RESEARCH INTERESTS
Thermal Stability Analytics of Lithium-ion Cells and Modules
State-of-Charge (SOC) Implications: A hierarchical TR modeling framework is developed to examine the TR behavior of Li-ion cells at various SOCs and probe its implications on the thermal runaway propagation (TRP) in a battery module. We perform accelerating rate calorimetry (ARC) experiments with 3.25 Ah cylindrical Li-ion cells at different SOCs and demonstrate the strong SOC dependence of TR characteristics. The thermo-kinetic parameters extracted from the ARC experiments are used to develop a TR model that captures the SOC-induced TR response in Li-ion cells. The mechanistic information from the cell-level model is used to examine the pathways for TRP in a battery module consisting of cells with uniform and imbalanced SOCs, thereby demonstrating the underlying role of SOC variability on the resulting TRP under abuse conditions. Please take a read here.
Thermal Runaway Propagation in Cell Modules: A hierarchical TRP analytics approach is developed, which includes cell-level thermokinetic and electrode crosstalk interactions derived from accelerating rate calorimetry characteristics of a representative high-energy 18650 cylindrical Li-ion cell with Ni-rich cathodes and Si–C anodes. The hierarchical TRP model, coupled with multimodal heat dissipation, demonstrated for an exemplar energy-dense Li-ion battery module configuration, determines TRP criticality at module level for a wide range of conditions, including ambient temperature, intercell spacing, trigger cell location, external heating power, and heat dissipation coefficients. Potential propagation pathways have been identified, and their underlying attributes in terms of propagation speed, heat release from exothermic reactions, critical thermal energy input, and heat dissipation to surroundings have been quantified. Please take a read here.
Design Optimization of Thermal Systems
Encapsulated Phase Change Material Heat Exchanger: The performance of a heat exchanger (HX) with encapsulated phase change material (EPCM) undergoing alternating melting and freezing is studied with the intent of analyzing the thermal behavior and optimizing its performance. The specific application of the EPCM HX is to cool the exiting hot water stream from the power plant steam condenser. The HX consists of a series of stacked flattened tubes with EPCM and arranged in lateral sections that are alternately exposed to orthogonal streams of the hot water and the ambient air. A systems-level model was developed to predict the quasi-steady state and transient HX performance. The HX capital cost is minimized using the genetic algorithm for a specific cooling duty and coefficient of performance (COP). Please take a read here.
Advanced Cooling Towers with Pressure Dehumidifying System: This study demonstrates an enhanced cooling tower technology (ECTT) under which the ambient air is precooled and dehumidified to improve the cooling tower efficiency. The reverse Brayton cycle, comprising a compressor, an air cooler, an expander, and a heat and mass exchanger, is used to precondition the ambient air before entering the cooling tower. A plant-scale thermodynamic model is constructed for ECTT, and the system parameters are optimized through genetic algorithm based on the maximum power gained from the system, including the effects of reducing the makeup water for the cooling tower. Compared to conventional cooling towers, the ECTT achieved sub-wet bulb cooling of water, which is 9.5°C below the wet bulb temperature of ambient air, reduces steam condensation temperature by 34%, and increases steam turbine power output by 4.3%. It also reduces evaporative losses in the cooling tower and reduces the makeup water by 36%. Please take a read here.
Computational Analytics of Falling Film Heat Exchangers
Hydrodynamics: Three-dimensional numerical simulations based on the Volume of Fluid method are conducted to investigate the hydrodynamic behavior of a falling film over horizontal round tubes with diameter and tube-to-tube spacings each of 16 mm. Using a mixture of water and ethylene glycol, the simulations comprise a range of flow rates (Re = 15 to 210) that covers all basic intertube flow modes i.e., droplet, jet (in-line and staggered), and sheet mode. InterFoam, a two-phase incompressible flow solver in OpenFOAM, has been used to simulate the flow field. The numerical results present the variation of the liquid film thickness and the film interface velocity over the tube surface for all the flow modes. A set of correlations have been presented to predict the film thickness and interfacial velocity over the tube surface for each of the studied modes of flow with reasonable accuracy. Please take a read here.
Sensible Heat Transfer: Numerical simulations are performed to explore the heat transfer characteristics of falling films over horizontal round tubes with uniform heat flux imposed for a range of Reynolds numbers spanning the droplet, jet, and sheet regimes. The study analyzes the local as well as average heat transfer behavior under the different flow modes. The numerical results show that the local Nusselt number (Nu) distribution depends on the flow features in each mode and varies substantially in all directions for the respective mode. The temperature distribution in the liquid film in each of the modes was examined to evaluate the mechanism of heat transfer process. This study also compares the local heat transfer coefficient distribution with the analytical heat transfer models derived to predict heat transfer performance over horizontal tube surfaces. Please take a read here.
Tube Surface Wettability: The primary aim of this numerical study is to characterize the influence of surface wettability on the film flow behavior and its associated surface heat transfer in the jet-flow mode. Surface wettability effects ranging from superhydrophilic to superhydrophobic are studied by varying the equilibrium contact angle from 2 deg to 175 deg. Two different liquid mass flow rates of 0.06 and 0.18 kg/m-s corresponding to the inline and staggered jet flow modes are studied. Results are presented in terms of the liquid film thickness, the contact areas between the different phases (solid–liquid and liquid–air), and the heat transfer coefficient or Nusselt number. The resistance imposed by the increasing contact angles inhibits the extent of the liquid spreading over the tube surface, and this, in turn, influences the liquid film thickness, and the wetted area of the tube surface. A significant decrement in the heat transfer rate from the tube surfaces was observed as the equilibrium contact angle increased from 2 deg to 175 deg. The local distributions of the Nusselt number over the tube surface are strongly influenced by the flow recirculation in the liquid bulk. Please take a read here.
Phase Change Heat Transfer: Numerical simulations are performed to investigate the interfacial heat and mass transfer from evaporating wavy falling liquid films in interaction with laminar gas streams. The OpenFOAM solver has been used to conduct the simulations where the liquid-gas interface is resolved using the Volume of Fluid method of solving for three phases, i.e., liquid, vapor, and air. The configuration considered is a falling liquid (water) film on a heated vertical plate with a confined laminar moist air (gas) flow that is either (a) co-current or (b) counter-current to the downward liquid flow. The evaporation at the liquid-gas interface is driven by the interfacial gradient of the vapor mass fraction. Interfacial waves are triggered using a monochromatic forcing disturbance that leads to sinusoidal or solitary waves forming at the liquid-gas interface under respective forcing frequencies. Correlations are proposed for predicting Sh under co-current and counter-current gas flow effects. Please take a read here.
Flow past bluff bodies
Unsteady flow past a square cylinder near a free surface: Two-dimensional flow past a square cylinder close to a free surface is studied numerically to investigate the effect of Froude numbers (0.0–0.6) and gap ratios (0.2–1.2) on the wake behavior and integral parameters. The surface deformation is found to increase with the increasing Froude number, which, in turn, produces substantial surface vorticity to cross-cancel the vorticity of the separated shear layer generated near the top surface of the cylinder. At high Froude numbers, merging of shed vortices in the cylinder downstream was encountered that possessed a period twice that in the formation region. For a fixed Froude number, with a decrease in the gap ratio, the amount of cross-cancellation between the top shear layer and the free surface increases. This enhanced interaction leads to diffusion of the vortices from the top shear layer to smaller downstream distances as the cylinder moves close to the free surface. Under such effects, the integral parameters (drag and lift coefficients and Strouhal number) are found to be a strong function of the Froude number and gap ratio. Please take a read here.