The Role Of Air Jets In Mine Ventilation

THOUGH air jets occupy only a minor role in the ventilation of mines and other enclosures, their importance in this respect is greater than is commonly recognized and may be considerably increased as our conception of their modes of operation becomes clearer. The object of this paper is to discuss some of the ways in which air jets are involved in mine ventilation and some possible applications to the ventilation of mines and other enclosed spaces. The mention of air jets ordinarily suggests the use of some form of an air injector, such as the Venturi blower occasionally used for auxiliary ventilation in mines, or the Saccardo (injector) system used to a limited extent for ventilating railroad tunnels. Actually, the term covers such common phenomena as the discharge of air from fan pipes and coal-mine line brattices, and even from smoke stacks and fans. A knowledge of the general mode of air entrainment by jets also helps to explain certain phenomena-some mine-gas explosions, for instance-that otherwise would remain obscure. Jet action has been involved in three recent series of Bureau of Mines experiments in which the author has participated: experiments on fan-pipe discharge, on the dilution of gases discharged from a stack and on the ventilation of a ship's hold. Available data on the discharge of jets into large openings and on the design and performance of air injectors have been analyzed in considerable detail and supplementary experiments are planned for after the war. The results of the fan-pipe experiments have been combined with the analyses of data on jet discharge into large spaces, and have been published.1 The results of the stack-gas dilution tests, which refer to a jet discharged into a crosswind, have also been published2 and a paper on the design and performance of injectors has been prepared for publication.3 These technical experiments and analyses form the background and the documented evidence for the following nontechnical discussion of the use of air jets in mine ventilation. JET CHARACTERISTICS The jets commonly encountered are those in which the air is in turbulent-as opposed to laminar, or streamline-motion. The characteristics under discussion apply only to such turbulent jets and have their basis in theories of the mixing of turbulent air currents. Such jets have basic characteristics that are present, though variable in form, regardless of whether the jet expands freely in space, whether the expansion is limited by adjacent walls, or whether the jet propels air through a flow circuit. They expand at definite angles and entrain the surrounding air, even though the entrained air must be obtained by recirculation of their own air streams. As the quantity of air in motion in the mixed stream increases, the average velocity of the stream decreases. In the case of entrainment in the free