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Integrated a fully developing three-dimensional heat transfer and flow model, a multi-objective optimization aims to fulfill the geometric design for double-tube heat exchangers with inner corrugated tube is investigated in this work with RSM. Dimensionless corrugation pitch ( p / D ), dimensionless corrugation height ( H / D ), dimensionless corrugation radius ( r / D ) and Reynolds number ( Re ) are considered as four design parameters. Considering the process parameters, the characteristic numbers involving heat transfer characteristic, resistance characteristic and overall heat transfer performance calculated by CFD, and are served as objective functions to the RSM ( Nu c , f c , Nu c / Nu s , f c /f s and η in this paper). The results of optimal designs are a set of multiple optimum solutions, called ‘Pareto optimal solutions’. It reveals the identical tendency of Nu c / Nu s and f c / f s reflecting the conflict between them that means augmenting the heat transfer performance with various design parameters in the optimal situation inevitably sacrificed the increase of flow resistance. According to the Pareto optimal curves, the optimum designing parameters of double pipe heat exchanger with inner corrugated tube under the constrains of Nu c / Nu s ≥ 1.2 are found to be P / D = 0.82, H / D = 0.22, r / D = 0.23, Re = 26,263, corresponding to the maximum value of η = 1.12. [ABSTRACT FROM AUTHOR]

The present work studies the enhancement of heat transfer on a cylinder impinged by a rectangular jet in presence of a metallic grid on the slot exit. The experiments are carried out to cool a smooth cylinder, electrically heated, with a rectangular jet of air, at three Reynolds numbers in turbulent flow, i.e. Re = 5100, 10,600 and 15,300, where the Reynolds number is defined with the cylinder diameter. Three metallic grids, identified by the French classification as NF06-10-20, are employed with the solidity ratio ranging from 24.3% to 67.7%, and the same diameter of the mesh filament (0.6 mm). Fluid dynamic measurements, performed with a hot film anemometer, are done to justify the thermal results. Heat transfer measurements are presented as local and mean Nusselt numbers. The comparison with the case without grid, indicated as NF00, is done at the same mass flow rate of the jet and the same power consumption of the wind tunnel to move the jet. The main goal of the paper is to evaluate how the solidity ratio of the metallic grids influences the heat transfer. [ABSTRACT FROM AUTHOR]

This paper investigates experimentally the correlation and the effect of the turbulence intensity on the pressure drop and the heat transfer mechanism of a heat exchanger with elliptic tubes in a staggered arrangement. The heat exchanger studied in this work is an air–water cross flow heat exchanger with air being the external fluid and water the internal one. The heat exchanger consists of 144 elliptic tubes placed in a staggered arrangement. The experiments were carried out for two setups. The first setup was referring to isothermal conditions for which only air was used, flowing around the tubes. The second setup was referring to non-isothermal conditions with air as the external fluid and water as the internal working fluid, flowing inside the tubes. The Reynolds number for the external air for the isothermal experiments ranged between 3100 and 5200 based on the maximum velocity between the elliptic tubes. The turbulence intensity values varied between 0.9% and 3%. For the non-isothermal measurements the Reynolds number took values from 3100 to 7700 and the turbulent intensity ranged from 0.9% to 3%. The measurements showed that the increase of turbulent intensity led to a decrease in the total pressure drop of the external flow of the heat exchanger together with an enhancement of the heat transfer mechanism. [ABSTRACT FROM AUTHOR]

In this paper the flow of an incompressible Newtonian fluid past a stationary unconfined spheroid under the mixed convective heat transfer regime is studied numerically. The spheroid is assumed to be maintained at a constant wall temperature. The effect of Reynolds number (Re), Prandtl number (Pr), aspect ratio ( E ) and Richardson number (Ri) on the average drag coefficient and Nusselt number is investigated. The range of parameters considered in this study is: 10 ≤ Re ≤ 100; 0 < Ri < 2 and 0.5 ≤ E ≤ 1.5 and for two Prandtl numbers of 1 and 5. The drag coefficient increases monotonically with the increase of surface temperature for all Reynolds number. Prolate spheroids have higher drag coefficient and Nusselt numbers compared to oblate spheroids. The variation of Nusselt number with Ri at a fixed Re is non monotonic at large Re and this effect is stronger for prolate spheroids when compared to oblates. For aspect ratios greater than unity and for Re > 60, Nusselt numbers for small Ri (∼0.5) are lower compared to Nusselt numbers for Ri = 0. This difference in Nusselt numbers becomes appreciable for larger Prandtl numbers. A correlation for average Nusselt number was developed using least squares regression analysis. [ABSTRACT FROM AUTHOR]

In this paper, a numerical investigation of thermal and thermodynamic performance of a receiver for a parabolic trough solar collector with perforated plate inserts is presented. The analysis was carried out for different perforated plate geometrical parameters including dimensionless plate orientation angle, the dimensionless plate spacing, and the dimensionless plate diameter. The Reynolds number varies in the range 1.02 × 10 4 ⩽ Re ⩽ 7.38 × 10 5 depending on the heat transfer fluid temperature. The fluid temperatures used are 400 K, 500 K, 600 K and 650 K. The porosity of the plate was fixed at 0.65. The study shows that, for a given value of insert orientation, insert spacing and insert size, there is a range of Reynolds numbers for which the thermal performance of the receiver improves with the use of perforated plate inserts. In this range, the modified thermal efficiency increases between 1.2% and 8%. The thermodynamic performance of the receiver due to inclusion of perforated plate inserts is shown to improve for flow rates lower than 0.01205 m 3 /s. Receiver temperature gradients are shown to reduce with the use of inserts. Correlations for Nusselt number and friction factor were also derived and presented. [ABSTRACT FROM AUTHOR]

This study presents a computational study of turbulent heat transfer through two-pass square-duct channel flows with several different turbulence models that were developed to improve near-wall predictions. To evaluate the performance of the advanced wall function (the analytical wall-function: AWF), the results were compared with experimental data, those by a low-Reynolds-number k-ε model and a conventional wall function (the log-low based wall-function: LWF). Furthermore, the study extended to examining three more extended forms of the AWF. The duct used in this paper was a square duct of 50.8 mm side length (hydraulic diameter is 50.8 mm) and three Reynolds number cases were examined (30,000, 60,000 and 90,000). The LWF showed much lower Nusselt number levels in complex turbulent flow regions such as separation and reattachment zones, because the log-low employed in the model is impossible in such regions. On the other hand, the AWF proved its better performance over the LWF in the width of Reynolds number flow range. In addition, the extended forms of the AWF also showed improvements in the heat transfer predictions. [ABSTRACT FROM AUTHOR]

This article presents an experimental investigation of the heat transfer characteristics in a typical rotor–stator system with cooling air inlet at low radius under different dimensionless flow rate (CW ) ranges from 1.32×104 to 4.87×104 at five different rotational speeds, i.e., 500, 1000, 1500, 2000 and 2500rpm with a large range of rotational Reynolds numbers 4.18×105 ⩽ Reω ⩽2.484×106. A transient thermochromic liquid crystal (TLC) technique is employed to obtain the detailed distribution of the Nusselt number (Nu) on the surface of the rotor with effective rotating radius of 350mm as well as with a maximum gap of 67mm between the rotor and stator. The relationship between the Nusselt number (Nu) and the turbulent flow parameter (λT ) which has been identified as the parameter governing heat transfer in the rotating disk system is experimentally explored. A numerical model is developed for the investigation of the flow structure inside the rotating cavity and provides a basis for further study in explaining the heat transfer behavior over the rotating disk. It is found that the heat transfer characteristics are strongly affected by the flow structure. Numerical results also clearly show the existence of three flow regimes inside the cavity, namely, viscous regime, co-dominated regime, and inertial regime. The heat transfer characteristics in a rotor–stator system are well explained by the flow structure obtained in the paper. [ABSTRACT FROM AUTHOR]

Abstract: Large Eddy Simulation (LES) of turbulent flow and convective heat transfer over a half-corrugated channel is presented in this paper. Simulations are performed for various ranges of the normalized wave amplitudes, (the ratio of wave height to wave length). The Reynolds number based on the bulk velocity is chosen as and the Prandtl number is Pr =0.71. A comparison between the DNS and LES results of a plane channel ( ) at is also performed. The obtained results indicate that the region of recirculating flow depends strongly on the wave amplitude. This study shows that the Nusselt number (Nu) increases by increasing the wave amplitude until a specific value then it remains approximately constant. The thermal performance parameter (JF) is used as a measure for the heat transfer enhancement relative to the pressure drop and it is found that the maximum values of Nu and JF appear at , which hence correspond to the optimum value of the wave amplitude. [Copyright &y& Elsevier]

