Acoustic streaming outside spherical particles and parameter analysis of heat transfer enhancement

Numerical simulation on acoustic streaming characteristics in boiler tube array

The distribution of acoustic streaming outside the tube in a planar standing wave field is calculated by numerical simulation using an elliptical heat exchanger tube at the low-temperature heating surface of a 600 MW unit as an example. The study discusses the impact of angle of attack (AOA), Strouhal number (St0), and Reynolds number (Re0) on the external acoustic streaming characteristics of the tube from structural, intensity, and scale perspectives. The results indicate that the strong nonlinearity of the second-order acoustic streaming in the Stokes boundary layer is driven by the first-order acoustic field. As the AOA increases, the external acoustic streaming vortices exhibit a phenomenon of inner and external vortex merging. Along the direction of the inner vortex center, the acoustic streaming velocity shows peak and valley fluctuations influenced by the vortices. In the normal direction, the acoustic streaming is significantly weakened, and the velocity distribution characteristics vary at different distances s. The inner vortices i = 1 and 2 achieve maximum and minimum values around AOA = 60° and 30°, respectively. The relative Nusselt number NU and relative shear rate SH quantify heat transfer and ash removal effects, respectively. Both small St0 and large Re0 enhance the heat transfer effectiveness. The SH shows a similar variation pattern to the scale of the acoustic streaming under the corresponding cases. The conclusion aligns well with the requirements of enhanced heat transfer and ash resistance for low-temperature heating surfaces, providing important reference for the selection of AOA for elliptical heat exchange tubes.

Currently, there is a problem of developing effective methods to calculate heat and mass transfer processes during steam condensation from a vapor-gas mixture in industrial devices. This is due to the need to create new reliable and highly efficient designs of heat exchangers for various purposes. The finite element method has been used in the ANSYS Fluent software package during the numerical simulation. An experimental plant is tested in the pipeline of an industrial enterprise at the production site of the Technopolis KHIMGRAD industrial park (Kazan-city). The applicability of the Lee model to solve problems of water flow in a pipeline with partial evaporation is proved. This model allows you to accurately account for the processes of evaporation and condensation, which is important for the design of cooling and air conditioning systems. The authors have designed a three-dimensional model of the flow area of the experimental module and a CFD model to calculate temperatures, phase velocities and their concentrations, considering the peculiarities of the ongoing processes of heat and mass transfer. A slight effect on the heat flow of a 180° rotation during the flow of a liquid medium in the range of the Reynolds numbers 1800–2600 is shown. The developed model is verified with the results obtained at the experimental plant. It is established that the model reproduces experimental data well. An analysis of studies of heat and mass transfer processes during the fluid flow in channels in the presence of phase transitions is carried out. The problems of calculating the parameters of heat transfer during the movement of a fluid in a transient mode are revealed, especially at values of the Reynolds number close to 2000. The experimental plant diagram is presented. It is found that in the range of Reynolds numbers of 2600–3600, the calculated temperature values differ from the temperature values obtained experimentally by less than 0,42 %. During the partial transition of water into steam in the range of Reynolds numbers of 1800–2600, the deviations were less than 6 %, which confirms the adequacy of numerical modeling of calculating the heat transfer process in a pipe. It is shown that considering the influence of temperature on surface tension, the coefficient of thermal conductivity of water and the coefficient of dynamic viscosity of water is important to obtain accurate results. The proposed approach can be used to optimize the operating parameters of condensers and evaporators, as well as to accurately calculate the heat transfer coefficient and determine the areas of vapor formation. It will improve the energy efficiency of industrial installations and reduce the cost of equipment operation.

The process of heat and mass transfer between the air flow and the natural source of cold - ice is considered. It includes heat exchange by sensible heat and moisture exchange through latent heat transfer. This type of heat and mass transfer is observed in seasonal refrigeration storages for air conditioning systems. Snow or ice frozen during the cold season is used as a natural source of cold. The experimental setup created conditions for the contact of the flow of thermal air and natural sources of cold. The air flow is uniform. For this, the setup was carried out. Based on the analysis of experimental data, equations were obtained that determine the process of heat and moisture exchange. These are the dependences of the thermal Nusselt number on the Reynolds number and the diffusion Nusselt number on the Reynolds number. The heat transfer equations were compared with the previously obtained equations for non-uniform air flow in the experimental setup. Heat transfer with a uniform air flow is less intense at high Reynolds numbers from the considered interval. However, at lower air flow rates, the thermal Nusselt numbers become higher than at nonuniform air flow. These dependencies make it possible to calculate the dimensions of the required surface of contact of air with a natural source of cold for designing seasonal refrigeration storages.KeywordsCold accumulationHeat exchange (transfer)Moisture exchangeConditioningThermodynamic parameters

