Table of Contents
Flow Rate Basics
Flow rate measures fluid volume or mass passing a cross-section per unit time. Volumetric flow (Q) in m3/s or L/min, mass flow in kg/s. The continuity equation (A1*v1 = A2*v2) states flow rate is constant in incompressible flow, so fluid speeds up in narrow sections and slows in wider ones.
Flow rate calculations are fundamental to plumbing, HVAC, chemical processing, hydraulic systems, and any application involving fluid transport. Proper pipe sizing ensures adequate flow without excessive pressure drop or erosion.
Formulas
Flow Regimes
| Reynolds # | Regime | Character |
|---|---|---|
| <2300 | Laminar | Smooth layers |
| 2300-4000 | Transitional | Intermittent |
| >4000 | Turbulent | Chaotic mixing |
FAQ
What is Reynolds number?
Re = rho*v*D/mu is the ratio of inertial to viscous forces. Below 2300, flow is smooth (laminar). Above 4000, it is chaotic (turbulent). This affects pressure drop, heat transfer, and mixing behavior.
How does pipe diameter affect flow?
Flow capacity scales with D^2 (area). Pressure drop scales with D^-5 (Darcy-Weisbach). Doubling diameter at same flow rate reduces pressure loss by 97%, explaining why upsizing pipes dramatically improves efficiency.
What is the continuity equation?
A1*v1 = A2*v2 for incompressible flow. Narrowing a pipe by half the diameter quadruples velocity. This explains why nozzles accelerate flow and why wind speeds up between buildings (Venturi effect).