【作者】 刘正刚；

【导师】 潘继红；

【作者基本信息】 山东大学 ， 热能工程， 2008， 博士

【摘要】 目前我国正在逐步推行供热体制改革,对热量表的需求量极大,目前大多数热量表都采用旋翼式基表测量流量,与国外产品相比,国产热量表在测试精度和可靠性方面还存在着一定的差距。为提高热量表的精度,应从基本理论入手研究热量表基表内部的水流特性,分析影响流量测量的相关因素。基于计算流体动力学(CFD)技术的数值计算是研究基表内水流特性的重要手段,与其他旋转机械相比,旋翼式热量表的特点是:1、基表内部水温变化大,要求在5℃—95℃的温度范围内能够准确测量流量;2、热量表机芯采用塑料材料制造,塑料的刚度、强度较小,热膨胀系数大,并且随温度的升高强度和刚度下降,在高温、大流量水流的冲击下,热量表内部薄弱部位的弹性变形会影响基表内部的水流特性和流量测量特性;3、旋翼式基表的叶轮旋转速度取决于水流特性,是数值计算方法需要求解的变量之一。以上特点决定了旋翼式热量表应当采用流固耦合模型求解叶轮转速,分析其关键结构的弹性变形对水流特性的影响。本文采用流固耦合模型进行数值计算,在流体区域采用RNG k-ε湍流模型,分析基表内的水流特性;在结构区域采用有限元方法分析结构的受力和变形,采用ALE描述(任意拉格朗日一欧拉描述)处理固体和流体分界面的位移,流固耦合问题的求解采用迭代方式进行。采用流固耦合模型计算时,既考虑结构在水流冲击下的受力和变形,也考虑结构变形对流场产生的影响。流固耦合数值计算的结果需用实验来验证,实验在热量表专用实验台上进行,实验时利用称重法测量流过热量表的体积流量,采用无磁式信号采集方式测量叶轮旋转速度并算出仪表系数K。通过将数值计算方法得到的仪表系数K与测量值比较,可以验证数值计算的有效性。本文研究了双流束基表内部的水流特性。双流束基表内设置有分流片,使水流分成两束,从不同角度冲击叶轮,驱使其旋转。叶轮室和分流片的材料采用工程塑料MPPO制造。实验研究表明,双流束基表的K系数随着流量的增大而增大,并且在80℃的高水温下K系数随流量增大的程度大于25℃水温下的情况。采用数值计算对双流束基表内部的水流特性进行了分析,当不考虑固体结构的弹性变形时,数值计算结果显示双流束基表的K系数不随流量、水温变化,基本上是一个定值,与实验结果不符,表明不考虑固体结构变形的模型不符合实际情况。经分析发现,双流束基表的分流片是一个薄弱结构,所选用的MPPO材料的刚度、强度随温度的升高而下降,因此认为在大流量的冲击下分流片可能产生明显变形从而影响了基表的性能,在进行数值计算需考虑结构变形的影响。采用考虑结构弹性变形的流固耦合模型重新对基表内部的水流特性和流量测量性能进行研究后,数值计算结果表明,采用MPPO材料制造叶轮室并且分流片结构不合理时,在高温、大流量水流冲击下,分流片会产生明显变形,在80℃水温、5m~3/h的流量下,分流片的水平方向最大变形量可以达到0.948mm,分流片的变形改变了基表内部的水流特性,计算所得的仪表系数K随流量的增大而增大。由于MPPO的刚度随温度升高下降,导致在同样流量下高水温时分流片的变形量大于低水温的情况,相应地在高水温时K系数随流量的增加值大于低水温时的情况,这些计算结果与实验结果一致,表明分流片在大水流冲击下的变形对基表性能产生了较大影响。为改善双流束基表的性能,本文对分流片的结构进行了改进,增大了分流片的厚度,并改用弹性模量更大的PPS材料制造叶轮室和分流片。改进后,经流固耦合模型计算表明分流片的刚度得到了增强,分流片的弹性变形量很小已不足以影响基表的测量性能,K系数不再随温度和流量变化,基本保持为一常数。在此基础上,采用数值计算和实验相结合的方式研究了各种结构因素对基表性能的影响。研究结果表明,在进水口不安装格栅时,表前直管段长度大于10倍管径后,表前的旋流等不均匀流动的影响基本消失,完全可以保证热量表的测量精度;在进水口安装3×3mm规格的格栅后,表前只需安装4倍管径长度的直管段就能够保证测量精度;改变调节肋条的角度可以调整基表的K系数值,有利于调节不同基表的K系数,保持流量测量的一致性;叶片的数量以6片或者7片为佳,叶片数量少于5片叶轮旋转较慢,多于7片时叶轮转速增加不明显;叶轮室底部肋片的存在可以起到增强流量测量稳定性的作用。压力损失也是热量表标准中要求的一项指标,根据热量表标准的要求,20mm口径的热量表在额定流量下的压力损失不能大于25kPa。数值计算和实验均证实双流束基表的压力损失符合热量表标准中的要求,在入水口安装格栅后的压力损失为22.26kPa,实验结果与数值计算结果相差不大,表明数值计算是研究基表内部水流特性的有力工具。本文也对多流束基表内的水流特性进行了研究。多流束基表在叶轮室沿圆周方向布置了导流片,水流流过导流片后均匀冲击叶轮驱使叶轮快速旋转。叶轮室和导流片仍采用工程塑料MPPO制造。实验结果表明在水温不变时多流束基表的K系数随流量的变化很小,当水温变化时基表的K系数随水温升高而增大。采用数值计算的方法对多流束基表内部的流场进行了研究,当不考虑结构变形时计算结果显示基表的K系数不随水温变化,与实验结果不符。而采用考虑结构变形的流固耦合模型实施计算时计算所得的基表K系数随温度的变化关系与实验结果基本一致,说明结构变形影响了基表的性能。研究结果表明在高温时的热膨胀是导致多流束基表结构变形的主要原因,而水流的冲击引起的应力和变形都很小,可以忽略。叶轮室导流片的弹性变形导致多流束基表的K系数随温度的升高而增加,必须根据温度对K系数进行修正。采用数值计算与实验相结合的方式研究了多流束基表内部的水流特性和测量性能,研究表明多流束基表的性能非常稳定,在入水口不安装格栅时,基表前保留4倍管径长度的直管段就可以消除入口旋流的影响。数值计算和实验还表明多流束基表的压力损失较大,在额定流量下的压力损失达到了28.2kPa,超过了热量表标准中的要求,必须采取措施降低压力损失。为降低压力损失,设计了4个开泻压孔降低压力损失的方案,并采用数值计算对4个方案的降低压损的效果进行了研究,数值计算的结果表明,4个开孔方案都可以使压力损失降到25kPa以下,但在表前安装4倍管径长度的直管段、入水口存在旋流干扰的情况下,在背对入水管位置处开2×8mm长方形孔的方案可以保持测量的稳定性,该方案为最佳的开孔降低压力损失的方案。通过与实验结果相对比,证实了数值计算的可靠性。本文的研究给出了结构在水流冲击和热膨胀作用下变形并影响流场的流固耦合计算实例,采用数值计算的方法定量研究了分流片变形对基表测量性能的影响,所采用的模型和计算方法可以为类似问题提供参考和借鉴,具备一定理论意义。本文深入分析了小型旋翼式流量计内部的水流特性,详细研究了影响流量测量精度的各个结构因素,给出了确定的热量表安装条件,研究结果可以用来指导热量表的设计,减少热量表设计中的盲目性和开发成本,有着很强的实用价值。更多还原

