Values Of Transfer Coefficient Research Articles

Temperature distribution analysis and mechanical property prediction of wire loops in the Stelmor air-cooling system

During the Stelmor air-cooling process, the temperature distribution has a significant impact on the wire loops' final mechanical properties. The temperature distribution during air-cooling process is accurately solved by establishing three-dimensional model and Computational Fluid Dynamics (CFD) simulation. It is also found that the heat transfer coefficient on the cross section of the wire loop is very different, with a difference of 70–100 Wm−2K−1. The average value of the surface heat transfer coefficient of the wire loops section is used to replace the surface heat transfer coefficient of the wire loops, which improves the temperature distribution calculation accuracy, which is not considered by previous scholars. The finite difference method is used to calculate the mathematical model of the temperature distribution during the Stelmor air-cooling process. It is found that the maximum temperature difference of the wire during the cooling process is 90–110 K. The prediction model of mechanical property of wire after air-cooling was established, and distribution of mechanical property of the wire loops was obtained, and the reason for the uneven distribution of mechanical property of the wire loops was found.

The neural network approach for estimation of heat transfer coefficient in heat exchangers considering the fouling formation dynamic

Routine maintenance for plate heat exchanger (PHE) cleaning improves the effectiveness of heat exchange network operation. Until recently, complex mathematical modelling was used to predict the value of the heat transfer coefficient after a certain period of operation of the heat exchanger, as well as the point in time when the coefficient reached the allowable limit. The applied mathematical tools included the systems of differential equations, matrices of heuristic coefficients, which needed a lot of computer resources. This paper offers an artificial neural network (ANN) technique for forecasting the following values: heat transfer coefficient at any time points during the operating period of PHEs; the time point, when the heat transfer coefficient reaches its lower permitted value. In this method, ANN uses the fuzzy logic techniques to expand the set of training parameters for the model, working with data of industrial measurements and data obtained from the mathematical modelling of the process. It allowed to train the developed feed-forward neural network (FFNN) with the coefficient of determination R2 equal to 0.99 and can predict the thermal resistance in PHE based on measurement data. To adequately predict the time-point to reach the limiting value of the heat transfer coefficient, it was proposed a recurrent neural network (RNN) with a hidden layer of long short-term memory (LSTM), where R2 value came up to 0.89.

The thermo-mechanical coupling method of brake disc based on dynamic convective heat transfer

With the increase of train speed, the brake disk receives more heat input, causing its own temperature to reach a higher level. Due to the complex structure of the brake disk, its convective heat transfer coefficient is difficult to calculate, which makes the analysis of the convective heat dissipation between the brake disk and the air complicated. In this article, based on the software FLUENT and ABAQUS, by combining the fluid-solid coupling method and the thermo-mechanical coupling method, the dynamic convective heat transfer and the transient temperature field of brake disk are simulated. The maximum value of dynamic convective heat transfer coefficient is 483 W/m2 ∙ K at the running speed of 350 km/h. As for the transient temperature field, not only the contact relationship between the brake disk and the brake pad is considered, but also the dynamic convective heat transfer between the brake disk and the surrounding air domain is applied. The simulation results are more in line with the actual situation and the engineering application.

Experimental investigation and modeling of the impact of random packings on mass transfer in fluidized beds

