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The present paper, for the first time, examines the influences of utilizing oil-ZnO nanofluid with different volume fractions φ = 0.5 %, 1 %, and 2 % in alternating flattened tubes (AFTs) with different alternating pitch angles of 30°, 45°, 60°, and 90° on the performance of a dual-tube heat exchanger (DTHE). This work is conducted experimentally and numerically for the Reynolds number (Re) range of 300 < Re < 1900 for oil-ZnO nanofluid and Re = 2000 for water. Based on the experimental results, the optimal case is selected for the numerical simulations of AFTs. The performance evaluation criterion (PEC) is defined for the simultaneous evaluation of pressure drop (Δp) and heat transfer coefficient (HTC). The results demonstrate that the overall heat transfer coefficient (U) and Δp are augmented with the inlet flow rate and the alternating angle between the pitches. Therefore, the maximum heat transfer (HT) and Δp correspond to the AFTs with the angle of 90° (AF4) at Re = 1900. The PEC amount of AF4 shows a 56 % enhancement compared to the circular tube. It is also observed that using copper oxide nanoparticles inside the oil improves the HT rate and Δp in the heat exchanger. Besides, an increment in φ increases U and Δp; however, the values of PEC show that the positive effects of the nanofluid are larger than their negative impacts in such a way that the PEC is improved by 64 % when the nanofluid with φ = 2 % is utilized in AFTs compared to the circular tube. [ABSTRACT FROM AUTHOR]

• In this paper is provided an calculation expression for heat transfer during condensation inside tubes that provides an adequate fit with experimental data available from a total of 22 fluids, including refrigerants, water, and a variety of substances organic. This paper presents a mathematical deduction of a new improved model for heat transfer during condensation inside tubes. This new model has been developed with the aid of the Gaussian Equation for an infinite straight line, considering this relation with the differential equations that govern the heat transfer process. The proposed model was verified by comparison with available experimental data of 22 different fluids, including various refrigerants, water, and organic substances, which condense inside vertical, inclined and horizontal tubes. The proposed model is valid for an reduced pressure values ranging from 0.0008 to 0.91, values of Reynolds number for single-phase between 68 and 84,827 and for Reynolds number for two-phase between 900 and 59,4373, a range of internal diameters ranging from 2 to 50 mm, vapor quality from 0.01 to 0.99, P r values for single-phase from 1 to 18 and mass flux rates in the ranges of 3 to 850 kg/(m2s). The mean deviation found for the analyzed data for horizontal tubes was 11.8%, while for the vertical and inclined tubes data the mean deviation was 13.0%. In all cases, the agreement of the proposed model is good enough to be considered satisfactory for practical design. [ABSTRACT FROM AUTHOR]

Abstract This paper presents analyses of complex flows and heat transfer induced by twin oblique turbulent slot-jets of different directions (divergent, convergent or parallel) impinging a heated wall. A comparison of the heat transfer characteristics between perpendicular and three cases of twin oblique jets (parallel, convergent and divergent). The twin slot jets are located on a confining adiabatic wall at a distance of 8 slot jet width. Convective heat is investigated numerically examining the effect of Reynolds number (Re) and jet inclination angle (α). This problem is relevant to a wide range of practical applications including nuclear engineering devices, manufacturing, material processing, electronic cooling, drying paper or textile, tempering of glass, etc. All computations are performed using two dimensional large eddy simulations (LES) approach with Smagorinsky sub-grid scale (SGS) models. For all directions and inclinations of the jets, the location of the stagnation points is changed and hence, the location and magnitude of the maximum Nusselt number on the heated wall vary. When Reynolds number increases, Nusselt number is improved for all types of inclination. The averaged Nusselt number shows that the perpendicular impingement gives better heat transfer than that of the oblique jets. The poor heat transfer is obtained for the parallel oblique jets. For the same angle, divergent jets give smallest heat transfer than the convergent jets. Graphical abstract Twin-jets flow configuration. (a) Perpendicular (b) Divergent (c) Parallel (d) Convergent. Image 1 Highlights • Four types of flow patterns of impinging twin jet are studied by Large Dissipation Simulation. • Nusselt number is enhanced for all types of inclination for increasing Reynolds number. • Averaged Nusselt number of the perpendicular impingement gives better heat transfer than that of the oblique jets. • The poor heat transfer is obtained for the parallel oblique jets. • For the same angle, divergent jets give smallest heat transfer than the convergent jets. [ABSTRACT FROM AUTHOR]