It is significant to develop an expression for the rarefaction correction to the continuum solution for heat transfer. An extended Reynolds analogy for slip and transition flow heat transfer in microchannels and nanochannels is proposed. A relationship between the rate of the dimensionless mean wall shear stress reduction and the rate of the dimensionless mean wall heat flux reduction may exist. The extended Reynolds analogy can be utilized to obtain an initial engineering approximation of the Nusselt number. As slip flow heat transfer is inherently complex, the proposed simple model may provide an insight into the mechanism of slip flow heat transfer. This paper cultivates the facility to perform the kind of engineering analysis which, although not exact, still provides useful information concerning real engineering needs. [ABSTRACT FROM AUTHOR]

Abstract: This paper proposes a new approach based on VSP method (Vorticity–Stream function–Poisson equation method), to study numerically fully developed laminar flow characteristics and heat transfer behaviors of helical ducts with rectangular cross section. The aim is to guide heat transfer enhancement for helical rectangular ducts. Based on the numerical results, synergies between velocity field and temperature field have been investigated through field synergy principle. The effects of curvature, torsion, aspect ratio, Reynolds number, and Prandlt number on velocity field, temperature field and field synergy have been examined. The results show that the secondary flow of the central regions is weaker than that in other locations of the cross section. With reference to the entire cross section, the best values of synergy between velocity field and temperature field can be found in the center of the section. Increasing the curvature, Reynolds number and Prandtl number causes the mean synergy angles of the cross section to increase. This result indicates that it is more important to improve the field synergy for high Prandlt number fluid, especially at high Reynolds number. As the torsion increases, the average synergy angles decreases. The mean synergy angles reach the maximum value when the aspect ratio is equal to one; therefore, one can conclude that the field synergy of helical ducts with square cross section is the worst. To enhance heat transfer performance of helical rectangular ducts, for small aspect ratio, the main important aspect is to improve the secondary flow in the center of the cross section. On the contrary, for moderate values of the aspect ratio, the most important aspect is to improve the field synergy of the region near the upper and the lower wall. The correlations of friction factor and Nusselt number have been also developed for helical ducts with rectangular cross section. [Copyright &y& Elsevier]

Taylor–Couette–Poiseuille flow commonly occurs in rotating machinery in the annulus between the outer casing and rotating shaft as part of lubrication or thermal management systems. To improve efficiency of these machines, industry is beginning to work with low viscosity, high density fluids operating at high pressures, such as supercritical carbon dioxide turbines. Within existing literature, vast high quality heat transfer data exists for air under Taylor–Couette–Poiseuille flow regimes. However, for other fluids, these data are limited in their experimental coverage, especially at high Taylor numbers (i.e. > 1 × 10 10). This paper presents first of its kind experimental data for supercritical carbon dioxide. The data are analysed to provide insight into the heat transfer processes and to provide a Taylor–Couette–Poiseuille flow heat transfer correlation. Operating conditions of the experimental campaign cover pressures of 7 and 10 MPa, a temperature range of 315 to 333 K, and rotational speeds of 10 to 25 kRPM. This equates to Taylor number and axial Reynolds number ranges of 9.52 × 10 10 to 8.73 × 10 11 and 3.41 × 10 4 to 5.4 × 10 4 , respectively. The range of conditions tested covers the operational envelope of those found in thermal management systems for supercritical carbon dioxide turbine designs. It is found that heat transfer of supercritical carbon dioxide is significantly higher than that of ideal gases (e.g. air) for the same Taylor number range. Further findings show that change in fluid properties near the critical point (i.e. heat capacity and density) increase the Nusselt number from 1.5 × 10 3 at 10 MPa to 3.1 × 10 3 at 7 MPa at an effective Reynolds number of 2.5 × 10 6. A dimensional analysis shows the importance of capturing the change in fluid properties when near the critical point and to account for tangential energy transport. To capture this we introduce a non-dimensional parameter, Ψ , being the ratio of tangential energy flux to inertial energy flux. Including Ψ in heat transfer correlations improves their validity by capturing additional mechanisms that influence heat transfer. The suitability of correlations including Ψ are demonstrated using air and supercritical carbon dioxide data sets. Finally, analysing heat transfer trends across the range of Ψ tested provides insight on how heat transfer mechanisms vary with operating condition. This work provides a vital step forward for the unification of Taylor–Couette–Poiseuille flow heat transfer correlations across multiple experimental data sets. [ABSTRACT FROM AUTHOR]

In this paper, a numerical investigation is performed to study the mixed convective flow and heat transfer characteristics past a square cylinder in cross flow at incidence. Utilizing air (Pr =0.71) as an operating fluid, computations are carried out at a representative Reynolds number (Re) of 100. Angles of incidences are varied as, 0°⩽ α ⩽45°. Effect of superimposed positive and negative cross-flow buoyancy is brought about by varying the Richardson number (Ri) in the range −1.0⩽ Ri ⩽1.0. The detail features of flow topology and heat transport are analyzed critically for different angles of incidences. The thermo fluidic forces acting on the cylinder during mixed convection are captured in terms of the drag (CD ), lift (CL ), and moment (CM ) coefficients. The results show that the lateral width of the cylinder wake reduces with increasing α and the isotherms spread out far wide. In the range 0°< α <45°, CD reduces with increasing Ri. The functional dependence of CM with Ri reveals a linear relationship. Thermal boundary layer thickness reduces with increasing angle of incidences. The global rate of heat transfer from the cylinder increases with increasing α. [ABSTRACT FROM AUTHOR]

Impingement cooling is an important turbine blades cooling technique, but it is often not as efficient as required due to the deflection of downstream jets caused by crossflow. In this paper, a novel corrugated target surface is proposed to convert downstream oblique impingement into orthogonal impingement for higher heat transfer. To prove this design, the flow and heat transfer characteristics of impingement cooling with conventional flat target surface (Baseline case), full-corrugated (FC) target surface, and semi-corrugated (SC) target surface are compared over the jet Reynolds number ranging from 15,000 to 45,000. Results show that the Nusselt number of full-corrugated target surface is almost the same as that of flat target surface at the region corresponding to the first five jets, but its peak value and uniformity are significantly improved at the region corresponding to the last four jets, especially under high jet Reynolds numbers. In addition, with the increase of corrugation depth (H), the total heat transfer capacity and area-averaged Nusselt number increase, but this comes at the cost of increased friction loss. Comprehensively evaluating heat transfer and friction loss, only the FC with H = 0.8 D case always has better thermal performance than Baseline case under all computed conditions, and the maximum improvement can reach 5.8%. Last but not least, the SC with H = 0.8 D case is proposed based on the FC with H = 0.8 D case to further reduce the friction loss, and results show that its thermal performance is improved significantly due to the decreased friction loss and almost unchanged heat transfer. [ABSTRACT FROM AUTHOR]

The heat transfer performances of pipes with twisted tape inserts having periodically spherical dimple-protrusion patterned walls are numerically investigated by Ansys Fluent under a constant heat flux of 5 W/cm2. The inserts having three different densities (N = 0, 30, and 45) and three different kinds of arrangement (composed of only dimples, only protrusions, and a combination of dimples and protrusions) with a constant twist ratio of 3 were tested for Reynolds numbers (Re) within the range from 3000 to 27000. Comparing the turbulence models proposed the validated realizable k-ϵ model coupled with enhanced wall function to employ on the numerical simulations. While the highest Nusselt number (Nu) and friction factor (f) values for the presented study were found for the dense protruded twisted tape (D-PT) arrangement at Re values lower than 18000 and for dense dimpled-protruded twisted tape (D-DPT) arrangement for the Re greater than 18000, dimpled-protruded twisted tape (DPT) and dimpled twisted tape (DT) sequences reached the lowest f values of 0.365 and 0.322 at Re = 3000, respectively. D-DPT and D-PT sequences reached the highest thermal performance factor (TPF) values of 1.508 and 1.478 at Re = 3000, respectively. All patterned twisted tapes performed better Nu than conventional twisted tapes in the analyzing range and performed better TPF than typically twisted tapes in the Re region of less than 6000. [Display omitted] • The effect of DT, PT, DPT, D-DT, D-PT, and D-DPT on heat transfer enhancement performance was numerically investigated. • All patterned twisted tapes performed better TFT than typically twisted tapes in the Re region of less than 6000. • D-DPT and D-PT sequences reached the highest TPF values of 1.508 and 1.478 at Re = 3000, respectively. [ABSTRACT FROM AUTHOR]

