Uniform Heat Flux Research Articles

CFD INVESTIGATION OF THE EFFECT OF MODIFIED TWISTED TAPE ON HYDROTHERMAL ENHANCEMENT IN VERTICAL PIPE

Heat transfer enhancement has been a significant focus in hydrothermal engineering for several decades. The current paper represents an investigation using computational fluid dynamics (CFD) for a vertical circular tube having twisted taper inserted (plain and modified) with different twisted tape ratios TR, i.e., 7.8, 3.9, and 2.6, at three uniform wall heat fluxes and ranges of Reynolds number as boundary conditions. The twisted strips were used as a system to produce a turbulent, powerful swirl flow with an increase in the intensity of the secondary flow at the radial direction of the tube. The (friction facto) f and (Nusselt number) Nu were two measures of pressure loss and heat transfer variation that have been studied and graphically depicted. So, to balance the heat transfer enhancement quantity with pressure losses, the performance evaluation factor PEF was tested. It was observed that the twisted tape insert enhances the radial secondary flow intensity. The findings demonstrated that, in every scenario, the increase in(Reynold number)Re at the inlet, heat flux quantity, and twisted ratio results from a proportionate increase in heat transfer. At the same time, the friction factor decreases with an increase in all boundary conditions, except the twisted ratio, where the friction factor increases with increasing heat transfer. After accounting for heat transfer and friction coefficient, it was discovered that the tube with modified twisted stripe and the twist ratio was 3.9 performs best out of all the configurations examined. The performance evaluation factor PEF of pipes inserted with modified twisted tape was higher, reaching 1.5. This study is based on enhancing a high-performance evaluation factor of heat exchangers for use in industrial settings.

Mechanisms of Enhanced Cooling by Sidewall Modification in a Microtube Cold Plate

Abstract The thermofluidic effectiveness of a microtube cold plate with sidewall modification has been disputed. This study settles the dispute by detailing how such sidewall modification—hemi-circular cavities arranged along a straight circular microtube embedded with a cold plate—enhances cooling. To this end, a series of numerical simulations for a single microtube cold plate in a practical range of Reynolds numbers (20 ≤ ReD ≤ 200) are carried out. For experimental validation, flow development and pressure drop in both reference and modified microtubes are characterized respectively by microparticle image velocimetry and pneumatic pressure measurements, while surface temperatures on both cold plates' substrates subjected to uniform heat flux are measured by infrared thermography. The results agree that the boundary layer development in each throat, a short straight coolant passage that connects successive cavities, indeed enhances the cooling in the cold plate with the wall modification. However, in contrast to its sole contribution argued previously, a non-negligible source of thermal enhancement is identified. The coolant flow approaching each throat noticeably bifurcates across the throat and cavity sidewall. Thereafter, a boundary layer develops along concave sidewalls and subsequently circulates within each cavity, generating high wall shear stresses upstream of each throat and significantly elevating local cooling. In conclusion, the local cooling both “upstream” and “in” throats plays a dominant role in enhancing the overall cooling in the cold plate with wall modification, up to 40% over the reference microtube cold plate in the Reynolds number range considered.

A comparative numerical evaluation of solar air heater performance having W-contoured, taper-contoured and reverse taper-contoured turbulators

The use of different turbulators in solar air heaters can significantly impact their thermal and hydraulic performance. This study compares solar air heaters equipped with W-contoured, taper-contoured, and reverse taper-contoured turbulators. It examines heat transfer coefficients, pressure drops, velocity contours, turbulent kinetic energy contours, and thermal perfor-mance factors for these systems under varying operating conditions. The air Reynolds number ranges from 4000 to 18 000, while design parameters such as relative roughness height and relative pitch ratio remain constant for accurate comparison. The simulations were conducted with a uniform heat flux of 1200 W/m2. The W-shaped contour roughness achieved the greatest heat transfer coefficient, surpassing both the tapered and reverse tapered configurations. In terms of friction factor, the tapered contour on the absorber plate led, followed by the reverse tapered and W-shaped contours. Overall, the W-shaped contour delivered the best performance. At lower Reynolds numbers, the reverse tapered contour outperformed the tapered contour, whereas at higher Reynolds numbers, the tapered contour showed superior performance compared to the reverse tapered contour.

