Search

In recent years, agricultural non-point source pollution (ANPSP) has become increasingly prominent, and nitrogen plays an important role in ANPSP. Therefore, we carried out traditional flooded irrigation (TFI) experiments in the paddy field, and applied HYDRUS-2D model to simulate the nitrogen transport in this study. Three observation points A1, A2, and A3 were arranged on the diagonal of the paddy field. We observed ponding water depth on soil surface and nitrogen concentrations in ponding water and soil water at 0.1 m, 0.2 m, and 0.3 m below soil surface. HYDRUS-2D model was proved to be effective in simulating the ponding water depth with root mean squared error (RMSE) = 0.717 cm and Nash-Sutcliffe coefficient (NSE) = 0.805 for the simulated and measured ponding water depth. The simulated and measured NH

X-ray free-electron lasers (XFELs) open a new era of X-ray based research by generating extremely intense X-ray flashes. To further improve the spectrum brightness, a self-seeding FEL scheme has been developed and demonstrated experimentally. As the next step, new-generation FELs with high repetition rates are being designed, built and commissioned around the world. A high repetition rate would significantly speed up the scientific research; however, alongside this improvement comes new challenges surrounding thermal management of the self-seeding monochromator. In this paper, a new configuration for self-seeding FELs is proposed, operated under a high repetition rate which can strongly suppress the thermal effects on the monochromator and provides a narrow-bandwidth FEL pulse. Three-dimension time-dependent simulations have been performed to demonstrate this idea. With this proposed configuration, high-repetition-rate XFEL facilities are able to generate narrow-bandwidth X-ray pulses without obvious thermal concern on the monochromators.

Minichannel technology has been shown to be a promising solution for the battery thermal management system (BTMS). Since the design space of the BTMS using minichannel technology consists a large number of parameters, performing parametric study and optimization of such a system is challenging. To overcome this challenge, a simplified numerical model is developed. In this model, effective convective heat transfer (heff) boundary condition is used at the cell-minichannel interface to circumvent simulating conjugate heat transfer and fluid flow in the minichannel. After validation with past work based on a three-dimensional conjugate heat transfer model, the simplified model is used to conduct a parametric study to determine the effect of various design and operation parameters on the performance of the BTMS. For the proposed BTMS, it is found that cooling half of the cell surface on single side of the cell is enough to keep the maximum temperature difference in the pack to be less than 3 °C at 2C discharge rate. At module level, it has been found that minichannel cooling on only one side of the cell or on every other cell can significantly increase the system level energy density. The results from the parametric study would be useful in designing a robust, effective, and economical minichannel-based BTMS for Electrical Vehicles.

Thermal management on lithium-ion batteries is a crucial problem for the performance, lifetime, and safety of electric vehicles (EVs) and hybrid electric vehicles (HEVs). Fire and explosions can be triggered by thermal runaway if the temperature of the lithium-ion batteries is not maintained properly. This work describes a minichannel cooling system designed at the battery module level and the investigation on its efficacy on the mitigation of thermal runaway. Nail penetration was employed to simulate the internal short circuits, which in reality may be caused by vehicle collisions and/or manufacturing defects. Two integrated models were utilized to study thermal runaway: the conjugate heat transfer model and the reaction kinetics model. Numerical simulations were conducted to understand the thermal runaway process and the effects of flow rate, thermal abuse reactions, nail penetration depth, and nail diameter. It is concluded that minichannel cooling at cell level cannot cease thermal runaway in a single cell, but it can prevent battery fratricide due to thermal runaway propagation between cells.