This study investigates the impact of two novel settings of bubbling fluidized beds equipped with random metal packings on the mass transfer. To do this, a comprehensive approach is applied that integrates experiments and modeling and explores the relevance of the different underlying mechanisms involved in interphase mass transfer in fluidized beds. Firstly, the mass transfer of water from moisturized silica gel particles to dry air is studied in both a packed-fluidized bed and a freely bubbling bed with no packing. The experimental set-up consists of a cylindrical bubbling fluidized-bed column with an inner diameter of 22 cm. Silica-gel particles with a mean particle diameter of 797 μm are used as bed material. The total bed amount ranges from 4 to 8 kg, while the fluidization number (F) varies between 1.7 and 2.3. Two types of packing, RMSR (stainless steel thread saddle rings) and Hiflow (stainless steel pall rings) are examined and compared to the reference case of a bubbling bed with no packing. The height of the packed section is maintained at 60 cm. The results show that, at all operating conditions, the use of packings enhances the amount of desorbed water in the fluidized bed. The increase is up to 17%, as compared to the bed without packing. The effect is believed to be inhibition of bubble growth in the packed-fluidized bed. To study this further, a mass-transfer model is introduced to analyze the different mass-transfer steps (intra-particle, particle surface to emulsion gas, and emulsion gas to bubble gas) in packed-fluidized beds compared to beds with no packing. TGA experiments are applied to describe the intra-particle mass transfer through a desorption kinetic model. Model analysis shows that the main resistance for mass transfer occurs across the bubble-emulsion boundary. The calculated value of the mass transfer coefficient for this, Kb, at reference conditions (6 kg of silica gel and F = 2.3) is 7.6e-5 s−1 with packings (in average) and 6.2e-5 s−1 without packings, i.e. a 23% improvement in the governing mass-transfer coefficient.

Proposals for new transfer coefficient (TC) values for worker re-entry activities in grape vineyards

New transfer coefficient (TC) values were derived for vineyard workers handling treated grapevines during harvesting and crop maintenance activities. Re-entry exposure and dislodgeable foliar residue (DFR) studies were performed in Europe, covering hand harvesting, pruning/training, pruning/tying and pruning/shoot lifting. Foliar applications of fungicides (iprovalicarb, dimethomorph, dithianon, pyrimethanil and fenbuconazole) were made and 73 workers at 16 sites were monitored over one working day. Exposure was measured on inner and outer dosimeters, face/neck wipes and hand washes. In concurrent DFR studies, leaf punches were taken at each site during the time of worker re-entry. Potential exposure values correlated well with DFR values. TC values were derived for various re-entry activities for potential and actual exposure, with and without gloves. The harvesting task resulted in lower TC values than the other crop maintenance tasks. Additional TC values reflecting the use of protective gloves can be derived from the results. The TC values are much lower than current European Food Safety Authority (EFSA) default values. This project addresses a data gap identified by EFSA for specific EU TC values to permit more realistic and reliable re-entry worker exposure estimates for grapes.

Experimental study on transpiration cooling through additively manufactured porous structures

Film cooling and thermal barrier coating are the most common external cooling techniques used for gas turbine components. In film cooling, coolant air is brought to the external surface via rows of discrete holes. This study investigates an alternative geometry, referred to as transpiration cooling. Here, the coolant air is brought to the external surface through an additively manufactured porous structure. An experimental study was performed to characterize the downstream cooling performance by obtaining film cooling effectiveness and heat transfer coefficient using thermochromic liquid crystals. The experimental setup was first validated using a cylindrical film cooling sample and the results were compared to results obtained from previously published film cooling studies. After validation, transpiration cooling samples were investigated. A total of three porous structure samples with different thicknesses were subjected to testing. The thickness of the porous structure had insignificant effect on both the heat transfer coefficient and the film cooling effectiveness. The samples were tested at blowing ratios ranging from 0.5 to 2.0. Results showed an increase in laterally averaged heat transfer coefficient with increasing blowing ratio and compared to cylindrical film cooling, transpiration cooling yielded significantly higher distributions. The spanwise averaged film cooling effectiveness displayed a similar trend with higher values for transpiration cooling, mainly due to a more even mixing in the lateral direction. Values of the heat flux ratio showed that the high heat transfer coefficient penalizes the porous cooling method and only for low blowing ratios may transpiration cooling be the beneficial choice. From visualizations with water and carbon dioxide, it could be concluded that the inclination angle is close to 90° which yields a chaotic flow field and, in turn, high values of the heat transfer coefficient. By changing the additive manufacturing's build direction, a more favorable inclination angle should be achievable and, thereby, an opportunity to utilize the porous structure's excellent mixing in the spanwise direction.

