Fabio Bozzoli

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.

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.

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.

Inverse estimation of the local heat transfer coefficient in curved tubes: a numerical validation

Wall curvature represents one of the most used passive techniques to enhance convective heat transfer. The effectiveness of wall curvature is due to the fact that it gives origin to the centrifugal force: this phenomenon induces local maxima in the velocity distribution that locally increase the temperature gradients at the wall by then maximizing the heat transfer. This fact brings to a significant variation of the wall temperature and of the wall heat flux along the circumferential coordinate. The convective heat transfer coefficient is consequently not uniformly distributed along the tubes perimeter and is characterized by higher values at the extrados wall surface in comparison to the ones at the intrados wall surface. Therefore, for predicting the overall performance of heat transfer apparatuses that involve the use of curved tubes, it becomes important to know the local distribution of the convective heat transfer coefficient not only along the axis of the heat transfer section, but also on the internal tubes surface along the cross section circumference. The present paper is intended to the assessment of a procedure developed to evaluate the local convective heat transfer coefficient, along the circumferential coordinate, at the internal wall of a coiled pipe.

Experimental Investigation on the Heat Transfer Performance of a Scraped Surface Heat Exchanger for Highly Viscous Foods

Scraped Surface Heat Exchangers (SSHEs) provide an interesting solution in order to actively enhance the convective heat transfer mechanism when highly viscous fluids have to be treated. The investigations available in literature about these heat exchangers are rare, especially under laminar flow regime conditions. Moreover, due to the specificity of each product, it is difficult to generalize the few data available, by making the thermal design of these apparatuses a critical point. This paper concerns with a pilot plant designed in order to evaluate the performance of a concentric SSHE, especially intended for highly viscous fluid foods. Preliminary experimental results are compared to the prediction of an analytical model and to the few empirical heat transfer correlations available in literature suitable for this application.Copyright

Short-time thermal response test based on a 3-D numerical model

To assist the design of ground heat exchangers the thermal response test method is usually performed. In its traditional version the thermal response test data are compared with the line source model solution to restore the soil thermal conductivity and the borehole thermal resistance. This approach requires that a steady-state thermal condition is approached and, consequently, it may need up to about 50 hours to be carried out. Promising approaches can be found within parameter estimation procedures supported by 3-D numerical tools. They allow the reduction of the test duration, since they perform a more realistic description of the systems behaviour especially in the early regime. In this paper, a versatile parameter estimation procedure based on a 3-D numerical model of a geothermal system is presented. The procedure is aimed to restore the value of both the soil thermal conductivity and the borehole thermal resistance, by using only the first few hours of the thermal response test data. The approach is validated by comparison with a medium-scale set of experimental laboratory data.