Sara Rainieri - Academia.edu (original) (raw)

Papers by Sara Rainieri

Food and Bioproducts Processing

Energies

Pulsating heat pipe is a promising two-phase heat transfer device that has many advantages such a... more Pulsating heat pipe is a promising two-phase heat transfer device that has many advantages such as a simple wickless structure and high thermal performance. Its thermal behavior is inherently time-dependent, and it can also be characterized by substantial spatial variations. However, there are few studies investigating the interaction or similarity of the local physical quantities, such as heat fluxes exchanged between the working fluid and the device wall in adjacent branches. In the present work, a new approach based on the application of cross-correlation analysis to local heat fluxes is proposed to deepen the understanding of the flow characteristics in pulsating heat pipes. The temperature distribution in the condenser of a seven-turn pulsating heat pipe was measured with an infrared camera, changing the power input. The local heat flux distributions were estimated by solving the inverse heat conduction problem in the tube wall. The cross-correlation of the heat fluxes at diffe...

Mathematical Modelling of Engineering Problems

The grape must temperature management in a fermenter tank is fundamental to guaranty a high-quali... more The grape must temperature management in a fermenter tank is fundamental to guaranty a high-quality wine. The fermentation is an exothermic process and a good cooling system in fermenter tanks is required. However, for reducing costs and increasing the efficiency of the cooling operation, a passive device based on the heat pipe technology was proposed. Since its thermal sizing must necessarily be accurate to allow a correct temperature of the product during fermentation, in this paper a thermal model of the proposed device was proposed and experimentally validated.

International Journal of Heat and Mass Transfer

Abstract A parameter estimation approach was applied to characterise the heat transfer inside a t... more Abstract A parameter estimation approach was applied to characterise the heat transfer inside a triple tube heat exchanger (TTHE) designed for the food industry. This type of heat transfer device represents a promising technology for the always increasing challenge of reducing the consumption of energy and raw materials. Especially, it provides a suitable solution for the heat treatment of highly viscous fluids; making the product flowing into the intermediate pipe, this kind of heat exchanger permits heat transfer both through the internal and the external surfaces, reducing the device size compared to traditional double tube heat exchangers. Moreover, this configuration reduces the problems related to product stratification, avoiding the risk of having part of the product burned and part not heated enough. However, the thermal design of these apparatuses is critical. Although they present a widespread industrial application, it is difficult to find correlations for their thermal behaviour. This study fills this gap by proposing an effective methodology for deriving a proper heat transfer correlation. Among the different methodologies that can be adopted to assess the performance of the TTHEs, the parameter estimation procedure represents a promising tool since it has been successfully applied in many disciplines of engineering. Following this approach, this study enables the successful and robust estimation of the heat transfer correlation for the product side Nusselt number starting from the temperature measurements at the inlet and outlet sections of the three tubes. The procedure was validated by adopting both synthetic and experimental data acquired from a TTHE for treating highly viscous fluid foods.

Tecnica Italiana-Italian Journal of Engineering Science, 2021

To assess the performance of triple tube heat exchangers, it is not possible to apply the traditi... more To assess the performance of triple tube heat exchangers, it is not possible to apply the traditional methods (e.g., Wilson plot method) because they can be only applied to the classical shell-in-tube configuration that can be easily modelled by the logarithmic mean temperature difference approach. Therefore, to overcome this limitation, some new methodologies have been proposed in the literature and, among them, a promising tool is represented by the parameter estimation procedure. Parameter estimation procedure is a powerful technique already adopted in many engineering applications. However, this procedure requires, for the application investigated in the present study, a detailed numerical model of the triple tube heat exchanger and the measurement of the temperature of the fluids in each tube at inlet and outlet sections. These elements make its application feasible only in well-equipped research labs, limiting its massive employment in industrial facilities. In this paper, a n...

International Journal of Heat and Mass Transfer, 2021

Abstract A Pulsating Heat Pipe (PHP), specifically designed to be hosted on board the Heat Transf... more Abstract A Pulsating Heat Pipe (PHP), specifically designed to be hosted on board the Heat Transfer Host of the International Space Station, is tested in microgravity conditions during the 67th Parabolic Flight Campaign promoted by the European Space Agency. The device consists in an aluminium tube (inner/outer diameter = 3/5 mm) closed in a 14 turns loop, half filled with FC-72. The PHP external wall temperature distribution are measured within the adiabatic section by means of a high-speed infrared camera. The resulting thermographic images are used as input data for the solution of the Inverse Heat Conduction Problem (IHCP) in the channels wall to estimate local time-space heat fluxes exchanged at the wall-fluid interface. The adopted post-processing method represents one of the first attempts to estimate the local wall-to-fluid heat flux in PHPs under microgravity conditions. A comprehensive investigation of the wall-to-fluid heat fluxes is performed on the overall device by means of an original statistical approach in order to study the PHP working regimes. The results highlight that such approach is capable of providing similar information regarding the fluid motion inside the PHP to those obtained by the traditional intrusive experimental methods (e.g. direct fluid temperature-pressure measurements) or transparent inserts (e.g. sapphire tube), which perturb the original layout of the PHP and require a complex experimental set-up.

