Research on Local Resistance Analysis in Fabric Ducts

In modern HVAC systems, fabric ducts (also known as fiber fabric ducts, textile ducts, or air dispersion systems) have been widely applied in fields such as food processing, pharmaceutical cleanrooms, sports venues, commercial spaces, and industrial plants due to their advantages of being lightweight, aesthetically pleasing, easy to install, and providing uniform air distribution. Unlike traditional metal ducts, fabric ducts not only transport airflow but also serve as the air supply terminal itself, achieving air distribution through fabric permeation or openings. However, during system design and operation, airflow inside the fabric duct encounters various local disturbances, generating non-negligible local resistance. These local resistances directly affect system static pressure distribution, fan selection, energy consumption, and air supply uniformity. Therefore, in-depth research on the characteristics of local resistance in fabric ducts is of great significance for optimizing system design and improving energy efficiency and comfort.
1. Basic Concept of Local Resistance
In fluid mechanics, local resistance refers to the energy loss caused when airflow passes through local obstacles such as sudden cross-sectional changes, directional changes, branch connections, or inlets/outlets, resulting in abrupt changes in velocity magnitude or direction. Unlike frictional resistance along the path, local resistance is concentrated at specific locations and is typically expressed by the "local resistance coefficient" (ζ). The corresponding pressure loss is calculated using the formula:

where
ρ is the air density, and
v is the characteristic velocity (usually the average velocity at the upstream or downstream cross-section). In fabric duct systems, although the overall structure is simple with few elbows, the following types of structures still constitute typical sources of local resistance.
2. Main Sources of Local Resistance in Fabric Ducts
1. Duct Inlet Connection
Fabric ducts are typically connected to the main duct (mostly metal) via flanges or flexible joints. If there is a sudden expansion, eccentric connection, or poor sealing at this connection, it can cause airflow separation and vortex formation, resulting in significant inlet local resistance. Especially when the main duct velocity is high and the initial section of the fabric duct requires high static pressure, inlet disturbances directly affect the starting static pressure level of the entire duct. Studies have shown that using tapered transition joints or guide vanes can effectively reduce the inlet local resistance coefficient by more than 30%.
2. Air Outlet/Slit Opening Area
Fabric ducts achieve directional jet air supply through preset circular holes, strip slits, or specialized nozzles. These openings are essentially "local outlets" on the duct wall, where airflow accelerates and detaches from the main flow to form a jet. Although the holes are functionally necessary, dense arrangement or unreasonable size design can cause local backflow, boundary layer separation, and even mutual interference around the openings, increasing additional resistance. In particular, in high-density small-hole arrays, the airflow coupling effect between adjacent openings can significantly increase local pressure loss. In engineering practice, CFD simulation is required to optimize hole size, spacing, and arrangement to balance air supply effect and resistance cost.
3. Duct Diameter Variation and Branching Structures
To maintain uniform static pressure along the path, long-distance fabric ducts often adopt variable diameter design (diameter gradually decreases along the airflow direction). Although this continuous diameter change is superior to sudden changes, it still belongs to geometric shape change, which will cause slight accelerated flow and boundary layer adjustment, generating certain local resistance. In addition, in large projects, a single main duct may branch into multiple branch ducts (such as Y-type or T-type branches). Such branch points become typical local resistance nodes due to uneven flow distribution and flow direction changes. Reasonably setting baffles or adopting branch structures with gentle curvature can effectively alleviate such losses.
4. Hanging Supports and Internal Obstacles
Fabric ducts rely on cables or steel ropes to hang below the roof. If the hanging points are arranged too densely or the ropes pass through the inside of the duct (some installation methods), they may form physical obstacles to the internal airflow, easily inducing local vortices at low velocity conditions. In addition, if installation tools, labels, or other foreign objects are accidentally left inside the duct, they will also become unexpected sources of local resistance. Therefore, standardizing installation processes and avoiding internal intrusions are the keys to controlling non-functional local resistance.
3. Impact of Local Resistance on System Performance
Although local resistance only accounts for a part of the total resistance, its impact is particularly prominent in low-pressure, low-speed systems like fabric ducts. First, local pressure loss directly increases the total pressure required by the fan, increasing energy consumption. Second, if local resistance distribution is uneven (such as excessive inlet loss), it will lead to insufficient static pressure at the front end of the duct, thereby affecting the permeation air supply volume at the rear section and destroying overall air supply uniformity. Furthermore, in multi-branch systems, differences in local resistance at each branch inlet may lead to unbalanced air distribution, requiring additional regulating valves for correction and increasing system complexity.
It is particularly worth noting that fabric ducts rely on internal static pressure to achieve permeation or jet air supply. Their working static pressure is usually only 50–250 Pa, far lower than traditional metal duct systems (often reaching 300–800 Pa). In such a low-pressure environment, even tiny local resistance may cause significant static pressure fluctuations, thereby affecting air supply stability. Therefore, local resistance control has a higher priority in fabric duct design.
4. Analysis and Optimization Methods for Local Resistance
Currently, research on local resistance in fabric ducts mainly relies on the following methods:
Theoretical Estimation: Referring to local resistance coefficients of similar components in ASHRAE or EN standards for preliminary estimation, suitable for conventional connectors.
Experimental Testing: Measuring pressure drops under different structures through wind tunnels or actual systems to obtain real resistance data, although the cost is high.
Numerical Simulation (CFD): Using computational fluid dynamics software to perform 3D flow field simulation on complex geometries (such as porous arrays, variable diameter sections), visualizing vortices, velocity distribution, and pressure loss. This is currently the most effective analysis tool.
Based on the above analysis, optimization strategies include:
Adopting streamlined inlet transition sections.
Reasonably arranging air supply holes to avoid high-density concentration.
Adding guide vanes at branch points.
Controlling installation methods to ensure no internal obstructions.
Implementing segmented air supply for long-distance systems to shorten single-segment length and reduce cumulative local impact.
As an efficient and flexible air distribution system, the performance of fabric ducts depends not only on materials and air supply methods but is also closely related to internal airflow resistance characteristics. As a key factor affecting system static pressure stability and energy efficiency, local resistance needs to be fully valued during the design stage. By deeply understanding the causes and impact mechanisms of various local resistances and combining modern simulation and optimization methods, the overall performance of fabric duct systems can be effectively improved, promoting their wider application in green buildings and healthy environment creation. In the future, with the development of intelligent sensing and adaptive control technologies, real-time monitoring and dynamic compensation of local resistance will also become possible, further releasing the technical potential of fabric ducts.