R134制冷剂闪蒸瞬态喷雾和传热特性的实验研究

制冷剂闪蒸瞬态喷雾冷却是皮肤激光手术中的重要辅助手段,既能够提高激光能量改善治疗效果,又可以保护表皮正常组织不受激光热损伤.针对制冷剂闪蒸瞬态喷雾冷却过程中所涉及的喷雾动力学和复杂沸腾相变传热等学术难点问题,本文建立了制冷剂闪蒸喷雾冷却实验台,对R134a制冷剂闪蒸喷雾冷却的喷雾特性和表面传热特性进行实验研究,得到了制...     

高温壁面液体射流冲击瞬态沸腾传热的实验研究

以水作为冷却介质,对高温壁面在射流冲击淬冷时的瞬态换热特性进行了实验研究,获得了介质在不同过冷度、不同射流速度下的完整沸腾曲线。实验结果表明。无论是增大工质过冷度还是提高射流速度,总会使得热壁面的冷却速率加大。在一定的过冷度和射流速度下壁温变化呈现快－慢－快的特点。临界热流密度随平均壁温变化率的增大而增大,二者之间存在线性关系．     

燃油液滴蒸发的一维导热模型与压力效应

考虑液滴内一维瞬态导热以及液滴,气流热物性随温度与组分的变化,建立了对流热环境中燃油液滴的蒸发计算模型.以柴油液滴为例,通过数值模拟,分析了蒸发过程中液滴内部的瞬态热响应,考察了不同条件下的环境压力效应.结果表明,对流蒸发过程中,燃油液滴内部温度梯度很大,导热作用明显;在温度不同的气流环境中,环境压力效应存在非单调性;压力效应发生转捩的气流温度与液滴初始粒径和气流速度相关.     

辐射传热空间非均匀性对燃油雾化的影响

将整个燃烧室部件(气缸盖-气缸套-活塞组-润滑油膜)作为一个耦合体,在对耦合体进行三维传热数值模拟的基础上,利用分区求解、边界耦合法建立缸内工作过程与燃烧室部件的三维耦合计算模型,并利用离散传递法实现缸内辐射传热与燃烧室部件的耦合三维仿真模拟,以此考察燃烧室部件辐射传热空间非均匀性对缸内燃油雾化的影响.结果表明,辐射换...     

激光手术喷雾冷却中单液滴蒸发理论模型的比较

制冷剂喷雾可对激光治疗葡萄酒色斑过程进行有效冷却以保护表皮不受热损伤,其瞬态喷雾冷却过程涉及复杂的液滴蒸发、强烈的对流换热和质量传递.本文对描述制冷剂喷雾平衡蒸发阶段单个液滴蒸发的理论模型进行改进,比较了基于各种假设的七个气相传质模型对R-134a喷雾冷却过程中单个液滴蒸发特性的影响,发现考虑过热影响的气相模型所预测的蒸发过程最接近实验结果,用该模型研究了液滴初始直径和速度等参数对液滴蒸发特性的影响.     

水分布对蒸发式冷凝器传热传质的影响

蒸发式冷凝器具有节水、节能、结构紧凑等优点而在工业制冷等领域得到了广泛的应用.在蒸发式冷凝器性能测试实验平台基础上,测试了水流参数如喷淋密度和水温对蒸发式冷凝器传热传质性能的影响.结果表明,在满足最小喷淋密度后,喷淋密度对蒸发式冷凝器传热传质性能影响很小;在稳定的操作条件下,蒸发式冷凝器水盘中的水温可以保持恒定;循环冷却水水温降低1.1～2.0℃,在相同的操作条件下,蒸发式冷凝器传热传质系数可以分别提高7.2%～16.9%和34.5%～63.4%.     

太阳辐照对静止水面稳态蒸发的影响

针对低风速下静止水面的蒸发过程,考虑水面与气流的对流传热传质、水体对太阳能辐射的容积内光谱吸收及内部传热,建立分析太阳辐照作用的水面稳态蒸发模型.采用控制容积法结合蒙特卡洛法和谱带模型数值求解水体内部、表面、气流之间的能量传递与质交换.分析空气湿度、温度、流速及正逆温差下,太阳辐照强度对于水面蒸发的影响.结果表明,太阳辐照强度对低风速下静止水面蒸发的影响很大.     

