EXPERIMENTAL INVESTIGATION BY IR-THERMOGRAPHY OF HEAT TRANSFER OVER RIB-ROUGHENED SURFACES

This paper presents experimental results on convective heat transfer of air flows over a not-thin plate with different rib-roughness patterns; data over a flat plate are also reported for comparisons. The paper first reviews the technique principles, the experimental setup, and the data processing to recover heat transfer characteristics from IR-thermographic images. Then, thermal performances of three differently ribbed surfaces, namely, with ribs tilted of 90° and 60° angles over flow direction, as well as with 60° V-shape ones are presented and discussed. In all the configurations, the plate is 200 mm long and 150 mm wide, whereas ribs have a square cross-section with 3-mm side, and a corrugation-pitch to rib-side ratio of 13.3. Air flows at room temperature with speed ranging from 2.3 to 11.6 m/s, corresponding to Reynolds numbers at the end of the heated-length between 50000 and 250000, and for a heat flux of near 650 W/m2. For all three enhanced surfaces, the average Nusselt number over the plate horizontal midline shows the same depend¬ence on the Reynolds number as the flat plate, but its values are from 50% to 130% higher with slight differ-ences between the different tested configurations

EXPERIMENTAL RESULTS BY IR-THERMOGRAPHY ON CONVECTIVE HEAT TRANSFER OVER ENHANCED SURFACES

This paper presents experimental results on convective heat transfer of air flows over a not-thin aluminum plate with transverse, straight ribs for different pitch-to-rib-side ratio values. Data over a flat plate are also reported for comparisons. The plate is 200 mm long and 150 mm wide, and it is heated by a thin foil heater; ribs have a square cross-section of 3-mm side. The paper first reviews the experimental setup and the data processing to recover heat transfer characteristics from IR-thermographic images; then, thermal performances of tested configurations, namely, pitch-to-rib-side ratio ranging from 10 to 40, are presented and discussed. Air flows at room temperature with speed from 2.3 to 11.6 m/s, corresponding to Reynolds numbers at the end of the heated-length between 50000 and 250000, and for a heat flux of near 550 W/m2. For the considered ribbed surfaces, the average Nusselt number shows an increase with respect to the flat plate varying from 45 to 130% as Re increases but with slight differences between them

A novel scale-invariant, dynamic method for hierarchical clustering of data affected by measurement uncertainty

An enhanced technique for hierarchical agglomerative clustering is presented. Classical clusterings suffer from non-uniqueness, resulting from the adopted scaling of data and from the arbitrary choice of the function to measure the proximity between elements. Moreover, most classical methods cannot account for the effect of measurement uncertainty on initial data, when present. To overcome these limitations, the definition of a weighted, asymmetric function is introduced to quantify the proximity between any two elements. The data weighting depends dynamically on the degree of advancement of the clustering procedure. The novel proximity measure is derived from a geometric approach to the clustering, and it allows to both disengage the result from the data scaling, and to indicate the robustness of a clustering against the measurement uncertainty of initial data. The method applies to both flat and hierarchical clustering, maintaining the computational cost of the classical methods

LOCAL AND GLOBAL HEAT TRANSFER CHARACTERISTICS INSIDE A RECTANGULAR CHANNEL OF 1:10 ASPECT-RATIO WITH RIBBED SURFACES

In this paper we present new experimental results on both local and global heat transfer characteristics of a forced air-flow through a 12-mm-height, rectangular channel of 1:10 aspect ratio, with square-cross-section ribs mounted on the lower wall, that is also the only one heated. As local heat transfer characteristics are evaluated for fixed heat flux boundary condition whereas the global ones are for uniform wall temperature, two different test sections are used; that one for local measurements is provided with a 120 mm x 120 mm Germanium window to allow optical access inside the duct to the IR-camera. The local convective heat transfer coefficient is evaluated by accurately assessing the dispersed non-convective heat flux components. A Matlab script was specifically developed to solve the radiative heat transfer problem in a 2D cavity involving partially transparent walls and multi-wavelength regions. Moreover, much attention was also paid to evaluate the heat flux conductively diffusing through the heater plane and the walls surrounding the cavity. Data here presented refer to convection over ribs of 4-mm side in transverse configuration with a pitch-to-height ratio of 20, and for Reynolds numbers, based on the duct hydraulic diameter, ranging between 700 and 8000. Finally, the stream-wise distributions of the local heat transfer coefficient are shown and compared to the flat channel

Experimental results on local heat transfer coefficient inside a rectangular channel with chevron-shaped ribs

A non-intrusive measurement technique of the heat transfer coefficient field, based on IR-thermography, is applied on a 12-mm-height rectangular channel of 1:10 aspect ratio, whose lower surface is enhanced by chevron-shaped square-cross-section ribs. The channel is operated at imposed heat flux while forced-air flows for Reynolds numbers ranging from 700 to 8000. In particular, the technique uses local temperature measurements to retrieve the radiative heat flux with a three-wavelength-band model for partially transparent enclosures, and the conductive flux towards the backside with a FEM model, which allow an edge-to-edge estimation of the local convective heat transfer. The resulting local Nusselt number maps are presented and compared to data obtained operating both at fixed temperature and inside a channel with flat surfaces