In this paper, the large eddy simulation of turbulence in coaxial cylinders determines the influences of slit structure on flow characteristics and heat transfer behavior. The cross-sections of slits are trapezoid, rectangle and ellipse respectively. The field synergy principle and entropy generation are utilized to comment on the heat transfer properties. The results demonstrate that slit shape influences the heat transfer capacity remarkably. The trapezoid and rectangle slit models show backflow in the slit region, while ellipse slit model has no backflow, thus being expected to result in the fluid exchange is more sufficient. In terms of field synergy principle, the ellipse slit model possesses the smallest field synergy angle under different Reynolds numbers which reveals the finest collaboration relationship of velocity with the temperature fields. The thermodynamic analysis indicates that thermal entropy generation has been promoted as a major contributor to total entropy generation, and the entropy production value of the ellipse slit model is small, leading to the energy utilization rate is better than other models. The finest heat transfer efficiency is found with the ellipse slit model for given Reynolds number range. [ABSTRACT FROM AUTHOR]

• Performances of TD, TP, TDP, D-TD, D-TP, and D-TDP were investigated numerically. • All cases of teardrop patterns have higher Nu values than TT in the tested Re region. • All cases except TP have higher TPFs than those of TT between Re 3000 and 5500. • The D-TDP case reached a thermal performance factor of 1.517 at Re = 3000. The teardrop pattern observed to be more efficient in straight tubes was examined for the first time in a twisted tape wall. The thermo-hydraulic performances of twisted tapes having periodically teardrop-shaped dimple-protrusion patterned walls are numerically investigated. The new designs have two different densities (N=30, and 45), three alternative types of layouts (composed of only protrusions, only dimples, and a combination of dimples and protrusions) align in two different flow directions. Designs have a constant twist ratio (TR) of 3, and calculations are conducted for Reynolds numbers (Re) between 3000 and 27000 by Ansys Fluent under a constant heat flux of 50000 W/m2. All cases have higher Nusselt number (Nu) values than traditional twisted tape (TT) in the tested Re region. Besides, all arrangements except the teardrop protruded twisted tape have higher thermal performance factors than the TT in the Re range from 3000 to 5500. All dense versions of the teardrop patterned twisted tapes have higher Nu and friction factor (f) than sparse versions of them. The dense teardrop dimpled-protruded (D-TDP) case reaches TPF of 1.517, the highest value among the tested twisted tapes, at Re = 3000. [Display omitted] [ABSTRACT FROM AUTHOR]

• Numerical simulations in a mixing tee to assess fatigue damage. • Non-dimensional temperature profiles along the centerline show a fairly overlapping behavior. • Case 3 predicts a usage factor of 1 for estimated number of around 1.1 million cycles. • The resulting alternating stress is coupled with the flow conditions by a linear profile. • The highest alternating stress has approximately the value 177 MPa. The present paper depicts a numerical study of flow and thermal features in T-junctions with circular cross-section. The numerical simulation is based on the experimental validated wall-resolved Large-Eddy Simulation method. The Reynolds number in the warm main pipe flow covers a range of R e m = 27000 to R e m = 134000 and the temperature difference of the mixing fluids is up to Δ T = 249 K. The location of maximum fluctuations does not change due to a nearly identical flow pattern type in all simulations. This condition allows the investigation of the flow evolution, mixing process and heat transfer for the same thermal flow pattern type. The numerical simulations are coupled with the heat conduction in the structure due to the thermal fluid-structure-interaction. Further, this study offers a deeper insight into the hidden physics of turbulent mixing flows and the transport as well as damping of thermal fluctuations in the near-wall fluid/solid region. Our study considers also thermal stress analysis by Finite-Element Method by utilizing the wall temperature distribution of previously performed numerical simulations. Therefore, fatigue damage is evaluated based on the stress received from the Finite-Element analysis. The results show that the highest Reynolds number case corresponds to the highest flux intensity and turbulence. This affects the flow field and the thermal features in the core flow and the transport of fluctuations close to the wall. The locations with peak fluctuation intensity in the fluid generates also high fluctuating temperature areas in the structure. The finite element analysis demonstrates that the highest temperature difference case leads to the highest alternating stress intensity which has potential for thermal fatigue. The generated maximal alternating stress is coupled with the flow conditions of the mixing fluids. With an increasing Reynolds number of the main and branch pipe flow as well as temperature difference raises also the resulting maximum alternating stress intensity. [ABSTRACT FROM AUTHOR]

• Introduction of air swirling can either enhance or reduce the heat transfer rate depending on operating condition. • Heat transfer enhancement is achieved with the appropriate level of air swirling when the complete combustion occurs in the inner reaction zone. • Impinging inverse flame jet with an appropriate level of air swirling possesses the advantages of both premixed and diffusion flame jets. • Impinging inverse flame jet is a desirable option for impingement heating applications. This paper presents an experimental study to explore the feasibility of utilizing a swirling inverse diffusion flame (IDF) jet for impingement heating. The investigation concentrates on the impinging flame structure and impingement heat transfer. The effects of air swirling, the air jet Reynolds number, Re a , the global equivalence ratio, Φ, and the non-dimensional burner-to-plate distance, H/d a , on the impinging flame shape and impingement heat transfer are investigated experimentally. The Experiments show that introduction of air swirling can either enhance or reduce the heat transfer rate depending on operating condition. Under-swirling produces no evident heat transfer enhancement due to insufficient air/fuel mixing, while over-swirling reduces heat transfer rate because of the dilution of the entrained cold ambient air. Heat transfer enhancement is achieved with the appropriate level of air swirling when the complete combustion occurs in the inner reaction zone around a stoichiometric condition. The investigation shows that the impinging inverse flame jet with an appropriate level of air swirling possesses the advantages of both premixed and diffusion flame jets with blue appearance, ultra-low NO x emission, high flame temperature, good self-stabilization capability, and enhanced heat transfer rate. Therefore, it is a desirable option for impingement heating applications. [ABSTRACT FROM AUTHOR]

• The most relevant novelty of this work is the development and investigation of simple and effcient mechanical mixing devices using the non-uniform finite difference approximation. • The non-uniform finite difference approximation is adapted and applied for the first time to a numerical investigation in uid mixing mechanics. • This study is the first investigation of its kind on the use of parallel wall jets for scalable heat/mass transfer enhancement. • Several configurations of the parallel wall jets as well as Reynolds regimes are studied. • Oscillatory motion of the ow downstream can be predicted via simulations and effciently boosted with certain configurations. • The best mixing performance is observed for the smallest wall jet width tested. [Display omitted] Many different approaches have been investigated in the literature to achieve efficient heat and mass exchangers in applications in several fields such as mechanical, aeronautical or biomedical engineering. Some of these engineering devices need to efficiently mix two fluids at different temperature or concentrations, achieving a decent mixing at a low power cost, and using passive mechanical elements if possible. This paper introduces and analyses numerically the novel application of two parallel wall jets to enhance mixing by vortex shedding induced mechanically. The numerical approach is the non-uniform finite difference approximation, used for the first time in the study of mixing mechanics. The parallel wall jets are constrained within a channel, which can be a simple and passive easily manufactured machine with high potential as scalable laminar mixing device. Different wall jet geometries and Reynolds numbers have been tested with a verified&validated CFD code based on non-uniform finite difference approximation. This analysis allowed to predict configurations that lead to unsteady oscillatory motion. In general, it has been observed that for a range of laminar wall jets, a small channel ratio seems a very efficient option for heat/mass transfer, thanks to the intense oscillation generated in the flow downstream, which enhances mixing with low power requirement. [ABSTRACT FROM AUTHOR]

A numerical method is utilized in this paper to study the effects of varying divider inclination angle on the flow structure and heat transfer characteristics in a ribbed two-pass channel for Reynolds number varying from 10,000 to 50,000. The studied divider inclination angles are − 3 ° , − 2 ° , − 1 ° , 0 ° , + 1 ° , + 2 ° and + 3 °. The ribs have an orientating angle of 45 °. The results show that the heat transfer in the bend region and in the passage upstream the bend is enhanced significantly and the heat transfer in the passage downstream the bend is reduced compared to a channel with a straight divider wall as the divider wall inclination angle varies from − 3 ° to + 3 °. Channels with inclined divider wall produce higher pressure drop and friction factors than a channel with straight divider wall. According to the results, a channel with − 1 ° inclined divider shows the best thermal performance among all the channels investigated. The flow structure and heat transfer characteristics for all the channels are depicted in detail and discussed in this study. [ABSTRACT FROM AUTHOR]

