圆形自由表面水射流冲击换热特性

对圆形自由表面水射流冲击换热特性进行了系统的实验研究,总结了喷距、射流出口速度以及相变等因素对换热的影响,并对其形成机理进行了分析,得到了驻点传热系数以及局部传热系数径向分布的关联式.     

不同流动方向上微型加热管内超临界CO2的换热特性

针对不同流动方向上超临界CO₂流体在微型加热管(管径为0.75 mm)内的对流换热特性进行实验研究。实验结果表明,当系统压力、质量流量、加热功率以及进口温度保持恒定时,局部对流传热系数在水平流动方向和垂直向上流动方向上的变化趋势相同,并且水平流动方向上的局部对流传热系数始终大于垂直向上流动方向上的局部对流传热系数。但在垂直向下流动方向上,局部对流传热系数随无量纲温度的升高而显著增大,并在无量纲温度最大时呈现出最佳的换热效果。不同流动方向上,局部对流传热系数均随质量流量增大而显著增大,但随着加热功率、进口温度的升高而显著减小。然而,在水平流动方向和垂直向上流动方向上,当流体温度低于其假临界温度时,局部对流传热系数随系统压力的升高而显著减小。当流体温度高于其假临界温度时,局部对流传热系数则随系统压力的升高而逐渐增大。     

不对称撞击流的实验研究与数值模拟

采用恒温式热线风速仪(CTA)测量不对称撞击流流场.用CFD软件对流场进行数值模拟,得到了和实验相吻合的结果.结果表明,不对称撞击流轴线上轴向速度除撞击面附近外,基本符合圆射流衰减规律.当喷嘴间距一定时,轴线撞击面驻点偏移量随气速比和喷嘴直径的增大而增大.当气速比一定时,轴线撞击面驻点偏移量随喷嘴间距增大而增大.     

气体射流冲击传热系数试验研究

气体射流冲击干燥是一种新的干燥方法。由喷嘴喷出的具有极高速度的气体直接冲击到需干燥物料的表面．因气流与物料表面之间产生非常薄的边界层，所以传热系数比一般热风传热要高出几倍乃至一个数量级。介绍了气体射流冲击原理与技术特点，通过研制的实验样机，利用集总热容法进行了不同风速下射流冲击传热系数的试验研究。实验表明，对流传热系数随风速的升高而提高。     

挤出塑料管喷淋冷却的数值模拟

冷却系统是塑料挤出管道生产工艺中的关键设备,其冷却的均匀性和效率直接影响管道产品的质量及生产速度。首先基于ANSYS对喷淋冷却瞬态传热进行模拟,结果表明:对流传热系数小于180 W·m-2·K-1时,冷却至目标温度(47℃)所需的时间随传热系数的变化明显;传热系数大于180 W·m-2·K-1时,冷却至目标温度所需的时间随传热系数的变化不大。然后基于FLUENT软件对喷淋喷嘴进行模拟,研究了喷淋入口速度、喷嘴高度对喷淋传热系数的影响,结果表明:随着喷淋入口速度的增加(6~15 m·s-1的范围内),总体对流传热系数增大,驻点处的传热系数由217 W·m-2·K-1增加到386 W·m-2·K-1;随喷淋高度的减小(68~128 mm范围内),壁面传热系数呈增加趋势,驻点处传热系数由227 W·m-2·K-1增加到311 W·m-2·K-1。基于以上研究,为真空定径喷淋冷却水槽的整体优化提出合理建议。     

撞击速度对连续液滴撞击热圆柱壁面局部传热特性影响的实验

借助高速摄像机捕获连续液滴撞击热圆柱壁面后的动力学行为,通过直接测试与数值计算方法相结合,获得了不同撞击速度下沿周向和轴向的局部对流传热特性。结果表明,当液滴撞击速度较小,液膜未发生飞溅时,由于圆柱面的各向异性,沿轴向的对流传热系数单调减小,而沿周向,对流传热系数先减小后略有增大;根据对流传热系数沿周向的变化,将圆周划分为撞击区域、热扩散区域和尾部脱离区域;增大液滴撞击速度主要提高撞击区域和热扩散区域的对流传热系数,而对尾部脱离区域对流传热系数的影响并不明显。当液滴撞击速度超过某一临界值(在本文的实验条件下约为1.53 m/s)时,液膜发生飞溅,此时继续增大撞击速度,壁温的降低不再明显。     

振动流化床宽筛分玻璃粒子与水平管局部传热特性

以宽筛分玻璃粒子为实验物料,测定了振动流化床与浸没水平加热管间的局部传热系数,研究了通气速率、振动频率和粒度分布对局部传热系数的影响.结果表明,浸没水平管局部传热系数随流化气速和振动频率的增加先增大而后逐步减小;小颗粒的存在有利于增强加热管与床层的传热;大颗粒比例的增加不利于管壁与床层的传热.由实验数据整理了预测局部传热系数的经验公式,经验公式与实验数据误差在20%以内,吻合较好.     

