The present study is devoted to the experimental investigation of the boiling process of promising dielectric refrigerants HFE 7100 and Novec 649 on silicon substrate at atmospheric pressure. The purpose of the research is to determine the key heat transfer parameters, specifically nucleation temperature, heat transfer coefficients, critical heat fluxes, as well as the growth dynamics of irreversible dry spots, using the high-speed IR thermography method. It has been found that the boiling of Novec 649 is characterized not only by higher heat transfer coefficients and a lower nucleation temperature head, but also by lower critical heat fluxes (151 kW/m2) compared to HFE 7100 (183 kW/m2). The propagation velocity of irreversible dry spots during the boiling crisis development on a silicon substrate has been measured. For Novec 649, the velocity (6.8 mm/s) turned out to be lower than for HFE 7100 (10.4 mm/s). The obtained experimental data have been generalized and methods for calculating the basic thermal characteristics based on known theoretical models for silicon–dielectric fluid systems have been proposed. The research results are also of practical importance, providing engineers with specific data for designing highly efficient direct cooling systems for silicon wafers using dielectric fluids.
Thermal conductivity and enthalpy increment of the intermetallic compound LiK_3Pb_4 were measured for the first time using the laser flash method and drop calorimetry in the temperature range up to 1100 K. Based on the measurement results, the specific heat capacity and thermal diffusivity were calculated. It was found that the thermal conductivity of the liquid phase of LiK_3Pb_4 ranges from 2.5 to 4.3 W/(m K), which is an order of magnitude lower than the values calculated using the additive rule, indicating substantial localization of conduction electrons. This finding correlates well with the hypothesis of the formation and dominance of ionic complexes such as [Pb_4]^4- in liquid systems of lead with heavy alkali metals. Tables of recommended data for the investigated properties were created across the entire measurement range, together with an estimation of uncertainties in them.
Experiments were performed to study water pool boiling at atmospheric pressure on biphilic surfaces with grooves (open microchannels) produced by laser ablation and hydrophobized owing to chemisorption of fluorinated methoxysilane from vapor at a temperature of 100–110 ◦C. Surfaces with arrays of parallel grooves, as well as with arrays of grooves intersecting at right angles (groove grids with square cells), were used. The experimental data were analyzed and compared with literature and previous data. The influence of the size and location of hydrophobic regions on boiling heat transfer enhancement was studied. It has been shown that the main parameter determining the heat transfer enhancement for a surface with an array of parallel grooves is the pitch between them. For groove grids, this is also true, but only for large heat fluxes. Removal of the hydrophobizer from the grooves leads to a significant decrease in heat transfer. A comparison of the results of this study with previous data for arrays of hydrophobic cavities and arrays of hydrophobic round spots on a flat surface shows enhanced heat transfer on biphilic surfaces compared to a flat smooth surface. These surfaces are characterized by a strong dependence of heat transfer on the pitch between the hydrophobic regions. The maximum effect on heat transfer was observed on surfaces with a 3D biphilic pattern, namely on arrays of microcavities and arrays of intersecting grooves (groove grids with square cells).
Developing clean energy technologies to harness ocean waves remains a key engineering challenge. In this context, the influence of geometry on the performance of an oscillating water column (OWC) wave energy converter through the Constructal Design method is investigated. The objective is to maximize available hydropneumatic power when the system is subjected to full-scale regular waves. An inclined plate apparatus was inserted at the bottom of the OWC to evaluate its performance at inclinations of 0°, 20°, 40°, 60°, 80°, and 90° relative to the vertical direction. A validated and verified computational model based on the finite volume method was employed. The multiphase volume of fluid model was applied to handle air–water interaction. Results showed that the highest hydropneumatic power was achieved at 80°, which is 30.47
The paper presents a nonlinear elasticity theory (with plastic deformations) taking into account the second derivates of a metric tensor that describes failure of fluid-saturated porous rock. The theory shows that the invariance principles, conservation laws, and the first law of thermodynamics determine equations of the theory for a medium with a characteristic scale of failure. The simplest model describing the deformation of elastic layers under external stress demonstrates that the characteristic scale of failure is strongly affected by stresses applied to the elastic matrix and by the fluid content.
