Efficient and precise proportional mixing method in jet flows: Liquid suction with Helmholtz nozzle

The precision and energy efficiency of foam proportioning systems significantly influence the quality, stability, cost, and extinguishment performance of foam-based firefighting equipment. Although cavitating venturis are widely studied for accurate proportional mixing, they often exhibit high energy consumption. This study proposes Helmholtz nozzle structures that generate stable negative pressure through cavitation, enabling high-precision proportioning with improved energy efficiency. Experimental investigations into the liquid suction behavior of Helmholtz nozzles indicated that the suction flow rate closely corresponds to the evolution of negative pressure, progressing through three phases: initial fluctuation, accelerated growth, and full development. With appropriate structural parameters, both the flow rate and negative pressure increase linearly with inlet pressure during the growth phase and stabilize in the full development phase. A maximum negative pressure of up to 900 kPa is achieved. Increasing the suction tube diameter elevates the critical inlet pressure and suppresses negative pressure development along the chamber wall, resulting in a quadratic relationship between the maximum suction flow rate and tube diameter. A predicted peak flow rate of 149.79 ml/s is attainable. These findings support the optimization of nozzle structures for the precise addition of foam concentrates under practical conditions. Unlike cavitating venturis, the Helmholtz nozzle operates effectively under cavitation without being limited by critical pressure ratios or back pressure, yielding a 20% improvement in energy efficiency. This research illustrates the potential of Helmholtz nozzles for reliable, energy-efficient liquid proportioning and provides theoretical guidance for improving the performance, mobility, and adaptability of foam firefighting systems in demanding environments.