Prefabricated, lightweight building elements are widely used in the building construction sector. Such elements consist of fibrous thermal insulation encapsulated between two metal sheets. Under various circumstances, moisture can appear in the insulation matrix. Since the temperature of the boundarz metal sheets changes dynamically with meteorological conditions, heat andy mass transfer between boundaries appear in this case. This paper presents a transient model of the heat and mass transfer, including the sorption and condensation processes. A numerical model considers the dynamical changing of the boundary temperatures. A parametric study considering different amplitudes of temperature change, different moisture masses and different thicknesses of the insulation matrix was made. It was found that a relatively small mass of water in the insulation matrix can result in a significantly increased average heat flux during a periodic cycle. The numerical code was verified with experiments, which showed good agreement with the numerics.
COBISS.SI-ID: 11903003
One of the technologies which help to reduce energy consumption is the thermal energy storage for cooling applications where the cold is stored in phase change materials (PCMs). Such materials would be suitable for use in buildings because they can store a large amount of cold and phase change occurs at a constant temperature, thereby increasing thermal comfort. The aim of the study was to investigate how and where PCMs are used in the cooling systems, how are these systems related to buildings, if they provide lower energy consumption, how the indoor temperatures change due to PCMs and if the indoor air conditions improve. In this article are firstly presented materials that are suitable for such applications and desirable properties for use in such applications. A review of cooling systems follows, which are divided into four groups, namely: free cooling applications, encapsulated PCM systems, air-conditioning (AC) systems and sorption cooling systems, both with integrated PCMs. All studies have shown that the use of PCMs helps to improve energy performance of buildings, the problems were encountered in heat transfer and the amount of PCM needed for storage. These topics are also worthy of further research.
COBISS.SI-ID: 12294683
A new, fast and flexible, time-dependent, one-dimensional numerical model was developed in order to study in detail the operation of an active magnetic regenerator (AMR). The model is based on a coupled system of equations (for the magnetocaloric material and the heat-transfer fluid) that have been solved simultaneously with the software package MATLAB. The model can be employed to analyze a wide range of different operating conditions (mass-flow rate, operating frequency, magnetic field change), different AMR geometries, different magnetocaloric materials and heat-transfer fluids, layered and single-bed AMRs, etc. This paper also presents an optimization of the AMRA news geometry, where the AMR consists of a packed-bed of grains (spheres) of gadolinium (Gd). The optimization of the mass-flow rate and the operating frequency of the AMR were performed by studying five different diameters of Gd spheres.
COBISS.SI-ID: 11935003
Experiments were performed to study the spatio-temporal temperature variation underneath growing bubbles on a thin platinum heating foil in saturated and subcooled nucleate pool boiling of water at atmospheric pressure. The transient wall temperature distributions were recorded with spatial resolutionof 40 m by a high-speed infrared camera at intervals of 1 ms, synchronised with a high-speed video camera to record bubble motion. Examples are presented of the transient distributions of wall temperature, heat flux and heat transfer coefficient underneath bubbles growing with the fast and slow bubble detachment mechanisms in saturated and subcooled pool boiling. Comments are made on the evidence for and against particular mechanisms of heat transfer.
COBISS.SI-ID: 10062363
The plunge test method and the self-heating test method represent two experimental techniques for identifying the dynamic properties of temperature sensors. The dynamic behaviour of a resistance temperature sensor can be described using transfer functions, which differ for the two test methods. It is possible to predict the sensor's dynamic properties for the plunge test with a proper transformation of the identified model for the self-heating test. The main contribution of the presented research work is the software, based on virtual instrumentation, developed to identify and predict the dynamic properties of resistance temperature sensors. The excitation signal and the sensor's response are utilized to identify its transfer function. The number of parameters for the approximation model is determined as a result of an optimization problem. The software was validated and then applied to identify and predict the dynamic properties of a commercial-grade Pt100 sensor. In this case study, the plunge test and the self-heating test were performed with a step change of the surrounding temperature and the supplied electrical power, respectively, under laboratory conditions. The relative difference between the predicted and the identified sensor's time constants for the plunge test equals -7.4%, which is within the acceptance interval of +/-10%. The tested resistance temperature sensor was therefore experimentally validated as being suitable for dynamic testing using the self-heating method.
COBISS.SI-ID: 12775451