A model for heat transfer coefficient (HTC) in film flow with consideration of tube surface roughness and nucleate boiling is developed. This model is based on a previously developed HTC-model for film flow on horizontal tubes, on Labunzov’s formula for velocity in vapor bubbles, and on Gogonin’s factor of roughness. The HTC is measured in an experimental setup containing a vertical row of slightly inclined rough tubes. The calculated HTC is compared with experimentally obtained data. A good correlation between theory and experiment is established.
The paper presents the results of an experimental investigation into heat transfer during boiling of subcooled water flow at a circular surface of 7 mm in diameter set up at the center of a channel with a rectangular cross-section of 14 mm in width and 50 mm in length. The aim of the investigation was to reveal the effect of the limitedness of the space for growth of a vapor bubble on heat transfer. The growth of a vapor bubble was only confined by the opposite wall of the channel, whereas the edges of the heat-releasing surface were in a flow of water underheated to saturation temperature. The dimensions of the gap from the heated surface to the opposite wall of the channel were 0.3, 0.6, and 3 mm. The investigations were performed at atmospheric pressure, mass velocity of 127–370 kg/(m2 s), heat fluxes of 50–4800 kW/m2, and water temperatures at the channel inlet equal to 25, 50, and 70◦C. Depending on the gap size, three boiling regimes were visualized: boiling with deformed bubbles, combined boiling with deformed and isolated bubbles, and boiling with isolated bubbles. The experimental heat transfer coefficients were analyzed with the use of eight correlations well known from the literature. The data obtained are most accurately predicted by the correlation of Lee and Mudowar [45] with a mean absolute error of 5.6
In this paper, a simple approach for the prediction of final state during solid-liquid phase transition of a phase change material (PCM) is proposed based on a mathematical model of heat conduction with phase transition in a cylindrical coordinate system. Generally speaking, models like this one assume a moving boundary between the liquid and solid phases. Myristic acid is used as a PCM, and the analysis is carried out on a 60-minute basis. The aim of the presented piece of research is an experimental validation of the usage of the mentioned models. The approach proposed has resulted in a good prediction in terms of the final melting interface position. On the other side, in the initial period, the model overestimates the melting front motion, and thus it cannot be used to precisely predict the motion and shape of the melting front.
The paper analyzes liquid cooling systems of high-performance microprocessors (Intel Core i7-13700K, i9-14900K), integrated with compression refrigeration machines, in transient and steady-state operating modes. Data on heat transfer coefficients for two microchannel heat transfer surfaces with plate and prismatic fins have been obtained experimentally. The conducted analysis has shown that the plate heat exchanger has a higher heat transfer coefficient, but its exergy losses are 15–40 ∝ Q^1.8 ). At a load of 300 W, losses deviate from linear growth by 18–25 Δ T in heat exchangers by 30–35
The fundamental scientific problem addressed in this work is associated with the development of effective means to measure dispersed phases through a multiphase barrier in experiments modeling a wide range of natural and technical hydrodynamic systems. In a number of multiphase flow problems, the dispersed phase concentration is significant throughout the entire volume of the medium and forms a multiphase barrier, which impedes passage of various diagnostic signals and introduces into diagnostic signals significant distortions, which limit the accuracy of experimental measurements. In this work, a probe ultrasonic method for diagnostics of flow velocity through a multiphase barrier has been developed; a mock-up for measurements has been created; the probe has been calibrated on a hydrodynamic stand; results of comparative measurements based on laser Doppler anemometry have been obtained.
