石蜡与低熔点合金双级联相变材料强化板翅式散热器换热性能的数值模拟

相变散热技术可以通过相变材料的物态变化,有效吸收或释放热量,从而实现被动式热管理。本文建立了一种板翅式相变散热器的结构模型,研究级联相变技术的相变传热过程及优化传热性能的措施,重点分析了相变材料(PCM)的组合方式、翅片结构以及不同的PCM体积比工况对相变散热器传热性能的影响。研究结果表明,改变PCM的组合方式对相变材料传热过程有显著影响,对于采用双PCM的级联相变散热器,PCM1和PCM2呈相变温度递减排列具备更优的热管理性能;当翅片数为16时,具备更低的工作温度,并且相比于四翅片结构的相变散热单元,两种级联PCM组合的相变散热单元的凝固时间分别缩短了25.3%和22.5%。因此,当翅片数为16及翅片厚度为0.5 mm时,热管理性能较优;采用石蜡和低熔点合金材料,当PCM体积比为1∶2时,两个临界温度下的热管理时间分别延长了17.2%和15%,温升速率也随着低熔点合金所占体积增多而逐渐减小,综合考虑热管理时间和温升速率,可知PCM体积比为1∶2时级联相变散热器的热管理性能较优。

非均匀翅片对级联相变储热系统热性能强化的研究

级联相变储热是一种具有重要商业前景的相变储热技术。利用数值模拟方法研究了一种采用非均匀翅片布置进行强化传热的级联相变储热系统,分析了各级相变储热模块采用均匀翅片数量和均匀翅片高度布置对系统的强化作用,进而研究了各级模块采用非均匀翅片数量和非均匀翅片高度布置下系统的充/放热性能。研究结果表明:相比于各级采用均匀翅片数量布置,采用非均匀翅片数量布置的级联相变储热系统可缩短完全熔化及凝固总时间约5.2%,同时提高系统充热平均功率约12.6%;采用非均匀翅片高度布置可略微缩短系统完全熔化及凝固总时间,但对系统充/放热平均功率影响较小。研究结果对级联相变储热系统的强化传热具有一定理论指导价值。

Realization of a Nine-Level Cascaded NPC/H-Bridge PWM Inverter Using Phase-Shifted Carrier PWM Technique

Cascaded H-Bridge inverter has been researched for the past two decades, but there are no explicit guidelines on how one can realize a cascaded NPC （neutral-point-clamped）/H-Bridge inverter. Past research has also concentrated on realizing a five-level NPC/H-Bridge inverter. This fails to address the principle of realizing a general cascaded N-level NPC/H-Bridge PWM inverter. This paper proposes an improved topology for achieving a nine-level cascaded NPC （neutral-point-clamped） H-Bridge inverter with reduced harmonic content. This new proposed topology requires a lesser number of separate dc sources as compared to conventional cascaded H-Bridge inverter. The whole system is considered as having four three level legs having two positive and two negative legs. By properly phase shifting the modulating wave and carriers, a nine-level output is achieved. A theoretical harmonic analysis of the proposed inverter is carried out based on double Fourier principle. The theoretical results are verified through MATLAB simulation.