Mass transfer across the interface of liquid and vapor in a cylindrical container of 100 mm diameter subjected to horizontal oscillation at different liquid depths has been investigated experimentally in the present study. Effect of forcing amplitude and frequency encompassing various flow regimes, viz., planar, chaotic, and swirl, which is observed in (1, 1) asymmetric mode, is the primary focus of the present study. Three frequencies are considered, which are omega f/omega 11 equal to 0.96, 1.00, and 1.02, respectively. Here, omega fis the wave frequency and omega 11is the natural frequency of that mode. All the experiments are conducted with n-pentane starting with active pressurization followed by a small relaxation period and then sloshing. When the liquid depth reduces, increased internal damping and wave damping affect the natural frequency and wave amplitude, which eventually reduces the sloshing Reynolds number. The objective of the present study is to test the validity of mass-transfer correlation of sloshing Reynolds number (Res, based on frequency of forcing and wave amplitude) and sloshing Nusselt number (Nus) at low liquid depth. Consequently, pressure variation with time at different liquid depths shows keeping all other parameters the same pressure changes magnitude decreases with decrease in liquid depth. This study reveals that swirl causes maximum drop in pressure in a pressurized vessel while, for planar wave, it is minimum. Increasing value of Res, has a substantial impact of pressure change and heat transfer. Nus, which is calculated based on that modified diffusion coefficient shows a power-law variation with Res.
The results of experimental study of the large-scale convective circulation over the flat surface and city layout, produced by extended localized heater for different heating intensity, are presented. The structure of the flow for these configurations, the mean, and pulsation characteristics were analyzed and compared. It is shown that urban roughness suppresses the large-scale circulation, which is mainly located above the urban terrain. The urban terrain leads to noticeable spatial non-homogeneity of the main circulation but velocity fluctuations are relatively low and do not exceed 5%. It is shown that a tall building can provide intensive fluctuations due to flow instability similar to the flow behind a bluff body. Correlation analysis shows significant correlation between terrain elevation and horizontal velocity. The correlation between velocity and building height is low.
In the present work, we study the evaporation of droplets of various suspensions on the surface of graphene film. Graphene sensors are silicon dioxide substrates with graphene film deposited on them with copper electrodes. Graphene conductivity changes while droplets deposited on the surface dry. The dependences of the resistance of graphene sensors on the geometric parameters of the droplets (heights, contact angles, contact areas of the droplets with the surface) are found. The sensitivity of the sensors to the presence of soot particles in the droplets, as well as to the presence and hydrophilic-lipophilic balance of surfactants is demonstrated. The variation of conductivity is associated with a change in the structure of the liquid near the graphene surface. The presence of water molecules and soot particles reduces conductivity by creating a tangentially directed electric field. The presence of surfactants, in turn, increases the conductivity of graphene by creating an electric field normal to graphene surface.
We study the flows arising in a liquid droplet placed on a heated solid surface. The flow arises due to simultaneous action of a nonuniform evaporation at the droplet surface and the Marangoni effect. Calculations are carried out with the lattice Boltzmann method taking into account the conductive and convective heat fluxes, the evaporation and the condensation, and the temperature dependence of the surface tension. We obtain the pictures of vortical liquid flow inside a droplet. We find that the contact angle can change due to the forces which arise during evaporation or condensation near the contact line. The influence of the droplet shape (which depends on the contact angle) on the possibility of the arising of vortical flows inside the droplet is shown.
The combined effect of a normal electric field and normal vibrations on the stability of the base state of a system consisting of two dielectric fluids separated by an interface, which in the base state is flat and horizontal, is being studied. It is assumed that the density of the upper fluid is small compared to that of the lower fluid. It is also assumed that the viscosity is small in a certain sense. A multiscale method is used to study the instability. A fast vertical coordinate is introduced to describe the viscous boundary layer. The influence of the normal electric field on subharmonic and synchronous resonance modes of instability is considered. The dependence of the critical dimensionless vibration amplitude on the dimensionless parameter describing the electric field intensity and on the dimensionless wave number has been derived. The influence of normal vibrations on the Tonks-Frenkel instability has been investigated. The dependence of the critical dimensionless parameter, describing the electric field intensity, on the dimensionless vibration amplitude has been obtained.
