Luca A. Tagliafico

Analysis of energy demand in residential buildings for different climates by means of dynamic simulation

The aim of the present paper is to propose an analysis of energy consumption of a standard building in different climates. The analysis is developed by simulating the dynamic behaviour of the building subjected to different climatic conditions according to the considered location. Simulations are performed by means of an in-house developed code, validated by comparison with the outcomes from leading software, particularly TRNSYS and EnergyPlus. The use of a self-developed code guarantees a high flexibility and allows the implementation of new capabilities if necessary. The impact on the energy consumption of various parameters, namely internal and external wall insulation, window surface areas, thermal capacity and orientation, is investigated. Results show that the insulation of external walls has a fundamental role in reducing energy consumption, because it allows to exploit the thermal capacity of the walls. This is particularly useful for buildings which necessitate to keep the internal temperature constant.

Borehole modelling: a comparison between a steady-state model and a novel dynamic model in a real ON/OFF GSHP operation

The correct design and optimization of complex energy systems requires the ability to reproduce the dynamic thermal behavior of each system component. In ground source heat pump (GSHP) systems, modelling the borehole heat exchangers (BHE) dynamic response is especially relevant in the development of control strategies for energy optimization purposes. Over the last years, several models have been developed but most of them are based on steady- state approaches, which makes them unsuitable for short-term simulation purposes. In fact, in order to accurately predict the evolution of the fluid temperatures due to the ON/OFF cycles of the heat pump, it is essential to correctly characterize the dynamic response of BHE for very short time periods. The aim of the present paper is to compare the performance of an analytical steady-state model, available in TRNSYS environment (Type 557), with a novel short-term dynamic model. The new dynamic model is based on the thermal-network approach coupled with a vertical discretization of the borehole which takes into account both the advection due to the fluid circulating along the U-tube, and the heat transfer in the borehole and in the ground. These two approaches were compared against experimental data collected from a real GSHP system installed at the Universitat Politecnica de Valencia. The analysis was performed comparing the outlet temperature profiles predicted by both models during daily standard ON/OFF operating conditions, both in heating and cooling mode, and the between both approaches were highlighted. Finally, the obtained results have been discussed focusing on the potential impact that the differences found in the prediction of the temperature evolution could have in design and optimization of GSHP systems.

Experimental and theoretical results on upward annular flows in thermal non-equilibrium

Abstract Some thermal and flow phenomena occurring in two phase annular flow regime have been analyzed by means of a simulation procedure coupled to an experimental investigation. After a discussion about the closure relationships adopted and the validation of the numerical model against available experimental data, some information have been obtained concerning the thermal behavior of the two phases involved in the non-equilibrium annular flow. A characteristic entry length required to reach the thermal equilibrium between the phases has been defined and the typical values have been calculated as a function of the flow operating parameters (liquid and gas superficial velocities, inlet temperatures). Finally a simple relationship has been proposed to obtain a first evaluation of this equilibrium entry length in terms of superficial velocities and mean values of the physical properties of the fluids involved.

An experimental approach for the dynamic investigation on solar assisted direct expansion heat pumps

Federico Scarpaa, Andrea P. Reverberib, Luca A. Tagliaficoa, Bruno Fabiano*,c aUniversity of Genoa, DIME/TEC Thermal Energy and Environmental Conditioning Division, Via Opera Pia 15a, Genoa, Italy bUniversity of Genoa, DCCI Department of Chemistry and Industrial Chemistry, via Dodecaneso 31, Genoa, Italy cUniversity of Genoa, DICCA Department of Civil, Chemical and Environmental Engineering, Via Opera Pia 15, Genoa, Italy bruno.fabiano@unige.it

Non equilibrium gas-liquid flows in variable cross section ducts

The behaviour of two-phase high velocity flows in variable cross section ducts was investigated using a one-dimensional numerical model developed for the study of the annular flow configuration. Heat, mass, and momentum transfer between the phases during the flow were considered. The validation of the calculation procedure was made with some experimental data for the air-water couple, while the main application concerned the evaluation of momentum transfer from an expanding gas to an entrained liquid stream in droplet form. A liquid metal-gas flow was considered to simulate the process taking place in a plant where electrical power is generated by a liquid metal flowing in a magnetic field (MHD). The effectiveness of energy and momentum transfer between the liquid and the gas phase during the expansion was evaluated and the influence of nozzles with different convergence angles was investigated.ZusammenfassungDas Verhalten von Zweiphasen-Hochgeschwindigkeitsströmungen in Kanälen veränderlichen Querschnittes wurde mit Hilfe eines eindimensionalen numerischen Modells untersucht, das ursprünglich für die Analyse von Ringspaltströmungen entwickelt worden ist. Wärme-, Massen- und Impulsaustausch zwischen den Phasen während des Strömungsvorganges fanden Berücksichtigung. Die Validifizierung des Berechnungsverfahrens erfolgte mittels einiger experimenteller Daten für die Paarung Luft/Wasser, während die Hauptanwendung auf die Ermittlung der Impulsübertragung an einen in Tropfenform mitgeführten Flüssigkeitsstrom gerichtet war. Ferner wurde eine Flüssigmentall/Gas-Strömung betrachtet, um den in MHD-Anlagen ablaufenden Prozeß zu simulieren. Diese Untersuchung lieferte den Effektivitätsgrad der Energie- und Impulsübertragung zwischen Flüssig- und Gasphase während der Expansion, sowie den Einfluß verschiedener Erweiterungsverhältnisse der Düsen.

