Nu Values Research Articles

Multireference calculations on bond dissociation and biradical polycyclic aromatic hydrocarbons as guidance for fractional occupation number weighted density analysis in DFT calculations

This study explores open-shell biradical and polyradical molecular compounds based on extended multireference (MR) methods (MR-configuration interaction with singles and doubles (CISD) and MR-averaged quadratic coupled cluster (AQCC) approach) using the numbers of unpaired densities NU. These results were used to guide the analysis of the fractional occupation number weighted density (FOD) calculated within the finite temperature (FT) density functional theory (DFT) approach. As critical test examples, the dissociation of carbon–carbon (CC) single, double and triple bonds and a benchmark set of polycyclic aromatic hydrocarbons (PAHs) have been chosen. By examining single, double, and triple bond dissociations, we demonstrate the utility and accuracy but also limitations of the FOD analysis for describing these dissociation processes. In significant extension of previous work (Phys Chem Chem Phys 25: 27380–27393), the assessment of FOD applications for different classes of DFT functionals was performed examining the range-separated functionals ωB97XD, ωB97M-V, CAM-B3LYP, LC-ωPBE, and MN12-SX, the hybrid (M06-2X) functional and the double hybrid (B2P-LYP) functional. In all cases, strong correlations between NFOD and NU values are found. The major task was to develop a new linear regression formula for range-separated functionals allowing a convenient determination of the optimal electronic temperature Tel for the FT-DFT calculation. We also established an optimal temperature for the semiempirical extended tight-binding GFN2-xTB method. These findings significantly broaden the applicability of FOD analysis across various DFT functionals and semiempirical methods.

Magnetohydrodynamic double diffusion natural convection of power-law Non-Newtonian Nano-Encapsulated phase change materials in a trapezoidal enclosure

PurposeThe present numerical investigation examines the magnetohydrodynamic (MHD) double diffusion natural convection of power-law non-Newtonian nano-encapsulated phase change materials (NEPCMs) in a trapezoidal cavity.Design/methodology/approachThe governing Navier-Stokes, energy and concentration equations based on the Cartesian curvilinear coordinates are solved using the collocated grid arrangement’s finite volume method. The in-house FORTRAN code is validated with the different benchmark problems. The NEPCM nanoparticles consist of a core-shell structure with Phase Change Material (PCM) at the core. The enclosure, shaped as a trapezoidal hollow, features a warmed (Th) left wall and a cold (Tc) right wall. Various parameters are considered, including the power law index (0.6 ≤ n ≤ 1.4), Hartmann number (0 ≤ Ha ≤ 30), Rayleigh number (104 ≤ Ra ≤ 105) and fixed variables such as buoyancy ratio (Br = 0.8), Prandtl number (Pr = 6.2), Lewis number (Le = 5), fusion temperature (Θf = 0.5) and volume fraction (ϕ = 0.04).FindingsThe findings indicate a decrease in local Nusselt (Nu) and Sherwood (Sh) numbers with increasing Hartmann numbers (Ha). Additionally, for a shear-thinning fluid (n = 0.6) results in the maximum local Nu and Sh values. As the Rayleigh number (Ra) increases from 104 to 105, the structured vortex in the streamline pattern is disturbed. Furthermore, for different Ra values, an increase in n from 0.6 to 1.4 leads to a 67.43% to 76.88% decrease in average Nu and a 70% to 77% decrease in average Sh.Research limitations/implicationsThis research is for two-dimensioal laminar flow only.Practical implicationsPCMs represent a class of practical substances that behave as a function of temperature and have the innate ability to absorb, release and store heated energy in the form of hidden fusion enthalpy, or heat. They are valuable in these systems as they can store significant energy at a relatively constant temperature through their latent heat phase change.Originality/valueAs per the literature review and the authors’ understanding, an examination has never been conducted on MHD double diffusion natural convection of power-law non-Newtonian NEPCMs within a trapezoidal enclosure. The current work is innovative since it combines NEPCMs with the effect of magnetic field Double diffusion Natural Convection of power-law non-Newtonian NEPCMs in a Trapezoidal enclosure. This outcome can be used to improve thermal management in energy storage systems, increasing safety and effectiveness.

