Abhijit Mukherjee

Experimental study of heat transfer in an evaporating meniscus on a moving heated surface

A stable meniscus is formed by a circular nozzle dispensing water over a heated circular face of a rotating cylindrical copper block. The nozzle is offset from the axis of rotation of the copper block and thus a moving meniscus is formed on the surface. The water flow rate, heater surface temperature, and the speed of rotation are controlled to provide a stable meniscus with continuous evaporation of water without any meniscus breakup. The study provides an important insight into the role of the evaporating liquid-vapor interface and transient heat conduction around a nucleating bubble in pool boiling

Flow Boiling Dynamics of Water and Nanofluids in a Single Microchannel at Different Heat Fluxes

The preferable cooling solution for micro-electronic systems could be forced flow boiling in micro heat exchangers. Nanoparticle deposition affects nucleate boiling via alteration of surface roughness, capillary wicking, wettability, and nucleation site density. In this study, flow boiling was investigated using water and nanofluids in a single rectangular microchannel at different heat fluxes. The observed change in flow regime transition revealed the effect of nanoparticles on the onset of nucleate boiling (ONB) and the onset of bubble elongation (OBE). The addition of nanoparticles was found to stabilize bubble nucleation and growth and increase heat transfer in the thin film regions.

Experimental Investigation of Nanofluid Flow Boiling in a Single Microchannel

The trends of decrease in size and increase in power dissipation for micro-electronic systems present a significant challenge for thermal management of modern electronics. The preferable cooling solution could be micro heat exchangers based on forced flow boiling. Nanoparticle deposition can affect nucleate boiling heat transfer coefficient via alteration of surface thermal conductivity, roughness, capillary wicking, wettability, and nucleation site density. It can also affect heat transfer by changing bubble departure diameter, bubble departure frequency, and the evaporation of the micro and macrolayer beneath the growing bubbles. In this study, flow boiling was investigated for 0.001 vol% aluminum oxide nanofluids in a brass microchannel and compared to results for regular water. For the case of nanofluid flow boiling, high speed images were taken after boiling durations of 25, 75, 125, and 150 min. Bubble growth rates were measured and compared for each case. Flow regime oscillation was observed and regime duration was split into two periods: single-phase liquid and two-phase. The change in regime timing revealed the effect of nanoparticle suspension and deposition on the Onset of Nucelate Boiling (ONB) and the Onset of Bubble Elongation (OBE). The addition of nanoparticles was shown to stabilize bubble growth as well as the transition of flow regimes between liquid, two-phase, and vapor.Copyright

Numerical Simulation of Bubble Growth During Nanofluid Flow Boiling in a Microchannel

In recent years, heat dissipation in micro-electronic systems has become a significant design limitation for many component manufactures. As electronic devices become smaller, the amount of heat generation per unit area increases significantly. Current heat dissipation systems have implemented forced convection with both air and fluid media. However, nanofluids may present an advantageous and ideal cooling solution. In the present study, a model has been developed to estimate the enhancement of the heat transfer when nanoparticles are added to a base fluid, in a single microchannel. The model assumes a homogeneous nanofluid mixture, with thermo-physical properties based on previous experimental and simulation based data. The effect of nanofluid concentration on the dynamics of the bubble has been simulated. The results show the change in bubble contact angles due to deposition of the nanoparticles has more effect on the wall heat transfer compared to the effect of thermo-physical properties change by using nanofluid.Copyright

Experimental Study of Forced Convection Heat Transfer of Water in a Short Microduct at a Turbulent Reynolds Number

The present study deals with experimental investigation of cooling of machining tools, by water flowing through a microduct at the tip of the tool. The average diameter of the microduct is 200 μm and the flow takes place at a turbulent Reynolds number. The outer wall temperature of the tool and the temperature of water at inlet and exit have been measured. The convective heat transfer coefficient is calculated at different wall temperatures and mass flux. The experimental results show that the average Nusselt numbers in the short microduct are higher than those predicted by conventional correlations for large-diameter ducts. This enhancement may be attributed to the micro size of the duct, entry effects, transition from laminar to turbulent flow at the microduct entrance, suspended microscopic particles in coolant water, and Prandtl number estimation based on the mean fluid temperature. A correlation has been proposed to compute convective heat transfer during turbulent flow through a short microduct of a particular geometry for a range of Reynolds and Prandtl numbers.