Numerical Solution of Simultaneously Developing Laminar Flow in a Plane Duct with Uniform Wall Temperature and Constant Heat Flux

Numerical Solution of Simultaneously Developing Laminar Flow in a Plane Duct with Uniform Wall Temperature and Constant Heat Flux

Thermodynamic analysis and performance evaluation of a parabolic trough collector receiver with external annular fins

ABSTRACT This paper investigates the influence of external annular fins on the thermodynamic performance of parabolic trough collector (PTC) receiver. The objective is achieved by developing thermal and fluid flow models with geometry of varied fin lengths and fin numbers and implemented in ANSYS Fluent software. With an assumption of evacuated annulus space, measured uniform heat fluxes were applied on respective halves. The results show an increase in thermal efficiency by 2.26% for T in = 350 K, 2.21% for T in = 400 K, and 2.22% for T in = 500 K for varying Reynolds numbers and fin lengths, t = 5 mm. It is also observed that entropy generation reduces by 1.44% for T in = 350, Re = 5000, and t = 5 mm. Low turbulence records higher heat transfer irreversibility as measured by the Bejan number. Exergy efficiency positively responds with an increase of 2.44% for Re = 5000, 2.30% for Re = 10000, and 2.30% for Re = 20000 for t = 5 mm. Due to the observed improvement, this passive enhancement technique can be utilized to design a high performing receiver system, thus maximizing the availability and reliability of PTC system.

Thermal challenges in heterogeneous packaging: Experimental and machine learning approaches to liquid cooling

In recent years, electronic packaging has evolved significantly to meet demands for higher performance, lower costs, and smaller designs. This shift has led to heterogeneous packaging, which integrates chips of varying stack heights and results in non-uniform heat flux and temperature distributions. These conditions pose substantial thermal management challenges, as they can create large temperature gradients, which increase thermal stress and potentially compromise chip reliability. This study explores single-phase liquid cooling for multi-chip modules (MCMs) through a comprehensive experimental and machine learning approach. It investigates the impact of chip spacing, height, fluid flow rate, fluid inlet location, and heat flux uniformity on chip temperature and the thermohydraulic performance of a commercial cold plate. Results show that increasing coolant flow from 1 LPM to 2 LPM decreased thermal resistance by 26 %, with heat losses remaining below 5 %. The left inlet configuration improved temperature uniformity compared to the right, though both yielded comparable thermal performance. Adjusting heater spacing impacted temperature distribution based on inlet position, and lowering one heater by 1 mm raised its temperatures by 15 °C due to increased thermal resistance from thermal interface material. A transient test demonstrated the cold plate’s quick response to power surges, in which there is only a 1 °C spike above steady state. Complementing these findings, an Artificial Neural Network (ANN) model was developed with optimized architecture specifically for the unique challenges of this study. The ANN model was rigorously validated using an independent dataset, achieving highly accurate temperature predictions (R2 = 0.99) within 2.5 % of experimental values, which demonstrates this framework’s potential for optimizing MCM thermal performance.

Thermal stress intensity factors in composite plates with cracks arising from a circular hole

In the present study, a novel analytical solution is introduced for the determination of thermal Stress Intensity Factors (SIFs) for a composite plate containing a cracked circular hole subjected to a uniform heat flux. Therefore, an analytical approach, based on the Complex Variable Method (CVM), is provided for the first time, employing the Schwartz integration method. To facilitate the solution of complex integrals using Schwartz’s theorem, a novel representation of mapping function is offered, providing a more straightforward solution. Thermal SIFs are calculated for circular holes containing one or two cracks. Also, the influence of factors like fiber orientation and dissimilar crack lengths is investigated. To verify the accuracy of outcomes derived from the current analytical solution, a comparison with results obtained through the Finite Element Method (FEM) is conducted. The comparative analysis highlights the robust accuracy of the present solution. In scenarios where fibers align with the crack length and the heat flux acts perpendicular to the crack direction, the Mode-I of SIF becomes neglected. However, when the fibers do not align with crack length, the Mode-I SIF deviates from zero. Furthermore, an increase in the length of one crack is observed to augment the SIF of another crack.

Numerical Assessment of the Thermal Performance of Microchannels with Slip and Viscous Dissipation Effects.