• Detailed parametric study of novel concentric microroughness configurations subjected to array jet impingement • Configuration with largest pin-fin height produced maximum pin-fin effectiveness of 3.23 • Prescribed correlation for pin-fin effectiveness based on 64 configurations Array jet impingement when combined with strategically placed short height (<1.25 mm) roughness elements on the target wall results in enhanced heat dissipation at high fin-efficiency (>95%) without much penalty on pumping power. Advances in metal-powder-based additive manufacturing enables the printing of roughness scales (< 0.5 mm) with acceptable accuracy. This paper presents an experimental and numerical study on the characterization of heat transfer performance of concentric-shaped micro pin-fins subjected to array jet impingement. The experimental study served the dual purpose of establishing manufacturability of such configurations and validating the computational model. Three concentric micro-pin fin configurations were additively manufactured with AlSi10Mg alloy through Direct Metal Laser Sintering (DMLS) with dimensions between 0.25 - 2.25 mm on the target wall subjected to a 5 × 5 array of jets with normalized jet-to-jet spacing of X/D jet =Y/D jet = 3 and normalized jet-to-target spacing Z/D jet of 1. The resultant Nusselt number (Nu) and fin effectiveness (ε) values have been reported for Reynolds number ranging between 3,000 and 12,000. The three target plate configurations produced effectiveness between 1.8 - 2.4 and the configuration with maximum wetted surface area (wall thickness of 250 µm) and void fraction produced highest effectiveness. Further, a validated numerical model was used to perform a parametric study on geometrical parameters of roughness element viz. pin-fin spacing (S), inner diameter (D 1), outer diameter (D 2) and height (H) at a Re Djet = 6,000. Significant enhancement in ε was observed with H being the dominant parameter affecting the heat transfer. The effect of void fraction (α), which denotes the free flow area between pin-fin elements (A b –A pin) was also explored. There exists an α opt at which maximum heat transfer was achieved. A correlation for fin effectiveness is proposed as a function of different non-dimensional variables derived from the parametric study and its predictive capabilities were found to be within ±10%. [ABSTRACT FROM AUTHOR]

• An optimization methodology is proposed for heat transfer enhancement studies. • MOO and MCDM are combined to find optimum design parameters. • Optimization of nanofluid flows in pipes with inserts are examined. • Effect of design parameters on thermal-hydraulic performance are investigated. A hybrid approach that combines Multi-objective Optimization (MOO) and Multiple-criteria Decision-making (MCDM) techniques has been introduced in the present paper for heat transfer augmentation in pipes. The combination of two passive heat transfer enhancement techniques namely the nanofluid and insert employment have been applied in the representative problem. In this context, the performance of water-based C u O , F e 3 O 4 , and C N T - F e 3 O 4 nanofluids and two different inserts as twisted tape (TT) and wire coil (WC) were evaluated and their optimum design and flow parameters were determined. The methodology introduced here has two major parts. Firstly, a MOO study was conducted employing the experimentally obtained correlations for the Nusselt number (Nu) and the friction factor (f) as objective functions. The optimum values of the selected design parameters (the Reynolds number (Re), nanoparticle volume fraction ((ϕ)), twist ratio (h/d), wire coil pitch ratio (p/d), and the Prandtl number (Pr)) that maximizes N u while minimizing f were found as Pareto front. Secondly, the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) method was used to help decision-makers making an optimal choice among the Pareto optimal solutions that meet their requirements. In the MCDM problems, two criteria (Nu and f) and fifteen alternatives were evaluated for each case. A sensitivity analysis was also conducted by changing the weights of criteria. It was found that the combination of TT and C N T - F e 3 O 4 with the values of design parameters (R e ; ϕ ; h / d ; P r)=(20,000; 0.27%; 10; 5.50) exhibited the best thermal-hydraulic performance for the equal weights of criteria. [ABSTRACT FROM AUTHOR]

• The particle concentration ranges from 5 to 95 g/m3, and the particle size ranges from 10 to 25 μm. • After particles are added, the increase rate of convective heat transfer coefficient (h) varies spatially along the flow direction. However, h is weakened in some local areas in the first half of the fins. • The presence of particles reduces the field synergy angle. • There exists an approximately linear mapping relationship between the particle concentration and the Colburn factor (j), and the friction factor (f), while a roughly exponential mapping relationship exists between the particle size and j , and f. • The thermal-hydraulic performance of wavy fins in a sand-dust environment is more sensitive to the particle size than the particle concentration. The thermal-hydraulic characteristics of aluminum wavy fins of compact heat exchangers in a sand-dust environment are currently unclear. Therefore, in this paper, the effects of particle concentration and size on the heat transfer and pressure loss characteristics of wavy fins, which are analyzed experimentally and numerically, are discussed. In addition, correlation functions of the Colburn and friction factors built using a multiple nonlinear regression based on orthogonal numerical simulation results are presented. It is found that the two-way coupling between the air and the particles can improve the synergy of the velocity and temperature gradient fields. In addition, this coupling can restrain the air turbulent kinetic energy in the near-wall flow zone. Note that the combination of these two effects increases the average Colburn factor. Moreover, as the particle concentration increases in the range of 5-95 g/m3, both Colburn and friction factors increase almost linearly. On the contrary, these factors decrease roughly exponentially as the particle size changes from 10 to 25 μm. It is worth noting that the Colburn and friction factors change with different rates under different Reynolds numbers so that smaller Reynolds numbers result in higher changing rates of both factors. Furthermore, the thermal-hydraulic performance (heat transfer versus pressure difference) of wavy fins decreases slightly in a sand-dust environment when compared with the conventional environment, but it is insignificant on the whole. [ABSTRACT FROM AUTHOR]

• Irreversibility due to winglets in fin-tube heat exchanger is analysed. • Entropy generation and exergy analysis are two pillars of irreversibility analysis. • Bejan number explain the dominancy of thermal and viscous entropy generation. • Comparison of heat transfer enhancement and irreversibility rate is done by N H. • Performance of the system is examined by exergetic efficiency, which is higher for upstream located RWPs. This paper investigates the irreversibilities caused by fluid flowing under the effect of isothermal walls of fin-tube heat exchanger in the presence of longitudinal vortex generators. Inline arrangement of circular tubes with fin-surface mounted rectangular winglet pairs (RWPs) arranged in common flow down configuration are selected for the present three-dimensional numerical study. Initially, thermodynamic performance analysis of different possible RWP locations corresponding to tubes has been examined for a fixed angle of attack. Further, the study is expanded for some specific locations by varying angle of attack ranging from 15° to 60° and flow Reynolds number ranging from 2000 to 4000. Thermodynamic performance of the present system has been evaluated by analyzing the irreversibilities caused due to RWPs and compared with the thermo-hydraulic performance. Thermodynamic analysis is categorized into two parts: the first 'entropy generation analysis' and the second 'exergy analysis'. For entropy generation analysis entropy generation rate, irreversibility distribution ratio and Bejan number are examined. The obtained results indicate that the irreversibility caused by entropy generation in the present system is majorly dominated by the entropy generation rate due to thermal effect. Exergy analysis is studied by calculating the irreversibility rate, exergy destruction number, heat transfer improvement number and exergetic efficiency. The observation of exergetic efficiency variation for different RWP locations demonstrates that the present system with upstream RWP locations delivers higher performance. Image, graphical abstract [ABSTRACT FROM AUTHOR]

In this paper, 3D analysis of forced convection of Newtonian and non-Newtonian nanofluids through a horizontal pipe using single- and two-phase mathematical models is performed. The models include the Newtonian single-phase model, non-Newtonian single-phase model, Newtonian two-phase mixture model and finally non-Newtonian two-phase mixture model. The geometry of the problem is a horizontal pipe that is filled with Al 2 O 3 /water nanofluid. The effects of particle volume fraction and the Reynolds number on the heat transfer and hydrodynamic characteristics of the flow are examined. Results indicate that for the volume fraction of 0.01, the non-Newtonian mixture model gives more accurate results in comparison with the other models. By increasing the solid volume fraction, the mixture models give a higher heat transfer coefficient in comparison with that of the corresponding single-phase model; however, by comparing the results of Newtonian and the corresponding non-Newtonian models, no meaningful relationship can be obtained. Furthermore, for all the investigated models, the Nusselt number is increased with increasing the Reynolds number. [ABSTRACT FROM AUTHOR]

