Natural Convection Heat Transfer Coefficient Research Articles

Numerical study of the melting process of a novel phase change material using a rectangular shell-coil heat exchanger

This paper presents the numerical study of the melting process of hydrogenated palm stearin as a new phase change material (PCM) using a shell-coil heat exchanger. A phenomenological model was proposed and validated using experimental results. After model validation, numerical simulation was used to study the influence of natural convection on the melting process and stored energy in a rectangular shell-coil heat exchanger. The influence of operational variables such as the inlet velocity and the temperature of the heat transport fluid (HTF) on the melting time was studied with an analysis of variance (ANOVA) with inlet temperatures of 65 °C, 75 °C and 85 °C and inlet velocities of 1 l·min−1, 2 l·min−1 and 3 l·min−1. The ANOVA indicated that the inlet temperature has the greatest influence on the melting time, while the inlet velocity has no significant effect. For the study of the influence of natural convection, the natural convection heat transfer coefficient was determined, showing that the upper zones have the greatest influence of natural convection, achieving coefficients of 61.8, 39.1 and 66.8 W·m−2·°C−1. In addition, using a rectangular shell-coil heat exchanger presents a more uniform melting process in the lower zones, allowing the melting of all the PCM. According to simulations, two streams of movement of the liquid phase of the PCM are formed in the center of the system. These results shows that hydrogenated palm stearin has a high capacity for energy store, reaching 306.46 kJ·kg−1.

Lattice Boltzmann simulation of effects of realistic boundary conditions on volumetric radiation‐conduction melting of a novel cylindrical enclosure filled with phase change materials

AbstractIn this research, a novel solar latent heat thermal energy storage (LHTES) system, including the cylindrical enclosures filled with a phase change material (PCM), is proposed, which can be installed on the building windows to alleviate the drawbacks of traditional PCM‐filled double‐glazed windows, such as daylight hindrance and leakage. The lattice Boltzmann method (LBM) is used to simulate the volumetric radiation‐conduction melting of the PCM within a single cylinder of the proposed LHTES system with considering more realistic conditions such as convective boundary condition, shadow effect, and variable solar radiation angle compared with the available works in the literature. As such, several boundary conditions are assessed, and parameters such as cylinder diameter, extinction coefficient, scattering albedo, solar angle, shadow effect, and natural convection heat transfer coefficient are studied on the time history of the melting fraction and charging time. The results revealed that considering the applied conditions, such as convection heat loss to the environment and shadow, significantly affects the charging time of the system. It is shown that the charging time for convective boundary condition with , , and increases, respectively, by 11%, 30%, and 50% relative to a case with the insulated boundary condition without the shadow effect and 38%, 91%, and 175% compared with the insulated case with a 90° shadow.

A comparative numerical study of three similar passive solar stills: Single slope, V-type, and greenhouse

In this paper, three solar stills with similar geometries are numerically investigated for analysis and comparison. The solar stills include single slope, V-type, and greenhouse stills. Steady-state laminar flow numerical simulations with different water surface temperatures and cover glass temperatures are performed. Simulation results were confirmed with data from analytical and experimental models to ensure reliability. The results showed that the V-type still has a higher number of recirculation zones than that of single slope and greenhouse stills. These vortex regions are small, but the velocity magnitude is no less than the other two stills. This difference makes the freshwater yield and convection heat transfer coefficient of V-type still the largest. Daily freshwater productions of single slope, V-type, and greenhouse stills are 0.592, 0.673, and 0.623 kg/m2, respectively. Productivity at 15 PM is 2.5 times higher than the hours from noon to 14 PM. The natural convection heat transfer coefficient seemed to be unvaried with the temperature difference but changed strongly with the still geometry.

