Values Of Transfer Coefficient Research Articles

Experimental and computational assessment into the heat transfer for the blade multicavity tips

In the gas turbine, the tip region of the shroudless blade is under severe heat load, and thus is seriously ablated. Therefore, an in-depth understanding of the tip heat transfer distributions is of great significance for tip cooling design. In the present work, the heat transfer coefficient distributions of the multicavity tips were measured by a steady-state thermal-analysis method with Infrared camera. The measurements were conducted under two inlet Reynolds number conditions (1.10 × 105, 2.23 × 105) and two tip gaps (1.6 %, 3.5 % Chord length). In addition, the computational simulations verified by the experimental data were conducted to attain the detailed flow field near the tip, and to gain insight into the heat transfer characteristics. Measured results show that additional hot spots are detected behind the rib for the multicavity tips with three or four cavities owing to the leakage flow reattachment. The heat transfer at the suction-side corner is also enhanced by the multicavity tips, whereas a low heat transfer coefficient region is found in the corner in front of the rib. Increasing the inlet Reynolds number would not change the heat transfer patterns, but would evidently increase the heat transfer coefficient values. Adopting a larger tip gap also increases the heat transfer coefficient values, but the effect is not so obviously as the inlet Reynolds number. Compared with the conventional one-cavity tip, the multicavity tips can reduce the aerodynamic losses, and the loss values are reduced by about 2 %.

Membrane assisted process intensification and optimization for removal and recovery of phenol from industrial effluents

The best method of preserving the environment is not to discharge any contaminant but to recover them as value from the effluent streams thereby adopting circular economy. This calls for developing new strategies including intensification of conventional processes to make the recovery of contaminants cost effective and environmentally friendly. In this context, experimental studies are reported with respect to conventional solvent extraction and membrane assisted solvent extraction for recovering phenol from effluent streams. Initial studies are carried out with aqueous solutions of phenol with concentrations up to 500 mg/L using hydrophobic polyvinylidene fluoride based hollow fiber membrane element in tandem with conventional solvent extraction using 1- hexanol as solvent. The operating parameters for the membrane assisted solvent extraction were optimized in parallel with conventional solvent extraction using the statistical tool, response surface methodology to provide a comparative assessment highlighting the advantages of membrane assisted solvent extraction in terms of energy consumption, operational simplicity and environmental friendliness. The studies were validated using real effluents obtained from a formaldehyde resin manufacturer. The comparative study highlights the specific advantages and demonstrates the superiority of membrane assisted solvent extraction using a real industrial effluent. It was observed that more than 98 % of phenol could be removed from effluent in membrane assisted solvent extraction and recover about 90 %, continuously allowing the recycle of the solvent. Being closed loop operation, no solvent is exposed to the environment making the process environmentally friendly operation. The mechanism of mass transfer in membrane contactors follows resistance in series model and the experimental values of mass transfer coefficient were estimated to be between 1.7 × 10 −7 and 2.2 × 10 −7 m/s, whereas the empirical values were around 1.3 × 10 −7 m/s.

Flow and thermal transport of supercritical n-decane in square minichannel featuring uniformly spaced tetrakaidecahedron-shaped unit cells

In the current study, a novel cooling channel configuration featuring periodically placed tetrakaidecahedron (TKD) unit cells is proposed and studied computationally. The concept can also be realized through the additive manufacturing process to mitigate the heat transfer deterioration and improve the overall thermal performance of combustor cooling channels with n-decane as the working fuel. The conjugate thermal performance of the TKD channel was investigated and compared with the corresponding smooth channel at 3 MPa operating pressure. Simulations were conducted at a fixed coolant mass flow rate of 2 g/s, wall heat flux of 1.5 MW/m2, and an inlet temperature of 423.15 K. TKD channel provided lower overall wall temperatures and higher heat transfer coefficient values than the smooth channel. Furthermore, the thermal performance of both the smooth and TKD channels were analyzed under accelerated flight conditions by employing accelerations ac=2 g, 4 g, and 6 g in the streamwise direction. For the smooth channel, with increase in the acceleration values, the peak temperature values were reduced, and the location of peak value shifted towards the channel outlet. The thermal performance of TKD channel was, however, not significantly influenced by the accelerated states, which ensures enhanced cooling performance at different operating conditions.

