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Abstract The heat transfer performance and friction loss of two-pass ribbed channels are highly influenced by the combined effect of rib orientation and bend shape. This paper, for the first time, numerically investigates this effect for four rib orientations, two rib angles and eight different bend shapes at Reynolds number of 30,000. The results show that the rib orientation determines the rotation direction and the transverse position of secondary flow in straight passages, while the bend shape determines the delivery of upstream secondary flow and the generation of local secondary flow. The key to heat transfer enhancement in the second passage is to promote local secondary flow using the energy from upstream secondary flow with the same rotation direction. The upstream secondary flow with the same rotation direction should be guided to a proper position, in order to enhance the dominant secondary flow in the second passage and reduce energy loss due to the undesirable secondary flow interaction. The impact of different bend shapes on the secondary flow has been summarized. The round inner corner for the bend is preferred for the significant friction loss reduction and the excellent secondary flow energy delivery capacity. Meanwhile, the secondary flow generated by the square outer corner can significantly enhance the cooling performance at the entrance of the second passage for some certain rib orientations. An optimized combination of 60° rib orientation and bend shape can either raise thermal performance factor by 10% in the second passage or reduce the friction loss by 22.3% in the whole channel, compared with the existing experimental design. By contrast, it is found cooling performance and pressure loss are not sensitive to bend shape for the rib angel of 45°. The optimized bend shape is inspired by an intuitive visualization of secondary flow evolution, which provides insight on the geometry optimization for heat and mass transfer applications. [ABSTRACT FROM AUTHOR]

Abstract To overcome the deficiencies of parallel-flow shell and tube heat exchanger with round rod baffles (RRB-STHX), STHX with a new type of anti-vibration baffle (hexagon clamping baffle) and equilateral triangle cross sectioned wire coils (HCBetsw-STHX) is proposed in this paper. Response surface methodology (RSM) is adopted to investigate its thermal-hydraulic performance. Baffle distance, coil pitch, coil diameter and Reynolds number are considered as four design parameters. Analysis is conducted based on CFD to acquire the objective functions (Nu H , Nu H / Nu R , f H , f H / f R and PEC) for different combination of design parameters. The RSM with Central Composite design is used to identify the relationships between the objective functions and the design parameters. Variances of the linear term, quadratic term and interactive term for design parameters in the response variables are analyzed. The mechanism of heat transfer enhancement of HCBetsw-STHX is illustrated from the view of field synergy theory. The results indicate that the heat transfer enhancement of HCBetsw-STHX is quite better than that of RRB-STHX. In the given design space, the Nu H / Nu R is in the range of 1.335–1.720; the f H / f R is in the range of 5.462–12.936. The most significant factor on Nu H is Reynolds number, while the most significant effect factor on Nu H / Nu R , f H , f H / f R and PEC is coil pitch. The sensitivity of three objective functions (Nu H , f H and PEC) to design parameters is explored. In addition, the design parameters optimization for the maximum value of Nu H and PEC and that for the minimum value of f H are also carried out and corresponding flow structures are shown and illustrated. Highlights • A new parallel-flow heat exchanger with hexagon clamping baffles and wire coils is proposed. • Parameter sensitivity of factors on thermal-hydraulic performance is explored. • Design parameters optimization is explored to maximize the Nu and PEC and to minimize the f based on the generated response surface. [ABSTRACT FROM AUTHOR]

• v 2 f turbulence model predict hydrodynamics and heat transfer of free surface liquid jet better compared to k − ϵ and k − ω models. • Lower jet to plate spacing (H / d) results in higher momentum before and after impact on impingement surface compared to high H / d. • For lower H / d , higher shear stress was observed throughout the impingement surface compared to higher H / d. • Nusselt number strongly dependent on jet to plate spacing for H / d ≤ 1. Numerical study is carried out to investigate hydrodynamic structures and heat transfer characteristics of vertical free surface liquid jet impingement on a horizontal flat plate. Different turbulent models (k − ϵ , k − ω , v 2 f) are compared to predict velocity field of the free surface jet, and the most suitable turbulence model is identified for prediction of flow physics of free surface jet impingement. A parametric study is performed to investigate the effect of Reynolds number (20000 − 60000), nozzle to plate spacing, H / d (0.5 − 2), and nozzle exit diameter (10.9, 15, and 23 mm) on the hydrodynamics, turbulence and heat transfer. The paper focuses on the underlying physics for jet impingement in the case of the lower jet to plate spacing, H / d (0.5 − 2). It was found that v 2 f turbulence model predicts flow field reasonably well compared to other turbulent models (k − ϵ and k − ω). Liquid jet radius before the impact and liquid film thickness after the impact have been examined. Velocity field, pressure and shear stress distribution on the flat plate are presented for free surface jet impingement. Further, turbulence intensity variation is examined and its effect on heat transfer is discussed. Jet to plate spacing, H / d = 0.5 resulted in around 20% higher Nusselt number compared to H / d ≥ 1 cases; and it is identified that a higher momentum before impact, compared to H / d ≥ 1, is the driving factor. Transition to turbulence is observed for higher Reynolds number and is explained by analyzing the variation in turbulence intensity along the flow streams of the free surface jet. [ABSTRACT FROM AUTHOR]

Highlights • Six heat transfer regimes identified. • Existence of a turbulent flow but laminar-like heat transfer regime for low Prandtl numbers. • Prediction of Nusselt number in turbulent flow and turbulent heat transfer regime. Abstract This paper investigates heat transfer regimes in fully developed plane channel flows without considering the buoyancy effect. Analyzing the governing thermal energy equation, aided by direct numerical simulation (DNS) data, six heat transfer regimes are identified including (i) laminar flow and laminar heat transfer (LamF-LamH), (ii) transitional flow and laminar-like heat transfer (TraF-LamH), (iii) transitional flow and transitional heat transfer (TraF-TraH), (iv) turbulent flow but laminar-like heat transfer (TurF-LamH), (v) turbulent flow and transitional heat transfer (TurF-TraH), and (vi) turbulent flow and turbulent heat transfer (TurF-TurH). One key result is the clarification of a TurF-LamH regime which exists only for low Prandtl number fluid (Pr ≪ 1). A critical non-dimensional number is determined as Re τ Pr 1 / 2 ≲ 50 where Re τ is the Reynolds number defined by the channel half-height δ and the frictional velocity u τ . In the TurF-LamH regime, the simplified thermal energy equation yields a prediction of Nusselt number as Nu ≈ 6.0. The clarification of the TurF-LamH regime provides valuable insight into the understanding of the Nusselt number data for liquid metals, which have very low Prandtl number. Another major finding is that, in the TurF-TurH regime, the Kader-Yaglom-style equation is shown to be better than the traditional power law correlations at predicting Nusselt number, for either low or high Prandtl numbers. No separate correlations are needed for the prediction of Nusselt number at low Prandtl numbers and high Prandtl numbers. Another advantage of the Kader-Yaglom-style equation is that the equation can be theoretically connected to the log-law for the mean velocity and the mean temperature. More importantly, in the TurF-TurH regime the prediction of Kader-Yaglom-style equation can be reliably extended to ultra high Reynolds numbers, for either low or high Prandtl numbers. [ABSTRACT FROM AUTHOR]