Fourier series-based modelling of the effects of thermal coupling on the transient dynamics of component loops in a coupled natural circulation loop

Energy efficiency in process industry and passive safety systems in nuclear power plants necessitate the use of buoyancy driven heat exchangers. The current study presents a 1-D numerical model of the buoyancy driven fluid system in a Coupled Natural Circulation Loop (CNCL) comprising water as the working fluid. In this study water at different temperatures is considered as the operating fluid in each of the loops. The study employs a 1-D model derived using a semi-analytical Fourier series. A validation study was carried out using the relevant literature to verify the mathematical model. A detailed parametric study on individual NCLs and their coupled system was conducted by varying the cross-sectional area keeping the other parameters constant to gain insights into the effect of thermal coupling on the long-term transient dynamics of each of the individual loops of the CNCL system, for stable, unstable – periodic, and unstable – chaotic conditions. The dynamics were analysed using the difference between transient buoyancy and viscous forces, and it was found that the overall heat transfer coefficient influences the coupling behaviour and the dynamics of the component loops in a CNCL. At lower values of the overall heat transfer coefficient, the component loops in the CNCL nearly retain their independent behaviour, i.e., the component loops hardly influence each other. It was also found that the temperature dependent fluid properties influence the stability of the CNCL system in some cases.

Influence of biochar on the remediation of cadmium-contaminated soil in an Eisenia fetida–Solanum nigrum system

Biochar, plants, and earthworms have good remediation effects on cadmium (Cd)-contaminated soils. However, few studies have combined all three technologies to explore the treatment of Cd-contaminated soils. This study investigated the effect of corn straw biochar addition (1% and 5% mass ratios) on soil Cd treatment in an Eisenia fetida–Solanum nigrum system. The addition of corn straw biochar increased soil pH, total nitrogen (TN), total phosphorus (TP), and soil organic carbon (SOC); adding 5% (w/w) biochar under Cd stress resulted in significant increases (P < 0.05) of soil pH, TN, TP, and SOC. Adding 5% (w/w) biochar under Cd stress increased Cd enrichment by E. fetida and S. nigrum and significantly reduced the soil total and available Cd contents (P < 0.05). The addition of biochar increased the metallothionein content of E. fetida, which functions to resist Cd stress in high-Cd environments (P < 0.05); with the addition of 5% (w/w) biochar, the metallothionein content was 1.55 times higher than in the 1% (w/w) biochar treatment, at 23.78 ng L−1. Adding 5% (w/w) biochar significantly increased the Cd enrichment coefficient and transfer coefficient values of S. nigrum under high-Cd stress (P < 0.05), reaching 7.37 and 1.89, respectively. Adding 5% (w/w) biochar significantly reduced the exchangeable and acid-soluble fraction of Cd, increased the oxidizable fraction, reduced Cd bioavailability, and mitigated physiological damage (P < 0.05). The present study demonstrated that adding biochar to the E. fetida–S. nigrum system could effectively reduce the soil Cd pollution level, providing a new method and scientific guidance for the remediation of heavy metal-polluted soil.

Metallurgical Method of Determining Heat Transfer Coefficient in Simulations of Twin-Roll Casting

We herein suggest a metallurgical method using pure aluminum with no freezing temperature range to derive appropriate roll/melt interfacial heat transfer coefficients in simulation of twin-roll casting process. This method is inspired by the concept that the position of the kiss points where two solidifying shells encounter and the roll nip coincides under the condition where the roll load becomes zero as the roll rotation speed decreases. The conditions where the roll load becomes zero under various melt supply temperature conditions in the actual TRC process are found experimentally. These conditions are then applied to simulation models to derive heat transfer coefficient values. When comparing these values with those derived previously from the empirical relation for roll rotation speed and heat transfer coefficient, the conclusion is drawn that the deviation was reasonably low, around 10%.

