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Micro fins on the internal surface of the tube improves its heat transfer ability. This paper investigates the thermal performance of tube flow numerically by adding axial straight micro fins (ASMF) on internal tube surfaces. The numerical method is experimentally validated using a tube featuring ASMF. The effects of fin height e , fin number N f , and fin shape of the tube with ASMF on the thermal performance are numerically studied within the range of Reynolds number (Re) from 1280 to 7687. Compared to a smooth tube, the Nusselt number (Nu) for tube with ASMF is reduced over the entire range of Re. In order to assess the performance of heat transfer, the ratio of the heat transfer coefficient (h) multiplying the heat transfer area (F) of ASMF tube (hF) to that of the smooth tube (h s F s) is utilized. Parametric studies were performed. The optimal fin height, fin number are found to be 0.0429 and 32. The optimal fin profile is found to be quadratic function profile. By combining the optimal fin height, fin number and fin profile, the maximum value of hF /(h s F s) can reach 1.32. Besides, the heat transfer enhancement mechanism induced by ASMF is also disclosed in this paper. [ABSTRACT FROM AUTHOR]

Five components of heat flux, viz: conduction, q c o n d ″ , sensible mass diffusion, q d i f f ″ (s) , formation mass diffusion, q d i f f ″ (o) , sensible convection, q c o n v ″ (s) , and formation convection, q c o n v ″ (o) have been analyzed extensively in this paper for the unwrapped model of the rotating detonation engine (RDE), using an explicit large-eddy simulation (LES) approach with a kinetic mechanism consisting of eight reaction steps and seven chemical species. The results shown are from averaged fields obtained after the attainment of the quasi-steady state condition. The maximum static pressure occurs at the transverse detonation wave, with a value that is one order of magnitude higher than the relatively uniform values that are found in most part of the domain. By y ≈ 0.0044 m , which is roughly one-tenth of the axial distance in the domain with (x , y) ε [ 0,0.1 ] × [ 0,0.04 ] m, most of the reacting species (H 2 , O 2) have been depleted at all transverse locations. From the mass averaged magnitudes of the heat flux components, it has been found that q c o n v ″ (o) , has the largest contribution, followed very closely by its sensible counterpart, q c o n v ″ (s) , whose magnitude is roughly half of that for the formation enthalpy heat flux. This is followed (in magnitude) by q d i f f ″ (o) and q d i f f ″ (s) , which are of comparable values and are over three orders of magnitude smaller than the convective heat flux components. The conductive heat flux q c o n d ″ is roughly an order of magnitude smaller than the mass diffusion values. The Nusselt number and heat transfer coefficient for the present problem are significantly higher than those encountered in low-speed, non-reacting flow fields. • Five components of heat flux in a model of the rotating detonation engine (RDE) were extensively investigated in this study using the large-eddy simulation (LES) approach within the context of combustion and detonation. • The critical structures in the RDE combustor (shock waves, slip lines, contact discontinuities, etc.) significantly determine the magnitudes and distributions of the three heat flux components that are based on molecular diffusion. • The formation convective heat flux has the largest contribution, followed very closely by its sensible counterpart, whose magnitude is roughly half of that for the formation enthalpy heat flux. The conductive heat flux has the smallest magnitude, which is roughly one order smaller than the mass diffusion values. • The maximum static pressure occurs at the transverse detonation wave, with a value that is one order of magnitude higher than the relatively uniform values that are found in most part of the domain. • The axial distance at which a significant amount of the reactions occur was determined. [ABSTRACT FROM AUTHOR]

This paper presents numerical simulation of graphe nanoplatelet (GNP) – sodium dodecylbenzene sulfonate (SDBS) – water nanofluid with volumetric mass fractions of ϕ equal to 0–0.1%. The new microchannel is made of 2 layers and has sinusoidal cavities along with the heat sink. The study was conducted for Re = 50, 300, 700 and 1000 (laminar flow). Based on the results, higher Re leads to more convective heat transfer coefficient. Therefore, the value of temperature gradients decreases and on the other hand the effect of cooling fluid temperature in warm areas near solid wall becomes important. Using a higher value of Re results in the penetration of heat in the fluid layer decreases which in general will reduce the temperature of the outlet section in larger Re values. Also, larger Re value results in larger development length which is also a cause for lack of thermal development and dominance of the temperature of the coolant flow in the heat sink resulting in decreased outlet section temperature for higher Reynolds numbers. For Re = 50 and 300 because of lower fluid velocity, the effect of surface geometry factors such as sinusoidal paths or hollow parts (cavities) has a smaller influence on the pressure loss or friction factor of the flow. [ABSTRACT FROM AUTHOR]

• Centrifugal buoyancy is the main mechanism that governs the secondary flows induced by rotation. • Rise in rotational speed doesn't necessarily result in increase in Nusselt number. • A dimensionless group is introduced as a criterion to judge the rotational effects. • The conditions in which the rotation positively affects the heat transfer is explored. In this paper, turbulent airflow and heat transfer are studied numerically in a square sectioned channel which is rotating about a parallel axis. Channel rotation may create secondary flows as a result of centrifugal buoyancy and Coriolis forces which may increase the heat transfer coefficient and wall friction. Under certain conditions that the axial flow is dominant over the rotation, the effects of rotation can simply be ignored which means that friction factor and Nusselt number can be calculated regardless of rotation. This paper explores a novel criterion to demonstrate how the rotation affects the fluid flow and heat transfer. Rotational Grashof number ( Gr Ω) and Reynolds number ( Re Ω) are chosen to describe the rotation effects in addition to Reynolds number (Re) taking the axial flow into account. The results show that at a given Gr Ω and, Re Ω a specific Reynolds number ( Re ∗ ) in which the effects of rotation vanishes can be defined. It is revealed that Re ∗ may be presented as a function of Gr Ω only. With this regards, a non- dimensional group as Gr Ω a 1 / R e is obtained which is utilized to judge the rotational effects. It is shown that for Gr Ω a 1 / R e > 0.0167 the rotation starts to become important while for Gr Ω a 1 / R e < 0.0167 the Nusselt number is approximately equal to stationary counterpart. [ABSTRACT FROM AUTHOR]

As a phase change material, ice slurry has been successfully applied in HVAC&R systems due to its high energy storage density. The heat exchangers are the key equipment realizing energy conversion of ice slurry. In HVAC&R systems, U-tubes are frequently used in shell and tube heat exchanger. However, there are few studies on thermo-fluidic characteristics of ice slurry in U-tubes. Therefore, the thermo-fluidic characteristics of ice slurry in U-tubes are numerically studied in this paper using Euler-Euler approach. The results indicated that as enlarging the inlet ice content and velocity, the ice concentration distribution becomes more uniform along in vertical profile. Due to the secondary flow in bend section, the ice concentration decreases in the inner wall and increases in the outer wall as the inlet velocity increases; At the high Reynolds number, the inlet ice content and ice particle diameter have a more significant fluence on pressure drop (Δ p). The local heat transfer coefficient (h lo) in bend section exhibits a tendency of first increasing and then decreasing, with the maximum value of h lo occurs at θ = 90° of the bend section. Finally, a correlation is suggested between the average resistance coefficient (f ave) and local Nusselt number (Nu lo). • The melting process of ice slurry in U-tubes is investigated. • The factors affecting thermo-fluidic behavior of ice slurry in U-tubes are discussed. • This peak value of h lo is located within the bending section at an angle of θ = 90°. • The correlation of f ave and Nu lo in U-tubes is obtained for ice slurry. [ABSTRACT FROM AUTHOR]

