Stefano Piva

Experimental and numerical investigation of the steady periodic solid–liquid phase-change heat transfer

A numerical and experimental investigation is presented of a periodic phase-change process dominated by heat conduction. In the experimental arrangement a plane slab of PCM is periodically heated from above. A one-dimensional control volume computer code has been developed for the solution of the corresponding mathematical model. The comparison between numerical predictions and experimental data shows good agreement, even though appreciable effects are produced by free convection and heat transfer to the environment, neglected in the model but unavoidable in the experiment. Finally, in order to study the energy stored in the process, parameters like amplitude and mean value of the oscillations are discussed as functions of the significant dimensionless numbers of the problem.

Summer Thermal Performance of Ventilated Roofs with Tiled Coverings

The thermal performance of a ventilated pitched roof with tiled coverings is analysed and compared with unventilated roofs. The analysis is carried out by means of a finite element numerical code, by solving both the fluid and thermal problems in steady-state. A whole one-floor building with a pitched roof is schematized as a 2D computational domain including the air-permeability of tiled covering. Realistic data sets for wind, temperature and solar radiation are used to simulate summer conditions at different times of the day. The results demonstrate that the batten space in pitched roofs is an effective solution for reducing the solar heat gain in summer and thus for achieving better indoor comfort conditions. The efficiency of the ventilation is strictly linked to the external wind conditions and to buoyancy forces occurring due to the heating of the tiles.

A Further Contribution to the Parametric Analysis of a PCM Energy Storage System

In recent years heat storage using phase change materials has been also considered in the thermal control of electronic devices. In a recent work we presented some results for a parametric analysis of an energy storage system with a phase change material undergoing a two-levels steady-periodic heat boundary condition, as happens in certain electronic equipments. In particular, a hybrid system composed of a finned surface partially filled with a PCM, was analysed. This solution, which combines both passive (PCM) and active (fins and fans) cooling solutions, is of interest in high power amplifiers characterized by different levels of power dissipation, as is the case of the telecom base station power amplifiers, where the power is proportional to the traffic load. In the present paper we analyze some parameter previously not investigated, in particular the dimensionless transition temperature, for the role played by this parameter in the limitation of the operating temperature reached during the peaks of the power input. The study has provided further useful information for the design of these hybrid cooling systems.

Transient Heat Conduction in a Wall Exposed to a Fire: an Analytic Approach

Fire safety engineering requires a detailed understanding of fire behaviour and of its effects on structures and people. Currently, advanced numerical codes for the prediction of the fire behaviour are available. However, they often require heavy calculations and long times. In this context analytical solutions can be useful for a fast analysis of simplified schematizations. It allows a more effective final utilization of the advanced fire codes. In this contribution, for a separation wall exposed to a fire the temperature history is analysed of the unexposed wall surface. Due to the limitations of the model, the temperature in the fire room changes stepwise, with a final value typical of a post-flashover condition. Nevertheless, with an appropriate choice of the heat transfer coefficient, the thermal action on the surface exposed to fire becomes that due to a fire following the standard temperature-time curve. The solution is then obtained by applying the separation of variables to the heat conduction equation. The problem is made dimensionless and the results are analysed in order to validate their significance. This simplified model allows to obtain useful information on the magnitude of the temperature reached.

Peltier cells cooling system for switch mode power supply

The results are presented of an experimental investigation in a liquid cooled Switch-Mode Power Supply (SMPS). The target is a quantitative analysis of the performance of a cooling system designed to dissipate the heat generated by the active and passive electronic components of this SMPS, in order to limit its maximum operational temperature. The active components are cooled with a liquid cold-plate. The passive components are cooled with an air flow. The temperature of this airflow is controlled with Peltier cells coupled to the cold-late. Measurements are made of temperature and of electric efficiency of the SMPS. The cooling system is placed in an experimental tool where it is possible to measure and control the cooling liquid flow. A detailed analysis of the thermal behaviour of this cooling system is given. Finally, the practical significance of the problem is discussed.

Numerical Analysis of Laminar Flow and Heat Transfer for an Abrupt Circular Contraction

Detailed predictions have been made of laminar thermo-fluid entry effects for the important practical case of an abrupt circular contraction. The Computational Fluid Dynamics code CFDS-FLOW3D (now CFX) was used, and special care was taken to achieve accurate data, by attention to issues of algorithm choice and grid fineness. Assuming constant properties, detailed local heat transfer predictions have been obtained, for a range of Prandtl and Reynolds numbers and contraction ratio β. The near-entry heat transfer is consistent with the flow behaviour, and gives a clear understanding of the process involved. Qualitatively the results are comparable with experimental and predictive data available in the literature, where however substantial effects due to property changes with temperature were evidenced. This study confirms the presence of flow separation and recirculation found and inferred by other authors, and investigates the phenomena in considerably more detail.Copyright