Convective Heat Transfer Rate Research Articles

Numerical simulations of third-grade fluid flow with exothermic reactions in porous media-saturated vertical microchannels

The aim of this article is to analyze the electro-osmotic flow behavior of a third-grade (TG) incompressible fluid flowing in a vertical microchannel saturated with porous material. The electro-osmotic flow is regarded as an essential part of many scientific and engineering applications used in microfluidic devices. The flow is subjected to joule heating, exothermic reactions, and asymmetrical convective cooling. The viscosity which is assumed temperature dependent via a Naham-type law and the convective heat transfer rates governing the asymmetric convective cooling at the microchannel walls are modeled via Newton’s law of cooling. The governing system of nonlinear partial differential equations is solved computationally using an efficient and reliable finite difference method. The spatial and temporal convergence of the numerical scheme is showcased graphically. Our study findings reveal that the combination of exothermic reactions and Joule heating significantly influences the electroosmotic flow. Higher values of variable viscosity parameter (α), Brinkman number (Br), and Buoyancy parameter (Gr) result an enhancement in the flow variables. Conversely, a reverse trend is observed when augmenting the Prandtl number (Pr), non-Newtonian parameter (γ), and porous media parameters (Ω2). Numerical results for temperature and velocity are also discussed qualitatively and validated with the data previously published.

Direct Numerical Simulation of Mixed Convection Flow in Lid-Driven Cavities

The mixed convection of heat transfer and fluid flow in a - lid drivencubical cavity filled with air is investigated numerically in this study. The computational procedure is based on the finite volume method and a full multigrid acceleration solver. The top wall of the cavity is maintained at a constant high temperature - Th, and it can move with a constant velocity U0. The bottom wall is immobile and maintained at a cold temperature Tc. While, the remaining boundary parts of the cavity are motionless and kept thermally insulated. Several numerical simulations were conducted to investigate mixed convection heat transfer in a sliding cubical cavity for a range of Reynolds numbers from 1000 to 5000 and Richardson numbers from 0.001 to 10. The influence of mixed convection parameters, Reynolds number, Richardson number, and heat transfer rate on the flow behavior was analyzed through parametric studies. The results include flow and heat transfer characteristics, iso-surfaces, and streamlines for the entire range of Richardson numbers and Reynolds numbers investigated. The study shows that as Reynolds number is increased beyond a critical value, the flow becomes unstable and bifurcates.

Experimental and numerical analysis of PV-PCM integrated with novel shaped corrugated fins

In this paper, performance of a PV-PCM system with novel internal corrugated fins is analysed by dynamic simulation and experiments for the inclinations of 0°, 15°, and 30° against a flux of 750 W/m2 using a solar simulator. The impact of fins (n) is investigated numerically, and the optimal number of fins is found. Performance parameters such as temperature, power output, and efficiency are compared for PV, PV-PCM, and finned PV-PCM systems. The validity of the numerical model is established by comparing the simulated and experimental PV temperatures, which reduces cost and time and allows the numerical models to optimise the design and performance of the system. Results showed that an increase in inclination leads to improved performance of the PV due to an increased convection heat transfer rate. The effect of the fins is pronounced at lower inclinations. Corrugated fins enhance the heat transfer rate compared to traditional flat fins, and the heat transfer enhancement characteristics of corrugated fins within the finned PV-PCM system can provide valuable insights for optimised performance. Compared to the PV-only system, the maximum PV temperature reductions for PV-PCM and finned PV-PCM systems are 30.1 °C and 38.6 °C for θ=0° and 25.23 °C and 34.1 °C for θ=30°. The optimal number of fins is 4 for all inclinations. The maximum efficiency enhancement for optimal systems decreased from 28.15 % at θ = 0° to 22.27 % at θ = 30°. Finally, the experimental performance of finned PV-PCM at a latitude corresponding to Calicut, India, is also conducted.

