Porous Media Flow for Phase-Change Heat Transfer
Liquid flow through micro/nanoscale natural and engineered porous materials has been widely applied to enhance liquid-vapor phase change heat transfer in diverse energy systems by leveraging the contact between the liquid/vapor and the solid. We study the fluid flow through porous media in different phase change conditions (i.e. evaporation and condensation) with numerical modeling, CFD simulation and experimental validation to predict the dryout heat flux in thin-film evaporation and maximum heat transfer coefficient enhancement in filmwise/dropwise condensation. The novel mode of condensation (i.e. capillary-enhanced filmwise mode) we proposed exhibits excellent enhancement in heat transfer performance and long-term robustness for condensation; Our modeling framework for porous media flow optimizes the design of the porous structure and serves as a general guideline for modeling steady liquid-vapor phase change processes. This work can potentially be applied to energy conversion, chemical processing, geological cycles and energy resource exploration and recovery.
1. Wang, R. and Antao, D.S., 2018. Capillary-enhanced filmwise condensation in porous media. Langmuir, 34(46), pp.13855-13863. [Link]
2. Wang, R., Jakhar, K. and Antao, D.S., 2019. Unified Modeling Framework for Thin-Film Evaporation from Micropillar Arrays Capturing Local Interfacial Effects. Langmuir, 35(40), pp.12927-12935. [Link]
Developing Robust Low Surface Energy Coatings for Enhanced Condensation
Dropwise condensation has been demonstrated to have better heat transfer performance due to faster condensate removal (i.e., droplets shedding), as compared to the traditional filmwise mode. Researchers have worked on developing distinctive thin low surface energy coatings (e.g., self-assembled monolayers or polymers) with new materials/chemistry to promote dropwise condensation with minimum coating thermal resistance for over half a century. However, all thin coatings degrade/fail within hours during condensation of water vapor, which is a major challenge and limitation for industrial applications, and the degradation mechanism remains unknown. Our work focuses on developing robust low surface energy coatings on typical metal oxide condenser materials, such as silicon and copper, for enhanced condensation heat transfer applications through understanding the mechanism of the coating degradation. Coatings deposited by our controlled synthesis protocol exhibit a higher coating conformality compared to traditional deposition techniques, and demonstrate lower contact angle hysteresis and better durability when condensing water vapor in the dropwise mode. This work provides durable solutions for industrial applications with enhanced heat transfer performance in the fields of power generation and conversion, water harvesting/desalination, and electronics thermal management.
Characterization of the Temperature Dependence of Interfacial Phenomenon and Fluid-Solid Interactions
We are inspecting the temperature dependence of interfacial interactions between fluids or fluids and solids which can result in a tailored process design for efficient energy and resource utilization in energy conversion processes and water treatment. Prediction of these interactions requires us to measure surface free energy of liquids and solid surfaces by measurement of interfacial tension and contact angle respectively. Through the proposed research effort, we seek to build a comprehensive database for temperature dependent data of liquid-liquid and liquid-solid interfacial tension and characterization of interfacial interaction dynamics.
Non-Equilibrium Plasma Discharges for Phase-Change Heat Transfer Applications
We are studying the non-equilibrium plasma discharges (specifically negative corona discharges) for potential application as (1) a probe for metrology, (2) an enhancement technique, in liquid-vapor phase-change heat transfer systems, and (3) surface modification.
Plasma is an ionized gas containing free electric charges – both electrons and ions, making them electrically conductive and strongly responsive to electromagnetic fields. A plasma can produce highly energetic and chemically active species (e.g., ions, electrons, radicals and excited states) and can be in thermodynamic non-equilibrium, forming high concentrations of the chemically active species at low bulk temperatures (as low as room temperature). The temperature in plasmas (similar to gases) is determined by the average energies of the plasma particles (neutral and charged, Ti for ions and Te for electrons) and their relevant degrees of freedom (translational (To), rotational (Tr) and vibrational (Tv)). Based on these temperatures/modes of internal energy, plasma discharges are broadly classified into thermal and non-thermal discharges. The electron temperature (Te) is usually higher than that of heavy particles (To), and subsequent collision of electrons with heavy particles can equilibrate their temperatures. The quasi-equilibrium plasma where the temperature of the electrons and heavy particles approach each other (Te ≈ Tv ≈ Tr ≈ Ti ≈ To) is usually called a thermal plasma. If time and energy are not sufficient for the equilibration of Te and To, ionization and chemical processes in such non-equilibrium plasmas are directly determined by the electron and vibrational temperatures and, therefore are not sensitive to thermal processes and temperature of the gas. This non-equilibrium plasma is usually called a non-thermal plasma (Te > Tv > Tr ≈ Ti ≈ To) with Te about 1 eV (~10,000 K) and the bulk gas temperature close to the ambient temperature.