Corrugated tubes are extensively unitized in enhancing heat transfer within tubular heat exchange devices. The efficiency of these corrugated tubes is closely linked to the geometric characteristics of the surface corrugations. In this study, we conducted an extensive series of Large Eddy Simulations on a representative type of enhanced tube featuring periodic inward corrugations (PIC). Our goal was to compile a comprehensive and precise dataset of the thermal and hydraulic properties specific to this design. The key design parameters that define the corrugated patterns are corrugation height (H), corrugation width (W), and the length of the straight section (P). We systematically explored these parameters to understand their impact on heat transfer and fluid dynamics. Among these parameters, corrugation height (H) has been identified as the most influential factor affecting flow and heat transfer properties. As the corrugation height varies from 1/8D to 3/8D, we observed a substantial increase in the friction factor and Nusselt number by a factor of 209 and 5, respectively, at Re = 1000; while the critical Reynolds number decreased roughly from 800 to 100. These findings underscore the significant role of corrugation height in shaping the flow and heat transfer performance of PIC tubes. The other two parameters, corrugation width and straight section length, were also found to be crucial in determining the overall heat transfer coefficient and pressure drop. When Re = 1000, variations in W/D from 1 to 3 resulted in a 1.3-fold increase in both the f and the Nu. Furthermore, altering the P/D from 2 to 6 led to a 2.1-fold increase in f and a 2.2-fold increase in Nu. The results of our study suggest that when utilizing this type of corrugated tubes, careful consideration is required to strike an optimal balance between heat transfer efficiency and pressure loss. The large-sets LES results presented in this paper can serve as valuable inputs for optimizing the design of PIC tubes. [ABSTRACT FROM AUTHOR]

Highlights • Effects of radiation and magnetic field on heat transfer and entropy generation. • Square cavity with a conductor fin containing nanofluid. • Increasing Rayleigh and reducing Hartmann increases Nusselt number. • Increasing radiation parameter increases Nusselt and entropy generation and decreases Bejan. Abstract In this paper, the effects of radiation and magnetic field on the convection heat transfer rate and the nanofluid entropy generation in a diagonal square cavity with a conductor fin have been numerically investigated. A fin with a thermal conductivity coefficient of k* = 100 is located on one of the walls of the cavity. A volumetric heat source is considered in the fluid that is producing heat in the form of radiation. The effect of this source is added as a source to the energy equation and its value is shown by the Rd radiation parameter. Mass, momentum, and energy conservation equations in two-dimensional mode are discretization with finite difference method based on the control volume and solved using the simple algorithm. The model used for the thermal conductivity coefficient is the phenomenon model, taking into account the Brownian motion of the particles. In this paper, the effects of Rayleigh numbers, Hartmann numbers, radiation parameter and volume percentages of nanoparticles on the entropy generation and heat transfer have been investigated. The results show that increasing the Rayleigh number and reducing the Hartmann number increases the Nusselt number. Alternatively, adding 6% of the nanoparticles to the base fluid in the absence of radiation increases the heat transfer rate and entropy generation by 5.9% and 16.6%, respectively. By adding the radiation parameter, Rd = 3, and the volume percentage of nanoparticles of 6%, the heat transfer rate and total entropy generation increased by 3.4% and 11.2%, respectively. It was also observed that increasing the radiation parameter at high Rayleigh numbers increases the Nusselt number and entropy generation and decreases the Bejan number. Increasing the heat transfer rate is more significant by increasing the radiation parameter at higher Rayleigh numbers. [ABSTRACT FROM AUTHOR]

In this paper, numerical investigations on heat transfer and flow resistance characteristics of a variable-direction twisted-oval tube (VDTOT) were performed. The effects of different geometric parameters (transverse and longitudinal tube pitches, ratio of major to minor axis, twist angle and pitch) on the heat transfer coefficient, pressure drop and heat transfer capacity per pump work were investigated. Contribution ratios of different factors were also analyzed. The results show that the transverse tube pitch and twist angle have a great impact on the overall performance of the tube bundle. VDTOT realizes the synergistic enhancement of flow and heat transfer performance. Compared to the circular tubes, the heat transfer capacity of VDTOT bundles is increased by 13.6%–20.6 % under the same pump power consumption. The average deviations of Nu and Eu correlations fitted by the multiple regression method are 0.99 % and 1.64 %, which is helpful for the rapid design and optimization of VDTOTs. • The thermal performance of the twisted oval tube heat exchanger are studied. • Effects of tube pitch, twist parameters and flow velocity are fully considered. • Overall performance of different cases is compared by performance evaluation plot. • Dimensionless correlations of Nusselt number and Euler number are obtained. [ABSTRACT FROM AUTHOR]

• The flow and heat transfer characteristics of single and multiple-synthetic jet impingement cooling is presented. • The equivalent Reynolds and Nusselt numbers were determined for the purpose of describing multiple orifice configuration. • The merging jet process is presented and the merging point is determined. In this paper, the comparison between synthetic jet actuator with single axisymmetric orifice and multiple axisymmetric orifices is presented. The aim of this paper is to compare the flow and heat transfer characteristics of single and multiple synthetic jet impingements cooling. The multiple orifice configurations had a central orifice and a satellite orifices placed in the pitch circle. The synthetic jet actuator had central orifice with 0, 3, 4, 5 and 6 orifices placed in the diameters of the relative pitch circle p / d = 2.5, 3 and 4. A total of 13 various geometric configurations were tested experimentally. In the multiple orifice configurations, the total cross-section area was approximately equal to the cross-section area of the single orifice configuration. The input power delivered to the synthetic jet actuator was maintained constant and equal to 3.00 W. The synthetic jet actuator energetic efficiency, Reynolds number, equivalent Reynolds number, radial and axial distributions of pressure coefficient, and heat transfer characteristic were presented. The synthetic jet issuing from multiple orifice plate merged at some axial distance, creating a single axisymmetric jet. The merging jet axial distances were determined for all investigated configurations. In the present paper, the integral quantities averaged over all orifices cross section area, or over impingement plate much larger than total orifices area, were analyzed for the purpose of objective comparison. From all various investigated parameters obtained from five different and independent experimental methods, the following conclusion may be made: if the total orifice cross section area in the case of single and multiple orifice synthetic jet is similar and real power deliver to the SJA is maintained at the same level the resulting equivalent Reynolds number and equivalent dimensionless stroke length achieved similar values and the impinging jet equivalent Nusselt number also achieved comparable values and distributions. [ABSTRACT FROM AUTHOR]

• Subcooled flow boiling heat transfer of water in the porous metal foam tubes was investigated experimentally. • Filling a tube with metal foam could improve the heat transfer and also pressure drop. • The thermal efficiency of metal foam tubes was investigated. • Based on experimental data, a correlation was presented to predict heat transfer in the metal foam pipes. This paper has investigated the effect of different porosities (0.80–0.90) of the metal foam pipes on heat transfer coefficient and pressure drop of subcooled flow boiling of water experimentally. The metal foam pipe with 0.80 porosity has increased the Nusselt number and pressure drop by 41 % and 21 % compared to the 0.90 metal foam pipe, respectively. The heat transfer of water phase change in a porous medium has received less attention due to the complexities of the problem. One of the challenges in this field is the absence of a correlation for predicting heat transfer in metal foams. In this paper, a systematic method based on dimensionless numbers considering the essential parameters, like mass flux, subcooling, and heat flux, was used to provide a correlation for calculating the Nu-number based on 106 experimental points. The mean absolute deviation (MAD) of the results predicted by the new correlation is 8.9 %, and it predicts 95 % of the database with ±20 %. [ABSTRACT FROM AUTHOR]

