• Energy Research

Abstract This work aims to study the devolatilization of pulverized coal particles under air and oxycombustion conditions. To do so, a newly developed experimental test bench has been used to stabilize coal jet flames with fuel heating rates similar to those found in industrial combustors. The thermal history of coal particles has been experimentally monitored by coupling pyrometric and particle image velocimetry (PIV) measurements. Char samples have been collected at different residence times to obtain devolatilization profiles that have been compared with data issued from 4 empirical models. New sets of kinetic parameters have been proposed to simulate coal devolatilization under high heating rate (> 10 6 K/s) with N 2 - and CO 2 -based atmospheres. The composition of the burnt gases has also been characterized at various heights above the burner (HAB). The analysis of the obtained results confirmed that an oxygen enrichment of the combustion atmosphere enhances the devolatilization process by favoring an increase of the coal particle temperature. From the comparison between measured and modeled data, it has been observed that apparent devolatilization rates and kinetics were influenced only by the thermal history of the fuel particles with no char-CO 2 gasification or CO 2 -cross linking reaction at the surface of the char in the conditions investigated here. CO releases were found to depend on the devolatilization rates and on the temperatures of the flames. An increase of the SO 2 emissions has been strongly correlated to the oxygen concentration in the medium for a given devolatilization yield. NO emissions were not significantly reduced during experiments conducted under oxycombustion conditions compared to those performed under air which is due to the fact that NO emissions have been mainly related to the formation of fuel-NO in this work.

Abstract The present study deals with thermalhydraulic characteristics and entropy production analysis in a two-rows Plate Fin-and-Tube Heat Exchanger (PFTHE). Using Unsteady- RANS simulations, the effect of the iso-sectional tube shape modification (from circular to elliptic) on the air-side thermalhydraulic characteristics and entropy production rate was studied in three geometrical configurations of PFTHE and two airflow velocities. Spanwise and time-averaged representative quantities as well as the flow structure were used for local characterization of heat transfer, pressure drop, thermal and viscous volumetric entropy production rates. A global analysis of the thermalhydraulic performances and entropy production was also performed. This study highlights the direct relationship between the flow structure, local heat transfer, wall shear stress and the different components of the volumetric entropy production rate. It is shown that the reduction of the tube ellipticity significantly increases the thermalhydraulic performance of the heat exchanger up to 80% when compared to circular tube shape. Similarly a reduction of the thermal and viscous irreversibilities respectively down to 15% and 50% was observed in the modified shapes when compared to circular ones.

Abstract Constructal theory has been used in several scientific fields since it has been established a decade ago. It mainly treats of the “area to point” flow problem, which has been first-written in the frame of the cooling of a finite-size volume generating heat. This problem, widely discussed in literature, has been optimized thanks to constructal theory formulated by Bejan, with a deterministic approach. However, some of the physical and mathematical assumptions made to simplify the problem are questionable, especially the ones regarding the high-conductivity material constituting the tree-shaped structure. In this paper, the performances of constructal designs are evaluated using a finite volume method, which allows making a comparison between different constructal geometries. An algorithm has been developed which has the ability for restraining constructal networks inside a finite-size area, without overconstraining the structure. All things considered, this paper examines constructal theory from a new point of view thanks to analytical and numerical investigations, in the frame of a finite-size volume generating heat.

Abstract Successive alternating wall deformations are applied to the external and internal walls of a coaxial tube. Through a numerical study, the flow structuration created by the combination of the wall deformations is investigated. The effect of the longitudinal phase-shifting between the external and internal deformations is more specifically studied by considering velocity vectors and normalised helicity fields. All the simulations are performed under the assumption of a laminar, incompressible and stationary flow. The convective heat transfer is also calculated and the performance evaluation criterion P E C = ( N u / N u 0 ) / ( f / f 0 ) 1 / 3 is used to assess the thermal–hydraulic performances. For a fixed temperature condition on both walls, it is shown that the heat transfer performances are increased compared with a smooth annular geometry. The phase-shifting value giving the best thermal–hydraulic performances is also identified: a phase-shifting of one eight of a wavelength between the external and internal walls helps increasing the performances of 43 % .

Global and local analysis of the heat transfer in turbulent vortical flows is studied using threedimensional numerical simulations. Vorticity is generated by inclined vortex generators in a turbulent circular pipe flow with twelve different configurations that fall into three categories. In the first category are rows of trapezoidal vortex generators in different arrangements; in the second category the vortex generators are fixed at certain distance from the tube wall, and the third category has vortex generator rows between which a row of small protrusions are inserted on the tube wall. First, a global analysis of the thermal performance is performed for all these configurations, which are also compared with other heat exchangers from the literature. New correlations for the friction factor and Nusselt number are then obtained. Local analysis of the effect of the flow structure on the temperature distribution is carried out for the four configurations showing the best performances. The local analysis involves studying the streamwise vorticity flux to characterize the convective transport process, the turbulent kinetic energy characterizing the turbulent mixing, and finally the local Nusselt number.

In this paper, we discuss the effect of self-sustained passive oscillations of multiple flexible vortex generators (FVG) in a two-dimensional laminar flow, on heat transfer and mixing. The FVG are located on two opposite channel walls in an alternating positions, inclined in the upstream direction with an angle of 30° with respect to the wall. The FVG oscillate freely without any external force except that provided by the flow itself. Five cases are studied and they differ by the number of alternating flaps and by the presence or absence of two co-planar flaps upstream. The Reynolds number is held constant with a value of 2000 based on the hydraulic diameter of the channel. The simulations are performed by considering a two way strongly-coupled fluid structure interaction approach. The effect of increasing the system degree of freedom, by increasing the number of flaps, resulting in a larger displacement oscillation, on heat transfer and mixing is numerically investigated. The mixing process is quantified by solving the passive scalar transport equation and calculating a mixing index. The results show that mixing is enhanced for larger flaps displacement achieving up to 99% in mixing homogeneity. Moreover, the high amplitude oscillations when compared to the results of an empty channel, show a great ability to reduce the thickness of the thermal boundary layer and to enhance heat transfer resulting in up to 275% increase in the global Nusselt number, 317% increase in the local Nusselt number and 34% increase in the thermal performance factor.