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Further Consideration On The Two-pass Channel Flow Is Still Necessary Due To The Complexity Of 180-deg Turn, Rotation, And Wall Ribs. Numerical Investigations Have Repeatedly Revealed Differences From Experiments, With The Primary Focus Of The Smooth Walls. Thus, This Work Deals With Newly Added Ribs And Three-equation Variant Of The Shear Stress Transport (Sst) K–ω Model To The Current Fluid Flow And Heat Transfer Depending On An Existing Experiment As A Reference. The Adapted Turbulence Model Thought To Be More Susceptible To U-bend Zone, Rotation, And Wall Corrugation Is Applied Using COMSOLMULTIPHYSICS Program. A Two-pass Profile With Leafy Characteristics, Derived From A Prior Work By The First Author, Is Implemented For The First Time And Contrasted Against Alternative Corrugation Designs. The Findings Demonstrated That Applying The Suggested Model Reduces The Percentage Error Between The Computational And Experimental Data To Less Than 20%. The Nusselt Numbers Computed At Different Leafy-corrugated Channel Divisions Are Augmented To 30% With 70% Surface Temperature Reduction; However, The Friction Penalty Rises Too. [ABSTRACT FROM AUTHOR]

The design of various biomedical, electronics cooling, and microfluidic devices relies on geometry-specific models and empirical correlations for flow and heat transfer through microscale pin fin geometries. Machine learning (ML) techniques are being used across many branches of science to develop more generalized surrogate models that can predict such transport processes. To collapse the simulation of flow and thermal properties across many different pin fin surfaces into a single predictive tool, the present study develops machine-learning-based surrogate models for the friction factor and Nusselt number (for constant wall temperature conditions) for fully developed low Reynolds number flow across pin fin geometries of differing cross section shape (circular, square, triangular) in aligned or staggered arrangements, oriented at any angle to the incoming flow, and for a range of transverse and longitudinal pitches, with water as the working fluid. The model training data are generated using an automated workflow that allows thousands of numerical simulations to be carried out on across different geometric and flow configurations. A total of ∼14,800 distinct simulation cases, for both friction factor and Nusselt number, are generated while varying the Reynolds number and aforementioned geometric parameters to train and test the machine learning models. The machine learning model architecture takes inputs of both image and vector data, and then outputs a scalar friction factor or Nusselt number. The trained models yield a goodness of fit (R2) value of 0.98 on unseen data. [ABSTRACT FROM AUTHOR]

The present study primarily investigates the exergy and entropy generation in a heat exchanger influenced by the combined effects of mechanical vibrations and magnetic fields. A rectangular channel with dimensions 40 mm in length and 4 mm in width was examined using magnets of varying strengths and subjected to vibrations and magnetic fields. The Reynolds number (Re) investigated in this study ranges from 150 to 300. Both, magnetic field and vibrations, generated intricate patterns and contours, highlighting their interaction with flow dynamics. As vibrational intensity increased, the Nusselt number amplified correspondingly. While the introduction of magnetic field also enhanced the Nusselt number (Nu), the impact of vibrations was more pronounced. A maximum Nu enhancement of 225.9% was achieved at a Re 300, under the influence of vibrations at 5 mm and 25 Hz, and a magnetic field strength of 2000 G. The study further revealed that exergy efficiency decreases progressively with increasing Re but improves with higher vibrational intensity, reaching a peak of 52.81% at 5 mm and 25 Hz. Additionally, it was observed that irreversibility (f) decreases with increasing vibrational and magnetic strengths. The ratio of entropy generation under the vibrational and magnetic influence to that of static case peaked at a value of 2.4 under vibrational intensity of 5 mm and 25 Hz, and magnetic field strength of 2000 G. [ABSTRACT FROM AUTHOR]

The study of flow behavior in the simultaneously developing transitional regime of mixed convection flows is rare. It has been believed that the transitional regime will give a good compromise between pressure drop and heat transfer compared to laminar and turbulent flow regime. In this experimental study, the friction factor (f) and Nusselt number (Nu) characteristics for buoyancy-assisted and opposed flows of water in concurrently developing transitional regime of mixed convection through a vertical tube are studied. Experiments were done for Reynolds numbers (Re) varying from 500 to 15,000, Grashof numbers (Gr) from 1.25 × 104 to 5 × 106, Prandtl numbers (Pr) from 3 to 7, and Richardson numbers (Ri) from 0 to 0.1 subjected to uniform heat flux boundary conditions. A flow visualization provision after the test section which confirms an early transition in buoyancy-opposing flow (Rec = 2264) compared to buoyancy-aiding flow (Rec = 2468) at a fixed Ri of 0.1. Further, with the increase in Ri from 0 to 0.1, the average f decreases, and the average Nu increases in both aiding and opposing flows. It confirms that the onset of transition gets delayed with the increase of heat flux supplied in both the flows. Based on the present outcomes, an efficient heat exchanging device can be operated either to delay or advance the transition in a vertical pipe flow for optimum heat transfer. [ABSTRACT FROM AUTHOR]

Several diseases like Sickle Cell Anemia, Thalassemia, Hereditary Spherocytosis, Malaria, and Micro-angiopathic Hemolytic Anemia can alter the normal shape of red blood cells (RBCs). The objective of this study is to gain insight into how a change in RBC deformability can affect blood heat transfer. The heat sink effect in a bifurcated vessel with two asymptotic cases (case 1: deformable and case 2: nondeformable RBCs) is being studied during hyperthermia treatment in a three-dimensional bifurcated vessel, whose wall is being subjected to constant heat flux boundary condition. Euler–Euler multiphase method along with the granular model and Kinetic theory is used to include the particle nature of RBCs during blood flow in the current model. To enhance the efficiency of the numerical model, user-defined functions (UDFs) are imported into the model from the C++ interface. The numerical model used is verified with the experimental results from (Carr and Tiruvaloor, 1989, “Enhancement of Heat Transfer in Red Cell Suspensions In Vitro Experiments,” ASME J. Biomech. Eng., 111(2), pp. 152–156; Yeleswarapu et al. 1998, “The Flow of Blood in Tubes: Theory and Experiment,” Mech. Res. Commun., 25(3), pp. 257–262). The results indicate that the deformability of RBCs can change both the flow dynamics and heat sink effect in a bifurcated vessel, which subsequently affects the efficacy and efficiency of the thermal ablation procedure. Both spatial and transient Nusselt numbers of blood flow with deformable RBCs are slightly higher compared to the one with nondeformable RBCs. [ABSTRACT FROM AUTHOR]

This paper employed a scaling analysis to represent the processing parameters, affecting the melting process in the dimensionless numbers, identify the relationships of these dimensionless numbers, and develop semi-empirical correlations to predict the width and depth of the melt pool. To develop the correlations, Ti-6Al-4V powder was used to print 38 tracks at various processing conditions. The correlations were then fit into this experimental data using python code to find the constants of the correlations. The correlations were then used to predict the depth and width of the melt pools. It was found that the mean discrepancy between the predicted melt pool dimensions and the experiment is 7%. To evaluate the accuracy of the correlation in predicting the melt pool dimensions of the materials never used during the development of the correlations, the melt pool depth of some tracks made out of stainless steel 316L printed at various conditions was predicted using the model, and it was found that the mean discrepancy between the predicted melt pool depth and experiment is 11%. [ABSTRACT FROM AUTHOR]

