جستجوی مقالات مرتبط با کلیدواژه « heat transfer » در نشریات گروه « فنی و مهندسی »

Heat exchangers are widely used in various industries, including power plants, solar cells, refineries, and automobiles. One of the simplest types of heat exchangers used in the industry is the double-pipe heat exchanger. In the present study, nanofluid flow in a double-pipe heat exchanger, with a square inner pipe and a circular outer pipe (sc), is simulated and investigated using computational fluid dynamics under constant heat flux and within the range of laminar and turbulent flow regimes. To verify the accuracy of the numerical study in both laminar and turbulent flows, the numerical calculation results for water flow with forced convection were compared with reference results. The results showed that with an increase in the Reynolds number, particularly in the turbulent flow regime, the Nusselt number in nanofluid flow increased more than in water flow. For instance, for aluminum oxide nanofluid flow with a Reynolds number of 500, the Nusselt number increases by nearly 5% more than water flow, and for a Reynolds number of 20000, this Nusselt number increase is close to 20% more. To investigate the impact of nanoparticle type on heat transfer and pressure drop, three types of nanoparticles were considered. The results indicated that the use of nanoparticles had a slight effect on the friction coefficient while significantly enhancing heat transfer.

Waste heat dissipation from spacecraft subsystems is crucial due to spatial limitations (no convective heat transfer, limited electrical power, lightweight, and high reliability). Radiators are often responsible for collecting and dissipating this waste heat into cold space. Panel radiators are widely used in various satellites, including scientific, communication, and remote sensing satellites. In scientific satellites, panel radiators are used to dissipate heat generated by scientific instruments such as cameras and spectrometers. In communication satellites, panel radiators are used to dissipate heat generated by power amplifiers. In remote sensing satellites, panel radiators are used to dissipate heat generated by sensors and processors. High efficiency, light weight, and high reliability are the advantages of using this equipment. The main challenge in using it is to provide sufficient heat dissipation area and uniformity of the surface temperature when radiating waste heat to the cold environment (space). The use of heat pipes in the panel radiator structure provides this uniformity. A heat pipe radiator consists of a sandwich panel with an embedded network of heat pipes. Increasing the number of heat pipes reduces the temperature gradient across the radiator surface but increases the radiator weight. Due to the importance of equipment lightness in space systems, optimization of the number of pipes and their geometric arrangement in the radiator should be such that maximum temperature uniformity on the surface and minimum radiator weight are achieved. The objective of this research is to optimize the performance of a radiator (maximum temperature uniformity on the surface) to achieve minimum weight while considering the weight and size constraints imposed by the system designer as requirements. Initially, a mathematical model is developed and solved numerically, and the effect of design parameters on the performance of a panel radiator, including face and core thickness, spacing between heat pipes, mass, and surface area, is comprehensively investigated. Based on the simulation results, considering the weight limitations and existing face and core thicknesses, the maximum allowable spacing between heat pipes is calculated to achieve maximum efficiency of the panel radiator. A network of heat pipes with this characteristic was produced and used in the panel sandwich. The results obtained from testing the manufactured panel radiator were compared with the design efficiency to validate the model. Based on the experimental results, an efficiency of 89% was obtained at a root temperature of 39°C. The error of this efficiency with the efficiency calculated from the theory is about 3%.

It is not possible to fabricate the complex geometry of coherent cooling channels with conventional machining methods, so channels can be created in the mold using additive manufacturing processes such as selective laser melting. Parts created by selective laser melting always have porosity, and the amount of porosity depends on process parameters. On the other hand, the ability to make porous materials with a selective laser melting process has made these materials with features such as lower density and better heat transfer in the aerospace industry, automobile, medical uses and heat exchangers to be the attention of researchers and according to Porosity Percentage, in addition to directly affecting mechanical properties, also affects heat transfer. In this research, the effect of porosity on heat transfer in the mold was investigated. First, the model and mold were designed, in order to investigate the effect of porosity, four simulation models with volume porosity percentage of 0, 10, 20 and 30% were performed and analyzed in the software. The analysis of the results shows that the increase in the percentage of porosity in the mold causes a faster increase in temperature in the mold, also with the increase in the percentage of porosity in the mold, the speed of temperature decrease in the mold increases. And the cooling of the part happens faster. Examining the results of the maximum thermal gradient of the non-porous material compared to the material with 30% porosity shows a 21% increase in the thermal gradient in the porous material

This paper aims to experimentally investigate the partial premixed combustion in a structure ceramic porous burner for cooking applications. The thermal efficiency, pollutant emission, heat transfer rate and burner performance map are assessed. Moreover, in the analysis of the thermal efficiency behavior, the contribution of the convective and radiative heat transfer rates is separately studied. In this study, aluminum containers with dimensions according to Iran's national standard have been used in a wide range of thermal power range of 1.83-9.83 kW and equivalence ratio of 0.5-1.2. Natural gas has been used as the most used and most common fuel in domestic gas stoves. The results show that with the increase of the equivalence ratio from dilute to rich combustion, the shape and color of the flame changes significantly and the blue flame turns into a yellow flame with longer radiation. The best working point of the burner in terms of thermal efficiency has a thermal power of 1.83 kW and an equivalence ratio of 0.7, which is 58.33%. At this working point, the emission rate of CO pollutants is lower than the national standard of Iran, and the amount of NO_x is below 1ppm in all tests.