Flow Boiling Heat Transfer in Enhanced Tubes With Composite Structure

Abstract To fill the research gaps in the study of flow boiling on composite surfaces, four heat exchange tubes with different structures were investigated: an enhanced tube with herringbone teeth (EHT_HB), a tube with a dimple structure (EHT_DIM), a tube with a composite herringbone tooth/dimple structure (EHT_HB/DIM), and a smooth tube as a baseline for comparison purposes. The experimental conditions were set as follows: saturation temperature 6 °C, mass flux 50–205 kg/(m2 · s), and vapor quality 0.2–0.8. After confirming the validity of the experimental results, the effects of the mass flux on the flow boiling heat transfer and pressure drop characteristics were examined. Results showed that the flow boiling heat transfer coefficient and pressure drop increase with the increase of the mass flux and the vapor quality in the tested tubes. The EHT_HB/DIM tube was found to combine the advantages of the two structures and to have the highest heat transfer coefficient. The average value of the heat transfer coefficient was found to be 43.75% higher than that of the smooth tube. Also, the average flow boiling heat transfer coefficients of the EHT_HB and the EHT_DIM tubes were found to be 15% and 35% higher than that of the smooth tube, respectively. From the perspective of the frictional pressure drop, it was found that the EHT_HB/DIM tube exhibited the maximum frictional pressure drop. The optimum working conditions of the enhanced tubes were determined by introducing a performance factor that captures the combined effects of heat transfer and pressure drop.

Mass variations and transfer process of shrimp farming pollutants in aquaculture drainage systems: Effects of DOM features and physicochemical properties

The expansion of aquaculture produces increasing pollutant loads, necessitating the use of drainage systems to discharge wastewater into surface water. To assess the mass variations and transfer process of aquaculture wastewater, an entire aquaculture drainage investigation lasting for 48 h was conducted, focusing on the nutrients, heavy metals, dissolved organic matter (DOM), and physicochemical properties of drainage in a commercial shrimp farm. The findings revealed that early drainage produced more heavy metals, total nitrogen (TN), dissolved organic nitrogen (DON), and feed-like proteins from aquaculture floating feed and additives, whereas late drainage produced more PO43--P and total dissolved phosphorus (TP). A few pollutants, including DON, Cu, and feed-like proteins, were effectively removed, whereas the contents of TN, dissolved inorganic nitrogen, and Zn increased in the multi-level aquaculture drainage system. Limited dilution indicated that in-stream transfer was the main process shaping pollutant concentrations within the drainage system. In the lower ditches, NO3--N, heavy metals, and feed-like proteins exhibited evident in-stream attenuation, while TN and NH4+-N underwent significant in-stream enrichment processes, especially in ditch C, with the transfer coefficient values (vf) of –1.74E-5 and –2.04E-5. This indicates that traditional aquaculture drainage systems serve as nitrogen sinks, rather than efficient nutrient purge facilitators. Notably, DOM was identified as a more influential factor in shaping the in-stream transfer process in aquaculture drainage systems, with an interpretation rate 40.79% higher than that of the physiochemical properties. Consequently, it is necessary to eliminate the obstacles posed by DOM to pollutant absorption and net zero emissions in aquaculture drainage systems in the future. Environmental implicationsNutrients, heavy metals, and dissolved organic matter are hazardous pollutants originating from high-density aquaculture. As the sole conduit to natural waters, aquaculture drainage systems have pivotal functions in receiving and purifying wastewater, in which the in-stream transfer process is affected by ambient conditions. This field study investigated the spatial variations, stage distinctions, effects of physicochemical properties, and dissolved organic matter (DOM) features. This finding suggests that the aquaculture drainage system as a nitrogen sink and DOM source. While the DOM is the key factor in shaping the in-stream transfer process, and obstacles for pollutant elimination. This study helps in understanding the fate of aquaculture pollutants and reveals the drawbacks of traditional aquaculture drainage systems.