磁场对熔盐射流冲击传热的影响

以永磁铁构建定磁场,进行外加磁场作用下熔盐射流冲击传热的实验研究,并得到Nusselt数Nu驻点关联式和径向分布。结果表明,在驻点区范围内,Nusselt数较无磁场作用时增大,传热得到比较明显的增强,而在壁面射流区,这种强化传热效果逐渐减弱。此外,当Reynolds数Re一定时,熔盐Nusselt数随着磁场强度的增加而增大,且驻点处强化传热效果最为显著。在Reynolds数Re=6400与磁场强度B=2800 Gs条件下,熔盐驻点Nusselt数Nu₀提高约6％,可见磁场作用对熔盐射流冲击传热具有一定的强化效果。     

柱形流化床传热特性的数值模拟

对Shedid等搭建的圆柱体流化床采用欧拉?欧拉法进行三维数值模拟,考察了颗粒球形度、表观进气速度和床料初始堆积高度对流化床内垂直加热壁面与流动床料之间对流传热特性的影响,采用有效导热系数分别计算气相和固相的对流传热系数。结果表明,随表观进气速度增大,流化床内颗粒物料湍流运动加剧,加热壁面平均温度和流体平均温度下降,壁面流体间传热平均温度差减小,壁面流体间对流传热系数增大;随初始床料高度增加,流化床内颗粒与加热壁面的接触面积增大,导致固相平均对流传热系数增大。     

微尺度通道内R134a的冷凝传热实验研究

建立采用射流冲击进行制冷剂冷却的冷凝传热实验系统,对当量直径为0.63 mm矩形微尺度通道内制冷剂R134a的冷凝传热特性进行研究。实验参数范围是制冷剂干度0~1,质量流率115~290 kg/(m²·s）,饱和压力0.35~0.5 MPa,实验获得了不同工况下微尺度通道的局部冷凝传热系数,并分析了制冷剂各参数对冷凝传热的影响。实验结果表明：冷凝过程中沿制冷剂流动方向,局部冷凝传热系数会随着干度减小而减小;在一定饱和压力下,局部冷凝传热系数与局部热通量相对应;冷凝传热系数随着饱和压力减小而增大。基于实验数据,整理出适用于本实验工况下微尺度通道内R134a的冷凝传热计算公式。     

圆形浸没射流冲击驻点时射流速度对传热影响的实验研究

以R113为工质系统地实验研究了圆形浸没射流冲击驻点时的传热情况 ,实验涉及单相对流传热、沸腾起始温度、核沸腾传热和临界热流密度等 .结果表明 :出口速度对沸腾起始温度和核沸腾传热没有影响 ;速度增加 ,单相对流传热和临界热流密度提高 ,而温度过头值减小 .当出口速度超过 10m·s^-1后 ,高速射流冲击驻点的核沸腾曲线需要修正才能位于相应过冷度下池核沸腾曲线的延长线上 .     

Influence of magnetic field on jet impingement heat transfer with molten salt

Experimental study of jet impingement heat transfer with molten salt under the influence of external constant magnetic field was generated by permanent magnets. Both stagnation correlation and radial distribution of Nusselt number under magnetic field were obtained. The results showed that the Nusselt number with magnetic field became higher than that without magnetic field at stagnation region and jet impingement heat transfer was comparatively enhanced, while in wall jet region, the enhancement of heat transfer was gradually weakened. In addition, when the Reynolds number was constant, the Nusselt number of molten salt increased with increasing of the intensity of magnetic field, and the most enhanced heat transfer existed at the stagnation point. Under the conditions of Reynolds number Re=6400 and the intensity of magnetic field B=2800 Gs, the stagnation Nusselt number of molten salt was about 6 % higher than that without magnetic field. It can be seen that the magnetic field may promote the jet impingement heat transfer of molten salt.     

Pulsating Jet Impingement Heat Transfer Enhancement

This article presents results from a numerical study of pulsating jet impingement heat transfer. The motivation is to seek conditions offering a significant enhancement compared to steady flow impingement drying. The CFD software package FLUENT was used for simulating slot-type pulsating jet impingement flows with confinement. The parameter study included velocity amplitude ratio, mean jet velocity, and pulsation frequency. The distance from nozzle exit to surface was three times the hydraulic diameter of the nozzle. The Reynolds number based on the nozzle hydraulic diameter and jet temperature was 2,460 with a mean jet velocity of 30 m/s, which is the base case of the numerical experiments. Results showed that time-averaged surface heat transfer increased with increasing velocity amplitude for the same mean jet velocity. Large velocity amplitudes helped enhance heat transfer by two mechanisms: high jet velocity during the positive cycle and strong recirculating flows during the negative cycle. For the cases with different mean jet velocities but the same maximum velocity, time-averaged surface heat flux decreased with decreasing mean jet velocity. As for the effects of pulsation frequency, with high-velocity amplitude ratio, time-averaged surface heat fluxes were at the same level regardless of frequency. However, at low-velocity amplitude ratio, high frequency caused stronger recirculating flows resulting in greater heat transfer compared to the cases with a lower frequency.     