Particle-based moving bed heat exchangers are increasingly playing a significant role in industrial energy conservation and solar energy utilization. However, the moving bed heat exchanger used for cohesive granular materials still needs to be developed. This paper investigates the feasibility of indirect heat transfer of cohesive granular materials through a moving bed heat exchanger. A vibrating moving bed heat exchanger is developed for the metallurgical slag of vanadium-titanium (MSVT) from industry. The experiment demonstrates that the vibrating moving bed heat exchanger can establish an approximate mass flow regime of MSVT in the experimental section. The vibration device designed for the experimental and discharge section effectively resolves bridging and the intermittent collapse of arching in cohesive granular materials. The outer side of the tube is tightly wrapped by MSVT with vibration. Significant differences are observed in the flow and heat transfer behavior of MSVT between the aligned and staggered tubes. The local heat transfer coefficient at the top and bottom of the aligned tubes exhibits much lower sensitivity to flow velocity compared to the staggered tubes. The relationship between Nu −1 and Pe −0.5 or MSVT around the aligned and staggered tubes follows a linear correlation. Compared to aligned tubes, Nu −1 for MSVT in staggered tubes shows an average reduction of 28%. [Display omitted] • A vibrating moving bed heat exchanger is developed for cohesive granular materials. • Special experimental design can resolve bridging and arching between tubes. • Rheological properties of vanadium-titanium granular are quantitatively evaluated. • The heat transfer of granular flow is tested around different tube arrangements. • The heat transfer correlations are built to predict the heat transfer. [ABSTRACT FROM AUTHOR]

• Novel opening combination rotor blade is proposed to enhance heat transfer effect. • The effect of opening combination rotor blade is optimized for overall performance improvement. • Enhanced heat transfer is revealed by field synergy principle. • The characteristics of opening combination rotor blade are revealed by numerical simulation. • The applicable conditions of heat transfer strategies are determined. Based on the passive enhanced heat transfer mechanism, this paper proposes the new opening combination rotor blade, which aims to improve heat and mass transfer performance by changing internal structure of cracking furnace tube, thus achieving efficient production of ethylene. Through mathematical modeling and simulation of heat transfer properties of opening combination rotor blade, heat transfer performance and fluid flow characteristics of ordinary rotor blade tubes and opening combination rotor blade with axial widths of W (W = 0.25 R, 0.5 R, 0.75 R) are analyzed using field synergy principle and comprehensive heat transfer evaluation coefficient (PEC).The results demonstrate that the Nusselt number of opening combination rotor blade is 19%-23% lower than that of ordinary rotor blade. The resistance coefficient of opening combination rotor blade with axial width of 0.25 R is significantly lower by 33%-43%. Opening combination rotor blade with axial width of 0.25 R has a significantly lower resistance coefficient of 72%-78%. Its PEC value is between 1.23–1.28 with significantly improved overall heat transfer performance. In summary, opening combination rotor blade with axial width of 0.25 R has the most significant heat transfer effect and has a high reference significance for engineering design. [Display omitted] [ABSTRACT FROM AUTHOR]

In the applied mathematical modelling of heat transfer systems, the heat transfer coefficient (HTC) is one of the most important parameters. This paper proposes a combination of the Method of Fundamental Solutions (MFS) with particle filter Sequential Importance Resampling (PF-SIR) to estimate the time-dependent HTC in two-dimensional transient inverse heat conduction problems from non-standard boundary integral measurements. These measurements ensure the unique solvability of the boundary coefficient identification problem. Numerical results show high performance on several test cases with both linear and nonlinear Robin boundary conditions, supporting the synergy between the MFS and simulation-based particle filter sequential analysis methods. [ABSTRACT FROM AUTHOR]

Highlights • The effect of moisture content on natural convection flow in a cavity with differentially heated vertical and conducting horizontal walls was investigated. • Natural convection flow and heat transfer are very sensitive to the level of moisture present in the air and even at low temperature gradient between the cavity vertical walls. • The presence of moisture causes changes in the energy equation, which has a significant effect on the heat transfer coefficient and influences the weak natural convection flows inside the cavity. • The vertical walls have been found to show greater sensitivity for heat transfer with changes in moisture content, while both horizontal walls are almost insensitive to moisture content variation. • The heat transfer coefficient increases proportionally with increasing moisture concentration, while the average relative humidity inside the cavity significantly reduces with increasing temperature gradients between the vertical walls of the cavity. Abstract This paper reports a numerical investigation of low turbulence buoyancy-driven flow of moist air and heat transfer inside a rectangular cavity with differentially heated vertical walls. The variations of the flow, temperature and moisture inside the cavity has been analysed together with heat transfer coefficients for a range of mass fraction of water vapour and temperature gradients between the vertical walls of the cavity. The accuracy of the numerical methodology was also scrutinised by conducting a rigorous validation study of benchmark experimental data for the average Nusselt number for similar buoyancy driven cavity flow. The results of this investigation showed that during the natural convection process, the change in moisture content in the moist air has a significant influence on the flow and temperature fields inside the enclosure and the variation of the vertical wall temperature gradients have also shown to affect the moisture concentration inside the cavity. The percentage change in the average heat transfer varied significantly depending on the mass fraction of moisture in the air and the temperature gradient between the vertical walls. The results also showed a 3.5% increase in the average heat transfer for every 0.02 kg/kg increment in the mass fraction of water vapour. The findings from the study are significance to scientists and practitioners who are responsible for moisture management in enclosed space at the design threshold and for optimisation of energy used in buildings. [ABSTRACT FROM AUTHOR]

Highlights • The heat transfer and flow characteristics of two-turn ribbed serpentine channel were analyzed. • Heat transfer distribution is studied with the transient liquid crystal measurement technique. • The inlet Reynolds numbers are varied from 5,000 to 20,000. • Pressure coefficient distribution of two-turn ribbed serpentine channel was researched. Abstract This paper experimentally and numerically investigates the heat transfer and flow characteristics of a two-turn ribbed serpentine channel with lateral outflow. The heat transfer coefficient was measured by a transient liquid crystal technique. Experiments were carried out at Reynolds numbers between 5,000 and 20,000 and rotation numbers of 0 and 0.03. The results indicate that with increasing inlet Reynolds number, the high-Nusselt number regions of the middle and lateral outflow channels gradually move to upstream ribs. The inlet channel shows the highest increase rate of the Nusselt number; whereas, the rate is lowest at downstream turning area. The rotation increases the trailing surface Nusselt number of the inlet and lateral outflow channels, and the increase rate is more prominent for higher values of the Reynolds number. However, the rotation has negative effects on the averaged area Nusselt number of the middle channel in cases of Re ≤ 17,000. The pressure coefficients decrease the in inlet and middle channels along the flow direction; whereas, they slightly increase in the lateral outflow channel. There is a long low-velocity vortex formed in the lateral outflow channel, and the vortex size is considerably reduced by the rotation. Accordingly, the rotation has the most positive effects on the pressure coefficient of the lateral outflow channel. [ABSTRACT FROM AUTHOR]

