A numerical investigation of total temperature probes measurement performance

Abstract

In almost every industrial application, the temperature is measured for development and condition monitoring purposes. The accuracy of these measurements is crucial to avoid misunderstandings about the current condition and misguidance in the development phase. The most practical mean of temperature measurement in industrial applications is using a thermocouple. Thermocouples are very flexible structures so they can be applied in many different regions for solid and fluid temperature measurements. It is also possible to design measurement probe geometries using thermocouples as sensing elements. In machines involving high-speed gas flow, the kinetic energy of fluid can't be neglected in energy interaction calculations so flow must be adiabatically stagnated before temperature measurement. The temperature a flowing fluid gains because of adiabatic stagnation is called stagnation or total temperature. A stationary probe geometry measures the total temperature of flow but there may be deviations in the temperature of the sensing point due to the flow physics. These deviations lead to errors in measurement. These errors are classified as recovery error, conduction error and radiation error. Recovery error originated from the non-adiabatic stagnation of flow on the surface of the thermocouple (TC) junction. Recovery error is characterized by a parameter called recovery factor which shows the degree of dynamic temperature recovery on the measurement. Conduction and radiation errors arise due to solid boundary conditions which are different from the flow total temperature around the probe. These different temperature zones cause heat interaction via conduction and radiation heat transfer modes between the TC junction and surroundings giving rise to deviations in measurement. Special probe designs are used to prevent these errors. In this study, an experimental case was selected from the literature to create a conjugate heat transfer (CHT) methodology. This CHT methodology served to investigate flow physics around and inside total temperature probes and the nature of heat interaction between flow and probe geometry. This experimental case contains a total temperature probe calibration setup which investigates the measurement performance of probe geometry under different Mach number flows. In the simulations, the measurement probe geometry was modelled and exposed to the flow at the same speed as the test conditions. The main observed parameter during simulations was TC junction temperature which determines the performance of the total temperature probe. The results of simulations were observed to be in harmony with experimental data. Then, flow structures around and inside the total temperature probe were investigated in detail using the outputs of simulations. The main aim of total temperature probe geometry is to decrease flow velocity inside the shield to decrease thermal conduction in the boundary layer. In simulations, this aim was observed to be accomplished. The flow velocity vectors were investigated to understand the nature of flow around and inside the total temperature probe. No flow separation was observed on the shield inlet.