The current article focuses on the examination of a hybrid nanofluid magnetohydrodynamic flow across two different geometries, specifically a cone or wedge, within a porous medium with considering non-linear thermal radiation and chemical reactions. Hybrid nanoliquids have numerous applications in engineering and industry. Cone and wedge geometries are commonly employed in processing of polymer data. Hybrid nanoliquids exhibit superior performance in heat and mass transport rates when compared to both nanoliquids and conventional liquids. This study introduces the concept of hybrid nanoliquid to enhance energy and mass transfer, demonstrating favorable outcomes in thermal and mass transfer performance. The current study is structured using partial differential equations, which are converted into ordinary differential equations through appropriate similarity transformations. Numerical solutions are derived utilizing the bvp4c function within the MATLAB environment. This simplification has several significant outcomes: increasing the non-linear thermal radiation parameter enhances the thermal profile of the hybrid nanoliquid; the mass transfer rate of the hybrid nanoliquid decreases with respect to the Schmidt number and the chemical reaction; hybrid nanofluids excel single component nanofluids in their thermal performance, which confirms their potential in enhancing the heat transfer efficiency.
This paper presents the results of an experimental study of the effect of surfactant on the motion characteristics of bubbles in a stagnant liquid within an inclined pipe. The measurements were performed in a round pipe with an inner diameter of 32 mm at gas flow rates of 3, 5, and 8 ml/min and pipe inclination angles of 30–60◦. The bubble characteristics were determined using shadow photography processing of the bubbly flow. Distributions of bubble sizes and their rising velocities were obtained as functions of the pipe inclination angle and the distance from the gas injection point to the measurement location. The addition of surfactants significantly reduced the probability of bubble coalescence upon collision, causing bubbles to form clusters. During cluster motion, bubbles in the core move slightly faster, while those at the periphery travel slower, enabling circulatory motion within the cluster. Larger bubbles also become entrained in this circulatory motion. Consequently, the rising velocity of bubbles becomes independent of their size and is governed by the group rising velocity of the bubble cluster.
This study aims to enhance the accuracy and reliability of temperature measurements in liquid films and droplets using an extended two-color/two-dye planar laser-induced fluorescence (2C/2D PLIF) method. The experiment employs Kiton Red (temperature-sensitive) and Rhodamine 6G (temperature-insensitive) dyes to compensate for concentration fluctuations and environmental effects. A calibrated setup with synchronized cameras, optical filters, and a pulsed laser is used to capture the fluorescence intensities of glycerol and oil emulsions. The key results demonstrate a successful reconstruction of the temperature fields with a spatial resolution of 138 pixels/mm and an RMS error below 5
An experimental study on the influence of through holes on the flow around an axisymmetric cylinder and the features of flow separation for different angles of inclination relative to the main flow is conducted with the aim of developing methods for passive control of separated flows at high Reynolds numbers (from 1.74×10^5 to 2.75×10^5 ). Flow velocity field measurements on a hydrodynamic test bench are performed using the high-speed PIV method. A comparative analysis of the mean flow and near wake characteristics is accomplished with varying hole inclination angles relative to the flow direction ( 0^∘ , 45^∘ , and 90^∘ ). It is shown that the presence of through holes located in the flow direction ( 0^∘ ) and at an angle ( 45^∘ ) leads to a significant decrease in the recirculation zone size with increasing Reynolds number, as compared to a regular cylinder without through holes. Moreover, the recirculation zone for a rotation angle of 0^∘ decreases by 1.5 times compared to a rotation angle of 45^∘ . In the case where the holes are arranged perpendicular to the flow ( 90^∘ ), a strong influence on the flow separation dynamics is observed, in particular, leading to a significant decrease in the size of the recirculation zone and an almost constant value of the dimensionless vortex shedding frequency, in contrast to the other cases considered.
In this work, turbulent mixing in a narrow gap between subchannels is investigated using simultaneous particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF). The effect of a sweeping jet on the flow characteristics and scalar mixing in a narrow gap is analyzed. The sweeping jet is generated by a fluidic oscillator installed in the gap. Mixing of a pure liquid with a solution of fluorophore Rhodamine 6G in the narrow gap is examined. From simultaneous measurements of instantaneous velocity and concentration fields, covariances between the concentration and velocity fluctuations are evaluated. Mean velocity and concentration fields, components of the turbulent scalar flux, and the local turbulent diffusion coefficient are obtained. It is shown that the sweeping jet enhances inter-channel exchange primarily by intensifying turbulent scalar transport while leaving momentum transport essentially unchanged.
