Although the gas turbine first made its appearance many years ago, it has never competed with the steam engine, steam turbine, and internal combustion engine as an important source of power. There are two major reasons for the very slow progress shown in development of a practical gas turbine.
First, the efficiency of the unit depends directly on the efficiency of the compressor supplying air to the combustion chamber, and for many years the compressor were so inefficient as to absorb most of the power developed by the turbine, leaving little for useful work. Advancement in the field of aerodynamics has made possible more efficient compressor, and the most significant development has been an efficient axial-flow compressor.
Second, the thermal efficiency that can be realized from the gas turbine depends on the temperature of the hot gases supplied to the turbine b the combustion chamber. Evidently, it is desirable to operate the turbine at temperature as high as possible, but at temperature high enough to make operation practical the strength of all known materials is greatly decreased, and the problem of stress due to high angular velocity is correspondingly magnified. Materials recently developed give relatively high strength at high temperature, and the gas turbine has come to be more widely considered as a practical source of power. Investigation of turbine operating conditions has become correspondingly important.
Although determination of the operating temperatures of hte turbine blades does not contribute directly to the development of materials capable of withstanding high stresses at high temperature, it does make possible utilization of the full strength of hte known materials at the operating temperatures. It is apparent that, if the strength of the materials at various operating temperatures is known and hte actual operating temperatures of the turbine cannot be predicted accurately, good engineering practice makes it necessary to be very conservative in designing the unit and in imposing maximum turbine inlet temperatures.
Turbine blade design would be greatly facilitated by a thorough knowledge of the temperature variations over the blades. High temperature gradients produce high stresses which are superimposed on the stresses due to high angular velocity. Such thermal stresses become particularly important when the blades are subjected to intermittent heating and cooling.
Attempts have been made to measure the actual temperature of the blades during operation and these attempts have been successful to some degree. However, in every method developed there have been limitations so important as to make it desirable to investigate other possible methods. One of hte most important methods has been the use of hte thermocouples imbedded in the turbine blades. This method has the disadvantage of measuring hte temperature only at the point of the thermocouple. The voltage developed must be impressed on the meter and transmission of this voltage presents many problems. Destruction tests may be used to give a indication of operating temperatures, but the limitation of this method are obvious. Use of paints with change color at a particular temperature will indicate that temperature only. The same is true of the use of small bits of alloys which melt at known temperatures.
In view of the importance of a thorough knowledge of blade temperature and because all methods used heretofore leave something to be desired, an attempt has been made to construct a device which will measure instantaneously the temperature of any point on the blade.
In 1944 a means of measuring instantaneously the temperature of the luminous gases present during combustion in a Diesel engine was developed at the University of Wisconsin (1). Since it was originally intended to measure temperature of a luminous flame recurring at reasonably low frequency, this pyrometer could not be applied to the gas turbine without modification. Because of the high rotational speed of the turbine, the pyrometer must have a very high frequency response (2). Very high gain is required, since the radiation from the turbine blades is of low intensity.
A turbosupercharger, fitted with burners and duckwork, was installed, and a pyrometer built to measure blade temperature of this unit.
Copyright 1948
Engine Research Center
University of Wisconsin-Madison