Anoop Kanjirakat

A CFD study on the effect of carbon particle seeding for the improvement of solar reactor performance

With the increasing concern of CO2 emissions and climate change, efforts have grown to include solar technologies in chemical processes to manufacture products that can be used both as a commodity and as a fuel, such as hydrogen. This study focuses on a technique, referred to as “solar cracking” of natural gas for the co-production of hydrogen and carbon as byproduct with zero emission footprint via the following reaction: CH4 →C(s)+2H2 (g). However, some portion of the incoming solar energy absorbed by the cavity greatly exceeds the surface absorption of the inner walls because of multiple internal reflections. Studies have shown that by seeding the reactor with micron-sized carbon particles, methane conversion improves drastically due to the radiation absorbed by the carbon particles and additional nucleation sites formed by carbon particles for heterogeneous decomposition reaction. This can maintain more heat at the core and can reduce the carbon deposits on the reactor walls. Present study numerically tries to investigate the above fact by tracking carbon particles in a Lagrangian frame-work. Initially, the numerical model is validated qualitatively by comparing the particle deposition on reactor window with the experimental observations. Effect of particle loading, particle emissivity, injection point location, and effect of using different window screening gases on a flow and temperature distribution inside a confined tornado flow reactor are studied. It is observed that the methane conversion substantially increases by particle seeding. The results of this research can be used in thermo-chemical reactor design.Copyright

Near-wall velocity profile measurement for nanofluids

We perform near-wall velocity measurements of a SiO2–water nanofluid inside a microchannel. Nanoparticle image velocimetry measurements at three visible depths within 500 nm of the wall are conducted. We evaluate the optical properties of the nanofluid and their effect on the measurement technique. The results indicate that the small effect of the nanoparticles on the optical properties of the suspension have a negligible effect on the measurement technique. Our measurements show an increase in nanofluid velocity gradients near the walls, with no measurable slip, relative to the equivalent basefluid flow. We conjecture that particle migration induced by shear may have caused this increase. The effect of this increase in the measured near wall velocity gradient has implications on the viscosity measurement for these fluids.

Effects of High Frequency Droplet Train Impingement on Crown Propagation Dynamics and Heat Transfer

The objective of this study is to investigate the hydrodynamics and heat transfer phenomena due to high frequency droplet train impingement on a pre-wetted solid surface for electronic cooling applications. The effects of crown propagation dynamics and surface heat transfer were investigated experimentally and numerically. Experimentally, a single stream of mono-dispersed HFE-7100 droplets was generated using a piezoelectric droplet generator at a frequency ( f ) of 6000 Hz with a droplet Weber number (We) of 280. Droplet-induced crater and crown were imaged using a high speed camera system. Numerically, the ANSYS Fluent CFD tool was used to simulate the droplet train impingement process. A reasonable agreement was reached between experimental and numerical data in terms of crown propagation dynamics. Numerical simulations reveal that at the instant of initial spot formation, the magnitude of droplet velocity is almost identical to the crown’s radial velocity. The instantaneous temperature field obtained by numerical simulations shows that heat transfer was most effective within the crown propagation region due to the radial momentum generated by the droplets, which leads to a large velocity gradient within the liquid film. A significant increase in surface temperature was observed beyond a radial position of 500 . In summary, high frequency droplet impingement leads to a very small temperature gradient in the radial direction within the droplet-induced impact crater. This study will benefit in understanding the relationship between the droplet parameters and surface heat transfer for different cooling applications involving impinging droplets.

Effects of High Frequency Droplet Train Impingement on Spreading-Splashing Transition, Film Hydrodynamics and Heat Transfer

The objective of this study is to investigate the effects of droplet-induced crown propagation regimes (spreading and splashing) on liquid film hydrodynamics and heat transfer. In this work, the effects of high frequency droplet train impingement on spreading-splashing transition, liquid film hydrodynamics and surface heat transfer were investigated experimentally. HFE-7100 droplet train was generated using a piezo-electric droplet generator at a fixed flow rate of 165 mL/h. Optical and IR images were captured at stable droplet impingement conditions to visualize the thermal physical process. The droplet-induced crown propagation transition phenomena from spreading to splashing were observed by increasing the droplet Weber number. The liquid film hydrodynamics induced by droplet train impingement becomes more complex when the surface was heated. Bubbles and micro-scale fingering phenomena were observed outside the impact crater under low heat flux conditions. Dry-out was observed outside the impact craters under high heat flux conditions. IR images of the heater surface show that heat transfer was most effective within the droplet impact crater zone due to high fluid inertia including high radial momentum caused by high-frequency droplet impingement. Time-averaged heat transfer measurements indicate that the heat flux-surface temperature curves are linear at low surface temperature and before the onset of dry-out. However, a sharp increase in surface temperature can be observed when dry-out appears on the heater surface. Results also show that strong splashing (We = 850) is unfavorable for heat transfer at high heat flux conditions due to instabilities of the liquid film, which lead to the onset of dry-out. In summary, the results show that droplet Weber number is a significant factor in the spreading-splashing transition, liquid film hydrodynamics and heat transfer.

