Stagnation curves

Figure 4.1 Typical stagnation curve of a metallic pipe material in contact with drinking water...

Plumbing design The final metal concentration at the tap is not just determined by water quality. Metal release by metal pipes is also strongly influenced by the type of domestic drinking water installation, such as materials and dimensions (length and diameter). The increase in metal concentration with stagnation time varies with the pipe diameter (Kuch et al., 1983). For instance for lead pipes, calculated and experimental stagnation curves show that 90 % of the maximum lead concentration is reached after 2 hours for pipes of 10-mm internal diameter but more than 24 hours for pipes of 50-mm diameter. 

During months 2 to 5 of the run, stagnation curves for the metals of interest were eonstrueted. This involved taking individual V2,2,4 and 8 hour stagnation samples. 

Information from reference rigs is more complete and closer to reality than is information given by a simple sit-and-soak test where only 24-hour stagnation samples are analysed over a short period of time (2 weeks). However, adaptations of sit-and-soak tests could be considered as getting more information on diiferent possible mechanisms (Table 4.13) or to produce stagnation curves. However, static tests cannot cover the influence of flow conditions, which appear also to be an important factor in authentic situations. 

Heating Facihty and El Aero Heating Facihty, NASA/Ames. Numbers on the curves indicate stagnation pressure in MPa — balhstic entry -, lifting... 

Figure 26.4 Surface temperature and surface fuel mole fraction (a), and NO (6) as functions of inlet composition, along the adiabatic curve, for the stagnation reactor (solid curves) and the PSR (dashed curves). The fuel-lean and fuel-rich regions are indicated. The conditions are pressure of 1 atm, inlet temperature of 25 °C, a strain rate of 1000 s (stagnation reactor), and a residence time of 1 ms (PSR)...

Figure 26.5 Surface mole fractions of fuel and NO as functions of stagnation surface (solid curves) and PSR temper-atirre (dashed curves), for 28% inlet H2 in air (a) and 12% inlet H2 in air (b). The maximum temperature indicates adiabatic operation. The conditions are the same as in Fig. 26.4...

Figure 26.6 Wall conductive flux and surface fuel mole fraction (a) and NOa, near the surface (6) vs. the inverse of the strain rate for a stagnation reactor with the surface at temperatures of 500 K (dashed curves) and 1000 K (solid curves). The conditions of pressure and inlet temperature are the same as in Fig. 26.4...

Consider the condition, which determines the velocity of the curved flame front propagation in the channel. Inside the stagnation zone filled by combustion products the pressure is constant and is equal to the value at infinity (when x = oo). Because of Bernoulli s integral along the streamline restricting the stagnation zone, the gas motion velocity remains unchanged. Since at x = oo the flow is plane-parallel (ptJO = const, v — 0), distributions of velocity u and of the stream function are associated with the vorticity distribution ... 

Under the conditions of turbulence, the time-averaged velocity field is symmetric with respect to the free stagnation plane, provided the flow rates from the two nozzles are equal. The mean axial velocity profile has a similar shape to the curve of uju ) vs x. The gradient of the time-averaged axial velocity takes the maximum at the stagnation plane, while it approaches zero near the nozzle. 

Figure 7-10. Electron ionization efficiency curves of Ar3+ ion in a neat argon expansion at different stagnation pressures (a) 1.2, (b) 2.0, (c) 2.5, and (d) 3.0 atm. Reprinted with permission from Vaidyanathan et al. 1992. Copyright 1992 American Chemical Society.

Conventional membrane surfaces arc cither flat (as a foil, plate or square channel) or regularly curved (as in a tube or circular channel). Potentially there may be stagnation zones in which carbonaceous materials arc deposited. Moreover, not every site on the membrane surface participates in the catalytic reaction. 

Figure 1. Fluorescence intensity of SO2 monomers as a function of stagnation pressure. Curve was fitted to Eq. (2.1) with = 0.08 and Icj = 1.9 x 10 ...

The uniquenessambient(external) flow of the curve = (ji) is broken at the beginning of the initial region or at the distance where a possible stagnation zone starts. 

In the layer 80-90 m, the see-saw shaped curve reflects the irregular sequence of smaller and bigger inflow events from the Kattegat by sudden leaps in sahnity, followed by an almost linear decay in the subsequent stagnation phase. The exceptional strength of the 1951 inflow has never been met again in the five decades of observation thereafter. Details of the various inflow events are described in Chapter 10. 

This analysis provides a lower anchor point for curves of impaction efficiency as a function of Stokes number. It applies also to non-Stokesian particles, discussed in the next section, because the point of vanishing efficiency corresponds to zero relative velocity between particle and gas. Hence Stokes law can be used to approximate the particle motion near the stagnation point. This is one of the few impaction problems for which an analytical solution is possible. 

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