1,720,965 research outputs found
Heat transfer measurements by IR thermography over enhanced surfaces with skewed ribs
No full textThis paper presents the experimental results on heat transfer characteristics of air forced convection over not-thin plates with transverse and skewed ribs; data are obtained by means of infrared thermography. Compari¬sons of heat transfer distributions for ribbed plates with data over a flat plate suggest the following observations. The periodic development of the boundary layer is very well displayed by the local distribution of the convective heat transfer coefficient; for both enhanced surfaces, the average Nusselt number shows the same dependence on the Reynolds number than the flat plate; the en¬hancement factor is about 1.4 for both ribbed configura¬tion, without appreciable differences between them
Numerical Simulations of the flow field within a 1:10 Aspect Ratio duct at intermediate Re
Full text linkNumerical simulations are carried on to study the fluid-dynamical features of a smooth duct with aspect ratio of 10. The duct is operated with an incompressible, newtonian fluid, whose Reynolds number, computed over the hydraulic diameter and bulk velocity, ranges from 470 to 14500, encompassing laminar and turbulent flow. To capture the details of all the flow scales, Direct Numerical Simulations are performed, by means of a code developed at Politecnico di Milano. The adopted code is a finite-difference, structured grid solver, that includes a mass flow rate correction. The latter guarantees high accuracy in the calculation of unsteady flows, or during the transition to turbulent regime, and it allows to check the consistency of numerical results. Both global parameters -including the friction factor and the identification of the laminar-to-turbulent transition and local flow features, e.g., corner vortexes, are investigated and presented in this work. Preliminary analyses agree fairly well with literature data and with experimental results obtained at ThermALab of Politecnico di Milano. The final goal of this work, including a deep integration between the numerical and the experimental setup, is to carry on detailed investigations of the fluid-dynamical and thermal characteristics of ribbed ducts, in the perspective of heat transfer enhancement and pressure drop reduction
Accurate contact resistance characterization for thermal conductivity measurement with the Heat Flow Meter method
Full text linkA common method to measure the thermal conductivity of low-conductive materials
is to impose a known temperature difference across the thickness of the specimen, and to measure
the resulting heat flow once steady-state conditions are reached. In particular, international
standards, i.e. ASTM C518 and ISO 8301, define the characteristics of the guarded Heat Flow
Meter apparatus and its measurement procedure. However, the actual measured quantity is the
overall thermal resistance, which is given by the series of the contact resistance between the
specimen and the temperature-controlled clamps, and the resistance of the specimen itself. Thus,
the contact resistance must be correctly quantified in order to retrieve an accurate measurement.
To this end, common practices are either to rely on a database of known contact resistances for
material classes, or to use the “double thickness” method, which allows to eliminate the
contribution of the contact resistance by carrying out the measurement on two specimens of the
same material, but different thickness. While the first method is rather useless for accurate and
reliable measurements, especially of unknown or innovative materials, the latter works only if
the contact resistances, and therefore the surface finish, are the same for every surface of the
specimen set. This paper presents an analysis of the uncertainties associated with the evaluation
of the contact resistance carried out on several samples, and proposes a method to reduce such
uncertainties, i.e. by inserting elastic thermal pads of known thermal conductivity between the
specimen and the instrument. The results of the validation of the method are also shown, with an
analysis on the improvement of the measurement accuracy for specimens with high roughness
and irregular surfaces, or with conductivities beyond the instrument declared range
Insight in thermal and fluid-dynamic properties of ribbed ducts by means of a novel clustering method
Full text linkThe analysis of experimental results on heat transfer by forced convection in diverse ribbed ducts showed that different geometries lead to comparable thermal and fluid-dynamic performances. Moreover, no evident layout has been observed in data, and therefore a statistical clustering analysis is performed to detect the rationale, if any, underlying experimental results. A novel, ad-hoc developed technique is used to disengage the clustering from the data scaling and to account for the measurement uncertainty, consisting of an agglomerative procedure, based on the definition of dynamically-changing bounding boxes, whose size depends on the Nusselt number and the pumping power. Additional informations, such as the the relevance of the diverse geometric parameters and the persistence of similarity among configurations over a range of operating conditions, can be retrieved by means of the developed technique. The described method is applied to a large dataset, obtained during an experimental campaign carried on at ThermALab of Politecnico di Milano, aimed at identifying the Nusselt number and the friction factor for diverse-rib configurations in a large-aspect ratio channel with low-Reynolds flows. The considerations originated from of the results of the clustering analysis suggest the existence of an underlying structure, pointing to a possible unique parameter, termed "generalized blockage", which is possibly able to describe the global effect of the ribs geometry on forced convection
