Atul Bhargav

Autothermal reforming of methane on rhodium catalysts: Microkinetic analysis for model reduction

Methane autothermal reforming has been studied using comprehensive, detailed microkinetic mechanisms, and a hierarchically reduced rate expression has been derived without apriori assumptions. The microkinetic mechanism is adapted from literature and has been validated with reported experimental results. Rate Determining Steps are elicited by reaction path analysis, partial equilibrium analysis and sensitivity analysis. Results show that methane activation occurs via dissociative adsorption to pyrolysis, while oxidation of the carbon occurs by O(s). Further, the mechanism is reduced through information obtained from the reaction path analysis, which is further substantiated by principal component analysis. A 33% reduction from the full microkinetic mechanism is obtained. One-step rate equation is further derived from the reduced microkinetic mechanism. The results show that the this rate equation accurately predicts conversions as well as outlet mole fraction for a wide range of inlet compositions.

Rack level transient CFD modeling of data center

Purpose The paper aims to capture the rack-level thermal dynamics in data center. It proposes the rack-level response experiments as well as transient Computational Fluid Dynamics (CFD) analysis to characterize the local thermal environment of the system. Design/methodology/approach A single sever simulator rack and its two neighboring racks with its cold and hot aisle containment have been modeled with known cold air supply temperature and flow rate for transient CFD analysis. The heat load was kept constant initially and varied case-to-case basis, which includes capturing the rack-level response with respect to changes in input. However, the response experiments on simulator rack were performed for 14 h by variation of server heat loads as step and ramp input. Findings The paper provides the detailed transient CFD analysis of data center racks. The local cold air flow rates and temperature at the vicinity of the racks showed significant effect due to changes in input. It was concluded that the rack-level dynamics impacts the thermal environment of data center and hence cannot be ignored. Research limitations/implications The high computing devices and faster internet demands have led to major thermal management concerns for data center operators. To tackle this issue, capturing the system thermal dynamics is imperative. However, the system-level CFD analysis is computationally expensive. Therefore, this paper deals with the rack-level transient CFD study using commercial tool STAR CCM+. Practical implications This paper includes the modeling of the servers as a porous media as well as the multigrid method to enhance the computational speed. The successful implementation of this approach validated through experiments. This would help to establish a base for research in any type of data center. Originality/value This paper provides the porous media approach to model servers and multigrid method to enhance the computational speed. At the same time, the thought of characterizing the local dynamics at the vicinity of data center racks is unique.

Role of carbon nanotube on the interfacial thermal resistance: A molecular dynamics approach

Very high thermal conductivity of carbon nanotube (CNT) makes it an obvious choice in electronic cooling applications. But at the nanoscale, these CNTs face a limitation due to the interfacial thermal resistance commonly known as Kapitza resistance, prevailing between the carbon nanotube and coolant molecules at the solid-liquid boundary. Vibrational mismatch at the interface gives rise to the Kapitza resistance which plays a dominating role in the heat transfer process. Current work puts an effort to investigate the impact of CNT diameter on the interfacial resistance between nanotube and water molecules through molecular dynamics. Molecular dynamics simulations have been performed using armchair single walled CNTs. Beginning with the initial configuration, the system of CNT and water molecules is equilibrated at 300 K and 1 atm. The temperature of the CNT is raised to 700 K and then allowed to relax in a bath of water molecules. The time constant of the CNT temperature response is determined based on the lumped capacitance analysis which is then used to compute the interfacial resistance. Present study illustrates that the interfacial thermal resistance is increases as the diameter of the single walled carbon nanotube increases. Therefore, in electronic cooling applications, CNT of smaller diameters should be preferred owing to its lower values of interfacial thermal resistance.

Molecular dynamics simulation of the effect of the solid gas interface nanolayer on enhanced thermal conductivity of copper-CO 2 nanofluid

The use of CO2 as a natural refrigerant in data center cooling, in oil recovery and in CO2 capture and storage is gaining traction in recent years. These applications involve heat transfer between CO2 and the base fluid, and hence, there arises a need to improve the thermal conductivity of CO2 to increase the process efficiency and reduce cost. One way to improve the thermal conductivity is through nanoparticle addition in the base fluid. The nanofluid model in this study consisted of copper (Cu) nanoparticles in varying concentrations with CO2 as a base fluid. No experimental data is available on thermal conductivity of CO2 based nanofluid. Molecular dynamics (MD) simulations are being increasingly adopted as a tool to perform preliminary assessments of nanoparticle (NP) fluid interactions. In this study, the effect of the formation of a nanolayer (or molecular layering) at the gas-solid interface on thermal conductivity is investigated using equilibrium MD simulations by varying nanoparticle diameter and keeping the volume fraction (1.413%) of nanofluid constant to check the diameter effect of nanoparticle on the nanolayer and thermal conductivity. A dense semi-solid fluid layer was seen to be formed at the nanoparticle-gas interface, and the thickness increases with increase in particle diameter, which also moves with the nanoparticle Brownian motion. Density distribution has been done to see the effect of nanolayer, and its thickness around the nanoparticle.

