Experimental study was conducted to determine the effect of velocity of axial fan,outlet vent height,position,area,and aspect ratio(h/w)of ventilated enclosure on convection heat transfer.Rectangular wooden ventilated...Experimental study was conducted to determine the effect of velocity of axial fan,outlet vent height,position,area,and aspect ratio(h/w)of ventilated enclosure on convection heat transfer.Rectangular wooden ventilated enclosure having top and front transparent wall was made up of Perspex for visualization,and internal physical dimensions of box were 200 mm×200 mm×400 mm.Inlet vent was at bottom while outlet vents were at the side and top wall.Electrically heated cylindrical heat source having 6.1 slenderness ratio was fabricated and hanged at the centre of the enclosure.To calculate heat transfer rates,thermocouples were attached to the inner surface of heat source with silica gel.Heat source was operated at constant heat flux in order to quantify the effect of velocity of air on heat transfer.It was observed that average Nusselt number was increased from 68 to 216 by changing velocity from 0 to 3.34 m/s at constant modified Grashof number i.e.5.67E+09.While variation in outlet height at the front wall did not affect heat transfer in forced convection region.However,Nusselt number decreased to 5%by changing the outlet position from top to the front wall or by 50%reduction in outlet area during forced convection.Mean rise in temperature of enclosure increased from 8.19 K to 9.40 K by increasing aspect ratio of enclosure from 1.5 to 2 by operating heat source at constant heat flux i.e.541.20 w/m^(2).展开更多
数据包络分析(dataenvelopment analysis,DEA)方法被广泛应用于相对有效性评价,明确数据无量纲化和指标相关性对DEA评价结果的影响机理有助于提升DEA方法应用的科学性。本文首先研究了线性数据无量纲化对DEA评价结果的影响,填补现有研...数据包络分析(dataenvelopment analysis,DEA)方法被广泛应用于相对有效性评价,明确数据无量纲化和指标相关性对DEA评价结果的影响机理有助于提升DEA方法应用的科学性。本文首先研究了线性数据无量纲化对DEA评价结果的影响,填补现有研究空白。研究发现,无截距项形式的线性数据无量纲化方法对DEA评价结果无影响,有截距项形式的线性无量纲化方法将使非DEA有效决策单元(decision making unit,DMU)的效率评价值发生偏差。在此基础上,控制数据无量纲化对DEA评价结果的影响,进一步研究了指标相关性对DEA评价结果的影响,深化了现有研究。研究发现,指标间简单的完全线性相关关系不会对DEA评价结果产生影响,实际评价中指标间复杂的相关关系会影响DEA评价结果,指标相关性可能会引起DEA有效的DMU的增多及具体效率评价值的升高。本文给出建议:DEA评价应用中应尽量避免做不必要的无量纲化处理,指标选取尽量具有代表性,避免相关性。本文最后对研究不足与下一步的研究展望进行总结。展开更多
文摘传统主元分析(Principal component analysis,PCA)方法因忽视量纲对系统的影响,从而使选取的主元难以具有代表性;而在进行量纲标准化后,又因得到的特征值常常是近似相等的而无法进行有效的主元提取.针对这一主要问题,本文通过引入相对化变换(Relative transform,RT)、相对主元(Relative principal components,RPCs)和分布"均匀"等概念,建立起一种相对主元分析(Relative principal component analysis,RPCA)的新方法.该方法首先对系统各分量进行量纲标准化;其次再根据系统的先验信息分析和确定各分量的重要程度;然后在系统能量守恒的准则下,赋以系统各分量相应的权值;最后利用已建立起的相对主元模型,对系统实施RPCA.同时运用数值例子,开展了RPCA在数据压缩和系统故障诊断中的应用研究.理论分析和仿真实验均表明,采用RPCA方法选取出的主元更具代表性和显著几何意义,加之选取主元的灵活性,将使新方法具有更广泛的应用前景。
文摘Experimental study was conducted to determine the effect of velocity of axial fan,outlet vent height,position,area,and aspect ratio(h/w)of ventilated enclosure on convection heat transfer.Rectangular wooden ventilated enclosure having top and front transparent wall was made up of Perspex for visualization,and internal physical dimensions of box were 200 mm×200 mm×400 mm.Inlet vent was at bottom while outlet vents were at the side and top wall.Electrically heated cylindrical heat source having 6.1 slenderness ratio was fabricated and hanged at the centre of the enclosure.To calculate heat transfer rates,thermocouples were attached to the inner surface of heat source with silica gel.Heat source was operated at constant heat flux in order to quantify the effect of velocity of air on heat transfer.It was observed that average Nusselt number was increased from 68 to 216 by changing velocity from 0 to 3.34 m/s at constant modified Grashof number i.e.5.67E+09.While variation in outlet height at the front wall did not affect heat transfer in forced convection region.However,Nusselt number decreased to 5%by changing the outlet position from top to the front wall or by 50%reduction in outlet area during forced convection.Mean rise in temperature of enclosure increased from 8.19 K to 9.40 K by increasing aspect ratio of enclosure from 1.5 to 2 by operating heat source at constant heat flux i.e.541.20 w/m^(2).
文摘数据包络分析(dataenvelopment analysis,DEA)方法被广泛应用于相对有效性评价,明确数据无量纲化和指标相关性对DEA评价结果的影响机理有助于提升DEA方法应用的科学性。本文首先研究了线性数据无量纲化对DEA评价结果的影响,填补现有研究空白。研究发现,无截距项形式的线性数据无量纲化方法对DEA评价结果无影响,有截距项形式的线性无量纲化方法将使非DEA有效决策单元(decision making unit,DMU)的效率评价值发生偏差。在此基础上,控制数据无量纲化对DEA评价结果的影响,进一步研究了指标相关性对DEA评价结果的影响,深化了现有研究。研究发现,指标间简单的完全线性相关关系不会对DEA评价结果产生影响,实际评价中指标间复杂的相关关系会影响DEA评价结果,指标相关性可能会引起DEA有效的DMU的增多及具体效率评价值的升高。本文给出建议:DEA评价应用中应尽量避免做不必要的无量纲化处理,指标选取尽量具有代表性,避免相关性。本文最后对研究不足与下一步的研究展望进行总结。