Microchannels are widely used across various industries, including pharmaceuticals and biochemistry, automotive and aerospace, energy production, and many others, although they were originally developed for the computing and electronics sectors. The performance of microchannels is strongly affected by factors such as rarefaction and viscous dissipation. In the present paper, a numerical analysis of the performance of microchannels featuring rectangular, trapezoidal and double-trapezoidal cross-sections in the slip flow regime is presented. The fully developed laminar forced convection of a Newtonian fluid with constant properties is considered. The non-dimensional forms of governing equations are solved by setting slip velocity and uniform heat flux as boundary conditions. Model accuracy was established using the available scientific literature. The numerical results indicated that viscous dissipation effects led to a decrease in the average Nusselt number across all the microchannels examined in this study. The degree of reduction is influenced by the cross-section, aspect ratio and Knudsen number. The highest reductions in the average Nusselt number values were observed under continuum flow conditions for all the microchannels investigated.

Analytical generalized thermal resistance model for conductive heat transfer in pebble beds based on heat flux weighted temperature difference

A novel model for inter-particle contact thermal resistance based on analytical solutions is proposed. By applying the concept of heat flux weighted temperature difference in thermal resistance modeling for the first time, this model offers greater physical significance than traditional thermal resistance models. It effectively describes the dissipation in the heat transfer process and the influence of particle radius on thermal resistance, making it a generalized thermal resistance model suitable for multi-dimensional systems. The impact of applying different levels of uniform heat flux boundary conditions on thermal resistance is analyzed from the perspective of energy dissipation, revealing that a more uniform heat flux input results in lower thermal resistance. The effects of contact radius and particle size on the dimensionless generalized thermal resistance of inter-particle contact were studied, showing that the dimensionless resistance increases with larger particle contact radius and smaller particle size. Using the thermal discrete element method, the thermal resistance model with uniform heat flux density boundary conditions was applied to calculate the effective thermal conductivity of the pebble bed. Considering the distribution of contact radius, differences in effective thermal conductivity at various heights of the pebble bed due to different contact radii were examined and compared with the traditional fixed-coefficient contact resistance model. It was found that the difference between the two models decreases as the contact radius decreases. Finally, multiple sets of fixed contact radii were established under three different particle sizes, revealing that as the contact radius increases, the deviation between the new generalized resistance model and the traditional fixed-coefficient model rises from 8.18 % to 14.64 % for different particle radii.

Enhancing Thermal–Hydraulic Performance in Nuclear Reactor Subchannels with Al2O3 Nanofluids: A CFD Analysis

In this paper, the performance of aluminum-based nanofluids with a possible application in pressurized water reactors is numerically investigated. A 605 mm long 4-rod array square (2 × 2) subchannel geometry with a uniform heat flux of 50 kW/m2 has been used in CFD simulation. This analysis has been carried out using the RNG k-epsilon turbulence model with standard wall function in ANSYS FLUENT 2022R1. The impact of various flow conditions and nanofluid concentrations has been examined. The effects of variable velocities on nanofluid performance have been studied using different Reynolds numbers of 20,000, 40,000, 60,000, and 80,000. The analysis was conducted with Al2O3/water nanofluid concentrations of 1%, 2%, 3%, and 4%. A comparison of the Nusselt number based on five different correlations was conducted, and deviations from each correlation were then presented. The homogeneous single-phase mixer approach has been adopted to model nanofluid characteristics. The result shows a gradual enhancement in the heat transfer coefficient with increasing volume concentrations and Reynolds numbers. A maximum heat transfer coefficient has been calculated for nanofluid at maximum volume concentrations (ϕ = 4%) and highest velocities (Re = 80,000). Compared to the base fluid, heat transfer was enhanced by a factor of 1.09 using 4% Al2O3. The Nusselt number was calculated with a minimal error of 3.62% when compared to the Presser correlation and the maximum deviation has been found from the Dittus–Boelter correlation (13.77%). Overall, the findings suggest that aluminum-based nanofluids could offer enhanced heat transfer capabilities in pressurized water reactors.

