جستجوی مقالات مرتبط با کلیدواژه « شار حرارتی بحرانی » در نشریات گروه « مکانیک »

Due to the growing need of industries to improve heat transfer methods, the nucleate boiling region is of special interest to researchers because fluid boiling makes it possible to achieve higher heat fluxes than processes that are based only on non-boiling convection heat transfer. In addition, the change of working fluid in boiling heat transfer systems from pure fluid to nanofluid improves the heat transfer characteristics and thus allows the transfer of higher heat flux at lower temperatures. At the critical heat flux point, where the heat flux is maximum, it is important to secure the boiling surface and protect it from overheating. The purpose of this study is to numerically simulate the pool boiling of a water-based hybrid nanofluid containing 30% multi-walled carbon nanotubes and 70% titanium oxide with a volumetric concentration of 0.5% on a polished copper circular surface and to investigate the heat transfer characteristics, especially the critical heat flux point. For this purpose, the boiling process of pure deionized water was first simulated by ANSYS-Fluent software and the results were compared with experimental data. According to the observed acceptable agreement, the mentioned water-based hybrid nanofluid, as boiling working fluid is numerically simulated by changing the density of nucleation sites and its heat transfer and heat flux characteristics are obtained. The results show that the critical heat flux for pure deionized water was occurred at 24.4 °C and is about 1.1 MW per square meter, while the critical heat flux for the hybrid nanofluid was occurred at 13 °C and is about 1 MW per square meter.

This study aims to investigate the effect of nanoparticle deposition on the boiling surface in the presence of microchannel on the characteristics of boiling heat transfer. In this experimental study, the copper boiling surfaces including polished circular surface, rectangular and trapezoidal microchannels were used. The microchannels include feeding sub-channels perpendicular to the main channel, which increases the boiling surface and separates the downward cool fluid flow and upward hot bubbles. Nuclear boiling experiments on microchannel surfaces in the presence of a hybrid water-based nanofluid containing 70% titanium oxide and 30% OH-based multi-wall carbon nanotubes in volumetric concentrations of 0.1% and 0.5% have been conducted. The results of nanofluid boiling experiments on both microchannel surfaces show that with increasing concentrations, critical heat flux and heat transfer coefficient increases and the highest increase in critical heat flux and heat transfer coefficient is related to the hybrid nanofluid with 0.5 % volumetric concentration on the surface with trapezoidal microchannel and their values are 64.64% and 344.76%, respectively, compared to pure water boiling on the polished copper surface. Also, in boiling of pure water on the deposited surfaces with nanoparticles, the greatest increase in critical heat flux and heat transfer coefficient is related to the surface with trapezoidal microchannels with 0.1% volumetric concentration  and 0.5% and volumetric concentration  and  their values are 120.16% and 149.4% respectively, compared to pure water boiling on the polished copper surface.

A milestone in linear parabolic solar power plants is the use of direct steam production systems. In these systems, due to the presence of water as a heat transfer fluid, we witness an increase in the working temperature. In a direct-production steam generator (DSG), the Sun's energy hits the tube where the water flows. Failure to control the flow of fluid in the boiling section leads to the arrival of the thermal flux of the fluid to the critical heat flux. The important point in designing these types of power plants is the critical point (with critical heat flux), which causes many problems, including thermal stresses in solar absorber. Therefore, the study of the boiling process in the absorber of these power plants is important. In this research, a homogeneous two-phase model is introduced for numerical simulation. In the first stage, an optical model for the reflector is simulated. In the next step, the heat transfer equations are simulated for radiation absorbent and heat transfer inside the pipe. Finally, the changes in the tube's wall and water temperature are investigated. The LUT method has been used to determine the location and amount of critical heat flux. In modeling, steam quality, wall temperature and fluid temperature and critical thermal flux point are obtained for different values of pressure and mass flow and the system efficiency is determined. The system's continuous function for different radiations (which is equivalent to different climatic conditions) has been examined.

Pool boiling has the ability to remove large heat flux at low difference temperature of wall and this can be further enhanced by using surface modification methods. This article investigates pool boiling heat transfer on 4 levels with different orientations. For this purpose, a laboratory device was designed and built. The main goal of providing a simple and cost-effective manner with high durability in industrial applications, to having the highest amount of critical heat flux at the lowest level of super-heated temperature difference. The results show that surface roughness factor causing a delay in connecting the bubbles and heat flux increased slightly. In addition to roughness factor, two factors separating bubbles from the fluid in the heat dissipation and more power nucleation sites and micro-bubble layer can be more important than the surface roughness. The surface polished in one direction with lower roughness has higher critical heat flux than circular rough surface. Ultimately to combine bubble separation and more feed the micro layer with made micro channel. With this method it could be increased 131% critical heat flux and 211% heat transfer coefficient.