A dimensionless heat transfer coefficient for free convection that is more appropriate than Nusselt number

Heat transfer in flows created by buoyancy, or natural convection, is a widely studied topic across various disciplines spanning natural flows as well as those with engineering applications. The convective heat transfer rate on a surface is commonly represented by the Nusselt number (Nu), a ratio of convective to diffusive transport, expressed often as RanPrm, where Ra is the Rayleigh number, the buoyancy forcing parameter, and Pr the Prandtl number. Motivated by the observation that n∼1/3 for turbulent convection, which implies the heat flux is independent of the length scale (L, characteristic length related to the geometry), we propose an alternate and physically more meaningful non-dimensional heat transfer parameter, denoted by Cq. Cq is derived using only the near wall variables and does not contain L. For n=1/3, Cq is constant. Even for laminar convection, where n∼1/4, Cq∼Ra−1/12, a weak function of Ra. We show that for natural convection over several geometries and a wide range of Ra, the Cq values within a narrow range while the corresponding Nu values span several orders of magnitude. We also show that Cq is akin to the non-dimensional representation of wall shear stress, skin friction coefficient Cf. We believe that just like Cf, Cq will be an equally useful non-dimensional measure of heat transfer in natural convection flows.

Computational Fluid Dynamics Analysis of Slip Flow and Heat Transfer at the Entrance Region of a Circular Pipe

In the era of sustainable development goals (SDGs), energy efficient heat transfer systems are a must. Convective heat transfer within circular pipes is an important field of research on a rarely addressed limitation of fluid flows. Vacuum solar tubes is one of many applications that could benefit from the existence of nanoparticles, Al2O3, for example, to enhance the heating of air or water steam. The current research investigates the impacts of the Reynolds number (Re), Prandtl number (Pr), Knudsen number (Kn), aspect ratio (x/Dh), and volume fraction of Al2O3 nanoparticles (ϕ) on the Nusselt number (Nu) under constant wall heat flux conditions. An axisymmetric computational fluid dynamics (CFD) analysis of the nanofluid flowing at the entrance region of a circular pipe was conducted under a slip flow at steady-state developing laminar conditions using the Ansys-Fluent 2018 software package. A mesh sensitivity analysis was conducted, and a proper number of mesh elements was selected. The results showed that an increasing Re and/or ϕ would result in an increasing Nu. The dependance of Nu on Kn was strong due to the high slip values and temperature jump. An increasing x/Dh ratio resulted in reduced Nu values. The major impact was due to Kn, which caused a reduction of up to 40% in the Nu value due to slip conditions. However, there was an enhancement of 2.5% in the heat transfer due to the addition of nanoparticles, which was found at Re = 250, Kn = 0.1, and ϕ = 0.1 (Pr = 0.729). Finally, Nuavg, Nux, U/Um, and ReCf were corelated with Kn, Pr, Re, and x/Dh with proper coefficient of determination (R2) values.

Correlations for the prediction of natural convection heat transfer of hollow hybrid fin arrays

A hollow hybrid fin (HHF) array is a staggered array of hollow pin fins concatenated with radially-extruded plate fins. Preceding research for a decade shows that natural convection HHF arrays could yield up to 30 % superior mass-based thermal performance and less orientation dependence compared to natural convection pin fin arrays. Hence, the HHF array could be a potential alternative to conventional pin fin arrays for lightweight high-performance thermal management of microelectronics in natural convection. To demonstrate potential and limitation of the natural convection HHF array, optimization is inevitable, and thus correlations for the prediction of natural convection heat transfer around the HHF arrays should be developed to provide design guidelines for the optimization. Nu correlations are developed for HHFs, HHF arrays, and oriented HHF arrays. Due to broad ranges of physical conditions, 168 cases for the HHF, 48 cases for the vertical HHF array, and 24 cases for the oriented HHF arrays are calculated and validated by the measurement and the literature data. Utilizing CFD thermal data, Nu correlations are formulated. Discrepancies between correlation-predicted and CFD-evaluated Nu values are examined. The results show that average discrepancies are 2.9 % for the vertical HHFs, 4.8 % for the vertical HHF arrays, and 5.5 % for oriented HHF arrays. These reasonable discrepancy values demonstrate the correlation validity. Validated Nu correlations are used to analyze parametric effects on convection heat transfer coefficients, h. It is seen that h increases asymptotically with the increase of fin spacing, S, mainly due to the porosity increase. The parametric study finds that h declines till a certain internal diameter, Di. They are 2 mm for external diameter, Do, of 4 mm, 4 mm for Do of 7 mm, and 6 mm for Do of 10 mm. The parametric study shows that after such Di, h consistently increases until the widest Di. The parametric study for the orientation shows that for the sideward orientation, h increases with the increase of porosity. The result also shows that h of the downward case is greater than that of the sideward case, and the difference is greater at lower Ra values.