Proceedings of the 2018 International Conference on Quantitative InfraRed Thermography, 2018

Displaced enhancement devices are inserts, positioned inside pipes, with the aim of increasing th... more Displaced enhancement devices are inserts, positioned inside pipes, with the aim of increasing the heat transfer rate. The actual work presents an original application of an inverse analysis technique to experimental infrared temperature data with the aim of estimating the local convective heat flux for forced convection flow in pipe when this kind of devices is present and, thanks to these results, with the aim of optimising insert geometry. In particular, butterfly-shaped inserts are investigated in terms of local and global thermal performances.

International Journal of Heat and Mass Transfer, 2020

The main goals of this paper are twofold: to provide an improved parameter estimation procedure t... more The main goals of this paper are twofold: to provide an improved parameter estimation procedure that enables the accurate evaluation of heat transfer correlations for the Nusselt number and to present a new procedure for identifying the presence of transition in the flow regime. In the first goal, it needs to be pointed out that although this kind of approach is based on temperature measurements, in the literature, the classical parameter estimation approach uses the overall heat transfer coefficient. To reduce uncertainty in the evaluation of internal and external heat transfer coefficients, in the improved procedure, the measured temperature data were directly used without any intermediate elaboration. The validation of the new procedure, carried out by means of synthetic data, highlighted that both the relative error on each estimated parameter and the amplitude of the confidence intervals were reduced by minimizing the objective function expressed in terms of outlet temperature of the tube side of the heat exchanger. With regard to the transition regime, the numerical outcomes revealed that both in the laminar and turbulent regimes, the amplitude of the confidence intervals of the estimated parameters was very small; in the transition regime, the amplitude became wide. Therefore, this parameter served as a useful indicator to identify the presence of transition in the flow regime. This finding enabled a new indirect methodology, called the transition alert procedure, to be developed for identifying the Reynolds number value at which transition of the flow regime occurred. This method is an iterative subsampling of the dataset based on the observation of the amplitude of the confidence intervals. To assess the robustness of the methodologies proposed here, the improved parameter estimation procedure and the transition alert procedure were applied to the obtained experimental data by analyzing a counter-flow heat exchanger with double corrugated tubes. In this kind of tube, the condition when the transition from the laminar to the turbulent flow regime occurs is not known a priori.

Journal of Physics: Conference Series, 2017

The Field Synergy Principle is widely applied to the evaluation of the convective heat transfer m... more The Field Synergy Principle is widely applied to the evaluation of the convective heat transfer mechanism. In fact, as highlighted in literature, the evaluation of the synergy between the velocity and the temperature gradient vectors could provide a better insight on the local convective heat transfer mechanism. In this paper, the field synergy approach is adopted to numerically investigate the fluid dynamic and thermal behaviour of a fully developed flow between parallel plates with asymmetric heating, when cylindrical inserts are present. To better evaluate the influence of the inserts on the convective heat transfer mechanism, different values of the insert diameter are considered, for a given pitch value. The numerical results in terms of Nusselt number point out that the convective heat transfer coefficient decreases as the insert diameter increases. The Field Synergy Principle allows to explain the cause of the convective heat transfer reduction identifying the regions in which the heat transfer mechanism is ineffective: the extent of these areas increases as the insert diameter increases.

