Increase In Convective Heat Transfer Coefficient Research Articles

The Scaling Analysis and Numerical Simulation of Single-phase Natural Circulation

In this work, scaling analysis and numerical simulation of the single-phase natural circulation loop with water are presented. First, grid-dependency testing and model validation are performed. Then, according to the assumption of the boundary conditions, the effect of the heat input and structural parameters on the heat transfer characteristic is studied, including the boundary condition and temperature of the steam fluid. The results show that the scaling model can accurately express the flow heat transfer characteristics of the original model, and can reach a steady state under different working conditions. The formation of stable natural cycles is the result of coupling the boundary conditions of the heating and cooling segments under the fixed structure reference. The mass flow rate on the steady state decrease with the increase of the external fluid temperature in the cooling section, and as the heat transfer coefficient of the external convection heat transfer coefficient increases.

Hydrocarbon Fuel Flow and Heat Transfer Investigation in Rotating Channels

Ram air turbines are used in the power generation systems of hypersonic vehicles, which can address the problem of the high power consumption of weapon systems. However, high incoming air temperatures can cause the turbine blades of power generation to ablate. At this point, the incoming air can no longer be used as a cooling source to cool the turbine blades. To prevent the ablation of the turbine blades of the hypersonic vehicle power generation, hydrocarbon fuel carried by the hypersonic vehicle itself is used to cool the turbine blades. Hence, hydrocarbon fuels under rotating conditions are investigated. The results show that the rotation leads to a strong pressure gradient that causes the density and dynamic viscosity of hydrocarbon fuel to increase dramatically. Compared to the static condition, the density and dynamic viscosity of the hydrocarbon fuel increase by a maximum of 65.1% and 405%, respectively, under the rotating condition. This leads to an obvious reduction in velocity. The comprehensive influence of the physical properties of the fuel, centrifugal force, and Coriolis force causes the convective heat transfer coefficient and Nusselt number of the channel to first increase and then decrease with the increase in the rotational speed. Compared to the static condition, the convective heat transfer coefficient and Nusselt number increase by a maximum of 69.7% and 45.6%, respectively, under the rotating condition. The critical rotational speed of the Nusselt number from rise to fall is 20,000 rpm for different inlet temperature conditions.

Convection heat transfer coefficient of building glasses under salt deposition conditions

Glass buildings in coastal regions and islands exposed to the hydrothermal marine environment, in addition to salt deposition and erosion, lead to a change in the surface convective heat transfer coefficient (CHTC) and gradual degradation of the building energy and thermal performance. However, limited research has been conducted on the influence of salt deposition on the CHTC of building glasses. Hence, in this study, the salt deposition characteristics of the glasses were obtained by conducting an accelerated salt spray test. In addition, the influence of salt deposition on the CHTC and heat transmission coefficient (HTC) was investigated. The results indicate that the sizes of the salt spots on the glass surface increased significantly and unevenly. The salt deposition per unit area (p) exhibited a logarithmic growth trend between 14.4 and 40.0 mg/m2, considering the spray time. Under the 10% saline condition, p was larger than that under the 5% saline condition, ranging between 15.6 and 44.3 mg/m2, with an average increase rate of 11.6%. The CHTC exhibited a monotonic increase in terms of the deposition amount on the glass. Compared with the salt-free scenario, the maximum increase in CHTC was 178.7%, which exhibited a linear relationship with p. When p increased to 40.0 mg/m2, the HTC increase rate of the single clear glass and hollow Low-E glass reached 20.1% and 5.2%, respectively. The findings of this study provide insights into the influence of salt deposition on the CHTC of building glasses and can serve as a basis for energy consumption studies in coastal regions.

Experimental study on boiling heat transfer in negative-pressure flowing water in a vertical annular tube

To study energy recovery from low temperature wastewater during industrial production, a water-boiling heat transfer experimental device was built. A casing evaporator with a length of 1450 mm, an inner tube diameter of 30 mm, and an annular space gap of 14.2 mm is taken as the main research object. The boiling heat transfer characteristics of water in the annular tube area of the casing evaporator were experimentally studied by adjusting the inlet temperature (60? 85?C) of hot water and the pressure in the annular tube (12.1-57.6 kPa). The results show that, as the pressure of the system decreases, the boiling phenomenon in the annular tube becomes more intense and the convective heat transfer coeffi-cient increases. The boiling flow and average surface convective heat transfer enhancement rates, ? , were 2.2 and 1.5 when an initial pressure of 1 kPa was used. When the flow rate of the working medium in the annular tube was 1.69 kg?m2/s, the convective heat transfer coefficient gradually increased with the temperature and then stabilized when the inlet temperature reached between 80?C and 85?C. These results reveal the characteristics of boiling-water heat transfer in an annular tube and aid in the design of heat pipe systems for waste-heat recovery.

