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This communication concerns some doubtful results included in the above paper. [ABSTRACT FROM AUTHOR]

It is a new practical method to apply external magnetic field in magnetic working fluid to enhance heat transfer. In this paper, the swirl flow and heat transfer characteristics of ferrofluid in elliptical tubes under thermal-magnetic-flow fields coupling have been studied by using the finite volume method. The flow structure and secondary vortices evolution process of magnetic nanofluid in elliptical ducts under the action of the magnetic fields have been obtained. The effects of magnetic induction intensity and the ratio of major axis to minor axis of elliptical pipe on the flow and heat transfer performances have been main investigated. The results show that there is obvious secondary flow (with four vortices or eight vortices) on the cross-section and the swirling flow is gradually formed due to the coupling of thermal-magnetic-velocity fields. With the increase of the ratio of major axis to minor axis, the heat transfer enhancement effect with the application of external magnetic field is weakened. The comprehensive performance of flow and heat transfer are better at lower Reynolds number and higher magnetic induction intensity. [ABSTRACT FROM AUTHOR]

This paper deals with the design of porous geometry of a heat transfer element. The proposed geometry combines a gyroid triply periodic minimal surface with the recursive principle of geometric body creation. The designed geometry is based on an attempt to increase the heat transfer surface while eliminating negative impacts on the fluid characteristics in the form of pressure loss or increase of the friction coefficient. The proposed geometry of the heat transfer element was compared with a pair of geometries based on the basic gyroid shape but with different channel size parameters. A numerical simulation was performed in Ansys Fluent 2020 R1 using the SST k-omega turbulence model for flow velocities in the range of 0.01 m.s−1 to 0.5 m.s−1, which covered a wide range of the Reynolds number and thus also flow forms in terms of the turbulence intensity. The presented results clearly show lower values of pressure loss and friction coefficient of the proposed geometry compared to the evaluated porous structures. Also, at the same time, they describe the factors positively influencing the mixing process of the liquid in the proposed element, which leads to an increase in the efficiency of the heat transfer process. [ABSTRACT FROM AUTHOR]

Today, impingement cooling structures with dimples can effectively ease the burden of turbine blades. This paper investigates the effect of dimple diameter and depth on the heat transfer of the target surface on a laminar-cooled turbine blade with a cross-flow and dimple numerically to find the mechanism behind it so that the dimple can be better used in turbine cooling. The commercial software ANSYS 19.2 and a baseline (BSL) turbulence model is used during the numerical computation. In this paper, the cross-flow Reynolds number varies from 15,000 to 60,000, while the jet Reynolds number remains at 30,000. When the cross-flow Reynolds number changes, due to the location change in vortexes generated inside or around the dimple, the two dimple parameters affect heat transfer differently. When the cross-flow Reynolds number is lower than the jet Reynolds number, dimples with smaller diameters and depths lead to better heat transfer performance. When the cross-flow Reynolds number exceeds the jet Reynolds number, dimples with bigger diameters and depths result in better heat exchange performance. The results also indicate that, while the dimple diameters remain constant, the rise of the cross-flow Reynolds number enhances the heat transfer of the dimple structure. [ABSTRACT FROM AUTHOR]

In this study, we propose a new type of small-channel plug-in, the double S turbulators, for passive heat transfer enhancement to improve the flow and heat transfer performance of the fluid in the channel. In the range of Reynolds number 254.51~2545.09, under constant wall temperature heating conditions, the effects of interpolated double S turbulators with different long axial radii (1mm, 1.5mm, 2mm) on the average Nusselt number, pressure drop, total thermal resistance and field synergy number in the rectangular mini-channel were studied. The simulation results show that compared with the smooth rectangular mini-channel, after interpolating double S turbulators with different long axial radii (1mm, 1.5mm, 2mm), the average Nusselt number increased by 81.74%~101.74%, 71.29%~94.06%, 67.16%~88.48%, the total thermal resistance decreased by 45.1%~50.72%, 41.72%~48.74%, 40.28%~47.2%, and the number of field synergies increased by 85.58%~111.65%, 74.1%~102.6%, 69.64%~96.12%. At present, there are few studies on the boundary condition of constant wall temperature, and this paper supplements the research on this aspect. At the same time, the heat transfer performance of the rectangular mini-channel of the interpolated double S turbulators is stronger than that of the ordinary smooth rectangular mini-channel, which not only provides a new idea for the manufacture of micro heat dissipation equipment, but also improves the heat transfer performance of micro heat dissipation equipment and improves its work efficiency. According to the simulation data, the prediction formula of average Nusselt number and pressure drop was established by nonlinear regression method, which can be used to predict the flow and heat transfer characteristics of the rectangular mini-channel of the interpolated double S turbulators. [ABSTRACT FROM AUTHOR]

The modern gas turbines need to run at very high inlet temperature to promote their power output. Thus, improvements in cooling technologies play a significant role for enhancing the gas turbine blade life. In this paper, thermohydraulic performance (THP) of a two-pass square channel with inclined ribs at 45° was scrutinized employing the k – ε realizable model with enhanced wall treatment in ANSYS Fluent. The calculations were performed for the rib pitch to height ratio (p / e) of 5-10, rib height to hydraulic diameter ratio (e / D h ) of 0.1-0.2, and Reynolds number (Re) of 20,000-40,000. Detailed analysis of the flow structure in a double passage square duct was carried out to understand the interaction of the rib and bend-induced secondary flows and its contribution to heat transfer enhancement for the rib configurations with distinct p / e and e / D h , which was not available in any other existing numerical or experimental investigations. The results revealed that the ribs with higher e / D h generated the stronger stream-wise secondary flows which led to the augmentation of the cooling performance with the disadvantage of pressure loss increment. The maximum THP of 26.55% was achieved with the ribbed configuration having p / e = 5 and e / D h = 0.1 at Re = 20,000. The new correlations were developed from the computational data to predict the normalized Nusselt number and friction factor (Nu / N u 0 and f / f 0 , where 0 is the correlation), taking the e / D h and flow Re into consideration. [ABSTRACT FROM AUTHOR]

