Reduce mass flow rate may increase air velocity?

In summary, Bernoulli's principle and equation relate the three different types of pressure to each other, while a venturi explains why velocity is higher in a restriction. The concept of a venturi applies, but there is usually a separation of flow and high velocity jets can be seen for a certain distance after the restriction.
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.

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.

1. How does reducing mass flow rate affect air velocity?

Reducing mass flow rate in a system will result in an increase in air velocity. This is because the same amount of air is being pushed through a smaller area, leading to a higher velocity.

2. What is the relationship between mass flow rate and air velocity?

The relationship between mass flow rate and air velocity is inversely proportional. This means that as mass flow rate decreases, air velocity increases, and vice versa.

3. Why is it important to consider mass flow rate when studying air velocity?

Mass flow rate plays a crucial role in determining air velocity in a system. By controlling the mass flow rate, we can manipulate the air velocity and ensure that it is at the desired level for optimal performance.

4. How can reducing mass flow rate affect the efficiency of a system?

Reducing mass flow rate can potentially increase the efficiency of a system. This is because when air velocity increases, it can lead to better mixing and distribution of air, which can result in improved heat or mass transfer processes.

5. What factors can influence the relationship between mass flow rate and air velocity?

The relationship between mass flow rate and air velocity can be influenced by factors such as the size and shape of the duct or channel, the properties of the fluid (such as density and viscosity), and the pressure drop along the system.

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