Thermal management of lithium-ion batteries with direct and counter flow channels: A comparative study of different cooling fluids

The study addresses the crucial issue of thermal management in lithium-ion batteries (LIBs) and the maintenance of safe operational temperatures. It emphasizes the importance of efficient cooling mechanisms in preventing damage. A comparative analysis was conducted to assess the impact of various cooling fluids on the thermal performance of LIBs using direct and counter fluid flow channels. The study involved the use of different cooling fluids including water, water-glycol, Novec7000, mineral oil, and silicon oil. The researchers examined temperature variation, surface Stanton number (SN), surface Nusselt number (Nu), surface heat transfer coefficient (HTC), battery surface temperature, and pressure drop to evaluate the battery cooling process with different cooling configurations. The battery cooling part proved highly effective in managing heat, achieving a significant maximum temperature reduction of 30.70 % and 37.43 % for direct and counter flow channels, respectively. The Nu values of various fluids showed notable differences, with water-glycol showing a 3.76 % increase, Novec 7000 with an 11.39 % increase, mineral oil with a 15.46 % improvement, and silicon oil with a 20.09 % increase. The HTC values in the direct flow channel proved a consistent decrease from 16.09 to 57.73 to 11.07–25.69 W/m2K, respectively. Similarly, the HTC values for the counter flow channel ranged from 15.88 to 55.38 to 11.07–25.69 W/m2K. Additionally, it was seen that silicon oil outperformed other cooling fluids with a 6.0 % improvement in HTC in both direct flow and counter flow channels. The SN showed increases of 17.34 %, 20.11 %, 26.76 %, 29.57 %, and 29.60 %, respectively, at all five liquid flow velocities through the direct channel, and 19.39 %, 22.18 %, 28.34 %, 36.58 %, and 37.64 %, respectively, through the counter flow channel. These findings clearly show the superior heat transfer capabilities of silicon oil compared to other coolants. However, it is important to note that the use of silicon oil also resulted in a lower pressure drop, a critical consideration for ensuring efficient cooling. These findings have significant implications for the advancement of battery thermal management systems for electric vehicles, as designers can optimize the cooling process by considering different cooling configurations to enhance the thermal performance of battery packs while ensuring safe and stable operation.

Comparative analysis of tube designs in heat exchangers: A numerical simulation study for enhanced thermal-flow efficiency, "optimizing wavy tubes bundle geometries for enhanced heat transfer in underwater applications"

