Page:Locomotive performance; the result of a series of researches conducted by the engineering laboratory of Purdue University (IA cu31924004619502).pdf/244

 velocity of both the jet of steam and the body of gas immediately surrounding it, together with the reaction of the latter, probably constitute the causes which operate to narrow the jet. The velocity curves, which in Fig. 108 are pretty evenly distributed throughout the body of the jet, are in Fig. 109 crowded together, giving evidence in the latter case of a very dense and powerful jet. The results show that the more rapidly moving, though smaller, jet (Fig. 109) is more effective in producing draft than that which is shown by Fig. 107.

101. The Effect upon the Jet of Changes in the Height of the Bridge.An essential feature of the single exhaust-pipe is the so-called bridge, which maintains a separate steam-passage for each cylinder for a portion of the length of the pipe. It is above the top of this bridge only that the steam exhausted from both sides of the locomotive intermingles. The purpose of the bridge is a twofold one: it prevents the exhaust of one side from blowing through into the exhaust-passage of the other side, and when properly designed it is the means of making that side from which the strongest stream is passing assist by induction the exhaust from the other side. To determine the best height of bridge exhaust pipes of four different designs were tested by the Master Mechanics’ Committee, all being the same except in the height of the bridge and in the proportions of those parts adjacent thereto. Two nozzles representing the extreme conditions with reference to this detail were employed in experiments designed to determine the character of the jet, with results which are to be seen by comparing Figs. 107 to 109 with Figs. 110 to 112. Figs. 107 and 110 represent identical conditions, except as to the height of the bridge, as do also Figs. 108 and 111,and Figs. 109 and 112. An examination of the velocity curves of these figures will show that for similar conditions of running the jet for the two groups is nearly identical. It will be of interest to add that, basing their conclusions upon efficiency tests made at Chicago, the committee concluded that, whenever the bridge was less than 12’, some loss of efficiency resulted, though such loss was not great even when the height of the bridge was reduced to 5, They recommend that the bridge be not less than 12. They recommend, also, that where long exhaust-pipes must be employed, the increased length should always be secured by extending that portion of the pipe which is above the bridge. This recommendation in effect contemplates a constant height of bridge for all lengths of pipes.

To the discussion with reference to the bridge it will be of interest to add some reference to the "choke."