Advanced Heat Transfer Research Articles

Chaotic convection in an Oldroyd-B nanofluid for the Horton–Rogers–Lapwood problem with nonlinear Boussinesq approximation and gravity modulation

The Horton–Rogers–Lapwood problem for a non-Newtonian Oldroyd B nanofluid with modulated gravity effects under isothermal boundary conditions is investigated. Both linear and nonlinear stability analyses are performed, with numerical results presented graphically. The nonlinear Boussinesq approximation (NBA) is applied to the buoyancy term in the momentum equation, providing a more accurate representation of fluid behavior under high-temperature gradients, which is essential in advanced heat transfer applications. Consequently, the governing system includes the continuity equation, the Navier–Stokes momentum equation, the equation of state, the energy equation, and the nanoparticle volume fraction equation. A comprehensive stability analysis is conducted for free-free, rigid-free, and rigid-rigid boundary conditions. Additionally, a weakly nonlinear stability study employing the spectral Fourier method under isothermal, tangential stress-free boundary conditions quantifies heat and nanoparticle transport. The analysis reveals the transition from periodic convection to chaotic convection and bifurcation. A reduced Lorenz model is developed to explore the underlying dynamics further, offering deeper insights into the onset and progression of chaotic convection within the system. The NBA can be interpreted as lowering the critical Rayleigh number, thereby facilitating the onset of convection. This behavior contrasts with findings in many studies based on the linear Boussinesq approximation commonly reported in the literature. Gravity modulation enhances heat transfer and induces chaotic patterns within the nonlinear domain. The Darcy number (Da) and scaled nanoparticle Rayleigh number (Rn) promote chaos, while the modified diffusivity ratio (NA) supports periodicity.

Flow microbubble emission boiling (MEB) in open microchannels for durable and efficient heat dissipation

Microbubble emission boiling (MEB) has been reported to be an advanced heat transfer mechanism due to its significant heat dissipation capacity at high heat flux. Microchannel heat sinks are considered to be effective heat transfer carriers, but MEB is difficult to be triggered in conventional microchannels due to insufficient mainflow subcooling and restriction of narrow channel walls. In this work, we conducted flow MEB experiments in plain and lasered open microchannels with various inlet temperatures (Tinlet) and open gap height (Hg). The open configuration can provide adequate mainflow subcooling and extra flow area to trigger MEB. The results showed that MEB occurred in plain open microchannels as a transition flow pattern between bubbly flow and flow reversal at Tinlet ≤ 25 °C and Hg = 0.3 mm, with a significant temperature drop after a one-time flow reversal, providing abundant vapor composition as MEB evaporation cores; MEB did not happen at higher Tinlet and larger Hg (1 mm). However, in lasered open microchannels, MEB was triggered at the beginning of flow boiling at Tinlet ≤ 65 °C with Hg = 0.3 and 1 mm, without temperature drop or flow reversal. This demonstrated that the required inlet subcooling, heat flux and temperature gradient in vertical direction for MEB initiation were simultaneously reduced in lasered microchannels. The two-phase heat transfer coefficient (htp) of MEB was significantly increased (up to 77.8 %) compared to conventional bubbly flow, due to the faster bubble nucleation frequency and rapid impact from the subcooled mainflow to channel walls, and was further enhanced in lasered microchannels. The durability of MEB in plain microchannel was unsatisfactory, as the persistent flow reversal dominated the flow pattern after ∼3750 s at G = 300 ml/min, Tinlet = 25 °C, Hg = 0.3 mm and qeff ∼1450 kW/m2. However, in lasered microchannels, MEB ran steadily for 22500 s at the same working condition. This study provided an effective and accessible method to achieve durable MEB in microchannels with excellent heat dissipation capacity, offering valuable insights for further thermal management engineering applications.