Combinatorial approach to predict synergistic ablation behavior of carbon fiber phenolic composites under aerodynamic loading

The study conducts a comprehensive analysis across 1-D, 2-D, and 2-D axisymmetric geometries, investigating temperature profiles, surface recession patterns, pyrolysis gas flow dynamics, and density fluctuations within the composite material. The findings highlight the effectiveness of the FEA technique in modeling complex multi-physical interactions associated with ablation phenomena. The carbon-phenolic composites, characterized by low thermal conductivity, high specific heat capacity, and substantial char yield, are shown to be particularly well-suited as ablative materials. The char layer formed during ablation acts as an additional insulating barrier, enhancing thermal protection. In a specific case study, a 2 cm thick composite laminate was subjected to an intense heat flux of 200 W/cm2 for 180 s. The results demonstrate that the Thermal Protection System (TPS) effectively maintains the back face temperature below 350 K for the initial 60 s under uniform heat flux conditions. However, beyond this period, the back face temperature gradually increases, reaching 1698 K at the end of the 180-s simulation. These insights into the material’s thermal performance under severe heat exposure conditions provide valuable guidance for applications requiring effective heat management and thermal protection.

Enhancement of turbulent heat transfer by using CuO nano-particle and twisted tape

AbstractA computational model is developed to investigate the convective heat transfer properties and the fluid flow characteristics of cupric oxide - water nano-fluid in a horizontal circular pipe aiming to provide insights into optimizing heat transfer in such systems. A twisted tape with varied twist ratios is inserted. This quantitative investigation used five Reynolds number from 4,000 to 12,000 under a uniform heat flux scenario of 25,000 W m−2. All experiments were performed as a single-phase fluid with cupric oxide values of 0, 0.4, 1, and 2% by volume. By reducing the twist ratio and increasing volume concentration, the average heat transfer coefficient of cupric oxide-water nano-fluid was improved. For a twist ratio of 4D, the maximum heat transfer improvement was 228% greater than the plain pipe. The presence of twisted tape with modest step ratios causes the friction factor to grow.

Performance evaluation and experimental verification of optimizing fin angles in adjustable- configuration heat sinks

Current heat sink designs typically assume uniform heat flux densities. However, applications such as integrated circuit chips and aircraft power modules often face nonuniform or even time-varying heat fluxes. Under these conditions, conventional fixed-configuration heat sinks are inadequate, and corresponding research is insufficient. This paper presents an adjustable-configuration heat sink with a rotatable fin and explores its flow and heat transfer characteristics under water cooling via simulations and experiments. Genetic algorithms optimize the fin angles for various heat flux densities to reduce the substrate temperatures, and compute numerical control (CNC) technology is used to fabricate and adjust the experimental components for optimal performance under different conditions. The proposed adjustable-configuration heat sink exhibited superior temperature uniformity and cooling performance under various heat flux density conditions. Under uniform heating, peak substrate temperatures are reduced by 15.68 and 14.01 K for the I-type and Z-type adjustable configuration heat sinks, respectively, compared to conventional multi-channel heat sink. For non-uniform heating, the optimized configurations lowered the temperatures in the high-heat regions by 2.8 to 5.1 K. This design, which incorporates an adjustable heat sink configuration, effectively adapts to varying heat flux density scenarios, making it a strong candidate for advanced thermal management applications.

An experimental study of the CHF in a square channel with non-uniformly heated rod under low pressure and low flow conditions

Experimental research on critical heat flux (CHF) was carried out in a vertical square channel with non-uniformly heated rod at low pressure and low flow conditions. The influence of inlet subcooling on CHF was also investigated. The results showed that the CHF position of the non-uniformly heat flux distribution mainly appeared in the middle and lower part of the heater rod. More significantly, it was found that CHF was enhanced with the rise of the inlet subcooling. The CHF data measured in this experiment were also assessed with the commonly used CHF empirical correlations suitable for low pressure and low flow. The results indicated that the general correlations predicting for CHF of uniform heat flux distribution were not applicable for prediction CHF of non-uniform heat flux distribution and these correlations usually overestimated the experimental values. In addition, The F-factor approach proposed by Tong was used to predict the experimental data and the error predicted by F-factor were mostly between 30 % and 50 %. Finally, F-factor was modified based on experimental data and the error between the predicted value and experimental results was between 0 and 30 %, and most of the data was between 0 and 20 %.