Conjugate Radiation and Convection Heat Transfer Analysis in Solar Cooker Cavity Using a Computational Approach

The heat loss caused by radiation and persistently laminar natural convection in a solar cooker cavity that has a rectangular cavity or a trapezoidal cavity are computationally explored in this paper. The hot bottom and the adiabatic side wall are both taken into account. Two possibilities are considered for the top wall: first, a cold wall, and, second, losses from wind-induced convection and radiation. The parameters of heat loss in various depth cavities have been investigated along with a variety of external heat transfer coefficient values above the glass surface were simulated. The emissivity of the bottom surface, the absolute temperature ratio, on heat loss from the considered geometries, are also calculated. Analysis of the cavity’s flow pattern and isotherms at different depths has been conducted, and it is discovered that the total rate of heat transfer from the top wall increases as the bottom wall’s emissivity, the absolute temperature ratio, the Rayleigh number, and the external Nusselt number all increase. While radiation heat transfer increases monotonically, convective heat transfer rates shift slightly as these values rise at different emissivities of the bottom, and the opposite occurs when Ra increases at the same emissivity. Furthermore, it has been discovered that as the aspect ratio of the cavity increases, the overall Nusselt number decreases. A trapezoidal cavity has a faster rate of heat transfer than a rectangular cavity for the same parameters. Generic empirical correlations were developed for the total average Nusselt number concerning all influencing parameters.

Preliminary Results of Heat Transfer and Pressure Drop Measurements on Al2O3/H2O Nanofluids through a Lattice Channel

A nanofluid is composed of a base fluid with a suspension of nanoparticles that improve the base fluid’s thermophysical properties. In this work, the authors have conducted experimental tests on an alumina-based nanofluid (Al2O3/H2O) moving inside a 3D-printed lattice channel. The unit cell’s lattice shape can be considered a double X or a double pyramidal truss with a common vertex. The test channel is 80 mm long and has a cross-sectional area, without an internal lattice with that has the dimensions H × W, with H = 5 mm and W = 15 mm. A nanofluid and a lattice duct can represent a good compound technique for enhancing heat transfer. The channel is heated by an electrical resistance wound onto its outer surface. The heat transfer rate absorbed by the nanofluid, the convective heat transfer coefficients, and the pressure drops are evaluated. The experimental tests are carried out at various volumetric contents of nanoparticles (φ = 1.00%, φ = 1.50% and φ = 2.05%) and at various volumetric flow rates (from 0.2 L/min to 2 L/min). The preliminary results show that in the range between 0.5 L/min ÷ 2.0 L/min, the values of convective heat transfer coefficients are greater than those of pure water (φ = 0) for all concentrations of Al2O3; thus, the nanofluid absorbed a higher thermal power than the water, with an average increase of 6%, 9%, and 14% for 1.00%, 1.50% and 2.05% volume concentrations, respectively. The pressure drops are not very different from those of water; therefore, the use of nanofluids also increased the cooling efficiency of the system.

CFD Modeling of Heat Exchanger with Small Bent Radius Coils Using Porous Media Model

The efficiency of heat transfer in air-cooled heat exchangers of various industrial facilities depends on the flow rate of the coolant, its inlet temperature and ambient temperature. These parameters are transient and depend both on the features of the technological process and on weather conditions. One option for a compact design of heat exchangers is the use of close-packed coils with a small bending radius. In this case, heat transfer in the complex geometry of the annular space cannot be described by simple one-dimensional dependencies. To solve this problem, it is necessary to consider the three-dimensional spatial structure of the heat exchange surface. Since the size of the grid elements will be several orders of magnitude less than the size of the facility, the size of the computational grids for CFD modeling full-scale heat exchangers will be billions of finite volumes, and even on powerful supercomputers, the solution time will be about a month. One way to reduce computational costs is to use reduced order models, in which the computational domain is not modeled directly; instead, simplified models, such as a porous medium model, are used to describe it. However, such models require additional closing relations and coefficients that characterize the actual channel geometry. This paper presents a technique for creating a digital twin of a heat exchanger with small bend radius coils based on a porous medium model. The values of heat transfer coefficients and hydraulic resistance depend on the speed of air movement in the space between the coils. The calculated value of the thermal power obtained using the strengthened model was 529 kW, which corresponds to the passport data of 500 kW, with less than 6% deviation for the heat exchanger under study. This confirms the correctness of the calculation with accepted simplifications. The calculation time in this case was only a few minutes when using a personal computer. The developed numerical model allows for the resolution of performance characteristics based on the temperature of the cooled medium at the inlet, air temperature, and fan speed. Analyzing the different modes of turning on the cooling fans made it possible to determine the values of the thermal power when turning off the fans or reducing the number of revolutions.

