Heat transfer and performance characteristics of axial cooling fans with downstream guide vanes

This study examines experimentally the effect of stators on the performance and heat transfer characteristics of small axial cooling fans. A single fan impeller, followed by nine stator blades in the case of a complete stage, was used for all the experimental configurations. Performance measurements were carried out in a constant speed stage performance test rig while the transient liquid crystal technique was used for the heat transfer measurements. Full surface heat transfer coefficient distributions were obtained by recording the temperature history of liquid crystals on a target plate. The experimental data indicated that the results are highly affected by the flow conditions at the fan outlet. Stators can be beneficial in terms of pressure drop and efficiency, and thus more economical operation, as well as, in the local heat transfer distribution at the wake of the stator blades if the fan is installed very close to the cooling object. However, as the separation distance increases, enhanced heat transfer rate in the order of 25% is observed in the case of the fan impelle

A comparative study of the local heat transfer distributions around various surface mounted obstacles

In many engineering applications, heat transfer enhancement techniques are of vital importance in order to ensure reliable thermal designs of convective heat transfer applications. This study examines experimentally the heat transfer characteristics on the base plate around various surface mounted obstacles. Local convection coefficients are evaluated in the vicinity of each individual protruding body with great spatial resolution using the transient liquid crystal technique. Five different obstacles of constant height-to-hydraulic diameter ratio (∼1.3) are considered. These include: a cylinder, a square, a triangle, a diamond and a vortex generator of delta wing shape design. The experiments were carried out over a range of freestream Reynolds numbers, based on the hydraulic diameter of each obstacle, varying from 4,000 to 13,000. The results indicate a negligible effect of the flow speed on the heat transfer topological structure and a considerable effect of the obstacle geometry on the level and distribution of heat transfer enhancement

Evaluation of Heat Transfer Coefficients for an Impingement Cooling Cascade: Experimental Challenges and Preliminary Results

In this paper, a modification of the widely used transient liquid crystal heat transfer measurement technique for impingement cooling is presented, in which a constant heat flux at the surface is used to drive the experiment, instead of a step change in the flow temperature. This method, already employed for film cooling, is used for the first time in a narrow impingement channel of an impingement cooling cascade which is characterized by complex and three dimensional vortical structures. The results are compared to the traditional heater mesh method (i.e. a temperature step in the flow drives the experiment). With both methods similar results are obtained, with the associated differences that can be attributed to entrainment effects or different surface roughness. An uncertainty analysis shows also that a transient experiment with constant surface heat flux improves the accuracy in the high heat transfer regions, e.g. stagnation points, which are the most uncertain regions in the heater mesh method

Detailed heat transfer distributions of narrow impingement channels for cast-in turbine airfoils

Gas turbine operation at elevated temperatures ensures increased thermal efficiency and useful power output. However, the industrial tendency to push further firing temperatures is limited by the capabilities of the current turbine blade cooling technologies which approach their limits. Nowadays, the capabilities of the foundry industry to produce high integrity and dimensionally accurate castings allow the production of integrally cast turbine airfoils with the so-called double-wall cooling technology. Cast-in turbine airfoils include a more distributed form of cooling, where the coolant can be injected in the form of impingement jets within the wall rather than the hollow of the airfoil increasing dramatically the heat exchanged capabilities. Narrow impingement cooling passages can be therefore generated close to the external hot gas flow. In this study detailed heat transfer distributions for all internal surfaces of narrow impingement channels are evaluated. The test models consist of a single row of five impingement jets investigated over a range of engine representative Reynolds numbers. Effects of jet-to-jet spacing, channel width and height and impingement jet pattern are independently examined composing a test matrix of 22 large-scale geometries. For the evaluation of the heat transfer coefficients, the transient liquid crystal technique is used considering also thermocouple thermal inertia effects. Given that the use of different impingement geometries could play an important role on the distribution of convection coefficients, and hence, on the homogeneity of blade metal temperatures, the experimental results of this study can be used along with the external heat load for the development of thermal design models for integrally cast turbine vanes and blades