Effects of Screen Laminates on Droplet-Induced Film Hydrodynamics and Surface Heat Transfer

Formation of dry-out area could lead to a sharp increase in surface temperature during droplet impingement cooling. The objective of this study is to figure out an effective way of suppressing dry-out formation in droplet impingement cooling. In this work, HFE-7100 droplet train was produced using a piezo-electric droplet generator at a frequency of 6000 Hz with a droplet Weber number of 280. A translucent substrate was coated with a thin film ITO, which was used as a heater in the experiments. A copper screen laminates with a single punched hole (diameter = 3 mm) was placed over the heater surface at a distance of 0.3 mm to enhance surface heat transfer. Optical images showed that screen laminates effectively suppressed the formation of the dry-out area. It was also found that heat transfer was greatly improved when screen laminates were used. The heat transfer improvement could be attributed to the enhanced surface tension effects, which keep the whole surface wet at high surface temperatures. In summary, the results show that screen laminates effectively suppress the formation of dry-out area and greatly improve surface heat transfer.

Viscosity Measurements of Nanofluids at Elevated Temperatures and Pressures

Engineered colloidal suspensions of nano-sized particles (less than 100nm) dispersed in a base fluid (nanofluid), have shown potential for industrial cooling fluids due to their enhanced heat transfer characteristics. Understanding the rheological characteristics of these suspensions is vital while employing them for flow applications. The effect of temperature on the viscosity of nanofluids at atmospheric pressure is well documented in literatures; however, there are no available data for viscosity measurements of nanofluids at elevated pressure and temperature. In this work, rheological characteristics of oil based nanofluids at high pressures and temperatures, order of 100atm and 100 °C, respectively, are investigated. Nanofluid is prepared by dispersing commercially available SiO2 nanoparticles (∼20nm) in a highly refined paraffinic mineral oil (Therm Z-32, QALCO QATAR) which has wide applications for heat exchangers in oil industry. The rheological characteristics of both the base fluid and the nanofluid are measured using a High Pressure High Temperature (HPHT) viscometer. During experimentation, viscosity values are measured at pressures varying from 10MPa to 40MPa and temperatures ranging from 25°C to 170°C for nanofluid with mass concentrations of 3 percent. The viscosity values of nanofluids as well as base fluid are observed to increase with the increase in pressure. From the pressure coefficient values evaluated for basefluid and nanofluid, it is evident that the effect of pressure on nanofluid and basefluid was similar with no additional effect with respect to particle loading.Copyright

Application of nanofluids in a shell-and-tube heat exchanger

Innovations in the field of nanotechnology have potential to improve industrial productivity and performance. One promising applications of this emerging technology is using nanofluids with enhanced thermal properties. Nanofluids, engineered colloidal suspensions consisting of nano-sized particles (less than 100nm) dispersed in a basefluid, have shown potential as industrial cooling fluids due to the enhanced heat transfer characteristics. Experiments are conducted to compare the overall heat transfer coefficient and pressure drop of water vs. nanofluids in a laboratory scale industrial type shell and tube heat exchanger. Three mass particle concentrations, 2%, 4% and 6%, of SiO2-water nanofluids are formulated by dispersing 20 nm diameter nano particles in desalinated water. Nanofluid and tap water are then circulated in the cold and hot loops, respectively, of the heat exchanger to avoid direct particle deposition on heater surfaces. Interestingly, experimental result show both augmentation and deterioration of heat transfer coefficient for nanofluids depending on the flow rate through the heat exchangers. This trend is consistent with an earlier reported observation for heat transfer in micro channels. This trend may be explained by the counter effect of the changes in thermo-physical properties of fluids together with the fouling on the heat exchanger surfaces. The measured pressure drop in the nanofluids flow shows an increase when compared to that of basefluid that could limit the use of nanofluids in heat exchangers for industrial application.Copyright

Heat Transfer Performance of SiO2-Water Nanofluid in a Plate Heat Exchanger

Over the last decade nanofluids, colloidal suspensions of nanoparticles (∼5–100nm) in a base fluid have created excitement, as well as controversy, due to the reported enhanced thermal properties. Most of the research in the past has focused on the thermal characteristics of nanofluids or their performance in micro systems and/or in simple fluid geometries. The objective of this study is to investigate heat transfer performance of nanofluids in an industrial type heat exchanger. Experiments are conducted to compare the overall heat transfer coefficient and pressure drop in a laboratory scale Plate Heat Exchanger (PHE) using nanofluids with that of water. SiO2-water nanofluids consisting of 20±2 nm diameter particles at three different particle mass concentrations of 1%, 3% and 5% are used as the working fluid. The experimental setup consists of the nanofluids in the hot stream and tap water in the cold stream. In addition, pressure drop across the heat exchanger inlet and outlet is also measured to estimate the flow performance of nanofluids. The results show a consistent increase in the total heat transfer coefficient of the heat exchanger for the nanofluids concentrations tested. However, the pressure drop in the hot (nanofluids) flow line also increases that effect can substantially limit the applicability of nanofluids in a PHE.Copyright

Effect of non-uniform, out-of-plane illumination, shear rate andparticle distribution on the accuracy of nPIV velocity measurement

AbstractNanoparticle image velocimetry (nPIV) uses evanescent-wave illumination to measure two velocity components, U and V, tangential to a wall in a region with thickness of the order of hundreds of nanometers. In this region the illumination intensity decays exponentially with distance normal to the wall, z, and hence tracers closer to the wall have ‘brighter’ and ‘bigger’ images than those that are further away, i.e. at larger z. Moreover, fluid velocity varies in this region with z and hence tracers at different distance from the wall move at different speeds. Furthermore, presence of the wall has a significant effect on particle distribution, and particle displacement due to local fluid velocity and Brownian displacement of particle tracers in this region. The variation in the displacement of particle images in this region, with different brightness and velocities, can bias the near-wall velocities obtained using standard correlation-based PIV method.Artificial nPIV images of nanoparticles in a flow...