# Thermographic Stall Detection by Model-Inspired Evaluation of the Dynamic Temperature Behaviour
###### Abstract
Since limiting measurement conditions exist for free-field measurements on distant wind turbines, active heating of the rotor blade is here excluded. The naturally available thermal difference between the rotor blade and the incoming flow exist solely due to solar radiation, which can be estimated to \(175\,\mathrm{W}\,\mathrm{m}^{-2}\) during summer on a cloud-free day [8]. Hence, the resulting low thermal contrast, together with the large distance between the rotor blade of a wind turbine and the thermographic camera on the ground, leads to the requirement of a thermographic measuring approach with enhanced sensitivity. The thermographic detection and characterisation of the laminar-turbulent transition in wind tunnel tests is a well known technique for wind tunnel experiments [12; 13; 14; 15]. A distinct difference in convective heat transfer exists between the laminar and turbulent flow, so that an evaluation of steady state temperature fields enables a distinction of the different flow regions [16; 17]. This measurement approach in a _thermodynamic steady state_ was successfully transferred to free-field measurements. First, [PERSON] et al. localized the laminar-turbulent transition on wind turbines in operation to quantify flow disturbances caused by the contamination or erosion of the leading edge [18]. They also investigated the effects on the measurement uncertainty for the distinction of different boundary layer flow regions [19]. Furthermore, [PERSON] et al. [20] proposed to evaluate the time series of thermographic images using the non-negative matrix factorisation in order to increase the ability to distinguish between different boundary layer flow regions by reducing temperature gradients inside each flow region. Another approach for the thermographic localisation of the laminar-turbulent transition is to consider the _transient thermodynamic behaviour_ on the rotor blade surface. Possible measurement setups to distinguish flow regions by heating rate are pitching rotor blades where the location of the different boundary layer flow regions changes [11; 21] or non-pitching rotor blades with transient internal or external heating intensities [22]. Both measurement setups can increase the thermal contrast between different boundary layer flow regions and enable the investigation of the transient behaviour of the laminar-turbulent transition [11; 22]. A transfer to wind turbines in operation with pitching airfoils or transient absorbed solar radiation seems feasible, but a study of the transient thermodynamic behaviour for localisation in particular separated flow regions is pending. In contrast to the laminar-turbulent transition, a thermographic distinction between the turbulent boundary layer and the turbulent-separated flow, as well as the interpretation of the turbulent-separated flow signature on IR thermograms, is still a challenging subject for wind tunnel experiments [12]. The heat transport mechanisms of the turbulent boundary layer and the turbulent-separated boundary layer flow are different [23] but have the same order of magnitude in the region of beginning separation [17]. Thus the thermal contrast, in the case of flow separation, is significantly lower compared to laminar-turbulent transitions. First, [PERSON] and [PERSON] [12] detected stall on an airfoil with internal heating by evaluating temperature fields on single frames. [PERSON] et al. [24] reported a statistical approach for stall detection on an intensively heated airfoil. They proposed creating stall maps for helicopter rotor blades by evaluating an increase in the spatial temperature variation. [PERSON] et al. [8] published an alternative thermography measurement method for stall detection on non-heated static airfoils, i.e., for low thermal contrast conditions. By evaluating the dynamic behaviour of the surface temperature by means of a standard deviation or Fourier amplitudes of the temperature time series, an increase in the distinguishability of the boundary layer flow regions was shown.