Investigation of forced convective heat transfer with magnetic field effect on water/ethylene glycol-cobalt zinc ferrite nanofluid

In this work, the magnetic field effect on the convective heat transfer (CHT) properties of nanofluids comprising cobalt–zinc ferrite nanoparticles were experimentally studied. The nanofluid was prepared by dispersing the cobalt–zinc ferrite nanoparticles in the water and ethylene glycol mixture (80:20) and subjected to heat transfer studies. The experiment result exhibited that the CHT coefficient increased with the increase in the nanofluid concentration. The maximum CHT coefficient exhibited at a concentration of 0.2 wt%, which was improved by about 23.9%. It was also found that the CHT coefficient at a magnetic field of 750 G was increased by about 2.64% compared to that at 0 G for 0.2 wt% cobalt–zinc ferrite nanofluid concentration. The nanofluid with 0.2 wt% cobalt–zinc ferrite concentration exhibited the maximum CHT coefficient and pressure drop increase rate by 17% when the magnetic field was increased from 0 to 750 G.

Parametric Study on Integrated Thermal and Mechanical Performance of Molten Salt Receiver for Solar Tower Power Plant

In this study, the mathematical and numerical models of a receiver tube are established for the integrated thermal and mechanical performance investigation of molten salt solar receiver. Effects of four parameters on heat transfer performance and thermal strain of the receiver are studied. The results indicate that when other parameters are settled, a smaller tube internal diameter can lead to a higher convection heat transfer coefficient and a smaller thermal strain. As the tube wall thickness increases, the convection heat transfer coefficient and thermal strain both increase. With the inlet molten salt flow velocity increased, the convection heat transfer coefficient increases and the thermal strain decreases. Similar to the effect of inlet flow velocity of molten salt, a higher inlet molten salt temperature can also improve the convection heat transfer coefficient and reduce the thermal strain. The thermal efficiency of the receiver can be improved by increasing the inlet flow velocity of molten salt, or by reducing the receiver tube internal diameter, tube wall thickness, or inlet molten salt temperature. All results have revealed the effect laws of the four parameters on the thermal and mechanical performances of the receiver tube, which may be helpful for the further research and development of molten salt solar receiver for solar thermal power plant.

Effect of different nanoparticle-dispersed nanofluids on hydrothermal-economic performance of minichannel heat sink

Hydrothermal and energy-economic performances of minichannel heat sink are experimentally compared by using water-based different nanoparticle-dispersed mono and hybrid nanofluids. Al2O3, AlN, CNT, Cu and capric acid as phase change material (PCM) are considered. Different nanoparticles combinations (oxide–PCM, oxide–nitride, oxide–carbon nanotube and oxide–metal) in 50/50 volume ratio with water (base fluid) are taken as working fluids. The effects of volume flow rate (0.1–0.5 LPM) or Reynolds number (50 to 500) and total particle volume concentration (0.01–0.1%) are investigated. Convective heat transfer coefficient and pressure drop increase by about 42.3% and 22%, respectively, for Al2O3 + CNT nanofluid. The maximum reduction of 26.6% in thermal resistance is obtained for Al2O3 + CNT nanofluid as compared to base fluid. Heat transfer effectiveness and figure of merit are above one for all the hybrid nanofluids, which conclude that hybrid nanofluid is a better option over base fluid for minichannel heat sink. Al2O3 + CNT hybrid nanofluid is better in terms of heat transfer effectiveness, but Al2O3 + AlN hybrid nanofluid yields higher heat transfer coefficient to pressure drop ratio and coefficient of performance. The lower nanoparticle volume concentration in nanofluid is preferable due to higher stability, lower clogging and lower cost per cooling capacity of heat sink.

Particle ratio optimization of Al2O3-MWCNT hybrid nanofluid in minichannel heat sink for best hydrothermal performance

