Pressure oscillating

Figure 5.3 gives an example of a combustion diagram recorded during knocking conditions. This is manifested by intense pressure oscillations which continue during a part of the expansion phase. 

The operation of flow dampers can cause pressure fluctuations in the ductwork system. Measurements by Melin indicate that pressure oscillations in an exhaust system can cause instabilities in the airflow through a fume cupboard sufficient to give rise to outward leakage of contamination, especially when a person stands in front of the cupboard. 

In this mode, acoustic-pressure oscillations are similar to those established in a closed organ pipe. The resulting pressure oscillations then couple with the pressure-sensitive combustion processes to further excite the oscillating pressure and thus produce the high-pressure amplitudes. 

Thus, the exponential growth constant of the pressure oscillation is directly related to the acoustic admittance of the propellant. Hence, the acoustic admittance can be evaluated directly from the growth rate of the pressure amplitude. Ryan (R5) has also desired this espression on the basis of acoustic-energy considerations. 

These studies have indicated that the independent parameters controlling the postulated solid-phase reactions significantly affect the resulting acoustic admittance of the combustion zone, even though these reactions were assumed to be independent of the pressure in the combustion zone. In this combustion model, the pressure oscillations cause the flame zone to move with respect to the solid surface. This effect, in turn, causes oscillations in the rate of heat transfer from the gaseous-combustion zone back to the solid surface, and hence produces oscillations in the temperature of the solid surface. The solid-phase reactions respond to these temperature oscillations, producing significant contributions to the acoustical response of the combustion zone. 

As more air was added to the channel, the slug flow became unstable, the slug bubble broke down, and eventually the churn flow occurred in the channel. As shown in Fig. 5.3d, the most significant feature of flow characteristics in the churn flow is that the pressure oscillated at a relatively high amplitude, since the gas plug and liquid bridge flowed through the test section alternatively. 

The flow patterns (expansion of the bubbly, slug and annular regions of flow) affect the local pressure drop, as well as the pressure oscillations in micro-channels (Kandlikar et al. 2001 Wu and Cheng 2003a,b, 2004 Qu and Mudawar 2003 Hetsroni et al. 2005 Lee and Mudawar 2005a). 

The capillary flow with distinct evaporative meniscus is described in the frame of the quasi-dimensional model. The effect of heat flux and capillary pressure oscillations on the stability of laminar flow at small and moderate Peclet number is estimated. It is shown that the stable stationary flow with fixed meniscus position occurs at low wall heat fluxes (Pe -Cl), whereas at high wall heat fluxes Pe > 1, the exponential increase of small disturbances takes place. The latter leads to the transition from stable stationary to an unstable regime of flow with oscillating meniscus. 

Taking into account Eqs. (11.42) and (11.44) we can present the pressure oscillations as follows ... 

In this section the influence of the pressure in the capillary and the heat flux fluctuations on the stability of laminar flow in a heated capillary tube is analyzed. All the estimations performed in the framework of the general approach and developed in the previous section are kept also in the present cases. Below we will assume that the single cause for capillary pressure oscillations is fluctuations of the contact angle due to motion of the meniscus, whereas heat flux oscillations are the result of fluid temperature fluctuations only. 

Taking into account that ml = l + l and ui < ml, we anlve at the following relation for capillary pressure oscillations ... 

Semilog plot of the real part of heat release response, Re[(q /cl)], p /p) of a ZFK flame to acoustic pressure oscillations [31]. Dotted line shows response of a simple two-step flame [47]. 

A plof of fhe real part of the relative heat release response for three Lewis numbers is shown in Figure 5.1.10. This plot was calculated for a reduced activation energy y3 = 10 and a burnf gas femperafure of 1800 K, represen-fative of a lean hydrocarbon-air flame. Note fhaf fhe order of magnitude of fhe relative response of fhe flame is only a little more than unity. This is a relatively weak response. For example, a sound pressure level of 120 dB corresponds to a relative pressure oscillation p /p = 2 X10 so fhe fluctuation in the heat release rate will be of fhe same order of magnifude. 

High-frequency pressure oscillations (10-100 Hz) related to time required for pressure wave propagation in system Low-frequency oscillations (-1 Hz) related to transit time of a mass-continuity wave Occurs close to film boiling... 

An acoustic wave (sound) is a propagation of pressure oscillation in medium such as air or liquid water with the sound velocity [1]. Ultrasound is inaudible sound and its frequency of pressure oscillation is above 20 kHz (20,000 oscillations per second) [2]. For convenience, an acoustic wave above 10 kHz in frequency is sometimes called an ultrasonic wave. 

A piston compressor (in case of a Stirling type PTR) or a combination of a compressor and a set of switching valves (GM type PTR) are used to create the pressure oscillations. The regenerator of a PTR stores the heat of the gas during a half-cycle and, therefore, must have a high heat capacity, compared to the heat capacity of the gas. 

In the GM-type PTR (mostly used at present), a compressor produces continuous high and low pressures and uses a rotary valve to generate pressure oscillations in the pulse tube. In the Stirling-type PTR, pressure oscillations are created by the movement of a piston, directly connected to the pulse tube and controlled by a motor. 

The buffer volume is a reservoir with a volume typically 10 times larger than the volume of the pulse tube. The pressure in the buffer is almost constant and close to the average pressure in the pulse tube. The combination of the orifice O, and the buffer provides a phase difference between the flow of the gas in the tube and the pressure oscillation such phase difference is necessary for the performance of the PTR. 

From the bubble size and the average flow rate, a first approximation of frequency (thereby total cycle time), as well as the limits between which the chamber pressure oscillates, is determined. The weeping time is calculated from the wave-form equations of chamber pressure, which are ... 

Rayleigh criterion chem The criterion for spontaneous pressure oscillations to accompany combustion, namely, that combustion progresses more rapidly or efficiently during the compression phase of the pressure oscillation than during the rarefaction phase. ra-le krT,tir-e-3n ... 

Current research on control of combustion is focussed not only to reduce combustion-induced pressure oscillations and instability but also to improve combustion performance. Attention is being paid to increased flame speed and improved flame lift-off limits. Flame speeds ranging from laminar to 3.5 times laminar values have been examined, using a Countercurrent Swirl Combustor... 

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