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The application of nanofluids has exploded in recent decades to improve the local number, mean Nusselt number, and rate of heat transfer. However, boundary layer equations of nanofluid across a flat plate with radiation have not been studied, and therefore this paper studies them mathematically for the first time. For water-based copper and aluminum oxide nanofluids, a similarity solution is presented in this study, and the subsequent system of the ordinary differential equation (ODE) is numerically solved by the Runge–Kutta method in MATLAB. Two different hydraulic boundary conditions are used in the simulations. In the first, the flow across a moving plate and the direction of the flow are analyzed, while in the second, the flow over a nonlinearly moving plate in a still fluid is investigated. The nanoparticle's boundary layer thickness is found less than the thermal and hydraulics boundary layers. The local Nusselt number and friction factor of both the nanofluids are calculated and compared with the base fluid. The results demonstrate that the friction coefficient is high and the Nusselt number is low for nanoparticles with a high volume fraction. It also revealed that the friction factor for water–aluminum oxide is 16% greater than that for the water–CuO whereas the local Nusselt number for water–aluminum oxide is only 5% more than that for the water–CuO. [ABSTRACT FROM AUTHOR]

Analyzing fluid dynamics and heat transfer holds significant importance in the design and enhancement of engineering systems. The current investigation utilizes the finite element method to explore natural convection and heat transfer intricacies within a novel cavity containing an inner circular cylinder under steady and laminar flow conditions. The principal aim of this study is to assess the impact of Rayleigh number (Ra), Bejan number (Be), and the presence of adiabatic, hot, and cold cylinders on heat transfer, entropy generation, and fluid flow. The range of Ra considered in this investigation spans from 103 to 106, while the Prandtl number for the air is fixed at 0.71. The findings illustrate that the presence of a cylinder leads to higher Be as Ra increase, compared to scenarios where no cylinder is present. This observation suggests that buoyancy forces dominate in the absence of a cylinder, resulting in significantly enhanced convective heat transfer efficiency. However, the presence of a heated cylinder within the tooth‐shaped cavity exerts a substantial influence on the overall thermal performance of the system. Notably, the average Nusselt Number (Nu) experiences a remarkable increase of 41.97% under the influence of a heated cylinder, when compared to situations where a cold cylinder is present. This elevated average Nu signifies improved heat transfer characteristics, ultimately resulting in an overall improvement in the thermal system's efficiency. [ABSTRACT FROM AUTHOR]

A parabolic trough solar collector (PTSC) converts solar radiation into thermal energy. However, low thermal efficiency of PTSC poses a hindrance to the deployment of solar thermal power plants. Thermal performance of PTSC is enhanced in this study by incorporating magnetic nanoparticles into the working fluid. The circular receiver pipe, with dimensions of 66 mm diameter, 2 mm thickness, and 24 m length, is exposed to uniform temperature and velocity conditions. The working fluid, Therminol-66, is supplemented with Fe3O4 magnetic nanoparticles at concentrations ranging from 1% to 4%. The findings demonstrate that the inclusion of nanoparticles increases the convective heat transfer coefficient (HTC) of the PTSC, with higher nanoparticle volume fractions leading to greater heat transfer but increased pressure drop. The thermal enhancement factor (TEF) of the PTSC is positively affected by the volume fraction of nanoparticles, both with and without a magnetic field. Notably, the scenario with a 4% nanoparticle volume fraction and a magnetic field strength of 250 G exhibits the highest TEF, indicating superior thermal performance. These findings offer potential avenues for improving the efficiency of PTSCs in solar thermal plants by introducing magnetic nanoparticles into the working fluid. [ABSTRACT FROM AUTHOR]

