Cooling channel flow characterization using Particle Image Velocimetry and Laser Induced Fluorescence
A detailed understanding of the flow and the heat transfer in cooling channels is important for designing optimal cooling circuits. This is particularly true for cooling channels with high heat loads and high thermal stresses, for example cooling circuits of rocket engines. Usually, the design process is based on RANS computations which often cannot be fully validated in detail due to missing availability of such information. Therefore, the design margins must be high to prevent structural failure due to cyclic heat loads (e.g. the Dog House effect). Hence, detailed measurements of the flow characteristics including local fluctuations of the temperature and the velocity field under well-defined conditions are necessary. This is possible using the high-quality, optical and non-invasive measurement techniques Particle Image Velocimetry (PIV) and Laser Induced Fluorescence (LIF) simultaneously. Such experimental results could be used as a validation platform for RANS and LES computations. A generic cooling channel experiment with water as the fluent was designed and constructed . The cooling channel has a rectangular cross section and a width of 6 mm and a height of 25.8 mm. The lower wall of the cooling channel is electrically heated and therefore provides a heat flux from the wall into the cooling fluid. The side walls and top are made from polymethyl methacrylate (PMMA) to provide optical access to the cooling channel. The channel consists of a feed line with a length of 60 times the hydraulic diameter to ensure having a fully established turbulent flow at the beginning of the test section. The test section itself also has a length of 60 times the hydraulic diameter to allow the full formation of the thermal boundary layer. A curved test section follows a straight test section to investigate the influence of the curvature on the flow, the secondary flow structures and the heat transfer. An overview of the cooling channel setup is given in Fig. 1a). The flow fields in two and three dimensions are measured by use of standard Particle Image Velocimetry (PIV) and Volumetric Particle Tracking Velocimetry (V-PTV), while Laser Induced Fluorescence (LIF) is used to determine the temperature fields. V-PTV is used to measure the flow field in all three dimensions whereas standard PIV is applied time synchronous with the LIF measurements. The seeding particles are silver-coated hollow glass spheres with a diameter of 10 μm. A two colour / two dye technique is used when applying LIF. LIF is based on the phenomena, that the fluorescence intensity of specific dyes which were excited by a laser is temperature sensitive. Hence, measuring the fluorescence intensity allows gathering information about the temperature field. Rhodamine B is used as the temperature sensitive dye for two colour LIF. The temperature insensitive dye Rhodamine 110 is used as the additional dye giving information about the local laser light sheet intensity. This two colour technique allows a more accurate temperature determination than the more commonly used one colour technique. The experimental setup is presented in Fig. 1b). The laser light source is a Nd:YAG double pulse laser with a wavelength of 532 nm and an energy of 60mJ per pulse. The laser light sheet with a thickness of approximately 1mm is focused by a plano-concave lens with a focal length of -50mm and a plano-convex lens with a focal length of 100mm. The light sheet is formed by a cylindrical lens with a focal length of -25mm. Three Imager pro / proX 11M cameras with Tamron SP AF 180mm lenses are used for acquisition of the PIV and LIF images. Depending on the requirements for the individual measurement, the cameras are used for the PIV images as well as the LIF images. Suitable band-pass filters transmit the wavelength of interest to each camera. The band- pass filter for the PIV camera transmits the 532nm laser light and the filters for the LIF cameras transmit the wavelengths in the regions of maximum emission intensity of the fluorescent dyes. Fig. 2a) shows the velocity distribution and Fig 2b) the standard deviation (RMS) of the velocity, recorded with standard PIV. The slow flow velocity near the walls is visible as well as the increase in the RMS values. A temperature calibration curve for a one colour LIF method is depicted in Fig.3a). The fluorescence decreases about 2 % per Kelvin, which correlates well with literature values. A temperature profile with a heated lower wall is shown in Fig. 3b). The temperature boundary layer as well as the uniform temperature in the bulk flow far away from the wall is visible. The fluctuations in the region between y = 3 mm and y = 10 mm are existent due to pollution on the cooling channel’s wall.