Effects of Brownian Motions and Fractal Structure of Nanoparticles on Natural Convection

The study simulated heat transfer in alumina-water nanofluid in a natural convection flow and Rayleigh-Benard configuration considering the Brownian motions and fractal structure of the nanofluids. The simulations were based on a two-dimensional, Eulerian-Eulerian method. Many simulations have been performed to examine the effect of aspect ratio, heat flux, and para-meters related to the structure of the nanoclusters including size, fractal dimension, and volume fraction on the natural convective heat transfer coefficient. The comparison between the simulation results and the experimental data of heat transfer coefficient indicates a good agreement. The simulation results indicated that the enhancement of aspect ratio, heat flux, and fractal dimension increases the heat transfer coefficient. On the other hand, the reduction of nanoclusters and nanoparticle size decreased this coefficient. Moreover, the simulation results showed that in high heat transfer fluxes, the heat transfer coefficient first increases by increasing the nanoparticles solid volume fraction and then decreases. However, heat transfer coefficient decreased steadily with the increase in the nanoparticles solid volume fraction in low heat transfer fluxes. The results suggested that using the nanoparticles Brownian motion mechanism along with their fractal structure can be well-applied in natural-convection heat transfer modelling of nanofluids.

Structure optimization and operation characteristics of metal gas storage device based on compressed air energy storage system

Compressed air energy storage (CAES) is a key technology for promoting the replacement of fossil fuels with renewable energy. Currently, CAES systems typically require an underground salt cavern, which is limited by geological and geographical conditions. To promote the rapid popularization of CAES, there is an urgent need to develop new gas storage devices with flexible layouts. Metal pressure vessel as a gas storage device is concerned with the benefits of high storage pressure and reliable operation. Considering both strength and fatigue, this study conducted a structural optimization of Tank-D2000 and Pipe-D400 to achieve the design operating life with the minimal quality. Furthermore, the optimized structure was used to study the operating characteristics of the gas storage device by the multi-domain fluid–solid thermal coupling method and the real gas model. The results indicated that the internal convective heat transfer coefficient of the gas storage device was a linear function of time during operation, whereas the external natural convective heat transfer coefficient almost remained constant. The pressure distribution was uniform; however, the temperature difference was large, particularly for D400. Analysis and prediction based on the theory of the polytropic process showed that the polytropic index of D400 was lower than that of D2000, so the heat transfer of D400 was more adequate and the effective capacity of gas storage device under the designed operating pressure range was larger. With an increase in flow, polytropic index increased and was lower at energy storage than at energy release. The pressure prediction error was basically within the range of ±4 %, and the temperature prediction error was within the range of ±3 %.

Experimental investigation on the impact of fin structure factors on the finned-Trombe wall thermal performance

By increasing surface area, fins can enhance the heat transfer and improve the Trombe wall thermal performance. However, there is still a lack of detailed experimental research on the impact of fin structure factors on the thermal performance of finned-Trombe wall (F-TW). This experiment establishes a platform under laboratory conditions to investigate the impact of fin height and spacing on the F-TW thermal performance. Electric heating is used to simulate the solar radiation. The results indicate that in areas with relatively low solar radiation intensity, the F-TW is more cost-effective for improving F-TW thermal performance. The F-TW thermal parameters including air temperature rise, mass flow rate, thermal efficiency and overall natural convective heat transfer coefficient are enlarged as the fin height increases and fin spacing decreases, while the heat loss coefficient of glass cover is opposite. The fin spacing has a more significant influence on the F-TW thermal performance and natural convective heat transfer in the channel than the fin height does. The maximum thermal efficiency of 63.75% is obtained at heat flux of 860W/m2, relative fin height of 0.75 and relative fin spacing of 0.06. Finally, for the engineering design and performance assessment of F-TW, the experimental correlation is provided to calculate the average natural convective heat transfer Nusselt number in the channel.