环境温度对掺水乳化柴油蒸发的影响研究

本文采用挂滴法研究了掺水质量分数30%(W30)的乳化柴油单液滴在573 K、673 K、773 K和873 K下的蒸发特性。结果表明,随着环境温度升高,归一化平方直径先增加后减小,首次微爆时刻逐渐小于进入准稳态时刻,而微爆频率先减小后增加,其微爆强度也先减小后增加,773 K是转捩点温度。本文还发现随着环境温度的升高,液滴气泡成核从单位点成核逐渐演变为多位点成核,并导致了大气泡形成过程从与强微爆过程重合过渡到与弱微爆过程重合,最终与强微爆和弱微爆过程分离。     

液滴蒸发对周围气相场的影响域特性分析

液滴在气体中运动蒸发的过程中,两相间会发生相互作用。为了弄清液滴对气相场的影响,基于液滴在气体环境中蒸发过程的现象和机理解释,提出液滴蒸发的影响域的概念,定义了影响域半径,并通过数值计算分析了不同条件下的影响域特性。结果表明,影响域内,由于液滴的存在使得其周围气相场参数变化较为剧烈;其他条件相同时,无量纲影响域半径几乎是一个定值。提出的影响域概念,对于研究气液两相间的相互作用机制具有重要意义。     

多孔介质结构对燃烧室内流场和喷雾特性影响的数值研究

以多孔介质发动机为背景,用数值模拟方法考察气缸内加入多孔介质蓄热体后对燃烧室内湍流流场及混合气形成的影响.计算基于Antohe和Lage的适用于多孔介质的κ-ε模型,其中引入了Darcy项和Forchheimer项.建立了一种可描述多孔介质随机结构的简化模型.针对燃油喷雾在不同结构多孔介质内的流场进行了数值计算.计算表明,多孔介质结构对燃烧室内燃油液滴的蒸发和混合气的形成过程具有重要的影响.     

Heat Transfer from a Hot Moving Steel Plate by Air-Atomized Spray Impingement

Air-atomized spray cooling of a hot moving AISI 304 steel plate of 6 mm thickness has been investigated experimentally by varying water flow rate and plate velocity at a fixed nozzle-to-plate distance. It is found that the heat transfer coefficient is a non-linear function of surface temperature. The result shows that the cooling rate increases with an increase in the water flow rate. The highest cooling rate has been found for the static plate, whereas for a moving plate, an increasing cooling rate trend has been observed with increasing plate velocity.     

Experimental Investigation Of Spray Cooling Heat Transfer On Straight Fin Surface Under Acceleration Conditions

A spray cooling heat transfer experiment on straight fin surface under acceleration conditions was conducted to investigate the effects of acceleration, flow rate, and nozzle height. The results show that the acceleration can improve the heat transfer performance in a limited way. In addition, whether in the acceleration or stationary condition, the flow rate as well as the nozzle height has the same impact on the spray cooling performance. It is also observed that the surface temperature can influence the effect of flow rate on spray cooling performance, and the cooling performance becomes worse with the increase of nozzle height.     

An Experimental Investigation of Liquid Jet Impingement and Single-Phase Spray Cooling Using Polyalphaolefin

Experiments on triangular and rectangular array jet impingement and single-phase spray cooling have been performed to determine the effect of both cooling techniques on heat transfer coefficient (h) and the coolant mass flux required for a given cooling load. Experiments were performed with circular orifices and nozzles for different H/D values from 1.5 to 26 and Reynolds number range of 219 to 837, which is quite lower than the ranges employed in widely used correlations. The coolant used was polyalphaolefin. The experiments simulated the boundary condition produced at the surface of the stator of a high power low-density generator or motor. For the custom fabricated orifices, commercial nozzles, and conditions used in this study, both cooling configurations showed enhancement of heat transfer coefficient as H/D increases to a certain limit after which it starts to decrease. The heat transfer coefficient always increases with Reynolds number. In keeping with previous studies, single-phase spray cooling technique can provide the same heat transfer coefficient as jets at a slightly lower mass flux, but with much higher-pressure head. Special Nu_d correlations that account for the range of parameters and coolant studied in this work are derived.     

Experimental Design and Methodology for Estimation of Local Heat Transfer Coefficient in Jet Impingement Using Transient Inverse Heat Conduction Problem

Abstract

This article aims to employ a two-dimensional inverse heat conduction technique in designing an experiment for accurately estimating the local convective heat transfer coefficient in slot jet impingement, given temperature measurements at some interior locations in the target plate. The method uses a sequential procedure together with the Beck function specification approach. Solution accuracy and experimental errors are examined using simulated temperature data. It is concluded that a good estimation of the space variable heat transfer coefficient can be made from the knowledge of the transient temperature recordings. The technique is used in a series of numerical experiments to provide the optimum experimental design for a slot jet impingement heat transfer investigation.     