Solar air heater (SAH) is one of many applications of solar energy, but it has low thermal performance. To cater to this issue, a novel corrugated jet-impinging channel has been proposed for the absorbing plate of SAHs. The Nusselt number, pressure loss pumping power, and friction factor were computed by experimental and numerical studies for three different arrangements of jets, i.e., 1 × 1 square, 2 × 2, and 3 × 3 jet array with three target plates, i.e., smooth, trapezoidal, and sinusoidal corrugated plates over the Reynolds number range of 1740-2700 and jet-to-target plate spacing of 5.1-14.6. It was observed that jet-to-target plate distance and multiple jets impinging can affect the heat transfer enhancement significantly. An optimization study has been performed using the statistical Taguchi method. The optimum value of Nusselt number was found for the trapezoidal corrugated target plate, with 3 × 3 multiple jet impingement, jet-to-target plate distance of 5.1, and Reynolds number value of 2700, while the optimum value of friction factor was found for 3 × 3 jet impingement. Computational fluid dynamics (CFD) and experimental data are found to be in good agreement. An increase of 11 times was observed in the thermal performance factor as compared to the base case. Trapezoidal corrugation seems to be the effective profile for SAHs. [ABSTRACT FROM AUTHOR]

The development of new technologies for energy generation and use has been increasing significantly. In this projection, the use of flat solar collectors to convert solar energy into thermal energy through water heating for residential and commercial purposes has grown due to the potential reduction of up to 40% in electrical energy consumption promoted by these devices. A promising but underexplored area in engineering is the study of the intensification of heat transfer in these devices by changing the dimensional and constructive characteristics of the elevation tubes, especially through a numerical approach by passive systems that operate under the thermosiphon effect. Thus, this work aims to investigate, by using computational fluid dynamics (CFD), the heat transfer process in a flat plate solar collector with a concentric plate to the elevation tube, evaluating different diameters, angles of inclination, and slope corrugation profiles subjected to a constant heat flux. The numerical modeling considers a single-phase, incompressible, permanent, three-dimensional, and laminar flow, in addition to the Boussinesq approximation. The results showed that significant increases in the heat transfer rate can be achieved with absorber plates in comparison to those configurations without absorber plates. Moreover, the increase in the tube diameter allowed gains of up to 5.1% in the heat transfer rate, while the increase in the angle of inclination did not promote significant improvements. The triangular profile R10 P20 configuration increased the Nusselt number by 8%, while the R5 P20 configuration promoted a 25% gain in thermo-hydraulic performance. [ABSTRACT FROM AUTHOR]

An experimental investigation is carried out to analyze the thermo-hydraulic characteristics of a triangular solar air heater duct having transverse ribs with gaps. The roughness parameters, such as non-dimensional pitch (̅P/e or) and non-dimensional height (̅e/Dh or) are kept in the range of 4.88-20 and 0.021-0.044, respectively. Reynolds number (Re) is kept in the range of 4000-18,000. Two and three gaps of each of 0.01 m are provided to each odd and even number ribs, respectively. Non-dimensional primary width (w1/W) and non-dimensional secondary width (w2/W) are kept constant at 0.29 and 0.4, respectively. A maximum heat transmission of 3.14 times that of the base model is achieved for the transverse ribs with gaps having non-dimensional pitch and height of 9.76 and 0.044, respectively, at Re = 18,000. In the parametric range, the highest friction factor of 3.88 times the base model is encountered for the non-dimensional pitch and height of 4.88 and 0.044, respectively, at Re = 4000. The highest thermal enhancement ratio of 2.31 is reported for the non-dimensional pitch and height of 9.76 and 0.044, respectively, at Re = 18,000. The correlation for the Nusselt number and friction factor is formulated, agreeing with experimental data within ±12% and ±8% deviation, respectively. [ABSTRACT FROM AUTHOR]

The objective of the study is to thoroughly analyze the flow and heat transfer of Bingham plastic fluids through an array of uniformly gapped rough surface cylinders embedded between two confined boundaries. Radial notches are used as the surface roughness in the model, evenly distributed. Due to the formation of front vortices in uniformly gapped cylinders, a negative pressure gradient is developed. The results of the numerical simulation analysis have shown that, when compared to the averaged Nusselt number, roughness has a minimal effect on the drag coefficient and pressure drop. As the degree of roughness increases, the size of the vortices decreases, resulting in a drop in heat transfer. Moreover, the analysis of each column shows that the first column array of cylinders has a higher total drag coefficient and average Nusselt number. [ABSTRACT FROM AUTHOR]

The entropy generation minimization principle is used as the criterion to optimize the flow and heat transfer of solar collectors and heat exchangers that use molten salts NaCl-KCl-MgCl2 and KCl-MgCl2. The Gnielinski correlation for the Nusselt number versus Reynolds number, as well as the Moody friction factor given by Petukhov, was used for the calculation of the convective heat transfer coefficient and pressure loss due to friction in smooth tubes. For twisted-tap-inserted tube, equations of Nu and friction factor provided by Manglik and Bergles were used. The objective function, the entropy generation rate of the heat transfer system, was expressed as the function of Reynolds number, Prandtl number, heating flux, tube diameter, etc. As a result of the analysis, the optimum Reynolds number was determined and thereby to determine the optimum Nusselt number, convective heat transfer coefficient, friction factor, and tube diameter, which also allows the calculation of optimum flow velocity. The analysis was conducted in the fluid temperature range of 500-700 °C, which covers the operation temperature for supercritical CO2 power cycles in concentrated solar power (CSP) system. Optimized results from the smooth tube and twisted-tap-inserted tube are compared, which is important to the design of solar receivers for CSP systems. [ABSTRACT FROM AUTHOR]

Recently, many studies have reported that a curved solar air heater (CSAH) performs better than a conventional flat SAH without using any extra material. It only requires geometrical modification. The present investigation is a two-dimensional numerical study of flow, heat transfer, and entropy generation characteristics of a CSAH having a sinusoidal profile absorber plate. Reynolds number (Re) and relative roughness pitch (λ/a) have been varied from 3800 to 18,000 and 7.143 to 17.857, respectively, while keeping the value of relative roughness height (a/Dh) at 0.042. The finite volume method (FVM) and SST k-ω model have been used to solve the governing equations. The average Nusselt number and average friction factor have been calculated to find the thermo-hydraulic performance parameter (THPP), which further helped determine the optimal arrangement of the number of sinusoidal waves in the absorber plate of the SAH. The maximum value of THPP developed with the proposed setup was found to be 5.9778. Turbulent flow features have been represented in the form of contours. Correlations have also been developed for Nuavg_r and favg_r as a function of Re and λ/a. Entropy generation per unit length due to heat transfer and fluid friction has been graphically represented. [ABSTRACT FROM AUTHOR]

