As a core component of the cooling tower heat dissipation system, the strength verification of the cooling tower fan blade must comprehensively consider the synergistic effects of various load conditions to ensure structural safety and functional stability in complex operating environments.
Under normal operating conditions, the cooling tower fan blade must withstand the combined effects of aerodynamic loads, gravitational loads, and centrifugal loads. Aerodynamic loads are formed by the pressure distribution generated when air flows over the blade surface, and their magnitude is directly related to the blade shape, rotational speed, and wind speed. Blade design requires optimization of the airfoil profile through fluid dynamics simulation to ensure uniform distribution of aerodynamic loads and avoid localized stress concentration. Gravity loads are generated by the blade's own weight, forming periodic bending moments during rotation, especially significantly affecting the root region of large blades. Centrifugal loads increase quadratically with increasing rotational speed and may become the dominant load at high speeds, requiring improved fatigue resistance through material selection and structural reinforcement (such as increasing the blade root transition radius).
Extreme environmental conditions place even higher demands on the strength of the cooling tower fan blade. Under strong wind conditions, blades must withstand instantaneous wind pressure impacts, especially under asymmetrical wind directions or gusts, where pulsating wind pressure can trigger resonance effects. In such cases, wind tunnel testing is necessary to verify the frequency misalignment design between the blade's natural frequency and the wind-induced vibration frequency, preventing flutter or vortex-induced vibration. In high-temperature conditions, material properties deteriorate with increasing temperature, requiring consideration of the combined effects of thermal and mechanical stresses. For example, fiberglass blades may experience resin softening at high temperatures, leading to a decrease in interlaminar shear strength; adding heat-resistant modifiers or optimizing the layup direction is needed to improve thermal stability. In low-temperature conditions, it is crucial to prevent material embrittlement, especially by conducting low-temperature toughness tests on the welds and edges of metal blades.
Impact loads during installation and transportation are a weak point in blade strength verification. During hoisting, blades may experience instantaneous overloads due to rope swinging or collisions; finite element analysis is needed to simulate hoisting conditions and optimize the blade root connection structure (e.g., using pre-tensioned bolts or flange connections). Transportation vibrations can trigger microcrack propagation, necessitating modal analysis of the blades to ensure transport support points avoid resonant frequencies and the addition of cushioning materials in the packaging design. For split blades, the positioning accuracy and connection strength of the splicing points must be verified to prevent stress concentration caused by installation errors.
Extreme loads under accident conditions require inclusion in safety redundancy design. In a runaway accident, the blade may overspeed due to control system failure, potentially resulting in centrifugal loads several times the design value. Overspeed testing is required to verify the blade's ultimate load-bearing capacity, and high-strength fibers (such as carbon fiber) or metal-based composite materials should be selected. In the case of cooling tower fan blade fracture, fragments from the broken blade may cause secondary impacts on adjacent blades. Collision simulation analysis is needed to optimize blade spacing and protective structures, and fall protection nets should be added to the inner wall of the tower. Lightning strike conditions require consideration of the coupling effect of electromagnetic force and thermal stress. Lightning positioning systems and grounding devices should be designed to reduce the probability of lightning strikes, and conductive coatings should be applied to the blade surfaces to disperse current.
The synergistic effect of combined load conditions is a challenge for strength verification. For example, the combined effects of strong winds and high temperatures can accelerate material performance degradation, necessitating accelerated life testing to verify the durability of cooling tower fan blades under multi-factor coupling. The combined effects of vibration and corrosion can trigger stress corrosion cracking, requiring anti-corrosion treatment of the blade surface (such as polyurethane spraying or thermal spraying of zinc-aluminum coating), and avoiding gaps and liquid accumulation areas in the structural design. For offshore cooling towers, the combined effects of salt spray corrosion and wave impact must also be considered, enhancing the blades' corrosion resistance through cathodic protection and sealing design.