Effect of Nozzle Geometry on the Rewetting of Hot Surface During Jet Impingement Cooling

An investigation has been carried out to investigate the effect of nozzle geometry on hot horizontal surface rewetting during water jet impingement cooling. The test surface of 800 ± 10°C initial surface temperature is cooled by water jet of 22 ± 1°C temperature. The water flow is varied to maintain the jet Reynolds number in a range of 5,000 to 24,000. The rewetting phenomena with sharp-edged and tube-type nozzles are compared on the basis of rewetting temperature, wetting delay, rewetting velocity, and maximum surface heat flux. The rewetting performance with tube-type nozzle is better than the sharp-edged nozzle particularly for the downstream spatial locations; however, maximum surface heat flux at the stagnation region is higher with the sharp-edged nozzle.     

涡轮叶片前缘阵列冲击冷却流动及传热特性数值研究

本文采用SST湍流模型模拟了类前缘通道内蒸汽射流阵列冲击冷却的流动与传热特性,分析了雷诺数(Re=10000～50000)、孔径比(d/H=0.5～0.9)和孔间距比(S/H=2～6)对流动及传热性能的影响规律,得到了相应的传热和摩擦关联式。结果表明:在不同雷诺数下,d/H从0.5到0.9变化时,通道压力损失系数降低了76%～79%,靶面平均努塞尔数降低了45%～49%;S/H从2增至6时,通道压力损失系数增加了1.64～1.92倍,靶面平均努塞尔数增加了54%～64%;增大d/H、减小S/H可有效提高类前缘通道蒸汽冲击冷却的综合热力系数。本文研究结果可为未来先进燃气轮机高温涡轮叶片蒸汽冷却结构的设计提供参考和借鉴。     

喷雾冷却相变传热的建模与模拟计算

喷雾冷却是一种高效的热控技术,为了探索形成完善的喷雾冷却技术设计流程,文章开展了喷雾冷却传热过程的建模研究.针对喷雾冷却传热过程的模拟计算,基于喷雾冷却相变传热的4个传热机制:液膜对流传热、池沸腾传热、二次气泡沸腾传热、二次气泡高频化机制,利用Monte Carlo方法描述了不同粒径与速率分布的液滴撞击液膜并生成二次气...     

高温空气燃烧炉内耦合传热的数值模拟

用自行开发的三维湍流流动、燃烧、辐射传热和NO_x湍流生成的计算程序,用离散坐标方法(DOM)模拟炉内的辐射传热并与其它传热方式相耦合,与各个输运方程共同求解,对燃烧室内的温度场进行了数值模拟.通过改变高温空气的预热温度,数值分析了高温空气燃烧炉内采用燃气直接喷射技术(FDI)的温度分布特性,检验了所采用的耦合数值模拟技术的有效性.模拟计算结果表明,提高空气预热温度,炉内温度的峰值相应增大,温度梯度降低,温度分布更均匀,火焰更长.相关的实验结果与数值预报的结果对比表明了相一致的规律.     

Impact of Surface Roughness on Flow Physics and Entropy Generation in Jet Impingement Applications

In this paper, a numerical investigation was performed of an air jet incident that normally occurs on a horizontal heated plane. Analysis of flow physics and entropy generation due to heat and friction is included using a simple easy-to-manufacture, surface roughening element: a circular rib concentric with the air jet. This study shows how varying the locations and dimensions of the rib can deliver a favorable trade-off between entropy generation and flow parameters, such as vortex generation and heat transfer. The performance of the roughness element was tested at three different radii; R/D = 1, 1.5 and 2, where D was the jet hydraulic diameter and R was the radial distance from the geometric center. At each location, the normalized rib height (e/D) was increased from 0.019 to 0.074 based on an increment of (e/D) = 0.019. The jet-to-target distance was H/D = 6 and the jet Reynolds number (Re) ranged from 10,000 to 50,000 Re, which was obtained from the jet hydraulic diameter (D), and the jet exit velocity (U). All results are presented in the form of entropy generation due to friction and heat exchange, as well as the total entropy generated. A detailed comparison of flow physics is presented for all ribs and compared with the baseline case of a smooth surface. The results show that at higher Reynolds numbers, adding a rib of a suitable height reduced the total entropy (St) by 31% compared to the no rib case. In addition, with ribs of heights 0.019, 0.037 and 0.054, respectively, the entropy generated by friction (Sf) was greater than that due to heat exchange (Sh) by about 42%, 26% and 4%, respectively. The rib of height e/D = 0.074 produced the minimum S_t at R/D = 1. As for varying R/D, varying rib location and Re values had a noticeable impact on Sh, Sf and (St). Placing the rib at R/D = 1 gave the highest total entropy generation (St) followed by R/D = 1.5 for all Re. Finally, the Bejan number increased as both rib height and rib location increased.