Highlights • Anisotropic heat transfer and natural convection of a ferro-nanofluid are studied. • Based on frame indifference rule, anisotropic thermal conductivity tensor is derived. • Due to the magnetic field, the isotherms show significant anisotropy. • The Nusselt number increases significantly as the Rayleigh number increases. • The Hartmann number may enhance or diminish the thermal performance. Abstract In this paper, effects of anisotropic thermal conductivity and the Lorentz force in a ferro-nanofluid induced by external magnetic fields are investigated. The nanofluid is modeled as a non-linear single-phase fluid, where the viscosity is considered to depend on the nanoparticle concentration; the thermal conductivity shows anisotropy due to the existence of the magnetic field. The anisotropic thermal conductivity tensor, which depends on the magnetic field vector, is derived by considering the principle of material frame indifference of Continuum Mechanics. Numerical simulations are performed in a horizontal annulus enclosure. Effects of the nanoparticle concentration, magnetic distribution, and the dimensionless parameters, such as the Rayleigh number and the Hartmann number, are studied. The numerical results indicate that due to the anisotropic thermal conductivity, the isotherms become elliptic and deviate significantly from a circle which is a typical distribution with isotropic thermal conductivity; furthermore the elliptic shape changes as the direction of the magnetic field varies while the heat transfer is always much faster in the direction parallel to the magnetic field. For the range of the dimensionless parameters studied here, the Nusselt number increases significantly as the Rayleigh number (ranging from 2500 to 50,000) increases; depending on the situation, the magnetic field (the Hartmann number which ranges from 0 to 1) may enhance or diminish the thermal performance. In addition, the streamlines are more organized and less wavy when there exists a magnetic field; this could be attributed to the suppressive effect of the Lorentz force. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A modular heat exchanger with microjet enhancement was fabricated and tested. • Effect of microjet array dimensions on the thermal performance was investigated. • Heat transfer characteristics were obtained via Wilsons plot method. • Experimental heat transfer database was compared against literature. Abstract The paper describes the comprehensive study on the effect of microjet array geometrical parameters on the heat transfer enhancement in the modular heat exchanger. The conducted experimental study provides an experimental database on single phase submerged microjet heat transfer. The Wilson plot method was applied to determine the heat transfer coefficients in the laminar and transition flow regimes of a liquid-to-liquid heat exchanger. The heat exchanger was capable of exchanging 296 W of thermal energy at LMTD of 44 K. The obtained heat transfer coefficient reaches over 24,000 W/m2 K. Average Nusselt number predictions of the Wen and Jang (2003) correlation were in best agreement with the experimentally determined average Nusselt numbers. In the whole tested flow range, Nusselt numbers were not well correlated by any of the correlations from the literature. The experimentally determined Nusselt numbers were significantly lower than expected, due to limited applicability of given literature correlations. The author also proposed own experimental correlation for jet impingement heat transfer coefficient, predicting the experimental results within 30%. [ABSTRACT FROM AUTHOR]

Abstract This paper deals with the modeling of weakly rarefied and dilute gas flows in micro channels by the continuum approach, valid for Knudsen numbers smaller than about 0.1. It particularly focuses on the modeling of the associated heat transfer. The models proposed in the literature for the forced convection of gas flows in long micro channels between two infinite plates are more specifically discussed. The complete model for such flows is reminded after a compilation and a brief description of their possible applications in industries. The compatibility of the pressure work and viscous dissipation in the energy equation with the power of the viscous stress at the walls is discussed in detail. A dimensional analysis is proposed in the context of long micro channels. Analytical solutions for the velocity and temperature fields and for the Nusselt number are provided in the case of compressible micro-flows in isothermally heated flat plate channels, with pressure work and viscous dissipation included. The choice of an appropriate Nusselt number, including the power of the viscous stress at the wall, is particularly discussed. It is shown analytically and numerically, by solving the complete model for an isothermal wall micro-channel, that the Nusselt number tends to zero when the hydraulic diameter decreases, that is when the Reynolds number decreases and the Knudsen number increases. This could theoretically explain the very small values of the Nusselt number obtained in the experiments by Demsis et al. (2009, 2010). Highlights • Isothermally heated gas slipping flows in long plane micro-channels are considered. • Analytical solution of flow and heat transfer established from an asymptotic analysis. • Viscous stress power at the wall must be included in the total wall heat flux. • Wall Nusselt number tends to zero or to very small values for this flow type. • The very small values of the Nusselt number obtained in experiments are explained. [ABSTRACT FROM AUTHOR]

This paper presents general types of correlation for the heat and mass transfer coefficients inside an air-solution contactor as expressions of Reynolds-Prandtl numbers and Reynolds-Schmidt numbers, respectively. These general equations summarize the physical and thermophysical properties of the air, the solution, and the contactor, which make them capable to be used for parametric studies provided they are fitted in a wide range of experimental data that include all the properties involved. In this work, a liquid desiccant system with an adiabatic structured packed bed as contactor and an aqueous lithium chloride as solution was constructed. The experimental data taken at various air superficial velocities and solution flow rates were fitted to the general correlations, and comparisons between the predicted and experimental results for both coefficients are within ±10%, for both dehumidification and regeneration processes. In addition, the calculated values of the outlet air humidity ratio and temperature agree well with the experimental data for both processes. The particular equations for the heat and mass transfer coefficients can be used to perform parametric studies at different air superficial velocities and solution flow rates with very good accuracy. Results from this study can help improve the system design and operation methods of air-solution contactors. [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]

• A transient TLC technique is used to measure the Nusselt number on the rotating cylindrical surface with flow confinement. • The effects of the rotational Reynolds number, jet Reynolds number, and jet angle on heat transfer characteristics are studied. • The mechanism of convective heat transfer for different parameters is analyzed. This paper presents an experimental study on the convective heat transfer characteristics of a circumferentially arranged oblique jet array impinging on the rotating cylindrical surface in the confined space. This cooling structure can be used for the cooling of rotating equipment, such as the cooling of rotating shafts of steam turbines, the quenching of metal materials, and the cooling of cement rotary kilns. The heat transfer coefficient on the rotating cylindrical surface is obtained with the transient thermochromic liquid crystal (TLC) method. Local and circumferential average heat transfer characteristics, overall heat transfer uniformity, and heat transfer capacity are analyzed comprehensively. The effects of rotational Reynolds number (R e r), jet Reynolds number (R e j), and jet angle (θ) on the heat transfer behaviors are investigated where the rotational Reynolds number is varied from 105 , 115 to 420 , 460 and the jet Reynolds number is varied from 20 , 000 to 35 , 000 at three different jet angles (θ = 20 ∘ , 30 ∘ , 45 ∘). The experimental results show that with the increase of the rotational Reynolds number, the overall circumferential average Nusselt number presents a trend of first decreasing and then increasing, mainly depending on the relative velocity of the circumferential velocity of the main flow and the tangential velocity of the rotating cylindrical surface. The average Nusselt number of the cylindrical surface decreases first and then increases gradually with the increase of the rotational Reynolds number and jet Reynolds number. As the jet angle increases from 20 ∘ to 45 ∘ , the average Nusselt number increases gradually. [ABSTRACT FROM AUTHOR]

In the present paper, a numerical and experimental study has been performed to investigate the heat transfer and pressure drop properties of Al 2 O 3 and SiO 2 /Base oil nanofluid flow in a helical tube under constant heat fluxes. The nanofluids were prepared by using SN-300 base oil as a working fluid for 0.05%, 0.1%, 0.3%, and 0.5% mass concentrations. The experiments were carried out with two constant heat fluxes of 3950 W/m2 and 5280 W/m2 at 30, 40, 50, and 60 °C. The impact of various parameters, including flow Reynolds number, fluid temperature, two different nanoparticles, and various weight concentrations, on the heat transfer factor and pressure drop of the flow, are investigated. To examine the impact of different geometries on the heat transfer rate, simulations were conducted over four different helical tubes by changing the pitch circle diameter and steps. The results indicated that the heat transfer factor and pressure drop were increased by utilizing nanofluid instead of the base fluid. In addition, in the same mass concentration, the Al 2 O 3 nanofluid increased the heat transfer coefficient more than the SiO 2 nanofluid. The maximum heat transfer rate corresponds to 0.5% mass concentrations of Al 2 O 3 and SiO 2 nanofluids, which are 41.4% and 27.3% more than base oil, respectively. The helical tube compared to the straight tube increased heat transfer by 19.5%. Moreover, heat transfer was improved by 6% and 16.5%, respectively, by reducing helical pitch and pitch circle diameter. [Display omitted] [ABSTRACT FROM AUTHOR]