Lithium-ion batteries are widely used for battery electric (all-electric) vehicles (BEV) and hybrid electric vehicles (HEV) due to their high energy and power density. An battery thermal management system (BTMS) is crucial for the performance, lifetime, and safety of lithium-ion batteries. In this paper, a novel design of BTMS based on aluminum minichannel tubes is developed and applied on a single prismatic Li-ion cell under different discharge rates. Parametric studies are conducted to investigate the performance of the BTMS using different flow rates and configurations. With minichannel cooling, the maximum cell temperature at a discharge rate of 1C is less than 27.8 °C, and the temperature difference across the cell is less than 0.80 °C using flow rate at 0.20 L/min, at the expense of 8.69e-6 W pumping power. At higher discharge rates, e.g., 1.5C and 2C, higher flow rates are required to maintain the same temperature rise and temperature difference. The flow rate needed is 0.8 L/min for 1.5C and 2.0 L/min for 2C, while the required pumping power is 4.23e-4 W and 5.27e-3 W, respectively. The uniform temperature distribution (

Many studies have revealed that Fourier’s law becomes invalid in the study of heat transfer in micro/nanoscale systems due to phonon-mediated nondiffusive heat transport. It is necessary to develop high-fidelity non-Fourier models and numerical methods to study nondiffusive heat transfer for both experimental data analysis in nanothermometry and thermal design in micro-/nano-systems. Starting from the phonon Boltzmann transport equation (BTE), we develop an enhanced Gray (EG) model by considering the second-order terms in Taylor expansion. In the proposed enhanced Gray BTE (EG-BTE), two parameters associated with inherent material properties, i.e., the diffusive relaxation time and the ballistic mean free time, are used to characterize the diffusive heat conduction and nondiffusive heat conduction, respectively. While the diffusive relaxation time is calculated directly from the thermal conductivity, the ballistic mean free time of nonmetallic materials can be determined from transient grating experiments. An implicit lattice Boltzmann method (ILBM) is developed to solve the EG-BTE, which is unconditionally stable. The EG-BTE model is validated by comparing predictions of thermal decay processes with transient grating experiments. After being validated, the EG-BTE model is applied to study heat conduction across thin films from ballistic to diffusive regime at room temperature as well as thermal wave propagation in crystal at low temperatures. The numerical results demonstrate that the EG-BTE provides a unified model to describe multiscale heat conduction covering both nondiffusive and diffusive regimes.

One approach to achieve longitudinally coherent hard X-ray free electron laser (XFEL) radiation is to introduce a single-crystal monochromator in the beam path to generate a co-propagating, time-synchronized self-seeding radiation pulse. For modern high-repetition rate XFELs, however, thermal loading on the crystal can degrade the quality of the seed pulse. To assess limitations on the maximum XFEL pulse repetition rate imposed by the seeding crystal, a comprehensive study of thermomechanical, and X-ray diffraction effects within the crystal is presented for two consecutive self-amplified spontaneous emission pulses incident on both reflective and transmissive crystal monochromators.

Significant influence from coking (aggregation and formation of a non-desirable carbonaceous porous layer on a solid metal surface) on the heat transfer in a cooling system of hypersonic aircraft with hydrocarbon fuel as the coolant is observed. In the present work, an explanation was proposed to understand the influence mechanism of coking on the heat transfer in cooling channel. In addition, a numerical model was further established to verify the explanation by comparing the heat transfer in cooling channel with different thicknesses of coke layer. The results indicated that critical thicknesses of coke layer with respect to effective thermal conductivity ( δ cl , λ c ) and heat transfer ( δ cl , h c ) existed. More specifically, the heat transfer was noticeably enhanced when δcl δ cl , h c , while the heat transfer is deteriorated as δcl > δ cl , h c . A transition thickness with length of ( δ cl , h c – δ cl , λ c ) existed between δ cl , λ c and δ cl , h c under the combined influences of convection of core flow and coke layer. By comparing the heat transfer in cooling channel after coke deposit at different inlet Reynolds number, it was further found that δ cl , λ c and δ cl , h c decrease with increasing the turbulent intensity. This implied that the influence of inlet Reynolds number on the coke deposition rate and turbulent heat transfer were dissimilar. This study is expected to provide further insight into the optimization of cooling channel for the purpose of reducing the heat transfer deterioration caused by coke.

