河南科技大学热能与动力工程专业英语考试.docx

1、河南科技大学热能与动力工程专业英语考试2.6 Natural Convection 自然对流 Heat transfer involving motion in a fluid caused by the difference in density and the action of gravity is called natural or free convection. Heat transfer coefficients for natural convection are generally much lower than for forced convection, and it i

2、s therefore important not to ignore radiation in calculating the total heat loss or gain. Radiant transfer may be of the same order of magnitude as natural convection, even at room temperatures, since wall temperatures in a room can affect human comfort. 由于密度差和重力作用引起流体运动而产生的热传递称为自然对流或自由对流。自然对流的传热系数一

3、般远低于强迫对流，因此在计算总的吸热量或放热量时，主要的一点就是不要忽略辐射热。辐射热传递与自然对流可以有相同的数量级，甚至在室温下也如此，因为室内墙体温度影响人体的舒适感。Natural convection is important in a variety of heating and refrigeration equipment: (1) gravity coils used in high humidity cold storage rooms and in roof-mounted refrigerant condensers, (2) the evaporator and co

4、ndenser of household refrigerators, (3) baseboard radiators and convectors for space heating and (4) cooling panels for air conditioning. Natural convection is also involved in heat loss or gain to equipment casings and interconnecting ducts and pipes. 在各种供热和制冷设备中自然对流是非常重要的：（1）在高湿度冷藏室内及室内安装的制冷剂冷凝器内使

5、用的重力盘管；（2）家用冰箱的蒸发器和冷凝器；（3）空间采暖用的踢脚板散热器和对流器；（4）空调用辐射对流护壁板。设备外壳及其连接风道和管道的放热量和吸热量中也包含自然对流。Consider heat transfer by natural convection between a cold fluid and a hot surface. The fluid in immediate contact with the surface is heated by conduction, becomes lighter and rises because of the difference in

6、density of the adjacent fluid. The motion is resisted by the viscosity of the fluid. The heat transfer is influenced by: (1) gravitational force due to thermal expansion, (2) viscous drag and (3) thermal diffusion. It may be expected to depend on the gravitational acceleration g, the coefficient of

7、thermal expansion , the kinematic viscosity (=/), and the thermal diffusivity =(k /c p ). These variables can be expressed in terms of dimensionless numbers: the Nusselt number, Nu, is a function of the product of the Prandtl number, Pr, and Grashof number, Gr, which, when combined, depend on the fl

8、uid properties, the temperature difference between the surface and the fluid, t, and the characteristic length of the surface, L. The constant c and exponent n depend on the physical configuration and nature of flow. 现在我们来考虑冷流体和热表面间由自然对流引起的热传递。与表面直接接触的流体由于导热而被加热、变轻，并且由于与相邻流体的密度差而上升。这种运动由于流体粘性而受到阻碍。热

9、传递受下列因素影响：（1）因热膨胀而引起的重力作用；（2）粘性阻滞；（3）热扩散。这种热传递被认为取决于重力加速度g、热膨胀系数、运动粘滞系数(=/)，以及导热系数=( k /c p )。这些变量可以用无因次数的形式给出：努谢尔特数Nu，它是普朗特数Pr和格拉晓夫数Gr乘积的函数，Pr与Gr的乘积，取决于流体特性、表面与流体间的温差t和表面特征长度L。常数c和指数n取决于物体外形和流动性质。The entire process of natural convection cannot be represented by a single value of exponent n, but can

10、 be divided into three regions: (1) turbulent natural convection for which n equals 0.33, (2) laminar natural convection, for which n equals 0.25 and (3) a region that has (GrPr) less than for laminar natural convection, for which the exponent n gradually diminishes from 0.25 to lower values. Note t

11、hat, for wires, the (GrPr) is likely to be very small, so that the exponent n is 0.1.自然对流的全部过程不能由单一的指数n表示，但可以分成三个区域：（1）紊流自然对流，n等于0.33；（2）层流自然对流，n等于0.25；（3）（GrPr）小于层流自然对流的区域，指数n从0.25逐渐减小到更低的值。注意，对金属线，（GrPr）可能会很小，所以n是0.1。To calculate the natural convection heat transfer coefficient, determine (GrPr) t

12、o find whether the boundary layer is laminar of turbulent, then apply the appropriate equation. The correct characteristic length indicated must be used. Since the exponent n is 0.33 for a turbulent boundary layer, the characteristic length cancels out, and the heat transfer coefficient is independe

13、nt of the characteristic length. Turbulence occurs when length or temperature difference is large. Since the length of a pipe is generally greater than its diameter, the heat transfer coefficient for vertical pipes is larger than for horizontal pipes. 为了计算自然对流传热系数，先算出（GrPr），确定边界是层流还是紊流，然后应用合适的方程。必须使

14、用已指出的正确的特征长度。由于紊流边界层的指数n是0.33，特征长度消去，传热系数与特征长度无关。当长度或温差大时就产生紊流。由于管道的长度一般大于它的直径，竖管的传热系数大于横管的传热系数。Convection from horizontal plates facing downward when heated (or upward when cooled ) is a special case. Since the hot air is above the colder air, there is no theoretical reason for convection. Some con

15、vection is caused, however, by secondary influences such as temperature differences on the edges of the plate. As an approximation, a coefficient of somewhat less than half of the coefficient for a heated horizontal plate facing upward can be used. 热面朝下（冷面朝上）的水平板对流是个特例。由于热空气在较冷空气之上，理论上不会产生对流。然而，一些次要