Abstract In the process of drilling, the tubing-casing annulus are easily to be eccentric. Heat transport phenomenon occurring in such tubes and annulus can be complex, and this paper investigates forced convection in a vertical eccentric annulus with different radius ratios and eccentricity (normalized by the radius difference). Physical experiments and numerical simulation have been conducted with constant inner tube inlet temperature and flow rate of hot water. Results show that with the increase of radius ratios, the convection heat transport coefficient increases. For a radius ratio of 1.875, the heat transport coefficient increases as the eccentricity increases when the Re > 2000 and it decreases when the Re < 2000. For a radius ratio of 2.25 and Re < 1500, the influence of eccentricity is trivial, but for Re > 1500, the increase of eccentricity strengthens the heat convection. For a radius ratio of 3.06, the heat transport coefficient increases as the eccentricity increases. An empirical correlation for Nusselt number is derived as a function of Reynolds number, Prandtl number and eccentricity. Highlights • For D o / D i = 1.875, the effect of eccentricity is identified into two zones. • For D o / D i = 3.06, the increase of eccentricity enhances the convective heat transport. • The increase of radius ratio strengthens the heat transport of annular channels. • With low eccentricity and low Reynolds number, the increase of radius ratio can hardly improve the heat exchange efficiency. • Three kind of empirical correlations are regressed through parametric analysis. [ABSTRACT FROM AUTHOR]

Highlights • Temperature distributions in electrokinetically driven microchannel flows with axially varying wall heat flux are obtained. • Both the Joule heating and axial conduction effects are considered. • Numerical results agree well with the analytical theory over practical ranges of problem data. • Results are of importance for efficient thermal management of microelectronics. Abstract There has been a growing interest in the development of microchannel heat sinks deploying electrically modulated fluid flow in recent years. The efficient design of such devices requires heat transfer models that can account for complex distributions of heat generation in microelectronics. In this paper, expressions are obtained for temperature distribution and Nusselt number of thermally developing mixed electroosmotic and pressure-driven flow through circular/slit microchannels of axially non-uniform wall heat flux. The heating section is considered to be of a finite length in order to simulate a physically more realistic situation. Both the Joule heating and axial conduction effects are considered in the model. By comparing the results for linear, sinusoidal, and exponential distributions of the wall heat flux with the predictions of full numerical simulations, it is shown that the analytical solutions presented are accurate up to a Péclet number of 10. This threshold is demonstrated to be larger than the maximum Péclet number encountered in practical applications involving electroosmotic pumping mechanisms. After justification of the model, a parametric analysis is executed, revealing that the average Nusselt number is a decreasing function of the EDL thickness and pressure-driven velocity, irrespective of the wall heat flux distribution. Moreover, whereas a higher Joule heating rate is accompanied by a smaller value of the average Nusselt number for pure electroosmotic and pressure-assisted flows, the opposite is true in the presence of a significant back pressure. [ABSTRACT FROM AUTHOR]

Highlights • Heat transfer and pressure loss in swirl cooling tubes with dimples were numerically obtained. • Total heat transfer performance of the swirl tubes with dimples can be increased. • Pressure loss of the swirl tubes with dimples can be decreased. • Dimple depths have appreciable influences on the heat transfer and pressure loss. • Detailed interactions of the swirling flow with dimples at the wall were provided. Abstract This paper presents a numerical study of heat transfer and pressure loss characteristics in a swirl cooling tube with five tangential inlet jets and with dimples on the tube inner surface. The swirl tube has a length of 20 times the tube diameter, and the Reynolds number based on the tube diameter ranges from 10,000 to 40,000. Polyhedral meshes as well as the SST k - ω turbulence model were chosen for the numerical calculations. A swirl cooling system with a smooth tube was investigated as baseline, and comparatively with dimples on the tube inner surface. The results showed different swirling flow and heat transfer patterns in the swirl tubes. Dimples on the tube inner surface can reduce the wetted-area averaged heat transfer coefficient on the tube inner surface. However, the total heat transferred in the dimpled swirl cooling tubes can be increased by up to 7.2% due to the increased heat transfer area and the interactions between the swirling flow and the dimpled wall. On the other hand, the pressure loss in the dimpled tube can be appreciably reduced by up to 17.6% compared with that of the smooth swirl tube. Detailed flow interactions between the jets, the tube wall and the dimples are illustrated to explain the heat transfer change and pressure loss reduction mechanisms. [ABSTRACT FROM AUTHOR]

Highlights • Falling film flows with natural and forced disturbances are simulated. • Velocity vector in a wave is obtained by LBM. • Dimensionalization to actual absorption by a velocity ratio. • Compare wave flow with laminar one on falling film absorption. Abstract The investigations on heat and mass transfer in the form of falling film flow were mainly focused on the simulation of heat and mass coupling adopting the continuity hypothesis of Navier-Stokes equations. Most of the previous works were done with laminar flow assumptions, or just deal with a very short smooth laminar section beyond the falling film entrance. In this paper, a study investigating the effects of wavy flow on stream absorption by falling liquid film is presented in the perspective of sorption refrigeration process. A multi-phase model lattice Boltzmann method is adopted to simulate the wavy falling liquid film flow, and the absorption process takes place in this flow field region. Absorption simulation was carried out using the laminar flow assumption with the semi-parabolic velocity distribution and the fluctuation results in the LBM simulation. When the overall simulation section is 1 m in length, with initial velocity 0.1 m/s for lithium bromide solution falling film flow, results show that the wave flow has apparent enhancement on heat and mass transfer. Local dimensionless numbers for mass (Sh) and heat (Nu) transfer with waves increase 4 times and 2 times, respectively, compared with the laminar flow. [ABSTRACT FROM AUTHOR]

Abstract The advancements in the materials science and the ability of manufacturing new materials, make it possible for engineers to use the porous medium as a solid body that provide maximum surface contact with the fluid. In this paper, to increase the contact surface area and decrease the flow resistance, the solid tapered fins replace with porous one and its effect on convective heat transfer is investigated for different Reynolds number. To do this, first the heat transfer of the rigid tapered fins is studied and validated, then, the solid body is changed to the porous one and the influence of the Reynolds number was investigated on the heat transfer and pressure drop. The results show that in laminar flow the Nusselt number of the flow with porous medium is 33% higher and the pressure drop is 9.35% lower than the rigid one with the same conditions. Therefore the application of the porous medium at low velocities is much recommended but with increasing the velocity, the porous media is not preferable. Also, for the problem of flow around a bundle of porous tapered fins, an equation for Nusselt number is presented based on the Reynolds number. [ABSTRACT FROM AUTHOR]

Abstract This paper discusses on the heat and fluid flow around two co-rotating cylinders in the tandem arrangement. The non-dimensional rotating speed (R.S) varies in the range of 0 ≤ R.S ≤ 4 and different non-dimensional gap spaces such as G/D = 1.5, 2.0, and 3.0 are considered between the cylinders. Computations are performed at the Reynolds number of 200 with constant Prandtl number of 7.0. It is demonstrated that rotating the cylinders deforms the recirculating regions of both upstream and downstream cylinders in which the rate of this deformation changes as a function of the R.S and G/D. On the other hand, co-rotating the cylinders shows some additional events such as the azimuthal displacement of the front stagnation points and development of the negative lift coefficient for both cylinders. It is found that the instabilities of the shear layer for both upstream and downstream cylinders are maximum at R.S = 1 and with increasing the R.S, the vortex shedding suppresses around the cylinders due to dominating the fluid rotating zone. Finally, it is revealed that at higher R.S values, a uniform Nusselt number distribution can be observed on both cylinders regardless of the gap space between the upstream and downstream cylinders. Highlights • Heat and fluid flow of both cylinders strongly depends on the RS and G/D. • Vortex strength of both cylinders against the RS is stronger for small G/D values. • At high RS values, a uniform Nusselt number distribution develops on the cylinders. • The RS = 1 is found as the critical RS at Re = 200. • Oscillations of the lift and drag coefficients become maximum at RS = 1. [ABSTRACT FROM AUTHOR]

