International Communications in Heat and Mass Transfer最新文献

In this work, the S-shaped manifold microchannel heat sink (S-MMHS) with a high surface-to-volume ratio (6690.89 m²/m³) is proposed to dissipate a high heat flux over 150 W/cm². There are 1550 micro-ribs distributed over a heating area of 3 × 3 cm², which contributes over 176.13 cm² of the total heat transfer area. Heat transfer characteristics of S-MMHS of three different structures were experimentally and numerically evaluated with the glycol aqueous solution under a series of heat flux and inlet flow rates. Experimental results depict that the lowest thermal resistance of S-MMHS is 0.22 cm²⋅K/W when the inlet flow rate is 2.89 L/min under a heat flux of 97.6 W/cm² and the overall convective heat transfer coefficient can reach up to 44,761.1 W/m²⋅K. The heat transfer processes of S-MMHS are divided into two parts: the jet-impingement heat transfer as the fluid flows into S-MMHS and the convection on micro-fins as the fluid flows through S-MMHS. Based on the analysis of heat transfer processes, a correlation for the overall Nusselt number is proposed, including dimensions of the microchannel heat sink and Reynolds number. The proposed correlation is used to optimize the heat sink structure, which achieves a reduction of thermal resistance by 33%.

Air handling unit (AHU) is the heat exchanger used for data center cooling. This article developed a detailed dimple/protrusion enhanced AHU modeling process and studied the flow and thermal performance of AHU in the Reynold number range from 5 × 10³ to 3.53 × 10⁴. Elliptic cylindrical dimples (ECD) and spherical crown dimples (SCD) are applied to enhance heat transfer. The entire channel (EC) and typical unit (TU) simulation domains are compared. The EC simulation domain is more reliable as the TU causes the Nu and f prediction relative deviations as large as 20.53% and 24.03%, respectively. The flow patterns in the channels are analyzed. The results show that the bigger SCD depth and smaller ECD depth make the mainstream bends closer to the “S” shape and the v velocity near the dimple/protrusion wall larger. Also, the second flow vortex distribution pattern becomes more complicated, and the velocity gradient near the wall is increased. These flow patterns enhance heat transfer. The dimpled surface has smaller local convective heat transfer coefficient compared with the protrusion surface in the same channel. With the decrease of ECD depth and the increase of SCD depth, the area-average convective heat transfer coefficient is increased.

The heat transfer characteristics of supercritical CO₂ heated in vertical smooth tube are experimentally and numerically investigated. The results show that the M-shaped velocity corresponds to heat transfer enhancement (HTE) at G = 100 kg/m²s, q = 20 kW/m², and the heat transfer coefficient (HTC) is 15% higher than that of normal heat transfer (NHT). While, the M-shaped velocity causes significant heat transfer deterioration (HTD) before the T_pc at G = 278 kg/m²s, q = 35 kW/m², and the HTC of HTD is 28%–42% of that in NHT. According to the numerical analysis on the M-shaped flow structure, it reveals that the zero-velocity gradient point of abNHT (HTE and HTD) always falls into the buffer layer (5 < y⁺ < 30). While, the zero-velocity gradient point of the heat transfer recovery (HTR) is in the log-law region (30 < y⁺ < 60). The u/u₀ of the zero-velocity gradient point of HTE are larger than 1.35. The cross-section turbulent kinetic energy structure shows that smaller TKE in the near-wall region is the dominant factor for HTD and larger TKE in the core region is the dominant factor for HTE.

The ability to control heat flux at the nanoscale opens up numerous exciting possibilities in modern electronics and the field of information processing. In this research, we propose a design with the focus on achieving efficient thermal rectification at moderate gap and relatively low temperature. This study centers on near-field thermal radiation between temperature dependent indium antimonide (InSb) and silicon carbide (3C-SiC) coated with bismuth selenide (Bi₂Se₃). Our investigation sheds light on the critical role played by the Bi₂Se₃ layer in enhancing various key parameters, including the net radiative flux, and thermal rectification efficiency (η). We achieved a substantial improvement in the η of a near-field radiative thermal rectifier (NFRTR) due to the presence of the Bi₂Se₃ sheet. This enhancement is contingent on factors such as the Fermi energy (E_f) of Bi₂Se₃, emitter temperature, and the vacuum gap (d). Our study culminated in the identification of an optimal design, achieving an impressive η of 75% at an emitter temperature (T_H) of 350 K, with vacuum gap (d) set to 20 nm. Furthermore, increasing T_H to 500 K resulted in even more promising outcomes, with the highest η reaching 93%. The need for operating the optimized device at moderate temperatures is to strike a balance between efficiency, safety, cost-effectiveness, and material compatibility. These findings represent a significant step forward in the development of efficient Bi₂Se₃-based NFRTRs, paving the way for future applications in thermal management, energy conversion systems, and thermal logic gates.