Predictive modeling for the boiling heat transfer coefficient of R1234yf inside a multiport minichannel tube

Existing prediction models of flow boiling heat transfer coefficient, such as the well-known superposition, asymptotic, and flow pattern models, provide an applicable method to attain the closest to the true value of the heat transfer coefficient in specific ranges. In this study, heat transfer coefficient data are collected through an experimental study of R1234yf inside a multiport minichannel tube within a mass flux of 50–500 kg/m2s, heat flux of 3–12 kW/m2, saturation temperature of 6 °C, and vapor quality up to 1. The assessment of the heat transfer coefficient is conducted by comparing the heat transfer coefficient of each model with that of R1234yf. In addition, a machine-learning prediction model is proposed to improve the prediction accuracy of the heat transfer coefficient. A machine-learning method could provide an accurate prediction result for the heat transfer coefficient by feeding the program with a factor from heat transfer coefficient data (e.g., a dimensionless number). Therefore, an alternative prediction method could be applied to predict the heat transfer coefficient with the lowest error by providing the setting parameter that fits the pattern of heat transfer coefficient data. In addition, a heat transfer coefficient correlation is proposed to define the only-value result of the machine-learning model.

Computationally efficient methodology of 3D thermal-hydraulic analysis of Diagnostic Shielding Module #2 of ITER Equatorial Port #11

The process of design justification of Diagnostic Shielding Modules (DSMs) of port plugs of the International Thermonuclear Experimental Reactor (ITER) includes a stage of thermal-hydraulic computational analysis. The ITER tokamak operates in a pulsation mode that means the nuclear heating affecting DSMs is unsteady, and transient analysis is required. Three-dimensional (3D) approaches for fully coupled thermal-hydraulic analysis, which imply simultaneous computations for fluid and solid parts of a 3D DSM model, lead typically to “heavy” numerical models being very expensive from the computational point of view, especially for transient simulations. In order to significantly reduce the consumption of computational resources, the present 3D thermal-hydraulic analysis of DSM #2 of ITER Equatorial Port #11 was performed using one-way coupling. Firstly, steady-state CFD computations of turbulent flow were performed for the front part of DSM cooling system, which is a high-density network of channels of complex geometry. Obtained in CFD analysis map of wall shear stress on surface of channels was then converted into a map of local heat transfer coefficient using a simple and conservative approach based on known empirical correlations for turbulent flow in a circular pipe. For the back part of DSM cooling system, which basically consist of many relatively long channels, an individual (single) value of heat transfer coefficient for each of the channels was directly evaluated using empirical correlation. The heat transfer coefficient maps obtained for both parts of DSM cooling system were used finally as a boundary condition in transient thermal analysis of the solid part of the DSM model. Details and justifications of the described computationally-efficient methodology are presented.

Heat Transfer Characteristics of Pool Boiling with Scalable Plasma-Sprayed Aluminum Coatings

To ensure adequate reliability in two-phase cooling systems involving boiling, it is essential to enhance the heat transfer coefficient and maximize the critical heat flux (CHF) limit. A key technique to avoid surface burnout and increase the CHF limit in pool boiling is the frequent coolant supply to the probable dry-out locations. In the present work, we have explored the plasma-spray coating as a surface modification technique for enhancing heat transfer coefficient and CHF value in pool boiling applications. Three plasma-coated aluminum surfaces (C-15, C-20, and C-25) are fabricated on a copper substrate at three different plasma powers of 15, 20, and 25 kW, respectively. Detailed surface morphologies of the plasma-sprayed coatings are presented, and their roles in pool boiling heat transfer mechanisms are analyzed. Plasma-coated surfaces exhibit wickability characteristics and enhanced wettability compared to the plain copper surface. Saturated pool boiling experiments are performed with DI (deionized) water at atmospheric pressure. Plasma spray-coated surfaces show favorable boiling incipience with less wall superheat and more active nucleation sites than the plain copper surface. Compared to the plain copper surface, enhancement values of nearly 68, 60.7, and 55.5% in the heat transfer coefficient are observed for C-15, C-20, and C-25 plasma-coated surfaces, respectively. Experiments could not be performed beyond the heat flux of 197 W/cm2 due to repeated failure of the cartridge heaters. Based on the experimental measurement of wickabilities, the CHF values of plasma-coated surfaces have been theoretically calculated. Compared to the plain copper surface, a maximum 2.39 times higher CHF value is observed for C-15 plasma-coated surface. Improved wettability and wickability are responsible for CHF enhancement in the case of plasma-coated surfaces. At higher heat flux, capillary wicking and frequent rewetting of the dryout locations delay the burnout phenomenon, enhancing CHF in plasma-coated surfaces. The plasma-spray coating is a robust and scalable process, which can be a potential candidate for high heat flux dissipation in various industrial applications.