Helically coiled pipes are widely used in many industrial and engineering applications because of their compactness, larger heat transfer area per unit volume and higher efficiency in heat and mass transfer compared to other pipe geometries. They are commonly encountered in heat exchangers, steam generators in power plants and chemical reactors. The most notable feature of flow in helical pipes is the secondary flow (i.e., the cross-sectional circulatory motion) caused by centrifugal forces due to the curvature. Other important features are the stabilization effects of turbulent flow and the higher Reynolds number at which the transition from a laminar to a turbulent state occurs compared to straight pipes. A survey of the open literature on helical pipe flows shows that a good deal of experimental and theoretical work has been conducted to derive appropriate correlations to predict frictional pressure losses under laminar and turbulent conditions as well as to study the dependence of the flow characteristics and heat transfer capabilities on the Reynolds number, the Nusselt number and the geometrical parameters of the helical pipe. Despite the progress made so far in understanding the flow and heat transfer characteristics of helical pipe flow, there is still much work to be completed to address the more complex problem of multiphase flows and the impact of pipe deformation and corrugation on single- and multiphase flow. The aim of this paper is to provide a review on the state-of-the-art experimental and theoretical research concerning the flow in helically coiled pipes. [ABSTRACT FROM AUTHOR]

This paper utilizes the Newton Raphson (NR) iterative algorithm to solve a detailed heat balance equation for overhead transmission conductors, for the purpose of calculating a Dynamic Line Rating or temperature-dependent impedance using real-time or forecasted weather data. Provided ambient weather conditions, conductor parameters and electrical current passing through the conductor, a starting guess is formed and the iterative solution method arrives upon a solution within a preset solution tolerance. A simple method to approximate the Rayleigh Number / Nusselt Number and Reynolds Number / Nusselt Number relationships is proposed to ensure successful convergence. The paper performs a thorough, numeric electrothermal conductor simulation to demonstrate proper solution convergence. This exercise identifies a minimum conductor temperature rise necessary for the solution method to converge successfully. Error related to the minimum starting conductor temperature rise, and error introduced by the approximated air property functions is illustrated. The temperature solution method is compared with several well-known thermal models. [ABSTRACT FROM AUTHOR]

Enhancements in heat transfer systems are being applied for recovery of heat in domestic as well as industrial applications in many areas. These are done by generating turbulence and swirl motion in the flow by inserting inserts to reduce the thermal resistance of heat transfer. This paper presents the experimental and numerical results of heat transfer enhancement of cooling of a confined space by inserting twisted tape in a pipe buried in concrete under the floor. The Reynolds number is fixed to 15000. Thirteen cases are considered varying the configurations and designs of twisted tapes and the effects of different types of perforations and slots on twisted tapes are studied. Water as the test fluid flowed in pipe inserted with twisted tapes. Heat is transferred from a heating plate to water along the pipe length through concrete to simulate the heat transfer from a room and the water temperature rise is studied. To validate the experimental results, ANSYS-FLUENT is used as the CFD tool with a Standard k-ε turbulence flow model and the SIMPLE algorithm is implemented. Flow physics behaviour is displayed using velocity and temperature profiles inside the pipe. Insertion of twisted tapes shows that the heat transfer is enhanced due to swirling action, friction and turbulence. The experimental differences in inlet and outlet water temperature validated the CFD simulation results. The temperature distribution on a single twisted tape with increasing twist ratio is found to be the most efficient for heat transfer enhancement of the cooling floor surface with a water temperature rise of 1.62 ºC along a pipe length of 1.8 m. [ABSTRACT FROM AUTHOR]

In this paper, the effect of solar intensity on the heat pipe tip temperature in a heat pipe type—evacuated-tube solar collector (HP-ETSC) was investigated. A simple relation was proposed, relating the solar intensity to the heat pipe tip temperature generated from the experimental data. This simple empirical relation was applied in a set of heat transfer equations derived to predict the heating medium temperature at the manifold outlet of the evacuated-tube solar collector. The calculated results corresponding to two types of heating medium, i.e., palm oil and water, were compared with experimental results from the literature. The results show that the average error was 6.41% for the case of palm oil and 4.66% for the case of water. Based on the case of water as a heating medium fluid, it was found that the flow rate of the heating medium fluid affected the accuracy of prediction, as the percentage error increased with the heating medium flow rate. The maximum percentage error increased from only 1.83% for a water inlet flowing at a Reynolds number of about 2.4 × 103 to 15.23% for a water flow rate at a Reynolds number of about 2.6 × 104. The correction factor was added into the correlation to predict the heat transfer coefficients of heating medium fluids. With this correction factor, the maximum error could be reduced from 11.78% to 7.29% for the palm oil case and from 15.23% to 5.57% for the water case. The average errors corresponding to palm oil and water cases could be reduced to 0.74% and 1.26%, respectively. [ABSTRACT FROM AUTHOR]

The aim of this paper is to propose an efficient numerical scheme to deal with heat transfer problems in pipe flow with a large length/diameter ratio. The generalized finite difference method (GFDM) is combined with a space stepping algorithm (SSA) for the solution. The SSA divides the solution domain into a number of overlapping sections in the length direction of the pipe. Due to the large Peclet number, the heat transport process is dominated by advection, which allows an approximate boundary condition to be applied at the downstream cross section. The problem is then solved section by section. Using the uniform distributed points, the solution matrix for each section does not change, and only the right hand side needs to be refreshed for the changing boundary conditions. This leads to a highly efficient scheme for problems with a large pipe length. To show the accuracy of the numerical scheme, a problem with available analytical solution is studied. Then the method is applied to problems of single pipe with different pipe wall temperature and flow Reynolds number, and concentric annular pipe. Numerical results confirm the stability and efficiency of the GFDM-SSA. The method can be applied to many real world transient heat transfer problems in which the pipe is heated or cooled along its length with arbitrary wall temperature for heat exchange and other purposes. [ABSTRACT FROM AUTHOR]