Use of single air extraction and injection to thermodynamically balance the combined heat and mass transfer process

The results of experimental studies of hydraulic resistance, heat and mass transfer characteristics of a prismatic lattice polymer regular packing for various options of its installation in space are presented. Based on the conducted experimental studies, the most rational design of packing installation was determined (taking into account the decrease in hydraulic resistance and the increase in the efficiency of heat and mass transfer processes), for which the geometric characteristics are given and empirical dependences are obtained, which make it possible to determine the value of the hydraulic resistance of the dry and irrigated packing, the values of the volumetric coefficients of mass transfer and heat transfer. The results of the study can be used in enterprises where different types of cooling towers or devices for conducting heat and (or) mass transfer processes, such as absorption, are used, as well as in the development of packed scrubbers.

The results of a numerical study of the influence of the thicknesses of dynamic and thermal boundary layers on turbulent separation and heat transfer in a tube with sudden expansion are presented. The first part of this work studies the influence of the thickness of the dynamic boundary layer, which was varied by changing the length of the stabilization area within the maximal extent possible: from zero to half of the tube diameter. In the second part of the study, the flow before separation was hydrodynamically stabilized and the thermal layer before the expansion could simultaneously change its thickness from 0 to D1/2. The Reynolds number was varied in the range of \(\text{Re}_{{{\text{D}}_{1} }} = 6.7 \cdot 10^{3} \,{\text{to}}\,1.33 \cdot 10^{5}\), and the degree of tube expansion remained constant at ER = (D2/D1)2 = 1.78. A significant effect of the thickness of the separated boundary layer on both dynamic and thermal characteristics of the flow is shown. In particular, it was found out that with an increase in the thickness of the boundary layer the recirculation zone increases and the maximal Nusselt number decreases. It was determined that the growth of the heat layer thickness does not affect the hydrodynamic characteristics of the flow after separation but does lead to a reduction of heat transfer intensity in the separation area and removal of the coordinates of maximal heat transfer from the point of tube expansion. The generalizing dependence for the maximal Nusselt number at various thermal layer thicknesses is given. Comparison with experimental data confirmed the main trends in the behavior of heat and mass transfer processes in separated flows behind a step with different thermal prehistories.

Nonequilibrium heat and mass transfer processes through liquid-vapor interfaces are studied through solutions of the Enskog-Vlasov (EV) equation and the corresponding system of EV26 moment equations. These models fully resolve liquid and vapor bulk regions and the diffuse interface connecting both. With that, evaporation and heat transfer processes can be studied without the need of modeled interface relations. Comparison of numerical results shows qualitative agreement of moment simulations with DSMC solutions of the EV equation, but quantitative differences. Interface resistivities for jump interface conditions are determined from the simulations, which show marked differences to those found from classical kinetic theory, where dimensionless resistivities are constants. In contrast, the EV models give temperature dependent resistivities, some negative off-diagonal resistivities, and indicate non-linear behavior where resistivities depend on mass and heat fluxes through the interface. In summary, the results point to the urgent need for systematic evaluation of resistivities over a wide range of conditions between weak and strong nonequilibrium, close to and far from the critical point.

Impact of working fluid properties on heat transfer and flow characteristics of two-phase loop thermosyphon with high filling ratios

Flow boiling in multi-microchannel evaporators is one of the most efficient thermal management solutions for high-power-density applications. However, there is still a lack of understanding of the governing two-phase heat and mass transfer processes that occur in these devices, which has resulted in a limited availability of applicable boiling heat transfer prediction methods based on first principles, and of reliable thermal design tools. This article presents a systematic analysis of the dynamics of bubbles and the surrounding liquid film during flow boiling in three-side-heated non-circular microchannels. The study is performed using a custom version of ESI OpenFOAM v2106 with a geometric volume-of-fluid method to capture the interface dynamics, also incorporating conjugate heat transfer through the evaporator walls. The hydraulic diameter of the channel is fixed to Dh=0.229mm and the range of width-to-height aspect ratios ϵ=0.25−4 is examined. We investigate different fluids, namely water, HFE7100, R1233zd(E), R1234ze(E), and evaporator materials, namely copper, aluminium, silicon, stainless steel, with base heat fluxes in the range qb=50−200kW/m2. The results show that conjugate heat transfer acts to make the temperature distributions around the perimeter of the channel cross-section more uniform, and that the topography of the lubricating film and the extension of the dry vapour patches that develop while the film is depleted both depend on the cross-sectional channel shape and influence the heat transfer performance significantly. For highly wetting conditions, channels with ϵ=0.25 tend to allow enhanced heat transfer rates, with a spatially-averaged Nusselt number that is 50% higher than that obtained for ϵ=1 (square channels) and 10% higher than that for ϵ=4. This arises thanks to an extended evaporating film that covers the vertical walls which, owing to the three-side-heated configuration, contribute twice to the spatially-averaged heat transfer performance. For more hydrophobic conditions, large dry patches develop over the vertical walls for ϵ=0.25 due to the lower evaporator temperatures, leading to reduced heat transfer, with thermal performance weakly dependent on ϵ in the range ϵ=0.5−2.