A simple model for the flow and temperature distribution analysis in a manifold-shaped direct cooling system

A fluid dynamic and thermal analysis has been carried out for the calculation of pressure, velocity and temperature distribution in a manifold-shaped cooling system of varying geometrical characteristics.A physical model of the general problem has been developed and solved by a numerical procedure. The results obtained are compared with some available experimental data on a box-shaped manifold, arranged with straight and inclined branches.The comparison has pointed out a good agreement between experimental and calculated data, allowing the method to be employed for the geometrical optimisation of manifold design.ZusammenfassungZur Berechnung von Druck, Geschwindigkeit und Temperaturverteilung in einem verzweigten Kühlsystem mit variierenden geometrischen Verhältnissen, wurde eine Analyse der Strömungsdynamik und der thermischen Verhältnisse durchgeführt.Es wurde ein physikalisches Modell für die zugrundeliegende Problematik entwickelt und durch ein numerisches Verfahren gelöst. Die erhaltenen Ergebnisse werden mit einigen verfügbaren, experimentellen Daten von einer kastenförmigen Verzweigungsstelle mit geraden und geneigten Abzweigungen verglichen.Der Vergleich zeigt eine gute Übereinstimmung zwischen experimentell ermittelten und errechneten Daten, was die Anwendung dieses Verfahrens zur Optimierung der Geometrie von Verzweigungsentwürfen erlaubt.

A Compact Dynamic Model for Vapor Compression Refrigerated Systems

In the paper a simplified dynamic lumped model for the simulation of a refrigerator working according to an inverse cycle between two thermal sources with finite thermal capacity is presented. A variable capacity compressor (VCC) is considered. The model is compact enough to be employed in actual regulation systems, but sufficient to describe all the underlying physical phenomena relevant to the transient response of the refrigerated cell. The dynamic behaviour of the system is simulated taking into account all the heat capacities involved in the heat transfer processes between the system, the refrigerating fluid and the outside. On the other hand the dynamic of the physical phenomena having time constants smaller than a few seconds has been neglected. The validity of this approach is proved by comparing the numerical results with the transient experimental data coming from an instrumented chest-freezer (one kind of small refrigerator often used in household and super-market applications).Copyright

Thermophysical Property Estimation by Transient Experiments: The Effect of a Biased Initial Temperature Distribution

The identification of thermophysical properties of materials in dynamic experiments can be conveniently performed by the inverse solution of the associated heat conduction problem (IHCP). The inverse technique demands the knowledge of the initial temperature distribution within the material. As only a limited number of temperature sensors (or no sensor at all) are arranged inside the test specimen, the knowledge of the initial temperature distribution is affected by some uncertainty. This uncertainty, together with other possible sources of bias in the experimental procedure, will propagate in the estimation process and the accuracy of the reconstructed thermophysical property values could deteriorate. In this work the effect on the estimated thermophysical properties due to errors in the initial temperature distribution is investigated along with a practical method to quantify this effect. Furthermore, a technique for compensating this kind of bias is proposed. The method consists in including the initial temperature distribution among the unknown functions to be estimated. In this way the effect of the initial bias is removed and the accuracy of the identified thermophysical property values is highly improved.

Transient Heat Transfer Processes in Batch Furnaces

The present paper describes a numerical model, based on a finite-difference technique, to simulate 2D thermal transients of solid and hollow cylinders in convective and radiant rectangular cavities, with participating media and time-dependent temperature-of-internal-gases. The model takes into account all the thermo-physical property variations with temperature of both gases and materials under treatment. These last are subject to strong non-linear convective-radiative thermal boundary conditions and undergo the needed solid-solid thermodynamic transitions. The study of the whole system includes the problem of conduction inside the multilayer furnace walls, solved by a 1D transient numerical model. The proposed model has been validated with respect to a numerical solution as well as to available full-scale experimental data. In particular a simplified radiation case is carried out comparing the solution obtained by the model to that given by a finite element commercial code (ANSYS® ). The calculated temperature curves have been compared with experimental measurements too, with reference to few particular points in a cylinder inside a batch furnace, during an actual heat treatment. The 2D finite difference model results to be an efficient tool for predicting the system thermal performance, well suitable for design and operating conditions optimization.Copyright