Effect of electrode properties on performance of miniaturized vanadium redox flow battery

Redox flow batteries have seen interest in electronic applications because of their potential to simultaneously deliver electric power and remove heat. For these applications, the flow battery has to be constructed on a side of a computer chip, with components such as flow channels, manifolds, supply tubes, electrodes, membranes and current collectors. Since experimentation with micro-scale components is especially expensive and time-consuming, there is a need to develop computational tools to understand trade-offs in the design and operation of these flow batteries. Computational fluid dynamics study of redox flow batteries is carried out using commercial software package COMSOL Multiphysics. This paper analyses the effect of flow rate and electrode thickness on current and voltage distribution in the flow battery. Tradeoff between flow rate and pressure drop also has been analyzed. We have quantified the effects of flow rate on pressure drop and thereby the parasitic pumping power and the effect of electrode thickness on the ohmic overpotential and thereby the performance of the cell.

Radiation heat transfer analysis of spectrometer's Dewar Cooling Assembly

Optical spectrometers have been of interest in remote sensing because of their ability to decipher an image scene based on its spectrum. Fore-optics, glass window, cold shield, order sorting filters (OSF), focal plane array (FPA) and cryo-cooler are integral parts of a spectrometer assembly. It can be divided into two parts: Fore optics & Integrated Detector Dewar Cooling Assembly (IDDCA). Optimum size of cryo-cooler ought to be known as weight constraints are critical in payload design. Cooling load on cryo-cooler depends on the ability of assemblys parts to absorb, transmit and radiate energy emanating from distant scene and incident on spectrometers aperture. The experimentation with small scale components is expensive and requires sophisticated measuring and calibration techniques. Efforts have been made to develop a computational model which can hypothesize thermal parameters of IDDCA and their interdependency. This study has been carried out for the assembly using Ray optics and Heat transfer modules of COMSOL Multiphysics in the wavelength range of 800 to 5000 nm. This model examines the distribution of temperature and total heat flux on FPA in order to maintain its low temperature 90 K) for maintaining image resolution.

Rack level forecasting model of data center

Due to the nature of their operations, electronic data centers are highly dynamic in their computational load handling, and concomitantly in the heat generated by various servers inside the data center. In the absence of physics-based control algorithms, air-conditioning systems are typically operated through conservatively pre-determined set points, thereby resulting in sub-optimal electricity consumption. Computational fluid dynamics based models that have been developed are too computationally intensive to be used in control algorithms. Here, we develop a validated transient model for a data center that can be used as the basis for model predictive control algorithms. We consider a common raised floor plenum data center in which cold air is delivered to the server racks and hot air is exhausted through the ceiling. We test and demonstrate the performance of the data driven model on this data center. Thermal and flow dynamics in a data center are strongly affected by rack inlet and rack outlet temperatures, in addition to the computational load (and thereby the heat generated by the racks). Using these variables, the response data were collected at the top, center and bottom position of server simulator rack inside a raised floor plenum data center. The server simulator rack consists of four server simulators, each with independent control of heat load & fan speed. The simulator was first allowed to reach steady state (by holding conditions constant for over 3 hours). Subsequently, the heat load was ramped up in steps. To reach steady state, all simulators were held at 2.5 kW constant heat output with 0.15 m3/s fan speed. The computer room air-conditioning (CRAC) unit was modeled as a constant air supply at 17 °C. The response data were collected for various cases with heat load as input to the system. This input-output data was used for data driven modeling to predict the rack outlet air temperatures. The data analytics techniques that were used include partial correlation and total least square approach. Results show good agreement with experiments and have prediction capacity of 15 minutes. These results indicate that data driven modeling can potentially be used to form the basis for model predictive control.

Design and testing of prototype data center

Real time experimentation without affecting the actual system performance is a big challenge and expensive for huge systems. The data centers are one of such systems as well as the highest power consuming entities in the world. In such cases the scaled down testing facility provides the platform to test the actual system with its all possible emergencies. There is no attempt of building the prototype testing facility of the data center in research community which will be useful for operator to manage the system efficiently. We have design and developed a unique prototype testing facility to achieve the efficient thermal management of data center. The dimensional analysis has been done by keeping Richardsons number constant and accordingly cold-air flow-rates range were defined for 20 times scaled down prototype. The CFD modeling of the actual system and scaled down system has shown the similar thermal behavior. Furthermore, the prototype model was developed for conventional raised floor plenum (2 rows each with 5 racks) and S-Pod (3 pods with 4 racks) rack structure with 4 CRAC units. To maintain the similar environment in prototype as the actual data center the structural modification includes the four server compartments each with porous sponge (around 60 % porosity), open tiles in front of racks, the hot air supply through pipes with pin holed at each server compartment. In the scaled down structure, the heating through servers has been emulated by supplying heated air (at 100°C) in each server rack compartments with 1.67×10-4 m3/s (10 SLPM). Similarly, filtered atmospheric air supplied with 10 SLPM through the CRAC in the plenum chamber as the cold air. The exhaust has been provided through a vent at the top of the system. Twelve thermocouples have been deployed to track the temperatures at server inlet-outlets, plenum, CRAC & overhead chamber. The whole system was insulated to avoid the leakages. It was observed that the S-Pod structure has superior performance than the conventional raised floor. The major parameters affecting the data center performance such as workload on servers (hot air passed through servers) has been considered in prototype by changing the inlet flow rate. The other complexities such as server fan, perforation of tiles will be introduced in near future. This study will be a breakthrough for actual testing the critical factors of any data center.

Conversion of a CNG Powered Auto Rickshaw to an Electric Rickshaw Designed for Indian Conditions

by Rounak Mehta, Preet Shah, Harsh Gupta, Prathamesh Bhat, Vaibhav Gandhi, Kimaya Kale, Madan Taldevkar, Akash Singh, Chinmay Ghoroi, Atul Bhargav and Amey Karnik