Nozzle and Diffuser MCQ Quiz in मल्याळम - Objective Question with Answer for Nozzle and Diffuser - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

• The maximum gas flow through a nozzle is determined by critical pressure. 
• Pressure ratio is defined as ratio of outlet pressure to inlet pressure.

Hence from the above two definitions we can say that critical pressure ratio is defined as ratio of outlet pressure to the inlet pressure of the nozzle only when mass flow rate per unit area is maximum.

Explanation:

Supersaturated expansion:

• When steam flows through the nozzle, the expansion of steam may be so quick which results in incomplete condensation of vapour due to less available time. The vapour expands as a superheated vapour up to the point at which condensation takes place suddenly and irreversibly. 
• The state of steam at this point is neither stable nor unstable. The steam in this state is said to be in a supersaturated state or metastable state.
• The super-saturation of steam occurs due to a delay in condensation which increases the flow rate of steam at the outlet of the nozzle.

Effect of super-saturation of steam through nozzle: 

The super-saturation-

• increase the mass flow rate through the nozzle,
• increase the entropy and dryness fraction of steam,
• increase the specific volume of steam,
• decrease the heat drop and velocity of steam at exit.

Thus, option (2) is the correct answer.

Explanation:

Nozzle: It is a device that is used to increase the velocity of flowing fluid at the expense of pressure drop.

Nozzle efficiency: It is defined as the ratio of the actual enthalpy drop to the isentropic enthalpy drop at the same pressure. 

\(\eta_{nozzle}=\frac{\Delta H_{Actual}}{\Delta H_{Isentropic}}\)

Factors affecting nozzle efficiency:

• Material of the nozzle
• Shape and size of nozzle
• Divergence angle
• Nature of fluid and its state
• Velocity of fluid
• Friction
• Turbulence in the flow path.

Thus, option (3) is correct answer.

Explanation:

The steam flow through the nozzle may be assumed as adiabatic flow and the work done is equal to the adiabatic heat drop. 

The mass flow rate or discharge equation for the nozzle:

\(\sqrt {\frac{2n}{n-1}{\frac{P_{1}}{V_{1}}}\left ( \left ( \frac{P_1}{P_2} \right )^{\frac{2}{n-1}} - \left ( \frac{P_2}{P_1} \right )^{\frac{n+1}{n}}\right)}\)

For maximum discharge, we need to differentiate the above equation. 

On differentiating the discharge equation we get the Critical pressure ratio which is:

 \(\frac{P_2}{P_1} = \left ( \frac{2}{n+1}\right )^{\frac{n}{n-1}}\) ,

where P₂ = pressure at the throat of the nozzle, P₁ = pressure at the inlet of the nozzle

• For saturated steam n = 1.135, \(\frac{P_2}{P_1} = 0.58 \)
• For superheated steam n = 1.3, \(\frac{P_2}{P_1} = 0.546\)
• After attaining certain value the flow becomes constant irrespective of the value of the pressure ratio that is called choking of the nozzle.

Rayleigh Flow: Flow in a constant area duct with heat transfer and without friction is called Rayleigh Flow.

From the T-S diagram for the Rayleigh flow, it can be observed that at M = 1 the entropy is maximum.

Fanno Flow: Flow in a constant area duct with friction and without heat transfer is known as Fanno Flow

Explanation:

Convergent-divergent nozzle:

• Convergent-Divergent nozzles are used to increase the flow of gas to supersonic speeds (as in the case of rockets).
• Their cross-sectional area first decreases and then increases. The area where the diameter is minimum is called the throat.
• As the gas enters the converging section, its velocity increases, considering the mass flow rate to be constant.
• As the gas passes through the throat, it attains sonic velocity (Mach number = 1).
• As the gas passes through the divergent nozzle, the velocity increases to supersonic (Mach number >1).
• The flow rate is maximum for a given nozzle if the flow is sonic at the throat. This condition is achieved by managing the back-pressure.
• For the compressible fluid flow, the Mach number is an important dimensionless parameter. On the basis of the Mach number, the flow is defined.