The mixed convection and heat transfer in enclosures with inner heat sources widely exist in building ventilation, cooling of nuclear reactors and cooling of electronic components. In this study, the heat transfer characteristics of mixed convection in an open arc-shaped cavity with double heat sources under the condition of bottom heating and top wall cooling are studied by two-dimensional numerical simulation using ANSYS FLUENT. The influence of the Reynolds number on flow structures, transient characteristics and heat transfer characteristics in the arc cavity was studied by changing inlet wind speed. As the Reynolds number changes from 2.19 × 105 to 4.38 × 107, the flow in the fully developed stage will evolve from turbulent flow to periodic flow and then to steady state flow. In this study, the critical Reynolds number intervals for three different flow states are given. The increase in the Reynolds number enhances the heat convection in the cavity. The difference between heat convection and heat conduction is increased, which has a linear relationship with the Reynolds number. As the Reynolds number increases, the heat removal capacity in the middle region of the double heat sources is not always enhanced, which is due to the sudden change in flow structure and flow state around Re = 1.18 × 107. [ABSTRACT FROM AUTHOR]

The transitional free convection flow and heat transfer within attics in cold climate are investigated using 3-D numerical simulations for a range of Rayleigh numbers from 103 to 106 and height-length ratios from 0.1 to 1.5. The development process of free convection in the attic could be classified into three-stages: an initial stage, a transitional stage, and a fully developed stage. Flow structures in different stages including transverse and longitudinal rolls are critically analyzed in terms of the location and strength of convection rolls and their impacts on the heat transfer. The transition unsteady flow and asymmetry flow in the fully developed stage is discussed for the fixed height-length ratio 0.5. Various flow regimes are given in a bifurcation diagram in the parameter space of Rayleigh numbers (10² < Ra < 107) for heightlength ratios (0.1 < A < 1.5). The time series of heat transfer rate through the bottom wall is quantified for different height-length ratios. The overall heat transfer rate for the low Prandtl fluid (Pr = 0.7) could be enhanced based on 3-D flow structure. [ABSTRACT FROM AUTHOR]

Heat transfer between inhaled hot/cool air and the lung surface within the human respiratory system is an intriguing topic that has not received enough attention. The lung can be considered an in vivo heat exchanger, balancing the inhaled air temperature by lowering the hot air temperature and increasing the cool air temperature. The current work studies the unsteady and incompressible airflow motion and heat transfer during inhalation between the surface of the lungs (37 °C) and the inhaled cool air (25 °C) in one case and inhaled hot air (43 °C) in another. Computerized tomography scan (CT-scan) images of the lung of a 39-year-old male patient were processed to generate the airway geometry consisting of 16 generations. The geometry was further modified in UG NX 12.0, and the mesh generation was carried out using Ansys Meshing. The shear stress transport (SST) k − ω turbulent model was employed in Ansys Fluent 20.2 to model the air/lung convective volume heat transfer utilizing a realistic breathing velocity profile. Temperature streamlines, lung volume temperatures, surface heat flux, and surface temperatures on all 16 generations were produced for both cases during the breathing cycle of 4.75 s. Several conclusions were made by studying and comparing the two cases. First, heat transfer between inhaled hot or cool air and the lung surface mainly occurred in the first few generations. Second, airflow temperature patterns are dependent on the inlet breathing velocity profile. Third, the lung volume temperature change directly correlates with the temperature difference between air and the lung surface. Finally, the surface heat flux strongly depended on the heat transfer coefficient. The density, viscosity, thermal conductivity, and specific heat of hot/cool air affected the Reynolds number, Nusselt number, heat transfer coefficient, and surface heat flux. [ABSTRACT FROM AUTHOR]

Natural convection from a duct into the quiescent ambient is investigated. Heat transfer and dynamics of natural convection from the duct are discussed using a simple scaling analysis and the corresponding scaling relations are obtained. Additionally, numerical simulation for a wide range of Rayleigh numbers from 100 to 106 and the duct aspect ratios from 0 to 4 with the Prandtl number of 0.71 (air) is performed. The development of natural convection may approach a steady or an unsteady state in a fully developed stage, which is determined by the Rayleigh number and the duct aspect ratio. It is demonstrated that heat transfer of natural convection is improved by the duct for high Rayleigh numbers but depressed for low Rayleigh numbers. The scaling relations of natural convection from the duct have been validated in comparison with numerical results. The scaling predictions are consistent with the numerical results. Furthermore, the formulae of the Nusselt number, the Reynolds number, and the flow rate quantifying natural convection from the duct are presented. [ABSTRACT FROM AUTHOR]

