Average Convective Heat Transfer Coefficient Research Articles

Temperature determination of tilted electronic assemblies equipped with basic and wire-bonded QFN16 and 32 devices subjected to free convection

The correlations proposed in this work allow to calculate the average temperature on different areas of electronic assemblies containing basic QFN16 and QFN32 packages and their wire-bonded versions named QFN16b and QFN32b respectively. The study is based on correlations proposed in recent works allowing calculation of the average natural convective heat transfer coefficient according to the power generated by the package and the tilt angle of the printed circuit board (PCB) on which they are welded. The considered generated power is within the range 0.1–1W for the QFN16 and 16b models. For the QFN32 and QFN32b, the correlations are extended to lower power ranging from 0 (off) to 0.1W. These power ranges are associated to an inclination angle of the PCB varying between 0° and 90° (horizontal and vertical positions respectively) corresponding to the operative conditions of these devices increasingly used in electronics. Temperature determination of every part of these assemblies through the proposed correlations easy to use, helps improving their thermal design and reliability.

Conjugate heat transfer analysis of turbulent forced convection of moving plate in a channel flow

The effect of wall thickness and heat conduction in the wall on turbulent forced convection flow characteristics in a vertical channel has been numerically studied. The effect of heated plate velocity related to fluid velocity in a vertical channel including heat conduction in the plate has been investigated numerically. The flow field is modeled as two dimensional unsteady state incompressible turbulent forced convection flow in a channel. Energy equation in the solid region is considered as two dimensional unsteady state heat conduction. Turbulence is modeled with a low Reynolds number k−ϵ model of Launder–Sharma. Conjugate heat transfer solver has been developed using high accuracy compact finite difference schemes. The effects of Reynolds number, wall thickness and thermal conductivity of the solid on the flow and heat transfer characteristics are reported. Results show that the interface temperature decreases and average convective heat transfer coefficient increases with increase in Reynolds number. The temperature decreases in the solid and fluid regions with increase in thickness of the solid region. The wall thickness and heat conduction in the wall significantly alter the convective heat transfer characteristics in channel flows. The heated plate velocity shows a significant effect on heat transfer characteristics in the solid and fluid regions. The average convective heat transfer coefficient increases with increase in speed of the moving plate. The present results are matching well with the experimental results available in the literature.

Thermal and fluid dynamic behaviors of confined laminar impinging slot jets with nanofluids

Heat transfer enhancement technologies play an important role in research and industrial fields; thus, they have been widely applied to many applications as in refrigeration, automotive, aerospace, and process industry. For example, heat transfer can be passively enhanced by increasing the thermal conductivity of the working fluids, adopting nanofluids, or actively by employing impinging jets.In this paper a numerical analysis on confined impinging slot jets working with pure water or water/Al2O3 based nanofluids is presented. The flow is laminar and a constant uniform temperature is applied on the target surface. The single-phase model approach has been adopted in order to describe the nanofluid behavior and different particle volume concentrations have been considered. Moreover, simulations have been performed for different geometric ratios in order to take into account the confining effects and Reynolds numbers. The behavior of the system has been analyzed in terms of average and local convective heat transfer coefficient, Nusselt number, and required pumping power profiles. Correlations for stagnation point and average Nusselt number for 100≤Re≤400, 0%≤ϕ≤5% and 4≤H/W≤10 are provided.

Effects of the wire-bonding technique on the QFN16b's thermal performance. New correlations for the free convective heat transfer coefficient

The technique of wire-bonding used in electronics is interesting for thermoregulation of the quad flat non-lead (QFN) packages during operation. To properly size these devices in the assemblies, it is necessary to know the surfacic thermal phenomena represented by the convective heat transfer coefficient. This is the objective of this work examining the thermal effects due to installation of wire-bonding on a QFN16. This particular device is named here QFN16b to differentiate it from the basic model without wire-bondings addressed in a previous study. During its actual operation, it generates a power ranging from 0.1W to 1.0W and it is welded on a printed circuit board (PCB) which could be inclined at different angles varying between the horizontal and vertical positions, according to the intended application. Calculations corresponding to 6 different areas of the considered assembly have been done by combining 20 powers generated by the QFN and 7 inclination angles. The distribution of the surface temperature and that of the source itself, as well as the free convective power exchanged by every area, are determined. Natural convective heat transfer is quantified by means of correlations which allow calculation of the average convective heat transfer coefficient in each area of the assembly and for all the treated configurations. They improve design, reliability, durability and performance of these conventional assemblies widely used in electronics in many engineering sectors.