The effects of topological configuration and geometric parameters on heat transfer and fluid flow characteristics of lattice-based heat sinks

Traditional pin-fin heat sinks could provide only thermal exchange. Whereas the topology and geometry of lattice-based heat sinks make them suitable for simultaneous thermal and structural applications. In this work, we numerically demonstrate the convective heat dissipation performance of periodic pyramidal and tetrahedral lattice-based heat sinks. The motivation for the study is to discover the effects of topological and geometrical parameters of the lattice structure on the heat and fluid flow characteristics of lattice-based heat sinks which are not yet presented in the literature. Hence, we consider pyramidal and tetrahedral lattice topologies with two different L/D ratios (8.8 and 5). Three-dimensional numerical simulations based on the finite volume method were carried out at constant pumping power with water as a coolant for applied heat flux in the range of 10–50 W/cm2. The thermo-hydraulic performance is characterized by the efficiency index and area goodness ratio as a function of pumping power in the laminar regime. The convective heat transfer coefficient values range from 2129 to 6510 W/m2 K for the operating conditions considered in this study. Also, in terms of overall combined thermo-hydraulic performance, the tetrahedral heat sink with the L/D ratio of 5 was found to be 50–53%, 18–25%, and 22–30% more efficient than the P8.8, T8.8, and P5 heat sinks, respectively, for the range of pumping powers considered. The thermo-hydraulic characteristics presented in this study enable the design of lattice-based heat sinks for specific applications with collective pressure drop and heat transfer constraints.

ДОСЛІДЖЕННЯ ПОКАЗНИКІВ РОБОТИ ЦИКЛОНА-УТИЛІЗАТОРА З ВИКОРИСТАННЯМ CFD-ПАКЕТА SOLIDWORKS FLOW SIMULATION

It is shown that the use of solid biomass as a renewable energy source is relevant for the production of thermal energy and electrical energy. but the burning of biomass is accompanied by the release of a significant amount of ash into the environment. The need to organize the primary cleaning of waste gases of heat generators in cyclone dust collectors has been established. The stages of solving problems of aerodynamics and heat transfer in the SolidWorks Flow Simulation CFD package are described. The nature of the distribution of pressures, velocities of gas and solid particles in cyclone filters, the values of heat transfer coefficients in the flow area, based on which the studies of cyclone efficiency, hydraulic resistance, power of the heat exchanger-utilizer and the temperature of heated water were performed.It was established that an increase in the temperature of the inlet gas stream reduces the efficiency of the cyclone in cleaning highly dispersed ash, the presence of a heat exchange surface increases the resistance of the cyclone, but improves its efficiency in capturing solid particles up to 10 μm in size. The obtained results make it possible to improve the design, reduce the metal content, and find rational modes of operation of cyclones in operating conditions

Experimental investigation on incipient boiling in narrow closed gaps with water

This research deals with characterization of the Onset of Nucleate Boiling (ONB) in an enclosed narrow gap system containing a stationary liquid. In the experimental system, subcooled water at atmospheric pressure is confined between two parallel vertical rectangular copper plates. One of the plates is heated electrically and the opposite one is cooled with an external water-cooling system, maintaining its constant temperature.The experimental gaps explored in this research are: 5 mm, 3 mm, 0.9 mm and 0.5 mm. In each gap, several experiments are performed with a different controlled cold plate temperature. Various heat transfer mechanisms are observed (convection, transition and conduction), depending on the gap size. In the convection regime, the results are in good agreement with a correlation from the literature. In the gap 0.5 mm wide, the heat transfer occurs by conduction through the water, whereas for the gap 0.9 mm wide, single-phase experiments show a “transition” heat transfer regime. The onset of nucleate boiling (ONB) is detected as the point at which the heat transfer coefficient behavior changes dramatically, and is revealed also by visualization of the experimental channel. The wall superheat and the heat flux are evaluated at ONB.An analytical model is developed, based on the [1] correlation and on the experimental values of the heat transfer coefficient. A good agreement for the ONB heat flux prediction is achieved with the model, whereas the ONB superheat is over-estimated, except for the really narrow, 0.5 mm, gap.