• This paper proposes a new method for heat transfer coefficient measurement. • The wall temperatures on the microchannel before and after the heater measured. • The temperature profile simulated with consideration of the axial conduction. • The measured wall temperatures compared to the simulated wall temperatures. • The Nu number in the laminar flow for a microchannel is independent of Re number. The measurement of heat transfer coefficients in microchannels is complicated due to the small sizes involved. Moreover, a heat transfer mechanism which is not usually considered, the axial conduction effect in the channel wall, must also be evaluated in micro-scale measurements. Previous heat transfer coefficient measurements have not accounted for the axial conduction effect, and those measurements showed the inconsistent result with the theory. In this paper, a new measurement method is developed to validate the theory that predicts a Nusselt number independent of Reynolds number in the laminar flow regime for microchannels. A numerical model is used to simulate heat transfer characteristics in a microchannel with wall conduction, and to predict the wall temperature difference between a location at the end of the heater and a location 3 mm away from the heater on the microchannel. The temperature difference is experimentally measured on a 160 μ m hydraulic diameter microchannel and compared with the numerical model. The comparison shows that the Nusselt number in the laminar flow regime for the microchannel is independent of Reynolds number at least down to Re=300. [ABSTRACT FROM AUTHOR]

This paper investigates a Taylor-Couette-Poiseuille flow in an annular channel of a slotted rotating inner cylinder, corresponding to a salient pole hydrogenerator. The purpose of this study is to improve the understanding of flow and thermal phenomena in electrical machines using a simplified scale model. The validation of the numerical model for a specific configuration is first shown by comparing the results with the experimental data. A parametric study is also performed to investigate all main flow regimes and to derive correlations in terms of the Nusselt number distribution on the rotor pole face and sides. The results show that the Nusselt number is proportional to the tangential Reynolds number to the power 1/7 in the pole and inductive faces trailing side. This relationship is similar to the one encountered in classical Taylor-Couette-Poiseuille flows between two concentric and smooth cylinders. [ABSTRACT FROM AUTHOR]

Predicting heat transfer is a primary task in the design of open-cell foams. When a Local Thermal Non Equilibrium (LTNE) model is employed, convection heat transfer between the solid and fluid phases is considered, and a volumetric heat transfer coefficient needs to be defined. Some recent studies pointed out that the effects of developing convection heat transfer between the fluid and the solid in a foam are to be taken into account. The developing thermal flow of air through an open-cell foam, with a uniform heat flux solid/fluid boundary condition, is investigated numerically in this paper. The geometry is modeled with reference to Kelvin's tetrakaidecahedron foam model. A correlation among the porosity, the cell diameter and the ratio of heat transfer surface to volume is derived. Three regions are identified along the flow direction: an impingement region, a thermally developing region and a thermally developed region. Dimensional and dimensionless convection heat transfer coefficients have been predicted numerically as a function of the axial coordinate of the foam, for different values of the Reynolds number and the porosity. A correlation is presented among the predicted values of the volumetric Nusselt number, the porosity, and the Reynolds number in the thermally developed region, which is in good agreement with experimental data and numerical predictions by other authors. Finally, the analysis of the convection heat transfer through a single foam cell, at a local pore-scale, is presented. [ABSTRACT FROM AUTHOR]

This paper develops a unified analysis of stagnation flow heat and mass transport, considering both semi-infinite domains and finite gaps, with and without rotation of the stagnation surface. An important objective is to derive Nusselt- and Sherwood-number correlations that represent heat and mass transport at the stagnation surface. The approach is based on computationally solving the governing conservation equations in similarity form as a boundary-value problem. The formulation considers ideal gases and incompressible fluids. The correlated results depend on fluid properties in terms of Prandtl, Schmidt, and Damköhler numbers. Heterogeneous chemistry at the stagnation surface is represented as a single first-order reaction. A composite Reynolds number represents the combination of stagnation flows with and without stagnation-surface rotation. [ABSTRACT FROM AUTHOR]

The aim of this paper is to develop a method of determining the heat transfer coefficient on the outer surface of the thermometer used to measure the temperature of a fluid flowing under high pressure. The method is based on the solution to the inverse heat conduction problem. The heat transfer coefficients are determined based on the measurement of the temperature of a cylindrical metal thermometer and the temperature of the wall of a cylindrical pipeline. The temperature sensor is located in the pipeline wall close to the inner surface. The correlations for the Nusselt numbers used to determine heat transfer coefficients on the outer surface of the thermometer and the inner surface of the pipeline contain unknown coefficients which are found using the least squares method. The unknown coefficients are selected so that the sum of the squares of differences between the fluid temperature determined based on the measurement of the temperature of the pipeline wall and the fluid temperature obtained from measurements inside the thermometer, calculated for several dozen set time points, is as small as possible. [ABSTRACT FROM AUTHOR]

• The condensation induced by a sub-cooled water injection in a circular horizontal pipe with a two-phase stratified flow is studied. • Three experiments are consolidated in a coherent database and the correlations found in the literature are assessed. • A new condensation model is calibrated and physically validated, significantly reducing the average standard deviation between evaluations and experimental data. In this paper, the condensation induced by a sub-cooled water injection in a circular horizontal pipe with a two-phase stratified flow is investigated. The focus of the work is to review the physical models or correlations predicting the condensation heat transfer coefficient and assess them against an experimental database. Three experiments, namely COSI, TOPFLOW-PTS and UPTF, are consolidated in a substantial database. They have different configurations and complexity, covering a wide range of injection mass flowrate, temperature and pressure. A thermal-hydraulic analysis is performed, resulting in reliable and coherent experimental data. The condensation models found in the literature are based on the modelling of the Nusselt number through several dimensionless numbers. The assessment of these correlations against the experimental database provides poor results. Thus, a new approach is proposed. The cold jet is modelled as a heat exchanger, which is described by a condensation potential. The analytical formula of the potential is found starting from an energy balance at the injection, showing that the condensation depends on the jet geometrical shape and a parameter η. A new correlation for the parameter η is calibrated against the COSI and TOPFLOW-PTS experiments, significantly reducing the average standard deviation between evaluations and experimental data. The new correlation is then applied to an independent database, i.e. the UPTF experiments. The results show good agreement between the calculated and experimental values, proving the capability of the new model to accurately predict the condensation at the injection. [ABSTRACT FROM AUTHOR]

In this work, a three-dimensional analysis is used to study the heat transfer and entropy generation inside a helically coiled tube-in-tube heat exchanger in laminar flow regime using two different types of nanofluids. Overall heat transfer coefficient, effectiveness, Nusselt number, and entropy generation of helically coiled tube-in-tube heat exchanger were investigated considering the nanoparticle volume concentrations between 0 and 2.0 vol.%. The mass flow rate of the nanofluid from the inner tube was kept and the mass flow rate of the water from the annulus was set at either half, full, or double the value. The present contribution is a companion paper of Huminic and Huminic [1]. The numerical results reveal that the use of nanofluids in a helically coiled tube-in-tube heat exchanger improves the heat transfer performances. Thus, the maximum effectiveness was 91% for 2% CuO nanoparticles and 80% for 2% TiO 2 nanoparticles. Also, the increase of nanoparticles volume concentration leads to the Nusselt number increase and the reduction of the entropy generation due heat transfer effects, the viscous effects to entropy generation being negligible. [ABSTRACT FROM AUTHOR]

The paper is concerned with a CFD numerical investigation using the Fluent code of the convective heat exchange between an impinging gas jet at room temperature and a structured solid surface with square-shaped grooves which is maintained at constant temperature of 80 °C. Reynolds averaged Navier-Stokes simulations coupled with the energy equation are performed for a planar jet on a 3D domain with periodic boundary conditions. The range of jet Reynolds numbers used for the simulations is in the interval from 2000 to 10000, while the jet standoff distance can take different values, ranging from 2 to 10 times the jet width b=2 mm. The detailed distribution of the convective heat transfer coefficient and the mean Nusselt number at the impingement of the air jet on the impacted plate will be inferred. Finally, a comparison between the numerical results, a set of experiments performed and relevant cases from literature is made and conclusions are drawn. One main result of the study is the confirmation of the numerical procedure for the domain used. [ABSTRACT FROM AUTHOR]