Interesting hydrothermal behavior of hybrid nanofluid containing combination of dissimilar particles (completely different in terms of shape, size and properties) by changing their ratio is unexplored yet, which is analyzed experimentally for minichannel heat sink. Nine parallel rectangular minichannels are used in the heat sink each of which has 1 mm width, 3 mm depth and 30 mm length. Al2O3–MWCNT/water hybrid nanofluid of 0.01% volume concentration with various nanoparticle mixture ratios is used as coolant to analyze the effect of Reynolds number and working fluid inlet temperature. Nanoparticles mixing ratios (5:0, 4:1, 3:2, 2:3, 1:4 and 0:5) have been taken as volume ratios. Convective heat transfer coefficient, Nusselt number, pressure drop and friction factor increase with an increase in fraction of MWCNT in hybrid nanofluid. A maximum enhancement of 44.02% has been observed for the convective heat transfer coefficient with MWCNT (5:0) hybrid nanofluid as compared to water. Maximum pressure drop has been increased by 51.2% over base fluid at an inlet temperature of 20 °C for the MWCNT (5:0) hybrid nanofluid. Inlet fluid temperature has negative effect on pressure drop and positive effect on heat transfer. The observed optimum volumetric mixing ratio of Al2O3 and MWCNT is around 3:2, for which hybrid nanofluid yields maximum heat transfer coefficient to pressure drop ratio. Performance evaluation criteria (PEC) has been evaluated greater than 1 for all the nanofluids, which concludes that nanofluids are better option as compared to conventional fluid (DI water). Based on the experimental data, correlations for Nusselt number and friction factor have been introduced with R2 values of 98.8% and 96.3%, respectively, and compared with available correlations.

A three-dimensional aircraft ice accretion model based on the numerical solution of the unsteady Stefan problem

This paper develops a three-dimensional aircraft ice accretion model based on the numerical solution of the unsteady Stefan problem. In this model, for both rime and glaze ice situation, the heat conduction within the ice layer is considered as an unsteady process and the evolution of temperature distribution is numerically solved by Variable Space Grid (VSG) method. Unlike the Myers' state-of-the-art model and its extended model which introduces a posteriori correction factor α by assuming a linear temperature profile, current model does not need to tune the parameter α to match the experimental results. A hierarchical overset grid strategy is applied to ease the grid generation and shorten the workload on constructing complex icing configurations. Only the grid around the iced surface needs to be regenerated at each ice accretion step and complex flow characteristics near the ice horn can be captured accurately with such high-quality grid. First, simulations on NACA 0012 airfoil are conducted to validate current ice accretion model. Under rime and glaze ice condition, the predicted ice shapes agree well with experimental results. The predicted temperature evolution at iced surface reveals that ice accretion is a complex process influenced by heat and mass transfer. Then the computations on GLC-305 swept wing are performed to study the ice accretion process on the three-dimensional surface. The results show a good agreement with the experimental data. At the leading edge, the overall convective heat transfer coefficient and droplet collection efficiency increase along the direction from the wing root to the wing tip. This leads to very distinct ice horns at different wing sections.

Experimental Study on LED Heat Dissipation Based on Enhanced Corona Wind by Graphene Decoration

To enhance the heat dissipation of high-power light-emitting diodes (LEDs), corona wind based on corona discharge was promoted. In this paper, we aimed at reinforcing the intensity of corona wind by modifying the needle electrode with graphene solution. To explore the differences in the graphene-coated electrode and the original bare electrode on the electrical characteristics, the intensity of corona wind, and LED heat dissipation, a corona wind cooling system and a thermal test system were built for the experimental study. The results showed that the graphene-coated electrode system could produce corona wind at a lower voltage. In addition, the discharge current of the graphene-coated electrode system is higher than that of the bare electrode system under the same discharge voltage. The graphene-coated electrode system can induce a faster corona wind and the highest speed is about 3.4 m/s, showing that it is more efficient than the bare electrode system in corona wind production. Moreover, the temperature drop of LEDs for the graphene-coated system is greater, which means that the corona wind cooling system can reduce the junction temperature of the LED chips effectively. Ultimately, as the discharge power increases, the average convective heat transfer coefficient increases. Indeed, the result of this paper can be used for the wide variety of the graphene-coated corona wind generator application as a promising technology for the LED heat management.

The convective heat transfer of fractal porous media under stress condition

The study of coupled convective heat transfer and deformation behavior in porous media is still a major scientific and engineering challenge, despite major technological advances in both theoretical and computational thermodynamics in the past two decades. Specifically, essential controls on convective heat transfer of porous media under stress condition are not yet definitive. In this paper, meaningful and reasonable quantitative models that manifest the most important fundamental controls on convective heat transfer of porous media under stress condition are proposed. Predictions of the normalized permeability using the theoretical models, derived from Hertzian contact theory and fractal geometry, agree well with available experimental data. The proposed model design specifically accounts for multiple key variables, including the influence of the microstructural parameters of porous media, including elastic modulus and Poisson's ratio, and as functions of rock lithology. The results presented here include (1) the heat transfer rate decreases with the increase of effective stress, and increases with the increased porous media elastic modulus or the increased power law index for a specific effective stress, (2) the heat transfer rate increases with the increase of pore fractal dimension Df0 or the increased ratio rmin0/rmax0 for a specific effective stress. However, the relationship between the convective heat transfer coefficient and effective stress with different pore fractal dimension Df0 is complicated and not monotonic, (3) The heat transfer rate and convective heat transfer coefficient increase as expected with the increase of Nusselt number (i.e. the parameter represents the convective heat transfer in the pore) for a specific effective stress. In general, fractal convective heat transfer models illustrate mechanisms that affect coupled convective heat transfer and deformation behavior of porous media. And, the model proposed here is intended to increase efficacy of reservoir development strategies.

