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Experimental results by IR-thermography on convective heat transfer inside a ribbed rectangular channel
No full textThis paper presents the first experimental results on local heat transfer characteristics of a forced air-flow through a rectangular channel with transverse ribs on the lower wall. The channel is 120-mm wide and 12-mm high, and it is operated at fixed heat flux on the lower wall while keeping thermally insulated all others; ribs have a square cross-section of 4 mm side and are made of a non-conductive material. Local heat transfer characteristics are investigated by measuring the temperature field on the surface of the ribbed heated wall by means of IR-termography (optical access in the infrared region to the inside of the duct is
provided by a germanium window mounted in the upper wall). Data here presented are obtained at Re=5400 for three values of the rib pitch-to-height ratio, namely, p/e =10, 20, 40; for all these configurations, the wall temperature shows similar streamwise-distributions with a minimum where flow reattaches; the distance of this point from the rib backside increases with p/e
HEAT TRANSFER CHARACTERISTICS IN FORCED CONVECTION THROUGH A RECTANGULAR CHANNEL WITH RIBBED SURFACES
No full textIn this paper we present new experimental results of investigation on average heat transfer characteristics of a forced air-flow through a rectangular channel with the lower and upper surfaces roughened by ribs; data for a rectangular channel with flat surfaces are also presented for comparison. The channel has a 120 mm wide, 12 mm high cross-section, and it is operated with the lower and upper walls at fixed temperature whereas the side ones are adiabatic. Ribs have a square cross section and are mounted transversely to air-flow direction; the tested configurations differ for the rib side dimension, namely, 2 or 4 mm, and for their pitch-to-side ratio equal to 10, 20 and 40. Upstream the test channel, there is an entry-section consisting of a 800 mm long, rectangular duct with the same transverse dimensions as the test section but with flat and adiabatic walls, in order to attain developed velocity conditions at the test channel entry. Air flow rates have been varied in order to investigate Reynolds numbers, based on the duct hydraulic diameter, between 700 and 7500. The average Nusselt numbers are evaluated on the basis of the air-flow bulk-temperature at entrance and exit from the heated zone, as well as of the surface temperature measured by eight T-type thermocouples plugged into the heated walls. The test section is also equipped with static pressure taps placed at the heated zone ends. Results show an increase of the average Nusselt number for the tested ribbed channels ranging between 1.1 and 4.6 with respect to the flat channel
FIRST EXPERIMENTAL RESULTS ON FORCED-CONVECTION HEAT TRANSFER INSIDE A WAVY CHANNEL
No full textIn this paper we present the results of an experimental investigation on average heat transfer characteristics and pressure drops of a forced air-flow through a rectangular channel with the lower and upper walls wavy configured. The test section is 880-mm long, 120-mm wide and 12-mm height, whereas waviness have in streamwise direction a triangular profile of 68-mm pitch and 161° apex angle; this geometry reproduces in a larger-scale a typical passage between two wavy-fins in a compact heat exchanger. The channel is operated with the wavy walls at uniform temperature whereas the side ones are adiabatic. Results here presented are for a wavy-wall temperature of 40 °C, air inlet-temperature of near 20 °C, and Reynolds numbers ranging from about 700 to 7500. Comparisons with the results specifically obtained in a flat rectangular channel, with the same dimensions and operating conditions, show that for the investigated wavy channel the laminar-to-turbulent transition starts at lower values of the Reynolds number, i.e., 1350, and heat transfer is enhanced from 40% to 80% but with a pressure penalization larger than 60%
EXPERIMENTAL RESULTS BY IR-THERMOGRAPHY ON CONVECTIVE HEAT TRANSFER OVER ENHANCED SURFACES
No full textThis 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
Accuracy in evaluating heat transfer coefficient by RANS CFD simulations in a rectangular channel with high aspect ratio - Part 1: benchmark on a channel with plane walls
Get PDFComputational Fluid-Dynamics (CFD) simulations are widely used for designing components and devices for heat transfer enhancement, and to this end the Reynolds Averaged Navier-Stokes (RANS) equations are often chosen, as they are computationally efficient. In this paper, several numerical simulations have been carried out on convective heat transfer of an air flow through a rectangular channel of 1:10 aspect ratio, 120 mm wide, 840 mm long. Numerical results have been compared to analytical values and experimental data. The configuration of the described numerical model will be used as starting point for detailed investigations of fluid-dynamic and thermal performances of ribbed channels in further analysis
