Relaciones ε–NTU para intercambiadores de placas con múltiples pasos y número reducido de placas

Los intercambiadores de placas presentan varias ventajas: alta compacidad, bajo ensuciamiento, altos coeficientes de transferencia de calor y alta flexibilidad. Dentro de esta última ventaja, destaca su capacidad para poder adaptar flujos con grandes diferencias de capacidades caloríficas y/o pérdidas de carga admisibles mediante configuraciones donde uno de los fluidos (el de mayor caudal o calor específico y/o menor pérdida de carga) debe realizar múltiples pasos en el intercambiador para acomodar la pérdida de carga admisible o para aumentar el coeficiente global de transferencia de calor. Cuando uno de los fluidos realiza varios pasos, el intercambiador se puede analizar como una serie de intercambiadores de flujo en contracorriente y de flujo paralelo alternos. Desde este punto de vista, Kandilkar y Shah [1,2] obtuvieron relaciones entre la eficiencia térmica P1=ΔT1/(Tce-Tfe) y NTU1=UA/C1 y la relación de capacidades caloríficas R1=C1/C2. Este método presenta un inconveniente de cara al cálculo de un intercambiador con herramientas informáticas, ya que existen diferentes expresiones en función del número de pasos del intercambiador. Además, para un número reducido de placas, la solución propuesta se presenta en forma de tablas y no de ecuaciones algebraicas. En este trabajo se obtiene la expresión algebraica de un intercambiador de placas en configuración 1 paso – N pasos mediante el método ε-NTU, dónde el parámetro NTU se define con la capacidad calorífica mínima, lo que lo hace más adecuado para los cálculos. Además se presenta una corrección de la solución analítica, en función del número de placas, para un número de placas reducido en diferentes configuraciones

Influencia de la corrugación en tubos de intercambiadores de calor mediante simulación numérica

La mejora de la eficiencia de transferencia de calor es un aspecto fundamental en el diseño y manejo de los intercambiadores de calor. Este aspecto adquiere mayor relevancia en numerosas aplicaciones industriales que utilizan fluidos con viscosidad elevada, como el caso de aceites y fluidos de tipo alimentario, donde el coeficiente de transferencia de calor se reduce debido al predominio de flujo laminar [2]. En estas condiciones, se han desarrollado diversos métodos para mejorar el proceso de transferencia de calor, como es el caso de la corrugación de los tubos en intercambiadores de calor, aspecto que contribuye a la formación de un régimen de transición o turbulento, aumentando el coeficiente de transferencia de calor [3,5]. En consecuencia, los tubos corrugados permiten, en comparación con los equipos que utilizan tubos lisos, reducir el espacio físico destinado a cada uno de ellos. En el presente trabajo se analiza el efecto de la corrugación sobre la transferencia de calor, así como la pérdida de carga en tubos de intercambiadores de calor. Para ello, se utilizarán herramientas de dinámica de fluidos computacional, analizando la influencia de distintos grados de corrugación sobre los parámetros indicados.Los autores agradecen al Ministerio de Economía y Competitividad de España por la financiación del proyecto “Planta piloto destinada a la caracterización de fluidos singulares en intercambiadores de calor de tubo corrugado” (UNCM13-1E-1832)

Numerical simulation of the heat transfer process in a fluidized bed with an inmersed tube

This work aims to perform a numerical simulation of the heat transfer process of an immersed spherical surface in a bubbling fluidized bed. The experimental conditions of Di Natale et al. [1] were numerically reproduced: a hot sphere of 28 mm diameter immersed in a fluidized bed of 600 mm height. The sphere was in the middle of the bed, at a height of 300 mm, and with constant surface temperature of 373 K. The numerical simulation was approximated to a 2-D geometry, with a thickness of the bed of only 15 mm, in which the hot sphere is replaced by a horizontal cylinder resembling a tube. The bed was filled with spherical glass particles with a mean particle diameter of 0.5 mm, which were fluidized with atmospheric air at 293 K and with an air velocity of 0.3 m/s. The numerical simulations were carried out with the software CPFD-Barracuda, which is based on multiphase particle in cell (MP-PIC) method. This methodology is specific to simulate granular flows and solve the motion of groups of particles called “clouds”. In this way, the computational cost is notably reduced in comparison with a full Lagrangian simulation, in which the motion of all individual particles is solved. The numerical results permited a detailed analysis of the local heat transfer coefficient between the hot tube and the fluidized bed. Different heat transfer rates were observed around the tube. According to the results, the heat transfer coefficient is high at the bottom and at both sides of the tube (with values close to 200 W/(m2·K)), where the bubbles motion continuously replaces the heated particles in contact with the tube surface with new cold particles, creating a high heat transfer rate in those regions. In contrast, on the top of the surface, the particles are not fluidized. This was clearly observed in the simulation results: on the top of the tube the resulting particle velocity was close to zero and the particle volume fraction was close to the one at minimum fluidization conditions (0.6). This means that particles on top of the tube are at rest and they are not replaced by new cold particles by the action of the bubbles. Consequently, the heat transfer in this region remains low all the time (with values around 20 W/(m2·K))

