Search

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]

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]

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]

• 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]

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]

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]

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]

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 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]

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 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]

• 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]

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 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 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 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]

• 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]

• Heat transfer performance of nanofluids in a plate heat exchanger is investigated. • The maximum enhancement in average Nu is 22.6% for 1.0 wt.% Fe 3 O 4 -water nanofluid. • The optimum concentration for thermal enhancement is 0.5 wt.% for CuO nanofluid. • Empirical formulas of experimental Nu are derived based on experimental data. • Fe 3 O 4 -water nanofluid is a promising heat transfer medium for solar energy systems. In this paper, a corrugated plate heat exchanger in solar energy systems is used to investigate heat transfer and fluid flow characteristics of various nanofluids. By adding various nanoparticles (Al 2 O 3 -30 nm, SiC-40 nm, CuO-30 nm and Fe 3 O 4 -25 nm) into the base fluid, effects of nanofluid types and particle concentrations (0.05 wt.%, 0.1 wt.%, 0.5 wt.% and 1.0 wt.%) on the thermal performance of the plate heat exchanger are analyzed at flow rates in the range of 3–9 L/min. Results indicate that both heat transfer enhancement and pressure drop for nanofluids show significant increases compared to the base fluid. The Fe 3 O 4 -water and CuO-water nanofluids show the best and the worst thermal performances of the plate heat exchanger, respectively. When 1.0 wt.% Fe 3 O 4 -water nanofluid is used as the working fluid, compared to DI-water, the convective heat transfer coefficient is increased by 21.9%. However, an increase of 10.1% in pressure drop is obtained for the 1.0 wt.% Fe 3 O 4 -water nanofluid. Finally, empirical formulas of experimental Nusselt number are obtained based on the experimental data. A new way to predict the thermal performance for various nanofluids in heat transfer systems is provided. [ABSTRACT FROM AUTHOR]

• The interaction between an isolated vapor bubble and heated square cylinder is numerically investigated in a microchannel. • The numerical model is validated against literatures and mesh-independent studies are conducted to ensure the model accuracy. • The heat transfer coefficients on all of the four cylinder sides are discussed in detail, and their dynamic evolutions with bubble motions are obtained. • Two important competing mechanisms that governing heat transfer enhancement on the heated cylinder are revealed. • The global heat transfer enhancement is found decreases monotonically with increasing Reynolds number and decreasing the bubble initial size. Two-phase flow boiling within pin-fin microchannel heat sinks has gain increasing attentions over the last decades owing to the demands for high efficiency cooling applications, while the detailed interactions between a vapor bubble and heated pillar structures are still not fully understood. In this paper, the saturated-interface-volume phase change model is coupled with VOF method to investigate the saturated flow boiling of an isolated vapor bubble traveling across a heated square cylinder in a microchannel (with Reynolds number Re ranging from 60 to 360 and dimensionless bubble diameter d b * from 0.2 to 0.6). The numerical model is first validated against literatures, and a mesh-independent study is conducted to ensure the accuracy of the present simulations. Then the bubble dynamics and its influences on flow structures are presented, where the symmetrical vortices attached behind the cylinder at small Re (60 and 120) are found replaced with the vortex-shedding flow and the deformed bubble (d b * = 0.6) squeezes the cylinder and forms evaporating thin film not only on the adjacent side but also the windward and leeward sides. The local wall superheat and Nusselt number are presented along all the four sides of the heated cylinder. Two competing mechanisms governing heat transfer enhancement, i.e. phase change heat transfer improved by the thin film evaporation and convective heat transfer suppressed by the wrap of the bubble, are revealed. Both the dynamic evolution and time-averaged heat transfer performance are obtained for each side of the cylinder and their sums. The global heat transfer enhancement is found decreases monotonically with increasing Re and decreasing d b *. [ABSTRACT FROM AUTHOR]

In this paper, an experimental investigation is carried out on internal convective heat transfer of SiO 2 /water nanofluids in a copper tube for a fully turbulent regime. Nusselt number and convective heat transfer coefficients of nanofluid at three different volumetric concentration (0.001%, 0.003%, 0.007%) of nanoparticles were estimated. Local convective heat transfer coefficient at different locations along the tube at varying Reynolds number was also observed. With volumetric concentration of 0.001% of SiO 2 nanoparticles, maximum enhancement observed was 8–9%. Whereas at 0.007% volumetric concentration of SiO 2 nanoparticles, a 27% enhancement in convective heat transfer coefficient was observed. The decrease in the heat transfer rate was observed in axial direction. Particle migration, chaotic movement of the nanoparticles and fluctuations in the fluid and increased thermal conductivity of the nanofluid are considered the possible reasons for this enhancement. • Internal convective heat transfer of SiO 2 /water nanofluid • Heat transfer coefficient at 0.001, 0.003 and 0.007 vol% of nanoparticles • Local convective heat transfer at different locations along the tube • Average and local Nusselt number at different locations along the tube • 0.007% SiO 2 nanofluid shows almost 27% enhancement in heat transfer. [ABSTRACT FROM AUTHOR]