Thin Film Evaporation Modeling of the Liquid Microlayer Region in a Dewetting Water Bubble

Understanding the mechanism of bubble growth is crucial to modeling boiling heat transfer and enabling the development of technological applications, such as energy systems and thermal management processes, which rely on boiling to achieve the high heat fluxes required for their operation. This paper presents analyses of the evaporation of “microlayers”, i.e., ultra-thin layers of liquid present beneath steam bubbles growing at the heated surface in the atmospheric pressure nucleate of boiling water. Evaporation of the microlayer is believed to be a major contributor to the phase change heat transfer, but its evolution, spatio-temporal stability, and impact on macroscale bubble dynamics are still poorly understood. Mass, momentum, and energy transfer in the microlayer are modeled with a lubrication theory approach that accounts for capillary and intermolecular forces and interfacial mass transfer. The model is embodied in a third-order nonlinear film evolution equation, which is solved numerically. Variable wall-temperature boundary conditions are applied at the solid–liquid interface to account for conjugate heat transfer due to evaporative heat loss at the liquid–vapor interface. Predictions obtained with the current approach compare favorably with experimental measurements of microlayer evaporation. By comparing film profiles at a sequence of times into the ebullition cycle of a single bubble, likely values of evaporative heat transfer coefficients were inferred and found to fall within the range of previously reported estimates. The result suggests that the coefficients may not be a constant, as previously assumed, but instead something that varies with time during the ebullition cycle.

Method for Intensive Gas–Liquid Dispersion in a Stirred Tank

This article presents the results of hydrodynamics and mass exchange in a stirred tank upon the introduction of gas from an open gas vortex cavity into local liquid regions with reduced pressure. It establishes conditions for the intensive dispersion of gas. Velocity fields and liquid pressure behind the stirrer paddles are determined by numerical simulation in OpenFOAM. The gas content value, gas bubble diameters, and phase surface are determined experimentally. The stirrer power criterion is calculated by taking into account the gas content and power input. The experimental mass transfer data based on the absorption of atmospheric oxygen into water during the dispersion of gas from the open vortex cavity in the local liquid regions behind the rotating stirrer paddles are presented. In this case, the energy dissipation from the rotating stirrer reaches 25 W/kg, with a phase surface of 1000 m−1 and a surface mass transfer coefficient of up to 0.3·10−3 m/s. These parameters are obviously higher than the data obtained in the apparatus for mass exchange through surface vorticity. The advantage of the given method for gas dispersion in a liquid is the functional stability of the apparatus regardless of how deep the stirrer is immersed in the liquid or the temperature or pressure of the gas. Apparatuses based on the intensive gas dispersion method allow for varying the mass transfer coefficient and gas content across a broad range of values. This allows establishing a dependency between the experimentally obtained mass transfer coefficient, energy dissipation, and phase surface values. An equation for calculating the mass transfer coefficient is formulated by taking into account the geometric parameters of the stirrer apparatus based on the stirring power and phase surface values.

Effect of Fruit Ripening Level and Roasting Temperature on Robusta Coffee Bean Quality

The purpose of this study is to determine the effect of roasting temperature on the chemical content of Robusta coffee at various levels of Robusta coffee maturity. The research will be conducted at roasting temperatures of 190℃, 200℃, 210℃, 220℃ and chemical content analysis is carried out bygravimetric method and UV-VIS spectrophotometry on fresh beans, green beans and coffee beans at each temperature variation and the maturity level of Robusta coffee. Maturity of coffee beans will be classified visually and use an RGB meter to help and facilitate the selection of coffee beans in accordance with the level of maturity. The variables taken and observed in the study are the temperature of the roasting process, the time required at the time of roasting, as well as the mass, chemical content and physical parameters of robusta coffee beans observed before and after the roasting process. Chemical content was observed in the form of water content, fat content, caffeine, and antioxidants, then the physical parameters observed were aroma, color, shape, taste, and characteristics of coffee beans at each level of robusta coffee fruit maturity. The results obtained value of the chemical content and organoleptic test at each level of maturity of the coffee fruit and roasting temperature variations because basically the roasting temperature will produce coffee beans that have their own characteristics and flavors. This study also determines the value of the mass transfer coefficient and heat transfer in the drying process by the roasting method.