With the growing challenges of heat transfer in marine vehicles and underwater power stations, the demand for an effective tube heat exchanger has surged. Numerical simulations were conducted on the constant, laminar flow of an incompressible viscous fluid inside a wavy tube while keeping the wall temperature consistent. The laminar flow was modeled in two dimensions using ANSYS FLUENT 19.2. For the water fluid model, an integrated algorithm was used to link the pressure and velocity fields. Primarily examined were circular tube banks organized in a staggered arrangement. The primary equations were approached via a finite volume method based on a combined technique. This study aimed to evaluate and contrast the heat exchange and flow traits of four distinct tube designs: asteroid (ASTB), hypotrochoid (HTB), Wavy edge circle (WCTB), and mixed bundle (MBT). The influence of tube design on a heat exchanger’s thermal-flow efficiency was measured across Reynolds numbers ranging from 100 to 400. The results indicated an improved Colburn factor and a decrease in both the friction factor and tube performance for all scenarios examined. Three key metrics determined the optimal tube design: the performance evaluation criterion (PEC), the global performance criterion (GPC), and the average Nusselt number (Nu). Regardless of the Reynolds number, the conventional HC had the least efficient PEC, while the STB registered the peak PEC value. The highest GPC was attained with the STB. Nu values remained relatively consistent for ASTB, HTB, WCTB, and MBT. In evaluating the average Nusselt number, the ASTB outperformed the STB (92–106% higher), HTB (36–48% higher), WCTB (21–32% higher), and MBT (57–64% higher).

Empirical correlation to analyze performance of shell and tube heat exchanger using TiO2 Nanofluid-DI water in solar water heater

Recent studies have examined the thermal stability and reliability of TiO2 nanofluid-based heat exchangers under long-term operation in solar water heating systems. This includes assessing nanoparticle sedimentation, agglomeration, and degradation over time, as well as their impact on heat transfer performance and system reliability. In this study heat transfer and fluid flow characteristics of TiO2-DI-H2O (deionized water) nanofluid used in a shell and tube exchanger with different baffle angles under turbulent regimes were numerically explored. The proposed setup was run with nanoparticle concentration of 0 %–0.2 % and Reynolds numbers (Re) from 5000 to 15,000. Baffle angles of 10°, 20°, and 30° were chosen for turbulence analysis. It was observed that nanoparticle concentration strongly impacted the thermophysical properties. The results indicated that adding modest concentrations of TiO2 to DI-H2O significantly improves heat transfer and the Nu. At the Re of 5000 and 30° baffles, the Nu was 58; however, at the Re of 15,000, the Nu value was 88. The improvement in heat transfer of 62.85 %, 70.64 %, and 75.06 % was recorded for baffle angles of 10°, 20°, and 30° respectively, Furthermore, a notable increase in friction factor of 13.6 % at Re 15,000 was observed. However, the ideal baffle angle for maximum thermal performance is 30°. Finally, with shell temperature, it was established that 30° baffle orientation could provide the best h and Nu values and improve the overall performance of the heat exchanger. The obtained Nu was also compared with the existing correlations of literature in the turbulent regime with the aid of nanoparticles and presented for case applications.

Analysis of thermohydraulic flow and enhancement heat performance in 3D dimple tube based on varying geometrical configurations

AbstractThis research work investigates how different dimple designs affect the flow field and thermal performance of three‐dimensional pipes. The study focuses on the effect of the number of improved dimples NOD (3, 4, and 5), different groups numbers DGNs (1, 2, and 3 groups), arranged around the pipe, and different distances between dimples (DBDs). Dimple geometry affects flow: Changing dimple parameters alters the velocity and pressure distribution within the pipe. Performance evaluation factor (PEF) varies with dimple configuration: The PEF, which balances heat transfer enhancement and pressure drop penalty, ranges from 1.187 to 1.23 for NOD and from 1.292 to 1.31 for DGN, and also from 1.26 to 1.302 for DBD. Reynolds number range, Re = 4000–15,000; turbulence model, standard k–ε model; numerical scheme, second‐order upwind scheme; test tube conditions, inlet temperature (Tin) = 25°C; pipe diameter D = 23 mm; thickness = 2 mm; heat flux q = 25,500 W/m²; and material (Cu). This research focuses on improving heat transfer efficiency in pipes using dimples. Dimple size and arrangement significantly impact flow dynamics and heat transfer. PEF is used to evaluate the overall performance considering both heat transfer improvement and pressure drop penalty. The study found a specific range for PEF under various conditions for different dimple configurations. The average enhancement in Nusselt number for model 2 was 15.16% compared with a smooth pipe and the heat transfer performance by 10.028%–28.963% at the effect of NOD, the DGN has slightly higher Nu values than smooth pipes, indicating improved heat transfer due to the dimples (around 7%–58% at Re 4000–15,000 and 9%–13% at Re 12,000), and at DBD (13.5%) at a Reynolds number of 12,000 and 4.6%–59% at Re 4000–15,000.