【Abstract】 With the reform of charge system of heating supply,the heat meter is used widely in China.Most heat meter use rotating-wing flux meter to measure flux. Compared with the foreign products,the technological level of the domestic heat meter is still quite low and can’t measure flux accurately.In order to improve the measurement accuracy of heat meter,the flow characteristic of heat meter must be researched.Using Computational Fluid Dynamics to carry out numerical calculation is the main method of researching the flow characteristic.Compared with other rotating machinery,heat meter have own specialty:first,the water temperature scope of heat meter is 5-95℃,the heat meter should measure flux accurately while water temperature changes obviously.Secondly,the inner parts of heat meter are made by plastic material,the stiffness and strength of plastic material can decrease with the rising of temperature.The elastic deformation of inner structure can obviously influence the flow characteristic and performance of heat meter while flux and temperature reach the maximum.Thirdly,the impeller rotate speed is determined by flow and must be calculated by numerical calculation.According to flow characteristic of heat meter,the fluid structure interaction(FSI) model must be used while numerical calculation is carded out.The research uses FSI model to carry out numerical calculation.The ALE (arbitrary Lagrange -Euler) method is used to deal with the interface between the fluid and structure domain.FSI model is used to calculate the elastic deformation of structure and research how elastic deformation influences the flow characteristic of heat meter.The results of numerical calculation can be proved by experimental results, using non-magnetic method to measure the flux signal in special experimental instrument for heat meter and calculate the meter coefficient K.This paper designs a new type two flow-channel flow meter,a triangle deflector is located at the inlet,and it changes the direction of flow and divides the water flow into two parts.The water flows into the impeller cell and pushes the impeller to rotate. The impeller cell and deflector are made by MPPO material.Experimental results show meter coefficient K increase with the increasing of water flux,when water temperature is 80℃,the meter coefficient K increase more quickly than water temperature is 25℃with the increasing of water flux.The numerical calculation that elastic deformation is not considered is used to research the performance of two flow-channel flow meter,the calculated results show the meter coefficient K don’t change when temperature and flux increase.The difference between calculated results and experimental results show the numerical model that elastic deformation is not considered can not reflect the real situation.According to the structure characteristic of two flow-channel flux meter,the deflector is weak and maybe be deformed obviously and influence the measure performance with the increasing the temperature and flow flux.So the FSI model that elastic deformation is considered is used to carry out numerical calculation to analyze the effect of elastic deformation of deflector on measure performance of heat meter.The calculation results of FSI model show the deflector deforms obviously as the water temperature and flux increase,the maximum horizontal deformation can reach 0.948mm while temperature is 80℃and flux is 5m~3/h.The deformation of deflector changes the flow characteristic and causes the meter coefficient K increases while flux increases.The stiffness of MPPO decreases while temperature rises,so the deformation of deflector is more obviously and the meter coefficient K increases more quickly when temperature rises.The results of experiments