This paper aims to provide a novel enhanced heat transfer method for the internal tip surface of a U bend channel of relevance for a gas turbine rotor blade. The DWVGs (delta-winglet vortex generators) pair is arranged at multiple locations on the tip surface. Two types of vortex generators are studied, including common-flow-up and common-flow-down configurations. The inlet channel Reynolds number varies from 10,000 to 50,000. The topological analysis method is used to determine the formation and evolution of the vortices and to better understand the mechanism of the heat transfer enhancement. Results of skin-friction lines, topological portrait, Nusselt number, friction factor, thermal performance are included. The results show that due to the interaction among the vortices surrounding the vortex generators, the DWVGs pair in common-flow-up configuration has a slight heat transfer improvement, and is not sensitive to the tip location. However, the DWVGs pair in common-flow-down configuration placed at the downstream of the tip surface improved the heat transfer significantly as the induced vortices between the turbulator pairs effectively reduce the thickness of the boundary layer. Compared with the smooth U bend channel, the optimal design shows that the heat transfer and overall thermal performance can be increased by up to 7.4% and 6.8%, respectively. This study elaborates the flow and heat transfer processes from the perspective of topology, which is helpful in the design of cooling procedures of turbine blades. [ABSTRACT FROM AUTHOR]

• Forced convection heat transfer from rectangular straight fins have been experimentally measured. • Three-dimensional conjugate heat transfer simulations are conducted. • The relationship between the Nusselt and Reynolds numbers is approximately linear. • Shorter fin channels were more effective in heat transfer than longer ones. • The heat transfer rate decreased linearly with the increase in channel length but remained approximately constant with the increase in number of fins. • Laminar and transitional-turbulent flows are characterized. This paper presents an experimental and numerical study of forced convection heat transfer from straight rectangular fins on a horizontal surface at low Reynolds numbers ranging from 2600 to 6800. Heat transfer for a fixed number of fins, fin spacing, and length was measured in a wind tunnel for varying inlet air velocity and heat input. Three-dimensional conjugate heat transfer simulations were also conducted to characterize important features of heat transfer and flow of the fin array and the effects of fin channel length on heat transfer. A maximum relative error of about 4.28% was found between the experimental and the numerical results for the average Nusselt number. The experimental results showed that forced convection heat transfer is characterized by an approximately linear relationship between the Nusselt and Reynolds numbers, which also agreed reasonably well with the available correlations for fully developed turbulent convective heat transfer for tubes with uniform heat flux. Further investigations of the effects of fin geometry were made numerically. Shorter fin channels, where the flow is thermally and hydrodynamically developing, were more effective in heat transfer than longer ones, where the flow approaches or reaches full development. It is shown that heat transfer rate per channel decreases linearly with the increase in channel length, but remains approximately constant with the increase in number of fins. It is also shown that the contributions of natural convection and radiation were negligible for the specific fin design and the range of inlet flow conditions and maximum fin temperature investigated here. [ABSTRACT FROM AUTHOR]

This paper numerically qualifies the effect of rib orientation on heat transfer and flow performances of mist/steam in square channels with inclined ribs. Four types of rib orientations are investigated. The study is conducted in contrast to steam with Reynolds number ranged from 10,000 to 60,000. Secondary flow patterns and droplet trajectories are analyzed to reveal the underlying reasons bringing about heat transfer enhancement due to mist/steam. The results indicate that rib orientation greatly determines secondary flow pattern in channels, and the distribution of secondary flow triggered by N type ribs is opposite to that by P type ribs. Such secondary flow forces droplets at different locations to assemble near different side walls. The places where water drops gather in a large quantity provide splendid heat transfer enhancement. And this effect gets more obvious at higher Reynolds number. For mist/steam, both heat transfer coefficient and overall heat transfer performance in the first passage are remarkably stronger than that in the second one. In general, NN channels and PP channels filled with mist/steam have the best heat transfer performance among the investigated cases. [ABSTRACT FROM AUTHOR]

An improved local correlation-based intermittency transition model is developed for high-speed flow heat transfer in this paper. Based on the intermittency transition model, two improvements have been proposed. One is introducing a new correlation of the momentum thickness Reynolds number for high-speed boundary layer into the transition onset function. In this new correlation, the free stream Mach number and the free stream Reynolds number are used to reflect the compressible effects. The other improvement is the modification of the intermittency factor transport equation to include physical information regarding the transition processes. Several test cases containing supersonic and hypersonic flat plates, a slender cone at different Reynolds numbers, the X-51A forebody, and the Hypersonic Inflatable Aerodynamic Decelerator are conducted to assess the applicability of the improved intermittency transition model. The numerical results show the ability of the improved model to describe the high-speed boundary layer transition processes. However, further research is needed to reproduce the overshoot phenomena and verify the capacity of the improved model on complex cases. [ABSTRACT FROM AUTHOR]

• New correlation between stagnation point nu and local re numbers during synthetic jet impingement heat transfer has been proposed. • The axial distribution of flow parameters under free and impinging synthetic jet reaches maximum at x / d ≈ 2. • The real (unmodified) axial distribution of stagnation nu number has been estimated. The paper presents the results of experimental investigations of heat transfer coefficients, velocity and pressure distributions of synthetic jets for two cases: constant Reynolds number and variable Stokes number and vice versa. Measurements were made for synthetic jet Reynolds number in the range of Re =5440–11,475, dimensionless stroke length L 0 =7.49–38.5 and dimensionless jet-to-plate spacing x / d = 0–20. The synthetic jet time-averaged and momentum velocity as well as the pressure for each case are presented. The centerline turbulence intensity and the Euler number are also reported. The centerline heat transfer coefficient distributions are compared with the distributions of the various velocity and pressure parameters. A new correlation between the local Nusselt number and the local Reynolds number is presented. Under synthetic jet impingement cooling the maximum of the stagnation point heat transfer coefficient appears mostly at axial distance of x / d ≈ 5. The location of the maximum Nusselt number at x / d ≈ 5 is caused by the modification of the heat transfer distribution due to the recirculation of hot air. With the decrease of the heat power released into the control volume by the impingement plate (heater) this modifying effects becomes less pronounced thus the maximum of the heat transfer coefficient shifts towards the location of about x / d = 2. The maxima of various flow parameters are generally observed at about x / d = 2 what was also reported by other independent researchers. Thus, it may be concluded that the purely mechanical phenomena does not full coincide with thermal ones in the case of synthetic jet impingement cooling. [ABSTRACT FROM AUTHOR]

• A critique viewpoint is provided fro effectiveness. • Experiments are carried out to support the said viewpoint. • All possible curve behaviors of effectiveness are described. • Probable previous imperfect effectiveness analysis was mentioned. In this study attempts are made to clarify some hidden features of effectiveness concept (the ratio of actual heat transfer to maximum possible heat transfer) of heat exchangers and provide a critique viewpoint and comprehensive description about that. It is shown in present paper that the effectiveness parameter requires much more attention when it is used in fluid-flow parametric or sensitivity studies of heat exchangers and lack of consideration of its hidden features may lead to incorrect or imperfect decision-makings on curve behavior of effectiveness. Indeed, it is shown that the curve trend of effectiveness against the Reynolds number of one side of heat exchanger can be ascending, descending, or ascending-descending depending on the mass flow rate (Re number) of the other side fluid. Although this critique viewpoint is not considered as a weakness of the effectiveness definition for heat exchangers, inattention may lead to wrong or imperfect inferences in sensitivity analysis of effectiveness parameter. Moreover, it is not logical to provide empirical correlation for effectiveness against flow parameters without the consideration of described features of effectiveness in this study. In order to support the described viewpoint some experiments are performed on double helical tube heat exchanger to evaluate the related thermal parameters and describe the mentioned features of effectiveness statistically. However, the provided analysis is valid for any other type heat exchanger as well. [ABSTRACT FROM AUTHOR]

• A combined experimental and numerical study for single and a row of threedimensional wall jets have been performed. • Four low Reynolds number turbulence closure models are accessed to select the suitable model that predicts the fluid flow and heat transfer characteristics of the three-dimensional wall jets accurately. • Measurement of flow characteristics like mean velocity profiles, growth of half jet widths, and velocity decay has also been performed. • For a fixed mass flow rate, at a given diameter, a smaller number of jets have higher heat transfer performance than that of the closely spaced jets. • Correlations are developed to predict local and average Nusselt numbers accurately. This paper reports the results of experimental and numerical studies to understand the heat transfer characteristics of three-dimensional wall jets exiting from a single and a row of circular jet openings over an unheated flat plate. Infrared thermography is employed to obtain the temperature distribution over the target surface, and the semi-infinite approximation methodology is used to estimate the heat transfer coefficients. Single wire constant temperature hot-wire anemometer is used to measure the flow characteristics. Additionally, computational studies have been performed to select a suitable low Reynolds number turbulence closure models among the following models, namely (i) Spalart Almaras (SA) (ii) Realizable k-ε with enhanced wall treatment (RKE-ewt) (iii) k- ω SST and (iv) Reynolds Stress Model with enhanced wall treatment (RSM-ewt) that predicts the experimentally obtained results for heat transfer characteristics accurately. Based on the investigations carried out, it is observed that RKE-ewt turbulence closure model is not only accurate but also faster in the prediction of heat transfer coefficient. Further, it is also seen that for a fixed mass flow rate, and at a given diameter of the jet opening, widely spaced jets show higher heat transfer coefficients. Furthermore, for a row of three-dimensional wall jets correlations based on the numerical results are developed for the Nusselt number for a Reynolds number range of 5000 to 15000. [ABSTRACT FROM AUTHOR]