Temperature distribution in two different fluidization technologies applied to directly irradiated fluidized beds

This work aims to compare two different fluidization technologies (bubbling and spouted beds) applied to directly irradiated fluidized beds, when both operate at similar conditions (mass of solid particles, airflow rates, radiation fluxes and medium bed particle heights). In both cases, the fluidized bed is irradiated from the top of the bed with a beam-down reflector with a 4 kW Xenon lamp working at 2 kW. There are different solid particles that can be used in fluidized beds. However, according to previous works [1], the most suitable material due to their optical properties is Silicon Carbide. Therefore, 7 kg and 10 kg of this material was necessary for spouted and bubbling beds, respectively. These masses of particles were exposed to similar radiation conditions from an optical point of view, using the same focal length between the top of the bed in both cases (Lfocal=1.29 m). The bubbling fluidized bed consists of a cylindrical geometry with an inner diameter of 31.5 cm. In this technology, the airflow passes homogeneously though the cross-sectional area of the bed and is supplied into the bed through a distribution plate with 89 holes at the lowest part, which separates the particles from the plenum. By contrary, spouted bed has a conical geometry with a bottom diameter of 10.8 cm, which corresponds to the inlet air diameter, and a top diameter of 31.5 cm. The movement of particles in each case is completely different due to the internal geometry. Bubbling fluidization presents particles agitation in the whole of the bed while in spouted bed case two clear regions are distinguished: the central or core region of the bed, where the voidage is very high, and the annular region around the jet. On the top of the spouted bed a form similar to a “fountain” appears, where the particles conveyed from the central jet are projected onto the top of the annular region. In this annular region, the particles move down slowly, while part of the gas percolates through the particles in a countercurrent configuration [2]. The results show how the spouted bed gets a similar behavior to the bubbling fluidized bed but only requiring one-third of its pumping costs, additionally, the thermal energy distribution in the center and periphery of the bed surface presented a behavior completely different. Furthermore, in both cases, higher airflow rates increase the mean temperature in the bed surface

Experimental study of different coatings on silica sand in a directly irradiated fluidised bed: Thermal behaviour and cycling analysis

The present work shows the experimental results obtained with sand particles, which where coated to increase the thermal absorptivity for CSP applications. The particles were tested in a lab-scale fluidised bed with concentrated irradiation on the top. The experimental results indicates that two of the three coatings tested, based on graphite and carbon black, worked properly and absorbed between 30 and 40% more energy than raw sand due to the higher thermal absorptivity. Both coatings were also experimentally tested during 10 cycles of charging/discharging without apparent deterioration of their thermal properties. Sand coated with graphite exhibited colour change, from an initial dark black to a greyish tone that did not have an impact on the particles thermal response. The extrapolation of the results observed at lab-scale, to the maximum expected temperatures in new generation of CSP plants with solid particles, up to 1250 K, shows that coated particles may enhance the energy effectively stored in the bed by 60–80% compared to raw sand.This work was partially funded by the Ministerio de Economía y Competitividad (Projects RTI2018-093849-B-C32 MCIU/AEI/FEDER,UE and ENE2016-78908-R) of the Spanish Government, the Regional Government of Castilla-La Mancha (project SBPLY/17/180501/000412), Generalitat Valenciana (PROMETEO/2020/029), the Universitat Jaume I (UJI-B2020-32) and the Ministerio de Ciencia, Innovación y Universidades - Agencia Estatal de Investigación (AEI) (RED2018-102431-T). The authors would like to thank the Catalan Government for the quality accreditation given to their research group (DIOPMA 2017 SGR 0118). DIOPMA is certified agent TECNIO in the category of technology developers from the Government of Catalonia. We also acknowledge financial support of the UCLM through the “Ayudas a Grupos de Investigación-Plan Propio” (ref. 2021-GRIN-31341 and 2021-GRIN-30978). Minerva Díaz-Heras is also a postdoctoral researcher Margarita Salas, with a contract within the framework of the call for grants for the requalification of the Spanish University system, funded by the European Union Recovery Instrument NextGenerationEU and obtained through the Universidad de Castilla-La Mancha (UCLM)