Synergistic effects of magnetic fields and Y-obstacles on a nanofluid-based split lid-driven thermal system

This study investigates the complex hydrothermal behavior in a novel split-driven square cavity filled with CuO-water nanofluid and featuring a Y-shaped obstacle. The research explores the interplay among mixed convection, magnetohydrodynamics (MHD), and geometric factors in a previously unexamined configuration. The upper wall is split into two isothermally heated, translating sections, while the bottom wall and Y-shaped obstacle provide cooling boundaries. Using finite element analysis, we examine the effects of Reynolds number (Re = 10–200), Rayleigh number (Ra = 103–106), Hartmann number (Ha = 0–70), and magnetic field inclination (λ = 0–150°) on flow patterns, heat transfer, and entropy generation. Energy flux vectors are utilized to analyze thermal energy transport dynamics. Key findings are as follows. (1) Increasing Re enhances fluid mixing, resulting in up to a 65 % increase in the average Nusselt number (Nu). (2) Nu peaks at Ra ≈ 105, then slightly decreases due to complex force interactions. (3) Increasing Ha suppresses convection, reducing Nu by up to 30 %. (4) Optimal heat transfer occurs at λ ≈ 75° (5) The Y-shaped obstacle enhances heat transfer by up to 30 %. These results provide insights for optimizing thermal management in applications requiring precise temperature control, potentially improving energy efficiency in advanced heat transfer systems.

Thermo-hydraulic performance characteristics of novel G-Prime and FRD Triply Periodic Minimal Surface (TPMS) geometries

Efficient thermal management is crucial for numerous applications, from electronics cooling and automobile systems to aerospace systems, necessitating the exploration of advanced heat transfer technologies like TPMS structures. Compared to conventional heat exchangers and heat sinks, lattice structures have improved by around 20–30 % performance. However, the variation in performance of the various lattice structures, such as FRD, G-Prime, etc., is yet unknown. Thus, this study investigates the thermo-hydraulic performance of various Triply Periodic Minimal Surface (TPMS) structures, including novel G-Prime and FRD geometries, through numerical simulations and experimental validation. The TPMS geometries were modeled using LattGen software and voxelized with 20 % relative density. Computational Fluid Dynamics (CFD) simulations were performed using Ansys Fluent with the k-epsilon turbulence model, and experiments were conducted on 3D-printed samples for validation. The numerical results reveal that G-Prime-2 and FRD Prime exhibit the highest heat transfer performance while demonstrating higher pressure drops than other TPMS geometries. Experimental validation agrees with numerical predictions, confirming the superior thermo-hydraulic performance of G-Prime-2 and FRD Prime for heat transfer enhancement applications. The results contribute to understanding TPMS thermo-hydraulic performance and enable informed geometry selection for thermal management applications.

Advanced heat transfer analysis using RSM-BBD for MHD hybrid nanofluid flow over a nonuniform stretching sheet with viscous dissipation

The recent studies focus on optimizing conditions to maximize the heat transfer rate while minimizing energy consumption, a key goal for various industries in both production processes and Automotive industry efficiency. Hybrid nanofluids play a crucial role in improving heat transfer phenomena. This study examines the enhanced heat transfer properties of a water-based hybrid nanofluid containing Al2O3 and SiO2 nanoparticles as it flows through a nonlinear stretching sheet in a porous medium. The convective flow is further influenced by a magnetic field and thermal radiation, adding complexity to the flow dynamics. To simplify the analysis, dimensional physical quantities are converted to non-dimensional parameters using appropriate similarity rules. These transformed equations are then numerically solved using the bvp4c function in MATLAB. To achieve optimal heat transfer rates, a robust statistical method known as response surface methodology (RSM) is employed, and validation is performed through an analysis of variance. Graphical representation is used to examine parametric behaviour, and a brief physical description is provided. Key findings include: the hybrid nanofluid shows a 10.61% increase in heat transfer rate compared to the base fluid at R = 0.2; the drag force coefficient for the Al2O3/H2O nanofluid is reduced by 13.98% compared to other hybrid nanofluids; and an increase in the nonlinear stretching sheet parameter improves velocity distribution while reducing temperature distribution. This study can be used in automotive transmission systems to enhance heat dissipation and lubrication, improving the efficiency and durability of the transmission.