Finned tube heat exchangers are among the most common types of heat exchangers that are used in engineering applications. Most of the tubes used in finned-tube heat exchangers are of circular type, but the oval tubes have more desirable features as they impose lower pressure drops with approximately the same amount of heat transfer. In this paper, a finned oval tube is studied experimentally and numerically to obtain temperature fields around the tube at different locations. Moreover, several 3D numerical simulations are performed to study the effects of fin length, fin spacing and diameter ratio of the tube cross section. Based on the obtained results, fin spacing of 1 mm is the optimum at all tube aspect ratios and fin lengths when Reynolds number is low while at high Reynolds numbers, fin spacing of 3 or 4 mm is the better choice. As to the fin length considerations, for a short fin, a spacing of 3 or 4 mm is the best choice at high Reynolds numbers. For larger fin lengths, sensitivity to fin spacing is less but a fin spacing of 3 or 4 is preferable at high Reynolds numbers. • The excellence of oval tubes with rectangular fins to circular tubes with rectangular fins in certain cases is observed. • Fixing major tube diameter and increasing tube aspect ratio culminates in increased pressure loss and decreased heat transfer. • Smaller fin spacing is desirable at low Reynolds numbers for all cases. • Larger fin spacing is desirable for high Reynolds numbers when tube aspect ratio and fin length are smaller. [ABSTRACT FROM AUTHOR]

• Heat transfer behavior between the supercritical carbon dioxide and water in the microtube heat exchanger is numerically investigated. • The effects of pressure, mass fluxes and buoyancy on the heat transfer of CO 2 flowing at low reynolds numbers are obtained. • The flow patterns of water in the shell side influence the secondary flow intensity of CO 2. • Separate correlations should be used to get a better prediction of heat transfer due to different flow forms of CO 2 in the microtube heat exchanger. A numerical study on the heat transfer between supercritical carbon dioxide and water in the microtube heat exchanger at low mass fluxes of CO 2 is reported in this paper. The inner diameter of the tube used in the heat exchanger was 1.6 mm. Four sections with different heat transfer performance are found during the heat exchange process between CO 2 and water. The operating pressure, mass fluxes of CO 2 and water, buoyancy effect are considered to analyze the heat transfer characteristics of sCO 2. The heat transfer coefficient is found to be the maximum when the temperature is near pseudocritical point and the peak value increases when the pressure is getting closer to the critical point. The heat transfer coefficient increases with the increase of mass flux of CO 2. The buoyancy effect is found to have a significant influence on the heat transfer at the different locations of the tube. The secondary flow of CO 2 results in the enhancement of heat transfer on the top side of the tube and weakens the heat transfer on the bottom side. Different flow patterns of water in the shell side are compared in this study. The counter-current flow achieves the best heat transfer effect, while the downward flow is slightly better than the upward flow on the total heat transfer due to the different intensity of secondary flow. Existing different correlations should be used to get a better prediction of heat transfer depending on the state of CO 2 in the microtube heat exchanger, especially when CO 2 flows at a low mass flux. [ABSTRACT FROM AUTHOR]

• Experimental and numerical parametric investigation for enhancement in heat transfer from the heated cylinder. • Effect of mist loading fraction, Reynolds number and nozzle-to-surface spacing on the enhancement in heat transfer are studied. • High enhancement in heat transfer with high mist loading fraction, low nozzle-to-surface spacing and low Reynolds number. The paper presents the study of experimental as well as the numerical study of air-water mist jet impingement cooling over a heated cylinder in the non-boiling region. The studies are conducted for various mist loading fraction, f = 0.0025, 0.0050, 0.0075 and 0.01; Reynolds number, Re mix. = 8000, 10,500, 13,000 and 15,500; and surface-to-nozzle spacing, H/d = 30, 40, 50 and 60. Enhancement in the heat transfer (η) for mist jet impingement as compared to air jet impingement is obtained from experimental and numerical analysis. The numerical study also helped in understanding the distribution and tracking of droplets in the computational domain. High enhancement in the heat transfer is observed with high mist loading fraction. Also, the lowest Reynolds number yielded the highest enhancement in heat transfer with comparison to the higher Reynolds number. At various surface-to-nozzle spacing, the enhancement in heat transfer is high for low surface-to-nozzle spacing near the stagnation zone to a certain point, after which point the enhancement reverses as higher enhancement in heat transfer is observed for high surface-to-nozzle spacing. As high as 408% and 775% enhancement in the heat transfer at the stagnation point is observed for f = 0.01, Re mix. = 8000and H/d = 30 during experimental and numerical analysis respectively. The correlation has been proposed to estimate the enhancement in heat transfer at the stagnation point. [ABSTRACT FROM AUTHOR]

In this paper, a combined internal and external blade cooling system is conducted numerically to study the effect of different nozzles arrangements on flow and heat transfer performance. The applied operating conditions are different values of Reynolds number ranging from 5000 to 25,000 and different temperature ratios varied from 0.5 to 0.85. Results indicate that Nusselt number increases with the increase of Reynolds number at constant temperature ratio for all cooling models. The staggered normal cooling model achieves the highest overall Nusselt number with a maximum increase of 14.3% compared to the other cooling models. Overall film cooling efficiency increases with the increase in the corresponding blowing ratios. The staggered normal cooling model achieves superior overall cooling performance compared with the other cooling models. A correlation for overall Nusselt number is derived as a function of Reynolds number and temperature ratio. [ABSTRACT FROM AUTHOR]

• Nusselt number measurements in a wide range of Reynolds and Rayleigh numbers resulting in a Richardson number between 0.1 to 100. • Particle image velocimetry (PIV) and Liquid crystal thermography (LCT) measurements using a refractive index matched porous medium. • Higher Nusselt number for the cavity with porous medium as compared to the pure-fluid cavity at high Reynolds numbers. • Three flow and heat transfer regimes depending on Richardson number. • New correlation for Nusselt number as a function of Reynolds and Rayleigh numbers. This paper reports on an experimental study of mixed convection flow and heat transfer in a vented, differentially side-heated cubical cavity filled with a porous medium consisting of relatively large solid low-conductivity spheres. Rayleigh numbers and Reynolds numbers are varied over the ranges 6 × 106 < Ra < 7 × 107 and 240 < Re < 4250, respectively, for a fixed Prandtl number of Pr = 6.75, thus covering more than three decades in Richardson numbers Ri = Ra/(Re2 Pr). Heat transfer measurements were combined with measurements of the velocity field (using particle image velocimetry) and the temperature field (using liquid crystal thermography) to better understand the dependence of the Nusselt number, Nu, on the Richardson number. We observed three different flow and heat transfer regimes depending on the Richardson number. For Ri < 10, the flow structure and the Nusselt number scaling are similar to those for the pure forced convection, i.e., the Nusselt number scales as Nu ~ Re0.61 independent of Rayleigh number. For Ri > 40, natural convection dominates the flow in the vicinity of the heating wall. The Nusselt number becomes less sensitive to the Reynolds number and is mainly determined by the Rayleigh number. In the intermediate regime for 10 < Ri < 40, the upward directed natural convection flow at the heating wall competes with the downward directed forced flow leading to a minimum effective Nusselt number. A Nusselt number correlation is derived that is valid in the range 0.1 < Ri < 100 covering all three regimes. [ABSTRACT FROM AUTHOR]