Preliminary Study on the Heat Transfer and Fluid Flow in Formed, Lap-Wound End Windings of Electric Machinery

Thermal analysis is a critical function in the design of electric machinery. While the core design discipline is electro-magnetics, other classic mechanical engineering expertise is required to create state of the art electric machines. Various components of electric machines present the thermal design engineer with obvious applications of classic, well-known solutions. These would include simultaneous thermal and hydro-dynamically developing flow when considering the long, narrow cooling ducts placed between stator laminations. However, the cooling of end windings of a formed, lap-wound electric machine is more challenging. This area features insulated copper coils that extend out of the stator or rotor and return to another section of the machine to complete the loop of a single coil. Effective thermal and flow analysis of this basket-like shape does not easily lend itself to well-known solutions. The present study explores the current literature of this type of machine end-windings as related to thermal and flow solutions. Simple correlations are proposed to aid machine designers that would accelerate the design process. These correlations can be used in thermal and fluid network programs to quickly predict flows and temperatures in a machine. Recent work to characterize heat transfer performance from the pressure drop in heat exchangers using the Generalized Leveque Equation can be of particular value for this effort. Finally, simple Computational Fluid Dynamics (CFD) analysis is presented for a simple geometry similar to a single, isolated stator bar.Copyright

Experimental Study of Forced Convection Heat Transfer of Water in a Short Micro Duct at Turbulent Reynolds Number

The present study deals with experimental investigation of cooling of machining tools, by water flowing through a microduct at the tip of the tool. The microduct is of diameter of around 200μm and the flow takes place at turbulent Reynolds number. The outer wall temperature of microduct and the temperature of water at inlet and exit have been measured. The convective heat transfer coefficient is calculated at different wall temperatures and varying liquid mass flux. The experimental results shows that the average Nusselt numbers for the short micro-duct are higher than those predicted by conventional correlations for large diameter ducts. A correlation has been proposed to compute convective heat transfer during turbulent flow through a short microduct of a particular geometry for a range of Reynolds and Prandtl numbers.Copyright

An Experimental Investigation on the Effects of Heat and Mass Flux on Vapor Bubble Growth Characteristics in a Microchannel

Micro heat exchangers are emerging as one of the most effective cooling technologies for high power-density applications. The design of micro heat exchangers is complicated by the presence of alternating flow regimes, which give way to flow boiling instability. Bubble formation inside microchannels can be correlated directly to flow boiling instability and can regulate flow characteristics and wall heat transfer when the bubbles grow to reach the microchannel hydraulic diameter. In this study, the growth of vapor bubbles in a single microchannel was examined using an experimental setup capable of measuring coolant flow rate, inlet and outlet liquid temperatures, and channel wall surface temperature. Liquid flow rate and wall heat flux were systematically varied while a high-speed camera was used to capture images of vapor bubbles forming in the channel. These images were used to compare bubble growth rates for a constant flow rate. The results provide fundamental understanding of the bubble growth process.Copyright

Molecular Dynamics Simulation of Homogeneous Nucleation in a Lennard Jones Liquid

Examination of metastable states of fluids provides important information pertinent to cavitation and homogeneous nucleation. Homogeneous nucleation, in particular, is an important topic of research. Molecular Dynamics simulation is a well-endorsed method to simulate metastabilitites, as they are limited to mesoscopic scales of length and time and this life-time is essentially zero on a laboratory time scale. In the present study, a molecular dynamics code has been used in conjunction with MOLDY to investigate phase change in a Lennard-Jones liquid. The Lennard-Jones atoms were subjected to different temperatures at various number densities and the pressure was recorded for each case. The appearance of a change of phase is characterized by the formation of clusters or formation of voids as described by the radial distribution function.Copyright