Enhancing thermal conductivity of novel ternary nitrate salt mixtures for thermal energy storage (TES) fluid

Efficient thermal energy storage (TES) is crucial for concentrated solar power (CSP) plants, necessitating the exploration of advanced heat transfer fluids with enhanced thermal conductivity. Conventional binary nitrate salt mixtures have limitations in thermal performance, prompting research into ternary mixtures and nanoparticle additives. This study investigates novel ternary nitrate salt mixtures comprising potassium nitrate (KNO₃), lithium nitrate (LiNO₃), and magnesium nitrate hexahydrate (Mg(NO₃)₂·6 H₂O) as high-performance TES fluids during the during the heat absorption phase. Seven different salt compositions were synthesized and characterized using the laser flash technique to evaluate their thermal conductivity over 100–400°C. Results revealed a significant influence of composition on thermal conductivity, with maximum values during melting ranging from 0.0777 W/m·K to 0.7373 W/m·K. Melting points varied from 335 K to 340.39 K, demonstrating tailorability through compositional adjustments. Furthermore, incorporation of 0.5 wt% aluminum oxide (Al₂O₃) nanoparticles resulted in substantial thermal conductivity enhancements, with the most significant increase observed in TSF10 (from 0.0777 W/m·K to 0.491 W/m·K). These improvements are attributed to enhanced phonon transport, increased surface area, and Brownian motion facilitated by Al₂O₃ nanoparticles. The study provides a comprehensive cost analysis, including raw material costs, and discusses the potential efficiency gains for CSP applications. The findings contribute to the development of high-performance and cost-effective TES fluids, advancing the efficiency and viability of sustainable energy generation.

Fructose-mediated single-step synthesis of copper nanofluids with enhanced stability and thermal conductivity for advanced heat transfer applications

A precisely controlled solution-phase approach was employed to synthesize copper nanofluid through the reduction of copper sulfate by fructose in the presence of cetyltrimethylammonium bromide, utilizing a mixture of water and ethylene glycol in 1:1 ratio as the base fluid. We delved into the nanofluid’s thermal conductivity and rheological properties, with a keen interest on particle size and reaction rates that exhibited significant sensitivity to variations in reaction parameters. The homogeneous dispersion of nanoparticles in the base fluid resulted in an augmentation of thermal conductivity to 2.31 Wm−1K−1 for particle loading fraction of 0.19%, with a never before achieved stability of 9 months. This method has proven to be not only straightforward and dependable but also efficient for the rapid synthesis of highly stable Newtonian nanofluids, underscoring the nanofluid’s potential for highly powerful cooling applications.

Radiation absorption, Hall and ion-slip current – driven MHD convection with spanwise cosinusoidally fluctuating temperature: a perturbative study

This study investigates the impact of radiation absorption, Hall and ion-slip current on magnetohydrodynamic free convective fluid flow past an infinite vertical porous plate with viscous dissipation and chemical reaction in the presence of Spanwise Cosinusoidally fluctuating temperature by time. Its significance extends across a wide array of applications, including advanced heat transfer systems for electronics, optimization of material processing kinetics, and fostering innovation for tailored material properties, with far-reaching implications spanning aerospace and renewable energy sectors. The governing equations, subject to stipulated boundary conditions are analyzed using the multiple regular perturbation method. The results are visually presented to evaluate the influence of various parameters on system dynamics. Graphical representations illustrate the physical significance of parameters including velocity, temperature, concentration, skin friction, mass transfer rate and heat transfer coefficients. The findings reveal that increased values of Hall and ion-slip current effects correlate with acceleration of velocity and skin friction. Moreover, higher radiation absorption parameter values lead to enhancements in velocity, temperature and Nusselt number. Conversely, different chemical reaction rates and Schmidt number values result in declines in velocity, concentration and Sherwood number. Additionally, incremental values of the Eckert number improve velocity and Nusselt number but have a reverse impact on temperature. The findings of this research align with those from prior examinations.

Comprehensive Performance Analysis of Hybrid Solar Water Heating Systems: Synergizing Photovoltaic and Thermal Technologies

