换热器中强化传热表面传热性能的评价

以热力学第二定律为基础，在熵产分析的基础上提出了新的传热性能评价判据——强化传热性能因数。

R134a表面增强型蒸发强化传热管的实验研究

介绍了国外空调冷冻设备中采用的一种新型商用表面增强型蒸发强化传热管 ,并对其进行了实验研究。发现对使用R134a环保制冷工质 ,在低热流密度的条件下 ,表面增强型蒸发强化传热管具有十分优异的强化效果。在实验的换热工况下 ,保持进口水温不变而改变管内水流速 ,这种强化管的总换热性能是普通低翅蒸发强化管的 2 .2到 2 .6倍 ,管外换热系数是普通低翅蒸发强化管的 1.3～ 1.9倍。

三维双侧强化管内R410a蒸发工程模型研究

为评估不同三维双侧强化管蒸发传热特性,采用实验方法对R410a在强化管内的蒸发传热特性进行了测试,并将结果与光滑管进行了比较。所采用的管型包括光滑管、人字纹管、螺旋纹管、人字/涟漪纹管、人字/疏水纹管等。R410a蒸发的饱和温度为279.15 K,质量流速为50—200 kg/(m^(2)·s),入口干度为0.2,出口干度为0.8。研究结果表明,对光滑管内蒸发传热系数,Liu和Winterton模型预测精度最高;强化管内蒸发流型均处于波状分层流。人字/疏水纹管的强化传热因子最高,这与增加成核点数量密切相关;人字/涟漪纹管没有带来明显的传热系数提升。人字/疏水纹管、螺旋纹管具有最佳的蒸发综合传热-阻力特性;人字/涟漪纹管综合性能最差,甚至逊于光滑管。已有关联式对强化管蒸发传热的预测值偏差较大,修正后的Kandlikar关联式以及Gungor和Winterton关联式对传热系数的预测偏差在±10%之内。

Pool boiling performance of porous surface tubes under vacuum conditions

Two types of tube bundles are designed,which are,respectively,composed of six tubes arranged in the boiling chamber.The nucleate pool boiling performance of smooth tube bundles and sintered porous surface tube bundles with deionized water as a medium are experimentally studied at atmospheric and sub-atmospheric pressures,respectively.The experimental results indicate that the boiling heat transfer coefficients of the two types of tube bundles increase with the increase in pressure under vacuum conditions as they behave under ordinary pressure.As the pressure varies from 10 to 100 kPa,it also can be seen that the heat transfer coefficient of the sintered porous surface tube is increased by 0.2 to 4 times compared with the smooth one under the same operating parameters.In addition,the experimental data show that a definite bundle effect exists in both sintered porous surface tubes and smooth tubes under vacuum conditions.

Enhanced Pool Boiling Heat Transfer on Copper Foam Welded Surfaces

Enhanced pool boiling heat transfer of the porous structure is critical to the thermal management technology.In this paper,pool boiling heat transfer experiments are performed on copper foam welded surfaces in de-ionized water to investigate the effects of basic parameters of copper foam on heat transfer enhancement.Boiling phenomenon is observed to facilitate the understanding of enhancement mechanism.The results show that copper foam welded surfaces can significantly enhance the pool boiling heat transfer performance,reduce the boiling incipience temperature by 7-9℃,and reach two times heat transfer coefficient compared with smooth plain surfaces due to numerous nucleation sites,extended surface areas,and enhanced turbulent effect.Pore density and thickness of foam have two side effects on heat transfer.

Investigations on Heat Transfer Enhancement in Pool Boiling with Water-CuO Nano-Fluids

The main focus of the present work is to investigate Critical Heat Flux (CHF) enhancement using CuO nanofluid relative to CHF of pure water. To estimate the effect of nanoparticles on the CHF, pool boiling CHF values were measured for various volume concentrations of CuO nanofluid and compared with pure water. CHF enhancement of 130% was recorded at 0.2 % by volume of CuO nano-fluids. Surface roughness of the heater surface exposed to three measured heating cycles indicated surface modifications at different volume concentrations of nanofluid. SEM image of the heater surface revealed porous layer build up, which is thought to be the reason for CHF enhancement.

A Comparative Study of the Local Heat Transfer Distributions Around Various Surface Mounted Obstacles

In many engineering applications,heat transfer enhancement techniques are of vital importance in order to ensure reliable thermal designs of convective heat transfer applications.This study examines experimentally the heat transfer characteristics on the base plate around various surface mounted obstacles.Local convection coefficients are evaluated in the vicinity of each individual protruding body with great spatial resolution using the transient liquid crystal technique.Five different obstacles of constant height-to-hydraulic diameter ratio(~1.3) are considered.These include:a cylinder,a square,a triangle,a diamond and a vortex generator of delta wing shape design.The experiments were carried out over a range of freestream Reynolds numbers,based on the hydraulic diameter of each obstacle,varying from 4,000 to 13,000.The results indicate a negligible effect of the flow speed on the heat transfer topological structure and a considerable effect of the obstacle geometry on the level and distribution of heat transfer enhancement.

Experimental investigation of anodized/spray pyrolysed nanoporous structure on heat transfer augmentation

This paper analyzes the effects of nanoporous surface on heat transfer temperaments of assorted thermal conductingmaterials. A phenomenal proposal of wielding the surface roughness to ameliorate the heat transfer ratehas been discovered. The maximum increase of heat transfer rate procured by nanoporous layers is 133.3% higherthan the polished bare metals of surface roughness 0.2μm. This plays an imperative role in designing compact refrigerationsystems, chemical and thermal power plants. Experimental results picture a formidable upswing of58.3% heat transfer in chemically etched metals of surface roughness 3 μm, 133.3% in nanoporous surface of porosity75-95 nm formed by electrochemical anodization, and porosity of 40-50 nm formed by spray pyrolysis increasesthe heat transfer by 130%. Effects of porosity, flow velocity and scaling on the energy transfer are alsoscrutinized. This paper also analyzes the multifarious modes of nanoporous fabrication, to contrive both prodigiousand provident system.