Numerical investigations of laminar heat transfer and flow performance of Al2O3–water nanofluids in a flat tube

This paper presents a three-dimensional numerical analysis to study the laminar heat transfer and flow characteristics of Al2O3–water nanofluids through a flat tube at constant heat flux boundary condition. New correlation models for thermal conductivity and viscosity of Al2O3–water nanofluids are developed and verified. Based on the single-phase approach, the effects of different parameters such as nanoparticle volume concentration, nanoparticle size, Reynolds number, temperature and tube flattening on the thermal–hydraulic performance of flat tube with Al2O3–water nanofluids as working media are discussed in detail. Then, this study provides an entropy generation analysis to evaluate the overall superiority of nanofluids. Numerical results show that the addition of nanoparticle enhances the heat transfer and pressure loss of base fluid in all of the flat tubes at various Reynolds number and temperature. Both the relative average convective heat transfer coefficient and press drop can be enhanced by increasing nanoparticle volume concentration and decreasing nanoparticle size. And the heat transfer and press drop enhancements of nanofluids are more obvious at smaller Reynolds number and higher temperature. In additions, it is also detected that the thermal entropy generation is the main part caused irreversibility, and the increase of nanoparticle volume concentration and size can decrease the total entropy generation of Al2O3–water nanofluids in a flat tube. Moreover, compared to the tube flattening, nanoparticle volume concentration has a slight effect on the relative thermal–hydraulic performance between flat tubes and circular tube.

Correlations highlighting effects of the PCB’s Copper ratio on the free convective heat transfer for a tilted QFN32 electronic package

The main objective of this work is to examine the effects of the Copper ratio constituting the upper face of a tilted Printed Circuit Board (PCB) on the natural convective heat transfer concerning an electronic equipment containing a quad flat non-lead type (QFN32) package. Calculations are done by means of the finite volume method for several positions of electronic device on the PCB which is inclined with respect to the horizontal at an angle ranging from 0° (horizontal position) to 90° (vertical position) with a step of 15°. The power generated by the QFN32 varies between 0.1 and 0.8W, and 10 Copper ratio varying between 0.26% and 39.45% are considered. These ranges correspond to the normal operating of the active electronic device for the intended applications. The study shows that the thermal behavior of the distinct areas of the active package is affected by the Copper ratio. Correlations are proposed, allowing determination of the average convective heat transfer coefficient on the different areas of the QFN32 device, according to the considered values of the Copper ratio, the PCB’s inclination angle and the generated power. They optimize its design while controlling its temperature during operation. The results of this survey provide a better modeling of this conventional arrangement widely used in electronic applications.