The article presents the results of an experimental study of the influence of the concentration of soluble solid impurities (NaCl, CuSO4, K2SO4, KH2PO4, and NaHCO3) on the processes of heating and evaporation of droplets of aqueous solutions at gas medium temperatures of 335–683 K. It is shown that the presence of impurities in concentrations of 1 to 5
A hydrodynamic model of the unsteady flow of a gas—liquid mixture in a pipe has been constructed, taking into account mass transfer and exchange process. An equation describing a gas—liquid mixture flow in a pipe is derived and its solution is obtained. Numerical computations are performed for the system parameters used in practice, and graphs are constructed. The results obtained show that the presence of mass transfer significantly changes the flow pattern, pressure gradients, and phase distribution in the flow. The research outcomes contribute to a better understanding of the two—phase flow behavior, which is extremely important for designing and optimizing industrial systems such as chemical reactors, pipelines, and heat exchangers.
The paper is devoted to elucidation of characteristic features of the heat transfer through a solid wall into a liquid heat-transfer agent in conditions of a confined space and a high heat flux density typical of modern mini-size systems. The objects of investigation were polypropylenglycol (PPG-425 and PPG-725) aqueous solutions with the lower critical solution temperature (LCST), transiently superheated with respect to the liquid—liquid equilibrium line and the liquid—liquid spinodal. Superheat was performed by the method of controlled pulse heat generation in a micro-size wire probe. The heating duration was from 20 to 100 ms, and the probe temperature varied from 373 to 773 К at a supercritical pressure. The attention is focused on a search for the explanation of the considerable changes in heat-transfer intensity accompanying the decomposition of an unstable solution, as functions of the changes of the water content in the initial solution. A physical model of the spinodal decomposition of aqueous solution of PPG has been suggested. According to this model, the effect of the asymmetry of heat-transfer intensity with respect to the critical concentration of the solution is related to the concentration dependence of the solution viscosity. The results will serve as a basis for choosing the optimum composition for transferring high-density heat fluxes in solutions with the LCST.
In this problem, heat transfer and the flow of water–alumina nanofluid inside a porous tube was studied. The constant heat flux was applied by three heat sources at different locations. The aim of this study was to find the optimal location for the three mentioned heat sources in order to facilitate heat transfer. Also the effects of Reynolds number (flow velocity), pipe porosity and different volume fractions of alumina nanoparticles in water on heat transfer were investigated. It was found that as the distance of other heat sources from the first heat source increases, the wall temperature reduces and therefore, for the same heat flux, the heat transfer coefficient and Nusselt number increase and heat transfer is improved. Also, it is observed that increasing the pipe porosity does not affect the temperature of the fluid mass at the outlet. In addition, by increasing the volume fraction of alumina nanoparticles, and heat transfer improves, but the bulk temperature at the outlet decreases, which is due to the significant increase in the mass flow rate of the inlet fluid with the addition of alumina nanoparticles.
Filmwise condensation of vapor remains the primary heat transfer mechanism in most modern industrial condensers. Although the use of finned tubes for heat transfer enhancement during condensation has a long history and proven effectiveness, the selection of optimal finning parameters is still often carried out empirically, without a rigorous theoretical basis. This review, being part of a series of brief reviews by the authors on studies of hydrodynamics and heat transfer during vapor condensation on smooth and finned tubes, systematizes the criteria for selecting geometric fin parameters depending on the properties of the working fluid and the characteristics of the finned tube material. The proposed empirical and semi-empirical correlations will make it possible to evaluate the finning efficiency of tubes for a specific heat exchanger and to determine the economic feasibility of its application. The findings presented will contribute to the further transition of the thermal power and refrigeration industries toward next-generation condensers.