Mixed convection laminar two-dimensional boundary-layer flow of non-Newtonian pseudo-plastic fluids is investigated from a horizontal circular cylinder with uniform surface heat flux using a modified power-law viscosity model, that contains no unrealistic limits of zero or infinite viscosity; consequently, no irremovable singularities are introduced into boundary-layer formulations for such fluids. The governing boundary layer equations are transformed into a non-dimensional form and the resulting nonlinear systems of partial differential equations are solved numerically applying marching order implicit finite difference method with double sweep technique. Numerical results are presented for the case of shear-thinning fluids in terms of the fluid temperature distributions, rate of heat transfer in terms of the local Nusselt number. [ABSTRACT FROM AUTHOR]

Numerical investigation on mixed convection of a two-dimensional incompressible laminar flow over a horizontal flat plate with streamwise sinusoidal distribution of surface temperature has been performed for different values of Ray leigh number, Reynolds number and frequency of periodic temperature for constant Prandtl number and amplitude of periodic temperature. Finite element method adapted to rectangular nonuniform mesh elements by a nonlinear parametric solution algorithm basis numerical scheme has been employed. The investigating parameters are the Rayleigh number, the Reynolds number and frequency of periodic temperature. The effect of variation of individual investigating parameters on mixed convection flow characteristics has been studied to observe the hydrodynamic and thermal behavior for while keeping the other parameters constant. The fluid considered in this study is air with Prandtl number 0.72. The results are obtained for the Rayleigh number range of 10² to 104, Reynolds number ranging from 1 to 100 and the frequency of periodic temperature from 1 to 5. Isotherms, streamlines, average and local Nusselt numbers are presented to show the effect of the different values of aforementioned investigating parameters on fluid flow and heat transfer. [ABSTRACT FROM AUTHOR]

• This study considers both conduction and convection heat transfer by introducing a uniformly thick top wall. • The thermal stratification between the top and bottom walls generates thermal convection. • This study indicates a 46.91 % rise in entropy generation for Re ranging from 500 to 1000. • A decrement of 88.31 % is observed in exergy generation with the greatest amplitude in the case of the lowest heat transfer. The main focus of this article is on analyzing the entropy generation characteristics of an air-filled hexagonal thermo-fluid system with an adiabatic clockwise rotating dynamic modulator through second law analysis. This modulator rotation creates striations that affect natural convection. This convection is generated by the thermal stratification between the top and bottom walls. To fully analyze the system, the study considers both conduction and convection heat transfer by introducing a uniformly thick top wall. Overall, the study looks at the behavior of a conjugate thermo-fluid system. The system is solved numerically using non-dimensional Navier Stokes equations as a moving mesh problem by the Arbitrary Lagrangian Euler finite element method. The parameters whose effects have been conceptualized here are 102 ≤ Re ≤ 103, 104 ≤ Ra ≤ 107, and 103 ≤ Bi ≤ 104. Air is chosen to be the working fluid with P r of 0.71. Effects of these parameters are visualized by spatially averaged total entropy generation, spatially averaged total exergy generation, spatially averaged Bejan's number, and fast Fourier transform analysis. The present study indicates a 46.91 % rise in entropy generation for R e ranging from 500 to 1000. Moreover, for the same case, a decrement of 88.31 % is observed in exergy generation with the greatest amplitude in the case of the lowest heat transfer. This phenomenon is further supported by the amplitude of the fundamental frequency, where the thermodynamic configuration with the lowest heat transfer has the highest peak. As far as the authors are aware, no previous research has been conducted to analyze the entropy generation characteristics of a hexagonal conjugate transient system that involves a dynamic modulator stirring the flow. Therefore, this study represents a novel contribution to the existing literature on the subject. Present study focused on only a single type of blade irrespective of its aerodynamic design, size, or thickness. Moreover, the blade is also considered to be a rigid body that is not affected by the hoop stress generated from its own rotation. [ABSTRACT FROM AUTHOR]