• The present work numerically investigates the flow field and heat transfer performance in stationary and rotating wavy channels with different shapes. • Three different geometries were generated through a desired phase-shift of ∅= 0, 90 and 180° between the two opposite sine-wave walls. • A cell-centered finite volume method was employed to solve the time dependent governing equations based on the unsteady SIMPLE algorithm technique. • Results showed that the wavy channel with a shift phase of ∅ = 0 d e g produces the highest surface-averaged Nusselt number and heat transfer enhancement in comparison to other geometries. • Rotation has a strong impact on the flow field and heat transfer performance. In this paper, flow field and heat transfer performance in stationary and rotating wavy channels with different shapes were numerically investigated. Three different geometries were generated through three different values of phase-shift angles of ∅= 0, 90 and 180° between the two opposite wavy walls. A cell-centred finite-volume technique was employed to solve the three-dimensional governing equations based on the SIMPLE algorithm technique. Besides, the Menter k − − ω SST turbulence model was used to simulate the turbulent flow in the current study. The wavelength and wave amplitude of the channel examined were Lw=20 mm and a = 2 mm, respectively. Numerical simulations were carried out over a range of design and operating conditions including the phase-shift angle of ∅= 0–180°, Reynolds number of Re =1,000–10,000, and rotating speed of Ω= 0–1000 rpm. The results showed that the surface-averaged Nusselt number increases as Re increases for all shapes of the wavy channel, however, at the expense of the raised pressure losses. Also, the wavy channel with a phase-shift of ∅= 0 deg showed the highest enhancement in the performance of heat transfer followed by that of ∅=90 and 180 deg, respectively. The rotation had a strong impact on the flow field and heat transfer performance. With the increase of rotating speed, lower wall heat transfer coefficient significantly increased, while the upper wall heat transfer coefficient exhibited a slight increase, indicating that those three different geometries of the wavy channels had a good versatility at various values of rotating speeds. The numerical results were compared with those available in the literature, and the results were in a good agreement. [ABSTRACT FROM AUTHOR]

• Pseudo-boiling is introduced to deal with supercritical heat transfer. • The similarity analysis yields the new dimensionless K number. • S-CO 2 heat transfer experiment is performed to expand the database. • The new correlation contains only one K number correction term except Re and Pr. • The correlation has the best prediction accuracy comparing with other correlations. The objective of this paper is to present a general supercritical heat transfer (SHT) correlation for advanced power cycles. In textbooks, supercritical fluid is considered to have single-phase structure. However, various SHT correlations incorporating buoyancy/acceleration effects fail to predict heat transfer coefficients. Here, pseud-boiling assumption is introduced to deal with SHT. Supercritical fluid is treated to have a heterogeneous structure consisting of vapor-like fluid and liquid-like fluid. Similarity analysis between subcritical boiling and SHT creates a new dimensionless parameter, the K number, representing evaporation induced momentum force relative to inertia force to govern the growth of wall attached vapor-like fluid layer thickness, which is key to dominate SHT. Thus, SHT is correlated in a simple form as N u = C R e b n 1 P r b , ave n 2 K n 3 , where Nu, Re b , Pr b,ave are Nusselt number, Reynolds number and Prandtl number, respectively. Totally, 5560 data points, including our newly obtained 2028 data points for S-CO 2 with pressures up to 21 MPa, and other 3532 data points cited from 18 articles for carbon dioxide, water and R134a, are used to determine the coefficients C and n1−n3, yielding the expression of N u = 0.0012 R e b 0.9484 P r b , ave 0.718 K − 0.0313 . The negative exponent −0.0313 for the K number explains the improved heat transfer by increasing pressures. By comparing with experiment database, the general correlation has better prediction accuracy than the widely cited correlations in the literature. The correlation is also compared with R22 data. Even though such data are not involved in the development of the SHT correlation, the correlation excellently matched the experimental data. This work paves a new road to understand SHT. The correlation ensures heat exchangers operating at supercritical pressures to be designed more accurately and safely. [ABSTRACT FROM AUTHOR]

