(40) Gas Turbine Development.
In the attempt to gain mechanical simplicity, small weight, and diminutive size of the steam turbine, many able experimenters have endeavored to obtain an internal combustion motor in which the energy of the expanding gas is converted into mechanical power by its reaction on a bladed wheel, but so far the problem is far from being solved. In 1906 two experimental turbines were built by René Armengand and M. Lemale, of the constant pressure type, one of which developed 30 Brake horse-power and the other 300 horse-power.
A 25 horse-power De Laval steam turbine was altered by Armengand says Dugald Clerk so that it operated with compressed air instead of steam. The compressed air was passed into a combustion chamber together with measured quantities of gasoline vapor, and the mixture was ignited by an incandescent platinum wire as it entered the chamber, thus maintaining a constant pressure with continuous combustion. Around the carborundum lined combustion chamber was imbedded a coil in which steam was generated by the heat of the burning gas, the steam being used to reduce the temperature of the gas from 1800°C to about 400° as it issued from the orifice and came into contact with the running wheel. The working medium was therefore composed of two elements, the products of combustion and the steam at the comparatively low temperature of 400°C.
The constant pressure maintained in the combustion chamber was about 10 atmospheres, and the hot gases were allowed to expand through a conical Lava jet in which the expansion produced a high velocity, and reduced the temperature of the fluid. At this reduced temperature and high velocity the gases impinged upon the Laval wheel, and rotated the wheel in the same way as steam would have done. The experiments showed that under these conditions the total power obtained from the turbine separate from the compressor was double that necessary to drive the compressor.
In the large 300 H. P. turbine the first part of the combustion chamber was lined with carborundum, backed by sand, but the second part was surrounded by a coil through which water was circulated. The water kept the temperature of the combustion chamber within safe limits, and after absorbing heat, it passed also around the jet nozzle, and was discharged into the passage leading to the jet, and there converted into steam by the hot gases. A mixture of products of combustion and steam thus impinged upon the turbine wheel. The expanding jet was arranged to convert the whole of the energy into motion before the fluid struck the wheel; the temperature was thus reduced to a minimum before the gases touched the blades. Notwithstanding this, the wheel itself had passages through which cooling water flowed, and each blade was supplied with a hollow into which water found its way. In the large turbine the compressor was mounted on the turbine spindle; it was of the Rateau type, and consisted of an inverted turbine of four stages, which delivered the compressed air finally to the combustion chamber at a pressure of 112 lb. per sq. in. absolute. The efficiency of this turbine compressor was found to be about 65 per cent. The total efficiency of the combined turbine and compressor was low, as the fuel consumption amounted to nearly 3.9 lb. of gasoline per B. H. P. hour. An ordinary gasoline engine with a moderate compression can readily give its power at the rate of 0.5 lb. of gasoline per B. H. P. hour. The combined turbine and compressor was stated to have run at 4,000 R. P. M. and to have developed 300 H. P. over and above the negative work absorbed by the compressor.
A gas turbine in which there was no compression was built in the following year by M. Karovodine which gave 1.6 horsepower at a speed of about 10,000 revolutions per minute.
It contained four explosion chambers having four jets actuating a single turbine wheel, which wheel was of the Laval type, about 6 inches diameter, having a speed of 10,000 R. P. M. The explosion chambers were vertical, and had a water jacket surrounding the lower end. The upper portion contained the igniting plug on one side, and the discharge pipe connecting with the expanding jet on the other. In the lower water-jacketed part there was provided a circular cover, held in place by a screwed cap. This circular plate was perforated with many holes, and it carried a light steel plate valve of the flap or hinging type, which pulled down by a spring contained within the admission passage. This spring could be adjusted, and the lift of the valve was regulated by means of a set screw passing diagonally through the water jacket. Air was admitted at one side by a pipe leading into the valve inlet chamber and a corresponding passage or pipe admitted gasoline and air or gas to mix with the air before reaching the thin plate valve. Adjusting contrivances were supplied in both air and fuel ducts. To start the apparatus, an air blast was forced through the valve, carrying with it sufficient gasoline vapor to make the mixture explosive. The electrical igniter was started, and the spark kept passing continuously. Whenever the inflammable mixture reached the upper part of the combustion chamber ignition took place, and the pressure rose in the ordinary way, due to gaseous explosion. The gases were then discharged through the pipe and nozzle on the Laval wheel. The cooling of the flame after explosion and the momentum of the moving gas column reduced the pressure within the explosion chamber to about 2 lb. per sq. in. below atmosphere. Air and gasoline vapor then flowed in to fill up the chamber, and as soon as the mixture reached the igniter, explosion again occurred. In this way a series of explosions was automatically obtained, and a series of gaseous discharges was made upon the turbine wheel. Diagrams taken from the explosion chamber showed a fall in pressure during suction of 2 lb. per sq. in.; ignition occurred while the pressure was low, and the pressure rapidly rose to about 1 1–3 atmospheres absolute. The pressure propelling the gas column and jet was thus only 5 lb. per sq. in. above atmosphere. The pressure rapidly fell, and the whole process was repeated again. According to the diagrams taken, a complete oscillation required about 0.026 second, so that about 40 explosions per second were obtained.