16、影响，诸如平板边缘的温差，会引起一些对流。作为一种近似，可以使用这样一个（传热）系数，该系数比热面朝上的水平板的系数的一半稍低。Since air is often the heat transport fluid, simplified equations for air are given. Other information on natural convection is available in the general heat transfer references. 由于空气经常是传热流体，空气的简化方程已经给出。其他有关自然对流的资料在一般有关传热的参考书中可以得到。Obser

17、ved differences in the comparisons of recent experimental and numerical results with existing correlations for natural convective heat transfer coefficients indicate that caution should be taken when applying coefficients for (isolated) vertical plates recommended by ASHRAE for situations with verti

18、cal surfaces in enclosed spaces (buildings). Improved correlations for calculating natural convective heat transfer from vertical surfaces in rooms under certain temperature boundary conditions have been developed.将最近的试验和数据结果与现有的自然对流传热系数关系式相比较，观察到的差别表明，在封闭空间（建筑物）内的竖直表面使用ASHRAE推荐的（独立的）竖直平板传热系数时应该小心。计

19、算房间内一定温度边界条件下竖直表面自然对流传热的改进关系式已建立起来。Natural convection can affect the heat transfer coefficient in the presence of weak forced convection. As the forced convection effect, i.e . , the Reynolds number, increases, the “mixed convection” (superimposed forced-on-free convection ) gives way to the pure fo

20、rced convection regime . since the heat transfer coefficient in the mixed convection region is often larger than that calculated based on the natural of forced convection calculation alone, attention is called to references on combined free and forced convection heat transfer. The reference given be

21、fore summarizes natural, mixed, and forced convection regimes for vertical and horizontal tubes. Local conditions influence the values of the convection coefficient in a mixed convection regime, but the references permit locating the pertinent regime and approximating the convection coefficient. 在较弱

22、的强迫对流情况下，自然对流会影响其传热系数。随着强迫对流效应，也就是雷诺数的增加，“混合对流”（加上自由对流的强迫对流）让位于纯强迫对流。由于混合对流的传热系数常常大于单纯自然对流或强迫对流的计算结果，故要参考有关自然对流与强迫对流综合作用的文献，文献概述了垂直和水平管道的自然对流、混合对流和强迫对流区。在混合对流中，局部条件影响对流系数值，但文献中允许选定适当的区域和近似取得对流系数。4.1TheIdealBasicVaporCompressionRefrigerationCycle11Theequipmentdiagramforthebasicvaporcompressioncycleis

23、illustratedinFig.4.1.2Minimumcomponentsofthiscycleincludecompressor,condenser,expansionvalveandevaporator.3Theidealcycleconsidersheattransferinthecondenserandevaporatorwithoutpressurelosses,areversibleadiabatic(isentropic)compressor,andanadiabaticexpansionvalve,connectedbypipingthathasneitherpressur

24、elossnorheattransferwiththesurroundings.4Therefrigerantleavestheevaporatoratpoint1asalowpressure,lowtemperature,saturatedvaporandentersthecompressor,whereitiscompressedreversiblyandadiabatically(isentropic).5Atpoint2，itleavesthecompressorasahightemperature,highpressure,superheatedvaporandenterstheco

25、ndenser,whereitisfirstdesuperheatedandthencondensedatconstantpressure,6Atpoint3，therefrigerantleavesthecondenserasahighpressure,mediumtemperature,saturatedliquidandenterstheexpansionvalvewhereitexpandsirreversiblyandadiabatically(constantenthalpy).7Atpoint4，itleavestheexpansionvalueasalowpressure,lo

26、wtemperature,lowqualityvaporandenterstheevaporator,whereitisevaluatedreversiblyatconstantpressuretothesaturatedstateatpoint1.8Heattransfertotheevaporatorandfromthecondenseroccurswithoutafinitetemperaturedifferencebetweenthefluidemittingtheheatandthefluidthatabsorbstheheat,exceptduringthedesuperheati

27、ngprocessinthecondenser.29Anenergybalanceandcertainperformanceparameterscanbederivedfromthefirstlawofthermodynamics.10Applyingthesteadyflowequationforthefirstlawtoeachofthecomponentsofthebasicvaporcompressioncycle,thefollowingrelationshipsarederived:12Compression23Condensing34ExpansionValve41Evapora

28、tor311Inapplyingthesteadyflowequation,kineticenergyandpotentialenergytermswereomitted;becauseflowvelocitiesarelowtoavoidfluidfrictionandundesirablepressurelosses,andheightvariationwithinainsignificant.12Sincethesystemiscyclic,theheatrejectedinthecondensermustbeequaltothesumoftheheatabsorbedintheevap

29、oratorandtheworkofcompression.413CoefficientofPerformance(COP)isusedtoevaluatetheperformanceofarefrigerationsystem.COP=refrigerationeffect/networkinput.514Forthebasicvaporcompressioncycle,fromEq.(4.1)and(4.3),theCOPisCOP=(h1-h4)/(h2-h1)615Inevaluatingcontributionsofthecompressortothermodynamicsystem

30、s,itisnecessarytoconsiderpropertiesoftherefrigerantsattheinletandoutletofthecompressor,withthechangeinstatebetweenthesepointsbeing(1)reversibleandadiabatic(isentropic)fortheidealcompressor;or(2)adiabaticandirreversible(withanincreaseinentropyinthefluidpassingthroughthecompressor)withthevariationfromtheidealcompressordescribedbytheadiabaticcompressorefficiency.716Animportantthermodynamicconsiderationforthepositivedisplacementcompressoristheeffectoftheclearancevolume,i.e.,thevolumebetweenpistonandcylinderheadwhenthepistonisinatop,centerposition.17Af