This paper proposes a combined method for two-dimensional temperature and velocity measurements in liquid and gas flow using a schlieren system. Temperature measurements are made by relating the intensity level of each pixel in a schlieren image to the corresponding knife-edge position measured at the exit focal plane of the schlieren system. The same schlieren images were also used to measure the velocity of the fluid flow. The measurement is made by using particle image velocimetry (PIV). The PIV software used in this work analyzes motion between consecutive schlieren frames to obtain velocity fields. The proposed technique was applied to measure the temperature and velocity fields in the natural convection of water provoked by a heated rectangular plate.

A method for measuring planar temperature fields of fluid flows is proposed. The focusing schlieren technique together with a calibration procedure to fulfill such a purpose is used. The focusing schlieren technique uses an off-axis circular illumination to reduce the depth of focus of the optical system. The calibration procedure is based on the relation of the intensity level of each pixel of a focused schlieren image to the corresponding cutoff grid position measured at the exit focal plane of the schlieren lens. The method is applied to measure planar temperature fields of the hot air issuing from a 10 mm diameter nozzle of a commercial Hot Air Gun Soldering Station Welding. Our tests are carried out at different temperature values and different planes along the radial position of the nozzle of the hot air. The experimental values of temperature measurements are in agree with those measured using a thermocouple.

In the schlieren method, the deflection of light by the presence of an inhomogeneous medium is proportional to the gradient of its refractive index. In the presence of temperature variations in a fluid flow, the refraction index is related to the gas density by the Gladstone–Dale constant, which depends on the nature of the gas and the wavelength of light propagating in the medium. The deflection of light in a schlieren system is represented by intensity variations on the observation plane. Then, for a digital camera, the intensity level registered in each pixel depends mainly on the refractive index variation of the medium and exposure time. Therefore, if we regulate the intensity value of each pixel by controlling the exposure time, it is possible to adjust the temperature value measurements. In this way, a specific exposure time of a digital camera allows us to measure a determined range of temperature values. For that reason, in this study we determine the range of temperatures that can be measured with a digital camera for different exposure times. By doing this, a wide range of average temperature value fields can be obtained by summing up the temperature contribution of each exposure time. The basic idea in our approach to measure temperature by using a schlieren system is to relate the intensity level of each pixel in a schlieren image to the corresponding knife-edge position measured at the exit focal plane of the system. Our approach is applied to the measurement of temperature fields of the air convection caused by a heated rectangular metal plate ($7.3\mathrm{cm}\times 12\mathrm{cm}$) and a candle flame. We found that the maximum temperature values obtained for exposure times of 31.3, 15.7, 7.9, 3.9, and 2 ms were 67.3°C, 122.6°C, 217.4°C, 364.3°C, and 524.0°C, respectively.

Visualizing two variables in the fluid flow requires a complex optical system to fulfill such a purpose. Furthermore, in these applications, measuring the temperature fields of the fluid is sometimes necessary. In this work, we use a color digital camera to record in one snapshot shadowgraph and two direction sensitivity Schlieren images. Furthermore, the Schlieren images obtained are used to measure temperature fields. The optical system, as main components, employs an rgb Light Emitting Diode (LED), two knife edges represented by shortpass and longpass ultra-thin filters, and a color digital camera. The results of this study show that shadowgraph images are contaminated with the crosstalk effect, and correction against it was applied. On the other hand, the reached resolution of the optical system to measure temperature values is approximately one degree.

The temperature field and chemiluminescence measurements of axisymmetric flame are obtained simultaneously in only one image. Digital Laser Speckle Displacement measures temperature fields, and direct image flame determines chemiluminescence values. Applying the Abel transform of axisymmetric objects for volume visualization requires smooth intensity profiles. Due to the nature of the experimental setup, direct image flame is corrupted with speckle noise and a crosstalk effect. These undesirable effects deteriorate the measurement results. Then, experimental data need crosstalk correction and speckle noise reduction to improve the measurements. This work aims to implement a methodology to reduce the speckle noise of highly noisy data intensity profiles to create smooth profiles appropriate to applying the Abel transform. The method uses a Four-Order Partial Differential Equation to reduce speckle noise and a Curve fitting utilizing a set of Gaussian functions to decrease residual undesirable effects. After this, correction of crosstalk is necessary to avoid this effect. The methodology is applied to premixed flames generated with Liquid Petroleum Gas for different mixes.

The efficiency of different fuels in combustion processes has been an ongoing quest. Temperature measurements and color spectrum of premixed combustion flames are the accepted parameters to study. This work’s aim is to do so simultaneously, with a very simple setup and saving computational resources. This method does not ask to synchronize two sets of measurements, avoid the need for complicated setup alignments, and use a single digital camera. Instead, the temperature and color spectrum are measured simultaneously using digital laser speckle photography and an image processing approach. The recorded data for both measurements are acquired using only a color digital camera, where the red (r) color channel is used to record data for temperature measurements, and the blue (b) and green (g) color channels are utilized to record the flames’ color spectrum. To calculate the flames’ temperature field, the Abel inversion technique was carried out, while the image processing required crosstalk image correction, image segmentation, and image filtering. The crosstalk image correction is the main feature that establishes an unequivocal relationship between image color and color spectrum of flames. The technique was tested on flames produced with a liquefied petroleum gas fed torch using different equivalence ratios. The temperature values obtained with this method are in good agreement with those obtained with a thermocouple.