Effective exhaust velocity

Since the thrust reaches a maximum value when pe = pa, a well designed rocket exhaust nozzle should exhaust gases at an exit pressure nearly or exactly equal to the ambient pressure. Near the point of maximum thrust the second term in the thrust equation (Eq. n. A. 9.) is like a small correction term. Thus it is appropriate to define another parameter, the effective velocity c, whose significance will be examined in more detail  

It follows from this definition and the maximum conditions for F, that c also reaches its maximum value at pe = pa, and, of course, at this point c = ue.  

Both ue and the term [(pe - pa)/w]Ae vary strongly with Ae, but in opposite 

From equation n. A. 12. one sees that c is a function of the chamber temperature Tc and the average molecular weight of the combustion products, as well as pc and the dimensions of the nozzle. Since TJ. and M are determined from the given propellant combustion, c is a parameter used for comparing various propellant combinations. However, such comparisons should be under optimum conditions that is, for pe = pa. For this reason  

Perhaps one of the most important performance parameters in rocket technology from point of usage is the specific impulse. It is defined as the propulsive impulse. 

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F = (H+ cosor) (wVe/g) + (Pe-P0)Ae where oc = half of the divergence angle of the nozzle, w - weight rate of proplnt flow, g = acceleration of gravity, Ve = exit flow velocity, Pe = nozzle exit pressure, PQ = external atm pressure, and Ae = cross section at nozzle exit plane. An effective exhaust velocity is defined by... 

II. A. 4. Variation of effective exhaust velocity with exit... 

Being simply the quotient of the thrust and the total weight flow, the specific impulse is a performance parameter readily measured experimentally with good accuracy. This fact accounts for its popular acceptance. With regard to convenience there is no greater merit in the use of I P instead of c. As for the effective exhaust velocity, the specific impulse is evaluated for optimum conditions when theoretical comparisons are made between various propellant combinations. For pe = pa then ... 

II. A. 2.. .. the effective exhaust velocity as determined by the ratio of F to w can be taken to be the optimum value of ue even if the actual experimental nozzle is somewhat off design. Herein lies the practical significance of the concept of the effective exhaust velocity. 

C =Vj/CF=gIs/CF=gF/tf (1/Cp) where Vj = effective exhaust velocity Cp= thrust coefficient g=gravitational acceleration (general) Is= specific impulse F = thrust... 

The specific impulse 7sp is the change of the impulse (impulse = mass x velocity, or force x time) per propellant mass unit. The specific impulse is an important parameter for the characterization of rocket propellants and can be interpreted as the effective exhaust velocity of the combustion gases when exiting the expansion nozzle ... 

By viriue of its vertical construction, the turbo-type tray dryer has a stack effect, the resulting draft being frequently sufficient to operate the dryer with natural draft. Pressure at points within the dryer is maintained close to atmospheric, as low as 0.1, usually less than 0.5 mm of water. Most of the roof area is used as a breeching, lowering the exhaust velocity to settle dust back into the dryer. 

The effective stack height (equivalent to the effective height of the emission) is the sum of the actual stack height, the plume rise due to the exhaust velocity (momentum) of the issuing gases, and the buoyancy rise, which is a function of the temperature of the gases being emitted and the atmospheric conditions. 

The velocities can be calculated by integrating Eq. (10.31) and (10.32) numerically. To perform the calculations, the exhaust opening is divided into. several area sinks and their effect on velocity at any point upstream of the exhaust opening is obtained by summing. For the centerline (r = 0), Eq. (10.32) yields along the x axis... 

An extractiOTi duct may not be the best choice for the extraction of powders and dust However, by placing the hood very close to the point where the dust is produced we can create a simation with high air velocity and an effective exhaust for dust In that case the exhaust duct is a dust remover, but evenmally without a dust collector bag for the dust A problem is that collected dust in the duct or the hood can fall back on (other) products. This is a serious risk for cross cmitamination and needs a proper cleaning schedule in place. 

Fume from hand soldering will rise vertically on thermal currents entering the employee s breathing zone as the person leans over the point of soldering. Control usually is normally achieved by means of effective high velocity and low volume local exhaust ventilation at the solder tip. 

Oxidation. Carbon monoxide can be oxidized without a catalyst or at a controlled rate with a catalyst (eq. 4) (26). Carbon monoxide oxidation proceeds explosively if the gases are mixed stoichiometticaHy and then ignited. Surface burning will continue at temperatures above 1173 K, but the reaction is slow below 923 K without a catalyst. HopcaUte, a mixture of manganese and copper oxides, catalyzes carbon monoxide oxidation at room temperature it was used in gas masks during World War I to destroy low levels of carbon monoxide. Catalysts prepared from platinum and palladium are particularly effective for carbon monoxide oxidation at 323 K and at space velocities of 50 to 10, 000 h . Such catalysts are used in catalytic converters on automobiles (27) (see Exhaust CONTHOL, automotive). 

In the SCR process, ammonia, usually diluted with air or steam, is injected through a grid system into the flue/exhaust stream upstream of a catalyst bed (37). The effectiveness of the SCR process is also dependent on the NH to NO ratio. The ammonia injection rate and distribution must be controlled to yield an approximately 1 1 molar ratio. At a given temperature and space velocity, as the molar ratio increases to approximately 1 1, the NO reduction increases. At operations above 1 1, however, the amount of ammonia passing through the system increases (38). This ammonia sHp can be caused by catalyst deterioration, by poor velocity distribution, or inhomogeneous ammonia distribution in the bed. 

The effect of exhaust performance on room air movement is limited compared to the effect produced by air jets. The distance from the opening to the point where air velocity drops to 10% of the initial velocity value (Fig. 7.14) is approximately equal to one characteristic si2e of the exhaust opening (D for the round duct) and 60 characteristic sizes for the supply outlet (60D for the round nozzle). 

Superposition of Flows Potential flow solutions are also useful to illustrate the effect of cross-drafts on the efficiency of local exhaust hoods. In this way, an idealized uniform velocity field is superpositioned on the flow field of the exhaust opening. This is possible because Laplace s equation is a linear homogeneous differential equation. If a flow field is known to be the sum of two separate flow fields, one can combine the harmonic functions for each to describe the combined flow field. Therefore, if d)) and are each solutions to Laplace s equation, A2, where A and B are constants, is also a solution. For a two-dimensional or axisymmetric three-dimensional flow, the flow field can also be expressed in terms of the stream function. 

This solution gives unequivocally the effective control range of both unflanged and flanged openings when the exhaust flow rate and velocity of the idealized cross-draft are known. The distance from the hood opening to the dividing streamline for a hood in uniform flow perpendicular to its axis is thus... 

Use of warm processes on a downdraft table should be avoided since the air velocity created by the exhaust is often lower than the velocity due to buoyancy effects. Effective use of a downdraft table for welding requires velocities high enough to counteract the buoyancy, which could result in disturbances of the welding process. 

The original design was based on a control velocity recommended by dust control design manuals. The original design of 28 m s exhaust flow induced an inward velocity of about 0.5 m s" through the enclosure entrance and trolley shots. This was not sufficient to overcome plume trajectories aimed outward, or to overcome the effect of moderate wind levels. 

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