• The effect of pulsating flow on heat transfer efficiency of ice slurry is studied. • The cycle-averaged heat transfer coefficient can be increased by 34%. • Heat transfer is more efficient for large particles and high ice volume fraction. • The prediction correlation for the time-averaged nusselt number is proposed. In this paper, the heat transfer enhancement of ice slurry in a horizontal pipe with constant heat flux has been investigated numerically. The model is validated with the experimental results from the literature. Then the axial distribution of the heat transfer coefficient, the bulk and wall temperature and the ice volume fraction are obtained. The factors that affect the heat transfer coefficient are analyzed. Furthermore, the heat transfer efficiency of ice slurry in pulsating flow is compared with that in steady state. It is found that the heat transfer efficiency of ice slurry can be significantly improved by pulsating flow. The maximum increment of cycle-averaged heat transfer coefficient reaches 34.32%. Moreover, the degrees of heat transfer enhancement increase with increasing pulsating frequency, relative amplitude of velocity, ice particle diameter and ice volume fraction, and decrease with increasing cycle-averaged velocity. And the heat transfer is more efficient to ice slurry with large particles and ice volume fraction under the pulsating flow conditions. The pulsating frequency is suggested to be not more than 10π Hz. Finally, the correlation for predicting cycle-averaged Nusselt number of ice slurry in pulsating flow is established, and obtaining the reasonably good prediction results. [ABSTRACT FROM AUTHOR]

• Micro-rib inhibits the near-wall turbulence burst and so decreases mean friction drag. • Enhanced streamwise vortices downstream of the micro-rib increase heat transfer. • Wall-normal turbulent scalar flux dominates heat transfer coefficient in turbulence. Blade internal cooling efficiency is of great significance for gas turbine. Among the blade internal cooling methods, micro perturbation elements show the potential of high heat transfer with low pressure drop but the mechanisms remain unclear. This paper investigated the influence of micro-ribs on turbulent momentum and heat transport in the near-wall region of a micro-ribbed channel flow by Large Eddy Simulation. The micro ribs were placed in the buffer region of the turbulent boundary layer, expected to effectively improve the near-wall turbulent transport. The results show that the micro ribs enhance heat transport and weaken momentum transport on average. The micro-ribs destroy the velocity streaks behind the ribs and inhibit turbulence burst, and so the momentum transport is weakened and the friction resistance falls by 9.4%. The micro ribs also induce spreading shear vortices that evolve into near-wall streamwise vortices downstream, leading to enhancement of regenerated streamwise vortices, and finally the Nusselt number increases by 5.0%. [ABSTRACT FROM AUTHOR]

This paper presents a new computational procedure for refrigerant condensation inside herringbone-type Brazed Plate Heat Exchanger (BPHE). A transition point between gravity controlled and forced convection condensation was found for an equivalent Reynolds number around 1600. At low equivalent Reynolds number (<1600) the heat transfer coefficients are not dependent on mass flux and are well predicted by a simple model based on the Nusselt (1916) equation for vertical surface. For higher equivalent Reynolds number (>1600) the heat transfer coefficients depend on mass flux and condensate drainage is controlled by the combined actions of gravity and vapour shear. A new model was developed for predicting the heat transfer coefficients in the forced convection condensation region. This new model was also applied to super-heated vapour condensation by using the equation of Webb (1998) to account for super-heating effects. The new computational procedure was compared against data from the literature: the mean absolute percentage deviation between experimental and calculated heat transfer coefficients was lower than 16%. [ABSTRACT FROM AUTHOR]

• A cross-corrugated triangular flow channel with trapezoidal baffles is proposed. • The flow and heat transfer characteristics of the proposed flow channel are studied. • The heat transfer in the flow channel is analyzed using the field synergy principle. Heat recovery ventilator (HRV) with cross-corrugated triangular flow channels has the advantages of high heat transfer coefficient and high mechanical strength, but it also has the disadvantages of uneven internal temperature distribution and large pressure loss. In this paper, a kind of cross-corrugated triangular flow channel with trapezoidal baffle is proposed, and the influence of the trapezoidal baffle on the flow and heat transfer characteristics of the cross-corrugated triangular flow channel is comprehensively studied by numerical simulation. The Nusselt number (Nu), friction factor (f) and Performance Evaluation Criteria (PEC) value of the flow channel with trapezoidal baffles and the original flow channel were compared. The synergistic effect of the trapezoidal baffle on the flow and heat transfer in the flow channel was analyzed. The results show that the trapezoidal baffle can promote fluid mixing and momentum exchange, thus enhancing the field synergistic effect and improving the comprehensive heat transfer performance of the flow channel. The Nu and f of flow channels with trapezoidal baffles are more than 1.7 and 3.5 times of those without baffles, and the PEC value can be increased by 30%. The data and empirical formulas of this study can provide a guideline for HRVs design. [ABSTRACT FROM AUTHOR]

• Lattice Boltzmann simulations have been conducted to study the heat transfer in particle-fluid beds. • The effects of flow condition, material properties and bed structure on the heat transfer are quantified. • The underliying mechanisms are analysed in terms of spatial and statitiscal distributions of relevant transport properties. • New correlation is formulated to describe, more accurately and generally, the heat transfer in particle-fluid beds. Non-isothermal fluid flows through packed beds are common in many industries. Such a system is very complicated because heat transfer is associated with the interaction between fluid and particles. Our current understanding of the phenomenon is still limited. For example, the effect of the Prandtl number on thermal flow and the significant inconsistency between experimental data and predictions at a low Peclet number remain unclear. To overcome these problems, this paper presents a comprehensive study of this system on a sub-particle scale. This is done by conducting about five hundred Lattice Boltzmann simulations under different conditions, covering a wide range of Reynolds number (Re), Prandtl number (Pr), and bed porosity (ε). The results show that the effects of Re, Pr , and ε on the thermal behavior of fluid flows are strong, varying significantly with the flow condition and fluid/bed properties. Based on the simulation data, a new correlation to calculate the heat transfer coefficient is established and validated against the experimental data in the literature. This correlation is more accurate and general than those reported in the past and is recommended for simulations of non-isothermal fluid-particle flows in the future. [Display omitted] [ABSTRACT FROM AUTHOR]

• Study the effect of condenser temperature on the performance of a wickless heat pipe. • Performance in microgravity declined as the condenser temperature decreased. • Temperature data was used to extract a heat transfer coefficient in the evaporator. • Nusselt number was found to scale with Marangoni number to the -1/3 power. • A single correlation using the Marangoni number collapsed all the experimental data. The Constrained Vapor Bubble (CVB), a simple, wickless, heat pipe design that depends on interfacial forces to drive corner flow in a square cuvette, was studied in the microgravity environment aboard the International Space Station (ISS). In this paper, we consider the effects of different condenser temperatures on the heat transfer and fluid flow behavior using pentane as the working fluid. As the condenser temperature was decreased, the performance of the system decreased. This performance decrease using the pure working fluid was opposite to the behavior observed when using a mixture of 94 vol% pentane and 6 vol% isohexane. The mechanism for the decline in performance as the condenser temperature was lowered was a stronger than expected increase in the apparent strength of Marangoni flows at the heater end of the system. A simple mathematical model was fit to the experimental data and used to extract an evaporator heat transfer coefficient for experiments where we held the condenser temperature constant while increasing the heater power and where we held the heater power constant while decreasing the condenser temperature. All the results could be collapsed onto a single Nusselt number vs. Marangoni number curve. In this formulation, the Nusselt number was found to decrease with increasing Marangoni number to the 1/3 power. [ABSTRACT FROM AUTHOR]