The paper presents the results of calculating the thermal-hydraulic heat exchange efficiency processes for direct and swirl (using an inserted twisted tape) pipe air flows at the same transferred heat fluxes and temperature conditions. The calculated energy efficiency, which characterizes the thermal-hydraulic heat exchange efficiency, demonstrated the ratio of transferred heat and energy consumption for pumping the coolant during the heat exchange for these two cases to be almost the same for coolant-flow Reynolds numbers up to 2.5 ⋅104. At large Reynolds numbers, the thermal-hydraulic efficiency of a direct flow heat exchange is higher. [ABSTRACT FROM AUTHOR]

A microchannel heat exchanger effectively evacuates heat from a confined space. This paper attempts to gain insight into the combinatorial repercussions of simultaneously coupling two factors that affect a microchannel's performance, of which channel size and micro-insert complexity are the two main contributors. With water as the working fluid, an ANSYS-based numerical analysis was carried out for two distinct channel sizes, 1 and 2 mm, both with and without micro-inserts. The Reynolds numbers varied between 125 and 4992 and between 250 and 9985 for the 1 and 2 mm channels, respectively. For the 2 mm diameter channel, adding micro-inserts raised the overall pressure drop with increased Reynolds number. The inclusion of micro-inserts increased the pressure drop in the 1 mm channel at first, and thereafter the pressure drop decreased. Incorporating micro-inserts into the channel resulted in enhanced heat transfer. The trade-off between enhanced heat transfer performance and a larger pressure drop was calculated by evaluating the channel's overall performance using the thermal performance factor. Micro-inserts were found to be most useful for improving overall performance in the low-to-moderate Reynolds number range, and their effectiveness increased with decreasing channel size. Changing the channel diameter and structure of the design can improve heat transmission through microchannels. [ABSTRACT FROM AUTHOR]

The heat transfer and flow characteristics of water in a copper foam tube, a nickel foam tube, and a smooth tube are investigated in this paper. The copper foam tube and nickel foam tube are produced, and an experiment for testing the heat transfer and flow characteristics of water in metal foam tubes is set up. Experimental sytem validation is done in this paper, and it is found that the experimental results have a good agreement with that of published literature. The effects of two kinds of metal foam tubes and different Reynolds numbers on the heat transfer and flow characteristics of water are investigated. It is found that copper foam tube shows the best heat transfer performance, followed by the nickel foam tube, and the smooth tube has the worst heat transfer performance. It is also found that comprehensive index of heat transfer and flow increases with Reynolds number. [ABSTRACT FROM AUTHOR]

This research paper focuses on CFD analysis for comparative evaluation of different hybrid nanofluids (NFs): MWCNT-Al 2 O 3 /H 2 O, MWCNT-MgO/H 2 O, Al 2 O 3 -CuO/H 2 O performance as working fluid (WF) inside parabolic trough solar collector (PTSC). The performance of the collector is evaluated using different performance indicators, including Nusselt number (Nu), friction factor (f), and Performance Evaluation Criterion (PEC) for different Reynolds number (Re) (10000-50000). Among the investigated hybrid nanoparticles (NPs), MWCNT-Al 2 O 3 emerges as the most promising choice for significantly enhancing the system's thermal performance with the highest PEC value of 1.9. This outcome signifies the superior efficiency of MWCNT-Al₂O₃/H₂O hybrid NF in enhancing convective heat transfer, thus underlining the paramount significance of adopting them in thermal systems operating within the specified Re range. [ABSTRACT FROM AUTHOR]

The article conducts an analysis of flow around spherical and non-spherical elements for a wide range of Reynolds numbers using direct numerical simulations based on the indefinite heat conducting equation (3D) and the Navier–Stokes equation. The research focuses on the influence of the shape of the element on the drag coefficient of the flow. It has been found that the total air drag coefficient around the considered elements depends on their shape and Reynolds number. The drag coefficient for the particular element example (sphere) has been compared with the experimental data. Further results have been presented in this paper. [ABSTRACT FROM AUTHOR]

In this paper, periodic vortices are generated by a fluid passing a cylindrical obstacle, d, near the micro-channel inlet. Two elastic pillars are arranged on the walls, and the effect of the pillar spacing on heat transfer performance is studied using the Arbitrary Lagrangian-Euler method. With the spacing of 10d, the small pillar amplitude of 2 µm is not conducive to the generation of vortices. The flexible vortex generator has higher heat transfer efficiency and lower pressure loss than the rigid vortex generator. The two pillars with no spacing generate isolated vortices, and the mixing of these vortices is insufficient downstream the pillars. It is found that with the pillar spacing of 5d, the overall performance factor is significantly higher than that with the pillar spacing of 0d and 10d in the Reynolds number range of 800 to 1100. The average Nusselt number with the spacing of 5d increases by 19.2% compared to that with the spacing of 0d at the Reynolds number of 1000. When the Reynolds number is 1100, the overall performance factor is 43% higher than that with a single rigid pillar. The vortices are periodically generated by the two pillars with the 5d spacing, and the disturbance to the boundary layer enhances the heat transfer downstream the region in the micro-channel. [ABSTRACT FROM AUTHOR]

