An essay on the front-end

A theory

for the design

of multiple exhausts

for steam locomotives

28-02-2000 version 1

Steam locomotive multiple exhaust theory

Introduction

Short history of front-end research.

From the first locomotive, Trevithick’s Pen-y-Darren creation, locomotives have exhausted through their chimney to augment the capability to burn more coal by producing a better draught. After the Rainhill trials of 1827 the device was universally adopted and over the years gradually improved. An overview of the do’s and don’ts was published in 1855 by D.K. Clark in his book "Railway Machinery". Popular belief used the piston theory, the puffs of exhaust steam pushed smoke through the chimney like a piston. However already in 1852 J.A. Longridge had another opinion which he read before a meeting of the North of England Institute of Mining Engineers: "1. That the action of a jet of steam was not an impulsive action …. but was the result of a frictional action between the surface of the jet and the surrounding air. 2. That a greatly increased action was obtained by subdividing the jets. This followed as a corollary from the preceding remark". He had found this while ventilating his mines.

One of the firsts to start research was Gustav Zeuner, then a professor at the Technical University of Zürich, Switzerland. In 1858 he used a little model of a locomotive front end in which he used different combinations of exhaust pipes, chimneys and pipes to the smokebox to draw air through. With the aid of a vacuum device, U-tube filled with water and a steam pressure meter, a U-tube filled with mercury he made a lot of measurements. He recognised the front-end for what it was, a momentum imparting device and formulated the theory of its working. In that way he formulated an equation that shows the amount of air expelled to the amount of steam used for it to be dependent on the relation of orifice to chimney throat. As a professor he was not really interested in design data, he wanted to show the principles and these principles are being used until now.

However in his, on average rather useless, data something was hidden that seems to have escaped further notice. He used two orifices of which the larger had twice the area of the smaller. If the relative vacuums in his data are compared they show that with the smaller orifice on average always more than half the vacuum of the larger orifice could be reached. Basically he had found that in his model each orifice of a possible pair was capable of drawing more than half the vacuum of a single exhaust with the same total orifice area.

Nozo and Geoffrey, who built a better-equipped model, reported similar results in 1863. They compared a design with eight little orifice/chimney combinations with a single one that had the same orifice area and found that this made not much difference. These findings have been used over and over to prove that an increase in contact-area with the flue gases did not matter much. What seems to have been overlooked however by most readers is their next test. In this the optimal relation was to be determined for a total of ten orifice/chimney combinations, showing that the combination used in the multiple exhaust test had the second worst performance of the ten tested. If anything, Nozo and Geoffroy proved that a multiple jet had a lot of capabilities for improvement. Because they used four orifices with systematically doubled areas with different pressures and resistance into the smokebox it is possible to compare the vacua from up to eight times more area against each other. Their results were used for a life test of the multiple chimney system on a Crampton locomotive of the French Nord.

Apart from these tests Prussmann should be mentioned, in 1860 he tested the distance of orifice to chimney and came to the conclusion that the further away the chimney was removed from the orifice the wider the chimney could be. He deduced from this that the best chimney was tapered upwards for the best draught. His reasoning was at best doubtful, his result however correct, 20th-century research showing that friction along the chimney sides results in an ever increasing stagnant sub-layer hampering the main flow. Slightly widening the chimney annihilates the effect.

Apart from others the last large research project was done by Troske in 1895. He used a model, which used practically real life dimensions for chimneys and orifices. His research was slightly chaotic and the data, although numerous because he had 30.000 readings, have not been re-used. He also showed that in his 1:1 model also a vacuum could be made with smaller orifices that was better than half of that produced with an orifice of twice the area using the same exhaust pressure. Also he showed that there was no change in shape of the exhaust jet between the different orifices other than a change in diameter equal to the change in orifice diameter.

The investigations were slightly revised and re-tested by Von Borries. In America a number of tests were made at Purdue University, showing a number of conclusions for American locomotives. Goss, in charge of the research, published these conclusions among others: ‘’ the jet acts on the smokebox gases in two ways, first by frictional contact as it induces motion in them, and secondly, it enfolds and entrains them. Another was "that the action of the jet is not dependent on the impulses from individual exhausts. Draft can as well be produced by a steady flow of steam as by intermittent action of the exhaust.’’ Goss was the first to use little tubes in his chimneys to try to measure the pressure. Huygen was in 1907 the first to explore the front-end with a then newly invented pitot-type pressure and velocity meter. He published in his doctor’s thesis a lot of data from these measurements. He was the first to find the large local vacuum near the chimney throat, near the orifice and in the jet above the orifice. In 1912 Strahl used the theory of Zeuner again, modified it for practical application and defined the resistance through the firegrate and the boiler tubes. In this form it has been used to our days.