Abstract In this paper the relative performance of two different differential stress and flux models for computing natural convection in an enclosure is examined. One model is the low-Reynolds number differential stress and flux model and the other model is a recently developed elliptic blending based differential stress and flux model. The primary emphasis of the study is placed on investigating the accuracy of the both models for the turbulent natural convection in an enclosure. Both models are used to predict turbulent natural convections in a square cavity with hot and cold side walls and conducting top and bottom walls (Ra = 1.58 × 109). The relative performance between both models is examined through comparing their solutions with the experimental data. It is shown that both models predict the vertical mean velocity component and mean temperature well. It is also shown that the low-Reynolds number differential stress and flux model slightly under-predicts the turbulent kinetic energy and vertical turbulent heat flux. It is also noted that the implementation of the elliptic blending second moment model is easier since this model does not contain any wall-related parameters and damping functions. [ABSTRACT FROM AUTHOR]

In a double-wall cooling system with multiple jet impingement the arrangement and size of effusion holes may change flow field and thereby change heat transfer characteristics. This paper stands in the view of internal cooling of turbine blades, and studied multiple-jet impingement heat transfer in double-wall cooling structures with narrow channels with pin fins and different-size effusion holes on the target wall. Five target plates were investigated including flat plate, pin fin plate and three pin fin plates with different-size effusion holes (effusion-to-jet diameter ratio of D e / D j = 0.5 , 1.0 a n d 1.5 ). Transient liquid crystal thermography experiments were conducted to explore the heat transfer characteristics on these target plates. The ratio of jet-to-plate spacing was fixed to be 1.5 and Reynolds numbers based on the jet diameter range from 15,000 to 30,000. The experimental results showed that the pin fins and effusion holes reduce the crossflow strength in downstream region, improve and uniform heat transfer on the whole target plate obviously. Compared with the flat plate, pin fin plate with effusion holes of D e / D j = 1.5 has highest averaged Nusselt number on the endwall. Numerical computations were carried out based on the experimental model, which revealed that the total heat transfer quantity on the pin fin plate with effusion holes of D e / D j = 1.5 can be increased by up to 51% comparing to that of the flat plate. Detailed interactional flow information between the wall jet flow, pin fins and effusion holes is expounded for the heat transfer improvement in the impingement-effusion structures. Moreover, conjugate heat transfer analyses were done to further investigate the overall cooling performance of the impingement-effusion structures. [ABSTRACT FROM AUTHOR]

In this paper, a numerical investigation is performed to study the heat transfer characteristics and the mixed convective flow past a square cylinder oscillating at different inclinations. The computations are carried out for a fluid with a Prandlt number of 0.7 at low Reynolds number ranging from 40 to 220. The tilt angle is varied from 0 (inline oscillation) to π/2 (transverse oscillation). The governing equations were solved using a finite-volume technique. The validity of the simulations is established by comparison with the available data from literature. The flow and heat transfer features are analyzed for different angles of oscillations. A critical oscillation angle at which the flow structure changes from inline oscillation mode to transverse oscillation mode has been determined. The effects of blockage ratio (β = h/H, the ratio between the side length of the cylinder and the height of the channel), Reynolds number Re , oscillating amplitude A m and oscillating frequency F c on the heat transfer characteristics are examined. The results show that the heat transfer is enhanced remarkably, when these parameters are increased. Furthermore, the rate of heat transfer from the cylinder increases when the angle of oscillation tends to π/2. For engineering applications, appropriate correlations have been proposed between the average Nusselt number and the different parameters. Finally, the interest of using the inclined oscillation is discussed. This approach has the advantage of widening the range of the oscillation amplitude. The results show that the heat transfer rate has been at least doubled. [ABSTRACT FROM AUTHOR]

In this paper, a numerical model is proposed to study the transition to three-dimensionality and the evolution of vortices in the wake of a blunt-headed cylinder considering air as the working fluid in the Reynolds number (Re) range 130–2000. An experimental setup is developed to validate the numerical model on the basis of heat transfer characteristics. Based on symmetry and time dependence characteristics of flow, it is categorized as dynamically steady symmetric flow (50 ≤ Re ≤ 200), dynamically steady unsymmetric flow (220 ≤ Re ≤ 270) and unsteady flow (Re ≥ 280). The vortex dislocation phenomenon showing intermittent low frequency pulsations during the transition to three-dimensionality is studied. Aerodynamic characteristics are studied by computing force coefficients and Strouhal number (St). Brief overview of heat transfer characteristics is given. Dynamic mode decomposition (DMD) analysis is carried out to determine quantitatively the critical Re at which secondary instability occurs and hence clear demarcation of the Re regime is obtained for the validity of two-dimensional analysis of blunt headed cylinder. [ABSTRACT FROM AUTHOR]

• Kagome truss structure of water-drop-shaped rods is proposed. • Flow and heat transfer characteristics of the composite structures are conducted. • The thermal performance of the new structure is better than the reference structure. • The best thermal performances of the composite structures with cross-flow are found. Jet array impingement providing high heat transfer are widely used in heat exchangers. However, the target plate is easily damaged impinged by high-speed air flow constantly. To solve this issue, Kagome truss structure is combined with jet array impingement to improve cooling performance in this paper. The flow and heat transfer characteristics of this composite structure in a heat exchanger unit with/without cross-flow were experimentally studied. Additionally, Kagome truss structures of circular and water-drop-shaped rods were used respectively in jet space and the results showed that the latter was beyond 7% higher than the former for the comprehensive thermal performance. When cross-flow was not added, the comprehensive thermal performance factor of the latter reached to a maximum of 1.656 at the Reynolds number of 5000. When cross-flow was added, the comprehensive thermal performance got worse at the cross-flow-to-jet velocity ratio of 1.0, while the best comprehensive thermal performances were found at the cross-flow-to-jet velocity ratio of 0.6 and at the mass flow rate proportion of cross-flow of 0.5, respectively. These results can provide a basic experimental data as a reference for designing new structures in advanced heat exchanger. [ABSTRACT FROM AUTHOR]

In this paper, the middle double swirl cooling (M-DSC) models for diverse nozzle aspect ratios (C ar) are established for flow and heat transfer behavior research. The k-ω turbulence model and the 3D viscous steady RANS equations are utilized for numerical calculation. The finite volume method and coupled implicit solver are chosen for simulation calculation. Influences of C ar values, namely 0.375, 0.67, 1.5, 2.67, 4.17 and 6 on the flow characteristics and thermal performance for the M-DSC are investigated when keeping vortex nozzle inlet area constant. Moreover, the cooling performance for diverse inlet Reynolds numbers 12,500, 22,500, 32,500 and 42,500 is studied with the model owning the optimal C ar. Results show that the velocity of the crossflow will increase along in the outlet direction. As C ar decreases, the velocity where the jet coolant scours the target will decay more significantly toward the outlet. There are four typical shapes of high heat transfer regions on the target. The uniformity of heat transfer will increase with the growth of C ar. When the C ar is 4.17, the thermal performance factor is the highest. The flow and heat transfer regulations are almost constant when the inlet Reynolds number is altered. • Middle double swirl cooling models with different nozzle aspect ratios are established. • Effects of different nozzle aspect ratios and inlet Reynolds numbers on flow and heat transfer characteristics are explored. • Optimal nozzle aspect ratio with the best cooling performance is obtained. [ABSTRACT FROM AUTHOR]

