range of Reynolds numbers, where different modes of self-sustaining acoustic oscillations occur. The presented results may be used in the development and design of heat exchangers.

Experimental study on bituminous coal blending in a down‐fired boiler with anthracite combustion system under low load

AbstractThe industrial experiments were carried out to study the bituminous coal blending in a 660‐MWe supercritical down‐fired boiler with anthracite combustion system under low load. The influence of the bituminous coal ratio on the boiler thermal efficiency, flue gas temperature, flue gas species, and heat transfer of various heating surfaces was analyzed in detail. The ignition distances of three experimental coals were measured, and the influence mechanism of bituminous coal blending on the combustion, heat transfer, and NOx emission in the furnace was proposed. The results show that increasing the bituminous coal blending ratio is beneficial to reduce NOx emissions. However, it leads to the rise of the exhaust gas temperature and the decrease of the boiler efficiency, which can be ascribed to the large amount of the cold primary air sent into the pulverizing system to control the air pulverized coal flow temperature at the mill outlet. The ignition distances of three experimental coals are different. The ignition distance from short to long follows the following order, that is, bituminous coal, lean coal, and anthracite. The earlier ignition of the bituminous coal leads to a sharp rise in the temperature of the airflow at the burner nozzle outlet, a rapid expansion of the airflow volume, and a sharp attenuation of the airflow rigidity, resulting in a short flame downward penetration depth and the upward movement of the flame center in the furnace. As the bituminous coal is blended, the change of the flame center and the flame fullness in the furnace affects the flue gas temperature distribution. Similarly, the change of the radiant heat transfer intensity and the convective heat transfer intensity affects the heat absorption distribution of the heating surface in the furnace.

Influence of the Fin Shape on Heat Transport in Phase Change Material Heat Sink with Constant Heat Loads

Nowadays, the heat transfer enhancement in electronic cabinets with heat-generating elements can be achieved using the phase change materials and finned heat sink. The latter allows to improve the energy transference surface and to augment the cooling effects for the heat sources. The present research deals with numerical analysis of phase change material behavior in an electronic cabinet with an energy-generating element. For an intensification of heat removal, the complex finned heat sink with overall width of 10 cm was introduced, having the complicated shape of the fins with width of 0.33 cm and height H = 5 cm. The fatty acid with melting temperature of 46 °C was considered as a phase change material. The considered two-dimensional challenge was formulated employing the non-primitive variables and solved using the finite difference method. Impacts of the volumetric heat flux of heat-generating element and sizes of the fins on phase change material circulation and energy transference within the chamber were studied. It was shown that the presence of transverse ribs makes it possible to accelerate the melting process and reduce the source temperature by more than 12 °C at a heat load of 1600 W/m. It should also be noted that the nature of melting depends on the hydrodynamics of the melt, so the horizontal partitions reduce the intensity of convective heat transfer between the upper part of the region and the lower part.

Intensification of convective heat transfer in new shaped wavy channel configurations

The paper presents the specific wavy heat exchange surfaces in order to intensify convective heat transfer in the channel. The study is focused on the analysis of local and average heat transfer coefficients and Nusselt numbers by experimental method of the holographic interferometry and numerical simulations. The results are presented for temperatures of the heat exchange surfaces Ts = 318.15 K, 328.15 K, 338.15 K, and 348.15 K, Reynolds numbers Re = 1386–4203, and height between the surfaces H = 30, 35, 40, 45, and 50 mm. New correlating equations for average Nusselt number for the in-line and the offset wavy channels are determined. The total heat transfer and pressure drop j/f and thermal performance η are calculated for the wavy channels compared with the smooth one. It is observed that the offset wavy channel achieves a higher j/f values compared with the in-line wavy channel for all H and Ts values. The thermal performance η increases with Re and decreases with height H. The offset wavy channel achieves higher thermal performance η by 5.7%–15.7% when changing Re and H compared with the in-line one.