Modeling temporal dynamics of a radiant Casson fluid near a steeped plate emitting heat and mass fluxes

AbstractIn today's rapidly advancing world, more deliberate development and implementation of energy‐efficient thermal management systems is emergent. In industrial and engineering contexts, the regulation of heat and mass transfer phenomena is heavily swayed by factors such as heat and mass fluxes, and wall temperature, and concentration levels. In cryogenic containers, understanding heat flux and wall temperature is essential for maintaining ultra‐low temperatures over extended periods. In this context, our paper embarks on modeling the transient dynamics of a radiant Casson fluid adjacent to a steeped plate, which is featured by its emission of both heat and mass fluxes. The plate is under two thermal conditions, namely uniform wall temperature (UWT) and uniform heat flux (UHF), which are crucial in modulating both upward and downward heat transport processes. The Laplace transform (LT) technique yields close‐form solutions for the model's governing equations. The distribution of flow and salient physical quantities are graphed and elucidated in response to intricate physical parameters. A comparative graphical analysis of UWT and UHF scenarios reveals stark differences. Notably, the fluid's flow rate appears considerably amplified in the UWT scenario compared to the UHF one. Furthermore, the UHF setting profoundly influences the heat and mass transportation within the fluid's flow realm. A pronounced Casson parameter tends to thin out the velocity profile. Concurrently, a heightened radiation parameter results in a reduction in fluid temperature. The theoretical framework explored in our study is not just an academic exercise but holds tangible applicability across varied sectors. This spans from rocket chamber cooling, where fine‐tuned heat flux conditions are paramount for safeguarding operations, to industries like cosmetics and food processing, which demand meticulous thermal regulation to ensure product excellence.

Thermoelastic fracture of two-dimensional hexagonal quasicrystal media weakened by a penny-shaped crack subjected to uniformly antisymmetric heat fluxes

This study focuses on the thermoelastic fracture behavior of quasicrystals, a class of solids that exhibit unique properties between traditional crystals and amorphous materials. The complex atomic structure of quasicrystals poses challenges in understanding their fracture mechanisms under thermoelastic loading conditions. Central to our investigation is an analytical examination of a penny-shaped crack in an infinite three-dimensional body composed of two-dimensional hexagonal quasicrystals, where the upper and lower crack surfaces are applied to a pair of uniformly antisymmetric heat fluxes. This crack problem requires simultaneous consideration of thermal-phason-phonon multiple field coupling. According to the symmetry of field variables with respect to the crack plane, the thermal-phason-phonon coupled crack problem is transformed into a mixed boundary value problem in the upper half space. The extended displacement discontinuities, encompassing both phonon and phason displacement discontinuities, as well as the temperature discontinuity, are chosen as the basic unknown variables to construct the boundary integral-differential equations governing the mixed boundary value problem. Based on these boundary governing equations and Fabrikant's potential theory method, the problem with the crack surface subjected to uniform antisymmetric heat fluxes is solved. The solutions of thermal-phason-phonon fields on the crack plane, and in the full space are given in closed-form. Numerical results are employed to validate the obtained analytical solutions and visually illustrate the spatial distribution of thermal-phonon-phason coupling fields in the vicinity of the crack. The study provides fundamental insights into the behavior of cracks in quasicrystals under thermal loading, with potential implications for the design of new materials and structures.

A crack in a confocal elliptical inhomogeneity embedded in an infinite matrix subjected to uniform heat flux

We derive a series-form analytical solution to the thermoelastic problem of an insulated and traction-free crack in a confocal elliptical isotropic elastic inhomogeneity embedded in an infinite isotropic elastic matrix subjected to uniform remote heat flux. Using complex variable techniques such as conformal mapping, analytic continuation and Laurent series expansions, the original thermoelastic problem is reduced to an infinite system of linear algebraic equations, the solution of which will yield the mode I and mode II stress intensity factors at the crack tip. An exact closed-form solution is derived when the inhomogeneity and the matrix have identical shear moduli. Detailed numerical results are presented to demonstrate the series-form and closed-form solutions with an emphasis on the particular case of a vanishingly thin inhomogeneity.

