Integrated modeling and validation for phase change with natural convection

Water-ice systems undergoing melting develop complex spatio-temporal interface dynamics and a non-trivial temperature field. In this contribution, we present computational aspects of a recently conducted validation study that aims at investigating the role of natural convection for cryo-interface dynamics of water-ice. We will present a fixed grid model known as the enthalpy porosity method. It is based on introducing a phase field and employs mixture theory. The resulting PDEs are solved using a finite volume discretization. The second part is devoted to experiments that have been conducted for model validation. The evolving water-ice interface is tracked based on optical images that shows both the water and the ice phase. To segment the phases, we use a binary Mumford Shah method, which yields a piece-wise constant approximation of the imaging data. Its jump set is the reconstruction of the measured phase interface. Our combined simulation and segmentation effort finally enables us to compare the modeled and measured phase interfaces continuously. We conclude with a discussion of our findings

Closed-loop control of product properties in metal forming

Metal forming processes operate in conditions of uncertainty due to parameter variation and imperfect understanding. This uncertainty leads to a degradation of product properties from customer specifications, which can be reduced by the use of closed-loop control. A framework of analysis is presented for understanding closed-loop control in metal forming, allowing an assessment of current and future developments in actuators, sensors and models. This leads to a survey of current and emerging applications across a broad spectrum of metal forming processes, and a discussion of likely developments.Engineering and Physical Sciences Research Council (Grant ID: EP/K018108/1)This is the final version of the article. It first appeared from Elsevier via https://doi.org/10.1016/j.cirp.2016.06.00

Numerical modelling and experimental validation of the effect of ultrasonic melt treatment in a direct-chill cast AA6008 alloy billet

In this work, we study how ultrasonic cavitation melt treatment (UST) affects the temperature distribution, sump profile, and resulting microstructure in the direct-chill (DC) casting of an AA6008 aluminum alloy. Two 152 mm diameter billets were cast; one was treated with UST (UST-DC casting) in the hot top while the other was not (conventional DC casting). To investigate the temperature distribution, temperature was measured at multiple points in both billets. The sump profile was visualized by pouring Zn into the sump during casting. The microstructure was analyzed by measuring the grain size of as-cast billets. A numerical model of DC casting and UST-DC casting has been validated with the temperature measurements across the billets, and the experimental results agrees well with the numerical model. It is found that the sump profile quantification with thermocouple measurements is more accurate and less prone to interpretation than with Zn tracing. Numerical simulation results show that UST application in the hot top with sonotrode position at 20 mm above the graphite ring level depresses the liquidus isotherm but does not affect the solidus isotherm, resulting in a thinner transition region compared with conventional DC casting. Grain structure analysis verifies that structure refinement with UST has been achieved at the given sonotrode position. The strongest grain refinement was at the center of the billet with the average grain size 50% smaller than that without UST. The results are discussed in terms of the known mechanisms of UST, i.e. dendrite fragmentation and deagglomeration of nucleating substrates

Continuum modeling and experimentation for solid-liquid phase change in binary systems with natural and mixed convection

Semi-empirical laws and microscopic descriptions of transport behavior have been integrated with principles of classical mixture theory to obtain a set of continuum conservation equations for binary, solid-liquid phase change systems. A widely accepted finite-difference scheme has been extended to accommodate the multiconstituent phase change process and used to solve the continuum equations. Numerical calculations have been performed for a binary aqueous ammonium chloride solution in both open and closed configurations and supplemented by bench scale experimental verification and comparison with previous qualitative experimental results. Advective transport of water enriched interdendritic fluids across the permeable liquidus interface has been identified as the primary mechanism for macroscopic species redistribution. The extent of this penetration is governed by the relative strengths of solutally driven mushy region flows and flows established in the bulk fluid by either natural or externally induced means. The redistribution of species which accompanies the mushy region outflow has been shown to significantly affect subsequent phase change behavior, contributing to localized growth rate variations, the formation of channel segregates, and the establishment of irregular liquidus front morphologies

Infrastructure Project Prioritization in Theory and Practice

The 2011 congressional ban on earmarks for infrastructure projects formally transferred responsibility for prioritizing federal infrastructure investments to the executive branch, and has redoubled the importance of how, exactly, the federal government evaluates and selects infrastructure projects that will receive federal funding. The Benefit-Cost Analysis (BCA) study is one such method of evaluating and prioritizing infrastructure projects or other policy alternatives which has been widely studied in literature and largely adopted by U.S. federal agencies. Despite their renewed and significant impact on the selection of infrastructure projects, however, the use and applications of BCAs in the U.S. varies significantly between sectors, agencies and levels of government. In this paper, we review the BCA and other project prioritization policies in U.S. federal agencies and compare them with other, international programs in the comparable economies of Australia and Canada