In this paper, we experimentally investigated the effect of turbulent intensity on heat transfer in a square rotating channel. The grid generated turbulence is measured by hot-wire, and the heat transfer coefficient is measured by TLCs. In the experiment, the Reynolds number, based on the channel hydraulic diameter ( D = 80 mm ) and the bulk mean velocity ( V m = 1.82 m / s ), is 10,000, and the rotation number ranges from 0 to 0.52. The mean density ratio ( d . r . = ( T w - T b ) / T w ) is about 0.1 in the current work using transparent heater glass (Indium Tin Oxide) to provide uniform heat flux. Two different turbulent intensity of inlet air (0.6% and 5.5%) are taken into consideration to investigate the heat transfer distribution on the leading and trailing side. The results show that turbulent intensity has an effect on heat transfer on both leading and trailing side, especially at rotating conditions. At static conditions, the effect of turbulent intensity on heat transfer is not obvious. However, with the increase of rotation number, in case B with a medium turbulent intensity (Tu) of 5.5%, the Nu/Nu 0 is about 10% higher than that in the case A with low turbulent intensity of 0.6% with the rotation number of 0.52 at X/D = 2 on trailing side. The enhancement of case B decreases along X/D directions. On the leading side, the turbulent intensity has same effect on heat transfer with that on trailing side, but not as prominent as that on the trailing side. In current work, the turbulent intensities at different X/D directions are also presented to explain the phenomenon of heat transfer in the channel. More detail of results will be presented in this paper. [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]

In this paper an attempt has been made with experimental as well as theoretical analysis to find the variation in heat transfer coefficient with Reynolds number and numerical simulation attempt has also been done with different turbulence models in forced convection turbulent flows. Experimental analysis was carried out by using forced convection experimental setup at constant power supply to the heater and by varying the different flow rates of air with the help of a blower. The tube heat exchanger is fully insulated. In turbulent flows, the heat transfer coefficient is greatly affected by the turbulence, hence in this paper numerical analysis was carried out with different turbulence models and analysis has been done with both experimental and theoretical values to find the best turbulence model. [ABSTRACT FROM AUTHOR]

• The heat transfer process of NH 4 Cl fouling was studied experimentally. • The local heat transfer characteristics around tube wall was analyzed. • The effects of flow rates and temperature on heat transfer process were investigated. • The fouling resistance and average fouling thickness on the HET wall were obtained. The crystallization fouling of ammonium chloride (NH 4 Cl) often occurs on heat exchange tube bundles in petroleum refining equipment, which seriously reduces the heat transfer and operational efficiency of the equipment. In this paper, the heat transfer characteristics of heat exchange tube (HET) with NH 4 Cl crystallization fouling was studied experimentally. The effects of velocity, temperature and mass flow rate of hot and cold fluids on the overall and local heat transfer characteristics were analyzed. The results indicate that the local heat transfer coefficient around the tube wall decreases first and then increases with the increase of the circumferential angle (θ) under clean condition. At the position of 0°-45°, the local heat transfer coefficient is the largest. At 90°-135°, the local heat transfer coefficient is the smallest. Under fouling condition, the local heat transfer coefficient increases with the increase of circumferential angle. In the region of 0°-45°, the local heat transfer coefficient is the smallest. The region with the largest local heat transfer coefficient is 135°-180° The heat transfer coefficient (h), fouling resistance (R f) and average fouling thickness increase with the increase of Reynolds number (Re), temperature of hot fluid (T h) and mass flow rate of cold fluid (q m,c), but decrease with the increase of temperature of cold fluid (T c). [Display omitted] [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]

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

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

This paper 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]

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]

