A confined jet impingement configuration has been investigated in which the matter of interest is the convective heat transfer from the airflow to the passage walls. The geometry is similar to gas turbine applications. The setup is distinct from usual cooling passages by the fact that no crossflow and no bulk flow direction are present. The flow exhausts through two staggered rows of holes opposing the impingement wall. Hence, a complex 3-D vortex system arises, which entails a complex heat transfer situation. The transient Thermochromic Liquid Crystal (TLC) method was used to measure the heat transfer on the passage walls. Due to the nature of the experiment, the fluid as well as the wall temperature vary with location and time. As a prerequisite of the transient TLC technique, the heat transfer coefficient is assumed to be constant over the transient experiment. Therefore, additional measures were taken to qualify this assumption. The linear relation between heat flux and temperature difference could be verified for all measurement sites. This validates the assumption of a constant heat transfer coefficient which was made for the transient TLC experiments. Nusselt number evaluations from all techniques show a good agreement, considering the respective uncertainty ranges. For all sites the Nusselt numbers range within ±9% of the values gained from the TLC measurement.

Laminar to weakly turbulent mixed convection in a square duct heated from the bottom side is highly strengthened by ionic jets generated by an array of high voltage points, opposite to the heated strip. Negative ion injection is activated within the dielectric liquid HFE-7100. Local temperatures on the heated wall are measured by liquid crystal thermography. Distributions of the Nusselt number are obtained at different forced flow rates, applied heat flows, and transiting electrical currents. In correspondence of the point emitters, higher Nusselt numbers in the impingement areas are measured and an analogy with the thermo-fluid dynamic behavior of an array of submerged impinging jets in a crossflow is drawn. The diameter of the ionic jets is evaluated and an electrohydrodynamic Reynolds number is employed for correlation and similarity purposes.

An experimental investigation has been performed to measure the cooling performance of the louver scheme using a two-dimensional cascade simulating the scaled vane of a high-pressure gas turbine. Two rows of an axially oriented louver scheme are distributed in a stagger arrangement over the pressure side. The effect of hole location on the cooling performance is investigated for each row individually, then the row interaction is investigated for both rows. The temperature distribution on the vane is mapped using a transient Thermochromic Liquid Crystal (TLC) technique to obtain the local distributions of the heat transfer coefficient and film cooling effectiveness. The performance of the louver scheme for each case is compared with that of two similar rows with a standard cylindrical exit at 0.9 density ratio. The exit Reynolds number based on the true chord is 1.5E5 and exit Mach number is 0.23. The local distributions of the effectiveness and the heat transfer coefficient are presented at four different blowing ratios ranging from 1 to 2. The louver scheme shows a superior cooling effectiveness than that of the cylindrical holes at all blowing ratios in terms of protection and lateral coverage. The row location highly affects the cooling performance for both the louver and cylindrical scheme due to the local pressure change and the variation of the surface curvature.

Simultaneous measurements of instantaneous velocity and temperature fields of air flows by means of Particle Image Velocimetry (PIV) and Particle Image Thermography (PIT) enables highly demanded studies on thermal plumes, their dynamics and the resulting heat transfer for Pr ≈ 0.7. Thereby, small particles of thermochromic liquid crystals (TLCs), which reveal temperature depending reflection properties are used as tracer particles for combined PIT and PIV. The feasibility of the method is demonstrated in a Rayleigh-Be´nard convection experiment in a cubical enclosure. Furthermore, a new particle generator being able to produce continuously very small monodisperse droplets of TLCs has been designed. The improvement of the developmental process for mixed and Rayleig-Be´nard convection studies is discussed. Thereby, special focus is laid on the production process of small TLCs, the generation of monodisperse acetone-TLC droplets and the temperature depending colour play of the produced particles.

A method is developed to capture the distribution of surface temperature while simultaneously imaging the bubble motions in diabatic flow boiling in a horizontal minichannel. Liquid crystal thermography is used to obtain highly resolved surface temperature measurements on the uniformly heated upper surface of the channel. High-speed images of the flow field are acquired simultaneously and are overlaid with the thermal images. The local surface temperature and heat transfer coefficient can be analyzed with the knowledge of the nucleation site density and location, and bubble motion and size evolution. The horizontal channel is 1.2 mm high × 23 mm wide × 357 mm long, and the working fluids are Novec 649 and R-11. Optical access is through a machined glass plate which forms the bottom of the channel. The top surface is an electrically heated 76 μm -thick Hastelloy foil held in place by a water-cooled aluminum and glass frame. The heat loss resulting from this construction is computed using a conduction model in Fluent. The model is driven by temperature measurements on the foil, glass plate and aluminum frame. This model produces a corrected value for the local surface heat flux and enables the computation of the bulk fluid temperature and heat transfer coefficient along the channel. The streamwise evolution of the heat transfer coefficient for single-phase laminar flow is compared to theoretical values for a uniform-flux boundary condition. Examples of the use of the facility for visualizing subcooled two-phase flows are presented. These examples include measurements of the surface temperature distribution around active nucleation sites and the construction of boiling curves for locations along the test surface. Points on the curve can be associated with specific image sequences so that the role of mechanisms such as nucleation and the sliding of confined bubbles may be discerned.

Viscous flows in a concentric annulus with rotating inner cylinder was characterized by Taylor-Couette flow. When the inner cylinder is replaced with a truncated conical rotor, the effect of imbalanced centrifugal force induces the meridional circulation flow with a single toroidal vortex. Under non-isothermal condition at cooling the rotor and heating the vessel wall, the flow pattern is drastically changed with the thermal conditions and the rotating speed. This paper describes the vortex flow-pattern transition and related heat transfer of the mixed convection in such a system. The temperature and flow field were visualized by means of thermo-sensitive liquid-crystal tracers. At the same time, the local heat transfer distributions on the cylindrical vessel wall were measured under a uniform heat flux by the electrically-conductive transparent film. Based on the experimental findings is described the interaction between the heat transfer and the thermal flow field during the unsteady mixed convection in the annular gap.