• Choked flow is a limiting condition where the mass flow will not increase with a further decrease in the downstream pressure environment while upstream pressure is fixed.
• For chocked flow in the convergent-divergent nozzle, the Mach number at the throat is equal to 1 and the pressure at the throat is equal to the critical pressure.
• Critical pressure ratio for a choked nozzle:
• \(\frac{{{p^*}}}{{{p_o}}} = {\left( {\frac{2}{{\gamma + 1}}} \right)^{\frac{\gamma }{{\gamma - 1}}}}\)

 where p* is the critical pressure and p0 is the inlet pressure.

Explanation:

Nozzle:

• A nozzle is a device, a duct of smoothly varying cross-section area, that increases the velocity of a fluid at the expense of pressure.
• The chief use of nozzle is to produce a jet of steam (or gas) of high velocity to produce thrust for the propulsion of rocket motors and jet engines and to drive steam or gas turbines.

Friction losses in a nozzle depend upon various aspects, the effects of nozzle friction are as follows:

Reduction in enthalpy drop:

• Friction in the nozzle affects its efficiency. As the efficiency of the nozzle is the ratio of the actual enthalpy drop to the ideal enthalpy drop in the nozzle, the friction in the nozzle decreases the enthalpy drop.

Reduction in exit velocity:

• The kinetic energy of the steam increases at the expense of its pressure energy in a steam nozzle. Some kinetic energy gets lost to overcome the friction in the nozzle. Therefore, the exit velocity of steam decreases due to nozzle friction.

Increase in specific volume:

• The specific volume of steam can be defined as the volume of steam per unit weight of the steam. Specific volume increases due to nozzle friction.

Decrease in mass flow rate:

• As the friction in the nozzle slows down the flow of steam in it, the mass flow rate also decreases due to nozzle friction.

Reheating of steam i.e., improving the quality of vapour at the exit:

The effects of steam nozzle friction are:

• Reduction in enthalpy drop.
• Increase in the specific volume
• Steam is reheated i.e. quality of vapour at the exit is improved means dryness fraction increases.

• The decrease in mass flow rate
• Reduction in exit velocity.

Concept:

All flows must satisfy the continuity and momentum relations as well as the energy and state equations.

Application of the continuity and momentum equations to a differential flow yield:

\(\frac{{dA}}{A} = \left( {M{a^2} - 1} \right)\frac{{dV}}{V}\)

This equation governs the shape of a nozzle or diffuser in subsonic or supersonic isentropic flow.

Diffusers: 

Diffusers are designed to decelerate fluids, therefore dV is negative, dV < 0

From the above equation, 

dV < 0, Ma > 1 (supersonic) ⇒ dA < 0, convergent

dV < 0, Ma < 1 (subsonic) ⇒ dA > 0, divergent   

Therefore, for diverging diffuser, Flow should be subsonic.

Nozzles:

Nozzles are designed to accelerate fluids, therefore dV is positive, dV > 0

From the above equation, 

dV > 0, Ma > 1 (supersonic) ⇒ dA > 0, divergent

dV > 0, Ma < 1 (subsonic) ⇒ dA < 0, convergent   

Explanation:

Coefficient of discharge of nozzle is the ratio of actual discharge to the theoretical discharge.

\(C_d=\frac{Actual \;discharg}{Theoretical\:discharge}\)

Coefficient of discharge for various devices are:

⇒ Venturimeter – 0.95 to 0.98

⇒ Orifice meter – 0.62 to 0.65

⇒ Nozzle  – 0.93 to 0.98

∴ Coefficient of discharge for nozzle meter lies between venturi meter and orifice meter.

Concept:

In a converging-diverging nozzle, when the back pressure ratio reaches a critical value, the flow will become choked with the subsonic flow in the converging section, sonic flow at the throat, and supersonic flow in the diverging section.

A further decrease in the back pressure ratio results in the formation of a shock wave within the diverging section.

The location of the shock is such that the pressure at the diverging section exit will equal the back pressure. As the back pressure ratio is decreased further, the shock wave moves downstream of the throat toward the exit of the device as shown below.