A central task in aviation technology is the development of efficient cooling techniques for thermal highly loaded engine components. For an optimal design of the cooling mechanisms, the heat transfer characteristics have to be known and need to be describable. As a cooling concept for low-pressure turbine casings, complex systems of impinging jets are used in order to reduce blade tip clearances during the flight mission. In order to improve established theoretical model approaches, this paper presents a novel method for the experimental investigation of such a complex system with 200 impinging jets using infrared thermography. The presented experimental method uses a thin electrically heated chrome-aluminum foil as target plate. Modeling the transient effects inside the foil, small structures and high gradients in the heat transfer coefficient can be reproduced with good accuracy. Experimental results of the local heat transfer characteristics are reported for jet Reynolds numbers of Re = 2000 ... 6000 . The influence of the jet-to-jet distance and the jet Reynolds number on the Nusselt numbers are quantified with Nu ∼ (S / D) − 0.47 and Nu ∼ Re 0.7 . The results indicate a dependency of the flow regime for the relatively low jet Reynolds numbers, as it is known from literature. [ABSTRACT FROM AUTHOR]

In this paper, the impact of an extended surface on the passage of a microchannel using cylindrical ribs with variable sector angles on heat transfer rate is presented using computer simulation. Extended surfaces in the form of cylindrical ribs of varying sector angles in the passage of microchannel in a staggered manner have been designed. The sidewalls of a new kind of microchannel incorporating five distinct ribs with sector angles ranging from 45° to 80° have been analyzed. Ansys Fluent workbench software has been exploited to simulate this novel design of a microchannel heat sink. A three-dimensional heat transfer and fluid flow model of the microchannel heat sink (MCHS) was developed, and the fluid and solid regions were discretized in very fine meshes. All CFD simulations were performed for Reynolds numbers between 100 and 900. Nusselt numbers are varied in the following ranges: 6.93 to 13.87, 6.93 to 14.38, 6.93 to 17.80, 7.15 to 27.86, and 7.20 to 37.38 at sector angles of 45°, 50°, 60°, 70°, and 80°, respectively. It is concluded that the Nusselt number is strongly influenced by the Reynolds number. At an angle of 80°, the maximum friction factor and pumping power requirements were observed. Additionally, a 45° angle has been proven to be the minimal friction factor and pumping power requirement. It is revealed that the THPP has all values larger higher than 1. At angles of 80° and 45°, the maximum and minimum values of THPP have been discovered, respectively. In addition, thermo-hydraulic performance parameters have been evaluated, which are greater than one for all sector angles. [ABSTRACT FROM AUTHOR]

The turbine inlet temperature is one of the most critical parameter that determines the thermal efficiency and thrust-weight ratio of the gas turbine. However, the higher gas turbine inlet temperature significantly increases the thermal stress on the blade, which necessitates effective cooling strategy to reduce the blade temperature. In this study, a united blade cooling strategy was developed by adopting a two-pass ribbed cooling channel with wavy tape for the reason that ribs are near-wall turbulence promoters while the wavy tape is core-region promoter with relatively lower pressure-drop penalty. Ribs and wavy tape may complement each other in the cooling channel. The synergistic effect of the combined near-wall and core-region heat transfer promoter was numerically studied for Reynolds numbers from 6000 to 30000. Two key geometrical factors, namely rib to wave crest phase shift and relative wave amplitude, have significant effect on the thermal performance. The results demonstrate that wavy tape with different phase shift configurations can eliminate the lower heat transfer areas and produce more uniform heat transfer. The 0° phase shift achieved best overall thermal performance, which provided 1.07-1.08 times the flat tape heat transfer enhancement with 1.09-1.11 times the pressure drop penalty. The better heat performance is due to the secondary flow induced by wavy tape, which enhances the fluid mixing between near-wall and core-region. [ABSTRACT FROM AUTHOR]

Development of the heat transfer surfaces on the tube inside makes it very difficult or even impossible to determine the heat transfer coefficient analytically. This paper presents the experimental determination of the coefficient in an internally rifled tube with spiral ribs. The tests are carried out on a laboratory stand constructed at the Institute of Thermal Power Engineering of the Cracow University of Technology. The tube under analysis has found application in a supercritical circulating fluidized bed boiler. The heat transfer coefficient local values are determined for the Reynolds numbers included in the range of ~6000 to ~50000 and for three ranges of the heating elements power. As the medium flows through internally rifled tubes with spiral ribs, the heat transfer process gets intensified compared to similar processes taking place in smooth tubes. Based on the obtained experimental data, a correlation is developed enabling determination of the dimensionless Chilton-Colburn j factor. The equation form is selected so that a comparison with existing results of tests performed on rifled tubes can be made. Comparing the Nusselt number values calculated based on the developed correlation with those obtained using other correlations described in the literature, it can be observed that the criterial number is about twice higher. The research results confirm the thesis that the element internal geometry has a substantial impact on the heat transfer process. [ABSTRACT FROM AUTHOR]

This study examines the performance of one point closure turbulence models in predicting of heat and momentum transfer of impinging flows. The scope of this paper is limited to impinging jet on a moving wall and heat transfer. The impinging distance is fixed to 8 thickness of the nozzle (8e) for this study. Two parameters are considered: the jet exit Reynolds number (10000 ⩽ Re ⩽ 25000) and the jet-surface velocity ratio (0 ⩽ Rsj ⩽ 4). The flow field structure at a given surface-to-jet velocity ratio is independent of the jet Reynolds number, a slight modification of the flow field is observed for low surface-to-jet velocity ratio (Rsj < 0.25) whereas at higher ratios Rsj > 0.25, the flow field is significantly modified. Good agreement with experimental results is obtained for surface-to-jet velocity ratio 0 ⩽ Rsj ⩽ 2. The purpose of this paper is to consider the case of higher of surface-to-jet velocity Rsj > 2. A further study of heat transfer shows that at the stagnation points the local heat transfer coefficient have a maximum value. The local Nusselt number at the impinging region tends to decrease significantly when Rsj ⩽ 1.5. The evolution of average Nusselt number is correlated according to the surface-to-jet velocity ratios for each Reynolds number. [ABSTRACT FROM AUTHOR]