International Journal of Thermal Sciences, 2019

Helical coil heat exchangers are commonly used in a wide variety of engineering applications beca... more Helical coil heat exchangers are commonly used in a wide variety of engineering applications because, due to curvature of the tube that generates secondary flows, this kind of geometry can generate an enhancement of the convective heat transfer coefficient. Since the distributions of the fluid velocity and temperature in coiled pipes are strongly asymmetrical over the cross-section of the tube, particularly in laminar flow conditions, it is important to monitor the local heat transfer phenomena, mainly with regard to the applications where the local behaviour plays a leading role, such as in food industry. Indeed, due to the high viscosity of fluid foods or to the low velocity in which they may operate, the flow regime is often laminar in this kind of devices. In laminar flow regime, the thermal boundary conditions considerably affect the convective heat transfer performance. In the present paper, a numerical investigation focused on the evaluation of the effect of the choice of the thermal boundary condition on the local heat transfer phenomena in coiled-tubes, within the laminar regime, is presented. To cover realistic configurations in which the helical coil heat exchangers are commonly used, the conjugate heat transfer problem in the tube was also considered. The momentum and energy equations were solved by means of the open-source CFD software OpenFOAM ® , with second-order accurate finite-volume schemes. The here adopted numerical model was validated by comparing the results with local experimental data. The numerical outcomes, obtained for several values of the Prandtl number, revealed that the thermal boundary conditions could significantly change the local distribution of the convective heat transfer coefficient. Finally, the influence of the thermal boundary condition on the average convective heat transfer coefficient were evaluated by considering the two definitions of the circumferentially averaged Nusselt number commonly used in literature.

Thermal Science and Engineering Progress, 2019

A promising solution in the field of passive two-phase heat transfer devices is represented by Pu... more A promising solution in the field of passive two-phase heat transfer devices is represented by Pulsating Heat Pipes (PHPs). These relatively new devices, which achieve resounding interest in terms of high heat transfer capability, efficient thermal control, adaptability and low cost, have been extensively studied in the last years by many researchers. Many authors have investigated the heat fluxes at the evaporator and the condenser area only in terms of the mean values. In this work a novel approach to investigate the local heat flux in PHPs is presented and tested: the temperature distributions on the external wall of the PHP acquired with a high-speed and high-resolution infrared camera were used as input data for the inverse heat conduction problem in the wall under a solution approach based on the Tikhonov regularization method. Infrared imaging is performed on a single loop PHP designed with sapphire inserts partially coated with a highly emissive paint, allowing to determine at the same time the external wall temperature and the fluid temperature. Results show that the technique is able to show when heat is transfer from the fluid to the sapphire wall, when the hot fluid is pushed from the evaporator towards the condenser; increasing the wall tube temperature. On the contrary, when a cold fluid flows back from the condenser, the tube releases the heat previously accumulated, thereby decreasing its temperature. This approach allows to analyze the thermal behavior of the device by investigating the direct interconnection between the thermo-fluid dynamic phenomena within the PHP and the local heat flux measurements. The results proposed in this work are a breakthrough for the improvement and validation of both VOF-based DNS simulations, for local physical phenomena, and 1D simulations of the global PHP behaviour.

Inverse Problems in Science and Engineering, 2018

A new regularization approach for the unconstrained linear least square problem is proposed and a... more A new regularization approach for the unconstrained linear least square problem is proposed and assessed. This approach improves the classical methods based on the singular value decomposition approach by employing the Gaussian filter, a kind of filter which has been proved to be effective in several applications related to noise suppression. In particular, as a benchmark, it is hereby considered the estimation of the heat flux density at the internal wall surface in a forced convection problem in ducts, by solving the inverse heat conduction problem in the solid wall only using temperature distribution available at the exterior boundary. Results on both synthetic and experimental data are reported with the aim of discussing the effectiveness of the proposed method in comparison to other similar solution approaches.

International Journal of Heat and Mass Transfer, 2018

Journal of Physics: Conference Series, 2016

Wall curvature is a popular heat transfer enhancement technique since it gives origin to the cent... more Wall curvature is a popular heat transfer enhancement technique since it gives origin to the centrifugal force in the fluid: this phenomenon promotes local maxima in the velocity distribution that locally increase the temperature gradients at the wall by enhancing the heat transfer both in the laminar and in the turbulent flow regime. This geometry produces an asymmetrical distribution of the velocity field over the cross-section of the tube which lead to a significant variation in the convective heat-transfer coefficient along the circumferential angular coordinate: it presents higher values at the outer bend side of the wall surface than at the inner bend side. Although the irregular distribution of the heat transfer coefficient may be critical in some industrial applications, most of the authors did not investigate this aspect, mainly due to the practical difficulty of measuring heat flux on internal wall surface of a pipe. In the present investigation the local convective heat-transfer coefficient is experimentally estimated at the fluid-wall interface in coiled tubes when turbulent flow regime occurs; in particular, temperature distribution maps on the external coil wall are employed as input data of the inverse heat conduction problem in the wall and a solution approach based on the Tikhonov regularisation is implemented. The results, obtained with water as working fluid, are focused on the fully developed region in the turbulent flow regime in the Reynolds number range of 5000 to 12000.

