Development of a temperature probe radiation shield which is heated by the combustion of gases

Accurate measurement of high gas temperatures is essential to the safe and efficient operation of steam generating units. With the trend toward using greater temperatures in combustion technology, instruments which can be used to obtain a more accurate temperature have become an important tool for the engineer. Difficulties in obtaining an accurate measurement occur as gas temperatures increase. It has been found in previous investigations that radiative cooling causes significant heat losses in thermocouples at temperatures as low as 1200° Fahrenheit, which increases with increased temperature. Detailed analysis of error causing effects for thermal exchange is not possible since a detailed knowledge of the thermal environment must be known. This information is rarely obtainable, and is further associated with its own amount of error making calculations often invalid. Therefore, the approach used in this investigation targets the factors involved in producing error, and designs a probe which will diminish errors for the type environment expected. Research of past investigations, along with developing a working knowledge of the various theory behind the mechanisms, which cause losses through heat exchange, led to design of the propane heated high velocity thermocouple probe (PHVT). Heating is accomplished by the combustion of propane and air, in a specially built, chambered, radiation shield thermocouple probe. A limited test program was run to show the viability of the PHVT probe. Tests show that shield heating with gas combustion can indeed extend the limiting temperature range encountered by previous types of electric resistance shield heated probes, while being more economical and easier to use. Experimental results show that there is a large departure, which increases with the increasing gas temperature, between lower high velocity thermocouple (HVT) readings and PHVT measurements, showing the effects of radiation. The magnitudes of error between HVT and PHVT measurements further compare with magnitudes of error found for the HVT probe and true gas temperature in previous investigations.