This paper reports the computational results of fluid flow and thermal characteristics in an isosceles trapezoidal solar air heater (SAH). By varying the base angle of the trapezoidal duct from 90 deg to 45 deg, six different models of solar air heater ducts are obtained. The six geometries of SAH ducts have cross sections of rectangular (90 deg), triangular (45 deg), and four isosceles trapezoidal (having base angles of 50 deg, 60 deg, 70 deg, and 80 deg) shapes. The solar radiation absorber plate width and the duct heights are maintained constant for all six models of SAH, i.e., 160 mm and 80 mm, respectively. The SAH is subjected to a constant and uniform heat flux value of 1000 W/m2 and Reynolds numbers varied from 5000 to 28,000. In all six cases, the size of the air heater is the same and having dimensions of 0.16 m width and 1 m length. For this investigation, a three-dimensional computational model has been developed and simulations are carried out using commercially available ansys fluent software. The numerical results are validated with the standard correlations and literature data, and a suitable model has been identified for the turbulence closure. A detailed analysis of the Nusselt number, temperature distribution over the SAH, and friction factor across the SAH duct is done. Empirical correlations for the estimation of heat transfer and friction factor have been developed as functions of the base angle of the duct and Reynolds number. An overall performance factor (λ) is adopted to get the combined effect of friction factor and Nusselt number with an intention to arrive at the optimum base angle of the SAH duct, and optimum geometry is identified. Based on the value of λ, it is concluded that the SAH duct with the highest base angle (90 deg) in this investigation, i.e., the rectangular duct, is the optimum among all the ducts considered in the study. [ABSTRACT FROM AUTHOR]

An effort is made to study the performance of a solar air heater having the absorber surface roughened by providing staggered, twisted, and V-shaped ribs roughness. The liquid crystal thermography technique is applied to get the Nu distribution over the surface. Experimentation is done for Reynolds number ranges from 3000 to 21,000, relative roughness to pitch ratio (P/e) from 7-11 and relative staggered distance (d/e) of 2-6, for the fixed angle of attack (a), relative twist length (y/e), and relative roughness length (S/e). It is found that the maximum Nusselt number and friction factor augmentation are 3.43 and 2.57 times than that of the smooth duct. The maximum thermo-hydraulic performance (THP) value obtained is 2.69 for the P/e of 9 and d/e of 4. It can be inferred that staggered twisted V-shaped rib roughness helps to get the enhanced Nu with minimum friction penalty. [ABSTRACT FROM AUTHOR]

The increase in the heat dissipation rate in heat sinks (HSs), the reduction of the occupied volume and mass, and the elimination of the lower heat transfer areas (LHTAs) behind the pins are the main parameters to be controlled in HSs design. For this purpose, this study is devoted to numerically investigating the effect of the combination between perforation technique and splitters inserts on the heat dissipation and turbulent fluid flow characteristics of pin fins heat sinks (PFHSs). The splitter is located in the back of the pin, and the cylindrical pin fins heat sinks (CPFHSs) are perforated with different pairs of hole numbers. These configurations are named PFHS-0 (without perforation) to PFHS-5. The results obtained for the PFHS-5 show an increase in Nusselt number by 34.91% and a reduction in the thermal resistance by 24.22%, compared with CPFHSs. For the same conditions, the occupied volume and mass of this case are also reduced by 70% and 47.5%, respectively. In addition, the PFHS-5 case ensures the highest hydrothermal performance factor (HTPF) of 1.42 at Re = 8,740. [ABSTRACT FROM AUTHOR]

Research results demonstrate the heat transfer effectiveness of an impinging synthetic jet toward cooling a plane normal to it. The utility of the synthetic jet lies in that the supply of coolant comes from the device itself as an alternating jetting flow that emerges from a plenum followed by a sink flow that returns to that same plenum. Experiments reported herein were conducted with the synthetic jet driven by an oscillating diaphragm powered by a rotating cam to expel fluid from the plenum out of a single hole, then return it through the same hole. The frequency of diaphragm oscillation and the distance from the synthetic jet's orifice to the surface being cooled are varied in the test program to determine their effects on cooling performance. A numerical study agrees with the results given by the experiment and flow visualization utilizing a smoke generator supports the data and numerical results. The local, time-average Nusselt numbers were measured in the experiment using the thermochromic liquid crystal technique and air as coolant. The color display of each test case was recorded with a fisheye camera. In the case of the highest frequency and shortest distance from orifice to cooled plate, a Nusselt number of nearly 40 was achieved within the central region of the cooled plate when the Reynolds number based upon jet maximum velocity and orifice diameter was 7500 and the distance from the orifice to cooled plate was 3.2 orifice diameters. [ABSTRACT FROM AUTHOR]

In recent years, a good number of research works have been conducted to elucidate the different aspects of photovoltaic thermal (PV/T) technology. However, in order to take a technology to its maturity level, it is important to explore its internal physics and identify the factors that control system performance. With this view, in the present research, thermal and heat transfer characterization, and pressure drop phenomena inside a parallel-plate PV/T collector have been examined numerically to portray the thermo-fluid physiognomy of the system. A three-dimensional mathematical model of the PV/T system has been developed, and the model is used to build a computer simulation of the system in COMSOL Multiphysics® software. Hence, the simulation model has been validated by outdoor experimental results and was found to be in good agreement. Thus, the simulation program is employed to produce temperature distribution and heat flow plots throughout the flow channel, wherein results have been evaluated for two different channel materials, e.g., aluminum and copper. Results show that heat flowrate through both aluminum and copper channels is virtually the same. On the other hand, pressure drop, thereby pumping power required to maintain flow, is greater for an aluminum channel. The developed heat transfer simulation model can be extended for other PV modules with diverse designs and materials of the heat exchanger. [ABSTRACT FROM AUTHOR]

In this study, the cooling of a gas turbine (GT) blade leading edge using a single row of five impinging air jets was numerically and experimentally investigated. The inner surface of the blade leading edge was depicted as a semicircle profile. The numerical study was performed using a shear stress transport (SST) k-ω turbulence model. On the other side, the experimental work was conducted using the infrared (IR) thermal camera. Additionally, the heat transfer characteristics were investigated with changing the Reynolds number (⁠Re ⁠) from 5000 to 40,000. The study was carried out for different cases based on the jet diameter (⁠d ⁠), named as fixed diameter (FD) and variable jet diameter in the streamwise flow direction, i.e., ascending diameter (AD) and descending diameter (DD). The local and average Nusselt numbers and heat transfer uniformity were studied at different operating and design parameters. The results revealed that for FD, increasing the jet diameter decreased the average Nusselt number while it enhanced the heat transfer uniformity. Furthermore, for all values of Re ⁠, the thermal performance of the AD was better than the DD, which has the worst heat transfer uniformity. Finally, predicted developed correlations to estimate the average Nusselt number, surface heat transfer uniformity, and performance evaluation factor (PEF) for all cases of jet diameter were introduced. [ABSTRACT FROM AUTHOR]

In this article, thermal performance of a solar air heater (SAH) having broken arc-shaped ribs has been investigated experimentally, and then, performance parameters have been optimized. Study reveals that the use of artificial roughness enhances both heat transfer rate and pumping power requirement, and hence, the overall performance of solar air heater increases. Thus, it is imperative to optimize the performance-defining criteria viz. Nusselt number enhancement ratio, friction factor enhancement ratio, and thermohydraulic performance parameter, to propose an optimal set of flow and roughness parameters. The hybrid entropy-VIKOR technique has been employed to outline the optimal set of parameters using performance-defining criterions, to obtain maximum profit with the minimum incurred cost. The optimal set of parameters attained using entropy-VIKOR approach among all the alternatives is as follows: relative gap width = 4, relative roughness pitch = 10, and arc angle = 45 deg at Reynolds number = 9000. [ABSTRACT FROM AUTHOR]

