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Heat exchangers consisting of bundles of horizontal plain tubes with boiling on the shell side are widely used in industrial and energy systems applications. A recent particular specific interest for the use of this special heat exchanger is in connection with Concentrated Solar Power (CSP) applications. Heat transfer and pressure drop prediction methods are an important tool for design and modelling of diabatic, two-phase, shell-side flow over a horizontal plain tubes bundle for a vertical up-flow evaporator. With the objective of developing a model for a specific type of cross flow evaporator for a coil type steam generator specifically designed for solar applications, this paper analyzes the use of several heat transfer, void fraction and pressure drop correlations for the modelling the operation of such a type of steam generator. The paper after a brief review of the literature about the available correlations for the definition of two-phase flow heat transfer, void fraction and pressure drop in connection with the operation of steam generators, focuses attention on a comparison of the results obtained using several different models resulting by different combination of correlations. The influence on the analysis of the performance of the evaporator, their impact on significant design variables and the effective lifetime of critical components in different operating conditions, simulating the daily start-up procedures of the steam generator is evaluated. The importance of a good calibration of the model based on the comparison with some experimental data is recognised. [ABSTRACT FROM AUTHOR]

The paper discusses about thermal management of PEM fuel cells. The objective is to define criteria and guidelines for the design of the air flow cooling system of fuel cells stacks for different combination of power density, bipolar plates material, air flow rate, operating temperature It is shown that the optimization of the geometry of the channel permits interesting margins for maintaining the use of separate air flow cooling systems for high power density PEM fuel cells. [ABSTRACT FROM AUTHOR]

Abstract: A methodological approach for analytical modelling of deep penetration laser beam welding (LBW) of stainless steels and its experimental verification is provided. After an analysis of the problem in general terms and a review of the modelling activity, a particular double source model is proposed and discussed. The model allows the derivation of penetration and width of melting zone caused by moving laser beam. Dependences of penetration length, width of the melting zone and aspect ratio of the zone were derived as a function of welding speed and laser power. The theoretical results obtained using the particular model are discussed and analyzed in comparison with experimental data obtained on a typical test case. Optimal conditions for obtaining a preliminary optimization of the process parameters were derived based on experimental results. The case study in the present paper, referred to the assembly of fuel injectors for automotive industry, demonstrates that when laser welding is performed at high speeds on thin wall components the energy released by the laser per unit of surface (energy density, ED) can be used to describe the heat transfer to the material and to shorten the experimental phase avoiding the dependencies on each single process parameter. [Copyright &y& Elsevier]

Highlights: [•] Heat and mass transfer performances of small size Closed Loop Two Phase Thermosyphons. [•] Evaluation of the systems in connection with renewable energy systems applications. [•] Experimental analysis on a new test rig using water and ethanol as working fluid. [•] Analysis of the influence of heat load, operating pressure and filling ratio. [•] Heat transfer analysis in evaporator and condenser. [Copyright &y& Elsevier]

Abstract: The optimization of the heat recovery steam generator (HRSG) is one of the key elements for increasing the efficiency of combined plants. According to the current technical practice, it can be organized at different levels of complexity with objectives sequentially defined: operating parameters, geometrical details and technological elements. According to this point of view, in the paper a complete strategy for the optimum design of the HRSG is outlined. The optimization is organized at two levels: the first one enables to obtain the main operating parameters of the HRSG, while the second involves the detailed design of the component concerning the geometric variables of the heat transfer sections. The output of the first-level optimization is the input of the second level. In particular, the second level of the optimization can be articulated in two different steps. The first step can be aimed to the minimization of the pressure drop for a given heat flow. The second step leads to a reduction of the overall dimensions, maintaining the imposed performance of the HRSG in terms of heat flow and pressure drop. The whole procedure is tested with reference to a case of existing HRSG structures; it shows the possibility of improving performance maintaining a constrained packaged size. [Copyright &y& Elsevier]

The optimum design of single longitudinal fins with constant thickness, considering different uniform heat transfer coefficients on the fin faces and on the tip, has been approached by means of an accurate mathematical method yielding the solution of constrained minimization (maximization) problems. Starting from the classical one-dimensional (1-D) model of the fin, the optimum design problem is reduced to a constrained optimization one by considering the limitations of the fin thermal convenience criterion, of the 1-D accuracy criterion, or of the geometric constraint on the primary surface of the fin array. The analysis, developed in dimensionless form, shows that the existence and the uniqueness of the solution are not ensured in any case, and the condition of the solution existence is often a consequence of the imposed constraints. A comparison between the results obtained and those achieved by applying to the fin optimization the half-thickness rule (HTR), based on the Harper and Brown approximation, has been carried out for some meaningful cases. Even if the HTR is usually satisfactory to design optimum fins, its uncritical use is very questionable. [ABSTRACT FROM AUTHOR]

The paper analyzes the problem of defining the potential of geothermal reservoirs and the definition of a sustainable size of a geothermal power plant in the preliminary design phase. While defining the size of a geothermal plant, the objective is to find a compromise between renewability, technical sustainability, and economic return-related issues. In the first part of the paper the simplified lumped parameter approach based on the First-Order methods and their further evolutions and limitations is proposed. Experimental data available for some geothermal reservoirs are used for critical analysis of the simplified approaches for estimating the renewability and sustainability of the production of geothermal plants. In the second part the authors analyze methods based on theoretical heat transfer analysis supported by experimental data acquired from the geothermal field (thermal properties of the rock, porosity of the reservoir, and natural heat flux) and finally consider the numerical simulation as a method to connect the two approaches discussed before. The sustainability of geothermal power production can be estimated taking into account the energy stored in the reservoir and the thermal and fluid dynamic analysis of the reservoir. From this perspective, the numerical simulation of the reservoir can be considered as an effective method for the estimation of a sustainable mass flow rate extraction. Some specific cases are analyzed and discussed. [ABSTRACT FROM AUTHOR]

Two-phase closed thermosyphons (TPCTs) are excellent thermal transfer devices that their integration into heat exchangers has been shown a strong potential for energy savings. The scope of this study is an experimental evaluation of the evaporation and condensation heat transfer of a TPCT for uniformly heated evaporator surface. Water as the working fluid is charged in a TPCT with a length of 500 mm and an inner diameter of 33 mm at different filling ratios (8–100%). The Thermosyphon heat transfer performances are compared with some predictive correlations of suppress the pool boiling insert uniquely and suppress the pool boiling combined with film evaporation for the evaporator section at different filling ratios. The experimentally obtained condensation heat transfer is also evaluated by available filmwise condensation model. Results show an agreement with the most of the selected correlations with tolerance ±30% and the appropriate set of correlations are introduced giving an accuracy within ±10%. [ABSTRACT FROM AUTHOR]