can prove the calculation results are valid and show the elastic deformation of deflector influence the performance of measure.In order to reduce the deformation of deflector,the thickness of deflector is increased and using PPS instead of MPPO produces impeller cell and deflector.The experiment and calculation results prove new structure and material of deflector improve the stiffness and the meter coefficient K don’t change when flux and temperature changes.Based on the improved structure and material of deflector,the effects of structure element on performance of heat meter are researched.The experiment and calculation results show the length of straight tube located at the meter inlet should be 10 times as long as tube diameter for guarantee the measure accuracy when no screen is located at the inlet.While 3×3mm screen is located at the meter inlet,only 4D(D is the diameter of inlet) straight tube can guarantee the measure accuracy of heat meter.The researches also show the adjusting rib located at the top of impeller cell can adjust meter K;the optimal number of blade should be 6 or 7;the rib located at the bottom of impeller cell can stabilities the rotate speed of impeller.The pressure loss of two flow-channel heat meter is also researched,the pressure loss is 22.26kPa when screen is located at the inlet and flux is 2.5m~3/h.The pressure loss of two flow-channel meter can meet the requirement of heat meter standard.The paper design a multi-channel flow meter,the water flow is deflected 90°to enter impeller cell and push the impeller to rotate.Experiments show meter coefficient K increases while water temperature rises.The FSI model is used to research the flow characteristic of multi-channel flow meter,calculated results show thermo-expansion is main factor of influencing measure performance of multi-channel flow meter.Acceding to the calculated results,meter coefficient K must be revised while water temperature changes.Numerical and experimental method is used to research the flow characteristic and measure performance of multi-channel flow meter.The researches show the influence of swirl in inlet can be eliminated and measure accuracy can be guaranteed when the length of straight tube located an inlet is 4 times as long as tube diameter. Numerical and experimental researches prove the pressure loss of multi-channel reaches 28.2kPa,exceed 25kPa-the value required by heat meter standard.In order to reduce pressure loss,4 schemes are designed and researched by numerical calculation. Numerical calculation show drill a 2×8mm rectangle hole at the backside that can not be impacted by inlet flow is the best way to reduce pressure loss.The experimental results prove the calculation results are correct.The paper uses FSI model calculate the elastic deformation caused by thermo-expansion and water flow impact,analyses the effect of elastic deformation of structure on flow characteristic and measure performance.The model and calculation method used in this paper can provide reference for similar problem.This paper also researches the flow characteristic of rotating-wing heat meter,analyzes the key factor influence measure performance,and determines the install condition of heat meter. The results of this paper can direct the design and reduce development cost of heat meter.更多还原