Population and Environment

Phase change materials (PCMs) play a pivotal role in various sectors, particularly in automotive engineering, electric vehicles, and building construction. In the automotive sector, phase change materials are crucial for thermal management systems, aiding in temperature regulation of components such as batteries and engines. In electric vehicles, phase change materials are instrumental in enhancing battery performance and lifespan by effectively managing thermal loads during charging and discharging cycles, thus ensuring optimal operating conditions. These materials offer significant energy efficiency benefits by absorbing and releasing large amounts of latent heat during phase transitions, which helps in maintaining stable temperatures and reducing the load on heating and cooling systems. Additionally, PCMs contribute to sustainable building practices by enhancing thermal regulation, thereby lowering energy consumption and associated costs. This study explores the diverse applications and properties of phase change materials for improving thermal management and energy efficiency in vehicles, residences, and buildings. This research provides a comprehensive review of innovative solutions, including PCM-based heat pumps, PCM-integrated cementitious composites, and hybrid active-passive battery thermal management systems.

Direct numerical simulation of heat transfer behind a spanwise obstacle was carried out in a steady channel flow. Reynolds numbers corresponded to transition to turbulence in the separation region behind the obstacle. The obstacle was mounted either on the channel wall or with a gap from the wall. Thorough verification of numerical results (visual flow pattern and flow statistics) against experimental data was carried out. Distributions of local coefficients of heat transfer and skin friction behind the obstacle were found to correlate with the vortical structure of the flow. For both positions of the obstacle relative to the channel wall, the study discovered principal regularities in the behavior of local and averaged, across the channel, values of heat transfer behind the obstacle with the varying Reynolds number of the oncoming flow. The effect of obstacle position on the total increase in the heat transfer coefficient on the wall behind the obstacle was estimated in comparison with the smooth wall.

Equipment used in the aviation industry heats up over time depending on working conditions. It is possible to preserve properties of equipment affected by heat by either cooling the system and returning it to initial conditions or producing the system from materials that are not affected by heat. One of the areas where nanocomposite materials might be used is avionic systems in the aviation and space industry. These systems are structures in which elements such as sensors, cabling, and processors, which form the basis of the electronic structure of flight, are brought together in very small volumes. It is important that the material used in these structures is light and has high strength as well as corresponding electromagnetic properties. In this study, the thermal analysis of vapor-grown carbon fiber (VGCF) nanocomposite materials produced by adding them to the epoxy matrix in terms of the thermal performance of avionic boxes was carried out by comparing them with thermal properties of aluminum. As a result of the findings obtained from thermal analysis studies carried out in four stages, it was observed that by using VGCF composite instead of aluminum, approximately 23% improvement in temperature output and 17% improvement in thermal load was achieved. Another outcome obtained from the analysis was the cooler capacity. If VGCF is preferred instead of aluminum in avionics box manufacturing, a 37.5% improvement is achieved in terms of cooler capacity. Another important finding is the time to reach critical temperature levels. VGCF reaches the steady state 163.5% faster than aluminum. Thus, it is anticipated that energy efficiency will be increased with the use of lightweight and high-strength nanocomposite materials, which is considered one of the most important goals of the aviation industry.

Spray cooling exhibits outstanding cooling performances compared to other liquid cooling techniques, which offers robust thermal management for numerous applications facing high heat flux challenges. In spray cooling, coolant droplets generated from a spray nozzle continuously impinge onto a hot surface at high flow rates. The interaction between the droplets and the surface - whether they land on a pre-existing liquid film or directly on the heated area - depends on the fluid saturation temperature and the surface temperature. Understanding the dynamics and heat transfer during droplet impact is crucial for advancing spray cooling research. The present work summarizes the recent advancements in the study of droplet impact dynamics and heat transfer in spray cooling from two aspects. The first aspect is about the statistical analyses of droplet behaviors and liquid film conditions in spray cooling, examining their influence on cooling efficiency. The second one is regarding the droplet-surface interactions in spray cooling, ranging from single droplet to spray by increasing the complexity of droplet condition and surface condition. It includes the single droplet impacting a dry heated surface, multiple droplets impacting a dry heated surface, and droplets impacting the heated flowing film.