Effect of humidification of combustion products in the boiler economizer with spiral geometry

In light of rising fuel prices and climate change in recent decades, improving the performance of heat recovery equipment such as boiler economizers is critical. Currently, numerical work is being done to improve the performance of the economizer with spiral geometry by humidifying combustion products. Simulations are performed using the finite volume method (FVM) and, in three cases, in a two-dimensional, steady, single-phase, and turbulent condition. In the first case, water vapor condensation and liquid water spray are not considered. In the second case, the impact of water vapor condensation is considered, and the effect of liquid water spray is not considered. In the third case, the water vapor condensation and the liquid water spray are simultaneously considered. Experimental correlation relations are used to predict the condensation rate in the second and third cases. The results are presented and discussed in the form of Nusselt number, convection heat transfer coefficient (h), outlet temperature, and heat transfer rate for different inlet temperatures of combustion products. Moreover, velocity, pressure, temperature contours, and velocity vectors are obtained and analyzed. The results showed that the cold-water outlet temperature in the third case was 2.85% higher than in the first case. In addition, a maximum improvement of 1.39% in the third case is obtained compared to the second case. Also, the heat transfer rate in the third case is improved by 8.2% compared to the first case and by 3.9% compared to the second case. The proposed method in the present work can be used to predict the effect of liquid water spray on the performance of boiler economizers.

Steady flow of non-Newtonian fluids: Momentum and heat transfer around a rectangular cylinder

This research aims to explore the impact of cross-sectional aspect ratio (AR), power-law index (n), Reynolds number (Re), and Prandtl number (Pr) on convective heat transfer rates using the Nusselt number (Nu) and coefficient of drag (CD) as a key metric. Numerical simulations employ the ANSYS FLUENT solver with the finite volume method. The SIMPLE scheme couples pressure and velocity, and the QUICK differencing scheme discretizes convective terms. This study varies the aspect ratio by adjusting the length-to-width ratio (b/D) to explore different configurations of AR = 1–5 and Pr = 0.7, 1, and 7, which investigate 5 ≤ Re ≤ 10 and 0.2 ≤ n ≤ 1, respectively. An increase in the Reynolds number is observed to diminish the thermal boundary layer thickness, resulting in an escalation of heat transfer rates. Additionally, the Nusselt number exhibits an upward trend with an increase in the Prandtl number. The investigation reveals a decrease in Nu with an increase in both AR and n. Specifically, higher aspect ratios and power-law indices are associated with reduced heat transfer rates. The AR emerges as a significant factor, influencing the coefficient of drag, with CD values showing an increase as AR rises, particularly at low Re. Moreover, CD is found to increase with an elevation in the n value. This study explores new ground by investigating power-law fluid behavior and heat transfer phenomenon across varied aspect ratios. These insights not only enhance the design of heat transfer systems with non-Newtonian fluids but also pave the way for future research in this less explored domain.

Experimental comparative study on thermal performance of latent heat storage tanks with pin, perforated, and rectangular fins at different orientations

The low thermal conductivity of phase change materials (PCMs) has limited their widespread use in practical applications. In the present study, different fin structures, namely, rectangular, perforated, and pin were examined to analyze the thermal performance of the melting process in rectangular latent heat storage tanks. Experiments were performed at both horizontal and vertical orientations to evaluate the effectiveness of different fin configurations. Visual observation of the phase change evolution at different time intervals was enabled through a transparent plexiglass shell. Instantaneous heat transfer rate and energy storage were measured using thermocouple readings and melting photographs. The results show that the maximum heat transfer coefficient between the heated wall and PCM is obtained by the pin-finned tank followed by perforated and rectangular-finned tanks. This thermal behavior is justified by the intensification of the upward convection flows through the voids provided by pin fins or perforated fins. Although the rectangular fin structure has the lowest convective heat transfer coefficient, its heat transfer rate is slightly higher than the other structures due to its larger heat transfer area. At a wall temperature of 70℃, the convective heat transfer coefficient and heat transfer rate obtained by the pin fin configuration are respectively 25% higher and 4% less than those of the rectangular fin. It reveals that the pin fin structure provides the most effective heat transfer area compared to its counterparts which have a significantly larger fin volume. In addition, it was found that regardless of the fin configuration, the melting rate in the horizontal tank was significantly higher than in the vertical tank due to the formation of more vortical flow structures within the molten PCM. The melting time in the unfinned horizontal tank was less than those of the vertical finned tanks implying that the tank orientation should be well-chosen to minimize the melting time along with adding fins of various configurations.