Accuracy in evaluating heat transfer coefficient by RANS CFD simulations in a rectangular channel with high aspect ratio - Part 2: channel with ribbed walls
Get PDFIn this paper, a Computational Fluid-Dynamics (CFD) analysis on heat transfer characteristic using Reynolds Averaged Navier-Stokes (RANS) equations on a rectangular channel with ribbed surfaces is presented. Several numerical simulations have been carried out on convective heat transfer of an air flow through a rectangular channel of 1:10 aspect ratio, 120 mm wide, 840 mm long, with 90° ribs. Ribs have a square cross section with 4 mm side and three different values of dimensionless pitch, namely, 10, 20 and 40. Results on convective global heat transfer coefficient, i.e., averaged over the whole channel, have been compared to experimental data obtained in a channel with the same geometry and boundary conditions. Agreement between numerical and experimental data is discussed for the three different pitches considered. As expected, comparisons show a decreasing reliability for increasing complexity. The aim of this work is to find a suitable configuration of a CFD model with RANS that will permit authors to apply it to the range of dimensionless pitches
FIRST NUMERICAL RESULTS ON FORCED-CONVECTION HEAT TRANSFER INSIDE A RECTANGULAR CHANNEL WITH STRAIGHT RIBS ON LOWER AND UPPER WALLS
No full textThis paper presents the first results of a numerical investigation on the fluid dynamics and heat transfer characteristics of a forced air-flow inside a rectangular channel with straight ribs on the lower and upper walls. The Reynolds number is 4180. The upper and lower walls are maintained at constant temperature whereas the side walls are adiabatic. The duct is 120 mm wide, 12 mm height and 840 mm long. The ribs have a square cross section of 4 mm2 and are 20 mm spaced. The calculations are carried out with a commercial code using a RANS approach with different turbulence models. A two-dimensional numerical analysis is performed on the entire fluid domain. The average heat transfer coefficient over the entire channel and the pressure drops are compared with the measured values from an experimental facility with the same geometry and operating conditions as the numerical model
PULSE PHASE THERMOGRAPHY ANALYSIS ON CYLINDRICAL GEOMETRY
No full textGoal of this work is the characterization of flaws in cylindrical geometry exploiting Pulse Phase Thermography (PPT), a Non-Destructive Testing method belonging to the Infrared Thermography category. The experiments are conducted on a hollow semicylindrical Plexiglas pipe section, with an outer and an inner diameter, respectively, of 150 mm and 130 mm. Defects are simulated by 5 cylindrical dummy holes, whose diameter range between 2 and 8 mm. Two operations are necessary to reconstruct the actual conditions of the analyzed specimen: morphological operations are carried on to reconstruct its actual surface, due to the distortion of the acquired image; furthermore, an evaluation how the directional emissivity affects the measurements is conducted. A FEM model has also been implemented to extend the analysis on other type of defects
First average and local heat transfer measurements on a forced air-flow at low Re-numbers through a rectangular channel with ribbed surfaces
No full textIn this paper we present new experimental results on average and local heat transfer characteristics of a forced air-flow through a rectangular channel with the lower and upper surfaces with continuous, square ribs. Comparisons with data inside a flat rectangular channel are also presented. The experimental duct cross-section is 120 mm wide and 12 mm height, and the channel is operated with the lower and upper walls at fixed temperature whereas the sides are adiabatic. Ribs are transverse to the flow or tilted with an angle of 45° or 60°; for the transverse
configurations, three values of the pitch-to-heigth ratio p/e, namely 10, 20 or 40, are investigated. Reynolds numbers, based on the duct hydraulic diameter, are varied between 600 and 8000. Accordingly, the average Nusselt numbers and the Darcy-Weisbach friction factor are presented.
In addition, first results on local temperature measurements by means of IR-thermography,
obtained in a properly modified test section operated at a constant heat flux, are shown for the transverse- rib configuration with p/e=10 and e=4-mm
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