The availability of reliable models for the prediction of the daily thermal behaviour of thermal solar collectors is of great relevance for a correct design of solar thermal systems. The most significant design parameters of a solar collector are outlet temperature and useful energy collection, both calculated on the base of the efficiency. The last, in turn, is commonly available on the base of experimental correlations obtained with the Steady State Testing method proposed by EN 12975-2. In these steady state tests, through the respect of very strict requirements, the contributions due to the thermal inertia of the solar collector becomes negligible. However, in its daily behaviour, it undergoes variable weather conditions with both regular and sharp changes of solar radiation. For these conditions the thermal inertia contributions become relevant in the correct prediction of its thermal behaviour. EN 12975-2 in Annex K suggests a modified equation for the prediction of the useful energy collection, based on a modification of the efficiency equation. A term dependent on the time derivative of the mean inlet/outlet water temperature is added. In this paper we examine experimentally the role played by this term for a Compound Parabolic Concentrator solar thermal collector.
A parametric analysis is presented for the performance of a thermoelectric generator based on its operating conditions. The mathematical model, consisting of non linear equations, is made dimensionless to allow the characteristic parameters to be evidenced. The proposed parameterization lends generality to the results obtained. In particular the relationships have been investigated between the temperature difference inside the thermoelectric cell and that on the outside of the generator, and the effect of the outer thermal resistances of the generator on the working conditions. These parameters have a significant effect on the efficiency and therefore on the optimization of the operating conditions of the thermoelectric generator.
Fire safety engineering requires a detailed understanding of fire behaviour and of its effects on structures and people. Many factors may condition the fire scenario, as for example, heat transfer between the flame and the boundary structures. Currently advanced numerical codes for the prediction of the fire behaviour are available. However, these solutions often require heavy calculations and long times. In this context analytical solutions can be useful for a fast analysis of simplified schematizations. After that, it is more effective the final utilization of the advanced fire codes. In this contribution, the temperature in a fire escape passage, separated with a thermally resistant wall from a fire room, is analysed. The escape space is included in a building where the neighbouring rooms are at a constant undisturbed temperature. The presence of the neighbouring rooms is considered with an equivalent heat transfer coefficient, in a boundary condition of the third type. An analytical solution is used to predict the temperature distribution during the fire. It allows to obtain useful information on the temperature reached in the escape area in contact with a burning room; it is useful also for a fast choice of the thermal characteristics of a firewall.
The results are presented of an experimental investigation of heat transfer in an air-liquid cooling system for a Switch-Mode Power Supply (SMPS) for TV digital power amplifiers. Since these SMPSs are characterized by high power and high compactness, thereby making the standard cooling techniques difficult to be used, a new cooling system is developed, using water and air as the cooling media. The active components (MOSFETs) are cooled with a liquid cold-plate, the passive ones (condensers, transformers, coils) with an air flow, in turn cooled by the cold-plate. By inserting the cooling system in an experimental tool where it is possible to control the cooling water, measurements are made of temperature in the significant points of the SMPS. The electric efficiency is also measured. The evaluation of the thermal performance of this cooling system is useful in order to limit its maximum operational temperature. The efficacy of the cooling system is demonstrated; the trends of efficiency and power dissipation are evidenced.
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.
An experimental study has been made of the fluid dynamics performance of electronic equipment designed to cool a heat sink in the form of a finned duct. The apparatus consists of a channel of rectangular section containing the finned duct. A forced airflow is driven by three fans placed in parallel in the inlet and in the outlet sections of the channel. In order to investigate a full range of flow rates, different sets of fans were used in the inlet section. Measurements were made of static pressures at different channel positions by pressure taps connected to a micro manometer, and of the flow rate by a Venturi meter. The experimental working conditions of the active fans were consistent with the manufacturers’ characteristic curves. Values of the local friction coefficient compared well with published correlations. The experimental working points agreed very well with the theoretical curve of the pressure drop and the hydraulic diameter was demonstrated to be of the appropriate size for this fluid dynamics problem.
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.
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