This study aims to numerically assess the heat transfer performance of water-in-glass evacuated tube solar water heater (WGET-SWH) for various parameters. It is observed that the natural circulation flowrate, velocity, and temperature distribution are strongly dependent on WGET-SWH geometry and operating conditions. The natural circulation flowrate is found to be increasing with tube length, diameter, inclination angle, and circumferential heat input. However, the heat transfer rate decreases with an increase in aspect ratio. The average Nusselt number increases with an increase in inclination angle till 45 deg and then decreases. Also, the study shows that thermal stratification in reservoir fluid is greatly influenced by circulation flowrate, flow momentum, collector outlet temperature, thermal acceleration, and buoyancy effect. [ABSTRACT FROM AUTHOR]

An experimental and theoretical study has been performed to analyze the heat transfer and pressure drop performance of a double-pass solar air heater having arc-shaped roughness. The investigations are performed for the Reynolds number ranging from 3000 to 12,000, relative roughness pitch (p/e) ranging from 8 to 12, relative roughness height (e/d) of 0.0433, and the angle of attack (a/90) of 0.3333. The investigation has covered different performance parameters such as Nusselt number, friction factor, and thermal performance factor of rough and smooth double-pass solar air heater. A significant enhancement is observed in the heat transfer coefficient by the authors and the theoretical and experimental data are found in good agreement. The correlations for Nusselt number and friction factor are obtained for the range of parameters investigated. The maximum improvement in the Nusselt number for the rough double-pass solar air heater is obtained as 1.7 times that of the smooth double-pass solar air heater for Reynolds number of 11,283 with a relative roughness pitch of 8 and relative roughness height of 0.0433. The corresponding thermal performance factor of 1.4 is obtained for this case. [ABSTRACT FROM AUTHOR]

Fluid flow and heat transfer of a gas stream in various ducts have been studied thoroughly before. However, in real applications, a gas stream usually contains dust particles, whose effects have typically been neglected. In this study, the effects of the dust particles on the flow and heat transfer characteristics in a parallel-plates duct were numerically investigated in detail. A lattice Boltzmann method combined with a modified immersed boundary approach was employed to calculate the velocity and temperature distribution in the duct. The effects of the particles on the development of the hydrodynamic and thermal boundary layers in the duct were predicted. The product of friction factor and Reynolds number (fRe) and local Nusselt number (NuL) along the flow direction were obtained for a particle-laden flow and compared with those for a pure gas flow. The results indicated that for particle-laden flows, the "fully-developed" flow was just an approximation. Both the flow and thermal boundary layers were disrupted by the accompanying particles. The particles would form a stable and dense particulate fouling layer at the walls; this could increase the local (fRe) and reduce the NuL in "fully developed" regions. Moreover, ducts with superhydrophobic properties would be less influenced by the particles due to decreased particle deposition because of the anti-dust property of the surface. [ABSTRACT FROM AUTHOR]

This investigation is devoted to analyze the buoyancy-driven flow behavior and associated thermal dissipation rate in a nanofluid-filled annular region with five different single source-sink and three different dual source-sink arrangements along the vertical surfaces. The remaining region on the vertical boundaries and horizontal surfaces are kept adiabatic. Numerical simulations have been performed by employing the finite difference method. To analyze the impacts of different nanofluids, nanoparticle volume fraction, Rayleigh number, size, and arrangement of sources and sinks, the results are graphically represented through streamline and isotherm contours, thermal profiles, average Nusselt number, and cup-mixing temperature. The results showed that identifying an optimum location and length of source-sink with a proper selection of other control parameters can lead to enhanced thermal transport and thermal mixing in the enclosure. In particular, middle-middle thermally active location and placing source-sink separately on the vertical walls lead to the production of maximum heat transport compared to other single and dual source-sink arrangements, respectively. Also, among the two nanofluids considered in the current investigation, larger enhancement in thermal transport has been achieved for Cu-water nanofluid. The calculated enhancement ratio of the heat dissipation rate enhances with an increment in nanoparticle concentration. [ABSTRACT FROM AUTHOR]

The increasing future energy demands can be met with solar-based devices such as solar air heaters (SAHs) only if the efficiency of these devices is enhanced with suitable design changes. Flow channel design of a SAH is an essential aspect for enhancing its thermohydraulic performance for a wide range of Reynolds numbers. In this article, a systematic approach has been adopted to investigate various nonrectangular channel designs numerically, and results are compared with the conventional rectangular design. The energy input to all designs is kept constant. The channel design that gives the best performance was further investigated by incorporating a sinusoidal wavy absorber having variable wavy roughness parameters. The flow and heat transfer characteristics have been evaluated in terms of friction factor (f) and Nusselt number per unit friction factor (Nu/f), temperature factor (To-Ti)/I, thermal effectiveness (ɛ), and Nusselt number (Nu). The results show that the SAH duct having a semi-ellipse cross section offers the best thermohydraulic performance and has maximum augmentation in the temperature factor of about 10% compared to conventional SAH. Moreover, semi-ellipse SAH with sinusoidal wavy absorber has a maximum value of f/fs and Nu/Nus at A/Dh = 0.12 and λ/Dh = 0.8 for the range of Reynolds numbers used, respectively. Here, A is the amplitude and λ is the wavelength of the absorber plate. New empirical relationships for Nu and f are established as a function of flow and geometric parameters that agree well with numerical results. [ABSTRACT FROM AUTHOR]

Microchannel heat sinks are potential devices that remove heat flux from high power density miniaturized electronic components. While the large surface-area-to-volume ratio and high heat transfer coefficient are the key features rendering benefits, the small flow rate and short channel lengths alongside high solid cross section to fluid free flow area make them susceptible to intense axial conduction loss. The conventional models for macrodevices based on the one-dimensional energy equation are often inappropriate in the microdomain. A novel multidimensional analytical model (capable of capturing axial heat transfer in microchannel heat sinks) has been used to study the thermal performance over a varied range of geometric and flow parameters. The effect of axial conduction has been seen in the solid-fluid temperature profiles, interfacial flux distribution and the average amount of heat transferred axially. The results indicate a skewed flux distribution at the fluid-solid interface leading to nonlinear temperature variation when axial conduction is dominant. Moreover, it has been shown that nonlinearity in the fluid temperature introduces significant errors in experimental data reduction, leading to apparently very low Nusselt number estimation. Moreover, this erroneous data interpretation is also linked to the prediction of a strong Reynolds number dependency of the average Nusselt number in the laminar flow regime. [ABSTRACT FROM AUTHOR]

Heat transfer modeling of large industrial ovens such as those used in automotive paintshops is difficult due to the large and multiple scales and long curing times. The flow inside a convective oven is turbulent, and the process includes large temperature gradients. An efficient simulation requires a simplified and localized model of the heat transfer coupling. We present a novel method with three ingredients: localization using local Nusselt numbers of the oven nozzles, projection of Nusselt number profiles onto the target, and efficient conduction modeling on a coarse background mesh. The approach, which was developed in a research project together with the Swedish automotive industry, makes it possible to accurately simulate a curing oven with close to real-time performance. The simulation results are demonstrated to be in close agreement with measurements from automotive production. [ABSTRACT FROM AUTHOR]