Convective heat transfer prediction in disk-type electrical machines

This paper presents correlations to assess the average convective heat transfer in axial flux permanent magnet machines based on numerical simulations. The rotor–stator system is enclosed in a cylindrical chamber, and all the surfaces within the machine are considered isothermal, each on their own temperature. CFD simulations have been performed for different ranges of the rotational Reynolds number, Re = ωR2/υ, the gap size ratio, G = s/R and the surface temperature of the rotor, the stator and the cover. The average convective heat transfer coefficients for all surfaces are defined. The bulk flow temperature has been taken into account as the reference temperature instead of the ambient temperature to calculate the average heat transfer coefficient. The latter is typically used in literature to describe the heat transfer between the rotor and the stator, but the heat transfer coefficient is then only applicable for a given surface temperature of the rotor and the stator, which is a serious drawback. Therefore, in order to calculate the bulk fluid temperature, we average the values of fluid temperature adjacent to the corresponding surface; by doing so, the average convective heat transfer in the gap becomes independent of the ambient temperature. Thereafter, for estimating the bulk fluid temperature, a linear correlation between the surface temperature of the rotor, the stator and the cover is made. The unknown coefficients of that linear equation are found with the aid of the least squares method. Afterward, the appropriate equations for the mean Nusselt number are given through curve fittings. The proposed empirical correlations are applicable to various geometries and boundary conditions. Results are compared with the available data in literature and good agreement has been found. It is shown that, for a given Reynolds number, there is a gap size ratio for which the average convective heat transfer of the stator surface in the gap reaches a minimum. Moreover, it can be concluded that the proposed correlation is a quite versatile tool for thermal modeling of disk type electrical machines.

Heat Transfer and Hydrodynamic Performance Analysis of a Miniature Tangential Heat Sink Using Al2O3–H2O and TiO2–H2O Nanofluids

In this article, thermal and hydrodynamic performances of a miniature tangential heat sink are investigated experimentally by using Al2O3–H2O and TiO2–H2O nanofluids. The effects of flow rate and volume concentration on the thermal performance have been investigated for the Reynolds number range of 210 to 1,100. Experimental results show that the average convective heat transfer coefficient increases 14 and 11% and the bottom temperature of the heat sink decreases 2.2°C and 1.6°C by using Al2O3–H2O and TiO2–H2O nanofluid instead of pure distilled water, respectively.

Thermal design of tilted electronic assembly with active QFN16 package subjected to natural convection

The natural convective heat transfer coefficient detailed in this study deals with the five elements of a typical assembly used in electronics constituted by a Printed Circuit Board (PCB) on which a quad flat non-lead (QFN16) is welded. This active package can be placed in different positions on the board and generates during its actual functioning a power ranging between 0.1W and 0.8W. Many configurations of the assembly are considered, corresponding to seven inclination angles of the PCB varying between the horizontal and vertical positions. Correlations between the average convective heat transfer coefficient, the generated power and the PCB inclination angle are proposed for the five assembly parts. They allow to optimize the thermal control of electronic devices incorporating QFN16 packages, which are increasingly used in engineering.

Heat Transfer Experiment and Simulation of the Verification Facility for High Power Rotating Tritium Target System

High intensity D–T fusion neutron generator (HINEG) is important for research and development work of fusion reactors, of which the rotating target system is one of the key components. The design of the cooling technology principle verification facility both for tritium target systems of the 3 × 1013 and 1014 n/s HINEG was proposed. The facility employed the jet array cooling method. The first stage heat transfer experiment was carried out and the heat transfer processes was simulated by CFD method, which aimed at the investigations of the new cooling enhancement technology. The experimental results, which had a good agreement with the numerical simulation results, show that the maximum temperature of the target surface was about 89.4 °C under 60 kW heating condition and the average equivalent convection heat transfer coefficient of the jet array cooling was about 57,000 W/(m2 K). All the results show that the verification facility could achieve the cooling requirement for 3 × 1013 n/s neutron yield (60 kW) design objective and the jet array cooling enhancement technology was effective.

Heat transfer of staggered bundles of flat oval tubes in transverse flow

The paper deals with investigating convective heat transfer for virtually unstudied staggered bundles of flat-oval tubes at their transverse air flow around. Experiments were conducted in an open-circuit wind tunnel with rectangular cross-section in the range of Reynolds numbers change from 2000 to 30000. In the course of experiments, the average convective heat transfer coefficients were determined. When processing and analyzing experimental data and dependencies of Nusselt numbers on Reynolds numbers of bundles of flat-oval tubes, much attention was paid to factors, which may affect the heat transfer intensity. During the experiments, several of these factors: operation factor - flow rate W, geometrical factors: back pitch between tubes S 1 , long pitch S 2 , the ratio of longitudinal to transverse tube size (profile elongation) d 2 /d 1 were identified.