In the last two decades, there has been a significant focus on improving the efficiency of heat exchangers by using nanofluids. Nanofluids have superior thermal conductivity and slightly increased specific heat. This review encompasses the studies conducted on improvements in the heat transfer efficiency of plate heat exchangers (PHE) for various nanoparticles (metallic, metal oxides, nonmetallic, hybrid) by different methods (experimental, empirical, and numerical analysis). Nanofluids have been found to have marginally increased viscosity and density and can cause little increase in pumping power. This review includes studies on increased power consumption in using nanofluids in PHE. Though nanoparticles are ultrafine particles, sedimentation and coagulation may affect the heat transfer after prolonged usage. This review includes studies conducted on sedimentation and coagulation of nanoparticles in PHE. Nanofluids may become unstable after continuous use, so this review explores the findings on the stability of nanofluids and various methods of increasing the stability of nanofluids, such as sonication and surfactants in PHE. This review also covers studies of nanofluids using artificial intelligence (AI) and machine learning (ML). By this review, it has been found that nanofluids undoubtedly increase heat transfer efficiency. However, the effects of coagulation, sedimentation, and stability over long-term use cannot be completely ruled out. Hence more comprehensive research is needed to assess the stability of nanofluids for prolonged use. Future research using AI and ML can help in developing commercially economical models that can work for a substantial period.
In this paper the thermocapillary stability of a thin liquid film coating the outside or the inside of a cylinder with a thick wall in the absence of gravity is investigated under the assumption of slip at the interface between the liquid and the wall. Analytical results are presented which show the regions in the parameter space where slip has the possibility to stabilize or destabilize the system. The linear results, which include the stability of azimuthal modes, show that slip may stabilize in two ways. First, from the point of view of the growth rate and, second, from the point of view of the maximum growth rate. These results are corroborated with the numerical analysis of the nonlinear evolution equation of the free surface deformation.
The liquid film driven by a turbulent gas flow in a flat minichannel under conditions of drop entrainment is investigated using a two-dimensional numerical model which employs a coupled volume of fluid and level-set (CVOFLS) method for interface capturing and the large eddy simulation (LES) approach with the Smagorinsky model to resolve turbulence. The results confirm that the transition from large-scale ducts to minichannels leads to a significant suppression of fast ripple waves on the rear slopes of disturbance waves, consistent with previous findings. However, the use of LES enables the resolution of these small-amplitude waves, revealing their increased frequency in minichannels. The analysis of the Q-criterion for vortex cores demonstrates that the generation of both initial waves near the liquid inlet and ripple waves on disturbance waves is driven by small-scale eddies in the gas phase. Furthermore, the model predicts the existence of vortical flow within the liquid phase of the disturbance wave itself and near the liquid inlet, which promotes the formation of ripple waves and initial waves, respectively. Thus, a qualitative similarity was observed between the formation of initial waves near the liquid inlet and the generation of ripple waves on the disturbance wave.
In the present study, we investigate the formation processes of methane and carbon dioxide clathrate (gas) hydrates in the presence of aluminum oxide (Al2O3) nanoparticles. Using molecular dynamics modeling, we analyze the influence of various nanoparticle concentrations and thermobaric conditions on hydrate nucleation, growth, and structural organization. The results show that an optimal nanoparticle content (approximately 1 wt.%) significantly reduces the induction period and accelerates the formation of clathrate cages, while simultaneously enhancing gas solubility through localized rearrangement of interfacial interactions in the water-gas-nanoparticle system. However, increasing the nanoparticle concentration above this optimal level leads to higher formation pressures and the emergence of an additional disordered liquid phase, thus constraining further improvements in process efficiency. These findings can be used to develop accelerated hydrate formation technologies for greenhouse gas capture and storage, as well as in cold supply and thermal energy storage systems.
Rotating packed beds (RPB) provide a more compact and efficient mass transfer compared to conventional packed columns by employing centrifugal acceleration to improve fluid flow and contact. Despite the analogy between heat and mass transfer processes, research into RPB applications in the field of heat transfer is currently limited. In the current study, the thermohydraulic performance of an RPB was experimentally analyzed for the evaporative cooling of water. Response surface methodology (RSM) was utilized for designing experiments and establishing regression equations between thermal performance parameters and operating parameters. Four crucial factors, including the flow rate of air, rotational speed, flow rate, and temperature of inlet water, were identified as input factors, and three performance indicators-pressure drop, cooling range, and cooling effectiveness-were chosen as the output responses. The central composite design (CCD) was utilized to generate the experimental runs with five levels of each factor. The resulting regression equations exhibit a strong fit to the data, with R-squared values exceeding 0.98. Plots of individual and interaction effects of significant factors on the performance parameters are presented.
This paper presents the results of an experimental study on the formation of carbon dioxide hydrate in a colloidal aqueous solution of aluminum oxide (Al2O3) and the anionic surfactant sodium dodecyl sulfate (SDS), which acts simultaneously as a kinetic promoter of hydrate formation and a stabilizing agent for the colloidal solution. Oxide nanoparticles were selected for the study due to their high specific surface area, which potentially facilitates the nucleation of gas hydrate. The presence of a large number of aluminum oxide particles increases the number of nucleation sites during crystallization. Additionally, these particles may participate in the process of gas transfer from the liquid-gas interface to the bulk liquid. The dependencies of the released energy and the conversion of the solution and carbon dioxide into the gas hydrate state were obtained.
