Gianpaolo Ruocco

Simultaneous visualization of flow field and evaluation of local heat transfer by transitional impinging jets

Abstract A combined approach has been employed to characterize the flow field and local heat transfer in jet impingement configurations, featuring a mass transfer experiment and a digital visualization technique. A jet velocity range is spanned to ensure flow regime transition. The well-known heat/mass transfer analogy has been used to infer on the local heat exchange on a infinite plate. In this experiment, a naphthalene film is ablated from a disk, due to jet exposure. Automated contact measurements of the variation of film depth in the stagnation region and beyond have been performed. From the local naphthalene loss rate the local heat transfer is then inferred. Coherent structures are created both at the interface between free jet and quiescent medium and upon impingement at plate, and need to be visualized in the vicinity of stagnation. To this end a particle image velocimetry system is exploited to extract the two components velocity instantaneous information. Ablation measurements confirm the non-monotonic progress of local heat transfer for small nozzle-to-plate spacings. The visualizations evidence that local heat transfer is strongly influenced by impingement structures: the maximum heat transfer coefficient offset which can be detected is due, even for laminar or transitional jet, to large-scale toroidal vortices impacting on the plate.

Entropy generation in conjugate heat transfer from a discretely heated plate to an impinging confined jet

Abstract In this paper the conjugate heat transfer from a finite thickness plate to a laminar confined, impinging planar jet is discussed in order to determine the solid-fluid coupling characteristics which minimize the rate of entropy generation. Velocity, pressure and temperature distributions are numerically determined using a semi-implicit finite-volume method. Velocity and temperature fields are further employed to simultaneously predict in both phases the distribution of local entropy generation. Understanding entropy generation mechanisms allows one to preliminary assess the conjugate arrangement based on solid-fluid coupling criteria. Such an approach can be used to optimize indirect cooling schemes in electronic thermal management.

Heat exchange in a multi-cavity volumetric solar receiver

The concept of a multi-cavity volumetric solar receiver is very attractive for the profitability of certain of its characteristics such as high efficiency and economy. The absorber is based on a pack of small ceramic cavities which intercept and absorb the inherent high solar flux reflected from an array of mirrors. Atmospheric air acts as a coolant medium when it is drawn through the pack. A model for an overall heat transfer performance of the receiver is given and numerically solved.

Thermal behaviour of a multi-cavity volumetric solar receiver: Design and tests results

Abstract After some years of basic studies and design, a prototype of a multi-cavity external flow air (or volumetric) receiver was tested at the Plataforma Solar de Almeria test facility. The absorber consists of a pack of SiSiC 9.0 × 4.8 mm (net) channels with a length of 150 mm with vertical and horizontal walls 5.0 and 1.6 mm thick, respectively. Experiments lasted for 30 days in the summer of 1990 and outlet air temperatures, power transferred to air, and efficiency were recorded as functions of solar incident flux and air flowrate. A good accordance between measured and predicted results was noticed, within the limits of accuracy of the data acquisition system of the facility and of the experimental system. Due to its compactness and despite its low mass, the structural behavior of the receiver was also satisfactory; the most important result was to confirm the validity of the concept and of the physical analysis on which the design is based.

Thermal storage in aquifers and energy recovery for space heating and cooling

Abstract An improvement of the energy efficiency ratio of air conditioning systems results when water-to-water dual-mode units are coupled to an aquifer of suitable characteristics which functions as a seasonal thermal storage. The evaluation of the system efficiency from a thermodynamic study is needed, to assess the interactions between the units and the aquifer. A comprehensive thermodynamic analysis is carried out in this paper. A procedure for a numerical evaluation of the system performance and optimization is presented in a convenient form for development and application of the system. A simplified method is also given for preliminary design purposes.

Flow Field Analysis in a Ventilated Large Enclosure Subject to Non-Homogeneous Boundary Conditions

Non-homogeneous thermal boundary conditions may play a relevant role in flow fields within ventilated enclosures. One such large-scale (23 m 3 ) test enclosure has been set in operation by termination jets under a combination of thermal unbalances in order to describe the dependence of secondary flow patterns on the bulk airflow. An integrated approach has therefore been attempted that features both measurements and calculations. An extensive and painstaking set of flow field measurements has been performed first, relative to different thermal and fluid dynamic working conditions, by means of a custom Laser Doppler Velocimetry apparatus. A commercial CFD software was then employed to perform the related calculations. A transitional regime is present (Re , 2000, based on grille diffuser equivalent diameter) at the given forcing inlet flow, as well as the competition between natural and forced convection. Some qualitative comparisons with respective experimental data are presented to delineate the role of thermal disturbances.

Final design of a multi cavity volumetric solar receiver

Abstract In order to design a multi cavity volumetric solar receiver, a model has been set up which allows the determination of both the transversely averaged surface and air temperature, as functions of the axial distance from the opening of a typical cavity. A procedure based on a ray-tracing technique which takes into account the geometry of the channel, as well as the directional characteristic of the irradiation resulting from the specular nature of the reflection from the mirror field has been used to calculate the distribution of solar radiation in the channel. Based on such analysis, the final design of the receiver has been completed and a prototype has been constructed and testing is in progress at an appropriate demonstration plant.

A performance model of bubbling fluidized bed hydrodynamics

Abstract A simple mathematical model of the fluid dynamics of a bubbling fluidized bed is reviewed and demonstrated. The goal of the model is to be one part of a computationally efficient performance model of a fluidized bed combustor. The intended application is dynamic, real time modeling of system performance. A one-dimensional model which is based on two-phase theory and bubble coalescence is employed. The model predicts important bubble parameters including size, velocity, and void distribution. A numerical solution technique is derived which is based upon an iteration for solid mass conservation in the bed. Validation calculations which compare model predictions to available cold reactor data are presented. Quantitative agreement with data is shown to be strongly dependent on the correlation of vertical bubble coalescence dynamics. Overall, a useful simulation methodology is produced.

Mass Transfer by Diffusion and Convection

Transfer of mass, in the sense of chemical or biological species, is the third and last physical mechanism we encounter: with the analysis of diffusion and mass convection a preliminary outlook on transfer phenomena is completed. Strong similarities exist between heat and species transport: we will use our acquired knowledge to describe the species transport in stationary media and a fluid stream that acts as a species carrier between two media in relative motion. After a brief reference to the basic physical mechanism, as we recognize that mass of a given substance is driven by a concentration difference, we start by exploiting first the macroscopic balance for diffusion in the simplest case of binary systems. So far our attention was drawn to the description of temperature and flow fields: now, we derive and integrate governing differential equations as usual in various cases, following the microscopic balance, but as one substance moves relative to another, we arrive to the distribution of a mass concentration scalar. Along the same line that we exploit so far, with few distinctions only, we will find that many concepts have been already laid down, leading to the numerical solution, so we will be ready to move to the last chapter to scrutinize some PCB phenomena of interest.

Transport Phenomena and Multiphysics Modeling

The motivation and principles of the book are outlined. The role of transport phenomena is presented, in the framework of actual and virtual experiments, with the focus on process engineering. The heat transfer, as an example of multiphysics transport, is briefly introduced with its different modes.