Evaluation of the electrical parameters and performance of floating PV generators

This study provides evaluation of floating photovoltaics (PV) in the Brazil tropical climate and discusses the specific technical and environmental benefits and limitations. This paper develops a model simulating the annual performance of the photovoltaic generator of a floating photovoltaic plant as a function of a given conditions. The reference is a 1.2-MWp floating-PV system commissioned in 2023 near the city of Grão Mogol, Brazil, in the reservoir of the PCH Santa Marta hydropower plant. The influence of the ambient meteorological and marine parameters on the PV module temperature, current, voltage, and power were evaluated. The simulation uses a reference crystalline-Si PV module and the Engineering Equation Solver (EES). Relevant experimental data, including incident solar radiation, ambient temperature, and wind speed were used as input data for the model. The effect of these parameters on the thermal end electrical parameters was assessed. Although small variations were found throughout the year, significant hourly and daily variations were observed, depending on solar irradiation and ambient and resulting module surface temperatures. The voltage at the maximum power decreases with the increase of the solar module surface temperature. The convective heat transfer rates are higher than the radiative heat transfer rates. This study provides a first-time complete energy and exergy analysis of a floating PV system (FPVS) incorporating the various heat transfer rates, electrical and irradiance parameters, under climate and meteorological conditions for this Brazil location.

Comparative Heat Transfer Performance of TPMS Structures: Spotlight on Fisch Koch S versus Gyroid, Diamond and SplitP Lattices

This study assesses the thermal behavior of different TPMS configurations, specifically examining the Fischer Koch S lattice in comparison to Gyroid, Diamond, and SplitP lattices. Utilizing numerical simulations alongside experimental validation, the research delves into the efficacy of different TPMS designs in enhancing convective heat transfer and fluid dynamics. Results indicate that the Fischer Koch S lattice outperforms other models, exhibiting enhanced convective heat transfer rates and increased Nusselt numbers due to greater areas of contact between the fluid and the solid surfaces and optimized flow patterns. Nevertheless, it also demonstrates significant pressure drops at high Reynolds numbers. These findings underscore the necessity for further research to refine performance based on these insights.

Heat Transfer Analysis for Combustion under Low-Gradient Conditions in a Small-Scale Industrial Energy Systems

The issue of maintaining low-gradient combustion in the conditions of high heat extraction has been investigated numerically in this work. The analyses include the application of a convective boundary condition at the wall (with estimated boiling heat transfer coefficient); analysis of the Internal Recirculation Device’s impact on combustion products and heat transfer under low-gradient conditions; and comparison of both traditional and low-gradient combustion modes. It was shown that the Internal Recirculation Device material and geometry has a significant impact on the nitrogen oxide (NOx) formation mechanism, as NO2 emission becomes predominant and can rise up to several hundreds ppm. What is more, along with decrease in thermal resistance of the IRD, CO emissions also increase rapidly, even achieving over 2000 ppm. Additionally, the convective heat transfer rate decreased by about 25% after switching from traditional to low-gradient combustion, whereas the radiative mechanism increased by ≈40% compared to traditional mode. It should also be mentioned that the low-gradient combustion applied in this work achieved approximately 10% higher efficiency than conventional combustion.

