Helmholtz共振的机理研究及应用
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摘要
换热器是广泛应用于化工、石油、动力、食品及其它许多工业部门的通用设备，在生产中占有重要地位。为了节能降耗必须开发出适用于不同工业过程要求的高效能换热设备。本课题将Helmholtz共振腔所产生的自激振荡脉冲射流引入换热器以实现强化换热。这是一种崭新的强化换热技术，流体脉动能够造成换热器全程的换热强化，而且自激振荡脉冲射流强化换热技术本身是一种无功强化的方法，无需输入外部能量来产生扰动。
本文首先介绍了Helmholtz共振腔产生自激振荡以及强化换热的机理。当前喷嘴流束中的不稳定扰动波在穿过腔内的剪切层时，剪切层对其有选择放大作用，形成涡环结构，剪切流动中涡环与碰撞壁撞击，在碰撞区域产生压力扰动波并向上游反射，在上游剪切层分离处诱发新的扰动的产生，当新扰动与原扰动匹配时，射流上游就被不断地受到周期性激励，腔内就产生流体自激振荡并在后喷嘴出口形成脉冲射流。将自激振荡脉冲射流引入换热器后，流体的脉动导致了壁面处旋涡的大量生成，增加了流体的掺混，实现强化换热的目的。
在Helmholtz共振腔的设计一章中，首先介绍了流体网络理论的基本知识，然后根据流通网络理论的相关知识推导了计算腔室的固有频率公式，并由此公式计算了本课题所使用的Helmholtz共振腔的固有频率，然后根据腔室的固有频率公式计算了有关结构参数，最后综合考虑影响自激振荡的各种因素设计了Helmholtz共振腔。
最后，本文运用设计的Helmholtz共振腔进行了自激振荡脉冲射流强化换热的实验研究，通过实验发现：对于本课题设计的Helmholtz共振腔，只要配合以适当的水力参数，就可以产生自激振荡脉冲射流。对于同一结构的Helmholtz共振腔，水力参数不同，所产生的自激振荡脉冲射流的强弱也不相同。将Helmholtz共振腔产生的自激振荡脉冲射流引入换热器后，当自激振荡的强度达到一定程度后，可以强化换热。不同的自激振荡强度，强化换热的效果也不同。对于同一Helmholtz共振腔，不同的出口结构尺寸对于换热效果的强化的影响不大，但是腔室长度对强化比的影响却是显著的。只要选定合适的水力参数和结构参数，Helmholtz共振腔可以将管内流动换热系数提高5%—30%。
This dissertation is devoted to the investigation of the influence of self-excited oscillation jet on heat transfer enhancement.
Firstly, the mechanism of self-excited oscillation was introduced. When the water jets from the inlet nozzle flows into the chamber, the momentum exchange between the jet and the liquid in the chamber occurs, thus to form an unstable shearing layer with certain thickness. The shearing layer is carried by the jet and the eddy is formed because of the instability and selective amplification of shearing layer which will propagate to the downstream. As the eddy impacts the impinging wall, a wave of pressure disturbance with certain frequency is induced. This wave then propagates to the upstream with high speed, and results in the overlap and amplification of the waves when the frequencies of them are close with each other. The repetition of the above process leads to the occurrence of the self-excited oscillation pulsating jet at the exit of the outlet of the back nozzle. When the pulsating jet was introduced in the heat exchanger, pulsating flow leads to the formation of an amount of vortices near the tube wall. The boundary layer was destroyed and the mixing of liquid was improved, so heat transfer enhancement was achieved.
Secondly, the Helmholtz oscillator was designed. Based on the flow network theory, the expression of intrinsic frequency was deduced. The main parameter of Helmholtz oscillator was calculated.
Finally, the experiment was performed in order to exam the effect. The pulsating liquid flows through a single pipe heated electrically. The heat transfer coefficients were measured under different conditions. Several important conclusions based on the experiment are drawn. (1) The performance of the oscillator is very sensitive to the geometric structure and the size of its parts. If the geometric structure and flow rate or pressure difference of oscillator are suitable, the expected oscillation can be obtained. (2) A remarkable enhancement of heat transfer was observed when the oscillator was installed. Experiments show that the heat transfer coefficient can increase 5-30%in comparing with the normal case when the oscillator was removed.
