Covalent modification of reduced graphene oxide with piperazine as a novel nanoadsorbent for removal of H2S gas

In the present research, piperazine grafted-reduced graphene oxide RGO-N-(piperazine) was synthesized through a three-step reaction and employed as a highly efficient nanoadsorbent for H2S gas removal. Temperature optimization within the range of 30–90 °C was set which significantly improved the adsorption capacity of the nanoadsorbent. The operational conditions including the initial concentration of H2S (60,000 ppm) with CH4 (15 vol%), H2O (10 vol%), O2 (3 vol%) and the rest by helium gas and gas hour space velocity (GHSV) 4000–6000 h−1 were examined on adsorption capacity. The results of the removal of H2S after 180 min by RGO-N-(piperazine), reduced graphene oxide (RGO), and graphene oxide (GO) were reported as 99.71, 99.18, and 99.38, respectively. Also, the output concentration of H2S after 180 min by RGO-N-(piperazine), RGO, and GO was found to be 170, 488, and 369 ppm, respectively. Both chemisorption and physisorption are suggested as mechanism in which the chemisorption is based on an acid–base reaction between H2S and amine, epoxy, hydroxyl functional groups on the surface of RGO-N-(piperazine), GO, and RGO. The piperazine augmentation of removal percentage can be attributed to the presence of amine functional groups in the case of RGO-N-(piperazine) versus RGO and GO. Finally, analyses of the equilibrium models used to describe the experimental data showed that the three-parameter isotherm equations Toth and Sips provided slightly better fits compared to the three-parameter isotherms

Direct numerical simulation of synthetic jet coupled to forced convection cooling in a channel flow

A synthetic jet (SJ) is a microfluidic device that uses the 'zero-net-mass-flux' concept to create a compact cooling solution and provide a net positive momentum flux to the local environment. SJs have been studied extensively for natural convection heat transfer, but there is a limited data available for SJs in cross flow regimes. This paper presents results based on direct numerical simulation of a SJ in a confined heat transfer channel with and without cross flow. Studied SJ had a deforming boundary that oscillated at 1000 Hz and was placed at a high orifice-to-plate distance ratio of 20. The flow field inside the device with a moving boundary was modeled in a coupled manner to the flow field outside of the device for 80 oscillation cycles. The coupled study of the flow fields inside and outside of the cavity revealed their interaction towards an unstable flow field. Moreover, comparison between SJ's and continuous jet's (CJ) cooling performance was performed with the same net mass flow rate and identical jet outlet temperatures. Without cross flow, CJ, and with cross flow, SJ outperformed in terms of heat removal. The remarkable difference in spatial evolution of CJ and SJ explains the better performance of SJ in cross flow regime. In the studied high orifice-to-plate distance, CJ stream was unable to penetrate effectively through the crossflow, while the vortical structures created by SJ were able to do so and impinge on the target surface with heat transfer augmentation at upstream. Furthermore, the SJ's cavity heating was found to be a limiting factor in its capability to achieve high heat transfer coefficients in confined channels, which needs to be addressed to maintain its reliable heat removal performance.Auburn University Samuel Ginn College of Engineerin

Vortex-enhanced jet impingement and the role of impulse generation rate in heat removal using additively manufactured synthetic jet devices

This article presents an approach to the design and fabrication of synthetic jet devices (SJDs) using rapid prototyping via additive manufacturing, marking the first study to employ this method for such devices. This manufacturing technique empowers researchers with complete design freedom, enabling the production of ultrathin SJDs-as thin as 4 mm-without mechanical fasteners and facilitating the rapid fabrication of multiple devices with varying geometries. To showcase the potential of this method, SJDs with conical and cylindrical cavities and orifices ranging from 1.6 mm to 7 mm were designed, fabricated, and tested. These devices achieved air jet exit velocities exceeding 106 m/s using a single piezoelectric diaphragm-among the highest reported in the literature-validating the effectiveness of this manufacturing approach. This high jet velocity is significant for practical applications requiring efficient thermal management, such as cooling high-power-density electronics, where compact and energy-efficient solutions are essential. Beyond achieving high velocities, it was revealed that maximizing jet velocity alone is not always optimal for heat removal. The hydrodynamic impulse generation rate was introduced as a more significant factor influencing heat transfer performance. By fabricating and testing multiple SJDs with different geometries, it was demonstrated that the impulse generation rate, which accounts for both jet velocity and flow rate, better correlates with enhanced heat transfer capabilities than jet velocity alone. This insight addresses an often-overlooked parameter in SJD design and has substantial implications for optimizing heat removal performance. Moreover, lumped element modeling, tuned solely on diaphragm deflection behavior, accurately predicted device performance and was validated using a hotwire anemometer. This model effectively characterizes center-axis orifice devices and confirms its applicability to thin-cavity designs, providing a valuable tool for future SJD development. Despite moderate volume flow rates (0.2 to 0.8 m3/h), the fabricated SJDs delivered significant improvements in heat transfer. Compared to natural convection, these devices achieved over 13 times greater heat removal rates, with an average heat transfer coefficient exceeding 120 W/m2 center dot K over a 30 mm x 30 mm heated surface. These findings demonstrate the practicality and effectiveness of vortex-enhanced synthetic jet impingement for targeted and efficient cooling of localized hot spots. This approach offers multiple advantages over traditional rotary cooling systems like fans, including increased reliability, lower profile, while consuming less than 100 mW. The ability to rapidly prototype and optimize SJDs using additive manufacturing accelerates research and development in this field, paving the way for advanced thermal management solutions in real-world applications.Auburn University Samuel Ginn College of Engineerin

