Flow instabilities

Many coating flows are subject to instabilities that lead to unacceptable coating defects. Three-dimensional flow instabilities lead to such problems as ribbing. Air entrainment is another common defect. 

Consider this eompressor operating in steady state at point A, with the reeyele valve elosed. If the resistanee in the eompressor diseharge system were to rise to point B, the eompressor would eneounter the surge region—essentially a region of flow instability. Catastrophie surge ineidents ean result in eomplete destruetion of the rotor. 

Implementation of advanced performance degradation models, necessitate the inclusion of advanced instrumentation and sensors such as pyrometers for monitoring hot section components, dynamic pressure transducers for detection of surge and other flow instabilities such as combustion especially in the new dry low NO combustors. To fully round out a condition monitoring system the use of expert systems in determining fault and life cycle of various components is a necessity. 

Loss of RCS flow (one loop) 22. Feedwater flow instability from mechanical can... 

If the constant speed characteristic of a pump is superimposed on a system curve, there is usually one intersection point, shown in Figure 32.39. If a flat system curve is being matched with a mixed or axial flow machine there can be flow instability, as illustrated in Figure 32.40, which is only corrected by changing pump speed or the static lift, or selecting a different pump. 

Hydraulic or flow instability is common in centrifugal pumps. In addition to the restrictions of the suction and... 

Positive-displacement pumps are subject to flow instability, which is created either by process restrictions or by the internal pumping process. Increases in amplitude at the passing frequencies, as well as harmonics of both shafts running speed and the passing frequencies, typically result from instability. 

Chapter 9 is devoted to regimes of capillary flow with a distinct interface. The effect of certain dimensionless parameters on the velocity, temperature and pressure within the liquid and vapor domains are considered. The parameters corresponding to the steady flow regimes, as well as the domains of flow instability are defined. 

The onset of flow instability in a heated capillary with vaporizing meniscus is considered in Chap 11. The behavior of a vapor/liquid system undergoing small perturbations is analyzed by linear approximation, in the frame work of a onedimensional model of capillary flow with a distinct interface. The effect of the physical properties of both phases, the wall heat flux and the capillary sizes on the flow stability is studied. A scenario of a possible process at small and moderate Peclet number is considered. The boundaries of stability separating the domains of stable and unstable flow are outlined and the values of the geometrical and operating parameters corresponding to the transition are estimated. 

Ozawa M, Akagawa K, Sakaguchi T (1989) Flow instabilities in parallel-channel flow systems of gas-liquid two-phase mixtures. Int J Multiphase How 15 639-657... 

Ozawa M, Akagawa K, Sakaguchi T, Tsukahara T, Fuji T (1979) Oscillatory flow instabilities in air-water two-phase flow systems. Report. Pressure drop oscillation. Bull JSME 22 1763-1770 Qu W, Yoon S-M, Mudawar 1 (2004) Two-phase flow and heat transfer in rectangular microchannels. J Electron Packag 126 288-300... 

The pressure spike introduces a disruption in the flow. Depending on the local conditions, the excess pressure inside the bubble may overcome the inertia of the incoming liquid and the pressure in the inlet manifold, and cause a reverse flow of varying intensity depending on the local conditions. There are two ways to reduce the flow instabilities reduce the local liquid superheat at the ONB and introduce a pressure drop element at the entrance of each channel, Kandlikar (2006). Kakac and Bon (2008) reported that density-wave oscillations were observed also in conventional size channels. Introduction of additional pressure drop at the inlet (small diameter orifices were employed for this purpose) stabilized the system. 

Kays WM, Krawford ME (1993) Convective heat and mass transfer. McGraw-Hill, New York Kennedy JE, Roach GM, Dowling ME, Abdel-Khalik SI, Chiaasiaan SM, Jeter SM, Quershi ZH (2000) The onset of flow instability in uniformly heated horizontal micro-channels. Trans ASME J Heat Transfer 122 118-125... 

Ozawa M, Akagawea K, Sakaguchi T (1989) Flow instabilities in paraUel-channel flow systems of gas-liquid two-phase mixtures. Int 1 Multiphase Flow 15 639-657 Peles YP (1999) VLSI chip cooling by boiling-two-phase flow in micro-channels. Dissertation, Faculty of Mechanical Engineering, Technion-Israel Institute of Technology, Haifa Peles YP, Yarin LP, Hetsroni G (2001) Steady and unsteady flow in heated capUlary. Int J Multiphase Flow 22 577-598... 

Peles el al. (2000) elaborated on a quasi-one-dimensional model of two-phase laminar flow in a heated capillary slot due to liquid evaporation from the meniscus. Subsequently this model was used for analysis of steady and unsteady flow in heated micro-channels (Peles et al. 2001 Yarin et al. 2002), as well as the study of the onset of flow instability in heated capillary flow (Hetsroni et al. 2004). 

Carey VP (1992) Liquid-vapour phase-change phenomena. Hemisphere, Washington, DC Collier SP (1981) Convective boiling and condensation. McGraw-Hill, New York Ha JM, Peterson GP (1998) Capillary performance of evaporation flow in micro grooves an analytical approach for very small tilt angles. ASME J Heat Transfer 120 452 57 Hetsroni G, Yarin LP, Pogrebnyak E (2004) Onset of flow instability in a heated capillary tube. Int J Multiphase Flow 30 1424-1449... 

The forced fluid flow in heated micro-channels with a distinct evaporation front is considered. The effect of a number of dimensionless parameters such as the Peclet, Jacob numbers, and dimensionless heat flux, on the velocity, temperature and pressure within the liquid and vapor domains has been studied, and the parameters corresponding to the steady flow regime, as well as the domains of flow instability are delineated. An experiment was conducted and demonstrated that the flow in microchannels appear to have to distinct phase domains one for the liquid and the other for the vapor, with a short section of two-phase mixture between them. 

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