Conjugate heat transfer analysis of micro-channel using novel hybrid nanofluids ([formula omitted

Laminar forced convection flow of nanofluids in a wide rectangular micro-channel has been numerically studied. The present study investigated the flow and heat transfer characteristics of Aluminium oxide (Al2O3), silver (Ag) and hybrid (Al2O3+Ag) nanofluids in a micro-channel. The conduction phenomena of the solid region show a significant effect on the heat transfer characteristics of nanofluid. Hence, the channel is considered with finite thickness on its bottom to accommodate heat source or electronic component and a uniform heat flux is applied to the three sides of the solid region. A two-dimensional conjugate heat transfer homogeneous phase model has been developed and results are reported for different Reynolds numbers. The governing equations are solved by Simplified Marker and Cell (SMAC) algorithm on non-staggered grid using finite volume method. The effects of Reynolds number, pure nanoparticles volume concentration, hybrid nanoparticles mixture volume concentrations and nanoparticles size on the flow and heat transfer characteristics are reported. The results show that the average convective heat transfer coefficient increases with increase in nanoparticles volume concentration and Reynolds number. The nanofluids obtained by dispersing nanoparticles such as Al2O3, Ag and hybrid (Al2O3+Ag) in the base fluid shows a significant enhancement of average convective heat transfer coefficient in comparison with pure water. It is also observed that 3 vol.% hybrid nanofluid (0.6 vol.% Al2O3+2.4 vol.% Ag) shows higher average convective heat transfer coefficient than that of pure water, pure oxide (Al2O3) and pure metallic (Ag) nanofluids. The study presents that hybrid nanofluids are the new class of working fluids with less volume concentration of metallic (Ag) nanoparticles. Moreover, use of hybrid nanofluids at high volume concentration reduces the cost of the working fluid and enhances the heat transfer characteristics in comparison with that of metallic nanofluids. The interface temperature between solid and fluid regions are reported for different nanofluids. The size of the nanoparticle shows significant effect on heat transfer characteristics. The present results are matching with the numerical and experimental results available in the literature.

Using nanofluid as a smart suspension in cooling channels with discrete heat sources

This paper aims to three dimensionally investigate flow and heat transfer characteristics of the Al2O3–water nanofluid in channels with discrete heat sources, in which variable thermophysical properties are used in the simulations. The effects of Reynolds number, aspect ratio of the channel, volume fraction and particle size are assessed. The results show that thermal conductivity of the nanofluid varies periodically along the length of the channels due to its dependency on temperature and the amplitude of these variations intensifies by increasing the volume fraction and decreasing size of the particles. As a result, in comparison with pure water, a more uniform temperature distribution is caused along the length of the channel since in the sections with heat source, thermal conductivity of the nanofluid increases, which leads to better cooling as compared to other sections. Due to having this superior characteristic, the nanofluid under study is called a “smart fluid.” It means that the nanofluid automatically possesses better heat transfer characteristics in sections where higher heat dissipation is required. Average convective heat transfer coefficient and pressure drop increase by raising Reynolds number, volume fraction and aspect ratio of the channel. Nevertheless, their relative values, which are calculated in comparison with water, do not vary considerably by changing the aspect ratio of the channel. In addition, a neural network model is developed to model the average convective heat transfer coefficient and pressure drop, which is able to predict the output variables with a great accuracy.

A Numerical Analysis on Nanofluid Mixed Convection in Triangular Cross-Sectioned Ducts Heated by a Uniform Heat Flux

In this paper, results obtained by the numerical investigation on laminar mixed convection in triangular ducts, filled with nanofluids, are presented in order to evaluate the fluid dynamic and thermal features of the considered geometry by considering Al2O3/water based nanofluids. The system is heated by a constant and uniform heat flux also along the perimeter of the triangular duct section in H2 mode as thermal boundary condition and the single-phase model has been assigned for a Reynolds number value equal to 100. Results are given for different nanoparticle volume concentrations and Richardson number values ranging from 0% to 5% and from 0 to 5, respectively. Results, presented for the fully developed regime flow, show the enhancement of average convective heat transfer coefficients values for increasing values of Richardson number and particle fractions. However, wall shear stress and required pumping power profiles increase as expected. The PEC analysis showed that the use of nanofluids in mixed convection seems slightly convenient. It should be underlined that, at the moment, experimental data are not available to compare the numerical proposed model for mixed convection in horizontal triangular ducts with nanofluids.

