The resistance of the fluid is responsible for energy loss (i.e. drag loss). One is the resistance loss along the way caused by the viscosity and inertia of the fluid; the other is caused by the sudden expansion or contraction of the pipeline-interface, etc. The blocking and disturbance effects of the solid wall facing the fluid are called local resistance. loss. The liquid resistance loss is usually expressed by the energy loss (or head loss) per unit weight of the fluid h1, and the gas is usually expressed by the energy loss (or pressure loss) of the fluid per unit volume p1.
(1) Resistance along the way and resistance loss along the way
(2) Local resistance and local resistance loss
(3) Laminar flow resistance and turbulent flow resistance change, showing irregularities, but the entire fluid still moves along the mainstream direction. In a circular pipe, the flow state of the fluid is related to the average flow velocity v and the kinematic viscosity coefficient of the pipe diameter d. The above three parameters are combined into a dimensionless number, called Reynolds number, represented by Re.
Experiments show that the critical Reynolds number is about 20,000. When the Reynolds number is greater than 2000, the flow state is turbulent flow; when the Reynolds number is less than 2000, the flow state is laminar flow. Turbulent flow resistance is much greater than laminar flow resistance.
(4) Total fluid energy loss: Based on long-term practical experience, the calculation problem of energy loss is transformed into the problem of finding the resistance coefficient. Write the energy loss as a multiple of the flow rate and head. When formulating the energy equation, you can combine it with the flow rate and head into one term to facilitate calculation. Due to the complexity of the influencing factors, the two dimensionless coefficients and strings in the formula must be obtained by analyzing some typical experimental results and using empirical or semi-empirical methods. Total fluid energy loss: The total fluid energy loss is equal to the sum of the losses along each pipe section and the local losses.
(5) Measures to reduce resistance: reduce the roughness of the pipe wall and replace the rigid side wall with a flexible side wall;
Prevent or delay the separation of the fluid from the wall, avoid the generation of vortex areas, or reduce the size and intensity of the vortex areas.
Measures to reduce the resistance of steel pipe fittings: Generally, for elbows with a smaller diameter d, rational use of curvature radius rulers can reduce resistance. Ventilation elbows with larger cross-sections need to be installed with reasonable guide vanes to reduce local The effect of resistance. For reducing pipes with varying pipe cross-sections, a certain length of tapered pipe or gradually expanded pipe should be used. Diversion partitions can be installed for tees or crosses. A very small amount of additive is added inside the fluid to affect the internal structure of the fluid movement to achieve drag reduction.
(6) Reduce energy loss of pumps and fans
The energy loss of pumps and fans is usually caused by three categories, namely hydraulic loss, volume loss, and mechanical loss.
Hydraulic loss: The size is closely related to the geometry of the flow-passing parts, the wall roughness, and the viscosity of the fluid. Hydraulic losses include inlet losses, impact losses, hydraulic losses in the impeller, dynamic pressure conversion, and casing outlet losses.
Post time: Oct-27-2023