Research on infrared nondestructive testing and thermal effect analysis of small wind turbine blades under natural excitation

In recent years, with the increase of wind power generation and the rapid growth of the number of wind turbines installed, the safety monitoring of wind turbine blades has also attracted wide attention. To further study the thermal effect of wind turbine damaged blades under natural excitation in summer in northern China, based on the outdoor infrared non-destructive testing of wind turbine blades, a numerical simulation of wind turbine blades with wear damage is carried out by using the fluid solid heat transfer theory in COMSOL multi-physical field coupling software. The best detection light condition is obtained, in which the detection effect of wear damage is the best when the light intensity reaches 1000 W.m−2 or above at noon. The simulation results are basically in agreement with the experimental results. And compared with the previous physical modeling method using natural convection heat transfer coefficient to replace wind speed, the physical modeling method considering wind speed and air humidity is more consistent with the experimental results, the maximum error value was decreased by 7.46 %, which verifies the rationality of the model and the feasibility of the method. In addition, the blade works in the natural environment for a long time, the thermal stress will have a significant impact on its fatigue strength, so on the basis of this model, the coupling analysis, and then analyze the change of thermal stress in different wind speed. With the increase of wind speed, the temperature on the blade surface gradually decreases and the wind pressure gradually increases. Because the influence of temperature on the stress is greater than that of wind pressure, the stress also shows a gradual downward trend.

Convective Heat Transfer Coefficient of Insulation Paper–Oil Contact Surface of Transformer Vertical Oil Channel

Insulation paper is the primary inter-turn insulation material for transformer windings. However, insulation paper is a poor conductor of heat and seriously impacts the natural convective heat dissipation in the winding oil channels. In order to study the convective heat transfer performance of the transformer vertical oil channel, the heat transfer characteristics of the two-dimensional boundary layer of the insulation paper–oil contact surface were analysed, and a characteristic number equation with a roughness correction factor was established. Based on the similarity principle and modelling theory, an experimental apparatus was designed to determine the convective heat transfer coefficient of the insulation paper–oil contact surface. Coefficients were obtained for different qualitative temperatures and characteristic lengths. Parametric fitting of the experimental data was carried out to obtain the correction factors. The results show that the natural convective heat transfer coefficient h of the insulation paper–oil contact surface is positively and linearly related to qualitative temperature and inversely related to characteristic length. For temperatures of 312.9 K to 328.1 K and characteristic lengths of 3 cm, h ranges from 116.15 to 144.38 W/(m2 · K). For temperatures of 319.0 K to 337.9 K and lengths of 6 cm, h ranges from 103.79 to 131.14 W/(m2 · K). The correction factor of the characteristic equation coefficient is 1.067. The maximum deviation of h calculated with a coefficient of 0.63 and an exponent of 0.25 is 5.93%. The results are essential for modelling the transformer space thermal circuit and solving the hot-spot temperature problem.

Calculation of natural convection heat transfer coefficient towards LNG based on finite element method

Calculation of natural convection heat transfer coefficient towards LNG based on finite element method

Reducing heat loss from solar hot water storage tanks using passive baffles

Solar water heating systems with thermal storage are one of the simplest ways of reducing energy demand for domestic water heating. Over the years, researchers have attempted to improve the thermal performance of storage tanks using various means, including baffle-type devices to control mixing during charging and discharging of the tank. However, one modification that has been overlooked is the use of passive baffles that suppress the natural convection inside the tank with a view to reducing their standing heat loss.Considering this, this study examined the use of concentric vertically mounted baffles on the natural convection inside a vertical cylindrical storage tank. To this end, a numerical model of a tank with a baffle was developed using computational fluid dynamics (CFD) and validated using experimental data. The results showed that a simple concentrically mounted baffle could reduce the natural convection heat transfer coefficient by up to 40%.

Numerical assessment of the impact of transverse roughness ribs on the turbulent natural convection in a BIPV air channel

The effect of varying rib geometry on the turbulent natural convective heat transfer coefficient in a BIPV air channel was assessed numerically. The CFD model was validated with experimental data from the literature to within reasonable accuracy. The study investigated parameters such as the rib shape, dimensionless rib height (e/D), dimensionless rib pitch (p/e), and channel inclination angle (θ). In each case, the heat flux is varied from 100 W/m2 – 1000 W/m2. The natural convection heat transfer coefficient on the heated wall is maximized for a vertical channel with triangular rib of e/D = 0.16 and p/e = 8.84. The enhancement in heat transfer afforded by the transverse ribs translates to a reduction of the summer heat gain of up to 35.5%. The natural convection heat transfer coefficient correlates with Ras*(S/H), e/D, p/e and θ to within 15% error.

