• Energy Research

A heat pipe can be considered a highly effective thermal conduction device, which is especially desirable in heat transfer operations in order to ensure high energy efficiency. However, the operation of heat pipes comprises different heat and mass transfer phenomena, such phase change, heat conduction and convection, solid-liquid and vapor-liquid surface interactions, surface vaporization, and nucleate boiling. Therefore, modelling heat pipes is a highly complex task that demands detailed knowledge of the physical phenomena involved and choosing suitable theoretical models to obtain a good representation of the real nature of the heat and mass transfer processes. In this study, some models and parameters available in the commercial CFD software ANSYS Fluent for turbulence, density, phase change, and phase interfaces were examined to determine their influence on the prediction of the heat and mass transfer in a two-phased closed thermosyphon (TPCT). The numerical results show that using a turbulence viscous model is not necessary and that a variable density model improves the temperature distribution inside the TPCT. Furthermore, using high mass and energy transfer coefficients during condensation makes the vapor remain close to the saturation temperature. Finally, a sharp interphase model is strongly recommended for this type of process.

Nanofluids have been introduced as an alternative to conventional fluids to improve energy efficiency in heat transfer systems. However, their stability problems before and after operation cycles can produce inconsistent results in different heat transfer technologies that use them. This review summarizes different experimental results obtained using nanofluids in heat pipes, particularly in two-phase closed thermosyphons, and it focuses on the role of preparation and stability issues of nanofluids before and after their use in these devices. Additionally, the effects of nanofluids on heat pipes’ thermal performance were compiled and compared from available experimental studies in the literature. Nanoparticles’ deposition on the evaporator surface and wick or groove structures were the most common mechanism to explain the reported increase or decrease in the thermal performance of heat pipes. This review also identifies the research problems that need to be solved in order to use nanofluids that outperform conventional fluids in heat pipes.

As global concerns surrounding climate change mount and fossil fuel reserves diminish, the application of additives in internal combustion engines is increasingly prevalent. Butanol and carbonaceous nanomaterials, such as carbon quantum dots (CQD), are being employed as additives to increase engine efficiency and mitigate the emission of pollutants. Nevertheless, understanding the impact of these additives on combustion behavior at the droplet scale through combustion assessments before their use in engines is crucial. In this study, our main objective was to assess the impact of incorporating CQD dispersed in n-butanol as additives to conventional diesel fuel on the combustion characteristics at the droplet scale. CQD were obtained from spent coffee grounds (SCGs) using n-butanol as a solvent. The product obtained was mixed with Colombian commercial diesel (10 % vol. palm oil biodiesel), and its combustion was evaluated using the droplet combustion method. Before the CQD synthesis, SCGs were characterized by thermogravimetric analysis (TGA) and field emission scanning electron microscopy (FESEM). CQD were characterized via Fourier transform infrared (FTIR) spectroscopy, Transmission Electron Microscopy (TEM), UV-vis, and fluorescence spectroscopy. Results indicated that adding n-butanol and CQD to commercial diesel leads to a 5.4 % and 16.5 % increase in droplet ignition delay, respectively. These additives also cause droplet contraction and expansion cycles, resulting in unstable combustion. However, CQD reduces the frequency of microexplosions caused by boiling n-butanol inside the droplet, which mitigates instabilities during droplet combustion. Including CQD can enhance fuel evaporation by increasing the density of nucleation sites for bubble formation and preventing micro-explosions, thereby leading to stable combustion. These attributes can significantly influence the performance of blends in Compression Ignition (CI) Engines.

Flameless combustion technology has proved to be flexible regarding the utilization of conventional fuels. This flexibility is associated with the main characteristic of the combustion regime, which is the mixing of the reactants above the autoignition temperature of the fuel. Flameless combustion advantages when using conventional fuels are a proven fact. However, it is necessary to assess thermal equipments performance when utilizing bio-fuels, which usually are obtained from biomass gasification and the excreta of animals in bio-digesters. The effect of using biogas on the performance of an experimental furnace equipped with a self-regenerative Flameless burner is reported in this paper. All the results were compared to the performance of the system fueled with natural gas. Results showed that temperature field and uniformity are similar for both fuels; although biogas temperatures were slightly lower due to the larger amount of inert gases (CO(2)) in its composition that cool down the reactions. Species patterns and pollutant emissions showed similar trends and values for both fuels, and the energy balance for biogas showed a minor reduction of the efficiency of the furnace; this confirms that Flameless combustion is highly flexible to burn conventional and diluted fuels. Important modifications on the burner were not necessary to run the system using biogas. Additionally, in order to highlight the advantages of the Flameless combustion regime, some comparisons of the burner performance working in Flameless mode and working in conventional mode are presented.