Heat removal and ventilation limitations of the solar chimney attached with a built enclosure: Correlations of thermal Rayleigh numbers, port arrangements and discrete heating elements

For energy efficient ventilation and low carbon design, solar chimney has been extensively applied on buildings. In the present work, three representative solar chimneys were introduced, i.e., Model 1 with top-horizontal outlet, Model 2 with upper-vertical outlet and Model 3 with middle-vertical outlet. Updraft ventilation from the built enclosure to the outlet and heat transfer from the absorber to the fluid by heat convection were numerically and theoretically investigated, and their limitations and correlations with governing parameters were also discussed. Streamlines and heatlines were used to visualize transport paths of air and heat respectively. Reverse flow could not be observed in the solar chimney channels of Model 2 and Model 3; Therefore, total volume flow rate induced by the solar chimneys Model 2 and Model 3 and convection heat transfer rate of the absorber increases positively with thermal Rayleigh numbers (Ra). Model 2 chimney produces greater thermal buoyancy than that by Model 3, and its air flow rate is higher than that of Model 3. Generally, volume flow rate induced in Model 1 is higher than those of Model 2 and Model 3 as Ra is no more than 2.5 × 106, and simultaneously, convection heat transfer rate Nusselt number (Nu) is also greater than those of Model 2 and Model 3. When Ra exceeded 2.5 × 106, reverse air flow was intensified further, Nu of Model 1 started to decline. The three models with discrete heat sources could induce more air from indoor than that respective model without discrete heat sources. The maximum increment rate is obtained by the Model 1 solar chimney with discrete heat sources, then Model 2 and the least done by Model 3. Their power relationships between Nu and Ra, V*and Ra are regressed respectively. The dependence of volume flow rate on broad thermal Rayleigh numbers covering all the three flow regimes showed that there were linear correlations between discrete numerical volume flow rates and the scaling predictions, except in Model 1 for high Ra (exceeding critical value 9.0 × 105), where volume flow rate was remarkably reduced due to reverse air flow. This research could provide useful theoretical information for applying small sized vertical solar chimneys attached to the building sidewall to effectively reduce built energy consumption.

MHD flow of third-grade fluid through a vertical micro-channel filled with porous media using semi implicit finite difference method

This study examines the complex dynamics of an incompressible, electrically conducting third-grade fluid (TGF) flow through a vertical micro-channel that is filled with porous media. The research utilizes asymmetrical convective cooling, a transverse magnetic field, and exothermic reactions to introduce a comprehensive model. The viscosity of the fluid, which is determined by the Naham-type law, changes significantly with temperature. Additionally, the convective heat transfer rates, which govern the asymmetric convective cooling at the surfaces of the micro-channel, are modeled using Newton's law of cooling, and Modified Darcy law is used to address the porous medium effect. To solve the complex system of nonlinear partial differential equations, we employ computational solutions derived from reliable and efficient finite difference methods (FDM). The method is tested for different time-steps and mesh sizes and it is found that the algorithms reliably produce the same results. Our investigation includes an evaluation of both spatial and temporal convergence of the FDM scheme. The study present qualitative discussions of graphical results, that highlight the impact of various flow parameters integrated into the system. The velocity and temperature profiles increases for higher values of thermal Grashoff number Gr and Reynolds number Re while opposite effect is noticed for increasing values of Hartman number M.

A general approach for predicting convective heat transfer coefficients in turbulent systems

A generalized approach for predicting turbulent convective heat transfer coefficients using large eddy simulation is presented. This model is based on semi-empirical transport theory, which links the local convective heat transfer rate to the local energy dissipation rate within the near-wall boundary layer. We apply these approaches to agitated tanks, pipe flow systems, cylinders in crossflow, and tube bundles. We validate the generality and reliability of the theoretical model against expectations from experimentally derived empirical design correlations.

The role of nozzle position and channel aspect ratio in thermal management of electronic modules

In this paper, cooling of electronic modules by a series of impinging jets is studied numerically and experimentally. The study investigates the effects of nozzle position (sj/b = 0 − 0.25 − 0.5) and channel aspect ratio (H/Dj = 3 − 4 − 5) on surface temperatures, local and average heat transfer characteristics and flow dynamics. The numerical analyses are conducted using the ANSYS Fluent software, and the numerical results are verified using experimental measurements. The study's findings indicate that the displacement of the nozzles has a notable impact on the location and sizes of the vortex cells around the modules. This alteration influences the rate of convection heat transfer from the modules’ surfaces. It is concluded that increasing the channel aspect ratio leads to a reduction in convective heat transfer. The highest cooling performance is achieved in the scenario where the aspect ratio (H/Dj) is 3 and the dimensionless nozzle position (sj/b) is 0.5. However, in terms of temperature uniformity, this is H/Dj = 3 with sj/b = 0.