Nowadays, utilization of micro/mini channel heat sinks (MCHS) has become a popular solution for cooling of electronic systems and maintaining their operating temperature in a desired range. Increasing the contact surface area between fluid and solid zones by geometrical modifications is a promising solution that significantly affects thermal performance and heat transfer in these systems. In this paper, considering thermohydraulic performance, the effect of creating grooves on the middle-placed barriers of a minichannel heat sink is investigated to access a criterion for favorable temperature performance. Evaluating different geometrical parameters of barriers and grooves including cross section, depth and position of horizontal groove as well as vertical groove distance, simultaneous employment of optimal horizontal and vertical groove in the channel at different Reynolds numbers is studied. Outputs including channel base temperature, friction factor ratio and total thermal resistance are studied to evaluate the performance of the proposed minichannel. Results indicate that vortex formation adjacent to solid walls considerably enhances the heat transfer and variation of each geometrical parameters offers a performance enhancement. Moreover, the best thermal performance is obtained by simultaneous employment of the horizontal rectangular groove (x = 0.45 mm, y = 0.2 mm and h g = 0.6 mm) and the vertical groove (d g = 0.4 mm). In the optimal case, the channel base temperature and the total thermal resistance reduces by 11.88 K and 27%, respectively. [ABSTRACT FROM AUTHOR]

The present paper compares measured and calculated heat transfer parameters for a small-size slot supersonic nozzle. Heat transfer coefficient and temperature recovery factor were examined under conditions of partial laminarization of boundary layer caused by moderate flow acceleration and low Reynolds numbers. The transient heat transfer method was implemented for estimation of these values for the nozzle subsonic and supersonic parts. Maximum values of flow acceleration parameter K lied in the range 1.3 ... 2.75∙10−6. Experimentally measured value was normalized to the developed turbulent boundary layer heat transfer coefficient and reached St/St Rex = 0.5–0.7 and indicated partial laminarization phenomenon. It was shown that the temperature recovery factor hasn't been affected by the flow acceleration. The results are compared to numerical heat transfer predictions using sst, γ-sst and γ-Re θ -sst turbulence models. Numerical calculations and experimental data were found to be in reasonably good agreement for low acceleration level with K less than 0.75∙10−6. Nevertheless, γ-sst turbulence model made slightly conservative estimation whereas in contrast γ-Re θ -sst model overestimates quantities for the moderate flow acceleration conditions. • Heat transfer in a small-size slot supersonic nozzle has been investigated. • Effect of flow acceleration on partial laminarization was assessed. • Temperature recovery factor does not depend on the flow acceleration. • Heat transfer coefficient reduces up to two times. [ABSTRACT FROM AUTHOR]

• Wall-resolved Large-Eddy Simulation of turbulent mixing flows in T-junctions. • Gravity effects can be neglected in cases with increased Reynolds number and decreased Richardson number. • Near-wall fluctuations increase due to raised Reynolds numbers. • Highest temperature fluctuations are 40% of the temperature difference between the mixing flows. • Highest thermal fluctuations in the solid region are generated in case 2C. This paper presents a detailed numerical study regarding the influence of Reynolds and Richardson numbers in T-junctions with circular cross-section. The numerical investigation is based on the wall-resolved Large-Eddy Simulation (LES), which has been experimentally validated. The Reynolds number in the main pipe covers a range from R e = 78400 to R e = 235200 , which is high enough to approach the engineering applications. This also belongs to the primary contribution of the current study. Furthermore, the temperature difference between the main pipe and branch pipe (T m − T b) is 180 K and the mass flow rate ratio (m ˙ m / m ˙ b) is 3. The primary focus is on thermal mixing characteristics in the downstream as well as on the evolution of the flow by consideration of gravity and buoyancy effects. Additionally, our study considers the thermal field and its corresponding fluctuations in the near-wall region. In addition, this paper also includes simulations with different T-junction configurations therefore we can examine the influence of the entrance orientation of the heavier cold branch pipe fluid on the mixing process and heat transfer. The analysis of our results indicates that gravity effects can be neglected in cases with increased Reynolds number and simultaneously decreased Richardson number. The area with high fluctuation intensities becomes larger by the higher Reynolds number and lower Richardson number of the mixing flow therefore they shows a higher potential for High Cycle Thermal Fatigue (HCTF). Furthermore, the highest near-wall thermal fluctuations are 40 % of the temperature difference between the mixing flows. [ABSTRACT FROM AUTHOR]

• The paper presents a novel approach for heat transfer enhancement using a cylindrical insert with impinging jet. • Twin jets are formed with velocity twice as high as that of a jet without an insert • The complex flow near the stagnationzone creates a plume like structure with high turbulent kinetic energy Periodic vortex shedding which forms when fluid moves over a circular cylinder creates a wake which causes fluctuating unsteady forces on structures. The oscillating wake may also serve to enhance heat transfer rates from a surface on which it impinges. In this paper we investigate the passive excitation of the impinging jet's velocity field using a cylindrical insert. The insert is placed just before where the jet issues from the circular pipe. The flow is examined using Large Eddy Simulations (LES). The jet's Reynolds number based on bulk inlet velocity is 23,000 and jet's outlet-to-target wall distance is two. It is found that using a cylindrical insert in the impingement pipe results in enhanced heat transfer rates for cooling applications. [ABSTRACT FROM AUTHOR]

In this paper, we apply a novel immersed boundary method to simulate pore-scale level fluid flow and convective heat transfer in realistic numerically generated open-cell solid foams in a Cartesian computational domain. Five different periodic foam samples of varying porosities ( ε = [ 0.877 , 0.948 ] ) are generated by numerically mimicking the actual foam formation process (minimizing surface area). The step-by-step procedure for generating the periodic foam geometries is presented. The specific surface areas of the generated foams of different porosities are compared with real foam geometries showing a reasonable agreement. The Reynolds number ( Re ) is varied from Re ≈ 0 (creeping flow) to Re ≈ 500 , and finally drag and Nusselt correlations have been proposed. A detailed analysis is presented on the local velocity and temperature field for the fluid-solid interaction in a complex cellular porous medium. [ABSTRACT FROM AUTHOR]

In order to improve the cooling process in impingement synthetic jets, it is necessary to evaluate the influence of dimensionless parameters and the geometry of the flow field and heat transfer. In this paper, the effects of geometry (confined and unconfined impingement synthetic jets), and jet-to-surface spacings, Reynolds number and dimensionless stroke length on a three-dimensional unsteady impingement synthetic jet are studied. For this purpose, two types of turbulence models, namely the v 2 − f and S S T / k − ω , have been employed. The simulation results have been indicated that the v 2 − f model comparing to the S S T / k − ω model has a close agreement with the available experimental data. The results show that the flow field and the corresponding heat transfer distribution of the impingement synthetic jet are affected by geometry such that an unconfined impingement synthetic jet is more efficient in a cooling process relative to the confined case. Also, increasing jet-to-surface spacing affects the vortex structure and consequently the heat transfer. The stagnation heat transfer rate reaches to the maximum value at an optimum impingement distance as a result of the appropriate ventilation and the coherence vortex structures. The stagnation heat transfer rate experiences several extrema as a result of the increasing stroke length and under the higher stroke length, the impingement synthetic jet acts similar to the impingement continuous jet. [ABSTRACT FROM AUTHOR]

This paper focused on the trajectory and flow patterns of submerged horizontal gas jet in stagnant ambient liquid. A rigorous mathematical model for the jet trajectory was developed based on geometric analysis, mass conservation, and momentum. Additional, multiple N 2 -Water experiments employing binary image processing were designed and performed. The experimental procedures and mathematical derivations are presented here in details. To assess the ability of the mathematical model to predict the jet trajectory, extensive experimental results were extracted for comparison with the mathematical results. The mathematical results were found to be in good agreement with the experimental results. Furthermore, a dimensionless jet spreading angle model was proposed and fitted to the experiment results. A flow pattern transition criterion based on the penetration depth of the buoyant jet in the water was also discussed, distinguishing the gas buoyant jet form bubbly or transitional flow patterns. [ABSTRACT FROM AUTHOR]