Abstract A characterization of the energy profile of certain ceramic industry enterprises in Colombia was carried out. An emphasis was placed on the barriers that prevent the implementation of energy efficiency actions, the degree of obsolescence of their production equipment, and the potential for energy efficiency savings. The barrier with the greatest impact in this sector was found to be hidden costs, followed by corporate values. There was a positive correlation between the potential savings and the degree of equipment obsolescence and the total barrier factor to the implementation of energy efficiency measures. Finally, it was concluded that the creation of programs that allow for more certainty in the appropriation of new technologies could help to overcome the barriers in Colombia's ceramic industry.

Abstract Nanofluids have been introduced as new-generation fluids able to improve energy efficiency in heat exchangers. However, stability problems related to both agglomeration and sedimentation of nanoparticles have limited industrial-level scaling. A fractional factorial experimental 2 k−1 design was applied in order to evaluate the effects of nanoparticle concentration, surfactant type and concentration, ultrasonic amplitude as well as ultrasonic time on the stability of alumina (Al 2 O 3 ) nanofluids. Commercial alumina nanoparticles (particle diameter 2 O 3 , CTAB, critical micelle surfactant concentration, 30% ultrasonic amplitude and 30 min of ultrasonication.

This paper presents an experimental study carried out in an industrial furnace for frits production using different configurations of burners based on different combustion techniques such as enriched air combustion, flat-flame oxy-combustion and preheater air combustion. The residence time of combustion gases inside the furnace also was modified. Several combustion configurations were tested and its effects on productivity and thermal energy specific consumption and efficiency were determined. The results show that higher residence time of the combustion gases can decrease significantly the specific consumption of fuel, while the change of the burners and combustion techniques did not show significant effects on decreasing the energy consumption. However, it is highlighted that the oxy-combustion flat-flame burners produced the lowest specific consumption of fuel. Even though the experiments were conducted in a furnace for frit production, the corresponding results can also be applied to guide or improve other industrial high temperature processes.

Abstract The cement industry is one of the world’s largest energy consumers. In this work, a walk-through study of representative cement companies was carried out to characterize the energy profile of the cement industry in Colombia, with emphasis on evaluating the degree of obsolescence of the technology used in the processes, as well as identifying the main barriers that prevent the adaptation of more efficient technologies. Perception-surveys were performed among the energy managers to determine the barriers that hinder the implementation of energy efficiency measures. A factor that quantifies the influence of each type of barrier was defined according to the results of the survey. The results show that the Colombian cement industry has a specific energy consumption that is competitive with the largest cement producers in the world, while the technological obsolescence found is at a medium - low level. Furthermore, the main barrier to the penetration of energy efficiency technologies and measures is identified as the hidden costs associated with the implementation of these technologies. Finally, some conventional and emerging technologies are proposed to improve the efficiency of thermal energy use in the processes studied.

Abstract This paper presents a three dimensional numerical simulation with experimental validation of a gas-fired self-regenerative crucible furnace. Turbulence, radiation and chemical reactions are simulated using the software Gambit V2 and Fluent V6.2. Different combustion models are used to assess their effects on the numerical results. Aerodynamics, temperature fields, species profiles and emissions are compared with the experimental data. The results indicate that k–e RNG model predicts the formation of two concentric swirls: the first one elevating up to the top of the furnace and the second one going down and reaching the outlet. In addition, it was found that is important to inject the fuel using certain vertical inclination of the nozzle in order to obtain a longer and flater flame. Finally, the use of PDF mixture fraction model for combustion causes overprediction of both temperature and CO, while Finite Rate/Eddy Dissipation model is rougher for temperature and species prediction.