Liquid desiccant system is more and more popular in the air-conditioning system. This paper focuses on the match property analyses of the internally-cooled liquid desiccant system. The unmatched coefficients both of the sensible heat transfer process between the desiccant and the cooling medium and the coupled heat and mass transfer process between the desiccant and the air were defined for the internally-cooled device. Numerical model of the dehumidifier was established for calculating the unmatched coefficients. Match properties of different kinds of liquid desiccant systems under various boundary conditions were then compared. The results show that the internally-cooled device has much smaller unmatched coefficients compared to the two-flow devices (adiabatic dehumidifier and sensible heat exchanger). The unmatched coefficients are good indicators for system improvement with identical heat and mass transfer area.

From a subsurface physical point of view, this paper discusses the necessity of considering the two‐phase heat and mass transfer process in land surface models (LSMs). The potential‐based equations of coupled mass and heat transport under constant air pressure form the basis of the proposed model. The model is developed considering dry air as a single phase, and including mechanical dispersion in the water vapor and dry air transfer. The adsorbed liquid flux due to thermal gradient is also taken into account. The set of equations for the two‐phase heat and mass transfer is formulated fully considering diffusion, advection, and dispersion. The advantage of the proposed model over the traditional equation system is discussed. The accuracy of the proposed model is assessed through comparison with analytical work for coupled mass and heat transfer and experimental work for isothermal two‐phase flow (moisture/air transfer). The influence adding airflow has on the coupled moisture and heat transfer is further investigated, clearly identifying the importance of including airflow in the coupled mass and heat transfer. How the isothermal two‐phase flow is affected by considering heat flow is also evaluated, showing the influence of heat flow only to be significant if the air phase plays a significant role in solving the equations of the water phase. On the basis of a field experiment, the proposed model is compared with the measured soil moisture, temperature, and evaporation rate, the results showing clearly that it is necessary to consider the airflow mechanism in soil‐atmosphere interaction studies.

This paper investigates the effects of activation energy and nonlinear thermal radiation on the flow, heat, and mass transfer of magneto-bioconvective Eyring–Powell nanofluid due to a stretching sheet. It analyzes the influences of magnetohydrodynamic forces, nonlinear thermal radiation, chemical reactions, and the activation energy of species within the Powell–Eyring nanofluid. Motile microorganisms are introduced alongside nanoparticles to prevent agglomeration and stabilize the suspension. The governing nonlinear partial differential equations (PDEs) are transformed into ordinary differential equations (ODEs) using similarity transformations and solved numerically with the RKF45 algorithm in MATLAB. Results indicate that increasing the Powell–Eyring fluid parameter and Grashof number enhances the velocity profile, while the Hartmann number and buoyancy ratio reduce it. Temperature distribution increases with higher thermal radiation, Hartmann, Eckert, Brownian motion, and thermophoresis parameters, but decreases with greater Grashof and Prandtl numbers. The nanoparticle concentration profile declines with increasing Lewis number, chemical reaction, and Brownian motion parameters but rises with thermophoresis and activation energy. The density of gyrotactic microorganisms decreases with higher bioconvection Lewis and Peclet number, concentration deference of motile microorganism parameter, while it increases with the Hartmann number. In table, we are observed that as the ε and Gr values are increases, then skin friction coefficient value are decreases. As the M , Rb , δ , Nr values are increases, then skin friction coefficient value are increases. In table, we are observed that Gr , R , θ w values are increases, then local Nusslet number increases. As the M , Pr , Ec , Nb , Nt values are increases, then local Nusslet number decreases. In table, we are observed that as the values of Le , Nb , Nt , σ , E , ω upsurges, then Sherwood number decreases. In table, we observed that as M values increases, then Local density number decreases. As the Lb , Ω , Pe increases, then Local density number increases. Main objective of the present study is observed from tables and the amount of heat transmission increases by increasing the values of M , R , θ w , Ec , Nb , Nt whereas it declines by increasing the values of Gr , Pr . It is noticed that the rate of mass transmission increases by increasing the values of Nt , E , ω whereas it declines by increasing the values of Le , Nb , σ . Shows an overview of the θ η ( 0 ) observations for different values of Nb & Nt . The activation energy significantly impacts the flow and heat transfer characteristics of Magneto-Bioconvective Eyring–Powell Nanofluid, affecting its velocity and temperature profiles. Higher activation energy reduces nanoparticle diffusion, slowing mass transfer, and concentration gradients, which is essential in applications requiring controlled diffusion, such as targeted drug delivery and biochemical processing. Nonlinear thermal radiation enhances the heat transfer rate near a stretching sheet and decreases thermal boundary layer thickness, particularly under high-temperature gradients, making it valuable for high thermal load applications, microfluidic cooling, and industrial heating. Magnetic fields influence bioconvection patterns of gyrotactic microorganisms, either enhancing or hindering heat and mass transfer based on magnetic field strength. The Hartmann number increases flow velocity and reduces temperature gradients, offering control over flow profiles in applications like magnetic-assisted manufacturing and microfluidic devices. The nanoparticle volume fraction notably affects the thermophysical properties and heat and mass transfer behavior of the nanofluid. The findings have critical applications in fields such as biomedical engineering, biomedical applications, and energy conversion devices, where controlled diffusion is essential, and in industrial microfluidics, where efficient heat and mass transfer are required. Simulation results provide insight into optimizing heat and mass transfer processes in practical engineering systems, thermal management systems.