Wacana Dinamis Nahdlatul Ulama: Analisis Kritis terhadap Perubahan Sosial

This research presents critical study of Nahdlatul Ulama’s dynamic discourse on social change. The background of this study is rooted in the significant role of NU in the history of Indonesia, particularly in the realm of cultural and religious ideologies. The research methodology employed encompasses a combination of qualitative approaches and document analysis to attain a comprehensive and profound comprehension, to understand how the NU tradition is integrated into moderate thinking and the extent to which the transformation of such thinking can act as a catalyst for societal progress. The theoretical framework of this research encompasses a profound understanding of the traditional values of NU, the concept of moderation as the cornerstone of thinking, and how thinking evolves in the face of contemporary challenges. The results of the analysis indicate that NU traditions not only persist in everyday life but are also effectively assimilated into the modern context. Moderation, as a pivotal aspect of NU thought, emerges as a crucial component in promoting interreligious dialogue and contributing to political stability. The transformation of NU's thinking is evident in its response to advancements in technology and science. Although NU thinking encounters challenges, particularly in preserving cultural heritage, this study underscores the opportunities for the development of NU thinking in the future. The conclusions of this study offer a critical evaluation of NU's impact on societal change and emphasize the significance of comprehending and permeating traditional values while remaining relevant in the face of evolving times. The implications of this research hold valuable insights for academics, practitioners, and policymakers in supporting the positive development of a society founded on the moderate values of NU.

Experimental heat transfer analysis of the effects of thermophysical properties on PCM at freezing conditions employing Particle Image Velocimetry

Cold chain logistics of fresh produce distribution are facing new challenges owing to the increasing consumer demand, often traveling long distances, and facing temperature variations during storage and distribution stages. Nowadays, solutions for improving food preservation by searching effective energy storage strategies, such as the use of PCM, are capturing more interest. Nevertheless, most studies are focused on organic PCM with ambient temperature melting points analyses. This leaves an important research gap in the experimentation of other types of eutectic materials with melting points below 0 °C, something of great interest for food preservation in the refrigeration industry. Velocity vector field and heat transfer via natural convection in a small cavity filled with two different PCM at below 0 °C conditions were experimentally investigated employing temperature measurements and Particle Image Velocimetry. In this regard, brine (18.8 wt%) and ethylene glycol-water (EGW) blend (30.5 wt%) PCM with equal melting temperature, but different thermophysical properties, were selected to compare their thermal behavior. It was investigated the effects of thermophysical properties such as density, specific heat capacity, thermal conductivity, and viscosity on melting front, heat transfer and flow patterns through the evaluation of liquid fraction, heat storage, Nu, Gr, Ra, and velocity field. The maximum Nu value for brine was almost the double of that of EGW. Pr values of EGW were in average 1.88 times higher than those of brine. Besides, Gr and Ra values of brine were in average 7.23 and 3.97 higher than those of EGW, respectively.

Comparisons on the thermal and entropic performance of helical tubes with varying cross-sections with constant wall temperature

This study investigates the thermal-hydraulic behavior of helical tubes with different cross-section types. The flow was considered to be in a turbulent flow regime, and the wall temperature of the tube was considered to be constant. Circle, horizontal oval, vertical oval, square, horizontal rectangle, vertical rectangle, equilateral triangle, and hexagons were eight cross-section types assessed in this study. Findings demonstrated that across the evaluated range of mass flow rates, cases with triangular cross-sections had the highest friction factor values. Also, it was observed that these cases have the largest and the most intense vorticity among all other cases. It was revealed that cases with sharp regions include vortices that increase the loss of kinetic energy. In circular cross-section and triangular cross-section examples, the Nu number was most varied. There is a 58% difference between maximum Nu values for circular and triangular cross-sections. The case with a circular cross-section and a mass flow rate of 2 kg/s exhibited the highest PEC factor value of approximately 1.001. Finally, it was determined that a circle cross-section is the best according to the first law of thermodynamics, but a triangular cross-section performed 52% better when compared to other cross-section forms according to the second law of thermodynamics.