The thermohydraulic performance of an enhanced PTC's tube is evaluated in this paper. A passive method is used by introducing transversal inserts on the bottom part of the receiver. The height of the inserts is investigated using MCRT method coupling FVM for Reynolds number range from 2.36 · 104 to 11.83 · 104. The heat transfer fluid used in this study is the Therminol®VP1. The numerical results show that the tube enhanced by inserts augments the reliability of the system, and the introduction of the inserts into the receiver tube decreases the temperature difference over the circumferential area of the absorber tube and minimizes the heat losses and also increase the lifetime of the receiver. [ABSTRACT FROM AUTHOR]

The focus of this paper is to investigate the effects of the addition of a connector between two serial microchannels. The idea of adding connector at the inlet of microchannels to enhance the random motion of molecules or nanoparticles in low Reynolds numbers was developed in our research group for the first time. It was experimentally determined that the shape of a connector between two microchannels has a significant impact on the enhancement of the random motion of molecules or nanoparticles. Consequently, the heat transfer coefficient is improved inside the second microchannel. The connector is large enough to refresh the memory of the fluid before entering the second channel, causing a higher maximum heat transfer coefficient in the second channel. It was also observed that the heat transfer coefficient can be increased at the end of the channel when the outlet temperature is relatively high. This may be explained by the fact that as temperature increases, the fluid viscosity tends to decrease, which generally drives an increase in the local random motion of base fluid molecules and nanoparticles. This causes an increase in the microchannel heat transfer coefficient. It was found that the addition of nanoparticles significantly modified the impact of the connector on the microchannel heat transfer coefficient. In addition, the effects of changing the Reynolds number and the shape of the connector were investigated through use of computational fluid dynamics (CFD) calculations. It was found that both factors have an important impact on the variation of velocity and enhancement of random motion of molecules and consequently significantly affect the heat transfer coefficient. [ABSTRACT FROM AUTHOR]

For propulsion systems using gel fuels, reducing the gel fuel viscosity is essential for achieving better atomization and combustion. In this paper, we investigate the flow and heat transfer in a water-gel with a temperature and shear dependent viscosity. We consider several different channels, mimicking the transport of gelled fuels in propulsion systems, and we also look at corrugation, which is a way of enhancing fluid mixing and thus improving the heat transfer characteristics. The rheological parameters in the constitutive model of the gel are fitted with experimental data. The influence of different corrugation profiles, corrugation configuration parameters and the Reynolds number on the mean apparent viscosity and the pressure drop are investigated. It was found that the flow recirculation formed in the valley of the corrugations enhances the heat transfer and thus the temperature of the main flow. We also noticed an increase in the pressure drop due to the stronger viscous dissipation. Furthermore, it was observed that the sinusoidal corrugation can achieve lower viscosity with a lower pressure drop compared with triangular and trapezoidal corrugations. A shorter wavelength and a deeper wave amplitude of the corrugation seemed to be better for reducing the gel fuel viscosity, while we must consider the adverse consequence of increased pressure drop. A larger Reynolds number was helpful for both lowering the pressure drop and for reducing the viscosity. In addition, compared with a smooth straight pipe, a Y-shape corrugated channel with a constant inlet velocity reduced the mean apparent viscosity by 70.8%, and this value increased to 72.6% by further applying a pulsed inlet velocity, which can greatly enhance the gel fuel atomization and thus improve the combustion efficiency. [ABSTRACT FROM AUTHOR]

The present paper investigates the thermal mixing and cooling processes in a passive micromixer, which is applicable for the cooling of electronic devices. Employing a porous block and testing different configurations for the mixing channel is considered to enhance the mixing process and cooling performance. A 2D lattice Boltzmann thermal model is utilized to investigate the thermal performance of a T-micromixer with a porous block. Two different types of mixing channel configurations, including a step-shaped and a zigzag-shaped channel, are considered, and the obtained results are compared with those of the simple mixing channel. The thermal mixing and cooling of two miscible fluids, at 50 and 25°C entering the micromixer, are investigated. The results show that changing the mixing channel configuration may create a chaotic laminar flow, which enhances the heat transfer rate between the mixed flow and the channel wall. Whatever the Reynolds number, the step-shaped mixing channel exhibits better mixing performance than the zigzag-shaped one. For the T-micromixer with a zigzag-shaped and step-shaped mixing channel, the cases with h/H = 0.5 and h/H = 0, respectively, exhibit better thermal mixing and cooling performance. [ABSTRACT FROM AUTHOR]

In this paper, the thermal performance improvements of a heat removal system like an electronic system have been analyzed. The studied case is a horizontal channel in which two partially inclined baffles are attached with variable height and number. The channel is crossed by a forced convective flow of a cooling fluid (air). This numerical work evaluates the influences of the height and number of the baffles on the enhancement of the heat transfer rate. The mathematical model of this system is composed of nonlinear partial equations that the analytical solution for them is very complex, hence the need for numerical analysis is mandatory with the aid of a finite volume method. Accordingly, The numerical results are presented in axial and transverse velocity, temperature, local and average Nusselt number, local friction coefficient, pressure drop, heat transfer rate, and turbulence kinetic energy. The results revealed that it is possible to improve the thermal performance of the considered system by adopting designs that allow the maximum heat transfer rate with the minimum energy loss. In addition, results show that at the lowest Reynolds number (Re = 10,000), as the height of baffles rises from 0.01 to 0.03 m (growth by 200%), the heat transfer rate augments about 59.09%. Moreover, at the highest evaluated Reynolds number (Re = 87,300), by increasing the height of baffles up to 200%, the heat transfer rate increases by approximately 50.53%. Furthermore, employing a higher number of baffles leads to more heat transfer rates and a significant pressure drop. [ABSTRACT FROM AUTHOR]

