共查询到19条相似文献,搜索用时 140 毫秒
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纳米颗粒悬浮液池内泡状沸腾的实验研究 总被引:7,自引:0,他引:7
本文对纳米颗粒悬浮液在平壁面上池内沸腾进行了实验研究。实验用的纳米粒子为26 nm的铁粉和13 nm的三氧化二铝纳米粉末,基液为去离子水。分别配成体积浓度为0.1%, 1%和2%的悬浮液。实验结果表明,纳米悬浮颗粒对液体沸腾换热过程的影响会随着纳米颗粒性质,颗粒浓度及热流密度大小的不同而出现不同的效果;加入纳米颗粒后, 对基液沸腾换热的影响存在着两个相反的作用机制,它们分别为:纳米颗粒增强了液体内部的热量迁移能力(热物性的影响)和改变了加热面的表面结构特性(加热面特性的影响)。 相似文献
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考虑在纳米流体中纳米颗粒做布朗运动引起的对流换热, 基于纳米颗粒在纳米流体中遵循分形分布, 本文得到纳米流体对流换热的机理模型. 本解析模型没有增加新的经验常数, 从该模型发现纳米流体池沸腾热流密度是温度、纳米颗粒的平均直径、 纳米颗粒的浓度、纳米颗粒的分形维数、沸腾表面活化穴的分形维数、基本液体的物理特性的函数. 对不同的纳米颗粒浓度和不同的纳米颗粒平均直径与不同的实验数据进行了比较, 模型预测的结果与实验结果相吻合. 所得的解析模型可以更深刻地揭示纳米流体对流换热的物理机理. 相似文献
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《工程热物理学报》2017,(2)
将羧基基团引入多壁碳纳米管,改善了碳纳米管在水中的分散性及稳定性。同时研究了不同质量浓度纳米流体的导热系数、加热表面颗粒沉积、接触角变化对核沸腾传热性能的影响。结果表明;羧基化碳纳米流体可强化核沸腾传热。在测试浓度范围内,强化率在低热通时,随着热通量的增加急剧增大,高热通时,趋于稳定;当质量比ω为0.10%,功率为210.6 kW.m~(-2)时,强化率达到最大为138.3%;流体的导热系数随着质量浓度的增大而增大,0.15%浓度导热系数是纯水的1.18倍。分析认为纳米流体表面张力,纳米颗粒沉积,纳米颗粒扰动和导热系数的变化均是影响水基羧基化碳纳米流体沸腾的因素。结论由0.05%的纳米流体沸腾过程高速成像得到验证。 相似文献
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采用分子动力学方法模拟纳米尺度下液体在固体壁面上发生核化沸腾的过程,主要研究壁面浸润性对气泡初始核化过程和气泡生长速率的影响以及固-液界面效应在液体核化沸腾的能量传递过程中所起到的作用.研究结果发现:壁面浸润性越强,气泡在固壁处越容易核化.该结果与经典核化理论中“疏水壁面易于产生气泡”的现象产生了明显的区别.其根本原因是在纳米尺度下,固-液界面热阻效应不能被忽略.一方面,在相同的壁温下,通过增强固-液相互作用,可以显著降低界面热阻,使得热量传递效率提高,导致靠近壁面处的流体温度升高,气泡核化等待时间缩短,有利于液体沸腾核化.另一方面,气泡的生长速率随着壁面浸润性的增强而明显升高.当气泡体积生长到一定程度时,会在壁面处形成气膜,从而导致壁面传热性能恶化.因此,通过壁面的热流密度呈现出先增大后减小的规律. 相似文献
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《Revue Generale de Thermique》1997,36(7):562-572
The aim of the present work is to identify the mechanisms of heat transfer enhancement during natural convective boiling in confined spaces. The influences of gap-size (0.3 to 2 mm), of pressure (1 to 3 bar), of heating surface orientation (vertical or horizontal) and that of the number of nucleation sites (no site, single site or several sites) for boiling R-113 in a narrow channel are investigated. Results for unconfined boiling are also presented as a reference. It is shown that latent heat transfer is improved when the bubbles are squeezed against the heated wall. The heat transfer enhancement is greater for a vertical than for a horizontal orientation because the two-phase fluid velocity is increased when the gap-size decreases, which enhances the sensible heat transfer. Finally, the confinement reduces the effect of an increase in pressure, which limits the influence of confinement. Indeed, both parameters lead to contrary effects on the bubble diameter. 相似文献
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In this paper, a fractal model for nucleate pool boiling heat transfer of nanofluids is developed based on the fractal distribution of nanoparticles and nucleation sites on boiling surfaces. The model shows the dependences of the heat flux on nanoparticle size and the nanoparticle volume fraction of the suspension, the fractal dimension of the nanoparticle and nucleation site, temperature of nanofluids and properties of fluids. The fractal model predictions show that the natural convection stage continues r... 相似文献
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The present work is an experimental investigation of the incipient boiling of R134a inside a circular glass minichannel mounted horizontally and equipped with a series of transparent indium tin oxide heaters. The effects of heat flux input levels and refrigerant mass fluxes on the onset nucleate boiling process and on the saturated boiling heat transfer rate are quantitatively explored. The flow pattern visualizations, carried on by means of a high-speed camera, show that the nucleation process is oddly non-uniform: the first vapor bubbles are always generated on the upper side of the tube and lead to a first wall temperature drop. A further increase in the heat flux values results in an increased wall superheat until bubble nucleation also originates on the lower side of the tube, causing a second wall temperature drop. Finally, at higher heat input levels, the boiling process becomes uniformly distributed on the inner tube surface. This phenomenon occurred also after a 180° rotation of the glass tube, and, after a critical analysis of the potential origins, it remains presently unexplained. An evaluation of heat transfer coefficients for low vapor quality regimes is finally presented. 相似文献