Infrared image processing for local convective heat transfer measurements in rib-enhanced channels
Full text linkIn the context of an experimental study designed to retrieve the local heat transfer characteristics in closed channels with ribs in several configurations by IR thermography, a filtering strategy must be adopted to calculate the laplacian of the temperature field, to remove not only Gaussian noise, but also unwanted local features due to the pattern of the conductors of the heater. In this paper, these issues are addressed with a comparative analysis on filtering and interpolation techniques, in particular mean filtering, local surface fitting and cubic smoothing splines, which are carried out by showing their influence on the smoothed thermogram and on the laplacian of the temperature field. The use of cubic smoothing splines gives the best approximation of the temperature field with respect to the mean filter, which alters the temperature field in proximity of the ribs, and to surface interpolation. However, the only filter among those tried that gives meaningful derivatives on the whole image is the mean filter, with a kernel size equal or bigger to that of the spatial feature to be filtered. The resulting derivative also need further filtering to reduce local spikes induced by the filtering operations
Heat fluxes distribution and uncertainty in low-Reynolds flow regimes inside a ribbed channel
Full text linkExperimental studies on the local convective heat transfer enhancement induced by ribs in channels are generally carried out in steady state, by imposing a known heat flux on the ribbed surface and subsequently retrieving the convective heat flux by means of a local heat balance, i.e. by subtracting all the non-convective heat fluxes from the source term. Such heat fluxes are retrieved from temperature measurements by means of models, whose complexity can range from simple algebraic expressions to complex simulations. In this context, an experimental technique based on FEM simulations, a radiative model for semi-transparent enclosures, and a filtering technique to retrieve the heat fluxes across the plane of the heated surface has been developed and tested in a 1:10 AR channel, for Reynolds number ranging from 650 to 7500 and a flow of air. This paper presents a discussion on the heat flux distribution as a function of the Reynolds number for a streamwise-pointing chevron (V) rib configuration with 60° angle of attack, with P/e = 20, showing the increasing weight of non-convective heat dissipations as the Reynolds number decreases, and their effect on the uncertainty of the Nusselt number measurement
Accuracy in evaluating convective heat transfer coefficient by RANS CFD simulations in a rectangular channel with high aspect ratio and 60° tilted staggered ribs
Full text linkA numerical analysis of heat transfer characteristics of an air flow through a narrow rectangular channel of 1:10 aspect ratio with ribbed surfaces has been carried out, by means of a CFD commercial code, exploiting Reynolds Averaged Navier-Stokes (RANS) equations. The channel is 120 mm wide, 840 mm long, with 60° tilted staggered ribs. Ribs have a square cross section with two different side heights (2 mm and 4 mm) and three different values of dimensionless pitch (10, 20 and 40). The numerical results have been compared with experimental data obtained by the authors inside a ribbed channel both with same geometry and operating conditions as one numerically modelled. Agreement between numerical and experimental data on convective global heat transfer coefficient, i.e., averaged over the whole channel, is discussed for the six different configurations considered. Moreover, performances are presented by considering at the same time both heat transfer enhancement and pressure-drop penalization, highlighting strengths and weaknesses per each configuration. This work is aimed at finding a suitable configuration of a CFD model with RANS that will allow authors to apply it to the range of dimensionless pitches and side heights during early design-phases of parametric studies
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
New data processing of local heat transfer coefficient inside a rectangular channel
Full text linkIn this paper, we critically reconsider and discuss the models used in one of our previous work to calculate the local convective heat flux for forced air-flows inside narrow rectangular channel, in order to evaluate the limitations which may be inherent with them. To this end, several numerical FEM models have been developed in COMSOLTM and used to analyze in depth previous data processing procedures; furthermore, experimental tests are made on the channel materials, to determine their thermal conductivities and radiative properties. Here, the results of this analysis are presented and discussed
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