• Water pool boiling in high porosity aluminum foams is studied. • The 10 mm thick foams show the best performance. • The 40 PPI foam showed the highest heat transfer performance. • The developed ANN model showed excellent predicting capabilities. This paper shows the promising capabilities that the aluminum metal foams offer in enhancing the water pool boiling on flat surfaces. Different metal foam samples, with pore density ranging between 5 and 40 Pore Per Inch (PPI), with similar porosity values, around 0.92, and two different thickness values, 5 and 10 mm, were tested to investigate their pertinent pool boiling performance. The saturated boiling curves at atmospheric pressure were obtained. Furthermore, using a high speed camera, heat and fluid flow inside and above the porous layer were investigated. The results showed that boiling heat transfer can be greatly enhanced by the use of metal foams partly because of an earlier onset of the nucleate boiling. Moreover, over a wide range of operating conditions foams lead to remarkably higher heat transfer coefficients, compared with a smooth aluminum reference surface. For a more comprehensive understanding of the problem, we tried to correlate the existing data in the literature for different foam geometries and base materials. However, the existing correlations and independent test data in the literature do not agree well; not even within the same order of magnitude. Hence, in order to offer a generic solution, predictions based on the results of an artificial neural network (ANN) are sought. A large database, comprising 758 experimental data points available in the open literature, was collected and then used to develop, train and validate a model based on ANN to estimate the heat transfer performance of metal foams during water pool boiling. Our data show that the predicted Nusselt number is within 10% of the measured experimental data. Moreover, we demonstrate that the developed ANN tool can be successfully implemented to predict the intrinsic complexity of water pool boiling inside metal foams that in most cases cannot be articulated by merely relying on conventional semi-empirical methods. [ABSTRACT FROM AUTHOR]

• New type of enhanced fin with convex-strips is numerically and experimentally studied. • The performance of wavy and enhanced fins with convex-strips are compared. • The increase magnitude of thermal performance is equivalent to that of pressure drop. • The effects of convex-strips on wavy surface are investigated. • The performance evaluation plot is adopted to describe the comprehensive performance. In this paper, a new kind of herringbone wave fin with convex-strips in fin-and-tube heat exchanger is experimentally and numerically studied. For comparison, a herringbone wave fin surface with the same dimension in fin-and-tube heat exchanger was also investigated. Four heat exchanger samples in staggered arrangement are tested: the herringbone wavy fin with two and three rows (F-W2 and F-W3) and the convex-strips herringbone wavy fin with two and three rows (F-C2 and F-C3). At low velocities (2.5 m/s for two rows and 2 m/s for three rows), the heat transfer coefficients of F-C2 and F-C3 increases by 6.6% and 4.6% compared to that of F-W2 and F-W3, respectively. And the pressure drops are correspondingly increased by 7.5% and 8.0%. At higher velocities (4.5 m/s below), 8.2% and 6.5% promotions for the heat transfer coefficients of F-C2 and F-C3 are achieved, respectively. And the pressure drops are correspondingly increased by 11.0% and 10.9%. Correlations of the Nusselt number Nu and friction factor f on the air side are summarized. Numerical results indicate that the induced vortices near the tube wall are the main reason for the enhancement of heat transfer performance. The performance evaluation plot is adopted to describe the comprehensive performance of the four heat exchangers. [ABSTRACT FROM AUTHOR]

In this paper, heat transfer performance was experimentally tested in a helically coiled pipe using three types of nanofluids, TiO 2 , ZnO and Ag water-based nanofluids. TiO 2 , ZnO and Ag nanoparticles were characterized by X -ray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscope (TEM). The XRD results showed that the purity of ZnO and TiO 2 were obtained. TEM and SEM results showed that the mean particles size of TiO 2 , ZnO and Ag are 14, 20 and 16 nm respectively. The three types of nanofluids, TiO 2 /water, ZnO /water and Ag /water were prepared with a volume concentration of 0.25%. To ensure a good mixture and measure particles distribution in each nanofluid, Zetsizer test was performed and showed a good distribution of nanoparticles in all tested nanofluids. The three different nanofluids have been thermally tested in helical coil with a flow rate from 1 to 3.5 L/s with different inlet temperatures in the range of 30 °C and 60 °C. The experimental results showed an enhancement in Nusselt number up to 27.31%, 16.03%, and 10.38% with augmentation in the heat transfer coefficient up to 32%, 21% and 16% utilizing TiO 2 , ZnO and Ag nanoparticles respectively. [ABSTRACT FROM AUTHOR]

The convective heat transfer coefficient on the rail surface is an important parameter to determine the temperature field of the wheel/rail contact region, especially at the condition that train operates normally and the condition that an eddy current brake is used. Because of the complex configuration of a real wheel/rail contact system, it is hard to build a model with similar geometry as the real one. In this paper, a simple experimental model is used to obtain the convective heat transfer coefficient on a circular track surface on which the wheels pass periodically. In the experimental processes uniform heat flux is applied on the track surface, and the six types of the wheels are used. The results show that when the heat transfer coefficient is presented regarding to the wheel center velocity or angular velocity, it is dependent on the wheel diameter. For the same wheel diameter, the heat transfer coefficient has a parabolic distribution characteristics regarding to the wheel center velocity, which means the heat transfer coefficient increases firstly and then decreases as the wheel center velocity increases. The maximum value of the heat transfer coefficient is independent of the wheel diameter. It is hard to find a correlation between Nusselt number and Reynolds number for the wheels with different diameters. The significant characteristic is that the frequency of the wheels pass takes a major role on the heat transfer coefficient. Therefore, the heat transfer coefficient can be correlated with the frequency of the wheels that run over the circular track. • The convective heat transfer coefficient on the track surface as wheels roll over periodically is studied. • The heat transfer coefficient changes along a parabolic line regarding the wheel rolling speed. • The heat transfer coefficient vs. modified frequency of wheels pass is independent of the wheel diameter. • The maximum heat transfer coefficient is independent of the wheel diameter. [ABSTRACT FROM AUTHOR]

The paper presents the specific wavy heat exchange surfaces in order to intensify convective heat transfer in the channel. The study is focused on the analysis of local and average heat transfer coefficients and Nusselt numbers by experimental method of the holographic interferometry and numerical simulations. The results are presented for temperatures of the heat exchange surfaces T s = 318.15 K, 328.15 K, 338.15 K, and 348.15 K, Reynolds numbers Re = 1386–4203, and height between the surfaces H = 30, 35, 40, 45, and 50 mm. New correlating equations for average Nusselt number for the in-line and the offset wavy channels are determined. The total heat transfer and pressure drop j / f and thermal performance η are calculated for the wavy channels compared with the smooth one. It is observed that the offset wavy channel achieves a higher j / f values compared with the in-line wavy channel for all H and T s values. The thermal performance η increases with Re and decreases with height H. The offset wavy channel achieves higher thermal performance η by 5.7%–15.7% when changing Re and H compared with the in-line one. [ABSTRACT FROM AUTHOR]