A non-Newtonian thermal lattice Boltzmann method for simulation of Rayleigh–Bénard convection of power-law fluids

Despite the widespread popularity of the Bhatnagar–Gross–Krook lattice Boltzmann (BGK-LB) model due to its simplicity and efficiency, its application in heat transfer involving non-Newtonian fluids (NNFs) has been limited by inherent constraints. This paper proposes a numerically stable BGK-LB model for the thermal flow of NNFs. The modified model incorporates the local shear rate into the equilibrium distribution function of the velocity field and addresses the numerical instability problems encountered in the traditional BGK-LB model under low viscosity conditions by introducing an additional parameter. In addition, a temperature evolution equation that can accurately recover the convective diffusion equation is adopted. The accuracy of the current method is validated by performing simulations of Rayleigh–Bénard convection (RBC) in a square cavity filled with Newtonian fluids and NNFs. Subsequently, simulations are conducted to investigate the behavior of RBC in power-law fluids. The analysis focuses on examining the impact of the Rayleigh number (Ra = 5 × 103 − 105) and the power-law index (n = 0.8–1.3) on the convective structure and heat transfer characteristics while maintaining a fixed Prandtl number (Pr = 7) and aspect ratio (L/H = 2). It is discovered that, for a given n value, the convection intensity and heat transfer rate increase with increasing Ra, which is supported by the increasing trend of the mean Nusselt number (Nū) with Ra. Furthermore, compared to NFs, pseudo-plastic fluids display a higher Nū value due to an augmented heat transfer rate, while dilatant fluids exhibit a lower Nū value owing to a diminished heat transfer rate.

Magneto-double-diffusive natural convection and irreversibility analysis of a nanofluid flowing in an annular concentric space

In the present article, magneto-double-diffusive density driven convection accompanied by an entropy optimization analysis of Al2O3-water nanofluid filled in a horizontal coaxial annular space is numerically addressed. The double-diffusive natural nanofluid flow which is produced due to the maintaining of constant and different temperature and solute on the inner and outer cylinders is affected by a uniform vertical magnetic field. Dimensionless governed equations which are developed through law of conservation of mass, momentum, and energy are numerically solved via finite volume approach, and parametric control analysis is developed. By using Corcione empirical correlations considering the Brownian diffusion of 4% of Al2O3 nanoparticles in water, the thermal conductivity and viscosity of the mixture nanofluid are determined. Results are performed and interpreted in detail for wide range of relevant variables: Rayleigh (102 ≤ Ra ≤ 105), Hartmann (0 ≤ Ha ≤ 30), Lewis numbers (0 ≤ Le ≤ 10), and Buoyancy ratio (−4 ≤ N ≤ 4) to evaluate the double-diffusive convective nanofluid flow, heat and solute transfer rates as well as entropies produced in the system.

Enhanced laminar heat transfer via magnetically driven ferrofluids

Many industries, such as microelectronics and power electronics, face challenges in removing large amounts of heat from small areas to prevent overheating. Recent research has shown that microchannel flow of ferrofluids driven by an external magnetic field (MF) has the potential to deliver efficient cooling systems for modern electronics. Despite this potential, robust optimisation of the MF has not been undertaken. This paper addresses this shortcoming by reporting the effect of the MF produced by three permanent magnets in various configurations on the heat transfer of ferrofluids undergoing laminar flow through a microchannel under constant heat flux boundary conditions at Peclét number Pe=73. Observations from numerical simulation show that the rate of convective heat transfer varies strongly with position and orientation of the magnets. We show that for the optimum magnet configuration, the maximum and average Nusselt numbers increase by more than 146% and 51% over the unperturbed flow, some of the highest values reported in the literature. These results show that significantly accelerated heat transfer is possible in magnetically driven ferrofluids, with potential for further enhancement with more rigorous optimisation.