Bioconvection due to the movement of the micro-organism cells is universal and affects many ecological and biological processes, including infection, reproduction, and marine life ecosystems. The impact of the bioconvection is more significant in nanofluids. In the present problem, we investigate the Marangoni triply stratified bioconvective flow of non-Newtonian (second-grade) nanofluid with the presence of motile micro-organisms over a permeable inclined plate. The problem provides an analysis of the impact of second-order effects, namely, viscous dissipation, radiation, and chemical reaction, allowing a set of similarity transformations to convert the governing PDEs into coupled nonlinear DEs. Thereafter, Runge-Kutta Fehlberg's numerical method is employed to find the solution of the DEs for some chosen values of different flow influencing parameters. The impact of crucial parameters on the velocity, temperature, nanoparticles volume fraction, the motile density of micro-organisms, and the quantities of physical interest, namely, local Nusselt number, local Sherwood number, and local motile micro-organism density number are illustrated through the plots and tables. It is revealed that the second-grade fluid parameter indicates a prominent correlation with the Marangoni convection in the bioconvective transport mechanism. Also, the Marangoni convection is significant in bioconvective flows for large Péclet numbers. [ABSTRACT FROM AUTHOR]

This paper numerically investigates the optimization of the geometric parameters and the coolant's inflow states of the perforated micropin-fins (MPFs) heat sink using an elitist nondominated sorting genetic algorithm-II (NSGA-II) coupled with a finite volume-based computational fluid dynamics (CFD) solver. Square-shaped MPFs with two circular perforations were considered for the investigations on the fluid flow and conjugate heat transfer using numerical simulations. Five design variables (two perforation diameters, their respective locations, and the inflow velocity) with the essential constrained equations were optimized to search for the optimal solutions. Two objective functions, viz., Nusselt number (Nu) and friction factor (f), were selected to evaluate the hydrothermal performances of the perforated MPFs heat sink. The optimization was performed for 52 generations with a population size of 30. We obtained the Pareto optimal solutions, which gave the design boundary of the important parameters. Some of the cases of the Pareto solutions were also investigated in detail to understand the underlying thermal physics and structural rigidity under thermal and hydraulic stresses. It is observed that the MPF's stiffness was not compromised upon introducing two perforations. This study identified different thermohydraulic features responsible for optimal performance at different inflow velocity regimes. The present paper demonstrates that this optimization technique has led to a better understanding of the underlying thermal physics of complex electronic cooling equipment while systematically exploring the design space. [ABSTRACT FROM AUTHOR]

This paper presents a comparison between the results of three thermoconvective flows of a Newtonian fluid over uniformly heated, undulated horizontal surfaces in a porous medium against the background of the results of a flat plate. The undulations are assumed to have sinusoidal, sawtooth, and triangular waveforms. A system of nonlinear coupled partial differential equations arising in the study is solved using the Keller-Box method. Streamlines and isotherms have been plotted and analyzed to examine the effect of parameters on the fluid dynamics and heat transfer. At large surface amplitudes, secondary flow is observed in the cases of sinusoidal and triangular waveforms, but not in the cases of a sawtooth surface and a flat plate. The magnitude of the slip velocity at the horizontal surface is greatest for the sine waveform, while it is least in the case of triangular. The flat plate does not support slip in the velocity to the extent seen in the case of undulated surfaces. The variation of the mean Nusselt number and mean skin friction with surface amplitude and the Rayleigh number indicate that heat transfer and viscous friction at the boundary increase with individual and collective increases in the values of the amplitude and the Rayleigh number. Further, the mean Nusselt number and mean skin friction are found to be maximum for the sinusoidal surface and minimum for the triangular one. The heat transfer and skin friction by the flat surface are much less than that of all three undulated surfaces. [ABSTRACT FROM AUTHOR]

A computational work is performed on laminar free convection from an isothermally heated spherical shaped open cavity with negligible wall thickness suspended in the air. Fluid flow and heat transfer are analyzed in detail by solving governing differential equations (continuity, momentum, and energy) numerically over wide ranges of the relevant dimensionless parameters, namely, Rayleigh number, 104 = Ra = 108; and height to diameter ratio, 0.15 = h/D = 0.95. The detailed behavior of thermal and flow fields is delineated by suitable visualization techniques for different Ra and h/D. The influence of Ra and h/D on the local and average Nusselt number is also predicted and it is observed that the average Nusselt number on both outer and inner surfaces decreases with the increase of h/D for a constant value of Ra. A suitable correlation for the net average Nusselt number is obtained for the spherical-shaped open vessel surface as a function of Ra, and h/D based on the computed data points, which is expected to be relevant for various academic and industrial operations. This study can be helpful in various industrial operations, such as heat treatment of foodstuffs, shield surfaces, thermal insulations, melting of polymer pellets, and fluidized reactors. [ABSTRACT FROM AUTHOR]

Flow boiling is a common phenomenon in modern technology and industries. A finite difference Front Tracking method is used to study film boiling on a flat plate subject to a constant heat flux. Large density ratios are examined (1000, 2000). The flow structure including the circulation zones, the vortex developing areas, and vortex pair generation are studied in detail. The numerical method is validated by comparing the result with the analytic solution for a simple problem. Large bubbles comparable to the computational domain are formed at large density ratios. The bubble either breaks up and departs from the rest of the gas, or a jet is formed depending on the heat flux imposed. It is found that the Nusselt number depends on the heat flux on the wall. Specifically, the Nusselt number decreases as the heat flux is raised. The Morton number also affects the Nusselt number in agreement with experimental correlations. The Nusselt number increases when the Morton number is raised. The results obtained agree with experimental correlations at relatively low heat fluxes. Agreement gets worse at large heat fluxes. Experimental correlations estimate larger Nusselt numbers compared to the present study. The flow enhances along the direction of the gravitational acceleration at large density ratios (2000). A wide neck is formed that stabilizes at a specific position inside the flow. The microstructure of the flow shows a vortex pair that develops beneath the root of the bubble at a large density ratio (2000). [ABSTRACT FROM AUTHOR]

Heat transfer characteristics of random suspensions of 0.25 aspect ratio (AR) cylinders are investigated for Reynolds numbers (Re) between 10 and 300 and solid fraction (φ) ranging from 0.1 to 0.3 using particle resolved simulations. The effect of particle inclination with respect to flow and particle clustering on heat transfer is investigated. The Nusselt number decreases with an increase in inclination angle and the dependence becomes stronger as φ and Re increase. On the other hand, while prolate ellipsoid suspensions of AR 2.5 follow the same trend, the Nusselt number increases with inclination angle as AR increases to 5 and 10 and as φ increases. Local particle clustering nominally decreases the Nusselt number because of the dominance of thermal wakes. At low φ⁠, this effect is felt only at low Re⁠, but as φ increases, the effect spreads to higher Re⁠. Similar but weaker trends are also found in suspensions of prolate ellipsoids of AR 2.5, 5, and 10. High AR, low Re prolate ellipsoids exhibit the greatest dependence of Nusselt number on local solid fraction. Implementation of two independent definitions of reference length, i.e., volume equivalent sphere diameter deq for ellipsoids and diameter dp of the cylindrical particle in the correlation of Tavassoli et al. (2015, "Direct Numerical Simulation of Fluid-Particle Heat Transfer in Fixed Random Arrays of Non-Spherical Particles," Chem. Eng. Sci., 129, pp. 42-48) provides good estimates of the respective suspension mean Nusselt numbers. [ABSTRACT FROM AUTHOR]