Proceedings of the 2006 International Conference on Quantitative InfraRed Thermography, 2006

The efforts made in the last decay in the field of infrared detectors technology are mainly devot... more The efforts made in the last decay in the field of infrared detectors technology are mainly devoted to the development of systems of high sensitivity and performance characterized, at the same time, by low cost and simple maintenance. These goals are hardly achieved with photon detectors which, in spite of the high sensitivity and signal-to-noise performance, need to be cooled in order to operate

Journal of Physics: Conference Series, 2015

In the last years, the attention of heat transfer equipments manufacturers turned toward helicall... more In the last years, the attention of heat transfer equipments manufacturers turned toward helically coiled-tube heat exchangers, especially with regards to applications for viscous and/or particulate products. The recent progress achieved in numerical simulation motivated many research groups to develop numerical models for this kind of apparatuses. These models, intended both to improve the knowledge of the fundamental heat transfer mechanisms in curved geometries and to support the industrial design of this kind of apparatuses, are usually validated throughout the comparison with either theoretical or experimental evidences by considering average heat transfer performances. However, this approach doesn't guarantee that the validated models are able to reproduce local effects in details, which are so important in this kind of non-standard geometries. In the present paper a numerical model of convective heat transfer in coiled tubes for laminar flow regime was formulated and discussed. Its goodness was checked throughout the comparison with the latest experimental outcomes of Bozzoli et al. [1] in terms of convective heat flux distribution along the boundary of the duct, by ensuring the effectiveness of the model also in the description of local behaviours. Although the present paper reports only preliminary results of this simulation/validation process, it could be of interest for the research community because it proposes a novel approach that could be useful to validate many numerical models for nonstandard geometries.

International Journal of Heat and Mass Transfer, 2015

Abstract The techniques for solving the Inverse Heat Conduction Problem represent useful tools fo... more Abstract The techniques for solving the Inverse Heat Conduction Problem represent useful tools for designing heat transfer apparatuses. One of their most challenging applications derives from the necessity of catching what happens inside a heat transfer apparatus by monitoring the temperature distribution on the external wall of the device, possibly by means of contactless experimental methodologies. The research presented here deals with the application of a solution strategy of the Inverse Heat Conduction Problem (IHCP) aimed at estimating the local heat transfer coefficient on the internal wall surface of a pipe, under a forced convection problem. The solution strategy, formulated for a 2D model, is based on the Quadrupole Method (QM) coupled to the Truncated Singular Value Decomposition approach, used to cope with the ill-conditioning of the problem. QM presents some advantages over the more classical domain or boundary discretization methods as for instance the fact that, being meshless, brings to a reduction of the computational cost. The analytical model, built under the QM, is validated by means of numerical simulations and the numerical outputs are then used as synthetic data inputs to solve the IHCP. The estimation methodology is also applied to experimental data regarding a forced convection problem in coiled pipes. Moreover, the adopted solution technique is compared to other two well-known and consolidated approaches: Finite Element Method coupled to the Tikhonov Regularization Method and Gaussian Filtering Technique. The comparison highlights that, for the problem here investigated, the Quadrupole Method coupled to the Truncated Singular Value Decomposition and Finite Element Method coupled to the Tikhonov Regularization Method perform better than the Gaussian Filtering Technique when the noise level is low, while, for higher noise level values, their efficiency is almost comparable, as it happens in the considered experimental study case.

Energy & Environment, 2014

The enhancement of the energy efficiency of existing buildings represents an important task, spec... more The enhancement of the energy efficiency of existing buildings represents an important task, specifically addressed by the European Directives. Among the currently available technologies to achieve this issue, the combined generation of heat and power (CHP) is found. Its higher efficiency with respect to the separate production of heat and power mainly depends on the occurrence of a profitable use of the cogenerated heat. This technology is now rapidly spreading because of its benefit from both the energy and the environmental sustainability points of view. However, in order to properly size the CHP system capacity and operation strategy, some specific design rules have to be identified. In particular, the CHP systems optimal design requires the building energy loads to be known on a hourly time scale. In the present paper a simplified procedure for estimating the hourly energy load for space heating and cooling of existing buildings is applied to the assessment of building cooling,...

International Journal of Heat and Mass Transfer, 2002

... This goal can be obtained by using different methods: spiral fins applied to the heat transfe... more ... This goal can be obtained by using different methods: spiral fins applied to the heat transfer surface, metallic twisted tape inserted into the tube, spirally roughened walls. ... [2] have experimentally investigated the forced convection of oils in integral straight and spiral fin tubes. ...

Heat Transfer Engineering, 2012

Scraped Surface Heat Exchangers (SSHEs) provide a versatile solution in the process industry for ... more 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 generalise the few data