Investigating the effect of nanoparticles diameter on turbulent flow and heat transfer properties of non-Newtonian carboxymethyl cellulose/CuO fluid in a microtube

PurposeAlthough many studies have been conducted on the nanofluid flow in microtubes, this paper, for the first time, aims to investigate the effects of nanoparticle diameter and concentration on the velocity and temperature fields of turbulent non-Newtonian Carboxymethylcellulose (CMC)/copper oxide (CuO) nanofluid in a three-dimensional microtube. Modeling has been done using low- and high-Reynolds turbulent models. CMC/CuO was modeled using power law non-Newtonian model. The authors obtained interesting results, which can be helpful for engineers and researchers that work on cooling of electronic devices such as LED, VLSI circuits and MEMS, as well as similar devices.Design/methodology/approachPresent numerical simulation was performed with finite volume method. For obtaining higher accuracy in the numerical solving procedure, second-order upwind discretization and SIMPLEC algorithm were used. For all Reynolds numbers and volume fractions, a maximum residual of 10−6 is considered for saving computer memory usage and the time for the numerical solving procedure.FindingsIn constant Reynolds number and by decreasing the diameter of nanoparticles, the convection heat transfer coefficient increases. In Reynolds numbers of 2,500, 4,500 and 6,000, using nanoparticles with the diameter of 25 nm compared with 50 nm causes 0.34 per cent enhancement of convection heat transfer coefficient and Nusselt number. Also, in Reynolds number of 2,500, by increasing the concentration of nanoparticles with the diameter of 25 nm from 0.5 to 1 per cent, the average Nusselt number increases by almost 0.1 per cent. Similarly, In Reynolds numbers of 4,500 and 6,000, the average Nusselt number increases by 1.8 per cent.Research limitations/implicationsThe numerical simulation was carried out for three nanoparticle diameters of 25, 50 and 100 nm with three Reynolds numbers of 2,500, 4,500 and 6,000. Constant heat flux is on the channel, and the inlet fluid becomes heated and exists from it.Practical implicationsThe authors obtained interesting results, which can be helpful for engineers and researchers that work on cooling of electronic devices such as LED, VLSI circuits and MEMS, as well as similar devices.Originality/valueThis manuscript is an original work, has not been published and is not under consideration for publication elsewhere. About the competing interests, the authors declare that they have no competing interests.

Experimental Investigation of Laminar Convection Heat Transfer of Al2O3-Ethylene Glycol-Water Nanofluid as a Coolant in a Car Radiator

In this experimental study, heat transfer of a coolant nanofluid, obtained by adding alumina nanoparticles to Ethylene Glycol-water (60:40 by mass), in a car radiator has been investigated. For this purpose, an experimental setup has been designed and constructed. Firstly, to investigate the accuracy of the results, the experiments have been done for the base fluid. Then the experiments have been performed for the nanofluid with different nanoparticles volume fractions of 0.003, 0.006, 0.009 and 0.012. To ensure laminar flow regime three coolant flow rates of 9, 11 and 13 lit/min have been tested. The thermophysical properties have been calculated using the recently presented temperature dependent models in the literature. According to the results, both the convective heat transfer coefficient and Nusselt number increase (about 9%) with increasing the coolant flow rate. Also, convective heat transfer coefficient increases with increasing the nanoparticles volume fraction. Although Nusselt number decreases when nanofluid is utilized, it enhances as the nanoparticles volume fraction increases. Based on the experimental results obtained, A new empirical correlation has been developed for average Nusselt number of Al2O3-EG-water nanofluid in developing region of flat tubes of car radiator for laminar flow and its maximum error is 3%.

Free surface flow and heat transfer characteristics of liquid metal Galinstan at low flow velocity

An effective method of obtaining a stable free surface flow with liquid metals is very important in terms of ensuring reliability of nuclear fusion devices. In this work, a miniature experimental apparatus was built and the characteristics of liquid-metal free surface flow and laminar convective heat transfer on different surface structures were experimentally investigated. The results show that, within a certain velocity range, Galinstan can flow in the form of an even liquid film on an acid-pickled stainless-steel surface. As the Galinstan flow rate increases, the convective heat transfer coefficient increases. Finally, a heat transfer correlation of free surface flow was proposed for the liquid-metal Galinstan.