This work investigates flow and heat transfer under an array of annular jets impinging on a heated moving surface. Numerical solutions of the full Navier-Stokes equation were attempted with a highly refined mesh. This study reports results for Reynolds numbers up to 500. In the surface movement direction, a periodic element from a jet-bank configuration was chosen, and the nondimensional surface velocity was considered from zero (i.e., a stationary plate) to two times the jet velocity. The impact of annular jet impingement over a moving surface on flow and heat transfer characteristics, including the development of the flow field, velocity profiles, skin friction coefficient and topology of skin friction lines, and local as well as surface averaged Nusselt number distribution are presented. It is observed that both the flow field and thermal performance are strongly affected by the surface motion. Heat transfer from the surface initially increases with the increasing surface motion, and after attainment of the highest value, heat transfer reduces with a further increase in surface velocity. However, higher surface velocity leads to higher uniformity in heat transfer, which may be beneficial for situations demanding uniformity in heat transfer. [ABSTRACT FROM AUTHOR]

The turbulent flow of binary hybrid nano-oils is investigated and Nusselt number correlation is developed for futuristic concentrated solar thermal application. Available Nusselt number correlations for water-based hybrid nanofluids are nanoparticle-specific property correlations dependent and substantially over-predict the values for hybrid nano-oils. Therefore, a generalized Nusselt number correlation for turbulent flow of water and oil-based binary hybrid nanofluids is deduced using the separation approach. Dissimilar to the available correlations, the developed correlation needs only the thermophysical properties of base fluid and nanoparticles. It is valid for the Reynolds number range 10,000-30,000 and the Prandtl number range 0.5-2000. It is found that the proposed correlation predicts the published experimental values for different hybrid nanofluids mostly within ±10-20%. Computational fluid dynamics simulation is also performed for turbulent flow of different hybrid nano-oils to assess the developed correlation. The comparative assessment also confirms that the developed correlation predicts the numerical values for hybrid nano-oils within ±10-20%. The deduced Nusselt number correlation will be useful for a realistic heat transfer analysis with different water or oil-based hybrid nanofluids. The need for experiments with different hybrid nano-oils is realized for further improvement. [ABSTRACT FROM AUTHOR]

The two-temperature model of local thermal nonequilibrium (LTNE) is utilized to investigate a weakly nonlinear stability of thermosolutal convection in an Oldroyd-B fluid-saturated anisotropic porous layer. The anisotropies in permeability, thermal conductivities of the porous medium, and solutal diffusivity are accounted for by second-order tensors with their principal directions coinciding with the horizontal and vertical coordinate axes. A modified Darcy-Oldroyd model is employed to describe the flow in a porous medium bounded by impermeable plane walls with uniform and unequal temperatures as well as solute concentrations. The cubic-Landau equations are derived in the neighborhood of stationary and oscillatory onset using a modified perturbation approach and the stability of bifurcating equilibrium solutions is discussed. The advantage is taken to present some additional results on the linear instability aspects as well. It is manifested that the solutal anisotropy parameter also plays a decisive role on the instability characteristics of the system. It is found that the stationary bifurcating solution transforms from supercritical to subcritical while the oscillatory bifurcating solution transforms from supercritical to subcritical and revert to supercritical. The Nusselt and Sherwood numbers are used to examine the influence of LTNE and viscoelastic parameters on heat and mass transfer, respectively. The results of Maxwell fluid are outlined as a particular case from this study. [ABSTRACT FROM AUTHOR]

In this work, rarefaction effect, heat transfer, and drag coefficient for gas flow around a square cylinder in transition flow regime are numerically studied using unified gas kinetic scheme (UGKS). To reduce computational cost, a mirror symmetry boundary treatment for UGKS is proposed and applied in this study. It is found that: (1) velocity slip is not obvious on upwind and downwind surfaces of square cylinder due to the tangential gas flow being weak in these surfaces; (2) with the increase of local Knudsen number, velocity slip on upper surface always increases, but temperature jump can decrease, which indicates that Knudsen number is not the decisive parameter to characterize temperature jump; (3) the heat transfer between gas and square cylinder enhances with the increase of inflow Knudsen number and cylinder temperature due to the increase of temperature jump and deteriorates with the increase of inflow Mach number on account of gas stagnation; and (4) the drag coefficient increases with the increase of inflow Knudsen number, the decrease of inflow Mach number, and the increase of cylinder temperature. To further predict the variation of average Nusselt number and drag coefficient, correlations for average Nusselt number and drag coefficient with inflow Mach number ranging from 0.05 to 0.3, inflow Knudsen number ranging from 0.1 to 10, and cylinder temperature ranging from 320 K to 380 K are proposed. This research can improve the understanding for mechanisms of gas flow and heat transfer in micro-electromechanical system (MEMS) devices. [ABSTRACT FROM AUTHOR]

This research work investigates the effect of point/line heat source/sink on free convection flow between two vertical channels with suction/injection. The Heaviside step function is used to model the constant heat source/sink, which is then transformed into line source/sink. The governing equations describing flow formation and heat transfer are obtained and solved using the Laplace transform approach. The effect of physical parameters such as suction/injection, Prandtl number, and heat source/sink on velocity and temperature fields are studied graphically, while the skin friction, Nusselt number, and mass flowrate are analyzed through tabular representation. Results indicate that as the value of point/line source parameters rise, velocity and temperature profile of fluid flow are enhanced. [ABSTRACT FROM AUTHOR]

A numerical investigation is performed to analyze air's steady natural convection phenomena in an enclosure with partial active sidewalls. A partition is attached to the hot wall for different active locations to study the effect of the partition position on the heat transfer. First, a finite volume method solves the coupled equations of continuity, momentum, and energy. The SIMPLE algorithm is used to solve the pressure velocities coupling iteratively. Second, based on the obtained dimensionless velocity and temperature values, the distributions of the local entropy generation due to heat transfer and fluid friction and the total entropy generation are determined for different parameters. The results are presented graphically in streamlines, isotherms, and average Nusselt numbers. Three different configurations of active region arrangement are considered in this study, while the partition length has been changed. In order to identify the optimum location of the partition for better heat transfer, the effect of entropy generation has been studied. The heat transfer rate decreases with the increased partition length, especially for l = L/2. Thus, the maximum average Nusselt number occurs for the middle-middle arrangement, while the minimum one occurs for the bottom-bottom arrangement. The numerical investigations in this analysis are made over a wide range of parameters, Rayleigh number 10³ to 105, dimensionless partitions length (l = L/8, L/4, L/2), and irreversibility coefficients 10-4< ϕ < 10-2; however, the Prandtl numbers was fixed to 0.71. [ABSTRACT FROM AUTHOR]

A comparative study on the enhancement mechanism by various twisted tapes (TTs) in association with the local flow pattern is investigated in detail via computational fluid dynamics (CFD). The effect of gap between TT and tube is further examined based on prior experimental validation. Result shows that the Nusselt number and friction factor will be increased by 12.6% and 18.1%, respectively, and the comprehensive thermal performance (CTP) will be improved from 1.05-1.07 to 1.12-1.15 when the gap is reduced from 1.0 mm to zero. The obtuse V-cut design in the experimental study shows superior performance, and the related enhancement mechanisms by windward, isosceles, and leeward V-cut configurations are numerically investigated. By comparing the local streamline at V-cut regions and the velocity distribution on the cross section, the windward V-cut might notably introduce fluid into the cut region and toward the tube wall subsequently, thereby resulting in effective heat transfer augmentation near the tube wall. Compared to the isosceles V-cut TT, the CTP of windward V-cut TT can be augmented from 1.15-1.18 to 1.16-1.21. However, the leeward V-cut TT will impair the CTP due to its inconsistent turbulence direction to the main helical flow. On the other hand, it is also indicated that fabricating three windward V-cuts at each helical pitch shows the highest improvement on CTP. Further increment of V-cut number may reversely reduce the CTP because of the rapid increase of flow resistance. Moreover, fabricating windward V-cuts on bilateral edges of the TT shows limited improvement. [ABSTRACT FROM AUTHOR]