The pulsating flow is one of the techniques that can enhance heat transfer, therefore leading to energy saving in tubular heat exchangers. This paper investigated the heat transfer and flow characteristics in a two-dimensional in-line tube bundle with the pulsating flow by a numerical method using the Ansys Fluent. Numerical simulation was performed for the Reynolds number Re = 500 with different frequencies and amplitude of pulsation. Heat transfer enhancement was estimated from the central tube of the tube bundle. Pulsation velocity had an asymmetrical character with a reciprocating flow. The technique developed by the authors to obtain asymmetric pulsations was used. This technique allows simulating an asymmetric flow in heat exchangers equipped with a pulsation generation system. Increase in both the amplitude and the frequency of the pulsations had a significant effect on the heat transfer enhancement. Heat transfer enhancement is mainly observed in the front and back of the cylinder. At a steady flow in these areas, heat transfer is minimal due to the weak circulation of the flow. The increase in heat transfer in the front and back of the cylinder is associated with increased velocity and additional flow mixing in these areas. The maximum increase in the Nusselt number averaged over space and time in the entire studied range was 106%, at a pulsation frequency of 0.5 Hz and pulsation amplitude of 4.5. A minimum enhancement of 25% was observed at a frequency of 0.166 Hz and amplitude of 1.25. [ABSTRACT FROM AUTHOR]

In this paper, a numerical investigation was performed of an air jet incident that normally occurs on a horizontal heated plane. Analysis of flow physics and entropy generation due to heat and friction is included using a simple easy-to-manufacture, surface roughening element: a circular rib concentric with the air jet. This study shows how varying the locations and dimensions of the rib can deliver a favorable trade-off between entropy generation and flow parameters, such as vortex generation and heat transfer. The performance of the roughness element was tested at three different radii; R/D = 1, 1.5 and 2, where D was the jet hydraulic diameter and R was the radial distance from the geometric center. At each location, the normalized rib height (e/D) was increased from 0.019 to 0.074 based on an increment of (e/D) = 0.019. The jet-to-target distance was H/D = 6 and the jet Reynolds number (Re) ranged from 10,000 to 50,000 Re, which was obtained from the jet hydraulic diameter (D), and the jet exit velocity (U). All results are presented in the form of entropy generation due to friction and heat exchange, as well as the total entropy generated. A detailed comparison of flow physics is presented for all ribs and compared with the baseline case of a smooth surface. The results show that at higher Reynolds numbers, adding a rib of a suitable height reduced the total entropy ( S t ) by 31% compared to the no rib case. In addition, with ribs of heights 0.019, 0.037 and 0.054, respectively, the entropy generated by friction ( S f ) was greater than that due to heat exchange ( S h ) by about 42%, 26% and 4%, respectively. The rib of height e/D = 0.074 produced the minimum St at R/D = 1. As for varying R/D, varying rib location and Re values had a noticeable impact on S h , S f and ( S t ). Placing the rib at R/D = 1 gave the highest total entropy generation ( S t ) followed by R/D = 1.5 for all Re. Finally, the Bejan number increased as both rib height and rib location increased. [ABSTRACT FROM AUTHOR]

The baffling technique is well-known for its efficiency in terms of enhancement of heat transfer rates throught channels. However, the baffles insert is accompanied by an increase in the friction factor. This issue remains a great challenge for the designers of heat exchangers. To overcome this issue, we suggest in the present paper a new design of baffles which is here called graded baffle-design. The baffles have an up- or down-graded height along the channel length. This geometry is characterized by two ratios: up-graded baffle ratio and down-graded baffle ratio which are varied from 0-0.08. For a range of Reynolds number varying from 104 to 2 x 104, the turbulent flow and heat transfer characteristics of a heat exchanger channel are numerically studied by the computer code FLUENT. The obtained results revealed an enhancement in the thermohydraulic performance offered by the new suggested design. For the channel with a down-graded baffle ratio equal to 0.08, the friction factors decreased by 4-8%. [ABSTRACT FROM AUTHOR]

This paper presents a numerical analysis of the thermal-flow characteristics for a laminar flow inside a rectangular microchannel. The flow of water through channels with thin obstacles mounted on opposite walls was analyzed. The studies were conducted with a low Reynolds number (from 20 to 200). Different heights of rectangular obstacles were analyzed to see if geometrical factors influence fluid flow and heat exchange in the microchannel. Despite of the fact that the use of thin obstacles in the microchannels leads to an increase in the pressure drop, the increase in the height of the obstacles favors a significant intensification of heat exchange with the maximum thermal gain factor of 1.9 for the obstacle height coefficient h/H = 0.5, which could be acceptable for practical application. [ABSTRACT FROM AUTHOR]

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This paper is the second of a two-part series that presents experimental data and analysis on the liquid nitrogen quenching heat transfer process of a stainless-steel tube with an inner surface low-thermal conductivity thin-film coating. This paper focuses on the effects of different flow directions and two tube wall thicknesses. Additionally, this paper also provides an analysis on the chilldown thermal efficiency of the quenching process. Three flow directions with four different inner surface coating modifications, and three tube wall thicknesses were examined. The experimental data covers the Reynolds numbers ranging from 3500 to 140,000. The chilldown efficiency, along with chilldown time and LN 2 (liquid nitrogen) mass consumption were analyzed to assess the overall performance of the LN 2 line chilldown process. For thin tube wall cases, the chilldown efficiencies cover a range between 3% and 41%, and the maximum chilldown efficiency value is found for the tube with 3 L coating at Re = 5278 in the vertical up flow direction. For thick wall tube cases, the efficiencies cover a range between 4% and 52%, and the maximum chilldown efficiency value was found for the tube with 4 L coating at Re = 5308 in the vertical up flow direction. [ABSTRACT FROM AUTHOR]