Investigating different types of fluids in pulsating heat pipes for improving heat transfer in an air-cooled heat exchanger

Air-cooled heat exchangers are utilized in various industries to cool process fluids. To have a better heat transfer, fins are used in the structure of exchangers. In this study, a set of experiments were performed to investigate the improvement in the amount of heat transfer in an air-cooled heat exchanger. For this purpose, a real case study was conducted on air-coolers in a natural gas compressor station; afterward, the finned tubes were replaced by pulsating heat pipes (PHPs), and a different type of fluid was applied to the pulsating heat pipes. The laboratory system was set up to measure the overall heat transfer coefficient under two conditions involving either finned tubes or a PHP with R134, R22, and filled acetone fluids. Finally, the results were verified by the theory method. The results showed that all the three fluid agents (R134, R22, and Acetone) with natural convection of 9.94 m/s, an inlet fluid velocity of 5.54 m/s, and an average feed velocity were able to enhance the heat transfer coefficient by 10, 13, and 11%, respectively. In addition, compared with finned tubes, R134, R22, and Acetone improved the natural convection heat transfer coefficient by 22.2, 18.6, and 21.7%, respectively.

Effect of structural characteristics on the natural convective heat transfer performance of copper foam

• Copper foam with highly controllable structure was manufactured. • The relationship between structure and heat transfer performance was studied. • When the porosity is > 78%, the heat transfer performance drops sharply. • The optimal porosity and pore size for convective heat transfer have been found. • Side-heated foams exhibit better heat transfer than the bottom-heated foams. The effects of porosity, pore diameter and heating direction on the heat transfer performance of copper foam under natural convection were experimentally investigated. The open cell copper foam with highly controllable porosity ranging from 65% to 85% and mean pore diameters of 330 μm, 560 μm, 850 μm and 1200 μm were manufactured using a space holder method. The natural convective heat transfer coefficient of the copper foam was measured using a purpose-designed device. The influence mechanism of porosity, pore diameter and heating direction on heat transfer was analysed and discussed. The results showed that when the porosity is less than 75%, the effect of porosity on heat transfer is not significant, but as the porosity increases further, the heat transfer performance decreases significantly. The optimal pore size was found to be 850 μm, while the heat transfer performance of other pore sizes depends on the porosity. It was also found that the samples heated from the side generally showed better heat transfer performance than the samples heated from the bottom. The influence of porosity and pore size on natural convective heat transfer performance is mainly caused by changes in thermal conductivity and permeability. This research provides important guidance for the design of future heat sinks.

Numerical analysis of biothermal-fluids and cardiac thermal pulse of abdominal aortic aneurysm.

Abdominal aortic aneurysms are serious and difficult to detect, conditions can be deadly if they rupture. In this study, the heat transfer and flow physics of Abdominal Aortic Aneurysm (AAA) were discussed and associated with cardiac cycle to illustrate the cardiac thermal pulse (CTP) of AAA. A CTP and infrared thermography (IRT) evaluation-based on AAA and abdomen skin surface detection method was proposed, respectively. Infrared thermography (IRT) is a promising imaging technique that may detect AAA quicker and cheaper than other imaging techniques (as biomarker). From CFD rigid-wall and FSI Analysis, the transient bioheat transfer effect resulted in a distinct thermal signature (circular thermal elevation) on the temperature profile of midriff skin surface, at both regular body temperature and supine position, under normal clinical temperature. However, it is important to note that thermography is not a perfect technology, and it does have some limitations, such as lack of clinical trials. There is still work to be done to improve this imaging technique and make it a more viable and accurate method for detecting abdominal aortic aneurysms. However, thermography is currently one of the most convenient technologies in this field, and it has the potential to detect abdominal aortic aneurysms earlier than other techniques. CTP, on the other hand, was used to examine the thermal physics of AAA. In CFD rigid-wall Analysis, AAA had a CTP that only responded to systolic phase at regular body temperature. In contrast, a healthy abdominal aorta displayed a CTP that responded to the full cardiac cycle, including diastolic phase at all simulated cases. Besides, the findings from FSI Analysis suggest the influence of numerical simulation techniques on the prediction of thermal physics behaviours of AAA and abdominal skin surface. Lastly, this study correlated the relationship between natural convective heat transfer coefficient with AAA and provided reference for potential clinical diagnostic using IRT in clinical implications.