Hole Staggering Effect on the Cooling Performance of Narrow Impingement Channels

This study examines experimentally the cooling performance of integrally cast impingement cooling channels which provide increased heat transfer area compared to traditional impingement configurations. For the evaluation of the heat transfer coefficient, the transient liquid crystal method was used. Full surface heat transfer coefficient distributions on the target plate and the side walls of the channel have been measured by recording the temperature history of liquid crystals using a frame grabber. Several impingement cooling geometries have been tested composing a test matrix of nine different geometrical configurations. The experimental data are analyzed by means of various post-processing procedures and aim to clarify and quantify the effect of hole staggering on the overall cooling performance, a variable which has been little addressed in the open literature. The experiments were carried out in a low speed wind tunnel over a wide range of Reynolds numbers between 15’000 and 100’000. The results indicated similarities with convectional multi-jet impingement cooling systems as well as a noticeable effect of the cooling hole pattern. Finally, an error propagation analysis of the experimental uncertainties was performed providing information for the significance of scatter on repeated experiments

Narrow Impingement Channels: Recent Advancements and Future Directions

This paper summarizes the state-of-the-art related to the heat transfer characteristics of narrow impingement cooling channels, which can nowadays be integrated within the external wall of turbine airfoils. Particular emphasis is given to the experimental outcomes of the research activities performed at the Group of Thermal Turbomachinery (GTT) of EPFL over the last 10 years. The influence of various geometrical factors on the heat transfer distributions is demonstrated, aiming to provide a summarized physical knowledge for the thermal design of such configurations. More specifically, effects of channel height and width, streamwise jet-to-jet spacing, impingement hole staggering arrangements, varying jet diameters, convergent and divergent channel geometries, as well as effects of number of holes, are addressed for all channel interior surfaces. Existing correlations are also presented and compared to empirical models derived from traditional multi-array impingement jet systems. Furthermore, future research directions in the form of sequential impingement cascades are also discussed

Effect of Stators on the Performance and the Thermal Capabilities of Small Axial Cooling Fans

This investigation examines experimentally the effect of stators on the performance curves and the heat transfer capabilities of small axial cooling fans. The experimental data indicated that the results are highly affected by the flow conditions at the fan outlet. Downstream guide vanes can be beneficial in terms of pressure drop and efficiency, and thus more economical operation, as well as,in the local heat transfer distribution at the wake of the stator blades especially when the fan is installed very close to the cooling object. However, the more jet spreading and mixing with the surroundings achieved due to the absence of stators at higher separation distances results in increased turbulent levels improving the average heat transfer rate in the stagnation region. The increase heat transfer rate, in the case of a single fan impeller (swirling jet),can be25% higher compared with the case of a typical fan stage (non-swirling jet)showing a similarity with the general trends of swirling and non-swirling impingement jets

High-Resolution Heat Transfer Measurements and Crossflow Regulation in Narrow Impingement Cooling Channels with Divergent Geometries

The present study describes an experimental investigation into narrow impingement cooling channels with divergent geometries. The following analysis aims to clarify the influence that this particular form factor exerts on the heat transfer distribution, as well as on the traverse crossflow of the cooling passages. The research is based on the outcome of a numerical heat transfer optimization algorithm which indicates that divergent cooling channels present favourable heat transfer traits. Based on this remark, three impingement channels are investigated experimentally – two of them have divergent geometries and one has a rectangular (uniform) geometry. Heat transfer coefficients are resolved for those channels using a variation of the transient liquid crystal measurement technique, based on a triple layer of thermochromic liquid crystals. The experiments verify that divergent channels present crossflow with reduced momentum, and more equable jet velocity. These two factors lead to a more homogeneous spatial heat transfer distribution across the channels’ endplates