Sara Rainieri

Convective heat transfer to temperature dependent property fluids in the entry region of corrugated tubes

Abstract The present experimental investigation is aimed to the analysis of the thermal performances of corrugated wall tubes employed in a broad variety of industrial applications in order to intensify the convective heat transfer. Both axial symmetrical and helical corrugations with different pitch values, have been considered in the present analysis. In particular, the phenomena arising due both to the wall roughness and to fluid property variation are investigated in the thermal entrance region in the Reynolds number range 90–800. The comparative analysis of the behaviour of the two types of corrugation enables to draw interesting conclusions about the effects of the corrugation pattern on the heat transfer enhancement mechanism. The data show that the helical corrugation induces significant swirl components to which, however, an as much significant heat transfer enhancement is not associated. The variation of the fluid physical properties with temperature, promotes the transition to an unstable flow. Regarding to this phenomenon a critical local Reynolds number, proportional to the dimensionless corrugation pitch has been identified. The dependence of the local Nusselt number on the corrugation pitch has been investigated, too.

Modeling approaches applied to the thermal response test: A critical review of the literature

This article provides a comparative review of various modeling approaches adopted in the open literature dealing with the parameter estimation procedure required in the geothermal thermal response test (TRT). First, the set of partial differential equations is introduced that describes the combined convective–conductive phenomena occurring in a borehole and in the energy storage system represented by the surrounding soil. The various approaches given in the literature for formulating approximate models are then illustrated. A model-based classification is adopted while introducing and reviewing the analytical and numerical methods found in the literature, including one-, two-, and three-dimensional approaches available for processing the experimental data resulting from the TRT. The various modeling procedures that have been applied to the TRT are discussed and compared to point out their strengths and weaknesses in relation to their differing extraction of information from the input data, represented by the time history of the experimental fluid temperature.

Data filtering applied to infrared thermographic measurements intended for the estimation of local heat transfer coefficient

Abstract A filtering technique has been applied in order to overcome the problem of random uncertainties in temperature measurement intended for the solution of the inverse heat conduction problem. To verify the best procedure for smoothening the surface temperature distribution whose Laplacian has to be estimated, several numerical filters have been tested on a simulated noisy two dimensional signal. The optimal method found, which is based on consecutive Wiener filterings, has been eventually validated by processing experimental data recorded with an infrared camera, with the aim of recovering the local heat transfer coefficient on a prototype of wavy fin commonly employed in compact heat exchangers.

Wiener filtering technique applied to thermographic data reduction intended for the estimation of plate fins performance

The present work deals with the optimization of a data processing technique aimed to the estimation of local heat transfer coefficient in plate fins commonly used in compact heat exchangers. When the assumption of uniform heat flux boundary condition cannot be made, the convective heat transfer coefficient distribution on thin fins may be recovered by solving the inverse heat conduction problem. The ill-posed nature of this problem has here been handled by pre-processing the raw temperature map, recorded with a high resolution infrared camera, with a filtering technique based on the Wiener filter. In order to increase the data processing efficiency, in the present work the algorithm by which the filter is usually implemented, has been modified. Basically, the filtering function here tested has been defined in such a way that only the pixels belonging to the measured temperature map are used, without any fictitious extrapolation of the signal. Besides, for the regions close to the boundaries, a variable size of the window used to perform the local statistical processing of the signal, has been used. This feature makes the filtering technique more suitable for processing low resolution images containing signal discontinuities. The estimation procedure has been validated throughout its application to experimental data regarding the usual plate fin and tube configuration subject to parallel air flow.

Numerical analysis of convective heat transfer enhancement in swirl tubes

Purpose – The aim of this paper is to investigate the convective heat transfer in swirl tubes, which are obtained by roto‐translating a circular section eccentric with respect to the rotation axis. The geometry is numerically investigated with the aim of evaluating the convective heat transfer enhancement effect due to the secondary flow induced by the centrifugal force.Design/methodology/approach – The governing equations, i.e. continuity, momentum and energy equations, are integrated numerically within Comsol Multiphysics® environment, under the assumption of incompressible Newtonian and constant properties fluid and of periodically fully developed laminar flow for what concerns both the hydrodynamic and the thermal problem under the uniform wall heat flux thermal boundary condition.Findings – The heat transfer performance of the geometry is discussed in relation to the flow pattern. In particular, the numerical results show that two different stable flow regimes may exist, according to the ratio of the...

Effect of a Hydrophobic Coating on the Local Heat Transfer Coefficient in Forced Convection under Wet Conditions

An estimation technique of the local heat transfer coefficient, based on the solution of the 2-D inverse heat conduction problem, has been adopted in order to investigate the effect of the surface wettability on the two-phase convective heat transfer in a dehumidifying process. The convective heat transfer coefficient distribution has been restored on aluminum plates coated with a hydrophobic oleic film on which the dropwise condensation of the water vapor carried by a humid air turbulent stream occurs. The refinement of the technique in relation to its ability of capturing the heat transfer local capability of the surface enables the appraisal of the heat transfer augmentation due to the hydrophobic surface coating.