Liquid cooling of data centers: A necessity facing challenges

The evolving data generation landscape requires faster and more efficient microprocessors, prompting innovative manufacturing methods for smaller and faster transistors. Transistor congestion and rising demand for parallel processing are pushing the thermal design power of microprocessors well beyond 280 W, a limit for air cooling, and are expected to surpass 700 W by 2025. Consequently, transitioning towards liquid cooling is necessary. This article is intended to serve as a comprehensive roadmap to understanding this shift. It covers four major liquid cooling techniques: indirect water cooling with rear door heat exchangers, direct liquid cooling using water blocks or evaporators, single-phase, and two-phase immersion cooling. Indirect water cooling with rear door heat exchangers is a simple water cooling adaptation for reducing the power consumption of existing air-cooled data centers, but it faces the same limitations as air cooling for high-power servers. With enhancements such as reduced hot air leakage, active rear door heat exchangers, and deployment in locations conducive to free cooling, this approach could provide highly efficient data centers for the foreseeable future. Direct liquid cooling is well suited to meet rising thermal design power demands with the highest heat transfer coefficient report of 25 W/ cm2-K, using water-based manifold microjet impingement on die. Emerging technologies are new thermosyphon systems, on-die/on-lid refrigeration/impingement, two phase impingement, and on/in die microchannel cooling. Air cooling is still required for peripheral equipment in this method, adding to the complexity and power consumption. Immersion cooling has the potential of reducing infrastructure size by one-third of air cooled data centers. Single-phase immersion cooling, while the most simple to implement, is limited by the low thermophysical properties of the dielectric liquids, and lack of flow control mechanism. In contrast, two-phase immersion cooling faces significant challenges related to the use of engineered fluids with global warming potential, health hazards, and long term reliability.Auburn University Samuel Ginn College of Engineerin

Effect of phase change materials on the optical path of LEDs for opto-thermal enhancement

A novel concept based on the encapsulation of transparent phase change materials (PCMs) into the optical packaging structure of light-emitting diodes (LEDs) is presented in this article. The concept was initiated by challenges of thermal management of photoluminescent particles in high-power optical systems. LEDs, white LEDs (WLEDs), and porous/network-based photoluminescent matrices can achieve improved thermal networks by embedding PCMs. In this article, paraffin is selected as a suitable PCM encapsulant, and aside from thermal perspectives, an unexpected optical benefit with melted paraffin after surface wetting of the chip was observed. Immersing an LED chip in a melted paraffin pool showed up to an 8% increase in light extraction efficiency and a 1.5% increase in power conversion efficiency (PCE). An accurate dynamic opto-electro-thermal monitoring of studied devices was used to support the proof of concept. This viable method can be integrated into current industrial packaging processes

Numerical simulation of thermal performance of radial manifold microchannel heat sink for electronics thermal management

Thermal management for electronics has continued to experience significant growth due to the rising demands for computing power along with a continuous decline in packaging volumes. Maintaining the electronics temperature within a predetermined limit is crucial since overheating can result in chip failure, performance deterioration, and even operational hazards in extreme circumstances. This study introduces a novel radial manifold microchannel (RMM) heat sink featuring an upper layer of curved, interdigitated channels designed to enhance flow distribution and reduce pressure drop. The single-phase flow and heat transfer behavior of three curved manifold lengths and three nozzle positions were numerically evaluated using DI water. Results show that the curved design improves thermal performance by up to 14.9% in the best-case scenario at higher flow rates. The Thermal Performance Index (TPI) demonstrated increases of 11% and 14% for the longest and shortest channels with right-edge nozzles, respectively. A full conjugate heat transfer analysis further confirmed that the RMM design reduces overall pressure drop by 19.7% compared to traditional manifold microchannel (TMM) heat sinks.Auburn University Samuel Ginn College of Engineerin