Liquid Crystal Thermography And Its Application In Film Cooling On Flat Plate

As an effective method of temperature measurement, liquid crystal thermography has been widely used in the field of heat transfer in many developed countries. This technique is just at the beginning of its applications in our country. It has the following advantages: high accuracy, fast response to temperature variations, suitable for temperature measurement on convective surface, etc. In this paper, a system of quantitative temperature measurement is based on the definition of hue. The type of the sprayable thermo-chromic liquid crystal for the experiment is SPN/R35C10W, made by Hallcrest Company in USA. This temperature measurement system is applied in the experiments of film cooling on flat plates for the first time interiorly.The relation between the hue of the color of the thermo-chromic liquid crystal and the temperature can be fitted with a six-order poly-nominal function. An error analysis shows that the measurement accuracy can be 0.8° C in the 95% confidence region. The calibration data show that the visual angle is an important factor to influence the calibration curve, however, this effect can be neglected when the visual angel is smaller than 30°. In the valid measurement region of temperature, light intensity has little effect on the calibration data, thus it can be set to achieve the best color appearance. The thermochromic liquid crystal should be re-calibrated when it is re-sprayed or deposited for a period of time.Film cooling is an important measure used to protect the blade in gas turbines. We configure our experimental setup of film cooling on flat plates, and utilize thermo-chromic liquid crystal to measure the temperature distributions downstream of the injection holes. In the thesis, we discuss the influences on film cooling brought by different blowing ratio, different injection hole length to diameter ratios, different injection angles and different compound angles. The adiabatic film cooling effectiveness is regarded as an important parameter to weigh the cooling performance at different experimental conditions.At a low blowing ratio, the secondary injection flow has low momentumcompared to the mainstream, so it is easily suppressed by the mainstream to a region near the cooper plate surface. Better cooling performance can be achieved in the region immediately out of the injection holes. When the blowing ratio increases, the phenomenon of "blowing off' and "re-attachment" happen downstream of the injection holes. At a low hole length to diameter ratio (L/D=2), the air flow cannot fully develop in the hole. The distribution characteristics of the adiabatic film cooling effectiveness are similar at different blowing ratios. The effect on the cooling performance due to different vertical velocity component is considered through experiments with different injection angles. The results show that the isolines still appear in a taper shape in the nearby region of the injection hole at a low blowing ratio when only vertical velocity exists, and the isolines change to near beelines at high blowing ratios. Different lateral velocity component can be produced through the introduction of compound angles. Its effect on film cooling is studied through experiments at three compound angles. The results indicate that a proper compound angle can yield better cooling performance than that of a simple angle. The regions between holes receive better cooling.