Abstract: Solar water heaters are an efficient and sustainable technology for meeting the growing demand for hot water in residential and commercial applications. This abstract presents a comprehensive overview of the performance evaluation of solar water heater systems through an extensive experimental study. The study aims to assess the effectiveness and efficiency of different solar water heater configurations under varying operating conditions. The experimental investigation involved the measurement and analysis of key performance parameters such as solar collector efficiency, heat transfer efficiency, thermal storage capacity, and overall system efficiency. A range of factors including solar radiation intensity, ambient temperature, water flow rate, and collector tilt angle were systematically varied to examine their impact on system performance. The results of the experimental study demonstrated the significant potential of solar water heaters in providing cost-effective and environmentally friendly hot water solutions. The solar collector efficiency was found to be highly influenced by the solar radiation intensity, with higher levels of solar irradiation leading to increased thermal energy generation. The heat transfer efficiency was optimized by adjusting the water flow rate and collector tilt angle, ensuring efficient transfer of heat from the collector to the water. Furthermore, the thermal storage capacity of the system played a crucial role in maintaining a consistent supply of hot water during periods of low solar radiation or high demand. Overall system efficiency was found to be strongly dependent on the integration of efficient heat exchangers and insulation materials. Based on the experimental findings, recommendations for system design and optimization were formulated, highlighting the importance of proper sizing, selection of suitable materials, and appropriate control strategies. The results also identified areas for future research, such as the integration of advanced heat transfer enhancement techniques and the utilization of hybrid solar water heater systems

Modeling of EMHD ternary hybrid Williamson nanofluid flow over a stretching sheet with temperature jump and magnetic induction

This study explores the heat transfer and fluid flow characteristics of an incompressible Williamson ternary hybrid nanofluid (WTHNF) over an unsteady stretching surface, influenced by external electric and magnetic fields, thermal radiation, and a porous boundary. The WTHNF consists of ZrO2, MoS2, and GO nanoparticles suspended in water. By applying suitable transformations, the fundamental equations are simplified and solved numerically using the Explicit Runge Kutta Method (ERKM). The research investigates how various physical parameters, such as electric and magnetic field strengths, thermal radiation, porous plate permeability, velocity slip, and temperature jump, impact the velocity, temperature, skin friction coefficient, and local Nusselt number. The findings offer valuable insights into the interactions between WTHNF and external factors, which can inform the design and optimization of advanced heat transfer and fluid flow systems in engineering. Additionally, this study demonstrates the potential of WTHNF to enhance heat transfer rates, making it a promising candidate for cooling systems, heat exchangers, and energy storage devices. The research contributes to developing more efficient and sustainable technologies in aerospace, biomedical engineering, and renewable energy. Moreover, the results provide a foundation for future research on the behavior of complex fluids under various external influences. This could lead to the creation of more advanced fluid flow systems, improving efficiency and performance across numerous applications.

Unraveling heat transfer mechanisms in MoS 2/SO−water nanofluid for flow over a stretching cylinder

Nanofluids, advanced heat transfer fluids with improved thermophysical properties, have proven effective in enhancing the efficiency of various devices. Widely applied in electronic devices and automotive industry, consisting of nanoparticles and base fluids, serve as efficient coolants to optimize heat transfer performance. This article investigates the physical effects of magnetohydrodynamic (MHD) boundary layer flow of H 2 O base fluid over a stretched cylinder subjected to heat generation and thermal radiation. The present nanofluid investigation incorporate Mo S 2 nanoparticles into a base fluid mixture of SO / H 2 O . The mathematical model, employing similarity transformations, is developed, leading to the formulation of similarity equations. Simulations are conducted using MATLAB’s bvp4c solver to analyze the resulting flow patterns. Simulation results for various physical parameters demonstrate that the inclusion of hybrid nanoparticles in the fluid mixture significantly enhances heat transfer compared to the conventional nanofluids. Our finding highlights the necessity of considering hybrid nanoparticles over single-type counterparts for creating efficient thermal systems. Moreover, investigation underscores the vital role of hybrid nanofluids in fluid transmission, showcasing their ability to achieve higher temperature distribution.

A numerical investigation of the chemically reactive maxwell nanofluid flow over a convectively heated rotating disk using Darcy–Forchheimer porous medium