Effect of stabilization method on the natural convection in an inclined cavity filled with MWCNTs/water nanofluids

The present work has been studied to investigate the effect of stabilization method on natural convection in an inclined cavity filled with MWCNTs/ water. Two cubic enclosures with different aspect ratio and slopes are tested. Non-covalent and covalent functionalization of MWCNT have been done by using gum Arabic (GA) and the amino acid Ethylene diamine (EDA) respectively. The natural convection heat transfer coefficient decreases with incrementing the concentration of nanoparticles. However, the slope of reducing of the heat transfer coefficient in lower concentrations is small and by increasing concentration, this slope enhances. The effect of changing the Rayleigh number is lower than the effect of incrementing the concentration of CNT on the Nusselt number. By reducing the inclination angle from 90° to 0°, the convection heat transfer increases for both covalent and non-covalent functionalized nanoparticles. Also, it was found that changing the inclination angle does not affect the Nusselt number at the low Rayleigh numbers. With incrementing the Rayleigh number and changing the inclination angle, changing of the heat transfer coefficient at Ar = 2 was more than Ar = 1. The free convection heat transfer coefficient of the non-covalent method had more reduction in comparison to the covalent method.

A numerical study of supercritical carbon dioxide as a medium for thermal energy storage applications under natural convection

This numerical study investigates supercritical carbon dioxide (s-CO2) as a medium for thermal energy storage (TES) systems operating under natural convection regarding heat transfer coefficient and energy density. The numerical domain comprises a horizontal heated cylinder surrounded by the supercritical fluid. The modeling employs a 2-D formulation due to the axial symmetry while the constitutive equations consider that the thermophysical properties vary spatially with temperature and pressure. The results revealed thermodynamic states that maximize the heat transfer coefficient and energy density for constant-pressure operation. Besides, considering the numerous studies on correlations for the natural convection heat transfer coefficient for s-CO2, the analysis compares simulation data with correlations based on constant and integrated-averaged fluid properties. The results suggested that the constant-property formulation fails in the thermodynamic region where properties change significantly. Thus, this work proposes a criterion for deciding when to consider integrated-averaged properties regarding the behavior of the isobaric thermal expansion coefficient. Finally, the study extended the analysis to air, helium, and nitrogen while employing operational conditions suitable for power cycles using such fluids. The comparisons revealed that, among these competitors, helium presents the highest heat transfer coefficient, but s-CO2 has the highest energy density.

The use of passive baffles to increase the yield of a single slope solar still

Despite their relative simplicity, single slope solar stills remain one of the most viable solutions to the production of freshwater from saline and non-potable water sources. Over the years, numerous researchers have attempted to improve the yield of single slope solar stills through changes to their geometry and the use of various design modifications. Despite this long history of research, one modification that has not been fully explored is the use of passive baffles to alter the natural convection inside them.Though baffles have frequently been used to suppress natural convection in enclosures, there may also be the opportunity to use them to enhance natural convection. This judicious placement of baffles to enhance the natural convection could potentially improve the yield from single slope solar stills, however it has not been well explored in the literature.To address this issue, this work examined the effect of vertically mounted passive baffles on the natural convection inside a single slope solar still geometry. In this vein, a computational fluid dynamics simulation was conducted for a still containing a baffle, with the results validated experimentally using particle image velocimetry. On this basis, subsequent simulations explored the effect of baffle length and position on stills with cover angles from 10–60°. The results showed that baffles did have a marked influence on the natural convection flow field and could increase the natural convection heat transfer coefficient, which is directly proportional to a solar still’s yield, by up to 20%.It was also found that short baffles enhanced natural convection at low cover angles, while baffles mounted close to the front of a still provided an improvement to the transfer processes over the widest range of conditions. Finally, the work led to the development of a relationship to describe the effect of baffles on natural convection, as shown in Equation (1), that it is hoped will aid designers of single slope solar stills in the future.(1)Nu'=Ra'0.188∗AR-0.29∗cosθ-0.5∗bB0.08∗dD0.09