Enhancing pool boiling performance is crucial for cooling high-power electronics. Inspired by the concept of liquid-vapor separation, we have developed a self-induced jet impingement device to enhance pool boiling, achieving notable results when combined with microporous copper surfaces in subsequent studies. This paper focuses on using sandblasted pin-fin surfaces as heating surfaces and explores their pool boiling performance under varied pin-fin and self-induced jet device parameters. Findings indicate that the self-induced jet device effectively mitigates the obstruction caused by nucleating bubbles to liquid replenishment, leading to improved q_CHF and h_NB@CHF performance compared to standard conditions. The impact of pin-fin sidewall characteristics, determined by the manufacturing process and parameters, is significant, particularly in enhancing boiling heat transfer performance for dielectric liquid cooling processes. Pool boiling performance is negatively affected by too short or too tall pin-fin heights, irrespective of the self-induced jet presence. Simple strategies like increasing guidance tube length or jet holes number are inadequate for enhancing q_CHF. However, increasing the number of jet holes with strategically placing it between pin-fins could still improve boiling performance. This study demonstrates q_CHF enhancements of up to 145.8%, achieving a q_CHF of 61.2 W/cm², which noticeably surpasses standard pool boiling conditions.

The poor thermal conductivity of phase change material (PCM) has limited its application to thermal energy storage system. The present work aims to improve the performance of PCM in a vertical shell-tube energy storage unit through unique hybrid fins. The enthalpy-porosity approach is used to numerically investigate the phase change phenomenon. Based on the straight and spiral fin results, the novelty designs of double side spiral fin and hybrid fins, i.e. spiral/straight hybrid fin and straight/spiral hybrid fin, are proposed to further optimize the PCM charging process. The effects of different fin structure, hybrid fin proportion and spiral fin angle on the liquid fraction, temperature and average thermal energy storage rate are discussed. The fin structures can reduce the melting time of PCM up to 100% compared to the no-fin case. Although double side spiral fin outperforms the straight fin for the PCM melting behavior, the hybrid fin configurations shows the best enhancement, especially for the straight/spiral hybrid fin. The optimal fin design for the straight/spiral hybrid fin case is that with 0.7 fin proportion and 180${}^{°}$ spiral angle, with up to 11.8% reduction of the melting time compared to the designs of other fin proportions and spiral angles. This work demonstrates the potential of this unique hybrid fin to be integrated with PCM for efficient thermal energy storage.

High-temperature solid particles contain relatively rich waste heat resources, and the moving bed heat exchanger has a significant advantage in the direct contact of particles to extract heat. Considering the varying surface roughness of the actual particles, the effect of particle surface roughness on heat exchange in heat exchanger tubes should not be neglected. To this end, a coupled CFD-DEM computational model of high-temperature solid particles flowing around a single heat exchanger tube is established, and the effect of particle surface roughness on heat transfer from a single heat exchanger tube is analyzed. The results show that f_s (static friction coefficient between particles) has a more significant effect on the heat transfer performance than f_r (rolling friction coefficient between particles). The mean heat transfer coefficients of the heat exchanger tube decrease with increasing f_s. When the f_s increases from 0.05 to 0.3, the mean heat transfer coefficients of the single tube decreases from 239.99 W/(m²∙K) to 234.59 W/(m²∙K), with a decrease of 2.25%. The f_r has little effect on the heat transfer of high temperature solid particles flowing around the heat exchanger tube.

Annotation. Currently, one of the problems significantly hindering the development and implementation of the water-coal technologies in the energy sector is the lack of reliable methods of spraying CWF to a finely dispersed state in the furnace space of the boiler units. The article provides a scientific justification for the new technology of spraying water-coal fuel. The results of the experimental studies of the processes of crushing single drops of water-coal fuel when the latter collide with a metal plate rotating at high angular velocity are presented. Based on the results of the experiments, the main modes of crushing fuel drops have been established depending on the impact velocity at the time of collision with an obstacle. Also, according to the results of the experiments, the distributions of the number of secondary drops of water-coal fuel in size depending on the collision velocity of the drop with an obstacle were established. The relationship between the efficiency of the crushing process and the impact velocity is shown.

To improve the heat dissipation of gear churning oil, six types of nanofluids are prepared using 2% concentration Al₂O₃, SiO₂, Fe₃O₄, TiO₂, CuO and graphene oxide (GO) nanoparticles and using castor oil as the base lubricant oil. The method of applying nanoparticles to lubricating oil to improve gear tooth surface heat dissipation is proposed. The 3D dynamic mesh technology and the computational fluid dynamics–volume of fluid (CFD–VOF) model were adopted to investigate the lubricant flow characteristics and gear heat dissipation performance. The numerical model of gear heat dissipation is verified by thermal imaging infrared experiment, and the simulation results are in good agreement with the experimental results. The effects of gear speed and oil immersion depth on gear heat dissipation are analyzed. The best heat dissipation performance of the gears is achieved at an oil immersion depth of l = 1.5 h and 420 rpm, its thermal performance is increased by 163.95% compared to l = 0.5 h and 1200 rpm. Based on this optimal working condition, the heat dissipation is analyzed under different nanofluid lubrication. The results show that GO has the best heat dissipation performance with a 50% increase in heat transfer coefficient compared to pure castor oil.