Heat treatment of aluminum extrusion dies and study of their heating by convection/radiation

This paper shows the study of the heating phase of the quenching procedure of dies in steel ALVAR 14 (DIN 56 NiCrMoV7) for the extrusion of aluminum. The heat treatment was executed in BMI vacuum hardening furnaces (model B84T) at the company F. Ramada, Acos e Industrias, S.A. The die had the shape of a massive disk and its heating was studied experimentally, by measuring and recording the relevant temperatures along the time using thermocouples, and was also studied analytically and numerically using the ABAQUS software. The experimental average convective heat transfer coefficient between the gas and the disk was found to be 10 W/(m2 K), with the convective heating representing 22 % of the total energy supplied to the disk. The coefficients of heat transfer by radiation found at the initial and final stages of the heating process ranged between 3 and 64 W/(m2 K), respectively, and were determined using the lumped system analysis in conjunction with experimental data. By acting essentially upon the apparent emissivity of the disks, it was possible to fit the numerically simulated heating curve of the disk to the experiments. The maximum value of the global heat transfer coefficient to the disk was fairly low, 64 W/(m2 K), resulting on Biot numbers below 0.1, and thus allowing the use of lumped system analysis. These Biot numbers also indicate that the temperature gradients inside the dies are smaller than expected, enabling the use of higher heating rates that reduces the total time and costs of the quenching treatment.

Turbulent heat transfer characteristics in a circular tube and thermal properties of n-decane-in-water nanoemulsion fluids and micelles-in-water fluids

We carry out experimental studies of turbulent convective heat transfer of several n-decane-in-water nanoemulsions and micelles-in-water fluids. We characterize the viscosity and thermal properties of the nanofluids, and the nanoparticle distributions and shapes with various techniques, including transmission electron microscopy. We find that the thermal conductivity of the nanofluids is lower than that of the base fluid, and the conductivity shows no anomalous behavior. Despite this we find that the average Nusselt numbers and convective heat transfer coefficients are enhanced for low volume fractions when the Reynolds number Re > 7000, in agreement with solid particle nanofluids. However, when the pressure losses are taken into account, we find that the nanofluids studied have a practical efficiency equal to or less than that of the base fluid. The highest heat transfer enhancements were obtained with volume fractions considerably smaller than those for the solid particle nanofluids. Our results indicate that the improved heat transfer is due to enhancement of turbulence in the large Re regime.

Thermal model design and analysis of the high-power LED automotive headlight cooling device

A study of the thermal performance of conventional plate-fin heat sinks and novel cooling device integrated with heat conductive plates (HCPS) for the application in high-power LED headlight was presented. The thermal resistance network model was built to analyze the factors which have impacts on the junction temperature. The analyses were based on the experiment combined with computations using the CFD software FloEFD. The prediction results were validated with the experimental one. Details of the heat transfer performance, particularly the temperature distribution of the LED module and lampshell, as well as the effect of HCPS length on the system thermal resistance and the average heat transfer coefficient were obtained. Since it could make some distance between the heat sink and the lampshell, the average surface convective heat transfer coefficient had been improved and the overall external thermal resistance was reduced effectively. The effects of the simulated ambient temperature, chip package depth, inclination angles and fan rotate speeds on the junction temperature were also examined. Results indicated that the airflow was weakened as the chip package depth increased. In addition, with the presence of the enforced velocity airflow, the junction temperature decreased gradually from 116.61 °C to 78.05 °C.

