Current Research

LASER-induced controlled generation of single bubbles

admin — Thu, 28 Jun 2018 19:29:10 +0000

LASER-induced controlled generation of single bubbles

admin Thu, 06/28/2018 - 12:29

Research Synopsis

In this project, we implement a LASER-based technique to controllably generate bubbles on a heated surface. A numerical model is developed to capture this process of artificial nulceation and bubble growth. The objective is to enable a more focused study of bubble behavior in boiling for enhancing this phase change heat transfer process.

Research Description

Phase change processes such as evaporation, boiling, and condensation are extremely important heat transfer mechanisms in many industrial applications. Of these, boiling is of fundamental importance in industries such as power generation, chemical processing, desalination, and electronics thermal management. Although the physics of the boiling process has been a subject of active research for more than a century, many aspects of this complex phenomenon, which involves coupled fluid mechanic, heat transfer and phase change processes, remain to be understood. This is especially important due to active ongoing research looking at various new techniques to improve boiling heat transfer performance parameters such as the critical heat flux and the heat transfer coefficient.

Although the boiling performance parameters are intricately linked to the the mechanisms of bubble growth and departure (or the bubble ebullition process), achieving a proper understanding of bubble behavior is challenging, especially in the presence of boiling surface modifications. There are several reasons for this. First, bubble ebullition is a really fast as well as short-timescale process requiring the use of advanced imaging techniques. Second, the bubble behavior is dictated to a big extend by the behavior of the contact line microlayer, which is difficult to capture due to the small length scale involved. Third, it is quite difficult to achieve proper visualization of a single bubble under actual boiling circumstances due to the statistical nature of boiling. Since bubbles nucleate randomly on the boiling surface, it is difficult to predict and therefore focus on a single bubble properly, and further, the neighboring bubbles often block the view path in addition to affecting the overall behavior of the bubble in question.

To enable a more focused study of single bubbles in boiling, in this project we implement a LASER-based technique to controllably generate bubbles under normal boiling conditions. The technique employs a combination of timed laser pulses and electrical heating to nucleate well-defined bubbles on the boiling surface. The effect of laser characteristics such as pulse power, pulse width, and pulse frequency on bubble ebullition behavior will be studied. A numerical model will be developed to explain artificial bubble nucleation using laser-induced transient heating of the substrate in contact with the liquid.

Researchers

Dilipkumar Choudhary

Mario Panfilo

Effect of Electric Fields on Evaporation of Stationary and Impacting Drops

admin — Thu, 28 Jun 2018 16:55:28 +0000

Effect of Electric Fields on Evaporation of Stationary and Impacting Drops

admin Thu, 06/28/2018 - 09:55

Research Synopsis

In this project, we perform droplet evaporation experiments and conduct numerical and CFD simulation studies to study the effect of an applied electric field on the evaporation of stationary and impacting drops on a heated surface. The objective is to develop new techniques for enhancing phase change heat transfer in industrial applications.

Research Description

Phase change processes such as evaporation, boiling, and condensation are extremely important heat transfer mechanisms in many industrial applications. Further, they automatically occur in other natural and man-made processes. The physics at the liquid-vapor interface and at the solid-liquid-vapor three-phase contact line is of primary importance in all of these processes. Although the rate of evaporation from or condensation to a liquid-vapor interface depends on the interfacial gas kinetics, it is primarily limited by the rate of heat transfer in the liquid thin film adjacent to the three-phase contact line. Further, the ultrafast transient nature of bubble (in boiling) or drop (in condensation) growth makes the dynamics of the contact line an extremely important factor in ultimately determining the values of different phase change heat transfer performance parameters. For example, the value of the critical heat flux (CHF) for liquid boiling on a hot solid surface is now widely believed to depend, among other factors, on the timely rewetting of a hot dry spot on the boiling surface by the surrounding liquid. Since this rewetting is achieved by an advancing three-phase contact line, the dynamic flow and thermal behavior of this region takes on increased importance. The phase change heat transfer performance parameters such as the critical heat flux (in boiling) and the heat transfer coefficient (in boiling and condensation) can potentially be enhanced by actively manipulating the dynamic flow and thermal behavior of the three-phase contact line and the surrounding region. In this project, we employ experimental and computational techniques to explore the effect of an applied high electric field on the behavior of a liquid drop impacting a hot solid surface. High-speed optical and infrared imaging techniques are employed for visualizing the flow and measuring the temperatures, respectively. An electromagnetic model is developed to explain the effect of the electric field on the liquid-vapor interface adjacent to the contact line. The results and insights obtained in this project can have a significant impact on several high-heat-flux heat transfer and thermal management technologies being used in industry and research.

Researchers

Abhishek Basavanna

Juan Estaban Viana

Effect of Phase Change Materials on Transient Heating of Impacting Drops

admin — Sat, 09 Jun 2018 21:41:55 +0000

Effect of Phase Change Materials on Transient Heating of Impacting Drops

admin Sat, 06/09/2018 - 14:41

Research Synopsis

In this project, we perform drop-impact experiments and conduct numerical and CFD simulations to explore the effect of a Phase Change Material (PCM) on the transient heating of a cold liquid drop impacting a heated substrate. The objective is to develop new techniques for enhancement of phase change heat transfer in industrial applications.

Research Description

The physics of the transient behavior of liquid drops impacting hot or cold surfaces are of significance in many different applications such as spray cooling, aircraft icing, etc. Further, the transient heating and cooling of vapor spots and liquid patches is of significance in determining the heat transfer performance parameters in phase change processes such as boiling and condensation. The thermal transients in all these processes are primarily dictated by the passive thermal properties of the solid substrate (e.g. thermal conductivity, specific heat) and by the flow conditions. An active control (or manipulation) of these ultra-fast thermal transients could provide a means to enhance the performance parameters in various phase change-based heat transfer processes. In this project, we use experimental and computational techniques to explore the effect of a solid-liquid phase change material (PCM) on the thermal characteristics of a liquid drop impacting a hot surface. High-speed optical and infrared imaging techniques are employed for visualizing the flow and measuring the temperatures, respectively. A numerical model is being developed to explain the effect of the PCM on the thermal transients in the liquid drop and the solid substrate. The insights obtained from these findings can have a significant impact on several technologies in the areas of phase change-based heat transfer and thermal management.

Researchers

Abdul Ahad Khan