Stagnation pressure

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

Pitot Tubes. The fundamental design of a pitot tube is shown in Eigure 9a. The opening into the flow stream measures the total or stagnation pressure of the stream whereas a wall tap senses static pressure. The velocity at the tip opening, lA can be obtained by the Bernoulli equation ... 

Choked and unchoked flow situations arise in pipes and nozzles in the same fashion for homogeneous equihbrium flashing flow as for gas flow. For nozzle flow from stagnation pressure po to exit pressure pi, the mass flux is given by... 

Total Pressure is the pressure that would occur if the fluid were brought to rest in a reversible adiabatic process. Many texts and engineers use the words total and stagnation to describe the flow characteristics interchangeably. To be accurate, the stagnation pressure is the pressure that would occur if the fluid were brought to rest adia-baticaUy or diabatically. 

What atmospheric characteristics are usually associated with stagnating high-pressure systems ... 

Korshover (3) studied stagnating anticyclones in the eastern United States over two periods totaling 30 years. He found that for stagnation to occur for 4 days or longer, the high-pressure system had to have a warm core. Korshover s criteria included a wind speed of 15 knots or less, no frontal... 

The primary cause of efficiency losses in an axial-flow turbine is the buildup of boundary layer on the blade and end walls. The losses associated with a boundary layer are viscous losses, mixing losses, and trailing edge losses. To calculate these losses, the growth of the boundary layer on a blade must be known so that the displacement thickness and momentum thickness can be computed. A typical distribution of the displacement and momentum thickness is shown in Figure 9-26. The profile loss from this type of bound-ary-layer build-up is due to a loss of stagnation pressure, which in turn is... 

Inlet and diseharge pressures are defined as the stagnation pressures at the inlet and diseharge, whieh are the sum of statie and veloeity pressures at the eorresponding points. Statie pressures should be measured at four stations in the same plane of the pipe as shown in the piping arrangements. Veloeity pressure, when less than 5% of the pressure rise, ean be eomputed by the formula... 

Stagnation pressure The pressure that would be observed if a flowing fluid were brought to rest along an isentropic path. 

There are several types of nozzle. The simplest is an open nozzle as shown in Fig. 4.34(a). This is used whenever possible because pressure drops can be minimised and there are no hold up points where the melt can stagnate and decompose. However, if the melt viscosity is low then leakage will occur from this type of nozzle particularly if the barrel/nozzle assembly retracts from the mould each cycle. The solution is to use a shut-off nozzle of which there are many types. Fig. 4.34(b) shows a nozzle which is shut off by external means. Fig. 4.34(c) shows a nozzle with a spring loaded needle valve which opens when the melt pressure exceeds a certain value or alternatively when the nozzle is pressed up against the mould. Most of the shut-off nozzles have the disadvantage that they restrict the flow of the material and provide undersirable stagnation sites. For this reason they should not be used with heat sensitive materials such as PVC. 

This states that the sum of the velocity pressure 0.5pv plus the static pressure / the total pressure, is constant along a streamline. In the case of standard air density (1.2 kg m ), 0.5pv becomes 0.6v. When a Pitot-static tube is immersed into the flow, as in Fig. 12.19, the velocity at the stagnation point at the tube nose is f = 0 and the local static pressure equals the total pressure p,. The flow static pressure p, is measured a short distance downstream from the surface of the tube. The flow velocity is obtained by applying Eq. (12.27) ... 

Absolute total pressure Sometimes called stagnation pressure, the algebraic sum of the velocity and static pressure at a given location in a moving system, in Pa. 

In addition to the irreversibilities associated with these components, pressure losses (Ap) may occur in various parts of the plant (e.g. in the entry and exit ducting, the combustion chamber, and the heat exchanger). These are usually expressed in terms of non-dimensional pressure loss coefficients, Ap/(p) N, where (/ )in is the pressure at entry to the duct. (Mach numbers are assumed to be low, with static and stagnation pressures and their loss coefficients approximately the same.)... 

From (a) and (b), the stagnation pressure and temperature can thus be calculated at exit from the cooled row they can then be used to study the flow through the next (rotor) row. From there on a similar procedure may be followed (for a rotating row the relative (7 o)r, i and (po)k replace the absolute stagnation properties). In this way, the work output from the complete cooled turbine can be obtained for use within the cycle calculation, given the cooling quantities ip. 

It has already been shown that (stagnation) pressure losses have an appreciable effect on cycle efficiency (see Section 3.3), so as well as obtaining the enthalpy change, it is important to determine the stagnation pressure change in the whole cooling proce.ss. 

A more sophisticated approach would not only take account of Eqs. (4.45) and (4.46) to give the two stagnation temperatures at exit from control surfaces A and B, but it would also not assume the total pressures of coolant and mainstream to be the same. Eor the first nozzle guide vane row these can be derived by accounting for losses as follows ... 

The simple approach described before involves approximations, particularly to obtain the stagnation pressure loss. The full determination of pf) m (7 o)5m from the various equations given above can lead to an approximation for the downstream entropy (.vsni). using the Gibbs relation applied between stagnation states. 

---