Showing papers on "Heat transfer coefficient published in 2018"

A comprehensive review of preparation, characterization, properties and stability of hybrid nanofluids

Abstract: Nanofluids are emerging as promising thermo fluids for heat transfer application. Nano fluids are two phase fluids of solid liquid mixture. The presence of solid nanoparticles in the base fluid increases significantly the effective thermal conductivity of the fluid and consequently enhances the heat transfer characteristics. Addition of single nanoparticle in base fluid, to improve the flow and heat transfer characteristics of the base fluid is a proven technology. In recent years, attention is focused on research studies involving impregnation of two or more nanoparticles in base fluids, called hybrid or composite nanofluids. Research studies on nanofluid containing composite nanoparticle showed better enhancement in thermal and rheological characteristics of base heat transfer fluid compared to mono nanoparticle based nanofluids. The research studies carried out on preparation, characterization, properties and stability of hybrid nanofluids are comprehensively reviewed in this paper. The models for properties such as thermal conductivity, viscosity, density, specific heat, friction factor and heat transfer coefficient of hybrid nanofluids are presented. The potential application and the challenges including stability methods and measures for hybrid nanofluids are discussed.

Thermal Management on IGBT Power Electronic Devices and Modules

Abstract: As an increasing attention towards sustainable development of energy and environment, the power electronics (PEs) are gaining more and more attraction on various energy systems. The insulated gate bipolar transistor (IGBT), as one of the PEs with numerous advantages and potentials for development of higher voltage and current ratings, has been used in a board range of applications. However, the continuing miniaturization and rapid increasing power ratings of IGBTs have remarkable high heat flux, which requires complex thermal management. In this paper, studies of the thermal management on IGBTs are generally reviewed including analyzing, comparing, and classifying the results originating from these researches. The thermal models to accurately calculate the dynamic heat dissipation are divided into analytical models, numerical models, and thermal network models, respectively. The thermal resistances of current IGBT modules are also studied. According to the current products on a number of IGBTs, we observe that the junction-to-case thermal resistance generally decreases inversely in terms of the total thermal power. In addition, the cooling solutions of IGBTs are reviewed and the performance of the various solutions are studied and compared. At last, we have proposed a quick and efficient evaluation judgment for the thermal management of the IGBTs depended on the requirements on the junction-to-case thermal resistance and equivalent heat transfer coefficient of the test samples.

The numerical modeling of water/FMWCNT nanofluid flow and heat transfer in a backward-facing contracting channel

Abstract: In recent years, the study of rheological behavior and heat transfer of nanofluids in the industrial equipment has become widespread among the researchers and their results have led to great advancements in this field. In present study, the laminar flow and heat transfer of water/functional multi-walled carbon nanotube nanofluid have been numerically investigated in weight percentages of 0.00, 0.12 and 0.25 and Reynolds numbers of 1–150 by using finite volume method (FVM). The analyzed geometry is a two-dimensional backward-facing contracting channel and the effects of various weight percentages and Reynolds numbers have been studied in the supposed geometry. The results have been interpreted as the figures of Nusselt number, friction coefficient, pressure drop, velocity contours and static temperature. The results of this research indicate that, the enhancement of Reynolds number or weight percentage of nanoparticles causes the reduction of surface temperature and the enhancement of heat transfer coefficient. By increasing Reynolds number, the axial velocity enhances, causing the enhancement of momentum. By increasing fluid momentum at the beginning of channel, especially in areas close to the upper wall, the axial velocity reduces and the possibility of vortex generation increases. The mentioned behavior causes a great enhancement in velocity gradients and pressure drop at the inlet of channel. Also, in these areas, Nusselt number and local friction coefficient figures have a relative decline, which is due to the sudden reduction of velocity. In general, by increasing the mass fraction of solid nanoparticles, the average Nusselt number increases and in Reynolds number of 150, the enhancement of pumping power and pressure drop does not cause any significant changes. This behavior is an important advantage of choosing nanofluid which causes the enhancement of thermal efficiency.

The study of heat transfer and laminar flow of kerosene/multi-walled carbon nanotubes (MWCNTs) nanofluid in the microchannel heat sink with slip boundary condition

Abstract: In this investigation, the laminar heat transfer of kerosene nanofluid/multi-walled carbon nanotubes in the microchannel heat sink is studied. The considered microchannel is two layers in which the length of bottom layer is truncated and is equal to the half of the length of bottom layer. The length of microchannel bottom layer is L = 3 mm, and the length of top layer is L 1 = 1.5 mm. The microchannel is made of silicon, and each layer of microchannel has the thickness of t = 12.5 µm. Along the external bottom wall, the sinusoidal oscillating heat flux is applied. The top external and lateral walls are insulated, and they do not have heat transfer with the environment. The results of this research revealed that in different Reynolds numbers, applying oscillating heat flux significantly influences the profile figure of Nusselt number and this impressionability is obvious in Reynolds numbers of 10 and 100. Also, by increasing the slip velocity coefficient on the solid surfaces, the amount of minimum temperature reduces significantly which behavior remarkably entails the heat transfer enhancement.

A review of heat transfer and fluid flow mechanism in heat exchanger tube with inserts

Abstract: Inserts are used to enhance the heat transfer rates between the two fluids in heat exchanger tubes. A variety of tube inserts such as twisted tape, wire coil, swirl flow generator have been investigated for their effect on heat transfer rates and fluid friction. This paper reviews the works pertaining to the application of different class of tube inserts in order to comprehend the prevailing mechanism of fluid flow and heat transfer. An attempt has been made to elucidate the fluid flow behaviour sustained by the particular class of insert that controls the heat transfer rates across the thermal boundary layer attached to the tube wall.

Thermal performance analysis of a flat heat pipe working with carbon nanotube-water nanofluid for cooling of a high heat flux heater

Abstract: Experimental investigation on the thermal performance of a flat heat pipe working with carbon nanotube nanofluid is conducted. It is used for cooling a heater working at high heat flux conditions up to 190 kW/m2. The heat pipe is fabricated from aluminium and is equipped with rectangular fin for efficient cooling of condenser section. Inside the heat pipe, a screen mesh was inserted as a wick structure to facilitate the capillary action of working fluid. Influence of different operating parameters such as heat flux, mass concentration of carbon nanotubes and filling ratio of working fluid on thermal performance of heat pipe and its thermal resistance are investigated. Results showed that with an increase in heat flux, the heat transfer coefficient in evaporator section of the heat pipe increases. For filling ratio, however, there is an optimum value, which was 0.8 for the test heat pipe. In addition, CNT/water enhanced the heat transfer coefficient up to 40% over the deionized water. Carbon nanotubes intensified the thermal performance of wick structure by creating a fouling layer on screen mesh structure, which changes the contact angle of liquid with the surface, intensifying the capillary forces.