For living spaces, radiant thermal mats are seen to be a good substitute for traditional heating systems. The natural convection heat transfer properties of radiant heating and cooling systems have been well studied, while the properties of radiant mats placed on surfaces have received relatively less attention. Mats of square and rectangular shapes (<i>a</i> × <i>b</i> = 0.5 m × 0.5 m, 1 m × 1 m, 1.2 m × 1.2 m, 1.4 m × 1.4 m, 1 m × 1.2 m, 1 m × 1.4 m, and 1 m × 1.6 m) are installed on the walls of an enclosure with floor dimensions <i>L</i> × <i>L</i> = 4 m × 4 m and a height of <i>H</i> = 3 m in order to address this gap in the literature. Upon analyzing the complete dataset of local convective heat transfer, it is evident that there is a steady decline in the local convective heat transfer coefficients. This decline commences at the initial point of mats, which corresponds to 3 W/m<sup>2</sup> K. This trend remains rather constant until the impact of turbulence becomes noticeably apparent, which occurs when the mat dimensions are 1 m by 1.6 m. Average convective, radiative, and overall heat transfer characteristics, which are important for building energy simulation programs, are found and correlated for different mat dimensions using the surface-to-surface (S2S) radiation model and the <i>k-ε</i> RNG turbulence model in the numerical program, with error ranges of ± 15%, ± 5%, and ± 5%, respectively.

This research explores the stability and rheological characteristics of hybrid nanofluids made from water-ethylene glycol (W/EG) and incorporating nanoparticles such as SiC, Al<sub>2</sub>O<sub>3</sub>, and multi-walled carbon nanotubes (MWCNT). The preparation involved a two-step method, and the nanoparticles were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Stability assessment showed that Al<sub>2</sub>O<sub>3</sub>-MWCNT hybrid nanofluids are optically more stable than SiC-MWCNT as W/EG-based Al<sub>2</sub>O<sub>3</sub>-MWCNT hybrid nanofluids took longer to sediment. Al<sub>2</sub>O<sub>3</sub>-MWCNT hybrid nanofluids exhibited superior stability in visual tests over a period of 19-21 days while SiC-MWCNT nanofluid took 12-14 days to sediment. The rheological analysis revealed that higher particle concentrations resulted in increased viscosity, with SiC-MWCNT and Al<sub>2</sub>O<sub>3</sub>-MWCNT hybrid nanofluids showing viscosity increases of 3.56 and 3.98 times, respectively, in comparison to the base fluid. Conversely, raising the temperature from 25°C to 55°C led to a decrease in shear stress, with reductions of 72.8% and 64.8% observed for SiC-MWCNT and Al<sub>2</sub>O<sub>3</sub>-MWCNT hybrid nanofluids, respectively. Furthermore, the viscosity versus shear rate trends indicated a pseudoplastic or shear-thinning nature for both hybrid nanofluids with particle volume fraction above or equal to 0.1%.

In the present paper, a rapid thermal management strategy (TMS) by combining composite phase change materials (PCM) and a vapor chamber (VC) is proposed to cope with high heat flux conditions. The performance of the heat sink with and without VC is experimentally investigated. Additionally, the low melting temperature alloy (LMTA) is applied to reduce the thermal contact resistance (TCR) between the VC base plate and copper foam, resulting in a significant improvement in thermal management performance (TMP). Our results reveal that the PCM heat sink exhibits poor thermal management in high heat flux conditions due to the low thermal conductivity of paraffin. However, the introduction of VC allows for initial heat diffusion of concentrated heat, demonstrating a higher equivalent thermal diffusion coefficient compared to using a copper plate of the same size. This extension leads to an effective thermal management time of up to 220 min. Furthermore, the application of LMTA substantially enhances temperature uniformity inside the PCM heat sink, reducing the average TCR between the VC base plate and copper foam by 71%, reaching 0.2 K/W. As a result, the overheat degree away from PCM melting temperature is alleviated from 37° to 29° during the quasi-steady state, and the effective thermal management time can be further extended by 11.4%, reaching 245 min. In practical applications, the rapid TMS not only extends the reliable operating time of electronic devices but also maintains the device at a lower temperature level compared to an individual PCM heat sink.

