Heat transfer from plastic film heated by thermal radiation

The heat transfer characteristics of a plastic film were experimentally studied using a two‐dimensional plane model apparatus of a double bubble tubular blown film process, which is one of the most important methods of polymer processing to produce a biaxially oriented film. In the model apparatus, a polyethylene film was heated by infrared heaters and cooled by plane air jets. The radiation absorptivity of the film was estimated on the basis of the temperature and irradiation measurements of the film that was cooled by natural convection, i.e., without the air jets. The convective heat transfer coefficient on the film cooled by the air jets was evaluated using the estimated absorptivity of the film. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(4): 265–278, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20059     

纵向涡强化竖直平板自然对流换热的实验研究

对纵向涡强化竖直平板自然对流换热进行了实验研究。结果表明，在一定的Rayleigh数范围内，直角三角翼纵向涡发生器的攻角、翼高、翼宽等几何参数是影响强化换热的主要因素。存在最佳攻角；宽高比一定时，翼高和翼宽的变化会影响换热的效果。发现在直角三角翼阵列中前排直角三角翼产生的纵向涡可以强化后排直角三角翼纵向涡的换热。将直角三角翼与矩形低肋换热表面的性能作了对比性实验，在其他条件相同的情况下，直角三角翼强化换热的效果优于矩形低肋。     

Effect of viscosity variation on natural convection flow of water–alumina nanofluid in an annulus with internal heat generation

Heat transfer enhancement in a horizontal annulus using the variable viscosity property of an Al₂O₃–water nanofluid is investigated. Two different viscosity models are used to evaluate heat transfer enhancement in the annulus. The base case uses the Pak and Cho model and the Brinkman model for viscosity which take into account the dependence of this property on temperature and nanoparticle volume fraction. The inner surface of the annulus is heated uniformly by a constant heat flux q_w and the outer boundary is kept at a constant temperature T_c. The nanofluid generates heat internally. The governing equations are solved numerically subject to appropriate boundary conditions by a penalty finite‐element method. It is observed that for a fixed Prandtl number Pr = 6.2, Rayleigh number Ra = 10⁴ and solid volume fraction ? = 10%, the average Nusselt number is enhanced by diminishing the heat generation parameter, mean diameter of nanoparticles, and diameter of the inner circle. The mean temperature for the fluids (nanofluid and base fluid) corresponding to the above mentioned parameters is plotted as well. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21016     

Enhancement of natural convection heat transfer from an inclined heated plate using rectangular grids

The effect of an inclination angle on the natural convection heat transfer from an inclined heated plate with rectangular grids is investigated. Heat transfer coefficients are measured in air when the plates are inclined at angles from ?30 to +60 from a vertical plane, grid heights are in the range of 5 to 10 mm, and diagonal lengths of the grid are 25, 50, 100, and 200 mm. For each configuration, the surface heat flux ranges from 50 to 200 W/m². It is found that the rectangular grids increase local heat transfer coefficients when the grids are applied to an inclined plate. The rectangular grids increase the average heat transfer coefficients along the horizontal centerline of the plate by up to 20% compared to those coefficients of a smooth plate, even when the angles of inclination are ±30° © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(5): 408–419, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10043     

Enhancement of natural convection heat transfer from a vertical heated plate using inclined fins

An enhancement technique is developed for natural convection heat transfer from a vertical heated plate with inclined fins, attached on the vertical heated plate to isolate a hot air flow from a cold air flow. Experiments are performed in air for inclination angles of the inclined fins in the range of 30° to 90° as measured from a horizontal plane, with a height of 25 to 50 mm, and a fin pitch of 20 to 60 mm. The convective heat transfer rate for the vertical heated plate with inclined fins at an inclination angle of 60° is found to be 19% higher than that for a vertical heated plate with vertical fins. A dimensionless equation on the natural convection heat transfer of a vertical heated plate with inclined fins is presented. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(6): 334–344, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20168     

