Reduce mass flow rate may increase air velocity?

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Discussion Overview

The discussion revolves around the relationship between mass flow rate and air velocity in a duct system, particularly when using a blower and a gate valve to control flow. Participants explore the implications of changing mass flow rates on air velocity and pressure measurements, seeking to understand the underlying physics.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant observed that decreasing the mass flow rate led to an increase in air velocity at lower flow rates, particularly when the gate valve was partially closed.
  • Another participant referenced Bernoulli's principle and the venturi effect to explain the relationship between pressure and velocity, noting that a valve may cause flow separation and non-uniform flow patterns.
  • A question was raised about the method of measuring mass flow rate and the units used, suggesting that the relationship between mass flow rate and velocity may not be linear, especially when adjusting orifice size simultaneously.
  • A participant clarified their measurement approach, indicating that they calculated mass flow rate using the formula involving density, velocity, and area, and acknowledged the influence of both venturi effects and orifice size on observed velocities.

Areas of Agreement / Disagreement

Participants express varying interpretations of the observed phenomena, with some agreeing on the relevance of Bernoulli's principle while others focus on the complexities introduced by the valve and orifice size. The discussion remains unresolved regarding the precise mechanics at play.

Contextual Notes

Participants have not fully clarified the assumptions regarding flow conditions, the nature of the valve's restriction, or the specific measurements taken, leaving some aspects of the discussion open to interpretation.

Adilah
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Hi,

I used a blower to flow air across an open duct through a nozzle. By using a gate valve, the mass flow rates were controlled. I took measurement in 3 different locations along the ducts.

But, why by decreasing the mass flow rate, the air velocity was increased? It is just happened for the lower mass flow rates (downsizing the gate near to the bottom of surface)!. but not when the gate is fully opened.

I also wondering if someone could explain what should happen to the total pressure, dynamic pressure and static pressures when decreasing and increasing the mass flow rate?

I'm really need someone to explain this simple physics. As I read more, I also confused more!

Thanks all.
 
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Welcome to PF!

Bernoulli's principle and equation relate the three different types of pressure to each other and the concept of a venturi explains why velocity is higher in a restriction. However, unlike a venturi, a valve doesn't provide a smooth restriction, resulting in flow separation, eddies, high velocity jets and other non-uniform flow patterns for a certain distance after the restriction.
 
How are you measuring mass flow, and in what units are you measuring it? For the same mass flow rate and a smaller orifice size, the velocity will be higher since you have to squeeze more mass per second through the opening. If you are reducing the mass flow rate and the orifice size at the same time, it will depend on how fast the mass flow rate is reduced compared to the rate the area of your orifice is reduced. That is likely to be a rather complex relationship when you are using a blower, and probably not linear. If you really are measuring a higher velocity with a smaller orifice, it just means that the mass flow rate has not fallen as fast as the area has.
 
Hi.

I was measuring the mass flow rate in kg/s, = density*velocity*area (orifice size). Thus same mass flow rate from the blower, but controlling the orifice size.
So, probably due to both venturi effects and squeezing more mass per second through the smaller opening.Thanks to you both Russ_watters & boneh3ad.
 

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