The results of a laboratory study of heat transfer processes in urban environments are presented. Nonstationary nocturnal cooling without external ventilation was modeled using three-dimensional models of two large cities characterized by substantially different natural terrain, namely, Krasnoyarsk and Perm. To evaluate the influence of topography and urban development on temperature distribution, correlation analysis was implemented. The values of correlation coefficients are significant and higher for Perm compared to Krasnoyarsk during the first stage of nocturnal cooling, implying a strong relationship between urban geometry and surface temperature for relatively flat cities. Land surface temperatures from Landsat 8 imagery were used to compare the results of laboratory-scale models and observations. Both experimental and observational data show the significant influence of the artificial terrain on the temperature distribution.
A cavitating venturi operates with an oscillating two-phase cavity that evolves through a combination of different flow mechanisms and is also closely linked to changes in geometrical configuration. Numerical prediction of the transient flow features of the venturi could become a convenient tool for venturi sizing and design for specific applications. The use of Eulerian two-phase mixture models, along with suitable cavitation models and Reynolds averaged Navier- Stokes (RANS) turbulence model, is known to over-damp the transient oscillatory nature of the cavitation zone due to the overproduction of turbulent viscosity. It is found from the literature that a modified turbulent viscosity equation formulated as a function of the two-phase density, after appropriate tuning of the model constant, is able to predict the transient phenomenon of cavitation in internal flows. However, tuning the model is highly case-specific, and generality regarding the correct frequency predictions is not always guaranteed. The current work presents the steady-state and transient numerical simulations using a two-fluid Eulerian model for the two-phase field, the Schnerr-Sauer model for cavitation and the RANS model for turbulence (without modifying the turbulent viscosity). Commercial software Ansys Fluent is used for the simulations. The model's steady-state predictability of cavitation length is benchmarked using the axisymmetric venturi data from the literature. A parametric study was also conducted to choose appropriate interfacial closure models. Systematic transient simulations were then carried out for a range of pressure ratios (Pr, the ratio of the absolute pressure at the outlet to that at the inlet of the venturi) representing the three different regions of experimentally obtained frequencies reported in the previous work of the present authors. The dynamic behavior predicted by the two-fluid modeling indicates two distinct regions of cavity oscillations. Although the numerically predicted frequencies deviate from the experimental predictions, distinct frequencies are predicted, indicating distinction in the dynamics at different pressure ratios. The current results from two-fluid models definitely provide pointers towards realistic dynamic predictions. Appropriate model improvements could offset the need for computationally exable, frequency of oscillations, transient behavior
The interface motion in a cryogenic propellant tank during the vehicle lift-off and in the accent phase disturbs the thermally stratified layer and results in variation of tank pressure and liquid temperature. A 1D mathematical model is developed here that can be used as a tool for engineering analysis of the effect of the sloshing/interface motion in cryogenic fluid. The model considers the ullage volume as a multicomponent gas mixture, and the pressure history predicted through this method matches with the flight data (within +/- 8%). The mathematical model reported in this paper incorporates the effect of interface motion by taking a multiplication factor (sloshing Nusselt number) for the liquid conductivity near the interface, which represents the effect of liquid mixing due to sloshing. The sloshing Nusselt number and Reynold number correlation proposed by the experimental work of Ludwig et al. (2013) [Ludwig, C., Dreyer, M.E., and Hopfinger, E.J., Pressure Variations in a Cryogenic Liquid Storage Tank Subjected to Periodic Excitations, Int. J. Heat Mass Transf., vol. 66, pp. 223-234, 2013.] is referred to here to estimate the heat transfer between the liquid and the interface. A parametric study is carried out by simulating the different types of slosh waves which can be generated inside the tank and the pressure collapse in each case is compared. It is found that the pressure collapse caused by stable planar waves is much less compared to collapse caused by breaking or swirl waves at the interface. The model reported here can be used along with results of ground testing (Nus) for predicting the pressure drop due to liquid sloshing in a cryogenic tank for a given flight disturbance profile.
We consider a circular dry patch formed in an evaporating liquid layer on a flat heated substrate. Moist air flow is induced by evaporation and thus depends on the distribution of vapor concentration. We solve a steady diffusion equation for vapor concentration above the dry patch formed in a liquid layer of infinite extent using integral transforms and validate our approach by comparing the analytical solution for vapor concentration profile along the patch surface with the previously reported numerical results. For the case when the extent of the liquid layer is finite, we develop a numerical solution using finite-element method with adaptive mesh and derive an approximate analytical solution which is shown to be in good agreement with the numerical results. Local evaporation-induced air flow structure is investigated using the approximate analytical solution for steady diffusion and then used to conduct parametric studies of trajectories of microdroplets flying towards the dry patch. Droplet dynamics is sensitive to changes in the layer-surface temperature and droplet size, but has only weak dependence on the air humidity.