The use of transport membrane condenser (TMC) technology to recover heat and mass from the flue gas has been increasing recently. The heat and mass transfer from the TMC tube bundle have been studied experimentally and numerically, and several numerical models have been proposed. Although many heat transfer and pressure drop correlations are available for single-phase flows over tube bundles of solid walls, to the best of our knowledge, there is a lack of heat and mass transfer and pressure drop correlations for the porous membrane tubes with condensing flue gas that cover a wide range of parameters. In this study, the heat transfer, mass transfer, and pressure drop imposed by the crossflow ceramic nanoporous tubes in TMC have been studied numerically within wide ranges of tube diameters (4.57-7.62 mm), number of rows (2-24 rows), and Reynolds number (170-8900), under flue gas condensation. The turbulent flow of the flue gas mixture was modeled by the shear stress transport SST k-ω turbulence model. A hybrid/mixed condensation model written in user defined functions was employed to calculate the water vapor condensation rate. Numerical results with condensing flue gas are compared to available correlations for single-phase Nusselt number and pressure drops in the literature. It was found that except for selected conditions, the single-phase correlations noticeably differed from the TMC numerical results. Empirical TMC correlations for heat transfer and pressure drops with respect to condensation rate, number of rows, and the nanoporous membrane geometrical properties were derived thereby. The derived correlations for TMC show a good agreement with numerical data for all investigated parameters and can predict the 96% of the convective Nusselt number, overall Nusselt number, and friction factor inside the TMC within ±10%, ±10%, and ±15%, respectively. The effects of key parameters on the heat transfer, mass transfer, and pressure drops are illustrated and discussed in detail. [ABSTRACT FROM AUTHOR]

This study is the examination of magnetohydrodinamics (MHD) flow and heat transfer between microparallel plates with constant wall heat flux using two different boundary condition coefficients, widely known in the literature, in a second-order slip flow and temperature jump boundary condition model. The stream of incompressible viscous liquid between rigid microtwin plates with electrical conductivity was assumed to be steady, laminar, hydrodynamically and thermally fully improved. The effects of dilution, viscous dissipation, and axial transmission, which are significant at the microscale, are also contained in the analysis. Momentum and energy equations are solved analytically under second-order slip velocity and temperature jump boundary conditions. Closed type answers for the temperature field and the fully developed Nusselt number are derived as a function of the magnetic parameter (MHD), Knudsen number and modified Brinkman number. Particular attention has been paid to the effects of the magnetic field on the second-order slip flow conditions. The use of the second-order boundary condition coefficients proposed in the study varies according to the proposed boundary conditions and tends to increase and decrease the heat transfer. Otherwise, MHD increases the heat transfer for both boundary conditions. [ABSTRACT FROM AUTHOR]

This paper presents the results of a numerical simulation for the natural convection inside an enclosure that has an inner open square at its center. The inner square is open at the top and connected to the ceiling of the enclosure. The open inner square distorts the convection patterns, slows down the flow, and provides a compartment to confine the fluid at the core of the enclosure. Ultimately, this lowers the local Nusselt number, Nu, along the hot wall, and reduces the heat flux through the enclosure. The analysis shows the effects of changing the dimensions of the inner square on the heat flux through the enclosure for a range of Rayleigh numbers-from 103 to 106. Short-sided inner squares work as flow deflectors while long-sided inner squares provide compartments to accommodate new flow circulation at the core of the enclosure. The inner square is most effective when the length of its sides equals the width of the stagnant core inside the empty enclosure at the same Rayleigh number, and the heat flux at this condition is the lowest. Inner squares made of thermally conducting materials can reduce the heat flux through the enclosure by 70%, while adiabatic inner squares can reduce the heat flux by 90%. Inner squares reduce the external heat load on buildings when fitted inside the holes of hollow bricks used in building facades. The external heat flux can be lowered by 30%-55% depending on the material of the inner square and outer side temperature. [ABSTRACT FROM AUTHOR]

Many engineering applications consist of rotating components that experience high-heat load. Applications like the gas turbine engine consist of rotating disks and the study of heat transfer over such rotating surfaces is of particular interest. In the case of gas turbines, the disk also needs to be protected from the ingress of hot turbine gases caused by the low-pressure region created due to the radially outward pumping of fluid close to the rotating surface. The present experimental study investigates the effects of introducing pin-fins on heat transfer over surface of a rotating gas-turbine disk. The experiments were conducted at rotational Reynolds numbers (⁠ReR⁠) of 5487-12,803 based on the disk diameter (⁠D⁠) and jet Reynolds numbers (Re) of 5000-18,000 based on the jet diameter. The effects of nozzle to target spacing (⁠z/d=2-6⁠), eccentricity of impingement (⁠ε = 0-0.67⁠), angle of impingement--both toward and away from the center (⁠θi and θo=0 deg-20 deg⁠), and the pin fin height (⁠Hf=3.05 mm-19.05 mm⁠) were studied. Steady-state temperature measurements were taken using thermocouples embedded in the disk close to the target surface, and area average Nusselt number (⁠Nu⁠) was calculated. The results have been compared with those for a smooth aluminum disk of equal dimensions and without any pin-fins. The average Nu was significantly enhanced by the presence of pin-fins. The enhancement was higher for lower values of Re⁠, and the maximum enhancement was found to be 3.9 times that of a smooth disk for Re=5000⁠. In the impingement dominant regime, the effect of disk rotation was minimal for a smooth disk, but the heat transfer increased with rotational speed in case of pin-fins. There was no impact of eccentricity on Nu for ε = 0 and 0.33⁠. For ε = 0.67⁠, the maximum reduction in enhancement over a smooth surface (21.95%) was observed when compared with coaxial impingement for stationary impingement for Re=18,000 and z/d=4⁠. The effect of inclination angle was insignificant, and no clear trend could be established. Higher heat transfer rates were observed for z/d=6 with the increasing Re⁠, and this effect diminished with the increase in the rotational speed. With the increase in pin-fin height, especially at higher values of Re⁠, there was in increase in the value of Nu. Qualitative visualization of flow field has been performed for smooth and the pin-fin case using the commercial simulation package Ansys Fluent to further understand the flow features that result in the heat transfer enhancement. [ABSTRACT FROM AUTHOR]

In this paper, we report on experimental results to measure the total temperature of nitrogen gas at the inlet and outlet of microtubes with constant wall temperature and to quantitatively determine the heat transfer rates. Experiments were conducted with nitrogen gas flowing in a stainless steel microtube with a diameter of 524 µm and a copper microtube with a diameter of 537 µm. The temperature differences between the inlet and the wall were maintained at 3, 5, and 10 K by circulating water around the inlet and the wall. The stagnation pressures were also controlled so that the flow, with atmospheric back pressure, could reach Reynolds numbers as high as 26,000. To measure the total temperature, a polystyrene tube with a thermally insulated exterior wall containing six plastic baffles was attached to the outlet. Heat transfer rates were obtained from the gas enthalpy difference by using the pressures and the total temperatures measured at the inlet and outlet. Heat transfer rates were also compared with those obtained from the ideal gas enthalpy using the measured total temperatures and from the Nusselt number of incompressible flows. It was found that the measured total temperature at the microtube outlet was higher than the wall temperature. Also, the heat transfer rates calculated from the total temperature difference were higher than the values obtained from the incompressible flow theory. [ABSTRACT FROM AUTHOR]