In this paper, the numerical simulation of combustion in one of the industrial furnaces of Kermanshah Oil Refining Company has been carried out in order to investigate the cause of high temperature damage to its damper and provide a solution to solve this problem. At first, the current situation has been investigated. For this purpose, the numerical model of furnace and burner has been produced and numerical simulations have been carried out to check the current situation. The agreement between the actual factory data and the numerical simulation data is acceptable. In the next step, by changing the burner and changing the shape of the flame, numerical simulations have been performed in order to solve the problem. The obtained results show that by changing the burner and using 4 smaller burners but with a longer flame, the shape of the flame becomes branched and with the increase of heat transfer caused by this situation, the temperature at the place where the damper is installed decreases by about 170 degrees Celsius. This decrease in temperature can solve the problem of damaging the damper and softening the structure connecting it to the flue.
 
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In this work, thermal performance improvement of the latent heat energy storage system using wavy fins and change in the heat transfer fluid tube geometry is numerically investigated. The investigated system is a shell and tube heat exchanger and RT35 paraffin is used as the phase change material. With the constant total cross-section area of the tube and fins as well as the volume of the phase change material, the effect of heat transfer fluid tube shape and the designed fin on the melting process of the phase change material is investigated. Numerical simulation results show that wavy fins accelerate the phase change process compared to conventional straight fins. The complete melting time of the phase change material is reduced by 9.33% compared to the straight fin. Changing the geometry of the heat transfer fluid tube from the circle to the petal shape is proposed to further improve the thermal performance of the latent heat energy storage system. The melting rate of the phase change material increases with the increase in the number of petals. The complete melting time for the tube with seven petals is reduced by 66.7% compared to the circular tube with wavy fins and 69.8% compared to the base case with straight fins.

In this study, the cooling of electrical panels was numerically and experimentally investigated to control the temperature distribution inside the sealed enclosures. Cooling systems usually operate in open cycles; however, in this study, a closed-air cycle cooling system is proposed, which protects sensitive electronic equipment from dust and moisture. To create a closed air cycle, a vertical duct was installed on the front side of the electrical panel and a fan was used to circulate the air inside it. The results of this research show that the airflow inside a sealed electrical panel can provide safe thermal conditions for internal parts owing to the elimination of high-temperature points. Under the appropriate conditions, the average temperature of the elements inside the panel decreased by 50%. In this situation, the average lifetime of electrical components, such as breakers, f

 The present study investigates the ability to improve the stability and heat transfer performance of water coolant fluid by dispersing aluminum oxide nanoparticles in water. Surfactant-free nanofluid with 4 different volume fractions of 0.05%, 0.5%, 1% and 2% were prepared by two-step method. The stability of the nanofluid was monitored by continuous imaging (visualization) and dynamic light scattering (DLS) techniques. Thermal conductivity, viscosity and density of aluminum oxide-water nanofluid were measured in 4 concentrations and at temperatures of 25, 35 and 45 °C. The results showed that the thermal conductivity and viscosity of aluminum oxide-water nanofluid increases with rising concentration. The viscosity of nanofluids in all concentrations were higher than the base fluid. This is while the increase in viscosity was almost independent of the increase in temperature. The highest percentage increase in the thermal conductivity of nanofluid is equal 49%, which was obtained at a concentration of 2% and a temperature of 45 °C. New correlations with high precision were presented based on the experimental data of thermal conductivity and viscosity of nanofluids. The heat transfer performance and pumping power of nanofluid were analyzed based on several performance criteria. The advantageous results of increasing the heat transfer capability made aluminum oxide-water nanofluid a potential candidate for cooling fluid in practical applications.

Condensation occurs in most heat transfer processes, from cooling electronic devices to heat removal in power plants. The overall heat transfer coefficient of dropwise condensation (DWC) is several times higher than that of filmwise condensation (FWC). Therefore, obtaining a stable DWC is very important for better performance. DWC stability depends on surface hydrophobicity, surface free energy, and surface tension of the condensed liquid. The properties required for DWC may be achieved by micro-scale surface modification. In this review, micro/nanoscale coatings such as noble metals, ion implantation, rare earth oxides, lubricant-injected surfaces, polymers, nanostructured surfaces, carbon nanotubes, graphene, and porous coatings have been reviewed and discussed. Surface coating methods, applications, and potential have been compared with respect to heat transfer ability, durability, and efficiency. In addition, common limitations and challenges for densification enhancement applications are summarized to provide future research directions.

In this study, the flow field and the impingement heat transfer of fluidic oscillators at narrow spaces are investigated numerically. Simulations are performed in 2-D, incompressible, and unsteady conditions and the aim is to analyze the effects of the jet to wall distance, the external nozzle angle, the Reynolds number, and removing the external nozzle on the heat transfer performance. Also, for a comprehensive review, the results of the fluidic oscillator are compared with the results of the steady jet. To ensure the validity of the numerical simulations, two experimental researches are applied for the fluidic oscillator and the steady jet and a good agreement is observed between the present simulations and the experimental data. The results show that increasing the distance in fluidic oscillators causes a maximum decrease of about 11% at the Nusselt number of the stagnation point, while employing the various distances does not have a significant effect on the steady jet. In addition, the configuration of the different external nozzle angles affects the Nusselt number, but this influence does not have a monotonic behavior. Furthermore, the Nusselt number increases by removing the external nozzle. When the Reynolds number increases for the fluidic oscillator and the steady jet, the Nusselt number of the stagnation point increases by at least about 22 and 28%, respectively.