This paper constructs a three-dimensional model to numerically investigate the heat transfer and fluid flow characteristics of combined microchannel with cone-shaped micro pin fins (MCPF) for inlet Re number ranging from 147 to 637. Firstly, the cases of micro pin fins located in the upstream (case1), middle (case2), downstream (case3) of microchannel and distributed throughout the microchannel (case4) are studied with fixed number of fins. Thereafter, the dimensionless parameters of relative fin diameter, relative space between the fins and relative height of the fin are studied to optimize the design. The variations of apparent friction factor f app,ave and average Nusselt number Nu ave with inlet Re number have been conducted, and the thermal enhancement factor ( η ) has been adopted as evaluation criteria to determine the best comprehensive thermal-hydraulic performance of MCPF. The results show that the microchannel inserted cone-shaped micro pin fins improves the heat transfer accompanied with a higher pressure drop as compared with the smooth rectangular (Rec) microchannel. Microchannel with uniform fins distribution (case4) has the largest η and the bottom wall temperature is the lowest and well-distributed among all cases. When Re < 500, the overall thermal performance of case3 is better than case1 but when Re > 500, it displays a contrary tendency. The geometric configuration optimization of MCPF suggests that there is an optimum pin diameter, fin space and fin height that satisfy the demands of heat transfer and pressure drop. [ABSTRACT FROM AUTHOR]

This paper numerically compares the flow structures and thermal performance in a swirl chamber with three different inlet chambers. The cross section of inlet chambers are constant, convergent, and divergent along the streamwise direction, respectively. Parametric studies involve Reynolds number and the temperature ratio between cooling air and the wall of swirl chamber. Results indicate that shear layer vortices, corner vortices, and large-scale vortices appear in different sections of swirl chamber. Besides, Görtler vortices appear in convergent inlet chamber. The alternation of the inlet chamber changes more than the value of the Nusselt number. It also changes the flow structures and the distribution of Nusselt number. In comparisons, the swirl chamber with convergent inlet chamber possesses the highest thermal performance coefficient. With the increase of Reynolds number, the intensity of heat transfer is significantly enhanced. Different from former research, the rise of temperature ratio leads to relatively high Nusselt number due to the addition of multiple jet nozzles and the inlet chamber. In addition, empirical correlation among global average Nusselt number, Reynolds number, and temperature ratio is obtained to assist relevant research and design. [ABSTRACT FROM AUTHOR]

This paper presents a two-dimensional numerical study of a novel flapping vortex generator mounted on a heatsink fin for airside heat transfer enhancement. The proposed vortex generator is made with a thin elastic sheet bonded to the inner wall of the heatsink channel with an inclined angle. Our investigations are focused on the effects of the Young’s Modulus of the vortex generator on the oscillations of the elastic sheet, vorticity fields, and heat transfer performances. The results are compared with the heat transfer performances of conventional rigid agitators at two different flow velocities (Reynolds nunbers). Our numerical results demonstrate that the vortex generator with a Young’s Modulus of 1 MPa has the best performance among the other three choices and can enhance the rejected heat by 140% at the same velocity and 87% at the same total pumping power. The developed flapping vortex generator can improve the average Nusselt number by 200% compared with a clean channel with the same Reynolds number. Modal analysis is performed with transient temperature and vorticity results using dynamic modal decomposition where it is found that a steady modal behavior directly influences the thermal performance of the system. Furthermore, creating more discrete patterns near the boundaries of the steady mode in the vorticity field can enhance the internal convective heat transfer rate. The numerical results presented can help to guide the design of the flapping vortex generators in future high-performance airside fins. [ABSTRACT FROM AUTHOR]

In this paper, the effect of vortex induced vibration (VIV) on convective heat transfer from an elastically mounted rigid circular cylinder in cross-flow is investigated numerically. The motion of cylinder is modeled by using a mass-spring-damping system. The effect of reduced velocity and damping ratio on the vortex formation, vortex shedding process, cylinder displacement amplitude, Nusselt number and the position of maximum local Nusselt number is studied. In this study the Reynolds Number and the mass ratio are 150 and 2, respectively. Furthermore, different values of reduced velocity and damping ratio are investigated ( U r = 3, 4, 5, 6, 7, 8 and   ζ = 0. 0.01, 0.05, 0.1). The numerical results demonstrate that the reduced velocity and damping ratio can affect the heat transfer considerably. The beating phenomenon is occurs at U r = 4 and ζ = 0.05 which leads to changes in the displacement amplitude and Nusselt number widely with respect to the time. In the beating phenomenon the total Nusselt number decreases in comparison with a stationary cylinder. The maximum heat transfer enhancement is obtained at U r = 4 , ζ = 0 . [ABSTRACT FROM AUTHOR]

The study of the forced convection in a channel has many practical applications. In this paper, the forced convection heat transfer from surface mounted blocks attached to the bottom wall of a horizontal channel with nanofluid is numerically studied by the second-order lattice Boltzmann method (LBM). The effects of Reynolds numbers and geometrical parameters of the blocks in different aspect ratios on the flow field and temperature distribution for various volume fractions of nanofluid (φ = 0, 0.01, 0.03 and 0.05) are analyzed. Also, the influence of these parameters is investigated on the local and average Nusselt numbers. It is concluded that heat transfer in channels can be enhanced by using the block on the walls and adding nanoparticles. There is a maximum value of 39.04% increase in average heat transfer coefficient for all the examined cases compared to the base fluid (i.e., water). [ABSTRACT FROM AUTHOR]

Perforated plates interspaced with spacers are effectively used in recuperative heat exchangers. In this paper, we present generalized correlations for Colburn factor and friction factor of perforated plates as functions of all geometrical variables and Reynolds number. Heat transfer and flow friction data are generated through CFD modelling on a stack of perforated plates. The model is validated with the published experimental data. Correlations are developed through regression analysis over a large number of computed data generated under varying parametric conditions. Effects of various geometrical parameters on the thermos-hydraulic characteristics of perforated plates are discussed in terms of Colburn factor, friction factor and modified area goodness factor. [ABSTRACT FROM AUTHOR]

In this paper, a two-dimensional numerical simulation is carried out to understand the effect of confinement (blockage ratio β ) on fluid flow and forced convective heat transfer characteristics past a blunt headed cylinder. Utilizing air as an operating fluid, flow simulations are carried out for wide ranges of blockage ratios ( 1 10 ≤ β ≤ 1 3 ) and Reynolds numbers ( 60 ≤ Re ≤ 200 ) . The flow characteristics and heat transport are analysed critically for different β . The functional dependence of C D (Drag Coefficient) and C L (Lift Coefficient) on blockage ratio is examined. It has been found that C D reduces with increasing Re, while the Strouhal number and the average Nusselt number show an increasing trend when the blockage ratio is increased. The average Nusselt number also increases with increasing Re . [ABSTRACT FROM AUTHOR]

Presented research paper is an investigation of water, paraffin and ethylene glycol based Al 2 O 3 nanofluid. Heat transfer coefficient is measured using shell and tube heat exchanger under the conditions of fully developed laminar and turbulent flow. Concentration of nanoparticles in the respective base fluids was varied from 0.01 vol% to 0.08 vol% in the step of 0.1%. Nanofluids were used as the working fluid and sent to tubular side. The effects of Reynold's number and concentration of nanoparticles in nanofluid on heat transfer characteristics were studied. A significant enhancement in heat transfer coefficient and thermal conductivity of nanofluid over base fluid is found in the study. As the concentration of nanoparticles was increased, heat transfer coefficient was also improved marginally. At greater turbulence, the improvement in the heat transfer coefficient of nanofluid over base fluid was found to be more than that at lower turbulence. This is due to homogeneous distribution of nanoparticles in a base fluid at higher temperatures was achieved. The various factors that were considered in the characterization of nanofluid heat transfer coefficients are: sonication time, temperature, pressure difference, base fluid (water, paraffin and EG). Pressure drop in case of nanofluid was higher than base fluid in the turbulent regime; however, no significant change was observed in the laminar region. [ABSTRACT FROM AUTHOR]