Influence of inclined unconfined circular air jet impingement on local heat transfer characteristics of smooth flat plate

The influence of inclined unconfined circular air jet impingement on local heat transfer characteristics of smooth flat plate is studied experimentally. The experimental investigation was conducted with jet inclination angles of 45°, 60° and 75°, jet Reynolds number of 12000–28000 and nozzle to target surface distance of 1–6. The Nu characteristics is studied in different radial direction angles on flat plate ranging from θ = 0° to 180° in stagnation and wall jet area for different z/d, Re and jet inclination configurations. The Nu characteristics are obtained by using an IR thermal imaging and thin foil approach. The Nu characteristics of different jet inclination angles (φ = 75°, 60° and 45°) was compared with vertical jet (φ = 90°) for different z/d and Re. Asymmetric in Nu characteristics increases with decrease in jet inclination about the geometric impingement point. With jet inclination, the maximum Nu value shifts away from geometric impinging point towards the uphill area of target plate. The displacement of maximum Nu from geometric impinging point towards the uphill area is highly pronounced at lower z/d and higher Re for a given jet inclination configuration. The isothermal Nu ring is reported at r/d ∼2 from geometric point in wall jet area. Within isothermal ring, Nu increases with radial direction angles from 0° to 180° and reverse behavior is observed outside the isothermal Nu ring. Correlations of Nu are obtained at geometric point and maximum Nu point with maximum deviation of ±10 %. Moreover, correlation for average Nu is also presented with and accuracy of ±3.4 %.

Effect of particle size on second law of thermodynamics analysis of Al2O3 nanofluid: Application of XGBoost and gradient boosting regression for prognostic analysis

In this study, the current research delves into the influence of nanoparticle size on turbulent forced convective heat transfer, entropy generation, and friction factor. The investigation focused on three different sizes (30 nm, 50 nm, and 80 nm) of Al2O3 nanoparticles (NPs) suspended in a water-based nanofluid (NF) with a 1 vol% concentration flow in a circular tube. The nanoparticles (NPs) were characterized using various characterization techniques. The stability and pH of the NF were determined, and its viscosity (VST) and thermal conductivity (TC) were measured at a temperature of 60 °C. Heat transfer experiments were conducted with varying particle sizes and Reynolds number (Re), maintaining a fluid inlet temperature of 60 °C. The results indicated that the NF containing 30 nm particles exhibited higher VST and TC compared to the other samples and the base fluid. The maximum enhancement in Nu for Al2O3 (30 nm) and Al2O3 (80 nm) NFs is 60.7 and 18.5 % greater than that of base fluid, respectively. The maximum and minimum total entropy generation (Sgen,T) value of 0.499 and 0.286 observed for base fluid and Al2O3 NF (30 nm), respectively at low Re. The highest friction factor enhancement for Al2O3 NF (30 nm) exceeded by 9.4 % compared to the base fluid, and the maximum thermal performance factor observed for Al2O3 NF (30 nm) was 1.57. Finally, regression analysis was employed to establish correlations for estimating Nu and friction factor values. Prognostic models were developed using two sophisticated machine learning algorithms, XGBoost and Gradient Boosting Regression (GBR). Both models demonstrated exceptional prediction abilities, achieving over 99 % accuracy rates based on the experimental data.

Free Convection Heat Transfer in Composite Enclosures with Porous and Nanofluid Layers