Introduction. Radiative heat transfer in porous insulating materials at high temperatures was investigated in [1–3], and compound heat transfer under unsteady-state conditions in a material was investigated in [4]. Radiation transfer in the gray medium approximation substantially limits the adequacy of the model used in [5]. The present work, which is a continuation of [5], was performed by us with a view to account for the dependence of the optical properties of gas-permeable materials on radiation wavelength. The article describes a computational investigation of compound heat and mass transfer in a semitransparent gas-permeable material in the presence of phase transition and oxygen chemisorption in pores. An innovation in the formulation of the problem is accounting for the interaction of the radiation fi elds of optical inhomogeneities (pores and the particles intruded into a material) when calculating the material�c s optical properties. The optical properties of particles are usually calculated by the Mie theory [6], but at high concentrations the radiation fi elds of individual particles overlap, which is not taken into account in the Mie theory and leads to a large error in the calculated properties. To avoid the occurrence of such an error, in the present work the optical properties at a high particle concentration are calculated by means of the Maxwell–Garnett approximation [7] that accounts for the interaction of the radiation fi elds of the indicated inhomogeneities. Physical Model. We studied quartz that contained pores fi lled with air and vanadium dioxide particles in which 2 nd -order dielectric–metal phase transformations occur with heat liberation during cooling and heat absorption upon heating. Heat exchange with the surrounding medium proceeds by radiation and convection. We consider the processes of heat and mass transfer in a thin gas-permeable plane layer in one-dimensional approximation under conditions where radiative heat transfer is comparable with heat conduction in the solid skeleton and gas convection in the pores. Under conditions of heating or cooling, 2 nd -order dielectric–metal phase transition occurs in the vanadium dioxide particles, which is accompanied by oxygen chemisorptions and by a change in the crystalline structure of particles, with simultaneous liberation or absorption of heat in the material. These processes can be modeled by volumetric heat and mass sources that are introduced into the effective heat capacity of the material [5]. The absence of large pressure gradients in the gas leads to a relatively slow gas fi ltration, which allows us to assume that thermal equilibrium was achieved between the solid skeleton and the gas. The problem formulated is solved in the following approximation: the material under study is considered as a set of three continua inserted into one another. The components of this set are the continua of air in the pores, quartz (the material skeleton), and of the vanadium dioxide particles introduced into the skeleton. The thermophysical properties of continuous components are the effective properties in relation to the corresponding properties of a real material. Gas fi ltration in a material is modeled as the motion of a continuous air component characterized by a velocity fi eld and by the fi eld of specifi c air fl ow rates. Radiative transfer in the system considered occurs through the solid skeleton and gas in the pores. In modeling the radiative transfer, we took into account the processes of the radiation absorption, emission, and scattering. We assumed that the optical properties of a material were dependent on the wavelength. The effective heat capacity of the material depends on temperature and

Heat and mass transfer processes in connected saturated He II baths