本文首先介绍了Helmholtz共振腔产生自激振荡以及强化换热的机理。当前喷嘴流束中的不稳定扰动波在穿过腔内的剪切层时，剪切层对其有选择放大作用，形成涡环结构，剪切流动中涡环与碰撞壁撞击，在碰撞区域产生压力扰动波并向上游反射，在上游剪切层分离处诱发新的扰动的产生，当新扰动与原扰动匹配时，射流上游就被不断地受到周期性激励，腔内就产生流体自激振荡并在后喷嘴出口形成脉冲射流。将自激振荡脉冲射流引入换热器后，流体的脉动导致了壁面处旋涡的大量生成，增加了流体的掺混，实现强化换热的目的。
在Helmholtz共振腔的设计一章中，首先介绍了流体网络理论的基本知识，然后根据流通网络理论的相关知识推导了计算腔室的固有频率公式，并由此公式计算了本课题所使用的Helmholtz共振腔的固有频率，然后根据腔室的固有频率公式计算了有关结构参数，最后综合考虑影响自激振荡的各种因素设计了Helmholtz共振腔。
最后，本文运用设计的Helmholtz共振腔进行了自激振荡脉冲射流强化换热的实验研究，通过实验发现：对于本课题设计的Helmholtz共振腔，只要配合以适当的水力参数，就可以产生自激振荡脉冲射流。对于同一结构的Helmholtz共振腔，水力参数不同，所产生的自激振荡脉冲射流的强弱也不相同。将Helmholtz共振腔产生的自激振荡脉冲射流引入换热器后，当自激振荡的强度达到一定程度后，可以强化换热。不同的自激振荡强度，强化换热的效果也不同。对于同一Helmholtz共振腔，不同的出口结构尺寸对于换热效果的强化的影响不大，但是腔室长度对强化比的影响却是显著的。只要选定合适的水力参数和结构参数，Helmholtz共振腔可以将管内流动换热系数提高5%—30%。
This dissertation is devoted to the investigation of the influence of self-excited oscillation jet on heat transfer enhancement.
Firstly, the mechanism of self-excited oscillation was introduced. When the water jets from the inlet nozzle flows into the chamber, the momentum exchange between the jet and the liquid in the chamber occurs, thus to form an unstable shearing layer with certain thickness. The shearing layer is carried by the jet and the eddy is formed because of the instability and selective amplification of shearing layer which will propagate to the downstream. As the eddy impacts the impinging wall, a wave of pressure disturbance with certain frequency is induced. This wave then propagates to the upstream with high speed, and results in the overlap and amplification of the waves when the frequencies of them are close with each other. The repetition of the above process leads to the occurrence of the self-excited oscillation pulsating jet at the exit of the outlet of the back nozzle. When the pulsating jet was introduced in the heat exchanger, pulsating flow leads to the formation of an amount of vortices near the tube wall. The boundary layer was destroyed and the mixing of liquid was improved, so heat transfer enhancement was achieved.
Secondly, the Helmholtz oscillator was designed. Based on the flow network theory, the expression of intrinsic frequency was deduced. The main parameter of Helmholtz oscillator was calculated.
Finally, the experiment was performed in order to exam the effect. The pulsating liquid flows through a single pipe heated electrically. The heat transfer coefficients were measured under different conditions. Several important conclusions based on the experiment are drawn. (1) The performance of the oscillator is very sensitive to the geometric structure and the size of its parts. If the geometric structure and flow rate or pressure difference of oscillator are suitable, the expected oscillation can be obtained. (2) A remarkable enhancement of heat transfer was observed when the oscillator was installed. Experiments show that the heat transfer coefficient can increase 5-30%in comparing with the normal case when the oscillator was removed.