Heat dissipation is becoming more and more challenging with the preface of new electronic components having staggering heat generation levels. Present day solutions should have optimized outcomes with reference to the heat sink scenarios. The experimental and theoretical results for plate type heat sink based on mathematical models have been presented in the first part of the paper. Then the parametric optimization (topology optimization) of plate type heat sink using Levenberg-Marquardt technique employed in the COMSOL Multiphysics® software is discussed. Thermal resistance of heat sink is taken as objective function against the variable length in a predefined range. Single as well as multi-parametric optimization of plate type heat sink is reported in the context of pressure drop and air velocity (Reynolds number) inside the tunnel. The results reported are compared with the numerical modeled data and experimental investigation establish the conformity of results for applied usage. Mutual reimbursements of greater heat dissipation with minimum flow rates are confidently achievable through balanced, heat sink geometry as evident by the presented simulation outcome. About 12% enhancement in pressure drop and up to 51% improvement in thermal resistance is reported for the optimized plate fin heat sink as per data manifested. [ABSTRACT FROM AUTHOR]

The paper investigates the heat transfer characteristics of the heat exchanger under two different configurations namely taper helical tube and spiral tube type heat exchanger. The working fluid is water which flows in tubes kept horizontally and the mass flow rate ranges from 0.04 kg/s to 0.16 kg/s. The effect of tube configuration parameters and Reynolds number on heat transfer coefficient and Nusselt number were studied. Regression equations have been developed for both the configurations under horizontal orientation using various process parameters and regression tools available in Minitab Software. A comparison has been made between the theoretical results and the results obtained from simulation gives an error of 15% which is well within the permissible limit. The result indicates that the highest value of Nusselt number is greater by 21% than taper helical tube heat exchanger which is attributed to better heat transfer characteristics of spiral tube heat exchanger and hence the former is thermally superior over the latter. In this study, temperature and velocity contour have been developed for both the configurations which clearly indicates that, when heat transfer is considered, spiral configuration should be preferred over taper helical configuration. [ABSTRACT FROM AUTHOR]

The investigation reported in this paper is dealt about the steady-state laminar flow and heat transfer of a lid driven staggered cavity with the heated block. Based on the aspect ratio (AR = H/L = 0.5, H/L = 1, H/L = 2) three different block shapes are introduced for numerical experiments. The solid block with no slip and stationary wall condition is considered and it is located at the geometric center of the cavity. The simulations are carried out for Reynolds numbers 50, 100, 200, 300, 500, and 1000 and temperature of the block is 300 K. A clock-wise momentum is converged to the fluid, by the two driving lids on the top and bottom side of the cavity, lids are set into an anti-parallel wall motion. The upper lid moves to the right, while the lower one to the left, both are consider as same velocities. The results are found to be in good agreement with existing published results. It was found that the dynamics and the structure of the primary vortex and the corner vortices were strongly affected by the Reynolds number. The investigation clearly describes that increasing the Reynolds number values the overall drag coefficient decreases, similarly the value of average Nusselt number also increases with an increasing Reynolds number for all the values of different blocks under studied. The study reveals the important flow physics such as flow separation, boundary-layer and recirculation. The results will be beneficial for similar situation occur in many industrial problems. [ABSTRACT FROM AUTHOR]

In the present paper, along with experimental study, CFD analysis of forced convection in a twisted tube is performed, using the transition SST model which can predict the change of flow regime from laminar through transition to turbulent. The differential governing equations are discretized by the finite volume method. The investigations are conducted for Reynolds numbers ranging from 100 to 50000 covering laminar, transitional and turbulent regimes, and for three length and three pitch ratios. The predictions are observed to show a good agreement with the measurements and published correlations of other authors. The analysis indicates that the large length ratio and small pitch ratio yields a higher heat transfer rate with relatively low performance penalty. The transition from laminar to turbulent regime is observed between Reynolds numbers of 2500 to 3500 for all cases. For almost all investigated cases the performance factors are greater than unity. [ABSTRACT FROM AUTHOR]

This paper studies numerically the influence of the louver's fin thickness on heat transfer and flow performance of a parallel flow evaporator, a comprehensive evaluation and analysis of the five structures at different Reynolds numbers are systematically carried out. Comparison of the numerical results with the experimental data shows good agreement with maximal errors of 12.16% and 5.29% for the heat transfer factor and the resistance factor, respectively. The results show that the heat transfer coefficient and the pressure drop increase with the increase of the thickness of the louver fins when the Reynolds number is a constant. The analysis of the comprehensive evaluation factor shows that the A-type fin is the best, and it can effectively strengthen the heat exchange on the air side and improve the heat transfer capacity of the system. The research results can provide reference for the structural optimization of the louver fins. [ABSTRACT FROM AUTHOR]

This paper presents a numerical simulation to determine the air-side heat transfer and the pressure drop characteristics of a flat tube heat exchanger with offset strip fin. The effects of the fin bending ratio such as 29%, 36%, 44%, 50%, and the fin spacing such as 2.10 mm, 2.35 mm, 2.60 mm on the performance of the heat exchanger are studied by using a commercial CFD software. The air having constant viscosity, thermal conductivity, and density enters the heat exchanger at 298 K and the wall temperature of the strip fins is considered as constant at 314 K. Variations of the heat transfer coefficient and the pressure drop in the airside are presented with respect to the frontal air velocity while Colburn j-factor and the friction factor f are presented with respect to the airside Reynolds number ranging from 200 to 1200. Finally, the thermal-hydraulic performance of all investigated cases is compared by using the volume goodness factor, j/f1/3. The results show that the air-side heat transfer coefficient and the pressure drop increase when the frontal air velocity ascends. The air-side heat transfer coefficient decreases with the increase of fin spacing. The fin bending ratio does not have a significant effect on the pressure drop in the considered fin spacing. Both the Colburn j-factor and friction factor reduce with the increment of Reynolds number and fin spacing [ABSTRACT FROM AUTHOR]