Методика расчета витых теплообменников, применяемых для термической обработки инструментов горного оборудования

Introduction. In modern conditions (depletion of deposits, complication of development conditions, the application of sanctions on the import of equipment, etc.), a special role is played by increasing the resource of equipment used in the mining industry, including processing drilling impact-rotary tools. One of these methods is cryogenic processing of machining tools, as a result of which the hardness and wear resistance of the cutting tool increase several times. For the production of cryogenic substances such as nitrogen, oxygen, methane, etc. in the liquid state, a method is proposed based on the use of adsorption-cryogenic technology implemented through the use of heat exchangers, the development of calculation methods of which is the subject of this article. The purpose of the research is to develop a methodology for calculating heat exchangers used as cold sources for cryogenic surface treatment of mining equipment tools in order to harden it. Research methods. The analysis of the current state of development of technical and technological solutions in the field of development and construction of heat exchangers was carried out through the use of such informal methods as: expert, morphological and factographic. To optimize the modeling calculations of the a priori selected configuration of the heat exchange surface, the method of uniform (blind) search was used. Research results and discussion: 1. In the course of the conducted research, the results of the analysis of the current state of development of technical and technological solutions in the field of development and construction of heat exchangers were obtained. 2. A method for calculating twisted heat exchangers has been developed. 3. It is determined that the presented method of calculating heat exchangers used as cold sources for cryogenic surface treatment of mining equipment tools in order to harden them allows optimization of modeling calculations a priori of the selected configuration of the heat exchange surface. 4. It is established that: heat exchangers with finned pipes should be used when the ratio of the heat transfer coefficient of the flow inside the pipes to the corresponding coefficient in the inter–tube space is 3 ...5 times greater. 5. The use of finned pipes in heat exchangers makes it possible to increase the area of the outer surface of the pipes due to finning, thereby improving the thermal characteristics of heat exchangers, while reducing its weight and size characteristics. 6. It has been revealed that heat exchangers with dense winding are less efficient compared to devices with sparse winding, and heat exchangers with smooth pipes should be used in cases of proximity of the values of the heat transfer coefficient of flows in the tube and inter-tube space. Resume: 1. The analysis of the current state of development of technical and technological solutions in the field of development and construction of heat exchangers was carried out. 2. A method has been developed for calculating heat exchangers used as cold sources for cryogenic surface treatment of mining equipment tools in order to harden them. 3. The developed method of calculating heat exchangers can be used to design heat exchangers consisting of pipes with different surface geometries that are wound onto the core in a spiral, used both in the extractive industries and in the engineering, chemical, oil and gas, rocket and space industries. The further development of the research topic is the optimization of the operating parameters of the heat exchanger according to the criterion of energy efficiency.

Performance Evaluation of a Fin and Tube Heat Exchanger Based on Different Shapes of the Winglets

Abstract The present work investigates numerically the performance of a fin- and tube-type heat exchanger using the finite volume method. The effect of different winglet geometries, namely, straight profile, concave profile, convex profile, and their combinations are extensively examined under turbulent flow conditions to evaluate the pressure drop and heat transfer performance. These winglets are also tested for smaller leading-edge and larger trailing edge, and vice versa—it has been observed that the former winglet configuration outperforms the latter ones. The convex profiled winglets yield the highest heat transfer performance as well as pressure drops, whereas the winglet with a concave profile has the lowest heat transfer coefficient and pressure drop values. The enhancement factor—defined as the ratio of enhancement in heat transfer to the enhancement in pressure drop penalty—has also been calculated for all models. Conclusive results indicate that the convex profile and the concave–convex (a hybrid winglet) configuration, with a smaller leading edge, deliver the highest enhancement factor compared to other profiles. Following this, the study is further elaborated to find the optimum height for the convex winglet profile. General correlations have also been developed to estimate the Colburn factor, friction factor, and enhancement factor for variations in the leading edge of the convex profile.