Dynamics of radioactive contamination of lingonberry (Vaccinium vitis-idaea L.) in the forests of Zhytomyr Polissia (Ukraine) since the Chernobyl Nuclear Accident

The article presents results of study of levels of radioactive soil contamination and 137Cs activity concentration in the aboveground part of lingonberry (Vaccinium vitis-idaea L.) phytomass and berries on permanent experimental plots in different years since the Chernobyl Nuclear Power Plant accident (forest site type conditions — moist fairly infertile pine site type (B3), soils — soddy-podzolic sandy-loam on fluvioglacial sands). The research was conducted at the Poliskyi Branch of the Ukrainian Research Institute of Forestry and Forest Melioration named after G.M. Vysotsky over the years 1998–2021, utilized methods generally accepted in forest science and radioecology were used. Results were processed using statistical analysis. To substantiate the research, an official reporting data from forestry enterprises of the Volyn, Rivne and Zhytomyr regions were used. An analytical review of the state of research on this problem revealed that at the last 10–15 years there were no scientific publications on the study of 137Cs contamination of lingonberry. It was demonstrated that there was significant reduction in the density of radioactive soil contamination and 137Cs activity concentration in the aboveground part of lingonberry phytomass and berries in the forests of the research region over the observation period. It was found that the main part of the total 137Cs activity is currently located in the upper (10 cm) layer of the mineral part of the soil — 74.7% where root systems of herbaceous and dwarf-shrub species are concentrated in forest ecosystems. It was established that a considerable content of the radionuclide is retained in the aboveground part of lingonberry phytomass even with low density of radioactive soil contamination. It was found that in 1998, at relatively low value of soil 137Cs contamination density (49±5.6 kBq∙m–2), specific activity of 137Cs in aboveground phytomass of lingonberry was 8043±511 Bq∙kg–1, while in 2021 (value of soil 137Cs contamination density 22±0.3 kBq∙m–2) — 1046±182 Bq∙kg–1. The values of radionuclide transfer coefficients in 1998 were quite significant and varied within wide range: in the air-dry aboveground phytomass from 33.4 to 164.1 m2∙kg–1∙10–3 and in the fresh berries, from 9.5 to 26.0 m2∙kg–1∙10–3. Dependencies between density of radioactive soil contamination and the content of 137Cs in the aboveground part of lingonberry phytomass and berries were calculated, which can be applied in the practice of their harvesting. It was found that lingonberry belongs to the group of plants characterized by the high content of 137Cs in the aboveground vegetative phytomass. Harvesting of lingonberry leaves for the production of medicinal products should be restricted in all areas contaminated with 137Cs. Lingonberry fresh berries can be harvested in areas with a radioactive soil contamination density up to 74 kBq∙m–2.

A novel numerical method to evaluate the heat transfer characteristics of complicated CICC structures

In this article, we propose a numerical model for calculating the external heat transfer coefficient between the bundle region and its wall for cables in conduit conductor (CICC). With the assumption of local thermal equilibrium, one macro equation describing the steady heat transfer on the cross section of the CICC can be obtained. A key parameter, the effective transverse thermal conductivity keff, which takes the contribution of both strands and flowing fluid into consideration, is introduced. To calculate the effective transverse thermal conductivity keff, we first obtain the true distribution of different phases (fluid, copper and superconducting material) on a cross section of CICC with the help of the image recognition technique. Based on this, the value of the effective transverse thermal conductivity keff can be calculated numerically. Due to the large difference among the component thermal conductivities (at 4.5 K, typical values of the thermal conductivity of liquid helium, superconducting material and copper are kHe=0.024Wm−1K−1, kNb3Sn=0.04Wm−1K−1 and kCu=708Wm−1K−1; and the maximal ratio of thermal conductivity can be as high as about 30,000), the high-performance finite analytical method (FAM) is recommended to calculate keff. After obtaining the effective transverse thermal conductivity keff of the bundle region, the heat transfer coefficient can be calculated directly by solving a simple Poisson equation under proper boundary conditions. Two case studies are performed, including the JT-60 Super Advanced (JT-60SA) CICC and the dual channel International Thermonuclear Experimental Reactor, Toroidal Field Performance Sample (ITER-TFPS). The calculated values of heat transfer coefficient are consistent with the experimental results, which verifies our proposed model.

Modelling and Thermographic Measurements of LED Optical Power.