This work conducts a numerical investigation of convection heat transfer within two composite enclosures. These enclosures consist of porous and nanofluidic layers, where the porous layers are saturated with the same nanofluid. The first enclosure has two porous layers of different sizes and permeabilities, while the second is separated by a single porous layer. As the porous layer thickness approaches zero, both enclosures transition to clear nanofluid enclosures. The study uses the Navier–Stokes equations to govern fluid flow in the nanofluid domain and the Brinkman–Forchheimer extended Darcy model to describe flow within the saturated porous layer. Numerical solutions are obtained using an iterative finite difference method. Key parameters studied include the porous thickness ( 0.0 ≤ S ≤ 1.0 ), the nanoparticle volume fraction ( 0.0 ≤ ϕ ≤ 0.05 ), the thermal conductivity ratio ( 0.5 ≤ R k ≤ 10 ), and the Darcy number ( 10 − 5 ≤ D a ≤ 10 − 2 ). Key findings include the observation that the highest heat transfer is achieved at the highest concentration, regardless of the porous layer configuration, permeability value, or thermal conductivity ratio. Specifically, an augmentation in values of N u ― I up to 22% is obtained as concentration is adjusted from 1% to 5%. Similarly, an augmentation in values of N u ― II up to 25% is obtained as concentration is adjusted from 1% to 5%.

Study on the performance of oil fume heat recovery heat exchanger based on micro heat pipe array

In China, the utilization of high-temperature flue gas waste heat recovery has been widely studied and developed, while the large number of low-temperature flue gas waste heat recovery has not been fully developed. Therefore, research has been conducted on the heat transfer characteristics of plate heat exchangers for low-temperature oil fume. The heat exchanger were 28–44 ℃ at the hot end and − 10 to 5 ℃ at the cold end. The outlet temperature, wind speed, etcetera were measured and recorded by changing indoor and outdoor temperature, variable flow rate and variable fin size. The impact of different working conditions on heat transfer and heat transfer efficiency of the heat exchanger was analyzed. The result showed that within the range of effective experimental test temperature, the variation trend of Nu and h was the same for heat exchangers with different finned fins and heat pipes with a different liquid-filled working medium The Nu value increased linearly with the increase of the Reynolds coefficient (Re), and the heat transfer coefficient presented a quadratic curve and increased with the increase of Re. Smaller fins had a higher efficiency and higher heat transfer. The heat exchange efficiency was stable between 0.75 and 0.97, which was higher than the value required by the national standard. At the same time, the heat exchanger also met the demand for high-efficiency heat recovery at low outdoor temperature, which has a wide application prospect.

Irreversibilities and heat transfer in magnetohydrodynamic microchannel flow under differential heating

This study investigates heat transfer and entropy generation in a microchannel subjected to differential heating, viscous dissipation, and Joule heating within a magnetohydrodynamic (MHD) fluid flow. A finite difference method with a fully implicit scheme is employed to accurately model temperature distribution and entropy generation. A comparison between the average Nusselt numbers (Nu) calculated using the classical method and the Bennett Formula reveals a notable discrepancy, particularly at the entry length (up to 14%). It has been found that when one plate is heated while the other is cooled and the Hartmann number (Ha) is low, the average Nu for both plates converges to 2. However, at high Ha values considering viscous dissipation and Joule heating, there is an 8% deviation between the Nu values of the two plates, with the higher Nu found on the cooling plate. Sensitivity analyses explore the impact of control parameters on entropy generation, emphasizing the significance of η as a key parameter that reflects the system's resistance to entropy generation. Increasing η from 0.1 to 0.5 results in a 32% reduction in entropy generation. In particular, for microchannels, substantial η high values imply reduced entropy generation, highlighting their efficiency in heat transfer.

Modified correlations for the Nusselt numbers at the boundaries of a bottom-heated molten metal layer in a stratified corium melt pool and an assessment of heat transfer conditions during a severe accident