• Obtain exact solutions for fluid and solid temperatures and Nu number under LTNE model together with different thermal interface conditions and no-slip interface condition. • Show the effect of stress jump at the interface on heat transfer under different thermal interface conditions. • Compare temperature fields and Nu numbers under three thermal interface conditions. This paper analytically investigates a fully developed forced convection in a channel partially filled with two porous layers symmetrically arranged at inner walls. The local thermal non-equilibrium (LTNE) model and Brinkman-extended Darcy model are employed as energy and momentum equations respectively in porous region. Three thermal boundary conditions (models A, B and C) and no-slip condition are adopted at porous-fluid interface. Explicit expressions of solid and fluid temperatures and Nusselt number are derived under each thermal boundary condition and variances between the three thermal boundary conditions in predicting thermal behaviors are discussed. It is shown that with present parameters, the temperature bifurcation phenomenon occurs in most cases under model B, while this phenomenon cannot be observed under models A and C. The LTNE model holds in almost any conditions under model B, but under models A and C the LTNE model holds with a low hollow ratio (S) and Biot number (Bi), a large effective thermal conductivity ratio (K) and Darcy number (Da). The increase of interfacial convective heat transfer coefficient (H s) makes the Nusselt number (Nu) of model C more close to but not exceed the Nu of model A, while the Nu under model B is lower than which under models A and C. By using different thermal boundary conditions, the stress jump coefficient (β) effect on heat transfer can be ignored, and with a high Da , a low β and S the Nu number predicted by stress jump interface condition has slight difference from that by stress continuity interface condition. [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]

In this paper, an industrial-scale experiment with ten testing heat exchangers is conducted to study the influences of structures and working conditions on the tube-side heat transfer and flow characteristics of spiral-wound heat exchangers. The experimental results show that, with the increase of the winding angle, the tube-side heat transfer coefficients increase because of the enhanced secondary flow and slightly larger velocity. The compactness of the whole heat exchanger changes with the variations of the layer and tube pitch. When layer pitch and tube pitch decrease, the flow channel in the shell side is narrowed down, which makes the overall heat transfer temperature difference decrease and finally results in deterioration of the heat transfer in the tube side. For the friction factors, they decrease when the winding angle decreases due to the different turbulence status and velocities in the tube. As a parameter of working condition, the tube-side inlet temperature has a slightly negative effect on the heat transfer. Based on the experimental data, the correlations of Nusselt number and friction factor are developed. Comparing the experimental data and the correlations, the average errors of the Nusselt number and friction factor are 8.04% and 6.28%, respectively. [ABSTRACT FROM AUTHOR]

New designs for ribbed microchannels (semi-circular and semi-elliptic) and filleted ribs are numerically studied in the present paper. Four presented configurations include: microchannel with semi-circular ribs (MC-SCR), microchannel with semi-elliptic ribs (MC-SER), microchannel with semi-circular ribs and filleted corner (MC-SCR-FR) and microchannel with semi-elliptic ribs and filleted corner (MC-SER-FR). Thermal and hydraulic performance of these configurations are evaluated by comparing Nusselt number, fanning friction factor, and performance evaluation criteria (PEC). Based on the results, the ribs corner curvature leads to a significant increase in heat transfer coefficient, accompanied by a minor increase in pressure drop. Compared to MC-SCR and MC-SER heat sinks, the two designs of MC-SCR-FR and MC-SER-FR enhance Nusselt number about 18–21% and 0.5–10%, increase the fanning friction factor about 0.1–7% and 1–9%, and improve the performance evaluation criteria (PEC) about 19–22% and 19–21%, respectively. In addition, because of the re-development of thermal boundary layer and higher mixing level, MC-SCR-FR configuration revealed the best performance among all other configurations. Moreover, the effect of relative fillet radius on thermal-hydrodynamic performance of MC-SER-FR configuration is investigated, and R* > 1.2 is selected as the appropriate value for design of microchannel heat sinks. [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]

This paper investigates the convective heat transfer characteristics of f-GnP (functionalized Graphene Nanoplatelets) suspended in distilled water. The effect of mass flow rate (5 g/s to 30 g/s) and the GnP concentration (0 vol% to 0.2 vol%) on various heat transport parameters such as convective heat transfer coefficient, temperature drop, Nusselt Number, Pressure drop have been discussed in this study. Experiments with GnP based nanofluids in the heat sink decreases the heat sink temperature by 10 °C and enhances the convective heat transfer coefficient and Nusselt number by 71% and 60% with a little increment in the pressure drop of about 12% higher as compared with water. The results prove that the functionalized GnP based nanofluids would effectively replace the existing nanofluids that improves the performance of the electronic chips due to enhanced thermal conductivity and convective heat transfer coefficient. • Functionalized graphene/water nanofluid was prepared and characterized. • Thermal conductivity of nanofluid increased by ~11% at 0.2 vol% of GnP. • Heat transfer coefficient and pressure drop of GnP/water nanofluid are reported. • Heat transfer coefficient enhanced by ~71% with 0.2 vol% of GnP. • The increase in pressure drop was significantly limited with 0.2 vol% of GnP. [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]

• Study MHD mixed convection from two rotating cylinders in a porous trapezoid. • Study on Cu-water nanofluid in a porous medium with different Darcy numbers. • Higher effect of vertical magnetic on decreasing the stream function. • High effect of magnetic field angle on temperature and irreversibilities. The numerical simulation of MHD mixed convection due to a rotating circular solid cylinder in a trapezoidal enclosure filled with a Cu-water nanofluid saturated with a porous media was presented in the current paper. The midpoint of the top wall of the enclosure is considered as a center of a cylinder which has a different radius. The remaining of this wall is considered adiabatic. The bottom wall is heated with the temperature of T h and moving to the right hand, while sidewalls are kept isolated and stationary. The Hartmann number, thermal conductivity ratio, angular rotational velocity, Richardson number, inclination angle, cylinder radius, Darcy number, nanofluid concentration vary in the ranges of 0 to 100, 1 to 10, -5 to 5, 0.01 to 100, 0° to 90°, 0.2 to 0.4, 10 −5 to 10 −1 and 0 to 0.1, respectively. While the convection heat transfer coefficient is considered constant at (h = 5). It was found that, the vertical magnetic field decreases the stream function values more than the inclined and the horizontal fields. Also, the inclination angle of the magnetic field has a profound influence on the thermal performance inside the enclosure. The average Nusselt number rises by raising the Hartmann number, thermal conductivity ratio, cylinder radius and Darcy number, while it decreases by increasing the Richardson number. It was found also that, when the cylinder rotates in a counter clockwise direction (i.e., Ω = 5) and (K r = 10), the Nu ave decreases by about (90 %) when the (Ri) increases from 0.01 to 100. While, it increase by about (35 %) when the (Ha) increases from 0 to 100. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Steady-state flow boiling heat transfer in a single rectangular minichannel. • Several refrigerants flowing in a different spatial oriented minichannel. • Enhanced heated plate surface produced by several methods. • Applicability of heat transfer correlations to flows in minichannels. • New correlation for subcooled flow boiling heat transfer in minichannels. This paper focuses on the applicability of heat transfer correlations to flows in minichannels. The data were taken from own research conducted at the experimental stand for subcooled flow boiling heat transfer in an asymmetrically heated rectangular minichannel oriented with seven inclination angles (0°, 30°, 60°, 90°, 120°, 150°, and 180°) relative to the horizontal plane. Various cooling liquids (FC-72, HFE-649, HFE-7100 or HFE-7000) flowing along a minichannel were heated by a thin plate. The plate had a smooth or enhanced surface on the side that was in contact with the fluid. The enhanced surfaces were obtained by vibration-assisted laser surface texturing or electromachining. Porous surfaces produced by soldering or sintering iron powder to the heated surface were also tested. Infrared thermography was used to determine changes in temperature on the outer smooth surface of the plate. Local heat transfer coefficients in the contact surface between the heated plate and the cooling fluid were determined using the one-dimensional mathematical method. A dozen or so heat transfer correlations selected from the literature have been used in calculations. It was found that the majority of correlations enabled the predicting of Nusselt number within error bands of ±25%. The smallest mean absolute percentage error in determining the Nusselt number was obtained using Mikielewicz J. correlation. A new correlation for subcooled flow boiling heat transfer in minichannels was proposed. The mean absolute percentage error of the Nusselt number, amount to 14%, was achieved according to own correlation. [ABSTRACT FROM AUTHOR]