This paper presents a simple engineering method for evaluating the optical power emitted by light-emitting diodes (LEDs) using infrared thermography. The method is based on the simultaneous measurement of the electrical power and temperature of an LED and a heat source (resistor) that are enclosed in the same plastic packaging under the same cooling conditions. This ensures the calculation of the optical power emitted by the LED regardless of the value of the heat transfer coefficient. The obtained result was confirmed by comparing it with the standard direct measurement method using an integrated sphere. The values of the estimated optical power using the proposed method and the integrated sphere equipped with a spectrometer were consistent with each other. The tested LED exhibited a high optical energy efficiency, reaching approximately η ≈ 30%. In addition, an uncertainty analysis of the obtained results was performed. Compact modelling based on a thermal resistor network (Rth) and a 3D-FEM analysis were performed to confirm the experimental results.

Towards an improved thermometric method: Convective and radiative heat transfer for heat flux measurement through an indirect approach

Actual thermal performance of building walls need to be assessed through on-site tests. Several measurement procedures have been proposed during the last decades. Nonetheless, among them the Thermometric (THM) method has been poorly explored in literature, hence opportunities for improvements can be highlighted. The THM technique is a quite a new and non-standardized procedure for wall thermal characterization, based on the well-known Newton’s law of cooling. This law needs the heat transfer coefficient knowledge and there is no agreement about the value to apply when the THM method is used. In this work, a low-cost experimental apparatus characterized by a heated wooden sample was realized. Sensors were installed to measure heat fluxes through a common heat flux plate. The obtained values were compared with the heat flows achieved via the THM method, where dimensionless groups analysis was performed for computing suitable heat transfer coefficients obtained from data processing. The aim of this work is to lay the groundwork for overcoming the disagreement related to the value of the total heat transfer coefficient in the THM method by proposing and consolidating an alternative approach. The results show that the proposed data processing for the THM method can provide satisfying results in terms of indirect heat flux measurements, showing extremely low percentage differences (ranging from + 1.16 % to + 3.56 %) if compared with the HFM method.

Method of parametric optimization of hydraulic structures for subzero temperatures

A method for optimizing design and geometric parameters using a "simplified" simulation of the water-cooling process is presented. This method allows solving the issues of operational manufacturability in the aspect of creating energy- and resource-saving constructions of water storage tanks. In the study to determine the size of underground and above-ground parts of the water storage tank the criterion of heat loss minimization is accepted and the mathematical model of water heat exchange with the ground under the condition of its homogeneity and stationarity of the thermal regime is compiled. The relationship between the height and the radius of the underground part at the minimum value of the heat transfer coefficient from water to the ground through the metal wall is established. Using the heat balance equation, an analytical expression is found that determines the aboveground fluid temperature for any instant of time. On the basis of the theory of differential calculus the condition of minimality of the cooling rate of water, which gives up heat through the side walls to the environment and air space under the roof of the vessel, was revealed, which made possible to optimize the size of its above-ground part. The relationship of the radius and the total volume of the water holding capacity was determined for the average values of ambient temperature and wind speed recorded in the region of the research. The developed algorithm of the method of "simplified" modeling and its representation in the form of a block diagram can be implemented at digitalization of calculation-analytical procedures for selection of structural and geometrical parameters of the vessel according to the criterion of minimum heat losses. The functions of the radius and the height of the underground and above-ground parts of the vessel from the argument of its total volume are graphically illustrated.

Quantitative characterization of the contribution of electrogenerated active species for NH+ 4-N oxidation on Ti/RuO2-IrO2 anode

The contribution of active species for ammonia nitrogen (NH4+-N) oxidation has received much attention in recent decades through electrocatalytic oxidation (EO) technology. Here we describe the strong NH4+-N removal capacity (more than 98 %) with NaCl as electrolyte of Ti/RuO2-IrO2 anode. Electrochemical tests were used to characterize the excellent NH4+-N degradation performance of Ti/RuO2-IrO2 anode in presence of chlorides. The values of electron transfer number (n = 1.02) and transfer coefficient (α = 0.42) show that the reaction is in the diffusion control stage, which is more conducive to the degradation of NH4+-N. Additionally, three active species (•OH, Cl•, and ClO•) formation is related to the adsorption energy (obtained by density functional theory (DFT) calculation) during the electrolysis process. The difference charge density and density of state (DOS) calculation indicate that the O − in ClO• is more likely to break and react with pollutants in water, which is consistent with the results of active species detection experiments. The combination of kinetics (Tafel) and thermodynamics (adsorption energy) shows that the variation of Tafel slope is affected by the coverage of active species on the electrode surface.