• Twelve slit ribs with various rib heights and open-area ratios are evaluated. • Flow structures and resulted heat transfer behaviors are analyzed and discussed. • Creating a slit in high-height ribs can improve the heat transfer performance. Augmentation of heat transfer must always be judged against an additional pressure loss. To this context, this paper explores slitted ribs on the turbulent flow and heat transfer characteristics in rectangular cooling channels with an aspect ratio of 4:1. A verified v2f turbulence model is applied in the present numerical simulations in the Reynolds number range from 20,000 to 80,000. The flow and thermal performance of various geometrical structures for the ratios of slit-length-to-rib-length (R l = 0, 0.20, 0.35 and 0.50) and the rib heights (e = 10 mm, 15 mm and 20 mm) are comprehensively compared and evaluated. Special thermal behaviors are carefully observed and analyzed with slits on ribs of different height. It is found that strip slits in low-height ribs provide both lower Nusselt number and friction factors than the corresponding solid ribs, while an opposite effect is captured for strip slits on high-height ribs. The present results suggest that introducing a short-length strip slit in ribs could be a beneficial way to enhance heat transfer of cooling channels with high-height ribs. [ABSTRACT FROM AUTHOR]

• Investigating forced convection of a nanofluid through a wavy-sinusoidal minitube using two-phase model. • Evaluation of the effect of geometrical and thermal parameters to achieve the most advantageous geometry. • Wavy amplitude, wavelength and phase shift have serious impact on wavy-wall minitube PEC. • PEC amount for optimum tube is about 1.670. In present study heat transfer and forced convection flow of non-Newtonian two-phase ethylene-glycol/water mixture based nanofluid through a wavy-sinusoidal minitube has been investigated. The 2D governing system of equations is solved numerically in the computational field using the Control-Volume-Approach founded on the SIMPLE technique. Flow velocities or Reynolds numbers are varied in the turbulent regime range from Re =6000 to Re =15,000 and the tube walls boundary condition is uniform temperature. The optimization was conducted considering five different constant and variable α(x) amplitudes, five various constant and variable λ(x) wavelengths and five various phase shifts in order to achieve the most advantageous geometry in view of the highest thermal-hydraulic performance evaluation criterion index. From obtained results in present paper, it has been realized that the wavy-wall minitube thermal-hydraulic performance is seriously affected with varying the wavy amplitude, wavelength and phase shift. The highest thermal-hydraulic PEC has been obtained for the amplitude of α=0.5 mm with increasing wavelength and phase shift of θ=90° at Re =6000 Reynolds number. After opting of the optimum geometry, Silicon-Carbide (SiC) nanoparticles were added to base fluid in volume fraction of 4% and sizes of 16 to 90 nm. Based on results, the PEC of EG/water based nanofluid with nanoparticle diameter of d np =90 nm has been conducted to be the most efficient among all models during investigated range of flow velocities. PEC amount for optimum tube using EG/water based SiC nanofluid with volume fraction of ϕ=4% and nanoparticle diameter of d np =90 nm at Re =15,000 is about 1.670. [ABSTRACT FROM AUTHOR]

This paper proposes a turbulence model correction method and an anisotropic k − ω model for predicting heat-transfer characteristics in a rotating state. We consider Reynolds numbers ranging from 3000 to 15,000, rotation numbers ranging from 0 to 1.31, and temperature ratios of 0.1, 0.17, and 0.22. Rotation corrections are made to the Reynolds stresses. Furthermore, the Reynolds stress ratio is expressed as a function of the turbulent kinetic energy Reynolds number and the rotation number to reflect the effects of the turbulent fluctuation on the Reynolds stress. Compared to the standard k − ω model, the calculation results of the anisotropic k − ω model, especially for the heat-transfer prediction at the trailing edge, are considerably improved, and in close agreement with the experimental data. The heat-transfer law at the leading edge gives a critical rotation number, which is inversely proportional to the value of the dimensionless position. By studying the buoyancy number on heat transfer, we demonstrate an increase in the rotating-to-stationary Nusselt number ratios at the trailing edge and a decrease at the leading edge, which are not obvious for buoyancy numbers less than 0.1. However, when the value is greater than 0.1, they start to increase, especially for the trailing edge. • An anisotropic turbulence model correction method was proposed. • The anisotropic correction method was applied to the standard k − ω model. • w ′ w ′ ¯ / u ′ u ′ ¯ is expressed as a function of R e d and Ro , the anisotropy is better reflected. • The anisotropic model can solve the problem of under-predicting heat transfer. • Critical rotation number was inversely proportional to the dimensionless position. [ABSTRACT FROM AUTHOR]

• Fluid flow and thermal performances of PHE are investigated experimentally. • The effectiveness and NTU of PHE are improved with increasing Rr. • Pressure drop and pumping power are increased by increasing Rr. • SHR is improved with increasing surface roughness particularly at low Reynolds number. • Empirical correlations are proposed to predict SHR as a function of Rr and Re. In the present paper, an experimental study of the effectiveness and pumping power of plate heat exchanger (PHE) with rough surface was performed. The experiments were developed for different values of relative roughness, Rr, of 2.6 * 10−4 (smooth surface), 3.4 * 10−4, 7.5 * 10−4 and 9.8 * 10−4 for Reynolds number, Re, varied from 500 to 5000. The average heat transfer rate, Q avg , hence the effectiveness of PHE, ε , and number of transfer units, NTU , as well as pressure drop, Δp , consequently pumping power, P, and specfic heat rate, SHR, were estimated. The results show that Q avg and NTU are enhanced with increasing Rr ; while, they are decreased with increasing Re. At Re = 5000, the effectiveness of PHE with relative roughness of 3.4 * 10−4, 7.5 * 10−4 and 9.8 * 10−4 is greater than that of the smooth surface by 11.5%, 16.6% and 18.6%, respectively. Furthermore, Δp and P are increased by increasing Rr and Re. Moreover, at Re = 500, SHR is enhanced with increasing Rr and its improvements compared with the smooth surface are 39.5%, 23.4% and 6.9% for Rr of 3.4 * 10−4, 7.5 * 10−4 and 9.8 * 10−4, respectively. In addition, empirical correlations are proposed to estimate SHR as a function of Rr and Re with a standard deviation of 5.1%. [ABSTRACT FROM AUTHOR]

The present paper is devoted to the first and second law analysis of a aqueous nano-fluid (NF) containing suspended Ag nanoadditives in two new microchannel heatsinks. The influences of nanoadditives volume fraction (φ) and Reynolds number (Re) on performance indicators based on the first and second law of thermodynamics are taken into account and analyzed. The outcomes reveal that the augmentation of Re and φ causes an intensification in the convective heat exchange coefficient and a decrease in the CPU temperature, thermal irreversibility rate and enhance the CPU temperature uniformity. However, grow of Re intensifies both the pumping power and irreversibility due to flow friction. Additionally, the outcomes depicted the superior performance of NF compared to the base fluid. Moreover, the irreversibility inside the studied heatsinks is mainly due to the heat transfer compared to the flow friction. Finally, the results demonstrate that the heatsink with more path changes is more efficient form the viewpoint of the first and second laws of thermodynamics. [ABSTRACT FROM AUTHOR]

The experimental system: (a) Schematic diagram. (b) Physical diagram. • Effects of round hole diameter on the flow and heat transfer are studied. • Effects of pitch-row on the flow and heat transfer are studied. • Thermal and exergy efficiency for thermo-hydraulic performances are studied. For investigating the thermal and hydraulic characteristics of water-based SiO 2 nanofluids in a triangular tube with different turbulators, an experimental system has been designed and verified in this paper. The effects of different round hole diameters (d = 3 mm, 4 mm, 5 mm) and round hole pitch-rows (l = 5 cm, 10 cm, 15 cm) of perforated turbulators on the thermo-hydraulic characteristics are researched. Meanwhile, the influences of Reynolds numbers (Re = 400–8000) and nanoparticles mass fractions (D-I water, ω = 0.1%, 0.3%, 0.5%) are also studied. These experimental results show that, under the same circumstance, the nanofluids in the triangular tube with ω = 0.5% have the largest positive influence on the heat transfer enhancement ratio which is up to 16.73%. For a comprehensive study of the flow and heat transfer, thermal efficiency (comprehensive performance index) and exergy efficiency are adopted. It can be found that the larger the diameter and the smaller the pitch-row of the holes is, the greater the comprehensive evaluation index can be. In addition, all working conditions exhibit the superior exergy efficiency. The highest exergy efficiency can be got when Re = 6000 and ω = 0.5%. [ABSTRACT FROM AUTHOR]