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The enhancement of the heat transfer assisted by ultrasound is considered to be an interesting and highly efficient cooling technology, but the investigation and application of ultrasound in minichannel heat sinks to strengthen the flow boiling heat transfer are very limited. Herein, a novel installation of ultrasound transducers in the flow direction of a minichannel heat sink is designed to experimentally study the characteristics of heat transfer in flow boiling and the influence of operation parameters (e.g., heat flux, mass flux rate) and ultrasound parameters (e.g., frequency, power) on the flow boiling heat transfer in a minichannel heat sink with and without ultrasound field. Bubble motion and flow pattern in the minichannel are analyzed by high-speed flow visualization, revealing that the ultrasound field induces more bubbles at the same observation position and a forward shift of the onset of nucleation boiling along the flow direction, as ultrasonic cavitation produces a large number of bubbles. Moreover, bubbles hitting the channel wall on the left and right sides are found, and the motion speed of the bubbles is increased by 31.9% under the ultrasound field. Our results demonstrate that the heat transfer coefficient obtained under the ultrasound field is 53.9% higher than in the absence of the ultrasound field under the same conditions, and the enhancement ratio is decreased in the high heat flux region due to the change of the flow regime with increasing heat flux. This study provides a theoretical basis for the application of an ultrasound field in minichannel heat sinks for the enhancement of flow boiling heat transfer. 相似文献
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The influence of nucleation on the flow boiling heat transfer coefficient of R-134a/R-290/R-600a refrigerant mixture is experimentally
studied in a smooth horizontal tube of 12.7 mm diameter. The heat transfer coefficients are experimentally measured for stratified
flow patterns under a varied heat flux condition; a condition found in the evaporator of refrigerators and deep freezers.
The experiments are conducted in a counter-current heat exchanger test section. By regulating the flow rate and inlet temperature
of acetone, which is the heating fluid flowing in the outer tube, a varied heat flux is provided to the refrigerant flowing
in the inner tube. The refrigerant mass flow rate is fixed between 3 and 5 g s−1 and its inlet temperature between −8.59 and 5.33°C, which corresponds to a pressure of 3.2 to 5 bar. The significance of
nucleate boiling prevailing in the above-mentioned evaporators is highlighted. The experimental heat transfer coefficients
are also compared with well known heat transfer correlations. 相似文献
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Sarit K. Das G. Prakash Narayan Anoop K. Baby 《Journal of nanoparticle research》2008,10(7):1099-1108
Pool boiling heat transfer using nanofluids (which are suspensions of nano-sized particles in a base fluid) has been a subject
of many investigations and incoherent results have been reported in literature regarding the same. In the past, experiments
were conducted in nucleate pool boiling with varying parameters such as particle size, concentration, surface roughness etc.
and all sort of results ranging from heat transfer enhancement, deterioration and no effect were reported. This work tries
to segregate a survey on pool boiling of nanofluids with respect to particle concentration. This is due to the fact that a
major drift in heat transfer behavior is observed at higher and lower particle concentration. But upon deep perusal it has
been found that deterioration in heat transfer coefficient are mainly observed at higher particle concentrations (4–16% by
weight) and enhancements mainly at lower particle concentrations (0.32–1.25% by weight). Moreover, the relative size of the
particle with respect to the surface roughness of the heating surface seems to play an important role in understanding the
boiling behaviour. Also, recent works have reported that change in ‘surface wetting’ of the heating surface due to nanofluids
and the formation of a porous layer modifiying nucleation site density can be of importance in predicting nucleate pool boiling
characteristics of nanofluids. In the present paper, attempts are made to make systematic analysis of results in literature
and try to bring out a common understanding of the results in literature. 相似文献
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This study presents new data on nucleate boiling heat transfer obtained in compact horizontal tube bundles with small tube gaps. The experiment investigates the heat transfer enhancement effects by the restricted spaces comprising the compact tube bundles and the enhanced heat transfer tubes for nucleate boiling heat transfer of R-11 at atmospheric pressure. A roll-worked tube was used as a new type of enhanced heat transfer tube. The experimental results show that the small tube gaps can greatly enhance boiling heat transfer in a smooth tube bundle, while enhancement effects of small gaps were not quite significant for the enhanced tube bundle. There is a compound effect from the enhanced surface and the restricted space only for the enhanced tube bundle with the tube gap of 0.5 mm. The effects of the tube positions within both compact tube bundles on the boiling heat transfer were minor. 相似文献