In this paper, an experimental study is performed to characterize the effect of constant and alternative magnetic fields on pressure drop and heat transfer coefficient for a ferrofluid in laminar flow in a circular straight tube. The ferrofluid considered in this study is prepared using Fe 3 O 4 nanoparticles. The verification of the size of nanoparticles is accomplished using TEM images and DLS distribution. A two-step preparation method is used to produce a ferrofluid with 0.5 and 1 wt%. The ferrofluid is stabilized using a surfactant and ultrasonic waves. The Zeta potential is employed to evaluate the stability of the ferrofluid, and the VSM to check its magnetization property. An uncertainty analysis is also performed where a maximum uncertainty of 1.8% is obtained. Experimental results are first compared with those of Shah and London equation for pure water as the working fluid. The Nusselt (Nu) number is enhanced using a ferrofluid instead of pure water for various Reynolds (Re) numbers. Next, two constant and alternative magnetic fields of 770 and 1300 G with four different Patterns are applied. The Nu number is improved by using a constant magnetic field. It is found that the Nu number is enhanced by 11.85 and 14.8% by using an alternative magnetic field with a frequency of 10 and 100 Hz, respectively. Further increase in the frequency (above 100 Hz up to 1000) does not lead to a positive outcome. • Magnetic field effects on the thermal entrance region heat transfer are studied. • Verification is performed by applying TEM, DLS, Zeta potential and VSM. • The Nusselt number is enhanced using a ferrofluid instead of pure water by 10%. • Constant and alternative magnetic fields with four different Patterns are studied. • The effect of magnetic field frequency of 10, 100 and 1000 Hz is investigated. [ABSTRACT FROM AUTHOR]

In the present paper, the heat transfer of slug flow in rectangular microchannels is studied numerically. The Al 2 O 3 -water nanofluid with the volume fraction of 1% is used as a homogeneous liquid. Three different configurations are considered to investigate the hydrodynamics and heat transfer in the microchannels. Boundary mesh adaptation method is used to capture the liquid film around gas bubbles. Slug flow characteristics are simulated with a very low computational time compared to previous researches. Slug and bubble lengths, liquid film properties, pressure drop, gas void fraction and heat transfer characteristics are studied. Results show that different inlet configurations impact on slug and bubble lengths. Gas hold-up is influenced by vortices and high frequency fluctuations that are created in the liquid film due to the difference between gas and slug acceleration. It is found that the Nusselt number is an increasing function of slug length and unit cell length. The results reveal that the hottest point on the wall is where the back tail vortex is generated. The nanofluid enhances the heat transfer coefficient by 10% in comparison with the pure water. • Slug flow heat transfer of Al 2 O 3 /water nanofluid in rectangular microchannels is investigated numerically. • Boundary mesh adaption method decreases the computational time in comparison with the other methods. • The results showed the generation of vortices and fluctuations in the liquid film. • The Nusselt number is a function of slug length and unit cell length. • The nanofluid enhances the heat transfer coefficient by 10% in comparison with the pure water. [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]

Helical coil heat exchangers are commonly used in a wide variety of engineering applications because, due to curvature of the tube that generates secondary flows, this kind of geometry can generate an enhancement of the convective heat transfer coefficient. Since the distributions of the fluid velocity and temperature in coiled pipes are strongly asymmetrical over the cross-section of the tube, particularly in laminar flow conditions, it is important to monitor the local heat transfer phenomena, mainly with regard to the applications where the local behaviour plays a leading role, such as in food industry. Indeed, due to the high viscosity of fluid foods or to the low velocity in which they may operate, the flow regime is often laminar in this kind of devices. In laminar flow regime, the thermal boundary conditions considerably affect the convective heat transfer performance. In the present paper, a numerical investigation focused on the evaluation of the effect of the choice of the thermal boundary condition on the local heat transfer phenomena in coiled-tubes, within the laminar regime, is presented. To cover realistic configurations in which the helical coil heat exchangers are commonly used, the conjugate heat transfer problem in the tube was also considered. The momentum and energy equations were solved by means of the open-source CFD software OpenFOAM®, with second-order accurate finite-volume schemes. The here adopted numerical model was validated by comparing the results with local experimental data. The numerical outcomes, obtained for several values of the Prandtl number, revealed that the thermal boundary conditions could significantly change the local distribution of the convective heat transfer coefficient. Finally, the influence of the thermal boundary condition on the average convective heat transfer coefficient were evaluated by considering the two definitions of the circumferentially averaged Nusselt number commonly used in literature. [ABSTRACT FROM AUTHOR]