Numerical Investigation on the Flow and Heat Transfer Characteristics of Supercritical Liquefied Natural Gas in an Airfoil Fin Printed Circuit Heat Exchanger

As a new kind of highly compact and efficient micro-channel heat exchanger, the printed circuit heat exchanger (PCHE) is a promising candidate satisfying the heat exchange requirements of liquefied natural gas (LNG) vaporization at low and high pressure. The effects of airfoil fin arrangement on heat transfer and flow resistance were numerically investigated using supercritical liquefied natural gas (LNG) as working fluid. The thermal properties of supercritical LNG were tested by utilizing the REFPROF software database. Numerical simulations were performed using FLUENT. The inlet temperature of supercritical LNG was 121 K, and its pressure was 10.5 MPa. The reference mass flow rate of LNG was set as 1.22 g/s for the vertical pitch Lv = 1.67 mm and the staggered pitch Ls = 0 mm, with the Reynolds number of about 3750. The SST k-ω model was selected and verified by comparing with the experimental data using supercritical liquid nitrogen as cold fluid. The airfoil fin PCHE had better thermal-hydraulic performance than that of the straight channel PCHE. Moreover, the airfoil fins with staggered arrangement displayed better thermal performance than that of the fins with parallel arrangement. The thermal-hydraulic performance of airfoil fin PCHE was improved with increasing Ls and Lv. Moreover, Lv affected the Nusselt number and pressure drop of airfoil fin PCHE more obviously. In conclusion, a sparser staggered arrangement of fins showed a better thermal-hydraulic performance in airfoil fin PCHE

Strength-Toughness Balance and Hydrogen Embrittlement Susceptibility of a Precipitation-Strengthened Steel Adopted Tempering Process

Two steels with different nickel (Ni) content were investigated to reveal the role of Ni on strength-toughness balance and hydrogen embrittlement susceptibility. Although they were similar in microstructure, i.e., nano-particles were precipitated on martensitic laths, different mechanical behaviors were exhibited. After tempering, the yield strength of 3.5 Ni steel reached a peak at 500 °C, while that of 2.5 Ni steel kept a downward trend, indicating that precipitation strengthening was significant in 3.5 Ni steel. Combined with thermodynamic and kinetic analyses, it was shown that when the Ni content increased, the rich-copper (Cu) precipitation transformation driving force would be enhanced and the reverse of austenite transformation accelerated to improve its stability. Moreover, the increase of Ni content also induced the increase in high-angle grain boundaries (HAGBs), which could inhibit crack propagation. Under the comprehensive effects of strengthening and ductility mechanism, 3.5 Ni steel exhibited excellent cryogenic toughness. Although it was not possible to obtain the ideal balancing of strength-toughness for the steel with lower Ni content, its hydrogen embrittlement susceptibility is satisfying. The results showed that the increase of grain boundary density caused by the grain refinement of 2.5 Ni steel is the key factor for its lower hydrogen embrittlement sensitivity index. Moreover, with the reduction of Ni content, the decrease of HAGBs and the increase in Σ11 boundary were conducive to reducing hydrogen-assisted cracking, while the residual Fe3C in 3.5 Ni steel would deteriorate the hydrogen embrittlement resistance

Soil Thermal Balance Analysis for a Ground Source Heat Pump System in a Hot-Summer and Cold-Winter Region

As a renewable and high energy efficiency technology providing air conditioning and domestic hot water, the ground source heat pump system (GSHPS) has been extensively used worldwide in recent years. Compared with conventional systems, GSHPSs with heat recovery reject less heat into the soil and extract more heat from it, which can help reduce soil thermal imbalance in hot-summer and cold-winter regions. In this paper, conventional GSHPS, and GSHPS with different heat recovery ratios, in a typical city were compared based on thermal imbalance ratios, average soil temperatures and soil temperature increases. The transient system simulation software was used to simulate the operation performance of GSHPS. The thermal imbalance ratio and soil temperature decreased with increasing heat recovery ratio. After 20 years of operation, the soil thermal imbalance ratios of the GSHPS were 29.2%, 21.1%, 16%, and 5.2%, and the soil temperature rises were 8.78 °C, 5.25 °C, 3.44 °C, and 0.34 °C, while the heat recovery ratios were 0, 18%, 30% and 53%, respectively. Consequently, a GSHPS with heat recovery is a potentially efficient and economical approach for buildings in hot-summer and cold-winter regions

Prediction of Human Brucellosis in China Based on Temperature and NDVI

Brucellosis occurs periodically and causes great economic and health burdens. Brucellosis prediction plays an important role in its prevention and treatment. This paper establishes relationships between human brucellosis (HB) and land surface temperature (LST), and the normalized difference vegetation index (NDVI). A seasonal autoregressive integrated moving average with exogenous variables (SARIMAX) model is constructed to predict trends in brucellosis rates. The fitted results (Akaike Information Criterion (AIC) = 807.58, Schwarz Bayes Criterion (SBC) = 819.28) showed obvious periodicity and a rate of increase of 138.68% from January 2011 to May 2016. We found a significant effect between HB and NDVI. At the same time, the prediction part showed that the highest monthly incidence per year has a decreasing trend after 2015. This may be because of the brucellosis prevention and control measures taken by the Chinese Government. The proposed model allows the early detection of brucellosis outbreaks, allowing more effective prevention and control