Improvement of Condensation Model With the Presence of Non-Condensable Gas for Thermal-Hydraulic Analysis in Containment

Steam condensation plays a key role in prediction of the pressure behavior and hydrogen distribution in the containment during a hypothetical loss-of-coolant accident or a severe accident in a light water nuclear reactor. The objective of this study is to evaluate and improve the condensation model in GASFLOW code. Reynolds analogy coupled with wall function and Chilton-Colburn empirical analogy is used to model heat and mass transfer in GASFLOW, which has requirements for dimensional distance of the first cell near the wall and some deficiencies in description of heat and mass transfer process in the stagnant zone. Based on the evaluation of original condensation, the results shows good agreement with COPAIN experiment cases where the mass fraction of air ranges from 76.7 to 86.4%. However, with the changes in geometry of the facility and the presence of helium, the original model has a large deviation in the prediction of pressure, temperature and gas distribution compared with MISTRA ISP47 (OECD International Standard Problem No. 47) experiment data. This work proposes a modified condensation model which uses McAdams correlation and Schlichting correlation with a weight factor to calculate natural, forced, or mixed convection heat transfer coefficient, and adopts Chilton-Colburn empirical analogy to model mass transfer. The modified model has no requirement for the dimensionless distance near the wall in heat and mass transfer calculation and improves the prediction performance of heat transfer in stagnant zone. The prediction result of the modified model shows good agreements with MISTRA ISP47 problem, and the error of it compared with COPAIN experiment data is within 25% which is the same as that predicted by the original model.

Heat Transfer Scale Effect Analysis and Parameter Measurement of an Electrothermal Microgripper.

An electrothermal microgripper is an important actuator in microelectromechanical and micro-operating systems, and its temperature field analysis is the core problem in research and design. Because of the small size of an electrothermal microgripper, its microscale heat transfer characteristics are different from those of the macrostate. At present, only a few studies on the heat transfer scale effect in electrothermal microgrippers have been conducted, and the heat transfer analysis method under the macrostate is often used directly. The temperature field analysed and simulated is different from the actual situation. In the present study, the heat transfer mechanism of an electrothermal microgripper in the microscale was analysed. The temperature field of a series of microscale heating devices was measured using microthermal imaging equipment, and the heat transfer parameters of the microscale were fitted. Results show that the natural convective heat transfer coefficient of air on the microscale can reach 60–300 times that on the macroscale, which is an important heat transfer mode affecting the temperature field distribution of the electrothermal microgripper. Combined with the finite element simulation software, the temperature field of the electrothermal microgripper could be accurately simulated using the experimental microscale heat transfer parameters measured. This study provides an important theoretical basis and data support for the optimal design of the temperature controller of the electrothermal microgripper.

Estimation of convective heat transfer coefficient in agricultural products under solar drying conditions

This research estimated convective heat transfer coefficients of agricultural products (banana, taro and chili) under solar (open sun) drying conditions using the Nu = h c(X/K v) = C(Ra) n equation. Values of constants C and n were obtained by linear regression analysis. Results showed that natural convective heat transfer coefficients of agricultural products were in the range 0.720-3.559 W/m2·oC. The C value of agricultural products was 1.0000-1.0011, while n was in the range 0.071-0.274, Pr = 0.697-0.705 and 2.931×105-1.803×108 < Gr < 6.139×108-2.230×109.