# Questions tagged [compressible-flow]

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### What are the boundary condition to simulate flow through vent mast?

Dear all, I am currently working on modeling compressible flows that occur when hydrogen needs to be evacuated (for purging, in case of fire, etc.). I am endeavoring to determine the appropriate ...
• 11
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### Dixon/Hall's "Fluid Mechanics and Thermodynamics of Turbomachinery" full of errors?

Note: This is no homework, but self-study. I have a thermodynamics and fluid mechanics background and am trying to work through this book in order to get better understanding of turbomachines (...
32 views

### Air flow rate through thin pipe from a tank

I would like to study the air pressure in a rigid box. One side of the box can be moved like a piston, so I can change its volume, which results in pressure change inside the box. Morover I installed ...
1 vote
93 views

### Fluid dyanmic - flow in a pipe

Dear fellow engineers, I have a question regarding a fluid dynamic problem which I have sketched out. The initial fluid velocity is 1.5 m/s at 1.5 bar and the fluid is water. The pipe line has a ...
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868 views

### What are the Units of flow coefficient Cv? And relationship between flow coefficient Cv formulas of incompressible and compressible fluids

I tried to understand flow coefficent Cv of a valve. There are many similar discussions on the Internet, e.g., https://www.pneumatictips.com/what-is-the-flow-coefficient-cv/ There are two formulas for ...
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129 views

### How to derive the formula for Expansion Factor Y in compressible flow?

I tried to understand of some formulas in ISA Standard "Flow Equations for Sizing Control Valves": http://integrated.cc/cse/ISA_750101_SPBd.pdf ISA standard likes to give formulas without ...
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363 views

### What would be the flow rate of 5 L hydrogen at 20 bar being vented through a 1/4 inch (assumed smooth) stainless steel pipeline to standard pressure?

As the title says, I have a 1 m pipe network of 1/4 inch diameter connected to a 5 L bottle of 20 bar hydrogen. If this system was to open suddenly, what would be the volumetric flow rate out the ...
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1 vote
154 views

### Why do Ejectors have a Venturi/Contraction?

Ejectors are flow devices that provide low back-pressure to a duct by passing a gas jet past the duct and taking advantage of the low pressure region this creates (See figure). A common usage of this ...
• 121
19 views

### In the process of spraying liquid from the nozzle, must the pressure at the outlet of the nozzle be equal to the atmospheric pressure?

During my studies, with high Mach compressible fluids, I always set the outlet pressure to 1 atm. In that case I can roughly calculate the exit velocity. However, in the case of steady flow, is the ...
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40 views

### Question about scaled models of compressors and turbines

Hello I am an engineering student and I have some questions about scaled models for compressors and turbines: When building scaled models of compressors or turbines to analysis them, what ...
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1 vote
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### Entropy in Euler equations for compressible flows

In compressible Euler equations, everyone knows the energy equation can be written as $$\dfrac{Ds}{Dt} =0$$ which implies the entropy is constant along the path line. However, we have shockwave as a ...
155 views

### Help needed understanding atmospheric tank vent physics principals

I try not to be a complete moron but am not an engineer so please bear with me. I have this LP record ultrasonic cleaner system that uses a solution stored in a "tank" which is a simple ...
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1 vote
36 views

### Confusion between Steady flow and Space-Time invariant density field

Let's take the example of a steady flow with velocity field as $u=βr(\tilde{r})$. Now, we can find a density field $ρ(r,t)$ that satisfies the mass conservation equation of $∂ρ/∂t+∇(ρu)=0$. First, ...
42 views

### Joule Thompson Effect and Isenthalpic Processes

We are trying to compute the Joule Thompson (JT) coefficient for air going through a valve from P1 to P2 (where P2 is a much lower pressure). Beginning this we were thinking that the water in the air ...
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### How are the conditions after the shock calculated for a shockwave formed on a cone?

How do oblique shockwaves differ from 2D to 3D problems? More specifically, how are the conditions after the shock calculated for a shockwave formed on a cone (e.g. the inlet cone on a ramjet)? I am ...
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399 views

### How do stators in axial compressors work?

How do the stators in axial compressors convert kinetic energy to pressure? Is this equivalent to a water hammer?
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800 views

### Determining required pressure drop for choked flow conditions to be met

I have pipework system with an inlet pressure, $P_0$, which passes from a pipeline of fixed diameter, $D_0$, through a tee with reducer to a smaller diameter pipe, $D_1$, before returning to another ...
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314 views

### Ideal Exhaust velocity of a rocket nozzle

In a youtube video, they defined exit exhaust velocity of a rocket nozzle as $$v_2 = \sqrt{\dfrac{2k}{k - 1}RT_1 \left[1 - \left(\dfrac{p_2}{p_1}\right)^{(k - 1) / k}\right]}$$ How can I derive this ...
899 views

### Why don't all gas pipe flows end up at sonic speed?

According to the 1D models of compressible fluid flow, the effects of pipe wall friction and the effects of heat addition from the environment both drive a flow toward Mach 1 (i.e. the speed of sound)....
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1 vote
33 views