This study investigates the convective heat transfer characteristics in the vicinity of a stagnation point for the flow of Maxwell nanofluid over a porous rotating disk. The analysis takes into account the complex inert-active effects arising from nonlinear thermal radiation, activation energy, and the presence of a Darcy–Forchheimer medium. Through numerical simulations, the enhancement of heat transfer due to the addition of nanoparticles is explored, considering their impact on heat transport. The rotational and porous characteristics of the disk, coupled with nonlinear thermal radiation and activation energy effects, are crucial factors in shaping the overall heat transfer behavior. The study aims to provide valuable insights into the complicated interactions of these phenomena, contributing to the understanding of advanced heat transfer processes and their potential applications in various engineering systems. Using suitable variables to convert the system of leading equations to dimensionless form has then been evaluated by employing the bvp4c approach. It has been revealed that Radial flow has retarded with an upsurge in Deborah number, inertial factor, and porous factor while has upsurge with growth in rotational factor. Angular velocity has declined with higher values of Deborah number, and porous factor and has upsurged with escalation in inertial and rotational factors. Azimuthal flow has weakened with an upsurge in porous factor and has augmented with growth in Deborah number, inertial factor, and rotational factor. Thermal profiles have augmented with an upsurge in rotational, porous, inertial, thermophoresis, Brownian, and radiation factors, and Deborah number has declined with growth in the Prandtl number. Concentration distribution has declined with an upsurge in Schmidt number, Brownian motion factor, rotation factor, and porous factor, while has grown with the escalation in chemically reactive, thermophoresis, inertial factors, and Deborah number.

Unsteady magnetohydrodynamic tri-hybrid nanofluid flow past a moving wedge with viscous dissipation and Joule heating

This study aims to boost thermal convection through careful selection and adjustment of nanomaterial volumes, focusing on the unsteady magnetohydrodynamic flow past a moving wedge with viscous dissipation and Ohmic heating in a ternary nanofluid of alumina (Al2O3), copper oxide (CuO), and copper (Cu) in water. Employing mathematical modeling and numerical analysis via MATLAB's BVP4C, it explores how discharge concentration influences flow characteristics and identifies critical conditions for single or dual solutions. Key parameters such as motion and wedge parameters, Eckert number, magnetic strength, and nanoparticle volume ratios were scrutinized for their impact on fluid dynamics and heat transfer. Results show enhanced convective thermal transfer with increased nanoparticle hybridity and volume fraction, alongside suction/injection parameter (S), unsteadiness parameter (A), Eckert number (Ec), and magnetic parameter (M), albeit decreasing with wedge angle adjustments. Stability analysis revealed the stability of the initial solution vs the instability of the secondary. Introducing a novel time variable, τ=cAt(1−ct), this research demonstrates that at λ=−4.7(a leftward wedge) with a 0.04 nanoparticle volume fraction, ternary and hybrid nanofluids significantly outperform mono nanofluid, achieving thermal efficiency gains of 25.6% and 7.5%, respectively. This foundation underscores the potential of optimized nanofluid mixtures for advanced heat transfer applications.

Thermal and hydraulic performance of volumetrically heated triply periodic minimal surface heaters

Advanced heat transfer surfaces, mainly Triply Periodic Minimal Surfaces (TPMSs) have great potential to increase heat transfer performance over traditional heat transfer surfaces due to their tortuous flow path and higher surface area-volume ratio. The emergence of additive manufacturing of nuclear fuel motivates experimental investigation of such advanced geometries in the context of a nuclear reactor core. This study employed a novel method to explore the performance of gyroid and diamond TPMS geometries. Volumetrically heated heaters were additively manufactured from conductive polymer filament, and airflow was used to simulate the convective heat transfer from the core within a nuclear reactor. The mass flow and volumetric generation rates were varied, local Nusselt number correlations were obtained using embedded thermocouples to measure the solid and fluid temperature, and friction factor correlations were developed using differential pressure measurements. The gyroid TPMS presented a friction factor 1.5 times higher than the diamond TPMS and 90 times that of the rod bundle. TPMS geometries showed Nusselt numbers 8–10 times higher than that of the rod bundle, also maintaining colder internal temperatures, which suggested that they could sustain 250–275 % higher power density than the rod bundle for a given centerline temperature. Hence, having the potential to greatly increase the safety margins and power output of a nuclear reactor core with similar volume.

