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侧载及加热方位对槽道内临界热流密度影响 总被引:1,自引:0,他引:1
为了研究侧载以及加热方位对矩形窄缝槽道内临界热流密度特性的影响,在旋转平台上进行了两相流的超重力实验.以蒸馏水为工质,通过改变质量流速、入口温度、侧载大小以及加热方位,获得了发生临界换热现象时的质量流速、实验段压降和壁温的变化趋势,考察了侧载和加热方位对临界热流密度的影响规律,并对侧载下两相流不稳定性进行了简析.实验表明:发生临界换热时,壁温迅速上升,有效加热热流开始减少,关闭加热电源后,实验段压降下降、质量流速回升较明显;侧载以及不同的加热方位对临界热流密度有明显影响;临界热流密度发生前后,蒸汽回流导致进口温度上升,增加了流动的不稳定性. 相似文献
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建立以等热流密度方式进行试验件加热的沸腾换热试验系统,分别对当量直径为1.28mm和1.59mm锯齿扁管内R134a工质的沸腾换热特性进行研究,试验参数范围:制冷剂质量流率为68.5~305.5kg/(m2·s),工作饱和压力为0.27~0.46 MPa,加热热流密度为9~42kW/m2。试验结果表明:相同结构的通道,当量直径小换热能力更强;热流密度和饱和压力对沸腾换热的影响与一个干度值有关。当干度小于此值时,沸腾换热系数会随着热流密度及饱和压力增大而增大;而当干度大于此值时,沸腾换热系数随着干度增大而急剧下降,热流密度和饱和压力对换热的影响较小;该干度值会随着热流密度或饱和压力增大而逐渐变小。质量流率对沸腾换热的影响与热流密度有关,随着热流密度增大,质量流率的影响趋向大干度区域。通过分析各参数对沸腾换热的影响,建立了一个预测试验工况下微小尺寸锯齿扁管的沸腾换热系数计算经验公式。 相似文献
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为研究激光功率与底面状态对选区激光熔化熔池流动的影响,基于离散单元法建立了选区激光熔化铺粉模型,采用粒径分布与实验相符的马氏体时效钢粉末分别铺展到平坦底面和增材底面上,将计算获得的粉末分布导入到基于有限体积法建立的选区激光熔化熔池计算流体力学模型中,研究激光功率和基板底面粗糙度对熔池流动和熔道表面形貌的影响。采用激光单道扫描实验验证铺粉模型和选区激光熔化模型。结果表明:随着激光功率的降低,单位长度的球化数量增加;由于增材底面使熔池润湿性变差,同时又对熔池流动行为产生扰动,使得增材粗糙底面上熔道的球化数量增加。选区激光熔化铺粉模拟及激光单道扫描模拟结果与实验结果吻合较好。本研究可为选区激光熔化工艺中工艺参数的选择提供理论指导。 相似文献
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This article presents the microstructure and hardness variation of an Al–8.5Fe–1.3V–1.7Si(wt%, FVS0812) alloy after selective laser melting(SLM) modification. Three zones were distinguished across the melting pool of the SLM-processed FVS0812 alloy: the laser melted zone(LMZ), the melting pool border, and the heat affected zone(HAZ) in the previously deposited area around the melting pool. Inside the LMZ, either an extremely fine cellular-dendritic structure or a mixture zone of the a-Al matrix and nanoscale Al_(12)(Fe,V)_3Si particles appeared. With a decreased laser beam scanning speed, the cellular-dendritic structure zone within the LMZ shrank significantly while the mixture zone expanded. The a-Al and Al_(12)(Fe,V)_3Si mixture zone was also observed in the HAZ, but another phase, submicron h-Al_(13)Fe_4 particles with rectangular or hexagonal shapes,formed along the melting pool border. Microhardness tests indicated that the hardness of the SLM-processed FVS0812 samples far exceeded that of the as-cast FVS0812 alloy. 相似文献
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Single track and single layer formation in selective laser melting of niobium solid solution alloy 总被引:1,自引:2,他引:1
Selective laser melting (SLM) was employed to fabricate Nb-37Ti-13Cr-2Al-1Si (at%) alloy, using pre-alloyed powders prepared by plasma rotating electrode processing (PREP). A series of single tracks and single layers under different processing parameters was manufactured to evaluate the processing feasibility by SLM, including laser power, scanning speed, and hatch distance. Results showed that continuous single tracks could be fabricated using proper laser powers and scanning velocities. Both the width of a single track and its penetration depth into a substrate increased with an increase of the linear laser beam energy density (LED), i.e., an increase of the laser power and a decrease of the scanning speed. Nb, Ti, Si, Cr, and Al elements distributed heterogeneously over the melt pool in the form of swirl-like patterns. An excess of the hatch distance was not able to interconnect neighboring tracks. Under improper processing parameters, a balling phenomenon occurred, but could be eliminated with an increased LED. This work testified the SLM-processing feasibility of Nb-based alloy and promoted the application of SLM to the manufacture of niobium-based alloys. 相似文献
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Oracle系统中,客户和服务器进程通过共享池进行自身对话或与其他进程之间的通信,在系统长期运行的过程中,共享池中因大量进程的调入调出操作而产生大量内存碎片,导致系统运行出错。本文给出了一种利用动态SQL及DBMS_JOB包来实现自动冲洗共享池的方法。 相似文献
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成分调整对DZ125合金凝固行为的影响 总被引:1,自引:0,他引:1
研究了标准成分的DZ125合金与以此为基础提高W、Al,降低Mo、Ti合金中的初生相的凝固顺序,确定了这两种合金在固-液范围内不同温度下的固-液相比例。结果表明,高W、Al低Mo、Ti合金显著缩小了合金凝固范围(Δ),使液相线温度提高了20℃,固相温度提高了70℃,并缩小失去技晶间毛细补缩能力时的温度与固相线之间的温度范围(ΔT2),从而减少了合金热裂倾向,提高了可铸性。Ti含量影响(γ+γ')共晶相尺寸、数量及形态,W、Wo对(γ+γ')共晶的数量、尺寸及形态均无影响。 相似文献
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空气辅助喷射闪急沸腾喷雾特性试验 总被引:1,自引:2,他引:1
在定容弹内利用高速相机和相位多普勒粒子分析仪(PDPA)研究了环境压力和燃油温度对空气辅助喷射系统喷雾特性的影响。试验环境压力的变化范围为0.01MPa到0.1MPa,燃油温度的变化范围为25℃到100℃。试验结果表明:随着环境压力的减小,燃油喷雾逐渐从冷态过渡到闪急沸腾状态,在喷嘴出口处喷雾气泡急剧增加。当环境压力小于0.02MPa时,喷雾处于闪急沸腾状态,液滴速度增加,粒径减小,在喷雾近端出现喷雾膨胀现象;随着燃油温度的上升,液滴表面张力减小,液滴易于破碎。当燃油温度达到100℃时,喷雾处于闪急沸腾状态,喷雾远端出现喷雾膨胀现象。 相似文献
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