Corona discharges are characterized with a non-uniform strong electrical field, ionization and luminosity around an electrode. The electrode is usually a sharp point or edge. Corona discharges require high voltage for ignition around a single electrode, the rest of the discharge gap is dark with a low electric field. If the voltage grows higher the remaining dark part of the discharge gap also breaks down and the corona is transformed into sparks. The mechanism for sustaining the continuous ionization level in a corona discharge depends on the polarity of the electrode where the high electric field is located. A corona discharge with a high electric field zone is concentrated around the anode, the discharge is referred as the positive corona. Conversely, the negative corona has a high electric field zone around the cathode. The ability of corona discharges to generate a high concentration of active species at atmospheric pressure without heating up the gas volume have made them appealing to different modern industrial applications.
Condensation heat transfer plays an important role in a variety of high-efficiency heat exchangers, such as heat pipes, and refrigeration and air conditioning radiators. A small amount of non-condensable gas (NCG) added to pure vapor can greatly affect the condensation heat transfer coefficient. Studies show that the presence of NCG plays a large (detrimental) role by reducing the heat transfer coefficient in liquid-vapor phase-change heat transfer processes. While most phase-change heat transfer studies attempt to reduce the presence of NCGs in the test systems, accurate in-situ quantification capabilities for these NCG species do not exist. NCG’s are traditionally measured using pressure gauges and this limits the measurement of species concentration (NCGs) to ~4000 ppm. Non-thermal pulsed corona discharges and optical emission spectroscopy can also be leveraged to detect species in a fluid.
DC negative corona discharges in the air have been observed to have a “diffuse glow” near the anode, however, the mechanism for its existence is not well understood. Temperature measured employing optical emission spectroscopy of the diffuse glow region showed lower rotational temperatures close to the ambient temperature. The low rotational temperature (potentially lower in pulsed excitation modes) makes the discharge attractive for non-contact temperature metrology at and near the anode surface with potential applications in phase change heat transfer.
Spectroscopic temperature measurements from the literature indicate higher vibrational temperature in the diffuse glow region of DC negative corona discharges indicating the presence of active species (or higher chemical activity due to higher vibrational temperatures in this region) which can be used for surface modification. Hence corona discharges can change the surface morphology of materials, and the originally smooth surface can become rough. Additionally, the wettability of surfaces can be modified by treating these surfaces with corona discharges and subsequent treatment with corona discharges also improves the adhesion of thin films.
Practical Design of a High-Voltage Pulsed Power Supply for Downhole Plasma Enhanced Geothermal Drilling
Energy demand worldwide has grown by 14% over the last decade driven by increasing industrialization and stronger heating and cooling needs in some regions. At a time when society is becoming increasingly aware of the declining reserves of fossil fuels along with environmental issues such as fossil-fuel-sourced greenhouse gas (GHG) emissions, use of renewable and sustainable energy sources is the appropriate and applicable choice. Geothermal energy is one form of sustainable source, which has certain advantages such as consistency, a vast amount of untapped potential, availability, and a wide range of possible applications that make it an interesting and viable solution for helping meet the world’s energy needs while reducing GHG emissions (especially CO2). Geothermal energy is a renewable resource, however, most geothermal drilling is done at depths near or below the transition zone, where the reservoir fluid remains in the supercritical conditions, i.e., at a temperature above 374oC and pressure around 221 bar, and drilling is done most often through hard rocks rather than through the softer, sedimentary rocks of petroleum-bearing formations. The harder rock lithology decreases the rate at which drilling occurs and increases the wear and tear on drilling tools. These factors add up to a lot of time and money that could be spent getting the resource sooner. In order to make geothermal drilling more efficient and cost-effective, new technologies need to be developed.
Our concept focuses on adapting the drill bits used in geothermal drilling to enable the bit to operate efficiently and over longer periods of time. We weaken the rock formation before the drill bit makes contact with it by locally directing high energy shockwaves at the rock formation using electrically induced microsecond plasma discharges in liquids. The shockwave results in microscale fractures in the rock formation, and the resulting lower compressive strength lithology is easier to drill. This lower drilling work requirement leads to reduced downtime for maintenance and bit replacements, as well as an increased Rate of Penetration (ROP).
The concept proposes to generate the required electrical energy downhole such that electrical power does not need to be supplied from the surface. To generate the microsecond high energy pulsed plasma discharge, our concept uses existing components of a drilling bottom hole assembly (BHA) such as the mud motor and alternator to generate power in situ. This project focuses on the design, assembly and characterization of the major electrical circuit required to generate the electrically induced microsecond high energy plasma discharge. To effectively fracture the rocks, the electrical requirements for plasma discharge are a microsecond time scale pulse with nanosecond time scale rise time, a direct current (dc) voltage of 40 kV and an electrical energy of 80 J per pulse. The major components of the overall circuit are a battery bank, an inverter, a step-up transformer (i.e., an alternating current (ac) power source), a rectifier (i.e., a Cockcroft-Walton generator or CWG) and a distributor. The CWG consists of voltage multiplier ladder network of capacitors and diodes to generate high voltage. We have assembled a single spark gap setup to test the circuit and we will characterize the CWG at two different conditions. The first condition will be at atmospheric pressure and room temperature (at different input ac power frequencies), and the second will be at atmospheric pressure and high temperature (~250 °C). The culmination of the project will be a characterization of the performance of the 80 J per pulse rectification (CWG) circuit at geothermal-relevant conditions.