Since the opacifier-loaded silica aerogel composites have strong ability to inhibit radiant heattransfer compared to the pure silica aerogel, they can improve effectively the insulation performance at high temperatur...
详细信息
Since the opacifier-loaded silica aerogel composites have strong ability to inhibit radiant heattransfer compared to the pure silica aerogel, they can improve effectively the insulation performance at high temperature conditions and have been used extensively as thermal insulation materials in a variety of industrial applications. This paper developed a theoretical model to predict the radiant performance of opacifiers by using the Mie scattering theory, Beer law and a radiant heattransfer equations. The model was used to calculate the radiant performance of carbon black opacifier with various particle diameters and the radiant performance of three opacifiers, carbon black, SiC and ZrO2. The results showed that the opacifier-loaded silica aerogel composites had much higher extinction coefficient than the pure silica aerogel in a range of 3-7.5 μm wave band or in medium and high temperature ranges, which significantly improved the insulation performance of composites. With a fixed opacifier mass fraction, the optimal particle diameter was 2-3 μm for carbon black opacifier. Among the three opacifiers, carbon black opacifier had better opaque property than that of SiC and ZrO2 opacifiers. Due to poor thermostability for carbon black opacifier at high temperature conditions, the optimal material design for opacifier-loaded silica aerogel composites should be as follows: SiC or ZrO2 are added on high temperature side of aerogel and carbon black is added on low temperature side.
Based on selecting the proper formulas of thermal conductivity and viscosity for nanofluids, a three-dimensional fluid-solid conjugated model was developed to analyze the effect of the heat sink structure, the nanopar...
详细信息
Based on selecting the proper formulas of thermal conductivity and viscosity for nanofluids, a three-dimensional fluid-solid conjugated model was developed to analyze the effect of the heat sink structure, the nanoparticle kind, diameter and volume fraction, and the base fluid kind on the cooling performance of microchannel heat sink. The results showed that: (1) thermal dispersion effect caused by nanoparticle random motion enhanced the thermal convection of nanofluid thus enhances significantly the cooling performance of heat sink;(2) the enhancement of nanofluids was closely dependent on the heat sink structure and the dependence was distinct from the pure fluid, hence the heat sink structure was needed to be optimized for nanofluids as coolants;(3) as the nanoparticle volume fraction increases, the thermal resistance reduced and the pressure increased, the water-based Al2O3 nanofluid with 0.5% volume fraction was the optimal coolant which caused 10.1% decrease in the thermal resistance and only 0.38% increased in the pressure drop;(4) although the nanoparticle size had a small effect on the thermal resistance, nanoparticles with small diameter were recommended with consideration of stability of nanofluids;(5) Al2O3 nanoparticle was superior to TiO2 and CuO, and water was the better base fluid than ethylene glycol and engine oil.
Hydrogen production by biomass supercritical water gasification is about to develop in the future because the process is closed and clean with high gasification efficiency in addition. Low concentration sodium carboxy...
详细信息
ISBN:
(纸本)9781424448128;9781424448135
Hydrogen production by biomass supercritical water gasification is about to develop in the future because the process is closed and clean with high gasification efficiency in addition. Low concentration sodium carboxymethylcellulose (CMC) (2-3% mass fraction) can be mixed with particulate biomass and water to form a uniform and stable viscous paste which can be pumped to the continuous high-pressure reactor. In this paper, the supercritical water gasification of CMC was investigated in the tube flow apparatus to produce gas products consisting essentially of H 2 , CO, CH 4 and CO 2 .
Spray cooling has received interest as a technique to dissipate extremely high heat fluxes. Due to its high heat flux cooling capacity, temperature uniformity and acceptable cooling medium inventory, it continually fi...