In this paper, a spin-etching method is proposed for wafer thinning using a mixture of hydrofluoric, nitric, and acetic acids (HNA solution), and experimental studies are conducted. In the spin-etching apparatus, the silicon wafer was rotated in a Teflon bath filled with HNA solution. The etch rate of HNA spin-etching can be determined by mixture ratio; mass transfer, both through and in boundary layer; and heat transfer. When the Reynolds number of the flow on the rotating disk is below 1.8 \times 10^5 , the flow is in the laminar region, and the boundary layer thickness and convective heat transfer coefficient do not vary with radius at the same rotational speed. In this study, the silicon wafer was rotated at <60 rpm, at which the flow was in the laminar region. Nevertheless, the local etch rate varied with the radial position. To investigate spin-etching characteristics of the silicon wafer rotated in the laminar region, experimental studies on etch rate and uniformity of etch of 6-in silicon wafers as the rotational speed was varied were carried out with various HNA mixture ratios. [2015-0031] [ABSTRACT FROM AUTHOR]

The mini-channel heat exchanger has better heat and flow transfer characteristics than conventional heat exchanger. In this study, we use the S-shaped turbulator inserted, which is simple to install and inexpensive, to improve the heat exchange. The fluid-flow and heat transfer characteristics in smooth and inserted S-shaped turbulator rectangular mini-channels are simulated by numerical simulation under constant wall temperature heating in the Reynolds number 198.77-1987.67. The results of the numerical simulation show that compared with the smooth mini-channel, the pressure drop of the three rectangular mini-channels with different inserted S-shaped turbulators with radii on the horizontal axis of 1 mm, 1.5 mm, and 2 mm increase by 334.8-774.3%, 275.9-606.4%, and 234.6-500.4%. The average Nusselt numbers grow by 28.7-98.6%, 18.8-92% and 11.1-88.5%. The total thermal resistances reduce by 22.43-50.15%, 15.91-48.40%, and 10.08%-47.45%. The field coordination numbers increase by 30.36%-115.29%, 19.72-106.94%, and 19.72-104.72%. Moreover, the non-linear regression method establishes the prediction formulas of the pressure drop and average Nusselt number. In most cases, the deviation between predicted and simulated values is between ±4.6% and ±14%. [ABSTRACT FROM AUTHOR]

Flow over shallow cavities is used to model the flow field and heat transfer in a solar collector and a variety of engineering applications. Many studies have been conducted to demonstrate the effect of cavity aspect ratio (AR), but very few studies have been carried out to investigate the effect of cavity height ratio (HR) on shallow cavity flow behavior. In this paper, flow field structure and heat transfer within the 3-D shallow cavity are obtained numerically for two height ratio categories: HR = 0.0, 0.25, 0.5, 0.75, and 1.0 and HR = 1.25, 1.5, 1.75, 2.0, 2.25, and 2.5. The governing equations, continuity, momentum, and energy are solved numerically and using the standard (K-ε) turbulence model. ANSYS FLUENT 14 CFD code is used to perform the numerical simulation based on the finite volume method. In this study, the cavity aspect ratio, AR = 5.0, and Reynolds number, Re = 3 × 105, parameters are fixed. The cavity’s bottom wall is heated with a constant and uniform heat flux (q = 740 W/m²), while the other walls are assumed to be adiabatic. For the current Reynolds number and cavity geometry, a single vortex structure (recirculation region) is formed and occupies most of the cavity volume. The shape and location of the vortex differ according to the height ratio. A reverse velocity profile across the recirculation region near the cavity’s bottom wall is shown at all cavity height ratios. Streamlines and temperature contours on the plane of symmetry and cavity bottom wall are displayed. Local static pressure coefficient and Nusselt number profiles are obtained along the cavity’s bottom wall, and the average Nusselt number for various height ratios is established. The cavity height ratio (HR) is an important geometry parameter in shallow cavities, and it plays a significant role in the cavity flow behavior and heat transfer characteristics. The results indicate interesting flow dynamics based on height ratio (HR), which includes a minimal value in average Nusselt number for HR ≈ 1.75 and spatial transitions in local Nusselt number distribution along the bottom wall for different HRs. [ABSTRACT FROM AUTHOR]

Results of heat transfer coefficient measurements in an agitated vessel heated or cooled by the liquid media flowing in a helical pipe coil are presented in this paper. The multistage impeller made of two pitched six-blade impellers and adjustable clearance was used in a vessel with conical bottom. The transient method based on measuring the temperature dependency on time and solving the unsteady enthalpy balance was used to determine the heat transfer coefficients between the agitated liquid and the helical pipe coil. The results are summarized by the Nusselt number correlations, which describe the dependency on the impeller Reynolds number. The second part of this paper introduces a theoretical analysis of the flow in the liquid batch near the helical pipe coil. Based on the known pumping capacity of the multistage impeller, the characteristic velocity near the helical pipe coil can be evaluated. This characteristic velocity can be then used to determine the Nusselt number describing the heat transfer in a flow around a cylinder with the same cross-section profile as the helical pipe coil. A clear correlation between this Nusselt number and the integral Nusselt number is presented in this paper and introduces an alternative approach for prediction of heat transfer characteristics on the basis of hydrodynamic parameters describing an agitated system. [ABSTRACT FROM AUTHOR]

This paper describes a detailed numerical investigation of a stationary high-aspect-ratio rib-roughed rectangular cooling channel with longitudinal intersecting ribs near the gas turbine blade trailing edge region. In order to overcome the heat transfer performance degeneration in the highaspect- ratio channel, longitudinal intersecting ribs are arranged on the channel bottom surface. The effect of the number of longitudinal intersecting ribs on the flow and heat transfer is systematically studied in the Reynolds number range Re = 10 000-30 000. The results show that a heat transfer augmentation region exists just downstream the junction between the longitudinal rib and the angled rib due to additional secondary flows. With more longitudinal intersecting ribs, the heat transfer distributions on the channel surfaces are more uniform. Though the pressure loss is also enlarged with an increase in the number of longitudinal intersecting ribs, the overall thermal efficiency increases in the entire range of Reynolds numbers investigated. The configuration with two sets of longitudinal intersecting ribs shows the best overall thermal efficiency. [ABSTRACT FROM AUTHOR]