### Linear pneumatic actuator opening time - pressurise connection pipe

I would like to estimate the time needed to open a linear single-acting pneumatic actuator. The actuator's piston is loaded with a spring pushing downwards and the operating fluid (air or nitrogen) ...
1 vote
63 views

### Can any Air rotameter be used for refrigerant gas by using equation conversion?

Currently, I am trying to find rotameter calibrated for R134A gas flow. However, most companies only sell rotameter or variable area flowmeter for water/Air/Nitrogen flow. Is it possible to use any ...
68 views

### How to balance flow and pressure in nitrogen purged chambers?

I have an instrument that is technically a humidity controlled chamber. It measures how much moisture is picked up by the sample through a microbalance(Zone 1). Nitrogen gas flows from zone 3 and ...
273 views

### Solving 1D Unsteady Homentropic Flow using Method of Characteristics

I am looking to solve the 1D Unsteady Homentropic Flow equations using the Method of Characteristics (MOC). I am using Zucrow and Hoffman's Gas Dynamics Volume 1 as a reference and I am currently ...
58 views

### What should be the density of a compressed gas while measuring the flow rate using Rotameter?

Say we have a required mass flow rate, in order to have equivalent volume flow rate, we need to consider density of the operating fluid (compressible fluid). To measure the volume flow rate in LPM, ...
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260 views

### If a fluid is incompressible,is it correct to say that the flow may or may not be incompressible?

From my understanding,when there is no change in volume due to pressure applied on a fluid,the fluid can be called as incompressible.As there can be still be change in volume due to other properties,...
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31 views

### Determination of solid bed fluidisation

How would one determine if a fluidised bed (with liquid flow) is determined to be a stable fluidised bed, which subsequently doesn't form a plug or bed at the top of a column (bounded by frits), when ...
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### Determination of blower mass flow from limited information

For a variable speed centrifugal blower, can a rough measure of mass flow be obtained from only motor speed and power when the outlet restriction is variable?
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220 views

### Why does exit pressure match back pressure in Converging Diverging Nozzles?

I am trying to understand why normal shocks are caused in Converging Diverging Nozzles. My textbook explains that since the exit pressure must match the back pressure, the normal shock is the only ...
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4k views

### Using pressure drop to correlate mass flow

I am looking for a way to double check readings I am getting through a mass flow meter that appear to be reading lower than expected. Once flow starts in the system, the pressure regulator at the ...
188 views

### Combustion Chamber Back Pressure due to gas injection

I am trying to calculate prior to testing the expected back pressure resulting from injecting gaseous oxygen through an injector orifice (choked) into a rocket combustion chamber before ignition. I ...
1 vote
73 views

### Water Valve Functionality

Before to present my question, I have to say that I'm not a hydraulic expert. I'm working on a water valve design and I found this model which work with a diaphragm and a spring. I want to consider ...
189 views

### Methane Gas Density in Adiabatic Compressible Flow

Hi.. The question above is one of the tutorial question from fluid mechanics textbook. In the textbook solution for this question, the density of the methane gas stream at downstream was computed with ...
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1 vote
235 views

### Isentropic Flow and Conservation of Mass

I have some doubt on fluid mechanics that need some confirmation. I am on the section that discusses about compressible flow and the effect of variation of flow cross section area. It stated that ...
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185 views

### Is the pressure of the gas identical to the static pressure of the liquid in the diagram?

In my application the liquid would be tapped from a fast-varying flowrate, and I want to measure the static pressure change during this by measuring the air pressure above it. Will this work?
456 views

### Shouldn't enthalpies be equal in isentropic process?

I have some questions about the enthalpies between two points during an isentropic process in a nozzle. By doing the energy balance between the two points, one gets: \$h_1 + \frac{v_1^2}{2}+gz_1=h_2+\...
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1k views

### Isentropic flow equation derivation

I am trying to derive the isentropic flow equations for a compressible gas by myself and at the end, I have different formulation than the one in the literature. Can you please tell me what am I doing ...
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### What are some concepts I can look into to aide in creating a 'compression funnel'(?) without the aide of a motor? [closed]

In short I'm designing a 'Humane fly swatter' for my University application into bachelor of Industrial Design. The idea is to utilise the energy of swatting a fly to capture air through an intake, ...
884 views

### Natural gas mass flow conversion

I have a question about gas valve selection and working out the CV. The equation I have needs the mass flow in kg/hr, but I have been given the volumetric flow rate in litres/minute. How do I go about ...
11k views

### How to determine the choke point in a compressed gas system?

I have a pressure regulator bringing an inlet pressure of 150 bar down to 10 bar which is then lead into a plenum followed by a nozzle of area 340 mm^2, exposed to atmospheric conditions. Weirdly ...
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130 views

### What makes something like water so hard to change its density?

So I'm taking a compressible flow class soon, and I've been thinking beforehand about the topic. A substance like water can easily be pressurized, but its density doesn't change easily, meaning there ...