In 1917-18 a number of articles in "The Railway Engineer" described the best possible front-end of that time. The orifice should be about six diameters away from the chimney throat. This should have a diameter of three times that of the orifice. The chimney should have a bell-mouth making it double the throat diameter. The chimney height above the throat should be about three times its diameter and should be tapered.

In 1924 Giesl-Gieslingen, still a student, used the data from Huygens book for his first articles. Later he enhanced it with Strahl's theory and worked out a number of cases for Austrian locomotives. In 1928 Chapelon published his first article about the front-end. In it the Strahl theory is reproduced, however without any calculation of the newly invented Kylchap exhaust. The Kylchap exhaust seems to be modelled along the lines of an injector, with a number of orifices in sequence. In the published form it is the result of experiments. In this publication Chapelon makes the remark that Nozo and Geoffroy had "proved" that the area of the jet in contact with the smoke gases did not matter. Contemporary critics contested this point of view.

In 1929 Giesl-Gieslingen published his doctors thesis about the subject, in which he calculated a marked difference between "shock loss" of a classical chimney and one with the orifice very near or within it. He repeated Chapelon's observation from the Nozo-Geoffroy test and stuck to it up to his last publication in 1987.

Young of the University of Illinois published in 1933 the results from an intensive investigation of a model of a locomotive front-end. Apart from the review of earlier work he defined the model-scale laws that made a comparison between model and real locomotive possible. Although the publication is a valuable source of information he succeeded in totally misunderstanding the nature of the exhaust jet which, at that time, in an academic environment is at least very amazing.

In 1935/1936 de Gruyter of the State Railways on Java (Indonesia) used the work of Young as a basis for theoretical work to improve the exhaust of the 4-6-4 tenderlocomotive 1300 series, resulting in larger power and a reduction in water and coal consumption.

After the war Chapelon went on to design his famous compound locomotives with improved front-end.

In the early fifties Giesl-Gieslingen succeeded in convincing the Austrian railways in testing his Giesl exhaust, then a very long tapered chimney in which five orifices exhausted. After extensive tests it was changed in the design with seven orifices which we know now.

Also in the early fifties the British Railways systematically tested all locomotive types resulting in a number of improvements in their front-ends. As a result the leader of the tests, Ell, wrote some publications in which he described the functioning of the front-end repeating the basic description of Goss. He improved the form of a standard exhaust/chimney combination a little from the one as published in 1917 and gave formulae with which the dimensions of a single or a double chimney could be calculated. After the B.R. tests interest gradually died. Giesl took a lot of trouble to interest a number of railway administrations into accepting his design. He succeeded in certain places and failed in others like South Africa and America. Only Porta in Argentina succeeded in enhancing the theory and improved the front-end of a number of locomotives. But basically the steam locomotive was replaced by other means of traction while the front-end design was far from complete.

In a review of about 150 years of design one aspect is very significant, practically all researchers with a railway background did not look beyond their own narrow field of engineering as if comparative problems did not exist elsewhere.

Non-locomotive directed research

However research of others in the field of jet theory is very interesting. The 19^th-century research into the velocity of gas from an orifice is well known. As a logical extension already in 1912 Trüpel measured the velocity distribution in a jet of air. He showed in his doctor’s thesis that the jet spread and that it had velocity profiles that resembled Gauss’s (probability) curve, however without naming this. In the twenties and the thirties of this century the then new turbulence theory was an invitation for a number of researchers to try and calculate the velocity profile of a jet, people like Prandl, Tollmien and Schlichting were busy with it and succeeded in calculating a Gauss type curve. Pre-war books about fluid dynamics like Eck’s mentioned this research and the results, however without anyone noticing in the locomotive community. After 1950 specialised books appeared like the ones from Pai (1954) while also in 1954 Barat published his pressure measurements in a free jet completing the data of Trüpel.

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