The paper reports an experimental investigation into the thermal performance and pressure drop characteristics of nanofluid-cooled heat sinks and then compares the results with the data for water-cooled heat sinks. Heat sinks with miniature circular-fin (MCFHS) and square-fin (MSFHS) structures are used and made from aluminum material with dimensions of 28 × 33 mm. Similarly, SiO 2 nanoparticles dispersed in deionized water with particle concentrations of 0.2%, 0.4%, and 0.6% volume are used as working fluids. The effects of pin fin configuration, particle concentration, and flow rate on the heat transfer performance and flow behaviors are presented. Reynolds numbers based on the hydraulic diameter of each flow channel ranging between 700 and 3700, fluid temperature of 15 °C, and heat flux ranging from 2 and 5 W/cm 2 are tested. Hydraulic diameters based on each flow channel are equally designed at 1.2 mm for both heat sinks. The experimental results indicate that the heat transfer coefficient increased with increasing Reynolds numbers and particle concentrations. The MCFHS gave greater heat transfer performance than that of the MSFHS by about 6–9%. For pressure drop data, the measured data showed that the pin fin configuration and particle concentration had small effects on the pressure drop and pumping power. [ABSTRACT FROM AUTHOR]

As an effective technology to reduce flow resistance, drilling holes on the surface of vortex generators had been studied extensively. In this paper, thermal performance of punched rectangular winglet vortex generators (PRWVGs) in turbulent regime (Reynolds number was in the range of 9,090 ∼ 21,210) was studied experimentally and numerically. Air was applied as the working fluid. The PRWVGs were inserted into the tube with three attack angles (α = 30 ° , 45 ° , 60 °). The punched holes were set in three different positions (g = 3 m m , 5 m m , 7 m m) in the vertical direction and at three different hole heights (d = 1 m m , 3 m m , 6 m m). The performance of PRWVGs was quantified by the heat transfer coefficient ratio (h / h 0), the friction factor ratio (f / f 0), and a combined thermal enhancement factor (T E F) of the two. The results revealed that h / h 0 as well as f / f 0 values of PRWVGs compared with those of planar VGs (without holes) were reduced. The flow and temperature fields of different cases were obtained by numerical simulations, and the experimental results were verified by analyzing the fluid flow behaviors. The results showed that the jet flow (defined as a jet emanating from the punched hole of a VG), would enhance the heat transfer efficiency in the recirculation area. And the T E F value of VGs was improved by punching holes on the surface of VGs. However, the shape of the jet flow depended on the position of the hole. When the position of the hole was lower in the vertical direction, the jet flow was effective in increasing its T E F value. Additionally, the size of the hole had a large effect on the T E F value. Besides, entropy generation analysis showed that PRWVGs were thermodynamically advantageous. When d = 1 m m , w = 5 m m , the Bejan number reached the minimum. Under this condition, the proposed PRWVGs performed best in practice. When the height of the hole was d = 1 m m , T E F was the maximum, which was 1.25 at α = 45 ° and R e = 9,090. [ABSTRACT FROM AUTHOR]

· Behavior of local heat transfer coefficient in plane diverging channels is described. • Local Stanton number should be based on flow parameters at diverging channel inlet. • Empirical equations for local Stanton number are based on Re and Kays parameter. • Kinematic structure and TKE in plane diverging channels are estimated experimentally. • Correlation between local heat transfer and local Reynolds stresses is revealed. Heat transfer in diverging channels is essentially different from the one in channels of constant cross section. The mechanism of heat transfer enhancement in diverging channels compared to the channels of constant cross section is based on increased turbulence intensity. The present paper suggests new approach to the analysis and generalization of heat transfer data. Heat transfer on the wall of a diverging channel should be considered similarly to heat transfer from a plate instead of heat transfer in a channel. The data on heat transfer coefficient on the wall of a plane diverging channel at different expansion angles can be generalized by the Stanton number as a function of the Reynolds number and Kays acceleration parameter, St∼ f (Re, K). This function has characteristic sections along the wall that are typical of laminar, transitional and turbulent boundary layers. Stanton and Reynolds numbers based on the distance from the inlet and the velocity at the diverging channel inlet can be employed for data generalization in all the considered ranges of geometries and flow regimes. Kinematic structure of flow is examined to reveal hydrodynamic mechanisms behind heat transfer formation. Optical measurements provide the profiles of velocities and turbulence parameters in characteristic sections of the channel. Mechanism of turbulent structure formation in diverging channels is examined. Correlation between hydrodynamics and heat transfer is analyzed. Correlation between local coefficients of heat transfer and near-wall Reynolds stress is revealed. [ABSTRACT FROM AUTHOR]

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

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

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

• 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]

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

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]

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