Heat transfer enhancement of turbulent natural convection adjacent to a vertical heated plate

Enhancement of heat transfer was investigated experimentally on natural convection adjacent to a vertical heated plate. In order to promote heat transfer from the heated plate, a V-shaped promoter of which the edge faced upstream was attached onto the surface of the vertical plate. The promoter redirects high-temperature fluids toward both sides of the promoter and also introduces low-temperature ambient fluids behind the promoter. These two mechanisms enhance heat transfer, in particular, in the region behind the promoter. Local heat transfer coefficients around the promoter were measured using water as the test fluid. These coefficients behind the promoter reached 2.5 times of those without the promoter. The optimal heights and attack angles of the promoter that maximize the heat transfer were also studied experimentally. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res. 25(1): 39–50, 1996     

辐射引起密闭容器内半透过性流体的自然对流对传热的影响

用数值方法研究了密闭容器内半透过性流体在辐射光的照射下产生的自然对流对传热的影响。基于二维假设下的流体流动和热量传递的动态数学模型,并用有限差分方向交替法进行数值求解,得到了稳态下的对流换热系数。结果表明,流体的自然对流受倾斜角度和光学厚度的影响,而传热又受自然对流的影响,对流换热系数的大小与密闭容器内自然对流的强度密切相关。局部换热系数沿透过板内侧的分布与密闭容器的放置角度有关,垂直放置时为从下至上逐渐增大,水平放置时,呈波浪形分布,倾斜角度从π／2减少至0时,由从下至上逐渐增大的分布变为波浪形的分布,垂直和水平放置时的平均换热系数都随光学厚度的增大而增大。     

Experimental study on heat transfer enhancement on natural convection in a vertical plate by using longitudinal vortex generators arranged in rows

Longitudinal vortices are capable of producing beneficial effects in heat transfer enhancement. Experiments in natural convection heat transfer enhancement were done on a vertical flat heating plate using delta‐winglet longitudinal vortex generators (LVGs) arranged in rows. In an experimental range of Rayleigh number, the height and width of the winglet of the longitudinal vortex generator (LVG), the array form of the longitudinal vortex generators on the heat transfer performance were experimentally investigated, and the best height of the winglet of the longitudinal vortex generator was obtained. The results showed the change of the array form of the longitudinal vortex generators could affect the heat transfer effect. Finally by arranging some longitudinal vortex generator arrays with the appropriate interval, the whole heat transfer effect of the interval could reach a prime value. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(5): 351–358, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20119     

Effects of changes in frequency of ultrasonic vibrations on heat transfer

The heat transfer coefficients during natural convection of water on a downward‐facing horizontal surface and that on a vertical surface were measured under the influence of ultrasonic vibrations. The multi‐type oscillation used in this experiment was able to operate at three frequencies—28, 45, and 100 kHz—not only continuously, but also intermittently. When ultrasonic vibrations of 28 and 45 kHz were applied to water, cavitation bubbles were generated and the heat transfer coefficient increased remarkably. When an ultrasonic vibration of 100 kHz was applied, the increase in the heat transfer coefficient was small compared to those at 28 and 45 kHz. At 100 kHz, cavitation was not observed at this low ultrasonic power. The uniformity of heat transfer, which depends on the profile of the standing wave due to the ultrasonic vibration, was improved by the method of repeatedly applying multiple vibrations. © 2000 Scripta Technica, Heat Trans Asian Res, 29(5): 358–372, 2000     