Partitions influence on rectangular latent heat thermal energy storage unit performance during melting and solidification

The growing use of renewable energy sources demands efficient storage solutions due to their variability. Thermal energy storage systems utilising phase change materials are emerging as viable options to address this challenge. This study evaluates the impact of various partition types on phase change duration in thermal energy storage systems. The well-known and validated enthalpy-porosity algorithm implemented in the Fluent 2021R2 software was used. The shortest melting time was observed in the case with four vertical tubes and three horizontal partitions, which was 82 % shorter than in the reference case. The highest change in specific enthalpy of phase change material (252.5 kJ/kg) during melting was also noted for this system. The shortest solidification time was observed for the case with four tubes and two diagonal partitions, which was 72 % shorter than in the reference case. The greatest increase in the specific enthalpy of phase change material (240.4 kJ/kg) during the solidification process was also observed in this system. In conclusion, due to the thermal conductivity of the partitions, the temperature distribution throughout the domain becomes more homogeonous, thus contributing to more uniform heat flux extraction and supply throughout the process.

Effect of viscous dissipation in heating/cooling of grade three fluid in a pipe subjected to uniform surface temperature

Forced convection in Newtonian and non-Newtonian fluids flowing through pipes maintained at uniform heat flux or uniform wall temperature are important for understanding heat transfer characteristics in design and thermal management of heat exchangers. Convective heat transfer in both Newtonian and non-Newtonian fluids flowing through pipes and parallel plates, subjected to uniform wall heat flux condition, were extensively studied by researchers. But for uniform wall temperature, studies on non-Newtonian fluids in pipes are rarely considered. Forced convective heating and cooling of a third-grade fluid, flowing in a pipe subjected to uniform (constant) wall temperature is considered. Effect of viscous dissipation is included in the energy equation. Separate energy conservation equations for heating and cooling are formulated and their dimensionless forms are obtained. Numerical solutions by shooting technique are obtained for the governing equations. The same equations are also solved by the least square method and semi-analytical solutions are yielded. Least square method is a widely used semi-analytical tool used for solving non-linear differential equations. Results of the numerical solution and semi-analytical solutions are compared and are observed to be in close agreement. This validates the numerical solution. Few important observations are presented. For heating, when the non-Newtonian parameter increases from 0 – 0.1, the peak temperature drops from 1.15 – 0.55 which occurs at the centre. In case of cooling, when non-Newtonian parameter increases from 0 – 0.1, the difference in central line temperature and wall temperature increases to 0.17 from 0.09. For change in the non-Newtonian parameter from 0.2 – 0.3, both for heating and cooling the peak temperature change is not drastic. Heat transfer coefficient, in case of heating, differs by nearly 3.5 when the non-Newtonian parameter increases from 0 – 0.1.

Turbulent pipe flow and heat transfer of a binary mixture at supercritical pressure: Influences of cross-diffusion effects

Cross-diffusion effects, including Soret and Dufour effects, are enhanced around the pseudo-critical temperature (Tpc) of a binary mixture. Their influences on heat transfer at supercritical pressure have been scarcely studied. To bridge this gap, large-eddy simulations (LES) are conducted to investigate forced convective heat transfer of a CO2–ethane mixture at supercritical pressures in a circular pipe subject to a uniform heat flux. Both heating and cooling conditions, along with varying initial concentrations and thermodynamic pressures, are included in the simulations. The LES results reveal that the Soret effect causes concentration separation, resulting in a concentration boundary layer. The magnitudes of the thermodiffusion factor (kT) and the radial temperature gradient control the intensity of separation, which is more pronounced at near-critical pressure and high heat flux. Since kT is significant only around Tpc, downstream decay of the concentration separation is observed as the loci of T=Tpc migrate away from the wall so that the local radial temperature gradient diminishes. The primary factors affecting heat transfer are the variations in thermal conductivity and isobaric specific heat resulting from concentration separation. In contrast, the Dufour effect and the accompanying inter-diffusion play negligible roles. In deterioration scenarios, the bulk Nusselt number (Nub) shows a maximum relative drop of 8%, whereas in enhancement scenarios, Nub shows a maximum relative increase in 10%, with both deterioration and enhancement decaying downstream. Cross-diffusion effects have negligible impacts on density and streamwise velocity, but noticeably alter streamwise velocity fluctuation and turbulent kinetic energy.