Impingement cooling can effectively disperse local heat load, but its downstream heat transfer is always reduced due to crossflow effect. In this study, the flow and heat transfer characteristics of impingement cooling with semi-circular (SC), semi-rectangular (SR), semi-diamond (SD), and semi-four-pointed star (SFS) crossflow diverters are compared over the ReD ranging from 3,500 to 14,000 by solving three dimensional Reynolds-averaged Navier-Stokes equations with SST k-ω turbulence model. It is found that the arrangement of crossflow diverters changes the distribution of local jet Reynolds number (ReD,j/ReD) and reduces the mass velocity ratio of downstream crossflow to jet (Gcf/Gj), so the impingement heat transfer is enhanced significantly. However, friction loss also increases. Overall evaluation reveals that all crossflow diverters can improve the comprehensive heat transfer performance parameter (Φ), and the maximum increases of Φ are 11.0%, 14.3%, 12.2%, and 14.7% for SC, SR, SD, and SFS cases, respectively. It is noted that the Nusselt number of heated SFS-shaped diverter surface is also the highest. Besides, the influences of streamwise location (L) and thickness (t) of SFS-shaped diverter are also investigated. Results show that when the L increases from 2D to 3D, the heat transfer and friction loss change slightly; when the L increases from 3D to 4D, the heat transfer decreases sharply, and friction loss increases seriously. As for the t, it has almost no effect on the flow field and heat transfer. [ABSTRACT FROM AUTHOR]

Open-cell metal foams are known for their superior heat dissipation capabilities. The morphological, pressure drop, and heat transfer characteristics of stochastic metal foams manufactured through traditional "foaming" processes are well established in the literature. However, employment of stochastic metal foams in next-generation heat exchangers is challenged by the irregularity in the pore- and fiber-geometries, limited control on the pore-volume, and an inherent necessity of a bonding agent between foam and the heat source. On the other hand, additive manufacturing (AM) is capable of printing complex user-defined unit cell topologies with customized fiber shapes directly on the substrates subjected to heat load. Moreover, the user-defined regular lattices are capable of exhibiting better thermal and mechanical properties than stochastic metal foams. In this paper, we present a numerical investigation on fully periodic unit-cells of three different topologies, that is, tetrakaidecahedron (TKD), rhombic-dodecahedron (DDC), and Octet with air as the working fluid. Pressure gradient, interfacial heat transfer coefficient, friction factor, and Nusselt number are reported for each topology. Rhombic-dodecahedron yielded the highest averaged interfacial heat transfer coefficient whereas Octet incurred the highest flow losses. Pore diameter, defined as the maximum diameter of a sphere passing through the polygonal openings of the structures, when used as the characteristic length scale for the presentation of Nusselt number and Reynolds number, resulted in a single trendline for all the three topologies. [ABSTRACT FROM AUTHOR]

Conjugate pure mixed convection in a differentially heated square cavity with two vertically placed heat conductive revolving cylinders has been analyzed in a computational approach applying the finite element method. This analysis has been implemented considering the upper and lower wall as insulated simultaneously and the left vertical wall as heated maintaining constant temperature (i.e., isothermally heated) and the right vertical wall as isothermally cooled. The outcomes of this study have been examined concerning streamlines, isotherms, average Nusselt number (Nu) which unveils a noteworthy fact that both the rotating cylinders' inclination patterns and Reynolds number (Re) have a vital role upon the average Nusselt number (Nu), flow pattern, and isotherms. From that perspective, best heat transfer phenomena have been observed for counterclockwise circulation of both cylinders so that the condition for these cases has been assessed from a distance variation between the two cylinders maintaining a constant speed ratio (Ω). Moreover, the best result from this assessment has been trialed changing the type of materials of the rotating cylinders and ultimately, the best one has been specified as well as tested for various speed ratios. In the end, the results have been compared with the stationary cylinders where a similar type of results have been observed, but for rotating ones, Nusselt number (Nu) has been observed higher than stationary ones. [ABSTRACT FROM AUTHOR]

Convective heat transfer for a rotating sphere around a vertical axis floating in stationary fluid is studied numerically using the model of volume of fluid (VOF). The effects of the immersion angle and rotating velocity on the streamlines, isotherm and volume fraction contours, mean and local Nusselt numbers, volumetric flow rate, and water film thickness are investigated for the angular rotational velocity, 1500≤Ω≤3500 and the immersion angle, 30 deg≤θi≤60 deg⁠. The results show that the sphere's rotation causes the liquid to be sucked from the lower pole of the sphere, which is thrown out after stopping in the equator. Due to the strong jet flow in the equatorial zone, heat is transferred by forced convection, but diffusion is dominant for heat transfer in other zones. At low rotational velocities, the liquid film is thrown out of the equator in the form of large droplets, but as the rotational velocity increases, its shape changes to a jet. Also, it is found that there is a direct relation between the Reynolds number and mean Nusselt number at different immersion angles so that an average of 27.5% increase for the mean Nusselt number is achieved by increasing the immersion angle from θi=30 deg to θi=60 deg⁠. In addition, at a constant rotational velocity, the volumetric flow rate increases with increasing immersion angle. [ABSTRACT FROM AUTHOR]

We present the deep learning model for internal forced convection heat transfer problems. Conditional generative adversarial networks (cGAN) are trained to predict the solution based on a graphical input describing fluid channel geometries and initial flow conditions. Without interactively solving the physical governing equations, a trained cGAN model rapidly approximates the flow temperature, Nusselt number (Nu), and friction factor (f) of a flow in a heated channel over Reynolds number ranging from 100 to 27,750. For an effective training, we optimize the dataset size, training epoch, and a hyperparameter ν. The cGAN model exhibited an accuracy up to 97.6% when predicting the local distributions of Nu and f. We also show that the trained cGAN model can predict for unseen fluid channel geometries such as narrowed, widened, and rotated channels if the training dataset is properly augmented. A simple data augmentation technique improved the model accuracy up to 70%. This work demonstrates the potential of deep learning approach to enable cost-effective predictions for thermofluidic processes. [ABSTRACT FROM AUTHOR]

The performance of air-cooled generators can be improved only if they have efficient system designs for heat removal. An air-cooled generator is composed of a pair of coaxial cylinders, namely, a fixed outer cylinder (stator) and a rotating inner cylinder (rotor); the rotor has axial slits. In this study, we experimentally and numerically clarified the flow behavior and the heat transfer characteristics of rotating coaxial cylinders by simulating a salient-pole rotor in an air-cooled generator. The flow behavior in the slit between the salient poles was observed by using a high-speed video camera. We measured the temperature on the slit walls to investigate the heat transfer. The velocity fields and the heat transfer coefficient between the rotor and the stator were obtained via a numerical simulation. From the results, we experimentally and numerically observed the vortex structure in the slit. The local Nusselt numbers on the front-side wall of the slits near the impinging flow were higher than those on the back-side wall near the separated flow. The local Nusselt numbers on the front-side wall were high because the gap flow between the cylinders impinged on the front-side wall and promoted heat transfer. By contrast, the local Nusselt numbers on the back-side wall were low because a separated flow appeared near the back-side wall, where the hot fluid was retained, thereby causing the separated flow to disturb the heat transfer on the back-side wall. [ABSTRACT FROM AUTHOR]