Engagement of modified heat and mass fluxes on thermally radiated boundary layer flow past over a stretched sheet via OHAM analysis

This work investigates advanced heat transfer mechanisms in boundary-layer flow of nanofluids over a stretchable sheet. The comprehensive model integrates radiation effects, Brownian motion, thermophoresis, Cattaneo-Christov heat flux and Magnetohydrodynamics (MHD). Nanofluids, composed of a base fluid with suspended nanoparticles, exhibit distinct thermophysical properties crucial for influencing heat and mass transfer dynamics. Through rigorous numerical analysis, key parameters such as nanoparticle volume fraction, stretching sheet velocity, Cattaneo-Christov parameter, radiation parameter, and magnetic field strength are examined. The findings reveal intricate interactions among these parameters, providing a thorough understanding of their combined impact on temperature and velocity characteristics inside the boundary layer. This research contributes significantly to fundamental knowledge in nanofluid dynamics and provides practical insights for optimizing heat transfer processes and fluid behavior over-stretching surfaces, especially in the context of advanced heat transfer mechanisms. The implications extend across diverse engineering disciplines and materials science, offering valuable guidance for designing and improving processes involving nanofluid flows over-stretching sheets. With respect to the magnetic field parameter, the fluid velocity exhibits a decreasing attitude.The temperature function enhances with higher R\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathcal{R}$$\\end{document} values but diminishes with increasing Pr\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathcal{P}\\mathcalligra{r}$$\\end{document} values.

Enhancing thermal performance in a magnetized square cavity: Novel insights from mixed convection of Ag-MgO nanofluid around a rotating cylinder

This numerical investigation considers the mixed convection of a magnetically influenced hybrid nanofluid in a two-dimensional square cavity containing a centrally positioned rotating circle cylinder with a radius of R = 0.12. The nanofluid within the enclosure is water-based and incorporates Ag-MgO nanoparticles, the chemical and physical interactions between MgO nanoparticles and Ag molecules play an important function in determining the thermal conductivity and dynamic viscosity of MgO-Ag nanofluid. These findings exhibit that MgO-Ag nanofluid has the potential to be used as an advanced heat transfer fluid in cooling systems and heat exchangers. Two distinct cases are examined: firstly, with all walls of the square cavity and the rotating circle cylinder being adiabatic, except for the bottom side, and secondly, with the right side of the cavity being maintained at an isothermal condition. A finite element software package (FLEXPDE) is used in the present study. The governing partial equations, rendered dimensionless and encompassing various physical parameters, are solved utilising the Galerkin-based finite element method. Novelty is injected into the exploration through a meticulous parametric study, delving into the influence of key parameters, including the angular velocity spanning from -25 to 25, Reynolds number (10 ≤ Re ≤ 100), and Richardson number (0 ≤ Ri ≤ 35). The outcomes are vividly portrayed through visualisation of the flow structure via streamlines, the depiction of the heat transfer process through isotherms, and the computation of average Nusselt numbers. A discernible trend emerges, demonstrating that an increase in the Reynolds number, angular rotation speed, and Richardson number corresponds to an elevation in the average Nusselt number. Increasing Reynolds number (Re) from 10 to 50 intensifies streamlines, indicating fivefold higher flow resistance, with isotherms at Re = 50 showing 80 % concentration near the hot bottom wall compared to Re = 10. Counterclockwise rotation induces 90 % curvature near the bottom, clockwise rotation near the top, highlighting directional effects on flow resistance and streamlines. Remarkably, an intriguing observation surfaces as the angular velocity intensifies counterclockwise, revealing a 10 % augmentation in the average Nusselt values compared to a clockwise direction. Furthermore, the adoption of nanofluids as cooling agents, in lieu of pure water, is shown to enhance the cavity's thermal and overall performance, surpassing previous results. This underscores the novel insight into the advantageous utilisation of nanofluids in improving the efficacy of thermal systems.

Impact of flow intermittency on heat transfer enhancement in serpentine channels

With liquid cold plates being widely applied in industries such as battery energy storage systems, advanced heat transfer enhancement technologies are urgently needed to efficiently dissipate the ever-increasing heat load. The present work numerically and experimentally explores the potential of flow intermittency in a laminar serpentine channel for thermal performance improvement. The numerical analysis shows that the dynamic Dean vortex evolution induced by the intermittent mainstream disrupts the thermal boundary layer more effectively than the steady-flow vortices and enhances local Nusselt number at the U-turns by 117% maximally. Such secondary vortices are transported intermittently to the straight segments, resulting in a 55% increase in the area-averaged heat transfer by promoting mainstream-boundary flow mixing. The optimization of the flow intermittency profile is achieved by matching the pulse-on and deceleration stage durations with the characteristic times of secondary vortex growth and transport. The numerical results are qualitatively validated by the experimental measurement conducted in a water bath. The current study novelly demonstrates the design concept of enhancing heat transfer in curved channels by actively controlling the intermittent flow and proposes the design criteria for the intermittency profile to achieve optimal performance.