This study provides an in-depth analysis of the heat transfer (HT) characteristics of a hybrid nanofluid (HNF) composed of Multi-walled carbon nanotubes (MWCNT), Iron oxide (Fe 3 O 4), and Ethylene glycol (EG), which is loaded in an enclosure with a heated cylinder. As per the exploratory report, this HNF exhibits Newtonian and non-Newtonian behavior depending on the percentage of nano-constituents suspended in the base fluid EG. The governing equations describing the convective HNF flow associated with the boundary conditions are numerically solved using the finite volume method. The study explores the HT performance of a loaded HNF by varying the volume fractions (ϕ), the driving condition of the vertical sides, and the aspect ratio (W ∗) of the enclosure. The study investigates the variation of the Richardson number (R i = 0. 01 ∼ 100) concerning the interactions between shear and buoyancy forces executed on the HNF. The study finds that the combination of the lower aspect ratio W ∗ = 2. 1 filled with non-Newtonian HNF with ϕ = 1. 8 % in the forced convection regime (R i = 0. 01) and the condition of both aiding and opposing forces applied to the side walls shows superior HT performance. The non-Newtonian HNF with ϕ = 1. 8 % shows HT enhancement of 9.27% for aiding forces and 39.63% for opposing forces when R i = 0. 01 , revealing ultimate enhancement in the average Nusselt number (N u a v g ). The study also investigates entropy production (S a v g) from a thermodynamics perspective to assess the system's thermal performance. The value of S a v g is about 11.54 for ϕ = 1. 8 % and R i = 0. 01 , and that value is comparable for both aiding and opposing force conditions. Results conclude that HT consistently enhances with nanoparticle addition in the forced convection regime. However, HT enhancement is only observed for non-Newtonian cases of HNF in the natural convection regime, suggesting the limitation of the HNF usage in the HT study. Additionally, the study discusses and compares different simulation results to comprehensively evaluate the HT characteristics and the system's thermal performance, guiding the best benefit of the studied hybrid nanofluids in a thermal engineering system. [ABSTRACT FROM AUTHOR]

Abstract: Numerical study has been performed in this study to investigate the turbulent convection heat transfer on a rectangular plate mounted over a flat surface. Thermal and fluid dynamic performances of extended surfaces having various types of lateral perforations with square, circular, triangular and hexagonal cross sections are investigated. RANS (Reynolds averaged Navier–Stokes) based modified k–ω turbulence model is used to calculate the fluid flow and heat transfer parameters. Numerical results are compared with the results of previously published experimental data and obtained results are in reasonable agreement. Flow and heat transfer parameters are presented for Reynolds numbers from 2000 to 5000 based on the fin thickness. [Copyright &y& Elsevier]

Abstract: Natural convection of a two-dimensional laminar steady-state incompressible fluid flow in a modified rectangular enclosure with sinusoidal corrugated top surface has been investigated numerically. The present study has been carried out for different corrugation frequencies on the top surface as well as aspect ratios of the enclosure in order to observe the change in hydrodynamic and thermal behavior with constant corrugation amplitude. A constant flux heat source is flush mounted on the top sinusoidal wall, modeling a wavy sheet shaded room exposed to sunlight. The flat bottom surface is considered as adiabatic, while the both vertical side walls are maintained at the constant ambient temperature. The fluid considered inside the enclosure is air having Prandtl number of 0.71. The numerical scheme is based on the finite element method adapted to triangular non-uniform mesh element by a non-linear parametric solution algorithm. The results in terms of isotherms, streamlines and average Nusselt numbers are obtained for the Rayleigh number ranging from 103 to 106 with constant physical properties for the fluid medium considered. It is found that the convective phenomena are greatly influenced by the presence of the corrugation and variation of aspect ratios. [Copyright &y& Elsevier]