相变材料融化储热中的自然对流增强效应研究

液相自然对流传热可显著提升相变材料的融化储热效率。为了明确相变材料融化储热中的自然对流增强效应,通过开展侧边恒定热源下的相变石蜡融化储热试验,获取相变石蜡融化前缘的倾斜特征,证实了液相自然对流对融化储热的影响。建立流-固-热三场耦合下的相变材料融化储热计算模型,对比试验测试与理论计算结果,验证计算模型的正确性。分析自然对流传热影响的尺寸效应和方向性,并进一步研究加热温度和初始温度等因素对储热效率的影响。结果表明,在侧边恒定温度热源下,方腔内液相自然对流运动将导致倾斜状的融化前缘,并提升了13.1倍的融化储热效率。液相自然对流影响存在尺寸效应,在方腔边长小于2mm后其增强效应基本可忽略不计。同时,液相自然对流运动受热源方向影响显著,热源方向角由90°向270°旋转时,液相自然对流效应将被逐渐激活,流动状态将由单一大环流向若干无规则小环流转变,最终在底部热源下达到最大储热效率,并转变为波浪形的融化前缘形态。加热温度由60℃增至100℃时,自然对流增强系数提升了20%。初始温度由25℃降至5℃时,自然对流增强系数降低了10.2%。     

双绝热圆柱对水平三角腔内自然对流影响的数值研究

针对含双绝热圆柱的底部加热水平等腰三角腔内空气的稳态层流自然对流开展研究.通过有限容积法对控制方程进行了数值求解,其中瑞利数的变化范围为104 ～106,圆柱体的尺寸比则分别为0（无圆柱体）、1/8和1/4.基于计算结果对自然对流的流动与传热特性随瑞利数和尺寸比的变化规律进行了分析和讨论.结果表明,双绝热圆柱的存在较大程度上改变了三角腔内自然对流的流型和温度场分布,但对整体传热影响较小,仅略微提高了平均努赛尔数,强化传热的效果在尺寸比为1/8时较为明显.     

Enhancement of natural convective heat transfer from tall,vertical heated plate to water

An enhancement technique was developed for natural convection heat transfer from a tall, vertical heated plate to water. Rectangular grid fins attached to the base plate were utilized as a heat transfer promoter. These grid fins redirect the high‐temperature fluid ascending along the base plate toward the outside of the boundary layer and introduce the low‐temperature ambient fluid toward the base plate instead. The heat transfer coefficients of thus‐treated surfaces were measured and compared with a nontreated surface and a surface with conventional vertical plate‐fins. The highest performance was achieved for the experimental surfaces. In particular, the experimental surfaces with 5‐mm‐high, nonconducting grids and with 10‐mm‐high, conducting grid fins show 27% and 80% higher heat transfer coefficients compared to the turbulent heat transfer coefficients of the nontreated surface, respectively. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(2): 178–190, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10083     

Natural convection heat transfer along a vertical flat plate with a projection in the turbulent region

In the present study, the heat transfer coefficients occurring with a projection in the turbulent region of a vertical flat plate were measured experimentally for various projection heights in the range of 0 to 20 mm. The wall temperature and fluid flow fields were also visualized using a liquid crystal sheet and nylon 12 powder, respectively. The average and local Nusselt numbers reach 1.07 to 1.22 and 1.2 to 1.7 times those for pure turbulent natural convection, respectively. The maximum enhancement rates of heat transfer are attained at a position of 2.3 to 3.3 times the projection height from the upper projection surface toward the downstream, and these positions are in good agreement with those of the reattachment of the fluid flow and with centers of dark red regions in the liquid crystal. On the other hand, the heat transfer coefficients in the just upstream and downstream regions of the projection are small compared with those for no projection. By introducing the nondimensional parameter R, the present experimental results are rearranged quantitatively and effectively. © 2001 Scripta Technica, Heat Trans Asian Res, 30(3): 222–233, 2001     

油浸式自冷变压器用三维肋管散热器换热特性研究

为研究三维肋管散热器的自然对流换热特性,分析不同温升下三维肋管散热器的强化换热性能,搭建了模拟油浸式自冷变压器工作过程的实验平台,在不同功率下分别测试了片间距为45 mm的片式散热器、排间距为45和55 mm的三维肋管散热器的进口温升,拟合得到散热器的温升-功率曲线,计算出3个散热器在相同温升下的散热量.研究表明:相比...     