引文
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[24] 汪志明，沈忠厚等，“自激振荡气蚀射流喷管内水声学理论及其应用”，水动力学研究与进展，1994，Vol.9，No.3
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[31] 唐川林，杨林等，“来流脉动对自激振荡脉冲射流的影响：，力学与实践，2001，Vol.23，No.3，pp24-27
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[2] 顾维藻等编，《强化传热》，科学出版社，1990
[3] 余德渊，“实用强化传热技术简介”，石油化工设备，1995，Vol.24，No.1，pp3-7
[4] 周士强，“强化传热在换热器中的应用”，黑龙江石油化工，1998，Vol.9，No.2，pp31-33
[5] 曹纬，“国外换热器新进展”，石油化工设备，1999，Vol.28，No.2，pp6-9
[6] King J. L., Doyle P., Ogle J. B., "Instability in slotted wall tunnels", Fluid Mech., 1958, Vol.4, pp283-305
[7] Crow S. C., Champagne F. H. "Orderly Structure in Jet Turbulence", Fluid Mech., 1971, Vol.48, pp547-591
[8] D. Rockwell, E. Naudascher, "Review----Self-Sustaining Oscillations of Flow Past Cavities", Transactions of the ASME, 1978, Vol.100，JUNE，pp152-165
[9] Nebeker E. B., Rodriguez S. E., "Percussive Water Jets for Rock Cutting", Proc 3rd Int Symp on Jet Cutting Technology, 1976, pp123-132
[10] Conn A. F., Johnson V. E., "the fluid dynamics of submerged cavitating jet cutting", Proc 5th Int Symp on Jet Cutting Technology, 1980, pp201-209
[11] D. Rockwell, "Prediction of Oscillation Frequencies for Unstable Flow Past Cavities", Transactions of the ASME, 1977, June, pp294-300
[12] Sammi, Anderson C., "Helmholtz Oscillator for the self-modulation of a jet", Proc 7th Int Symp on Jet Cutting Technology, 1984, pp131-143
[13] T. Moschandreou ，M. Zamir, "Heat Transfer in a Tube With Pulsating Flow and Constant Heat Flow", Int. J. Heat Mass Transfer. 1997, Vol. 40, No.10, pp2461-2466
[14] M. A. Habib, A. M. Attya, A.L.Eid, A. Z. Aly, "Convective heat transfer characteristics of laminar pulsating pipe air flow", 2002, Heat Mass Transfer. Vol. 38, pp.221-232
[15] Zhixiong Guo，Hyung Jin Sung, "Analysis of Nusselt number in pulsating pipe flow", Int. J. Heat Mass Transfer. 1997, Vol. 40, No.10, pp2486-2489
[16] 蒋海军，"自激振荡脉冲射流机理探讨"，西南石油学院学报，1998，Vol.20，No.3，pp55-58
[17] 汝大军等，“自激振荡腔频率特性及腔室设计研究”，西南石油学院学报，1999，Vol.21，No.4，pp78-81
[18] 杨秀夫，廖荣庆，“脉冲射流信号分析软件的研制与应用”，西南石油学院学报，1994，
No.2，pp23-27
[19] 张德斌，李根生等，“围压作用下自振空化射流脉动特性实验研究”，中国安全科学学报，1999，Vol.10，pp6-11
[20] 杨林，李晓红，“结构参数对自激振荡脉冲射流固有频率的影响”，流体机械，2001，Vol.29，No.2，pp26-28
[21] 廖振方，唐川林，“自激振荡脉冲射流喷嘴的理论分析”，重庆大学学报（自然科学版），2002，Vol.25，No.2，pp24-27
[22] 杨林,李晓红等，“自激振荡脉冲磨料射流中波速对频率的影响”，重庆大学学报（自然科学版），2000，Vol.23，No.5，pp4-7
[23] 张焱，陈平，“自激振荡脉冲喷嘴的研究”，探矿工程，1997，Vol.3，pp38-40
[24] 汪志明，沈忠厚等，“自激振荡气蚀射流喷管内水声学理论及其应用”，水动力学研究与进展，1994，Vol.9，No.3
[25] 周长山，沈忠厚，“自振空化水射流冲击压力脉动的实验研究”，石油大学学报（自然科学版），1996，Vol.20，No.5，pp35-39
[26] 汤一波，"脉动压力的紊流分形特征"，水科学进展，1998，Vol.9，No.4，pp361-365
[27] 田茂林等，“汽—水换热器内流体诱导振动强化传热实验”，化工学报，2001，Vol.52，No.3
[28] 刘建清等，“流体诱导振动强化换热的试验研究”，华中理工大学学报，1998，Vol.26，No.11，pp85-87
[29] 李学来等，“振荡管内冷热气体分流的实验研究”，大连理工大学学报，1999.5，Vol.39，No.3
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