Performance Testing and Modeling of Printed Circuit Heat Exchangers for Advanced Nuclear Reactor Applications

The primary mission of very-high-temperature reactors (VHTRs) is to generate electricity and provide high-temperature process heat for industrial applications with high efficiency, which relies on an effective intermediate heat exchanger (IHX) that transfers heat from the primary fluid (i.e., helium) to a secondary fluid. A printed circuit heat exchanger (PCHE) is one of the leading IHX candidates to be employed in VHTRs due to its compactness and capability for high-temperature, high-pressure applications with high effectiveness. In this study, the thermal-hydraulic performance of a fabricated zigzag-channel PCHE was investigated. New pressure drop and heat transfer correlations were developed based on the experimental data. Local thermal-hydraulic performance of the PCHE indicated that fully-developed flow conditions were never achieved, which was attributed to the wavy nature of zigzag flow channels as well as the large temperature variations along the flow direction. This study concluded that heat transfer discrepancies between the hot side and cold side were caused by the differences in both thermal boundary conditions and thermophysical properties. Several effects on the PCHE’s thermal-hydraulic performance were also studied, including the fluid and solid thermophysical properties, radiuses of curvature at zigzag bends, channel configurations, channel pitch lengths in the fluid flow direction, and zigzag pitch angles. It was found that the mean pressure loss factors and mean Nusselt numbers in zigzag channels with sharp bends were 4–6% larger than those in zigzag channels with bends with a curvature radius of 4 mm. A multi-objective optimization for the PCHE’s geometry was conducted based on the numerically obtained correlations. A total number of 142 points on the Pareto front were obtained. Users can select the optimal geometrical parameters for the zigzag-channel PCHE designs from the obtained Pareto front based on their needs. In addition, the stress field of a simplified three-dimensional geometry was obtained. It was observed that the highest stress occurred at diffusion-bonded interfaces and that it was less than the maximum allowable stress of the structural material. Furthermore, a computer code was developed to predict both the steady-state and transient behaviors of both straight-channel and zigzag-channel PCHEs. Comparisons of the numerical results with the experimental data indicated that the dynamic model was successful in predicting the experimental transient scenarios. The numerical results could also provide useful insight on control strategy development for an integrated high-temperature reactor system with process heat applications, for instance, using the helium mass flow rates or helium inlet temperatures variations to adjust the heat exchanger effectiveness and heat transfer rate. Finally, the heat exchanger model was implemented into a system dynamic code to simulate the transient behavior of a 20-MWth Fluoride-salt-cooled High-temperature Reactor (FHR). Results were obtained for three initiating events: a positive reactivity insertion, a step increase of the helium flow rate, and a step increase of the helium inlet temperature to the secondary heat exchanger (SHX). The results demonstrated that the FHR reactor, for the three transient scenarios analyzed, had inherent safety features. The results also showed that the intermediate loop consisting of the IHX and SHX played a significant role in the transient progression of the integral system. This study provides critical insights into the thermal-hydraulic performance of PCHEs that can be applied to nuclear power, offshore industry, solar power, dual cycles for process heat applications, and cooling of electronics and fuel cells.PHDNuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/143945/1/chenum_1.pd

Two Photon Transition Form Factor of cˉc\bar{c}c Quarkonia

The two photon transition of $\bar{c}c$ quarkonia are studied within a covariant approach based on the consistent truncation scheme of the quantum chromodynamics Dyson-Schwinger equation for the quark propagator and the Bethe--Salpeter equation for the mesons. We find the decay widths of $\eta_{c}^{} \to \gamma\gamma$ and $\chi_{c0,2}^{} \to \gamma\gamma$ in good agreement with experimental data. The obtained transition form factor of $\eta_{c}^{} \to \gamma\gamma^{\ast}$ for a wide range of space-like photon momentum transfer squared is also in agreement with the experimental findings of the BABAR experiment. As a by-product, the decay widths of $\eta_{b}^{},\chi_{b0,2}^{} \to \gamma\gamma$ and the transition form factor of $\eta_{b}^{}, \chi_{c0,b0}^{} \to\gamma\gamma^{\ast}$ are predicted, which await for experimental test

BERT with History Answer Embedding for Conversational Question Answering

Conversational search is an emerging topic in the information retrieval community. One of the major challenges to multi-turn conversational search is to model the conversation history to answer the current question. Existing methods either prepend history turns to the current question or use complicated attention mechanisms to model the history. We propose a conceptually simple yet highly effective approach referred to as history answer embedding. It enables seamless integration of conversation history into a conversational question answering (ConvQA) model built on BERT (Bidirectional Encoder Representations from Transformers). We first explain our view that ConvQA is a simplified but concrete setting of conversational search, and then we provide a general framework to solve ConvQA. We further demonstrate the effectiveness of our approach under this framework. Finally, we analyze the impact of different numbers of history turns under different settings to provide new insights into conversation history modeling in ConvQA.Comment: Accepted to SIGIR 2019 as a short pape

Pre-seismic ionospheric anomalies detected before the 2016 Taiwan earthquake

On Feb. 5 2016 (UTC), an earthquake with moment magnitude 6.4 occurred in southern Taiwan, known as the 2016 (Southern) Taiwan earthquake. In this study, evidences of seismic earthquake precursors for this earthquake event are investigated. Results show that ionospheric anomalies in Total Electric Content (TEC) can be observed before the earthquake. These anomalies were obtained by processing TEC data, where such TEC data are calculated from phase delays of signals observed at densely arranged ground-based stations in Taiwan for Global Navigation Satellite Systems. This shows that such anomalies were detected within 1 hour before the event

AutoML from Software Engineering Perspective: Landscapes and Challenges

Machine learning (ML) has been widely adopted in modern software, but the manual configuration of ML (e.g., hyper-parameter configuration) poses a significant challenge to software developers. Therefore, automated ML (AutoML), which seeks the optimal configuration of ML automatically, has received increasing attention from the software engineering community. However, to date, there is no comprehensive understanding of how AutoML is used by developers and what challenges developers encounter in using AutoML for software development. To fill this knowledge gap, we conduct the first study on understanding the use and challenges of AutoML from software developers’ perspective. We collect and analyze 1,554 AutoML downstream repositories, 769 AutoML-related Stack Overflow questions, and 1,437 relevant GitHub issues. The results suggest the increasing popularity of AutoML in a wide range of topics, but also the lack of relevant expertise. We manually identify specific challenges faced by developers for AutoML-enabled software. Based on the results, we derive a series of implications for AutoML framework selection, framework development, and research