Spray cooling has received interest as a technique to dissipate extremely high heat fluxes. Due to its high heat flux cooling capacity, temperature uniformity and acceptable cooling medium inventory, it continually finds new cooling applications from semiconductor to laser diode array, from high precision forging to space shuttle’s flash evaporator system. These applications are all concerning the phase change heattransfer. However, phase change happens in a relative high temperature which inevitably increases energy consumption and decreases life span of electronics. So the heattransfer research in non‐boiling spray cooling is worthwhile and urgent. Experimental investigation of non‐boiling spray cooling with low surface temperature and big flow rate was conducted. At the condition of room temperature and atmospheric pressure, water was used as working medium and water flow rate ranged from 0.58–0.95 L/min. It shows that the bigger the spray pressure, the better of the heattransfer performance. When the nozzle‐to‐surface distance is within a certain value, there exists an optimum position where the spray footprint is inscribed to the heated surface. TiO2‐water Nanofluids was used as cooling medium. The mass concentration of nanofluids was set to 0.1%, 0.5% and 1%, respectively. heattransfer performance was found to be significantly enhanced and heattransfer coefficient was increased with the increased concentration.
Based on the maximum CHF (critical heat flux) criterion, an optimal heattransfer criterion, which is called H criterion, was proposed. Experimental apparatuses were conducted. Distilled water was used as the working ...
详细信息
Based on the maximum CHF (critical heat flux) criterion, an optimal heattransfer criterion, which is called H criterion, was proposed. Experimental apparatuses were conducted. Distilled water was used as the working fluid. Three different DANFOSS nozzles with cone angles being 54deg, 50deg and 54deg respectively were used to cool a 30times30 mm 2 square copper surface. Experimental results indicated that the volumetric fluxes were pro-portioned to P 0.5 , where P is the pressure drop across the nozzles. The optimal distance between the nozzles and the heated surface were derived. The results indicated that the optimal heattransfer appeared while the outside of the impellent thin spray film inscribed in the square heated surface. Based on the H criterion aforementioned, two DANFOSS nozzles of the three were used to study the temperature distribution of the heated surface experimentally while there were spray inclination angles. Distilled water was also used impacting on the 30times30 mm 2 square copper surface aforementioned and a circular heated copper surface with diameters being 30 mm respectively. The heat flux of the surface was kept in constant (about 26-35 W/cm 2 ). The inclination angles were 0deg, 10deg, 20deg, 30deg, 40deg and 50deg respectively. Experimental results indicated that the grads of the temperature of the surface increases first and then decreases with the increase of the inclination angle.
Slug flow is a highly intermittent flow regime in which large bubble slugs flow alternately with liquid slugs at randomly fluctuating frequency. A one-dimensional model for the interfacial structure in bubble slug was...
详细信息
Slug flow is a highly intermittent flow regime in which large bubble slugs flow alternately with liquid slugs at randomly fluctuating frequency. A one-dimensional model for the interfacial structure in bubble slug was developed based on mass and momentum conservation for bubble nose, body and tail as well as for a hydraulic jump at the rear of the bubble. The model can reproduce experimental results accurately. The results indicate that the interfacial structure is strongly dependent on the Froude number defined by gas/liquid mixture velocity, pipe inclination, and bubble length.
The upward multiphase cross flow and heattransfer in the vertical tube may occur in oil production and chemical facilities. In this study, the local flow patterns of an upward gas-water two phase cross flow in a vert...
详细信息
The upward multiphase cross flow and heattransfer in the vertical tube may occur in oil production and chemical facilities. In this study, the local flow patterns of an upward gas-water two phase cross flow in a vertical tube with a horizontal rod have been investigated with an optical probe and the digital high speed video system. The local flow patterns are defined as the bubble, slug, churn and annular flow patterns. Optical probe signals are ana- lyzed in terms of probability density function, and it is proved that the local flow patterns can be recognized by this method. The transition mechanisms between the different flow patterns have been analyzed and the corresponding transitional models are proposed. Finally, local flow pattern maps of the upward gas-water two-phase flow in the vertical tube with a horizontal rod are constructed.
暂无评论