– Forced convection heat transfer in a cavity is of great importance due to its strong relevance to the practical aspects. Most of the studies focus on this topic from a steady perspective. But adding a flow modulator enhances a system's thermodynamic performance and makes such a system more realistic and widens the field of application. Such studies are unsteady and often quite expensive. As a result, only a few literatures are available with very idealistic geometric configuration and boundary conditions. – Two-dimensional unsteady continuity, momentum and energy equations are used for the mathematical modeling of the problem incorporating the Boussinesq approximation. A free triangular discretization scheme is adopted to solve the moving mesh problem by formulating the Arbitrary Lagrangian Euler (ALE) finite element approach. – Numerical studies are carried out for a fixed Prandtl number (Pr = 0.71) and fixed geometry of the rotating blade while varying the other parameters i.e. Rayleigh number, Reynolds number and Biot number. This dynamic boundary problem encompasses a wide range of parameters i.e. (100 ≤ Re ≤ 103), (103 ≤ Bi ≤ 104) and (104 ≤ Ra ≤ 107) to evaluate the thermodynamic behavior of the thermo-fluid system. External flow condition is taken into consideration in terms of Biot number. Effects of these parameters are visualized through streamlines, heatlines, spatially average Nusselt number evaluated on the heated surface and system effectiveness. – Present computational study focuses on the transient analysis of the conjugate forced convection flow and heat transfer characteristics in a hexagonal, air filled cavity. This cavity is equipped with a floor heater of constant heat flux under the rotational influence of an adiabatic flow modulator. And the blade is placed in the central position of the cavity which is rotating in the clockwise direction. Also, the whole computational domain is composed of four different domains, one convective domain and three solid domains. The solid domains are made of brick and glass as per their practical aspects. – A Fast Fourier Transform (FFT) analysis is presented to comprehend the thermo-oscillating system response. FFT plots indicate that for all of the cases the fundamental frequency of the system response conforms to the blade frequency. Moreover, present numerical results show that the heat transfer effectiveness has an inverse relationship with the Biot and Rayleigh numbers while improves significantly with the increase of Reynolds number and reaches a critical state for Re cr = 650. Higher Reynolds number also attenuates the degree of power spectrum. – To attain a higher heat transfer effectiveness the thermo – fluid system should be operated at low Rayleigh number with higher Reynolds number. [ABSTRACT FROM AUTHOR]

Natural convection and heat transfer in triangular cavities are widely used in the thermal engineering of buildings and heat dissipation in electronic devices. In this study, we analyze dimensionally the development of thermal flow for Pr > 1 in a triangular cavity under top-cooling conditions. The formation and development of the boundary layer under a sloping wall are given in terms of instantaneous flow and heat transfer characteristics. The dimensional dependence of the thickness of the thermal boundary layer on time, as well as the timescale of the balance between convection and heat conduction, are also presented. We obtain the volume flow rate relation for the bottom horizontal intrusion flow. The variation of the flow dynamics and heat transfer control mechanisms over time is presented, the thickness and velocity dimensions of the horizontal intrusion flow at different stages are obtained, and the horizontal distance travelled by each mechanism is predicted. For the development of the initial plume, the virial relation between the initial plume thickness and the velocity is presented for different mechanisms. The dynamical and heat transfer control mechanisms of horizontal intrusions and plumes are summarized, the timescales of convective and heat conduction equilibria are given for different dynamical mechanisms, and the flow laws of intrusions and plumes in transient processes are discussed. It provides a reference for natural convection in triangular cavities. [ABSTRACT FROM AUTHOR]