Federico Scarpa

The Role of Adsorption and Phase Change Phenomena in the Thermophysical Characterization of Moist Porous Materials

Phase change phenomena in moist porous media with low liquid content, the typical condition of a porous body at ambient conditions and far from the contact of liquid water, are controlled by the shape of the adsorption isotherms and by the effective liquid-vapor thermodynamic condition within the pores. Usually, heat and mass transfer models are developed under the assumption of thermal and hygrometric equilibrium. This gives rise to an expression of the evaporation source that is too complex in view of the dynamic identification of thermophysical and transport properties of a porous material. In this study, the hypothesis of hygrometric equilibrium is dropped. The phase change rate is considered proportional to the amount of local nonequilibrium through an appropriate delay coefficient. This approach leads to a simple representation of the process and makes manageable the formulation of a coupled heat and mass transfer inverse problem. A comparison with a first group of experiments performed with an open-pore light insulating material (expanded perlite board) confirms the suitability of the proposed approach. However, the analysis shows that, for this material, phase change occurs not far from the hygrometric equilibrium.

INFLUENCE OF SENSOR CALIBRATION UNCERTAINTY IN THE INVERSE HEAT CONDUCTION PROBLEM (IHCP)

In the inverse heat conduction problem (IHCP) of finding the temperature-dependent thermophysical properties of materials, the evaluation of the confidence region of the unknown functions is at least as important as the numeric values of the function itself. The uncertainty of the reconstructed properties arises from various disturbances affecting the measurement instrumentation and from an imprecise mathematical description of the experiment. The aim of this work is to consider, besides the effect of random noises superposed onto the signals, the bias arising from the calibration of the sensors. By conceiving the calibration process as an integral part of the experiment it is possible to quantify the effect of the above bias and to obtain better estimates and correct confidence regions for the reconstructed properties.

Ten Years of Parameter Estimation Applied to Dynamic Thermophysical Property Measurements

The parameter estimation theory is considered the best way to estimate thermophysical properties from dynamic experiments. This approach deals with measurement and model errors in a statistical context and provides useful information to optimize the experiment. The experience gained in ten years of implementation of inverse algorithms based on the parameter estimation theory (OLS, MAP, and Kalman filtering) is summarized and presented. Several examples of estimation of thermophysical properties using transient and pulse techniques are reported and discussed. The thermal conductivity, specific heat capacity, and total hemispherical emissivity of different materials (light insulators, Pyrex, and niobium) are presented and compared with data obtained with consolidated techniques and with literature data.

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.

Evaluation of reliable confidence bounds for sensor calibration curves

When calibrating a sensor, the evaluation of the uncertainty of the estimated function is at least as important as the numeric values of the function itself. The calibration process usually involves a least-squares fitting, performed, for example for a thermocouple, by comparing some voltage readings with corresponding temperatures from a reference thermometer. The aim of this work is to encourage the use of improved algorithms, based on an extended minimization criterion, which can overcome some limitations of the classic method.

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

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.