Investigation of convective heat transfer in a cold flow pebble bed nuclear reactor using advanced pebble probe heat transfer sensor

The aim of this study was to investigate the local heat transfer coefficients between the pebbles and the coolant gas in a PBR using an advanced measurement technique that consists of a heated pebble probe, a micro-foil heat flux sensor mounted flush on the surface of the heated pebble probe, and a thermocouple in the center of the bed void in front the sensor. In this work, the local heat transfer coefficients were derived at various axial levels, radial and angular locations and at different superficial inlet gas velocities that cover both transitional and turbulent flow conditions. The local heat transfer coefficients were found to be 22-32%, 34-39%, and 18-20% higher near the wall at superficial inlet gas velocities of 0.3,1.2, and 2m/s, respectively. This is due to higher volumetric flow at the wall where larger void in the pebble bed exists compared to the center region of the bed where the flow of the gas in the packed bed follows the least resistance path. Large deviations were obtained between the experimental overall heat transfer coefficients in the bed and the predictions of seven correlations that were selected from the literature. Furthermore, these correlations cannot predict the local heat transfer coefficients inside the PBR. This necessitates the development of new correlations for the prediction of the local heat transfer coefficients using the data obtained in this work. A pseudo-3D correlation was developed and was found to provide predictions that are in good agreement with the experimental values for the conditions used, with an averaged absolute relative error (AARE) of 3.33% at high Reynolds numbers for our operating and design conditions.

Direct Numerical Simulation of liquid metal forced and mixed convection in a square rod bundle

This paper reports results of Direct Numerical Simulations of fully developed, forced and mixed convection of Liquid Lead Bismuth Eutectic (LBE) around four vertical cylindrical rods arranged in a square lattice at Pr = 0.031. The solutions provided here are placed in a context where very few data are available for low-Prandtl flows, especially in mixed convection conditions, and can hence be useful for the development and validation of advanced turbulent heat transfer models. The equations are discretized using a Finite Volume implementation of a second order projection method. The irregular cylindrical boundaries are handled with an original Immersed Boundary technique. A single friction Reynolds number value is set (Reτ = 180) and buoyancy effects are accounted for by imposing the Rayleigh number, under the Boussinesq approximation. A Ra-value Ra = 2.5 ×104 is selected in order to investigate the main differences between forced and mixed convection at low Prandtl number values. Time-averaged velocity and temperature fields are shown, in order to discuss the main features of the flow and thermal fields. First order statistics are also presented, highlighting the effect of aiding buoyancy on turbulent heat and momentum transport.

Plastic Waste Pyrolysis Reactors with Integration of Rocket Stove and Advanced Heat Transfer System: An Analysis of its Design

The development of efficient and sustainable pyrolysis reactors is critical for advancing bioenergy technologies and waste management. This study presents the design and stress analysis of a pyrolysis reactor incorporating a rocket stove and an advanced heat transfer system, using Autodesk Inventor 2016. The integration of a rocket stove ensures efficient combustion and high-temperature stability, while the advanced heat transfer system maximizes thermal distribution within the reactor, enhancing the pyrolysis process. Stress analysis revealed critical stress points and informed design optimizations to ensure durability and safety. This innovative design approach, validated through advanced simulation tools, offers a significant advancement in the development of sustainable and efficient pyrolysis reactors. The safety factor is defined as the ratio of maximum allowable stress to equivalent stress. The permissible stress value for stainless steel components used in pyrolysis tubes is 187.5 MPa. The stress analysis test revealed that the reactor tube experienced a maximum equivalent stress of 17.72 MPa. The reactor’s design, incorporating materials capable of withstanding high temperatures and stresses, ensures safe and reliable operation. Stress analysis and the implementation of appropriate safety factors play a critical role in maintaining the reactor’s structural integrity.