Pulsating Jet Impingement Heat Transfer Enhancement

This article presents results from a numerical study of pulsating jet impingement heat transfer. The motivation is to seek conditions offering a significant enhancement compared to steady flow impingement drying. The CFD software package FLUENT was used for simulating slot-type pulsating jet impingement flows with confinement. The parameter study included velocity amplitude ratio, mean jet velocity, and pulsation frequency. The distance from nozzle exit to surface was three times the hydraulic diameter of the nozzle. The Reynolds number based on the nozzle hydraulic diameter and jet temperature was 2,460 with a mean jet velocity of 30 m/s, which is the base case of the numerical experiments. Results showed that time-averaged surface heat transfer increased with increasing velocity amplitude for the same mean jet velocity. Large velocity amplitudes helped enhance heat transfer by two mechanisms: high jet velocity during the positive cycle and strong recirculating flows during the negative cycle. For the cases with different mean jet velocities but the same maximum velocity, time-averaged surface heat flux decreased with decreasing mean jet velocity. As for the effects of pulsation frequency, with high-velocity amplitude ratio, time-averaged surface heat fluxes were at the same level regardless of frequency. However, at low-velocity amplitude ratio, high frequency caused stronger recirculating flows resulting in greater heat transfer compared to the cases with a lower frequency.     

强润湿性液体在圆形断面喷嘴浸没射流冲击下沸腾的热滞后

系统地研究了液体在圆形断面喷嘴浸没射流作用下沸腾热滞后与射流冲击速度大小、液体流动方向、液体过冷度和喷嘴直径等因素的影响 .结果表明 :沸腾起始过热度随液体过冷度增加而减小 ,与其他因素无关 ,而温度过头值则随射流速度和液体过冷度减小而增加 .从强润湿性液体沸腾机理的角度对起始沸腾点的影响给出了相应的解释 .最后总结出了几种减小或消除沸腾热滞后的方法     

Influence of Impingement Temperature and Nozzle Geometry on Heat Transfer—Experimental and Theoretical Analysis

Based on the earlier studies the most common empirical correlations in the literature predict the impingement heat transfer coefficients rather well at low (close to 100°C) temperatures, but get inaccurate at higher temperatures. Impingement temperatures used in paper drying applications are typically 300 to 700°C, and there has been a need to improve the existing heat transfer correlations at high temperature area. This study presents experimental results of impingement heat transfer measurements with a laboratory-scale heat transfer test rig. Five nozzle configurations were measured. The parameters varied in the investigation were nozzle-to-plate distance, nozzle open area, nozzle diameter, impingement velocity and impingement air temperature. Regression model of heat transfer was developed based on the measurement data. A correction factor including the impingement and heat receiving surface temperatures is proposed. The correction factor can be used to improve the existing correlations at large jet to heat receiving surface temperature differences.     

Influence of Impingement Temperature and Nozzle Geometry on Heat Transfer—Experimental and Theoretical Analysis

Abstract

Based on the earlier studies the most common empirical correlations in the literature predict the impingement heat transfer coefficients rather well at low (close to 100°C) temperatures, but get inaccurate at higher temperatures. Impingement temperatures used in paper drying applications are typically 300 to 700°C, and there has been a need to improve the existing heat transfer correlations at high temperature area. This study presents experimental results of impingement heat transfer measurements with a laboratory-scale heat transfer test rig. Five nozzle configurations were measured. The parameters varied in the investigation were nozzle-to-plate distance, nozzle open area, nozzle diameter, impingement velocity and impingement air temperature. Regression model of heat transfer was developed based on the measurement data. A correction factor including the impingement and heat receiving surface temperatures is proposed. The correction factor can be used to improve the existing correlations at large jet to heat receiving surface temperature differences.     

翅片板式传热器双流道传热与流动数值模拟

焊接型板式传热器的紧凑性好、质量轻、传热性能好、初始成本低等优越性已越来越被人们所认识,因此人们纷纷对板式传热器内流动状态和传热机理展开研究。鉴于此,本文运用数值模拟软件Fluent对全焊接翅片板式传热器双流道进行模拟,在此基础上又进行了实验研究及实验数据与数值模拟的对比分析,得出不同结构参数和操作参数下翅片的传热系数和压力降,并分析翅片高度和翅片间距对翅片传热性能与流动阻力的影响。结果表明：①固定冷侧的入口速度和温度,热侧的传热系数和压降随之热侧入口速度增加而增大;②板间距一定时,翅片高度并非越高传热性能越好;③翅片间距越小,传热性能越好。