The paper presents experimental studies on heat transfer phenomena which occurs during the impingement cooling with the use of synthetic jet. The synthetic jet was created by a special actuator consisting in a plexiglass cavity and a loudspeaker. An air was a working fluid. The effect of synthetic jet actuator geometry and supply parameters on heat transfer enhancement has been investigated. The experiment was performed with the use of classical thermal balance method. Heat transfer coefficients at a stagnation point have been obtained. Measurements were carried out in the range of excitation frequency f = 5 ÷ 600 Hz and of root mean square voltage E = 1, 2, 3, 4, 6 V. Changes of actuator geometry included: orifice diameters d = 9, 15, 24 mm, orifice thickness t = 3, 5, 20 mm and cavity depth H = 20, 40, 60 mm. On the basis of measurements results the correlations for the stagnation point Nusselt number of an axisymmetric impinging synthetic jet characterized by large Reynolds numbers have been obtained. Nusselt number has been determined in the function on scaling parameters: Reynolds number, jet-to-surface spacing x/d and the dimensionless stroke length L 0. Reynolds numbers were ranging from 3600 to 22950, x/d = 1 ÷ 20 and L 0 = 0.84 ÷ 170.5. The scaling parameters have significantly extended the range of research so far. The obtained Nusselt number correlations provide heat transfer calculations at large Reynolds numbers and are expected to have practical applications. [ABSTRACT FROM AUTHOR]

• Infrared thermography to resolve transient jet behaviour. • Impinging heat transfer analysis. • Non-swirling (S = 0) and highly swirling jets (S = 1.05). • Correlations presented. A multitude of research has already been undertaken to examine the effects of imparting swirl (S), Reynolds number (Re), and nozzle-to-plate distance (H/D) on the steady-state heat transfer characteristics of turbulent impinging jets. However, no studies to date have compared the transient development of such jets in both swirling and non-swirling conditions. The current paper addresses this gap by using highly resolved (time series) imaged (infrared) data in conditions spanning Re = 11,600, 24,600, and 35,000. The experiments are based on an electrically heated foil (0.025 mm) with jets over S = 0–1.05 and nozzle-to-plate-distances of H/D = 2, 4, and 6. For non-swirling impinging jets, at greater Reynolds numbers, the location of the highest Nusselt number remains fixed for all the time steps in contrast to jets with low Reynolds number. The transient characteristics for H/D = 4 are distinct compared to H/D = 2 and 6, with impingement heat transfer taking more time to reach steady-state for H/D = 4. It is also seen that heat transfer distribution over the impingement plate changes significantly over the small interval of time. For swirling jets, the impingement plate heat transfer characteristics develop simultaneously for the stagnation and wall jet region. The peak Nusselt number shifts to the wall jet region as the intensity of swirl increases. Two correlations (first for non-swirl and low-swirl, later for moderate to high swirl) are proposed to predict the time needed to reach steady-state for Re = 35,000. [ABSTRACT FROM AUTHOR]

This paper numerically compares the flow and heat transfer behavior in a set of variable cross-section vortex chambers with the different vortex chamber draft angle β. Simulations are carried out by solving 3D RANS equations coupled with the standard k-ω turbulence model. Cooling performance for vortex chambers under different β , the inlet to target wall temperature ratio σ and Reynolds number Re are compared in detail. Results indicate that flow and heat transfer behavior of vortex cooling in variable cross-section vortex chambers is distinctly different from that in the uniform cross-section vortex chamber. The heat transfer intensity increases and pressure drop and aerodynamic loss decrease with an increase in β , which leads to a superior flow and heat transfer performance for the vortex chamber with higher β. The cooling air velocity and heat transfer intensity increase with the increasing σ , while aerodynamic loss decreases with the increasing temperature ratio. When Re increases, both of the pressure drop and heat transfer intensity increase. Compared with the uniform cross-section vortex chamber, the vortex chamber of β > 0°, presents more intense heat transfer and lower aerodynamic loss under the same Re and σ. [ABSTRACT FROM AUTHOR]

• Heat transfer characteristics of falling film evaporation were experimentally studied. • The test evaporator is a plate-fin falling film heat exchanger with perforated fins. • The effects of the heat flux, inlet Reynolds number and vapor quality were analyzed. • A semi-empirical prediction model of the heat transfer coefficient was developed. In this paper, the heat transfer characteristics of falling film evaporation of pure refrigerant HCFC123 were experimentally investigated in a pilot scale plate-fin heat exchanger with perforated fins. The experiments have been carried out under the conditions that the mass flux is in the range of 23 to 49 kg/m2s, the corresponding inlet Reynolds number from 200 to 500, the heat flux from 3.2 to 9.5 kW/m2, and the evaporation pressure about 0.3 MPa. The variation laws of the local heat transfer coefficient versus the main parameters, including the inlet Reynolds number, heat flux and vapor quality were discussed. By introducing the two-phase convective multiplier F as a function of Lockhart-Martinelli parameter, a correlation for the two-phase forced convective component of the local boiling heat transfer coefficient was developed, and compared with correlations in open literature. By using a superposition method, the local boiling heat transfer coefficient that includes the contributions of forced convective boiling and nucleate boiling was analyzed. The predictions of local heat transfer coefficients were found to be in good agreement with experimental data. [ABSTRACT FROM AUTHOR]

• Unsteady cavitation dynamics of liquid hydrogen in different modes are analyzed. • Lagrangian methods are utilized to investigate the evolution of shedding dynamic behaviors. • Entropy transport equation is utilized to analyze the thermodynamic characteristics for different cavitation modes. This paper presents a numerical study regarding the evolution of cavitation dynamics in liquid hydrogen cavitating flows with special emphasis on the shedding process and thermodynamic characteristics. The liquid hydrogen cavitating flows around an ogive body in different thermal cavitation modes (the quasi-isothermal mode, the transition mode, and the thermo-sensitive mode) were numerically investigated. Both 2-D and 3-D computations were compared and validated. The results show that the thermal transition of cavitation dynamics is mainly affected by the combined effects of thermodynamic effects and Reynolds number. Lagrangian methods were utilized to investigate the shedding process. When cavitation dynamics changes from the quasi-isothermal mode to the thermo-sensitive mode, the distribution of Lagrangian Coherent Structure (LCS) is closer to the ogive surface. Several vortexes exist between the LCS and the ogive surface. The free-stream liquid flows into the attached cavity because of the smaller vapor volume fraction, and the impact of the re-entrant jet on attached cavity becomes slight and finally disappears. The thermodynamic characteristics of liquid hydrogen cavitating flows were further investigated by the entropy transport equation. The analysis indicates that the distribution of entropy production rate caused by direct viscous dissipation S pro.D becomes mushy and gets closer to ogive surface while the distribution of the entropy production rate caused by heat transfer S pro.C is always along the liquid-vapor interphase with the increasing temperature. [ABSTRACT FROM AUTHOR]

• Optically accessible test section designed for high speed junction flow measurements. • Turbulent kinetic energy increases underneath primary vortex with increasing momentum thickness Reynolds number. • Increasing randomness in primary vortex position with Reynolds number. • Symmetry plane endwall heat transfer scales with incoming momentum thickness Reynolds number. Turbulent junction flow is commonly seen in various turbomachinery components, heat exchangers, submarine appendages, and wing-fuselage attachments, where the approach boundary layer separates and rolls up into a coherent system of vortices upstream of a wall-mounted bluff body. One of the signature features of this flow is its tendency to switch randomly between two semi-stable states. The highly unsteady behavior causes high pressure fluctuations on the wall and obstacle surfaces and high heat transfer. Despite its prevalence, few studies have examined the Reynolds number dependence of the dynamic junction flow behavior. In this paper, the flow physics as well as heat transfer of the turbulent junction flow are investigated using high speed particle image velocimetry and time-average infrared thermography measurements. A wide range of approach momentum thickness Reynolds numbers are studied, ranging from 550 to 5740. Although the time-mean flowfield does not show significant Reynolds number dependency, the normalized turbulent kinetic energy in and around the vortex core increases with Reynolds number, and the high heat transfer associated with the junction flow moves closer to the junction. These effects are linked to the increasing randomness in the position of the primary junction flow vortex as the Reynolds number increases. [ABSTRACT FROM AUTHOR]

The performance analysis of baffle implanted Plate-Fin Heat sink (PFHS) is considered in this analysis, which provides physical understanding of heat transfer characteristics. The baffles are made up of copper strip which act as turbulator, placed in between PFHS channel to induce turbulence in cooling medium. The study variables are heat intensity and cooling air velocity. Thermal characteristics of Baffles implanted PFHS are compared with plain PFHS by experimental and simulation methods. The experimental result shows that about 12.9% higher heat transfer in baffles attached heat sink than plain heat sink. The effect of baffle deflector on PFHS is studied and recommended to vary dimensional changes to improve heat transfer. The experimental and simulation results discover that baffles attached PFHS has more heat transfer than plain PFHS. The turbulator attachment increases pressure drop and Reynolds number, leads to reduced thermal resistance and increased heat transfer. [ABSTRACT FROM AUTHOR]