Fig. 15. Cross-Section of the Combustion Chamber of the Holzwarth Gas Turbine. From the Scientific American.
The most promising type of turbine that has been built to date is that designed by Hans Holzwarth, an engineer of some prominence in the steam turbine field. A 1000 horse-power machine has been built at this writing and as experimental machines go has made most remarkable performance.
The turbine in general arrangement outwardly resembles the Curtis steam turbine, in that the turbine wheel rotates in a horizontal plane, the spindle or shaft is vertical and a dynamo is mounted on this spindle above the turbine. In the Holzwarth turbine ten combustion chambers are provided, each of a pear or bag shape. They are arranged in a circle around the wheel, and are cast so as to form the base of the machine. The wheel is of the Curtis type, with two rows of moving and one row of stationary blades.
In this turbine the energy of the fuel is liberated intermittently by successive explosions, instead of by continuous combustion, and in much the same way that the explosions occur in a reciprocating engine. Tests made on the new machine have shown that it is in no way inferior in efficiency to the ordinary type of motor, and that at full load, the weight per horse-power is only about one-quarter of that of the reciprocating engine. The weight factor, as is well known, is of the utmost importance in marine service and should prove of value to the marine engineer, if this alone were its only characteristic.
Any of the ordinary power gases may be used with success, as well as vaporized liquid fuels, and the lower grade oils such as crude and kerosene have given much better results in the turbine, than in reciprocating engines, even at this early stage of its development. As the heat losses are much smaller than met with in ordinary practice, the temperature is higher, which, of course, greatly facilitates the vaporization of the lower grade liquids.
Mr. Holzwarth does not give the dimensions of his turbine wheel, but from the drawings and some of the velocities given by him it appears to be about 1 m. in external diameter. The lower part of each combustion chamber carries gas and air inlet valves, and the upper part carries a nozzle arranged to cause the gases to impinge upon the first row of moving blades. This nozzle is connected to and disconnected from the combustion chamber by means of an ingeniously operated valve. The explosion chambers are charged with a mixture of gas and air, which appears to attain a pressure of about two atmospheres within the chamber before explosion. The air and gas are supplied under sufficient pressure from turbine compressors, actuated by steam raised from the waste heat of the explosion and the gases of combustion, so that whatever work is done in compression is obtained by this regenerative action, and does not put any negative work upon the turbine itself. The combustion chambers are fired in series, by means of high-tension jump spark ignition.
Referring to the cut, the explosion chamber A is filled intermittently with the explosive mixture at a low pressure (about 8 to 12 pounds per square inch). When ignition has occurred, the pressure of explosion opens the nozzle valve F, allowing the compressed gases to flow through the nozzle G to the bladed turbine H, on which the energy is to be expended. The expansion of the heated gases in the nozzle reduces the pressure to that of the exhaust, with the resulting increase in the velocity of the gas. By means of fresh air, the nozzle valve F is kept open throughout the expansion and scavenging periods.
After the expansion has been completed, the air that is forced through the valve D, at a low pressure, thoroughly scavenges or removes the residual burned gases left in the combustion chamber and nozzle, forcing it into the exhaust. When the scavenging has been completed, the nozzle valve and the air valve D are closed. The combustion chamber A is now filled with pure cold air, which not only enables a fresh charge of gas to be introduced into the chamber but which also aids in keeping the chamber cool.
Pure fuel gas, or atomized oil, is now injected through the fuel valve E, forming an explosive mixture ready for the ensuing cycle of events. A number of these chambers are arranged around the turbine wheel in order to have a uniform application of power, by having the several chambers working intermittently. This is in effect, the same proposition as increasing the number of cylinders on a reciprocating engine.