skip to main content

Search for: All records

Creators/Authors contains: "Najafi, Ahmad"

Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher. Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?

Some links on this page may take you to non-federal websites. Their policies may differ from this site.

  1. Abstract

    Microvascular materials containing internal microchannels are able to achieve multi-functionality by flowing different fluids through vasculature. Active cooling is one application to protect structural components and devices from thermal overload, which is critical to modern technology including electric vehicle battery packaging and solar panels on space probes. Creating thermally efficient vascular network designs requires state-of-the-art computational tools. Prior optimization schemes have only considered steady-state cooling, rendering a knowledge gap for time-varying heat transfer behavior. In this study, a transient topology optimization framework is presented to maximize the active-cooling performance and mitigate computational cost. Here, we optimize the channel layout so that coolant flowing within the vascular network can remove heat quickly and also provide a lower steady-state temperature. An objective function for this new transient formulation is proposed that minimizes the area beneath the average temperature versus time curve to simultaneously reduce the temperature and cooling time. The thermal response of the system is obtained through a transient Geometric Reduced Order Finite Element Model (GRO-FEM). The model is verified via a conjugate heat transfer simulation in commercial software and validated by an active-cooling experiment conducted on a 3D-printed microvascular metal. A transient sensitivity analysis is derived to provide the optimizer with analytical gradients of the objective function for further computational efficiency. Example problems are solved demonstrating the method’s ability to enhance cooling performance along with a comparison of transient versus steady-state optimization results. In this comparison, both the steady-state and transient frameworks delivered different designs with similar performance characteristics for the problems considered in this study. This latest computational framework provides a new thermal regulation toolbox for microvascular material designers.

    more » « less
  2. Free, publicly-accessible full text available December 1, 2024
  3. Free, publicly-accessible full text available November 1, 2024
  4. Van Mullem, T. ; De Belie, N. ; Ferrara, L. ; Gruyaert, E. ; Van Tittelboom, K. (Ed.)

    The goal of this research is to develop innovative damage-responsive bacterial-based self-healing fibers (hereafter called BioFiber) that can be incorporated into concrete to enable two functionalities simultaneously: (1) crack bridging functionality to control crack growth and (2) crack healing functionality when a crack occurs. The BioFiber is comprised of a load-bearing core fiber, a sheath of bacteria-laden hydrogel, and an outer impermeable strain-responsive shell coating. An instant soaking manufacturing process was used with multiple reservoirs containing bacteria-laden, hydrophilic prepolymer and crosslinking reagents to develop BioFiber. Sodium-alginate was used as a prepolymer to produce calcium-alginate hydrogel via ionic crosslinking on the core fiber. The dormant bacteria (spore) ofLysinibacillus sphaericuswas incorporated in hydrogel as a self-healing agent. Then, an impermeable polymeric coating was applied to the hydrogel-coated core fibers. The impermeable strain-responsive shell coating material was manufactured using the polymer blend of polystyrene and polylactic acid. The high swelling capacity of calcium-alginate provides the water required for the microbially induced calcium carbonate precipitation (MICP) chemical pathway, i.e., ureolysis in this study. The strain-responsive impermeable coating provides adequate flexibility during concrete casting to protect the spores and alginate before cracking and sufficient stress-strain behavior to grant damage-responsiveness upon crack occurrence to activate MICP. To evaluate the behavior of developed BioFiber, the swelling capacity of the hydrogel, the impermeability of shell coating, the spore casting survivability, and MICP activities were investigated.

    more » « less
  5. Van Mullem, T. ; De Belie, N. ; Ferrara, L. ; Gruyaert, E. ; Van Tittelboom, K. (Ed.)

    Vegetative cells used for the concrete bio self-healing process often face threatening environmental conditions such as extreme temperature, pH, salinity, shear stress, and starvation during the hardening process and the service life of the concrete. These conditions can eventually lead to cell death. Since endospores are likely to remain dormant for prolonged periods and can survive, germinate, and grow under inhospitable conditions, they are a suitable bacterial phenotype to introduce into concrete for microbial-inducing calcite precipitation. This study investigated how different endosporulation methods affect the endosporulation ratio (i.e., the fraction of vegetative cells that are converted to endospores during endosporulation), as well as the germination ratio (i.e., the fraction of endospores that are converted to vegetative cells following germination) and the microbial-induced calcite precipitation (MICP) performance of germinated endospores after facing harsh conditions of concrete, specifically, freeze and that cycling. Results from this study show that thermal shock followed by cell incubation in alkaline conditions leads to increased sporulation and germination ratios. It was also observed that freeze and thaw cycling had negligible effects on calcite production by endospores, while exposure of vegetative cells to these harsh conditions led to not only less biomass and calcite production but also to a lower mass of calcite produced per mass of cells, as determined by thermogravimetric analysis (TGA). The results from this study provide key insights into improving methods for endosporulation and germination to effectively use them for bio self-healing applications in concrete.

    more » « less