A steam nozzle is a passage of varying cross-section, which converts heat energy of steam During the first part of the nozzle, the steam increases its velocity. Flow of steam through nozzles, shapes of nozzles, effect of friction, critical pressure Friction exists between the steam and the sides of the nozzle; heat is . When the steam flows through a suitably shaped nozzle from zone of high Properties of steam at various pressures when expanding dry saturated steam from.
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The steam nozzle is a passage of varying cross section by means of which the thermal energy of steam is converted into kinetic energy. The shape of the nozzle . Discontinuities in nozzle flow: normal and oblique shocks, expansion fans. .. A steam flow of kg/s expands isentropically in a nozzle from a chamber at nozzles usaascvb.info - Free download as PDF File .pdf), Text File .txt) or read online for free.
This is because of thetime lag in the condensation of the steam during the expansion. The area of such duct having minimum cross-section is known as throat. A fluid is calledcompressible if its density changes with the change in pressure brought about by the flow. If the density changes very little or does not changes, the fluid is said to be incompressible. Generallythe gases and vapors are compressible, whereas liquids are incompressible.
Types of Nozzles: There are three types of nozzles Convergent nozzle Convergent-divergent nozzle. Convergent Nozzle: A typical convergent nozzle is shown in the Fig.
In a convergent nozzle, the cross sectional area decreases continuously from its entrance to exit. It is used in a case where the back pressure is equal to or greater than the critical pressure ratio. Fig 1. Convergent nozzle Divergent nozzle: The cross sectional area of divergent nozzle increases continuously from its entrance to exit.
It is used in a case where the back pressure is less than the critical pressure ratio. Fig 2. Divergent nozzle Convergent — Divergent nozzle: In this condition, the cross sectional area first decreases from its entrance to the throat and then again increases from throat to the exit. This case is used in the case where the back pressure is less than the critical pressure. Convergent-divergent nozzles can therefore accelerate fluids that have choked in the convergent section to supersonic speeds.
This CD process is more efficient than allowing a convergent nozzle to expand supersonically externally. The shape of the divergent section also ensures that the direction of the escaping gases is directly backwards, as any sideways component would not contribute to thrust.
Main article: Propelling nozzle A jet exhaust produces a net thrust from the energy obtained from combusting fuel which is added to the inducted air. This hot air passes through a high speed nozzle, a propelling nozzle, which enormously increases its kinetic energy. However, momentum considerations prevent jet aircraft from maintaining velocity while exceeding their exhaust jet speed.
The engines of supersonic jet aircraft, such as those of fighters and SST aircraft e. Concorde almost always achieve the high exhaust speeds necessary for supersonic flight by using a CD nozzle despite weight and cost penalties; conversely, subsonic jet engines employ relatively low, subsonic, exhaust velocities and therefore employ simple convergent nozzle, or even bypass nozzles at even lower speeds.
Rocket motors maximise thrust and exhaust velocity by using convergent-divergent nozzles with very large area ratios and therefore extremely high pressure ratios. Mass flow is at a premium because all the propulsive mass is carried with vehicle, and very high exhaust speeds are desirable. Magnetic[ edit ] Magnetic nozzles have also been proposed for some types of propulsion, such as VASIMR , in which the flow of plasma is directed by magnetic fields instead of walls made of solid matter.
Main article: Spray nozzle Many nozzles produce a very fine spray of liquids. For a steady flow process without any accumulation of the fluid between inlet and exit. In this equation. Convergent — Divergent Nozzle In the converging portion From inlet to throat. Steam becomes more dry due to increased dryness fraction and hence specific volume of steam increases and mass flow rate decreases.
Isentropic heat drop or Rankine heat drop.
The friction between steam and walls of nozzle. If the steam enters the nozzle in a super heated condition. If the effect of friction is neglected. Effect of friction in a Nozzle Point A represents the initial condition of steam which enters the nozzle in a dry saturated state. Internal friction of steam itself. The whole expansion from A to C is isentropic. The effects of friction are: The useful heat drop is less than the isentropic heat drop.
Most of the friction in a convergent divergent nozzle occurs in the divergent portion -between throat and exit. The effect of friction is shown on the h-s diagram or Mother chart in fig. Shock losses. The heat drop hA.
Due to the effect of friction. Due to this. In actual practice. Let point B' represent the final condition of steam. The kinetic energy gets converted into heat due to friction and is absorbed by the steam.
The enthalpy drop is reduced and hence the final velocity. AB' represents the actual expansion Adiabatic expansion.
Dryness fraction at B' is more than at C. The ratio of actual or useful heat drop to isentropic heat drop is known as Coefficient of nozzle or nozzle efficiency. The efficiency of a nozzle depends upon the following factors: Fluid velocity. Nature of the fluid and its state. Finish of the nozzle. Material of the nozzle. The initial velocity compared to exit velocity is so small and is generally neglected.
Angle of divergence. Turbulence in the flow passages. Coefficient of discharge. It is defined as the ratio of actual mass flow rate to mass flow rate corresponding to isentropic expansion.
Size and shape of the nozzle. Considering the effect of friction.
For wet steam. This reduces the enthalpy drop by percent and hence the exit velocity of steam is also reduced correspondingly. Velocity coefficient is defined as the ratio of actual exit velocity to exit velocity when the flow is isentropic for the same pressure drop. V2 Volume of 1 kg of steam i. The mass flow per unit are has maximum value at 'throat' which has minimum area.
The pressure at throat is known as. The area of throat of all steam nozzles should be designed on this ratio. Critical pressure ratio and its value depends upon the value of index n. The following table gives approximate values of index n and corresponding values of critical pressure ratio. Convergent Nozzle. Critical pressure and the ratio of pressure at minimum cross section i. Then no flow takes place.
We know that velocity of steam at any section in the nozzle is For maximum discharge. The ratio of exit pressure p2 to inlet pressure p1 is called.
If p2 is reduced below this critical value. Let the pressure p2 in vessel B initially is equal to pressure p1. The critical pressure gives the velocity of steam at throat equal to velocity of.
When friction is present. When the pressure p2 approaches a 'critical' value. Sonic velocity. Critical Pressure Let the vessel A contains steam at pressure p1 while pressure p2 in vessel B is varied at will.
For a convergent. Through A. Considering the flow of steam from inlet to throat: It is a point where the initial pressure p1 line meets the given dryness fraction x1 line. AB represents expansion of steam from inlet to throat. C from the chart. Mass of steam discharged — kg The first step is to estimate the critical or throat pressure p2 for the given initial conditions of steam.
Extend the line AB to meet the exit pressure line p3 at C. To increase the velocity of steam above sonic velocity To supersonic velocity by expanding the steam below critical pressure. In a convergent. Now on the Mollier chart h-s chart. BC represents expansion from throat to exit. Knowing the value of 'm'. Length of Nozzle In the divergent portion of the nozzle.
If the divergence is rapid. The convergence of the walls tends to stabilize the flow as shown in fig.
As a result. Expansion of Steam under Thermal Equilibrium The point S in expansion lies on saturation line and represents the point at which condensation within the vapour just begins.
In normal condensation. When steam is free of foreign particles. In practice. A certain time interval is essential for the collection of these molecules to form droplets.
This type of expansion is in thermal equilibrium and is shown in Fig. This requires gradual increase in area. The condensation of steam occurs when steam passes through certain distance in the nozzle and after certain short interval of time. When steam flows through the nozzle. This is known as.
As expansion continues below this line into wet region. When certain degree of super saturation is reached. Upto the point at which condensation occurs.
The line A'. Point C represents the meta stable state.