I have a question regarding traffic changes
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Why does the flow rate of a liquid remain unchanged when it passes through such a reducer – for example, when it goes from a larger pipe to a smaller one, the flow rate stays the same – but it changes when a control valve is used? When the valve is closed, the flow rate also decreases. Aren’t both cases related to a change in the cross-sectional area through which the fluid flows? Then why does the flow rate remain unchanged in one case while it decreases in the other? Could someone explain it?
The pressure drop when fluid flows through a reducer is small, resulting in a minimal throttling effect that is hardly noticeable; in contrast, the pressure drop of a control valve is large, giving a significant throttling effect
Dear, you’re only focusing on flow rate; there are other parameters as well, such as pressure changes. For example, when a pipe undergoes a change in diameter, if the pressure difference remains unchanged, the flow rate will also decrease. If the flow rate remains unchanged, the pressure difference increases. The control valve works on the same principle: when the valve is closed more, the flow rate decreases, while the pressure difference remains unchanged.
So, is that continuity equation still valid? That is, u1*A1=u2*A2?
This post was last edited by jujiangliu on 2017-1-23 at 10:23. According to the principle of fluid continuity, the flow rate remains constant regardless of the path it takes, but the flow velocity and pressure can vary. A control valve changes the flow rate because a change in its opening degree leads to a change in resistance. When the total differential pressure in the piping system remains constant, any change in resistance results in a corresponding change in flow rate, just as in Ohm’s law in electrical circuits.
When the power source is a centrifugal pump, with all other conditions remaining unchanged, two different reducer pipes are selected (Reducer Pipe 1, inlet diameter 100, outlet diameter 90； For the reducer 2, with an inlet diameter of 100 and an outlet diameter of 10, balance is maintained before and after the reducer: the flow rate at the inlet and outlet of reducer 1 is Q1_in = Q1_out, and the flow rate at the inlet and outlet of reducer 2 is Q2_in = Q2_out. This indeed conforms to the principle of conservation of mass. However, Q2 is smaller than Q1 (because changing the outlet diameter of the reducer alters the characteristics of the pipeline) ； When the energy source is a positive displacement pump (such as a piston pump), for a pipeline equipped with a control valve, changing the opening degree of the control valve (under condition 1, the opening degree is 90%) ； Under condition 2 with an opening degree of 10%), a balance is calculated for the areas before and after the valve; in condition 1, the flow rate Q1 in = Q1 out, while in condition 2, Q2 in = Q2 out, and Q1 = Q2. (Although the pipeline characteristics change when the valve opening degree is altered, the flow rate of the positive displacement pump is independent of these pipeline characteristics.) A control valve can be understood as a reducing pipe with an adjustable outlet diameter; if a comparison is made under the same conditions, the original poster would not have this confusion
One must understand the essence: in reality, the pressure drop across the entire pipeline is not the same. Without changing the pressure drop, how does a control valve achieve its regulating function?