Pipeline flow calculation problem

The calculation of flow rate usually uses the formula Q=Av, where Q is the flow rate, A is the cross-sectional area, and v is the flow velocity. In practice, the relationship between flow rate and pressure difference requires consideration of the fluid properties and pipe characteristics. For incompressible fluids such as water, we can use Bernoulli’s equation, but local resistance and friction still need to be taken into account. Regarding your question, the inner diameter of a DN20 pipe is approximately 20 mm; therefore, the cross-sectional area A = π*(20/2)^2/1000 is about 0.000314 square meters. The density of water is ρ≈1000 kg/m³, and the pressure difference ΔP=0.2 MPa=200 kPa. According to Bernoulli’s equation (also known as the energy equation), under the ideal conditions of no friction and no heat exchange, we have ρgh + 1/2ρv² + P = constant, where g is the acceleration due to gravity, h is the height, v is the velocity, and P is the pressure. Assuming the pipe is horizontal, then h=0. We can solve for v using the pressure difference ΔP=1/2ρv², and then use Q=Av to calculate Q. But this result will be highly idealized; the actual flow rate is affected by many factors. For more accurate calculations, it is recommended to use flow meters for measurement, or employ more complex calculation methods (such as empirical formulas or computational fluid dynamics simulations), which usually require specialized software and expertise. .

If the flow rates at the inlet and outlet are different, is there a solid medium in between or is it a fluid medium?

I’m not sure whether I didn’t explain it clearly or if you didn’t understand what I meant – the traffic in and out is definitely the same, but since there’s no flow meter to show it, it needs to be calculated.

Since there is a pressure difference in the pipeline, a throttling device is needed to change the pressure (such as a control valve). Once the pressure changes, the flow characteristics of the pipeline also change. Without altering the cross-sectional area of the pipeline after throttling, the flow rate cannot increase; it can only decrease.

Put it this way: Container A is connected to Container B by a pipe, and the pressure in Container A is higher than that in Container B (creating a pressure difference). Now, we want to know the flow rate at which fluid flows from Container A to Container B; how can we calculate this flow rate given only the pressure difference, the pipe diameter, and the properties of the fluid?

Haha, I misunderstood; I didn’t realize it

Although 2L is an AI, it has a stronger understanding ability than me. . . And also for: Q

This is a high-flow condition similar to venting, calculated based on resistance

I have to give you a thumbs up – your analysis is well-founded; I now understand it. Thank you.

Ideally: Taking the inlet and outlet of the pipeline as reference points, since the pipe diameter is the same, the flow velocity is also the same (in steady-flow conditions, the flow rate remains constant at any cross-section at any given time). According to Bernoulli’s equation, ΔP/ρg (pressure head difference) + ΔZ (height difference) = ΣHf (pipe friction loss) = λ*l*u^2/(2*g*d). Here, λ is the friction coefficient, l is the sum of the length of the straight pipe and the equivalent lengths of various local resistances (valves, bends, etc.), u is the flow velocity, g is the acceleration due to gravity, and d is the pipe diameter. Since the pressure difference at the inlet and outlet of the pipeline as well as the head difference are known, the left side of the equation is a constant value. Based on the material of the pipe, it is possible to determine the approximate range for ε (absolute roughness), and a value is selected from within this range. And ε/d is called relative roughness. First, assume a flow velocity u. The Reynolds number Re is calculated using d, u, ρ, and μ (viscosity), as Re = d*u*ρ/μ. Using Re and ε/d, it is possible to determine λ from the Moody chart; this value is then substituted back into the original equation to calculate a new value of u1. If u1 is not significantly different from the assumed value of u (usually not more than 5%), then the assumption is correct; otherwise, u needs to be reassumed and the process repeated through iteration.