The present paper deals with entropy generation study of CuO/water nanofluid flow over a spherical heat source that is installed inside a cylindrical channel with sudden expansion and contraction. To estimate the entropy generation rate; first, the governing equations are solved numerically to find the velocity and temperature fields in the domain where the flow is steady and laminar. Next, using the simulation data, the entropy generation number is calculated theoretically. The effects of nanofluid concentration, nanoparticle size, blockage ratio, and Reynolds number on the entropy generation have been investigated. The outcomes of this research elucidate that entropy generation number rises with an increase in the nanoparticle volume fraction and surface temperature of the heat source. On the other hand, it was found that the entropy generation is diminished by implementing finer nanoparticles and lowering the blockage ratio. In this study, a new correlation has also been presented to calculate the Nusselt number that is a function of Reynolds number, Prandtl number, and blockage ratio. [ABSTRACT FROM AUTHOR]

Inserts are used to enhance the heat transfer rates between the two fluids in heat exchanger tubes. A variety of tube inserts such as twisted tape, wire coil, swirl flow generator have been investigated for their effect on heat transfer rates and fluid friction. This paper reviews the works pertaining to the application of different class of tube inserts in order to comprehend the prevailing mechanism of fluid flow and heat transfer. An attempt has been made to elucidate the fluid flow behaviour sustained by the particular class of insert that controls the heat transfer rates across the thermal boundary layer attached to the tube wall. [ABSTRACT FROM AUTHOR]

In this paper, a numerical study of heat and mass transfer in counter-flow indirect evaporative air coolers is presented using an approach proposed by previous work. The mathematical model equations of continuity, momentum, energy and concentration are solved by the finite volume method to obtain the value of primary outlet dimensionless temperature. This mathematical model developed is validated against published data acquired from literature. The key target of presented analysis is to develop a performance correlation based on five dimensionless factors: ratio of channel length to half width of secondary air channel, ratio of water to secondary air mass flow rate, primary air inlet Reynolds number, secondary air inlet dimensionless temperature and water film inlet dimensionless temperature. Numerical simulations are performed by changing the value of each dimensionless factor under a constant value for other parameters. The simulation results are regressed to obtain the empirical constants of the correlation. The value of outlet dimensionless temperature is varied from 0.47 to 0.92. The developed correlation can predict the performance of the counter-flow indirect evaporative air coolers within a discrepancy of 12.6%. The correlation would be helpful to provide a simpler equation to improve the calculation speed within the engineering allowable accuracy range. [ABSTRACT FROM AUTHOR]

This paper presents a numerical modeling and simulation of incompressible laminar mixed convection in rotating channels using parallel lattice Boltzmann method. Individual distribution functions with D3Q19 and D3Q6 lattice types were considered to solve fluid flow and heat transfer problems, respectively. The Reynolds number was set to 100, and wall-to-inlet fluid density ratio was set to 0.2. Two rotation modes namely orthogonal and parallel modes were considered with rotation number equal to 0.2. LBM code was written in C language and was parallelized using OpenMP libraries. Domain decomposition method of data parallelism was adopted here, and simulations were conducted on a workstation with dual processors and 64 GB RAM. Predicted velocity and temperature fields were found to agree well with velocity and temperature obtained from Fluent. [ABSTRACT FROM AUTHOR]

This paper investigated the frost formation on louvered folded fins in microchannel type heat exchangers and the effects of frost accumulation on the time dependent air side convective heat transfer coefficients during transient heat and mass transfer processes. Seven fin geometries, which were commonly used in outdoor microchannel type heat exchangers of air-source heat pump systems, were investigated for a wide range of temperatures and humidity promoting frost growth. The air-side heat transfer coefficient was depended on the rate of frost growth and it sometimes increased during early frost formation and then it decreased sharply. Frost thickness on the fin leading edge, air face velocity, and corresponding heat transfer coefficients and pressure drops were measured and semi-empirical correlations were developed. The frost thickness and air face velocity were key variables for estimating a local time-dependent air-side Reynolds number and for describing the transient behaviour of the local air-side heat transfer coefficient of the fins. The newly developed correlations identified the main parameters that describe frost growth and heat transfer on louvered folded fins under frosting operating conditions and they agree with the experimental data of the present work and from the literature. [ABSTRACT FROM AUTHOR]

The present paper presents fluid flow and mixed convective heat transfer characteristics past a semi-circular cylinder at incidence with a tandem square cylinder in cross flow. Using air ( Pr = 0.71 ) as an operating medium, numerical simulations are performed for the range of Reynolds numbers, 10 ≤ Re ≤ 45 and incidence angles, 0 0 ≤ α ≤ 180 0 . The effect of thermal buoyancy is brought about by varying the Richardson number in the range 0 ≤ R i ≤ 2 . The mathematical model is firstly validated with the experimental and numerical results from the literature and found to be in good agreement. The steady separated flow is observed to become unsteady periodic under the superimposed thermal buoyancy. Furthermore, apart from thermal buoyancy effect, angle of incidence is found to play a pivotal role in bringing hydrodynamic instabilities and thereby vortex shedding for such steady mixed convective flows. Functional dependence of drag ( C D ) , lift ( C L ) , and moment ( C M ) coefficients on the combined influence of α and R i , is explored and analysed in detail. Additionally, other global quantities, such as local and average Nusselt number distribution, Strouhal number ( S t ) are determined with respect to the various ranges of parameters considered in the present investigation. [ABSTRACT FROM AUTHOR]

In this paper, 2-D numerical investigation and sensitivity analysis are performed on heat transfer rate and heat exchanger effectiveness of a double pipe heat exchanger filled with porous medium. The Darcy–Brinkman–Forchheimer model is applied to model the flow field in the porous zone. The sensitivity analysis is performed utilizing the Response Surface Methodology. The studied parameters are: Reynolds number (50 ≤ Re ≤ 250), Darcy number (10 −5 ≤ Da ≤ 10 −3 ), temperature difference between hot and cold fluids (30 ≤ ΔT ≤ 70) and the porous substrate thickness (1/3 ≤ δ ≤ 1). The obtained results showed that enhancement of the Nusselt number due to the increase in Reynolds and Darcy numbers is in the vicinity of the 77.84% for the case with δ = 2/3 and Da = 10 −5 to 10 −3 , and 203.25% for the case with δ = 1 and Re = 50 to 250. Furthermore, increasing porous substrate thickness reduces the mean Nusselt number until δ = 2/3 and then increases it. In addition, it is found that the heat exchanger effectiveness increases with the Re number and reduces with enhancement of the Da number. The sensitivity analysis showed that the sensitivity of the mean Nusselt number to the Re and Da numbers and the porous substrate thickness is positive, while the sensitivity of the heat exchanger effectiveness to the Re number is positive but to the Da number is negative. [ABSTRACT FROM AUTHOR]