The paper discusses new correlations for the Nusselt numbers at the side and bottom surfaces of a bottom-heated molten steel layer under the conditions of mixed natural convection of the melt at Rayleigh (Ra) numbers from ∼ 106 to 1012. The melt layer under consideration is characterized by radial non-uniformity of the temperature distribution, when the averaged temperatures of the top and side surfaces of the layer show a difference in value. This thermal pattern of the molten metal layer is typical when a stratified corium molten pool forms during a severe accident (SA). Upper metallic melt layer goes above the oxide phase of corium pool, and it is heated along its bottom surface. For an adequate prediction of heat transfer at the side and bottom surfaces of a layer in a SA, we developed special correlations for the Nu numbers. They take into account Rayleigh number and additional factors that determine geometric parameters of the layer and the temperature conditions at the bottom, side and top surfaces of the layer. Parameters in the developed correlations for the Nu numbers were determined via CFD simulation and mathematical processing of the simulation results. CFD simulations by means of the ANES code were completed in accordance with a specially developed experiments matrix. Two-parameter k-ω turbulence model by Wilcox, which includes Menter’s improved near wall functions, has been implemented in the CFD code and applied accordingly. To evaluate the accuracy and predictability of the developed correlations we ran a comparative analysis of the Nu numbers at the sidewall and bottom surfaces of the melt layer, as defined on the basis of the developed correlations for the Nu numbers and as defined in CFD simulation. The comparison showed a strong correspondence of the results within a wide range of Rayleigh number values of ∼ 106 < Ra < 1012. The analysis of the developed correlations allowed shaping a number of critical arguments that define heat transfer conditions at the melt layer boundaries. It was found that the Nu numbers at the side and bottom surfaces are largely dependent on the difference in the temperatures of the top and side surfaces of the layer. As the difference between the these temperatures widens, the Nu numbers increase drastically (by tens of percent or more) in comparison to similar Nu numbers defined on the basis of the well-known correlations. It is specified that when the temperatures at the top and side surfaces of the layer are equal, the heat transfer conditions at the side surface are defined by “Globe and Dropkin” correlation used for the horizontal surfaces of the layer. We also evaluated the significance of the geometric parameters on the Nu values for the sidewall and bottom surfaces of the melt layer. Multiplicative effect of this parameter and of the parameter for temperature conditions at the boundaries of the layer mentioned above may lead to a significant (by times) increase of the Nu values. That must be considered in the thermal analysis of the corium pool and the processes of the SA.

Image denoising and segmentation model construction based on IWOA-PCNN

The research suggests a method to improve the present pulse coupled neural network (PCNN), which has a complex structure and unsatisfactory performance in image denoising and image segmentation. Then, a multi strategy collaborative improvement whale optimization algorithm (WOA) is proposed, and an improved whale optimization algorithm (IWOA) is constructed. IWOA is used to find the optimal parameter values of PCNN to optimize PCNN. By combining the aforementioned components, the IWOA-PCNN model had the best image denoising performance, and the produced images were crisper and preserve more information. IWOA-PCNN processed pictures have an average PSNR of 35.87 and an average MSE of 0.24. The average processing time for photos with noise is typically 24.80 s, which is 7.30 s and 7.76 s faster than the WTGAN and IGA-NLM models, respectively. Additionally, the average NU value measures 0.947, and the average D value exceeds 1000. The aforementioned findings demonstrate that the suggested method can successfully enhance the PCNN, improving its capability for image denoising and image segmentation. This can, in part, encourage the use and advancement of the PCNN.

Numerical investigation of the thermal performance optimization inside a heat exchanger tube using different novel combination of perforations on Y-shaped insert

Metallic inserts have been receiving considerable interest in recent research as a means of enhancing passive thermal performance. Nevertheless, little research has been conducted on Y-shaped inserts with different perforations. The purpose of this study is to seek out optimum performance in terms of heat transfer employing hexagonal, trapezoidal, kite, and hybrid shapes with Perforation Indexes (PI) of 10%, 20%, and 30%. While conducting the analyses, air is utilized as the working fluid, using Reynolds numbers (Re) ranging from 3000 to 21000. The findings demonstrate that Nusselt number (Nu) values of up to 225 can be achieved at Re = 21000 for trapezoidal designs with a PI of 20%. In contrast, such case seems to have the lowest friction factor (f) of 0.07, yielding in the most optimized thermal performance. With PI = 10%, hexagonal perforations have the maximum f of 0.24, making it particularly prone to pressure drop. When compared to the smooth tube, Nu improved up to 6.15 times and f enhanced up to 5.03 times. The trapezoidal configuration with PI = 20% yielded the highest thermal performance factor, a TPF of 3.68, making it the best optimal case in terms of both thermal and hydraulic performance.