This work presents new information on the interaction of a promising biofuel - furan and the products of its decomposition with molecular oxygen under conditions simulating combustion processes. The investigations were carried out using the precision method of atomic resonance absorption spectroscopy on a high-purity shock tube behind reflected shock waves in an ultra-dilute mixture of 10 ppm C<sub>4</sub>H<sub>4</sub>O + 10 ppm O<sub>2</sub> in Ar in the temperature range 1600-4000 ± 50 K at pressures of 1.5-3 bar. During the oxidation of the studied fuel mixture time-resolved concentration profiles of the formation and consumption of atomic oxygen were obtained. Based on new experimental data, the predictive efficiency of the modern kinetic model of biofuel combustion developed by the CRECK Modeling Group was assessed, which was also used to demonstrate the key reaction pathways that determine the dynamics of furan oxidation and the corresponding thermophysical processes under the studied chemical and thermodynamic conditions. By comparing experimental and numerical data, a detailed analysis of the pathways for the formation of products and the sensitivity of the rate constants of the occurring elementary reactions was carried out. As a result, refinements to the rate constants of key reaction pathways were proposed and implemented, which significantly increased the predictive abilities of the tested model. The accurate data obtained provide a valuable tool for verifying new kinetic and thermophysical combustion models of multicomponent hydrocarbon fuel mixtures involving promising biofuels.

In this work, based on the compressible SIMPLE algorithm, a calculation model of the combined thermoacoustic engine was established. The results presented the changes in the thermoacoustic engine evolution process before and after adding the refrigerator part. Subsequently, the flow field in the oscillation period of the thermoacoustic engine-driven refrigerators was analyzed, and it was found that during the flow velocity transformation, the velocity interface was formed near the middle position of the resonance tube. Additionally, the performance optimization of the thermoacoustic engine-driven refrigerator was studied. It can be found that the added refrigerator part will increase the start-up temperature difference by more than 25 K, as well as decrease the vibration amplitude at the stable stage. The temperature difference between the two ends of the refrigerator part increases with the addition of the temperature difference of the engine. This work provides a useful reference for the application of thermoacoustic engine to drive the same type of refrigerator.

The suboptimal photoelectric conversion efficiency of light-emitting diodes (LEDs) leads to increased temperature. There is a growing interest in using microstructure ionic wind pumps to regulate the chip temperature. But the ionic wind flow and thermal transfer characteristics of thin-plate electrode pumps used for cooling LED chips is unclear. This study proposes ionic wind pumps equipped with wedged and zigzag emitters to effectively manage the heat generated by high-power LED chips. Experimental investigations were conducted to analyze the electrohydrodynamic characteristics of pumps with different emitter types. A two-dimensional model with a wedged electrode and a three-dimensional model with a zigzag electrode were developed for flow distribution analysis and energy efficiency comparison. The cooling capacity of pumps with different configurations was examined. The results show that the pump equipped with a zigzag electrode exhibits improved stability in corona discharge and approximately 1.53 times higher energy efficiency compared to the pump with a wedged electrode. Moreover, the pump with the zigzag electrode covers a larger ionic wind flow area, generating a higher intensity of ionic wind. The angle between the emitter and the grounding electrode significantly affects the ionic wind flow characteristics. The optimal angle is 70° for pumps with wedged emitters and 30° for those with zigzag emitters. Both pumps can produce a steady wall jet at their optimal angle, causing significant disruption in the surrounding area. The pump with a zigzag electrode exhibits superior cooling performance and is more effective with low power consumption.