In the present work, the effect of channel cross section on the heat transfer performance of an oblique finned micro-channel heat sink was investigated. Water and Al 2 O 3 /water nanofluid of volume fraction 0.25% were used as a coolant. The oblique finned microchannels are designed with three channel cross-sections namely square, semicircle and trapezoidal. The primary work of this paper is to study the heat transfer and hydrodynamic characteristics in the oblique finned microchannel. The experimental setup and procedure are validated using water as coolant in a micro-channel heat sink. Heat transfer and flow characteristics are examined for three cross-sections of varying mass flux. The trapezoidal channel cross-section increases the considerable heat transfer rate improvement for both water and nanofluid by 3.133% and 5.878% compared to square and semicircle cross section. Also, the pressure drop is higher in the trapezoidal cross-section over the square and semicircle cross section. This is due to increase in friction loss of trapezoidal cross section. The results indicate that trapezoidal cross-section oblique finned micro-channel is more suitable for heat transfer in the electronic cooling application. [ABSTRACT FROM AUTHOR]

Heat transfer enhancement inside ribbed channels for turbine blades is a critical phenomenon that impacts overall performance and life of the gas turbine. Present study investigates heat and fluid flow in a rectangular duct with heat transfer enhancement features, under rotating and stationary conditions. The heat transfer data obtained experimentally has been explained using numerical prediction of flow features. Detailed heat transfer coefficients have been measured on the walls of two-pass rectangular duct (AR = 1:2) featuring V-shaped rib turbulators, using transient liquid crystal thermography (TLCT). The first pass and second pass featured nine V-shaped ribs each and the bend featured a 90° rib connecting the blade tip underside and the two-pass divider wall. The flow in the first pass was developing in nature. The rib-pitch to rib-height ratio (p/e) was 9.625 and the rib-height to channel hydraulic diameter (e/d h ) was 0.125. The baseline case for the rib roughened duct was geometrically identical smooth duct (with no heat transfer enhancement features). Stationary experiments were carried out for Reynolds numbers ranging from 25000 to 75000. The rotation experiments were carried out at 400 RPM ( Ro = 0.036) and 700 RPM ( Ro = 0.063), at Reynolds number of 25000 ( Ro = Ω d h / V , Re = Vd h / ν ) . Also, numerical simulations were performed for a similar test model under similar flow conditions, using realizable k - ∊ turbulence model. Detailed discussion on rib induced secondary flows and rotational effects on heat transfer in smooth and rib roughened duct are presented in this paper using results obtained from detailed heat transfer measurements from experiments and fluid dynamics predictions from numerical simulations. [ABSTRACT FROM AUTHOR]

This paper reports the hydrodynamic and thermal performance of stainless steel wire mesh blocks in a vertical channel, for different aspect ratios. Commercially available stainless steel wire meshes are arranged side by side to act as a porous block. Convective heat transfer experiments in a vertical channel using an isothermal flat plate are conducted for a Reynolds number range of 60–2380. The plate is sandwiched with wire mesh block on either side so that the assembly completely fills the channel. The pressure drops across the filled region are measured for the velocity range discussed. The plate temperature is measured under steady state conditions. The effect of mesh block thickness on the hydrodynamic and thermal performance is analyzed, keeping the porosity fixed. Plain channel experiments are also conducted under similar conditions for purposes of comparison. The novelty of the present study is that a simple and cost effective method without any permanent bonding is developed for different mesh block thicknesses and its overall performance has been studied. This situation represents the least performance of the porous structure. The performance of the porous blocks is also compared with other porous structures available in literature, based on a performance factor. Nusselt number correlations are developed based on the experimental data. The thermal performance has been compared with brass wire mesh blocks of identical porosity and thickness. [ABSTRACT FROM AUTHOR]

In this paper, the thermal-fluid characteristics of a pin-fin heat sink with delta winglet vortex generators in a cross flow are investigated experimentally and numerically. The effects of the Reynolds number of the flow, the angle of attack of the vortex generators, the shortest distance between the vortex generators, the distance between each vortex generator and the heat sink, the height of the vortex generators, and the configuration of the vortex generators on the performance of the heat sink are explored. The results show that the thermal resistance decreases as the Reynolds number increases, but that the magnitude of the decrease tends to diminish with increasing Reynolds numbers. By considering the thermal resistance and the pressure difference simultaneously, it was determined that an angle of attack of 30° is a better arrangement for the vortex generators. The thermal resistance produced with the shortest distance between the vortex generators equal to the length of the heat sink is lower than that produced with the shortest distance between the vortex generators greater than the length of the heat sink. The heat transfer of the heat sink is higher when the vortex generators are installed against the middle of the heat sink on both sides of it. Although increasing the height of the vortex generators can enhance the heat transfer, the pressure difference also increases. The thermal resistance of the heat sink with the vortex generators arranged in the common-flow-up configuration is lower than that of the heat sink with the vortex generators arranged in the common-flow-down configuration. [ABSTRACT FROM AUTHOR]

In this paper, heat transfer in a carbon nanotube based fluid between two parallel rotating disks is investigated. The governing equations are solved by Least Square Method (LSM) using a semi-analytical code. Water is considered as the base fluid and two kind of carbon nanotube (CNTs), single-walled carbon nanotube (SWCNT) and Multi-walled carbon nanotube (MWCNT) are added as the second phase or additives to the base fluid. The problem is solved for different nanoparticles volume fraction and Reynolds numbers to study the effect of some parameters such as scaled stretching parameters (A 1 , A 2 ), Rotation parameter (Ω), Radiation parameter (Rd) and thermal Biot numbers (γ 1 ,γ 2 ) on the skin friction factors and Nusselt numbers. Results show that in most cases, SWCNTs leads to more heat transfer and temperature profiles compared to MWCNTs. [ABSTRACT FROM AUTHOR]

In this paper, some novel structures aimed to optimize the airside performance of H-type finned tube heat exchangers, such as longitudinal vortex generators, dimples/protrusions and grooves, are examined numerically. The effects of geometric parameters on the heat transfer, flow resistance and comprehensive thermal–hydraulic performances are investigated by means of Taguchi method. The optimum configuration of compound designs is determined on the basis of the optimizations of these structures. According to the results, for longitudinal vortex generators, dimples/protrusions and grooves, the heat transfer characteristics are significantly affected by the winglet type, vertical distance and groove diameter while the attack angle, dimple depth and groove type have prominent impacts on the flow resistance characteristics. The winglet type, vertical distance and groove number make the most contributions to the comprehensive thermal–hydraulic performance, respectively. In compound designs, the longitudinal vortex generator plays the most important role in the performance of heat exchangers. The comprehensive thermal–hydraulic characteristic of the optimal compound design exerts the improvements of 0.9–23.8%, 24.5–57.1% and 8.3–37.5% than those of LVGs, dimples/protrusions and grooves, respectively, when Reynolds number ranges from 4650 to 28,300. The enhancement of 4.5–16.6% can be achieved compared with the case built based on references. • The optimal parameters of longitudinal vortex generators, dimples/protrusions and grooves on H-type fins are determined by Taguchi method. • The innovative compound structures are designed based on the optimizations of each novel structure. • Longitudinal vortex generators have the primary impact on comprehensive thermal–hydraulic performances among three structures. • The optimal compound design can achieve significant improvements compared to single structure. [ABSTRACT FROM AUTHOR]

Jet impingement cooling is usually used in zones with extremely high thermal loads. The major factor that limits impingement heat transfer uniformity is the crossflow. The impingement cooling with return hole is applied to reduce the negative effect of crossflow on the target surface heat transfer in present paper. The influences of jet-to-plate spacing (H / D), impingement target surface curvature (α), temperature ratio (TR) and Reynolds number on flow field and heat transfer performance are investigated in detail. The results indicate that the correlation between the overall area average Nusselt number of target surface and H / D , the overall area average Nusselt number decreases by approximately 5.76% with H / D increases by one unit. Increasing the impingement target surface curvature is beneficial to expand the spanwise heat transfer. The overall area average Nusselt number increases by 295.5% for Reynolds number varying from 10,000 to 50,000 at TR = 0.910, and the overall area average Nusselt number increases by 8.68% for the TR varying from 0.668 to 0.910 at Re = 30,000. The results indicate that impingement cooling with return hole can achieve high heat transfer coefficient and uniform heat transfer distribution. [ABSTRACT FROM AUTHOR]

