Exhaust gas total temperature is a main gas turbine parameter to be measured, especially in combined cycle applications, district heating and cogeneration, because it is directly related to the thermal energy delivered to the final application. For this reason, it is crucial to achieve an accurate measurement of the gas temperature in the chosen cross section of the exhaust duct.
Exhaust gas temperature
The exhaust gas of a gas turbine usually has high speed and non-uniform flow and temperature distributions over the measurement cross section.
To get a complete mapping of the total temperature distribution over the duct cross section, measurement points shall be distributed on different radiuses and located at different depths from the duct wall to its centre.
A CFD analysis of the gas in the duct is required to know the flow and velocity distributions. This allows to determine the correct number of measurement points, which is strongly dependent on the spatial accuracy desired for the measurement result.
Figure 1 – Measurement points
For example, by averaging the measurements from equally-spaced sensors along a radius, we will obtain a result which will be affected by low spatial uncertainty if the temperature is uniform.
On the contrary, if the temperature has a steep gradient radius-wise, the lack of information about temperature distribution between one sensor and the following one is compensated with a higher spatial uncertainty.
Once the temperature is collected for each measurement point, the average total temperature shall be calculated on a mass weighted average basis, using the flow distribution provided by the CFD analysis.
The rakes to be installed inside the duct will be designed to place a sensor in the exact location of each measurement point. Moreover, they will be equipped with devices, such as radiation shields, whose purpose is to:
locally reduce the flow velocity in the measurement point to obtain an accurate measurement of the gas total temperature;
protect the sensorâ€™s tip from any radiation heating which could worsen the measurement.
Total Temperature Rakes
Starting from our experience in gas turbine field testing, SINT Technology can help you in measuring exhaust gas temperature by providing total-temperature measurement rakes to be installed in the turbine exhaust duct.
Figure 2 – Total temperature rakes
Our engineers will carefully analyse construction drawings of the duct and the CFD analysis of gas distribution in order to provide the best solution for the type and number of sensors to install and where to place the traverse measurement points.
Rakes are designed and manufactured to resist to the drag forces caused by the high-velocity flow in the duct. Finite element modelling and analysis are made to verify the tensional strength of rakeâ€™s material (stainless steel AISI 316) and the maximum displacement of the probes in the flow direction.
The hollow body of the rake acts as a perfect housing for the thermocouples and protects them from the stream. Only the sensing tip of the thermocouple exits and is bent towards the flow.
The shield protects the tip of each sensor from radiation heating and reduces the flow velocity. Anti-blockage holes are machined on the shields so that the flow does not stagnate inside the shield. All sensors come with calibration certificate traceable to ILAC-MRA.
The outer-most end of the rake has a suitable connection flange which is used to fix it on the duct wall and allows the rake to be aligned in the flow direction. On the flange, compression fittings are used to tight the thermocouples.
According to your needs, we can take care of the measurement campaign, providing the Acquisition System and collecting all the data. A detailed report will be issued with the temperature recordings in each measurement point, the average total temperature in the exhaust duct and the measurement uncertainty calculated according to ASME PTC 19.1
Theory: stagnation (total) and static temperature
The gas stagnation (total1) temperature is defined as the temperature the gas would reach if it was adiabatically2 slowed down to zero local velocity.
is the gas static temperature, K
is the gas specific heat at constant pressure, J/(kg K)
is the gas speed, m/s
Stagnation temperature can be expressed in terms of the Mach number
Where: is the specific heats ratio
1The total temperature of the gas is determined by all energy contributions to its thermodynamic state. Among these, also the mass potential energy should be considered. For gases, the mass potential energy contribution is negligible, due to the low density. For this reason, stagnation and total temperatures are assumed to be the same.
2An adiabatic deceleration is also isentropic only if it is reversible.
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