Experimental and numerical investigation of turbulent nanofluid flow in helically coiled tubes under constant wall heat flux using Eulerian–Lagrangian approach

In the present study, turbulent flow in helically coiled tubes under constant wall heat flux is numerically and experimentally investigated. Pressure drop and convective heat transfer behavior of water and water–silver nanofluid are studied. In experimental section, the pressure drop measurements as well as the average convective heat transfer coefficient calculations are carried out. The numerical computations are performed by Eulerian–Lagrangian two-phase approach in connection with an RNG k–ε turbulence model accounting for four-way coupling collisions using ANSYS CFX software. The Brownian motion of nanoparticles is taken into account. Single-phase approach (homogeneous model with constant effective properties) is also used. Two-phase approach predicted much more accurate results than the homogeneous model. More enhanced heat transfer was observed for tubes with greater curvature ratio. The results showed that the nanoparticles did not change the axial velocity and turbulent kinetic energy significantly, while the micrometer-sized particles increased mean axial velocity and suppressed turbulence. It was found that the utilization of the base fluid in helical pipe with greater curvature ratio compared to the use of nanofluid in straight tubes increased heat transfer more effectively. Based on the numerical results validated by the experimental ones, two correlations were developed to predict the ratio of the mean heat transfer coefficient and the pressure drop of nanofluid to water in helical tubes.

Prediction of convective heat transfer of Al2O3-water nanofluid considering particle migration using neural network

Purpose – The purpose of this paper is to attempt to investigate the particle migration effects on nanofluid heat transfer considering Brownian and thermophoretic forces. It also tries to develop a model for prediction of the convective heat transfer coefficient. Design/methodology/approach – A modified form of the single-phase approach was used in which an equation for mass conservation of particles, proposed by Buongiorno, has been added to the other conservation equations. Due to the importance of temperature in particle migration, temperature-dependent properties were applied. In addition, neural network was used to predict the convective heat transfer coefficient. Findings – At greater volume fractions, the effect of wall heat flux change was more significant on nanofluid heat transfer coefficient, whereas this effect decreased at higher Reynolds numbers. The average convective heat transfer coefficient raised by increasing the Reynolds number and volume fraction. Considering the particle migration effects, higher heat transfer coefficient was obtained and also the concentration at the tube center was higher in comparison with the wall vicinity. Furthermore, the proposed neural network model predicted the heat transfer coefficient with great accuracy. Originality/value – A review of the literature shows that in the single-phase approach, uniform concentration distribution has been used and the effects of particle migration have not been considered. In this study, nanofluid heat transfer was simulated by adding an equation to the conservation equations to consider particle migration. The effects of Brownian and thermophoretic forces have been considered in the energy equation. Moreover, a model is proposed for prediction of convective heat transfer coefficient.

Study on heat transfer and friction factor characteristics of γ-Al2O3/water through circular tube with twisted tape inserts with different thicknesses

An experimental study was carried out to investigate heat transfer and friction factor characteristics of γ-Al2O3/water nanofluid through circular tube with twisted tape inserts with various thicknesses at constant heat flux. In this work, γ-Al2O3/water nanofluids with two volume concentrations of 0.5% and 1% were used as the working fluid. The twist ratio of twisted tape remained constant at 3.21, while the thicknesses were changed through three values of 0.5 mm, 1 mm and 2 mm. The experiments were performed in laminar flow regime from 150 to 1600 Reynolds numbers. Results indicated that twisted tape inserts enhanced the average convective heat transfer coefficient, and also more the thickness of twisted tape is more the enhancement of convective heat transfer coefficient is. Also, the highest enhancement was achieved at maximum volume concentration. Results showed that nanofluids have better heat transfer performance when utilized with thicker twisted tapes. At the same time, the increase in twisted tape thickness leads to an increase in friction factor. In the end, the combined results of these two phenomena result in enhanced convective heat transfer coefficient and thermal performance. Finally, two new correlations were offered for Nusselt number and thermal performance based on our experimental observation.