• Energy Research

Abstract In this study, the thermal performance of an unsteady, one-dimensional model, and irreversible of a CH4-air reacting process is investigated. Due to the fundamental differences between the free flame and porous structures such as flame thickness and the length of the preheated zone, it is necessary to employ a chemical kinetic which in accordance with the structure of a porous duct made of cellular ceramic, even if it is very simple. To achieve this aim, CH4 oxidation with the five-step reaction mechanism is considered to simulate the combustion phenomenon in the porous burner. In order to solve the governing equations, a finite volume method is used to discretize the governing conservation equations of the problem. Transient, displacement and diffusion terms were respectively solved using completely implicit schemes, Upwind, and discretized central difference and the resulting algebraic equations by the TDMA repeated procedure. In this research, the segregated method was used to solve the set of equations, in which the suitable convergence condition for the governing variables of the problem was used. After validation of the obtained results with available experimental data, the effects of the solid matrix porosity density, porosity, length, extinction coefficient as well as the effects of fire rate, equilibrium ratio, and inlet temperature are investigated. The results show any changes in solid matrix properties and inlet conditions lead to significant changes in combustion structure.

AbstractEfficiency of annular elliptical fin has been studied numerically. It has been shown that constant temperature lines do not preserve their circular shape in fins with high aspect ratio and therefore, the common methods such as equivalent fin area or sector method may not be applicable. For this reason, a new simple correlation has been proposed to approximate annular elliptical fin efficiency and then the fin geometry optimized, to maximize the rate of heat dissipation for a specified fin volume, when there is a space restriction on the fin minor axis. Results have been presented graphically and a correlation has been deduced also.

Abstract 3D turbulent flow numerical simulations were performed in order to develop correlations for heat transfer and flow friction characteristics in louvered fin and tube heat exchangers. A detailed sensitivity analysis was done on the effects of Reynolds number, fin pitch, longitudinal tube pitch and transversal tube pitch on parameters j/j0 and f/f0. Developed correlations are based on ratio of Colburn and friction factors of louvered fin heat exchanger to that of flat fin heat exchanger (j/j0 and f/f0) respectively which are much simpler than other correlations. Also, as it is expected, when louver angle approaches zero (flat fin), parameters j/j0 and f/f0 reach 1. This approach cannot be seen in previous developed correlations. The developed correlations can describe 100% and 86% of parameters j/j0 and f/f0 respectively among 186 numerical simulation data within ±15%. Optimum louver angle was also obtained by genetic algorithm using the developed correlations.

Porous burner offers attractive advantages including high combustion performance and power ranges, and approximately zero pollutant emission. In this paper, the thermal behavior of premixed flame o...

In this work, the two dimensional transition of H2-O2 deflagration-to-detonation was examined in some porous closed-ducts by an in-house high speed reacting flow solver in OpenFoam package—a pressu...

The use of annular fins in air-cooled heat exchangers is a well-known solution, commonly used in air-conditioning and heat-recovery systems, for enhancing the air-side heat transfer. Although associated with additional material and manufacturing costs, custom-designed finned-tube heat exchangers can be cost-effective. In this article, the shape of the annular fins in a multi-row air heat exchanger is optimised in order to enhance performance without incurring a manufacturing cost penalty. The air-side heat transfer, pressure drop and entropy generation in a regular, four-row heat exchanger are predicted using a steady-state turbulent CFD model and validated against experimental data. The validated simulation tool is then used to perform model-based optimisation of the fin shapes. The originality of the proposed approach lies in optimising the shape of each fin row individually, resulting in a non-homogenous custom bundle of tubes. Evidence of this local-optimisation potential is first provided by a short preliminary study, followed by four distinct optimisation studies (with four distinct objective functions), aimed at addressing the major problems faced by designers. Response-surface methods – namely, NLPQL for single-objective and MOGA for multi-objective optimisations – are used to determine the optimum configuration for each optimisation strategy. It is shown that elliptical annular-shaped fins minimise the pressure drop and entropy generation, while circular-shaped fins at the entrance region (i.e., first row) can be employed to maximise heat transfer. The results also show that, for the scenario in which the total heat transfer rate is maximised and the pressure drop minimised, the pressure drop is reduced by up to 31%, the fin weight is reduced up to 23 %, with as little as a 14 % decrease in the total air-side heat transfer, relative to the case in which all the fins across the tube bundle are circular. Moreover, in all optimised cases, the entropy generation rate is also reduced, which shows a thermodynamic improvement in tube bundle performance.