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International audience; Fouling buildup in circular heat exchanger pipes is a common problem in the industry. Various methods have been proposed to increase the shear stress of the fluid near the pipe wall. This is known to reduce fouling rate, as well as increasing the heat transfer rate of heat exchangers. In this article, the effect of fouling-rate reduction is investigated by increasing friction in a circular pipe. This is done by inducing swirling flow at the pipe entrance, which in turn increases velocities close to the wall and consequently the shear stress. Results are obtained from a three-dimensional finite-volume computational fluid dynamics (CFD) code, where the pipe is modeled along with a swirling device at the entrance. Flow conditions are set to the laminar regime, where the effects of swirling flow are much more influential than in turbulent flow. It is concluded that considerable increase in friction can be obtained, but at the cost of increase in pressure drop.

The objective of this work is the development of an automatic solar water heater (SWH) fault diagnostic system (FDS). The latter consists of a modelling module and a diagnosis module. A data acquisition system measures the temperatures at four locations of the SWH system (outlet of the water tank; inlet of the collector array; outlet of the collector array; inlet of the water tank). In the modelling module a number of artificial neural networks (ANN) are used, trained with the very first values when the system is fault free. Then, the neural networks are able to predict the fault-free temperatures and compare them to actual values. When the differences are low, the corresponding networks are unchanged. On the contrary the networks are retrained. Then the diagnosis module analyses the difference between the current connection weights and the initial weights. When a persistent significant modification occurs, a flag is set to signify that a default is present in the SWH. The system can predict three types of faults: collector faults and faults in insulation of the pipes connecting the collector with the storage tank (to and from the tank) and these are indicated with suitable labels. It is shown that all faults can be detected well before the end of the drifts, without any false alarm, when the networks and thresholds are well tuned and that the observation window has the right size. It is shown that this does not depend on the draw off profile.

The objective of this work is to present the development of an automatic solar water heater (SWH) fault diagnosis system (FDS). The FDS system consists of a prediction module, a residual calculator and the diagnosis module. A data acquisition system measures the temperatures at four locations of the SWH system. In the prediction module an artificial neural network (ANN) is used, trained with values obtained from a TRNSYS model of a fault-free system operated with the typical meteorological year (TMY) files of Nicosia, Cyprus and Paris, France. Thus, the neural network is able to predict the fault-free temperatures under different environmental conditions. The input data to the ANN are the time of the year, various weather parameters and one input temperature. The residual calculator receives both the current measurement data from the data acquisition system and the fault-free predictions from the prediction module. The system can predict three types of faults; collector faults and faults in insulation of the pipes connecting the collector with the storage tank and these are indicated with suitable labels. The system was validated by using input values representing various faults of the system.

This report deals with the study of an electrical heater for corrosive fluids that is built from a crossflow graphite beat exchanger. The first tests have shown a maldistribution of the temperature in the heater core and in the fluid in the outlet header. We have consequently studied the heating elements, the heater core and the fluid distribution. As we cannot know experimentally the thermal characteristics of the heating elements that have been used, we have found them by using a simulation based on well known heating elements. It results from the study of the heater core that the ratio of the highest speed to the smallest speed is a fundamental parameter and must stay under an upper limit to keep the heater in good working order. We have determined the actual value of this ratio experimentally and by using a software. As we have found this value up the upper limit, we have looked for a way to homogenize the fluid flow in the core of the heater. We have experimentally reached good results by placing a porous baffle in the plenum chamber of the inlet header. We have suggested a law for the fluid distribution that helps, with simple hypothesis, to determine the porosity of the baffle that leads to an optimal use of the heater. We have also shown that a software can also correctly predict the improvement due to the porous baffle. We have modified our prototype and carried on new tests with success. The results that have been obtained could be extrapolated to «normal fluids» heater built around a core that could be in a less expensive material.; Cette étude concerne un réchauffeur électrique de fluides corrosifs conçu à partir d'un échangeur de chaleur en graphite, à courants croisés. Les premiers essais ont montré une mauvaise répartition de la température du bloc d'échange et du fluide en sortie d'appareil. Nous avons donc étudié les éléments chauffants électriques, le bloc d'échange et la répartition du fluide dans celui-ci. Les caractéristiques des éléments chauffants utilisés ne pouvant pas être obtenues expérimentalement, nous les avons déterminées en utilisant une modélisation que nous avons validée sur d'autres éléments dont les résultats expérimentaux étaient connus. L'étude du bloc d'échange a révélé l'existence d'une limite supérieure du rapport des vitesses maximale et minimale dans l'appareil. Nous avons recherché la valeur de ce rapport en étudiant expérimentalement et par modélisation la répartition du débit. La valeur trouvée étant trop élevée nous avons cherché à homogénéiser l'écoulement du fluide dans l'appareil. Nous avons obtenu expérimentalement de bons résultats en plaçant un obstacle dans la boite à eau. Nous avons proposé une loi représentative de la répartition du fluide qui, sur la base d'hypothèses simplificatrices, nous permet de déterminer l'obstacle qui permet une utilisation optimale de l'appareil. Nous avons montré que l'utilisation d'un logiciel permet également de bonnes prévisions. Nous avons modifié notre prototype et nous avons repris nos essais avec succès. Les résultats obtenus pourraient être extrapolés à des réchauffeurs de fluides ordinaires dont le bloc d'échange serait en un matériau moins noble et donc moins couteux.