The Representative Thermal Conductivity of Wickless Heat Pipes in a Thermoelectric Generator Module Design via Computational Numerical Analysis

Thermoelectric generator (TEG) cells are capable of producing electricity from the flow of heat through semiconductor materials via the Seebeck effect. A TEG module (TEM) was developed through the integration of TEG cells, heat pipes, and heat sinks with the function to recover low-temperature waste heat for combined heat and power outputs. The mechanics of heat transport through the components have a distinctive impact on overall performance. Wickless heat pipes were used in the design, and it is difficult to directly evaluate the convection heat transfer coefficient within the heat pipes. Computational numerical analysis usually simplifies the heat pipe by modelling it as a solid copper pipe, where the thermal conductivity of the pipe replaces the actual value of heat transfer coefficient. To determine the representative thermal conductivity of the wickless heat pipe, a computational numerical model of the TEM was developed. Experiment data of the TEM operation, where the cold-side air inlet temperature inlet was at 20.7°C while the hot-side waste heat inlet temperature was at 70.0°C, was used to validate the computational model. The representative thermal conductivity value for the wickless heat pipe was found to be 500 W/m.K, at which the percentage difference for the hot-side and cold-side TEG surface temperatures, and the TEG surface temperature differences were below 5%. The approach has been proven suitable to analyse the representative heat pipe thermal conductivity for simplified thermoelectric generator modules.

Preparation of HfNbTiTaZr Thin Films by Ionized Jet Deposition Method

The ionized jet deposition (IJD) method is applied to the preparation of thin films composed of refractory HfNbTiTaZr high-entropy alloy (HEA). Due to its stoichiometric reliability, the IJD method provides a flexible tool for deposition of complex multi-element materials, such as HEAs. Scanning electron microscopy, energy-dispersion spectroscopy, confocal microscopy, and X-ray diffraction methods are used to characterize the influence of the applied accelerating voltage of the IJD deposition head ranging from 16 to 22 kV on the resulting morphology, chemical composition, thickness, crystalline structure, and phase composition of the layers prepared as 10 mm-wide strips on a single stainless-steel substrate. With a low accelerating voltage applied, the best surface homogeneity is obtained. Transfer coefficient values characterizing the elemental transport between the bulk target and the grown layer are evaluated for each constituting element and applied voltage. With the IJD accelerating voltage approaching 22 kV, the coefficients converge upon the values proportional to the atomic number of the element. Such voltage dependence of the IJD elemental transport might be used as a suitable tool for fine-tuning the elemental composition of layers grown from a single deposition target.

Calculation of ablation instabilities during MCCI with the TIM model

The Transient Interface Model (TIM) (Seiler and Combeau, 2014; Seiler and Jamet, 2019) dedicated to Molten Core-Concrete Interaction simulation is used to investigate ablation instabilities. The ablation instability leads to concrete ablation in preferential directions, potentially jeopardizing the mitigation criterion based on residual concrete thickness. The ablation instability occurs when all the corium/concrete interfaces do not undergo the same ablation regime. These regimes are specific of the interface temperature; in the homogeneous Low Interface Temperature (LIT) regime, the interface temperature is close to the liquidus temperature of concrete matrix, whereas in the homogeneous High Interface Temperature (HIT) regime, the interface as well as the pool temperatures are close to the pool liquidus temperature. When these different ablation regimes are present simultaneously (non-homogeneous situation), only the surfaces under LIT regime are ablated. It is shown that ablation instabilities are favored at low power dissipation and at small test scale. Calculations suggest that the limit between HIT and LIT dominant regimes can be represented in the pseudo-binary phase diagram. The surfaces initially under HIT regime (not ablated) may return to the LIT regime (with ablation) after a time delay which depends on the extension of the surface initially under HIT regime, on the value of the mass transfer coefficient and, to some extent, on the melt to interface heat transfer. Finally, the calculation at reactor scale with siliceous concrete indicates that an ablation instability may occur in the initial stage of the MCCI transient. Such ablation instability does not induce significant preferential ablation in the reactor configuration if it is limited to the horizontal surface of the reactor pit. If it affects the lateral walls, and due to the small thickness of the corium layer, a significant ablation depth could be reached (∼1 m in a reactor pit of 6 m inner diameter).