Numerical thermal analysis of water's boiling heat transfer based on a turbulent jet impingement on heated surface

In this study, a numerical method for simulation of flow boiling through subcooled jet on a hot surface with 800°C has been presented. Volume fraction (VOF) has been used to simulate boiling heat transfer and investigation of the quench phenomena through fluid jet on a hot horizontal surface. Simulation has been done in a fixed Tsub=55°C, Re=5000 to Re=50,000 and also in different Tsub=Tsat−Tf between 10°C and 95°C. The effect of fluid jet velocity and subcooled temperature on the rewetting temperature, wet zone propagation, cooling rate and maximum heat flux has been investigated. The results of this study show that by increasing the velocity of fluid jet of water, convective heat transfer coefficient at stagnation point increases. More ever, by decreasing the temperature of the fluid jet, convective heat transfer coefficient increases.

Statistical Analysis of the Effect of Nanoparticles Volume Fraction on Turbulent Forced Convective Heat Transfer Coefficient of Nanofluid in a Circular Tube

In this paper, the statistical analysis of the effect of nanoparticles volume fraction on one of the most important thermal characteristics turbulent flow of nanofluid i.e. convection heat transfer coefficient, inside a circular tube with uniform wall heat flux is investigated numerically. Also, water as a base fluid and Al2O3 as suspended particles with a diameter of 36 nm are considered. Heat transfer characteristics are computed using the solution of elliptic equations based on discrete the finite volume method and the second order upwind. The relationship between pressure and velocity using SIMPLEC algorithm is established. In this study, the variation of volume fraction of nanoparticles is assumed in the range of 0 to 6%. The best probability distribution function of the heat transfer parameters are selected using chi square test that various probability distribution such as: Gamma, Normal, Lognormal, Gumbel, and Frechet are evaluated based on numerical analysis of tube flow. After reviewing the results, it was found that with increasing volume fraction of nanoparticles, the convective heat transfer coefficient increases. On the other hand, the convective heat transfer coefficients with regard to variation of volume fraction of nanoparticles follow Gumbel Max probability distribution function.

Numerical Study of the Effect of the Reynolds Numbers on Thermal and Hydrodynamic Parameters of Turbulent Flow Mixed Convection Heat Transfer in an Inclined Tube

A numerical investigation of the effect of the Reynolds number on the thermal and hydrodynamic parameters of mixed convection heat transfer of the water-Al2O3 nanofluid turbulent flow in an inclined circular channel is the subject of this article. The upper wall of the channel was under non-uniform heat flux and its lower part was isolated. The two-phase mixture model, the finite volume method, and the second-order upstream difference scheme were used to solve the governing equations. After reviewing the results, it was found that by increasing Reynolds number, the convective heat transfer coefficient and shear stress increase, but the surface friction coefficient decreases. In the case of Nusselt number and the surface friction coefficient, some equations were extracted.

Effect of ventilation on the thermal performance of tunnel lining GHEs

A tunnel lining ground heat exchanger (GHE) uses surrounding rock as heat source and transfers heat through absorber pipes buried in the tunnel lining. In order to evaluate the effect of ventilation on the thermal performance of tunnel lining GHEs, thermal response field tests considering ventilation were performed in Linchang tunnel, and a 3D numerical model of tunnel lining GHEs was built and verified with the field monitoring data. The study results show that (i) the ventilation has a significant influence on the temperature field of surrounding rock; (ii) the heat exchange rate rises exponentially as convective heat transfer coefficient (CHTC) increases, and it increases significantly with the decreasing thickness of secondary lining; (iii) the heat exchange rate presents a linear variation as the inlet temperature increases, the growth gradient of the heat exchange rate rises exponentially as the CHTC increases and decreases gradually with the running time increase.

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.

Experimental Study of Local Heat Transfer Coefficient of R113 Flow Boiling in Vertical Square Tube

This article is based on the experimental investigations on flow boiling heat transfer of R113 with the equivalent diameter of the square tube 15.66mm, mass flow rate 500 ~ 2000kg / (m2•s), steam quality from 0.1 to 1, heat flux 50 ~ 90kW/m2, and pressure 100 ~ 250 kPa. The experimental results show that with the rise of heat flux and mass velocity the local convection heat transfer coefficient increases, and with the enhancement of steam quality the local convection heat transfer coefficient increases firstly and then decreases. By comparison of the experimental data and the calculation results of predecessor’s correlations, a correction coefficient applicable to this experiment was obtained.