This report deals with the study of an electrical heater for corrosive fluids that is built from a crossflow graphite beat exchanger. The first tests have shown a maldistribution of the temperature in the heater core and in the fluid in the outlet header. We have consequently studied the heating elements, the heater core and the fluid distribution. As we cannot know experimentally the thermal characteristics of the heating elements that have been used, we have found them by using a simulation based on well known heating elements. It results from the study of the heater core that the ratio of the highest speed to the smallest speed is a fundamental parameter and must stay under an upper limit to keep the heater in good working order. We have determined the actual value of this ratio experimentally and by using a software. As we have found this value up the upper limit, we have looked for a way to homogenize the fluid flow in the core of the heater. We have experimentally reached good results by placing a porous baffle in the plenum chamber of the inlet header. We have suggested a law for the fluid distribution that helps, with simple hypothesis, to determine the porosity of the baffle that leads to an optimal use of the heater. We have also shown that a software can also correctly predict the improvement due to the porous baffle. We have modified our prototype and carried on new tests with success. The results that have been obtained could be extrapolated to «normal fluids» heater built around a core that could be in a less expensive material., Cette étude concerne un réchauffeur électrique de fluides corrosifs conçu à partir d'un échangeur de chaleur en graphite, à courants croisés. Les premiers essais ont montré une mauvaise répartition de la température du bloc d'échange et du fluide en sortie d'appareil. Nous avons donc étudié les éléments chauffants électriques, le bloc d'échange et la répartition du fluide dans celui-ci. Les caractéristiques des éléments chauffants utilisés ne pouvant pas être obtenues expérimentalement, nous les avons déterminées en utilisant une modélisation que nous avons validée sur d'autres éléments dont les résultats expérimentaux étaient connus. L'étude du bloc d'échange a révélé l'existence d'une limite supérieure du rapport des vitesses maximale et minimale dans l'appareil. Nous avons recherché la valeur de ce rapport en étudiant expérimentalement et par modélisation la répartition du débit. La valeur trouvée étant trop élevée nous avons cherché à homogénéiser l'écoulement du fluide dans l'appareil. Nous avons obtenu expérimentalement de bons résultats en plaçant un obstacle dans la boite à eau. Nous avons proposé une loi représentative de la répartition du fluide qui, sur la base d'hypothèses simplificatrices, nous permet de déterminer l'obstacle qui permet une utilisation optimale de l'appareil. Nous avons montré que l'utilisation d'un logiciel permet également de bonnes prévisions. Nous avons modifié notre prototype et nous avons repris nos essais avec succès. Les résultats obtenus pourraient être extrapolés à des réchauffeurs de fluides ordinaires dont le bloc d'échange serait en un matériau moins noble et donc moins couteux.