Experimental investigation on the convective heat transfer enhancement for highly viscous fluids in helical coiled corrugated tubes

In the present analysis, the forced convective heat transfer in smooth and corrugated helical coiled tubes was experimentally studied in the Reynolds and Dean number ranges 50÷1200 and 12÷295 respectively, by adopting Ethylene Glycol as working fluid. The primary aim of the investigation is to study the combined effect of the wall curvature and of the wall corrugation in the thermal entrance region for highly viscous fluids. Two coiled tubes with a curvature ratio of about 0.06, one with smooth wall and the other with spirally corrugated wall, were investigated under the uniform heat flux boundary condition. The main conclusion is that in the Reynolds number range analyzed, both curvature and corrugation enhance the heat transfer. For Dean number values lower than about 120 the wall curvature effect prevails, and the heat transfer enhancement reflects Nusselt numbers that are approximately 2–3 times higher than the straight smooth section. For greater Dean number values, the wall corrugation instead prevails. In fact the corrugated coiled tube reaches Nusselt number values which are up to 8 times higher than the ones expected for the smooth straight tube. The smooth coiled tube shows instead thermal performances at maximum 3.6 times over the straight section.

Thermal characterization of intumescent fire retardant paints

Intumescent coatings are now the dominant passive fire protection materials used in industrial and commercial buildings. The coatings, which usually are composed of inorganic components contained in a polymer matrix, are inert at low temperatures and at higher temperatures, they expand and degrade to provide a charred layer of low conductivity materials. The charred layer, which acts as thermal barrier, will prevent heat transfer to underlying substrate. The thermal properties of intumescent paints are often unknown and difficult to be estimated since they vary significantly during the expansion process; for this reason the fire resistance validation of a commercial coatings is based on expensive, large-scale methods where each commercial coating-beam configuration has to be tested one by one. Adopting, instead, approaches based on a thermal modelling of the intumescent paint coating could provide an helpful tool to make easier the test procedure and to support the design of fire resistant structures as well. The present investigation is focused on the assessment of a methodology intended to the restoration of the equivalent thermal conductivity of the intumescent layer produced under the action of a cone calorimetric apparatus. The estimation procedure is based on the inverse heat conduction problem approach, where the temperature values measured at some locations inside the layer during the expansion process are used as input known data. The results point out that the equivalent thermal conductivity reached by the intumescent material at the end of the expansion process significantly depends on the temperature while the initial thickness of the paint does not seem to have much effect.

Numerical 2-D Modeling of a Coaxial Scraped Surface Heat Exchanger Versus Experimental Results Under the Laminar Flow Regime

Scraped surface heat exchangers (SSHEs) provide a versatile solution in the process industry for treating highly viscous fluids that may also contain particulate matter. Although SSHEs are frequently used in industrial applications, literature on this topic, particularly on the laminar flow regime, is limited. Moreover, due to the specificity of each product, it is difficult to generalize the few data available, and this makes the thermal design of this type of apparatus a critical point. Regarding the numerical approach, several studies based on a two-dimensional (2-D) approximation are available in the open scientific literature, but there is a lack of experimentally validated models. To test the numerical modeling approach, an experimental investigation that focused on the behavior of a coaxial SSHE in the presence of laminar flow was conducted. The appropriateness of the 2-D numerical approach is discussed here. Comparison of the numerical results with the experimentally measured Nusselt number values demonstrates the limits of the 2-D approach in describing the behavior of this type of apparatus.

Estimation of the local heat transfer coefficient in coiled tubes: Comparison between Tikhonov regularization method and Gaussian filtering technique

Purpose Most of the passive techniques for enhancing heat transfer inside pipes (e.g. rough surfaces, swirl-flow devices and coiled tubes) give origin to an irregular distribution of the heat transfer coefficient at the fluid–wall interface along the wall perimeter. This irregular distribution could be critical in some industrial applications, but most of the available research papers, mainly due to the practical difficulty of local measuring heat flux on the internal wall surface of a pipe, present the results only in terms of Nusselt number averaged along the wall circumference. This paper aims to study the application of inverse problem solution techniques, which could overcome this limitation. Design/methodology/approach With regard to the estimation of the local convective heat transfer coefficient in coiled tubes, two different inverse heat conduction problem solution techniques were considered and compared both by synthetic and experimental data. Findings The paper shows the success of two inverse problem solution techniques in the estimation of the local convective heat transfer coefficient in coiled tubes. Originality/value This paper fulfills an identified need because most of the available research papers present the results only in terms of average thermal performance, neglecting local behavior.