This paper presents a numerical investigation of the flow-induced pitching, heaving or surging of an airfoil in a rectangular channel for heat transfer enhancement. The flow dynamics, airfoil fluttering response, and heat transfer performance are analyzed and discussed at four different Reynolds numbers (Re = 700, 1000, 1300 and 1500), three rotational axis locations (a x / C =0.25, 0.33 and 0.42), two airfoil perturbation modes (passive vs. active), and three combinations of multiple airfoil freedoms (pitch only, pitch and heave, and pitch and surge) with various stiffness coefficients. It is found that after Reynolds number reaches 1000, the pitch only airfoil starts the similar self-fluttering pattern, and the fluttering effect becomes the dominant influencer for increasing the Nusselt number compared to the Reynolds number effect. Placing the rotational axis at a x / C =0.42 achieves the highest thermal enhancement factor η of 1.4, while when a x / C =0.25, the airfoil oscillation diminish effect is vast because of its aerodynamic center. The passive pitching airfoil can achieve more than 6% of the thermal enhancement factor than the active ones. Adding another heave or surge degree of freedom to the pitching airfoil, the heat transfer enhancement of the channel can also be improved. The present work can facilitate future optimizations of airfoil-based flow-structure interaction system for heat exchanger channels. [ABSTRACT FROM AUTHOR]

• Thermal and hydrodynamic characteristics of staggered drop-shaped tubes were studied. • The effect of the number of tube rows N R on the performance was investigated. • The effect of increasing Reynolds number Re on the performance was analyzed. • The investigation covered 1.78 × 103 ≤ Re ≤ 18.72 × 103 and 1 ≤ N R ≤ 20. • New correlations for N u ¯ , f , and ε considering N R were developed. In this paper, for the first time, air-side correlations were developed and introduced to describe the average Nusselt number N u ¯ , friction factor f , and thermal − hydraulic performance ε of compact heat exchanger employing drop-shaped tubes considering the number of tube rows (N R). Drop-shaped tubes were arranged in a staggered formation and placed in 1 to 20 consecutive rows in the flow direction, each consisting of 7 tubes in the transverse direction. The Reynolds number (Re) varied from 1.78 × 103 to 18.72 × 103. The calculations were carried out using Ansys Fluent software package. Tube row and Reynolds number effects on the thermal and hydrodynamic characteristics of the studied heat exchangers were analyzed. The simulation results were in a good agreement with the available literature. The results showed that N u ¯ and ε increase, while f decreases as Re increases. Moreover, an increase in N R leads to an increase of f and a decrease of ε. The maximum values of N u ¯ were obtained for N R = 2, which were higher by 3.95–6.69% and 19.91–24.62% than those for N R = 1, and N R = 20, respectively. Heat transfer was stabilized only from the 19-row onwards. In addition, a 1-row heat exchanger had better thermal − hydraulic performance compared to that with a larger N R (ε for N R = 1 was about 22.32–27.24 times higher than those for N R = 20). The maximum deviation between the numerical results and the obtained correlations developed from them was ± 8.87%, which provides a useful reference for future studies and designs of heat exchangers with different row numbers of drop-shaped tubes. [ABSTRACT FROM AUTHOR]

This paper compares the thermohydraulic performance of varying curvature wavy microchannels, namely elliptical-wavy and sinusoidal-wavy microchannels. The amplitude of microchannels is fixed at 0.6 mm, and wavelengths are varied from 3 mm to 8 mm. Simulations are performed for the laminar flow of water (160 ≤ Re ≤ 900) considering three-dimensional conjugate heat transfer using ANSYS Fluent version 2020R2. The computational model has been validated by performing experiments on a serpentine-wavy microchannel, a special case of the elliptical-wavy microchannel, using micro-particle image velocimetry (μ-PIV) test facility. Both Nusselt number and friction factor increase with the increase in waviness. Elliptical-wavy microchannels have superior thermohydraulic performance than sinusoidal-wavy microchannels except for the channel of highest waviness at Re = 160. A microchannel with lower entropy generation has a higher thermohydraulic performance. Further, we observe centrifugal instabilities in both sinusoidal-wavy and elliptical-wavy microchannels at higher Reynolds numbers. Centrifugal instabilities in the elliptical-wavy differ from those in the sinusoidal-wavy microchannel by the asymmetric formation of additional vortices about the horizontal midplane in the channel cross-section. [ABSTRACT FROM AUTHOR]

This paper explores the effect of curved shape fin heights on open microchannel heat sink performance. Different geometries of rectangular, plano-convex, and plano-concave fins have been investigated by employing three-dimensional simulation. A variety of parameters including fin height, curve range coefficient, Reynolds number, and heat flux have been examined. At the same heat flux, Reynolds number, and fin height, the Nusselt number of the rectangular fin is lower than the two shapes of plano-convex and plano-concave. In Comparison of the two curved shapes, due to their geometries and fluid flow behavior, in the lower fin heights, the Nusselt number of the plano-concave is greater than the plano-convex. In the higher fin heights, the Nusselt number of the plano-convex is greater than the plano-concave. Even though at higher fins, where the Nusselt number of the plano-convex fins is greater than the rectangular and plano-concave shape, the plano-convex pressure drop is less than these two shapes. In a heat flux of 450 k W / m 2 , Reynolds number of 600, and a fins equivalent height of 0.6 mm, that the volume and mass of the fins are the same, there is an 18% difference between the maximum and minimum Nusselt number of plano-convex and plano-concave, by different curve coefficients. The obtained results are beneficial in designing novel shape fins in enhanced open microchannel heat sinks. [ABSTRACT FROM AUTHOR]

Mini-channel heat exchangers are extensively used to achieve heat dissipation with high heat flux, while the problem of flow maldistribution seriously impairs the thermal efficiency and operational reliability of heat exchangers. In this paper, the flow distribution characteristics of mini-channel heat exchangers with the conventional manifold model are numerically studied, and the influence of flow maldistribution on heat transfer is analyzed. To suppress the problem of flow maldistribution, an innovative approach for improving fluid uniformity by the built-in spiral baffle in the inlet manifold is proposed, and the effects of spiral baffles with different pitches and radii on flow distribution are discussed. Besides, the heat transfer characteristics and pressure drop under different manifold models are also compared. Finally, the variable-pitch spiral baffle is introduced to realize the stepped regulation of flow distribution. The results show that the conventional manifold model has serious flow maldistribution and causes the deterioration of heat transfer in front-end channels, especially at high Reynolds numbers. The improved manifold model greatly improves the uniformity of the fluid and temperature distribution inside mini-channel heat exchangers on the premise of sacrificing a small amount of pressure drop. The optimal pitch and radius of the spiral baffle are 20 mm and 9 mm respectively, and the flow maldistribution in almost all channels is controlled within the range of ±15% at this case. In addition, the variable-pitch spiral baffle further improves the flow uniformity of the mini-channel heat exchanger, and shows better overall performance than the equal-pitch spiral baffle. • The influence of flow maldistribution of the conventional manifold is analyzed. • An innovative method for improving the fluid uniformity is proposed. • The performances of spiral baffles with different parameters are compared. • Variable-pitch spiral baffle shows better comprehensive performance. [ABSTRACT FROM AUTHOR]