Study of thermal convection in chemically reactive carbon nanotube suspension saturated in anisotropic porous enclosure

This study aims to investigate the influence of chemical reactions and anisotropic porous material on the convective instability, heat and mass transfer rate of water-based carbon nanotube suspension. Flow governing dynamics are modeled using the modified Brinkman–Buongiorno model. The effects of pertinent flow characterizing parameters such as chemical reaction parameter, porosity parameter, mechanical anisotropy parameter and thermal anisotropy parameter on the threshold of convection, heat and mass transport rate are discussed and compared for three types of enclosures: shallow, square and tall. The study concludes that nanoliquid suspended with single-walled carbon nanotubes has higher heat and mass transfer capability than the multi-walled carbon nanotubes when saturated in a tall porous enclosure and also tall enclosure allows the convection to set in earlier. Anisotropic effect and destructive chemical reaction delay the starting of convection. Further, it is observed that the heat transfer rate decreases with the chemical reaction parameter.

Influence of Nanoparticles and Magnetic Field on the Laminar Forced Convection in a Duct Containing an Elastic Fin

In the present paper, an investigation of the effect of a magnetic field and nanoparticles suspended in pure water on the forced flow in a duct containing an elastic rectangular fin is performed. The nanofluid, i.e., CuO nanoparticles suspended in water, flow in the duct with an inlet fully developed velocity profile and a cold temperature. The lower boundary of the duct is kept at a hot temperature, while the upper boundary is adiabatic. According to the ALE formulation, numerical simulations of the laminar flow are carried out, by employing the software package Comsol Multiphysics, to solve the governing equation system: mass, momentum, energy, and deformation. The behavior of the Nusselt number, of the temperature and velocity fields as well as of the stress profiles are presented and interpreted. As a result, the addition of CuO nanoparticles to pure water improves the local and global heat transfer rate by up to 21.33% compared to pure water. On the other hand, it causes an additional deformation of the elastic fin as well as the increase of the stress due to the presence of the nanoparticles, leading to an increase of its maximum displacement of 34.58% compared to the case of pure water flow. Moreover, the enhancement of the flexibility of the fin (and thus its deformation) leads to a relative reduction in terms of convective heat transfer rate, especially downstream of the fin.

Computational Framework Development for Heat Transfer Studies in Liquid Metal-Cooled Small-Scale Heat Sinks With Non-Circular Cross Sections

Abstract The present work provides a reliable computational framework to investigate the laminar and turbulent forced convection of sodium and sodium–potassium (Na, NaK) in small-scale heat sinks with hydraulic diameters between 1 mm and 5 mm. Na and NaK flow and heat transfer are studied numerically for a wide range of Reynolds numbers from 600 to 9000 in three sharp-cornered miniature heat sinks with rectangular, pentagonal, and hexagonal cross sections. For a fixed surface area to volume ratio, it is observed that the rectangular minichannel heat sink provides the highest convective heat transfer rates. The rectangular miniature heat sink is shown to provide 280% higher convective heat transfer rates in comparison with the pentagonal heat sink. Moreover, the obtained convective heat transfer coefficients for NaK are almost 20% higher than the ones for Na in the investigated pentagonal heat sink. For the same flow Peclet number in the rectangular and hexagonal heat sinks, both Na and NaK provide nearly identical average Nusselt numbers. However, NaK shows greater local and average Nusselt numbers compared to Na at the same Reynolds number.

The effect of fin structure on forced heat convection in a pentagonal channel

he main purpose of this study is to examine the effect on forced heat convection properties of fin structures in a pentagonal duct. For this purpose, rectangular shaped fins are placed in the selected duct. The study was carried out under the constant heat flux (q’’=500 W/m2) and fully developed turbulent flow (10000<Re<40000) conditions. As a result of the study, an improvement in velocity distribution and convective heat transfer rate was observed with increasing Reynolds number and the use of rectangular fins. In addition, with the use of fins, the movement area of the flow was narrowed and an increase in the Darcy friction factor was observed.