Thermal performance of three concentrating collectors with bifacial PV cells. Part II – parametrical study

One of the problems in using PV cells to extract energy from sunlight is the temperature effect on PV cells. As the solar panel is heated, the conversion efficiency of light to electrical energy is diminished. Moreover, successive temperature elevations can cause dilatations in the array of cells which may also contribute to the degradation of the receiver. Some of the operating temperature mitigation approaches may include air-flow ventilation. In this study, data obtained by experimental and numerical simulations of a collector with bifacial PV cells is compared to the expressions found in the literature for the estimation of the heat transfer coefficient. Forced ventilation was applied to the studied collector as it accounts for much better heat dissipation. A new correlation for the estimation of the heat transfer coefficient is developed for such a geometry, for inlet velocities ranging between 3 and 8 m/s. Values of heat transfer coefficient estimated in the present work have been compared with studies of other researchers.

A numerical model for calculating velocity distribution in cross-section of an open channel

Flow velocity in open channels is a fundamental hydraulic parameter with wide-ranging applications, including the development of rating curves and the study of sediment transport. While some river engineering projects may only require the calculation of average flow velocity, others, such as the design of hydraulic structures and stable channels, as well as the assessment of boundary shear stress, necessitate a more comprehensive understanding of flow dynamics, including the two-dimensional velocity distribution within open channels. To address this, various mathematical models have been proposed for estimating the two-dimensional distribution of flow velocity across transverse and depth directions. However, these models often come with complexities that hinder their practical application. In this research, we introduce a simplified numerical model that combines simplified Navier–Stokes equations with an eddy viscosity formula. This innovative approach aims to estimate velocity distribution in both rectangular and trapezoidal channels. The accuracy of our developed model hinges on the momentum transfer coefficient used in the eddy viscosity formula. Through the execution of the numerical model and the utilization of observational data, we determined the optimal value of the momentum transfer coefficient to be 0.241. To validate the effectiveness of our numerical model, we compared its predictions with laboratory data encompassing diverse hydraulic conditions. The results demonstrated a high level of accuracy, with the calculated velocity distribution and flow discharge differing by no more than 7.6% and 6.8%, respectively.

Investigations of Flow Boiling in Mini-Channels: Heat Transfer Calculations with Temperature Uncertainty Analyses

The article aims to explore boiling heat transfer in mini-channels with a rectangular cross-section using various fluids (HFE-649, HFE-7000, HFE-7100, and HFE-7200). Temperature measurements were conducted using infrared thermography for the heated wall and K-type thermocouples for the working fluid. The 2D mathematical model for heat transfer in the test section was proposed. Local heat transfer coefficients between the heated wall and the working fluid were determined from the Robin condition. The problem was solved by means of the finite element method (FEM) with Trefftz functions. The values of the heat transfer coefficient that were obtained were compared with the results calculated from Newton’s law of cooling. The average relative differences between the obtained results did not exceed 4%. The study included uncertainty analyses for temperature measurements with K- and T-type thermocouples. Expanded uncertainties were calculated using the uncertainty propagation and Monte Carlo methods. Precisely determining the uncertainties in contact temperature measurements is crucial to ensure accurate temperature data for subsequent heat transfer calculations. The results of the heat transfer investigations were compared in terms of fluid temperature, heat transfer coefficients, and boiling curves. HFE-7200 consistently exhibited the highest fluid temperature and temperature differences at boiling incipience, while HFE-7000 demonstrated the highest heat transfer coefficients. HFE-649 showed the lowest heat transfer coefficients. The boiling curves exhibited a typical shape, with a notable occurrence of ‘nucleation hysteresis phenomena’. Upon the analysis of two-phase flow patterns, bubbly and bubbly-slug structures were observed.