Heat transfer on a horizontal rotating cylinder near a flat plate in a cross‐flow

An experimental study of heat transfer on a horizontal rotating cylinder near a flat plate was performed. The cylinder and plate were set in a cross‐flow. Temperature distribution and coefficients of local heat transfer were measured by a Mach–Zehnder interferometer. Flow visualization was made using smoke. Rotating Reynolds numbers (Re_r) and cross‐flow Reynolds numbers (Re_d) were varied from 0 to 2000. The spaces between cylinder and plate were varied from 1 × 10^?3 m to 5 × 10^?3 m. The rotating direction of cylinder was changed clockwise or counterclockwise. The following results are obtained: When the space between the rotating cylinder and flat plate is the same as the displacement thickness on the plate, the heat transfer on the cylinder near the plate has the best performance. We have procured the empirical equation of heat transfer from a rotating cylinder near the flat plate in the cross‐flow. 8 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20329     

基于烟囱效应的通信基站热沉优化设计

利用数值模拟的方法,探究了烟囱效应对通信基站热沉自然对流散热的强化作用,对影响热沉散热性能的主要因素及其机理进行了分析,并以热沉热阻作为优化目标,通过优化翅片间距与隔板间隙的取值提升了热沉的散热性能。在优化设计过程中,通过模糊均值聚类对拉丁超立方抽样所得的样本点进行筛选,快速并有效的缩减了设计区间,使用Kriging模型对新设计区间内的均匀样本点进行拟合,构建了热沉热阻与设计变量间的代理模型,并结合遗传算法寻优,确定了最优设计参数取值。在最优参数布置下,相比于初始热沉,热沉的发热面温度降低了15.23 K,总热阻降低了34.29%。     

A numerical study of natural convection heat transfer in a composed thermal diode

The effects of inclination on the steady natural convection local heat transfer characteristics in an air-filled enclosure, which is composed of rectangular and parallelogrammic portions, are studied numerically. In this investigation, two geometrical aspect ratios are introduced: one for a parallelogrammic portion of an enclosure, the other for a rectangular one. The governing equations for a two-dimensional, laminar, natural convection process in an enclosure are discretized by the control volume approach which ensures the conservative characteristics to be satisfied in the calculation domain, and then solved by a modified SIMPLE algorithm. The momentum and energy equations are coupled through the buoyancy term. Computations are carried out for Prandtl number Pr = 1 and Rayleigh number Ra = 2.7 × 10⁸. In order to obtain a greater understanding of the flow and heat transfer behaviors, flow patterns with streamlines and isotherms at different inclination angles are shown. Also, the effects of numbers of installed guide vanes in a composed enclosure are studied to consider the enhancement of heat transfer of the inner diode. © 1999 Scripta Technica, Heat Trans Asian Res, 28(7): 573–582, 1999     

Natural convection and radiation heat transfer in a vertical porous layer with a hexagonal honeycomb core (Part 2: heat transfer experiment)

Combined natural convection and radiation heat transfer characteristics in a vertical porous layer with a hexagonal honeycomb core were investigated experimentally. The temperature distributions on the honeycomb core wall and the combined heat transfer rates through the porous layer were measured. The measurements of the heat transfer were accomplished using the guarded hot plate method. The honeycomb core wall was made of paper and large-mesh foamed resins were inserted into the honeycomb enclosures. The measurements were performed by varying the radiation parameters between 0.5 and 0.65, varying the temperature ratios between 0.01 and 0.1, and varying the Darcy-Rayleigh numbers between 5 and 80, and for a fixed aspect ratio H/L = 1. The experimental results for Nusselt numbers agreed well with our available numerical results. © 1999 Scripta Technica, Heat Trans Asian Res, 28(4): 295–306, 1999