A two-dimensional model is developed to study the flutter instability of a flag immersed in an inviscid flow. Two dimensionless parameters governing the system are the structure-to-fluid mass ratio M⁎ and the dimensionless incoming flow velocity U⁎. A transition from a static steady state to a chaotic state is investigated at a fixed M⁎=1 with increasing U⁎. Five single-frequency periodic flapping states are identified along the route, including four symmetrical oscillation states and one asymmetrical oscillation state. For the symmetrical states, the oscillation frequency increases with the increase of U⁎, and the drag force on the flag changes linearly with the Strouhal number. Chaotic states are observed when U⁎ is relatively large. Three chaotic windows are observed along the route. In addition, the system transitions from one periodic state to another through either period-doubling bifurcations or quasi-periodic bifurcations, and it transitions from a periodic state to a chaotic state through quasi-periodic bifurcations.

A hybrid phonon gas model was developed to describe heat pulse propagation in dielectric crystals. In this model, heat energy is described as a mixture of longitudinal and transverse phonon gases. The novel aspects of this model include: (1) the distinction between longitudinal and transverse phonon excitations is taken into account; (2) transitions between ballistic and second sound transport, ballistic and diffusive transport, and second sound and diffusive transport are successfully captured. For the first time, benchmark cases of heat-pulse experiments in NaF at low temperatures in ballistic, ballistic–diffusive, and diffusive regions have been completely reconstructed in numerical simulations, which demonstrate the validity of the new model. It was elucidated how heat pulses are transmitted by longitudinal, ballistic transverse and dispersive transverse phonons at low temperatures. The numerical results not only yield new insight in physics of transient ballistic–diffusive heat conduction with effects from different phonon scattering processes, but also provide numerical tools to study similar transient ballistic–diffusive heat conduction in nanoelectronics and modern optoelectronics.

In the laminar-turbulent transition of boundary-layer flows, the spatial development of boundary-layer waves, especially that of unstable waves, is important and indispensable. In this paper, the response of a Mach 8 flow over a 5.3° half-angle sharp wedge to wall blowing―suction is studied by numerical simulations. Steady base flow is obtained by solving the compressible Navier―Stokes equations with a combination of a fifth-order shock-fitting method and a second-order total-variation-diminishing scheme. In stability simulations, wall blowing―suction is introduced through an actuator on the wedge surface. The unsteady flow simulation is carried out using the shock-fitting method. The results show that mode F, mode S, acoustic waves, and entropy/vorticity waves are simultaneously excited by wall blowing―suction and coexist in the boundary layer just downstream of the actuator. For blowing―suction at a specific frequency, the results also show that mode S is strongly excited when the actuator is located upstream of the corresponding synchronization point. There is no significant amplification of pressure perturbation when the actuator is downstream of the synchronization point. This result represents a mixture of the receptivity and the downstream growth of mode S. The exact cause and mechanism of this result are not clear. However, such a result is obtained for wall blowing―suction at all frequencies considered in the current study. To excite a strong mode S at a specific frequency, the result indicates that it is necessary to place the blowing―suction actuator upstream of the corresponding synchronization point. Further theoretical analysis is needed to reveal the mechanism behind the numerical results.

A new treatment of pressure boundary conditions for the DSMC method is proposed for flow prediction in microchannels. Validity and accuracy of the new method are verified by comparing to the analytical solutions of the micro-Poiseuille flow under slip condition. The new method shows better convergence compared with previous boundary treatments. This advantage becomes more remarkable as the geometry of the microchannel becomes more complex. A study on a microchannel with sudden expansion is demonstrated using the new DSMC method. Wall temperature in the expanded region of the microchannel independently varies from 200 to 800 K to study the effects on the pressure distribution, velocity, mass flow rate, and heat flux of the microchannel flow. The results show that the wall temperature in the expanded region significantly affects the microchannel flow. Some unique phenomena are observed to be quite different from those of the macroscopic flow and the mechanism of these interesting phenomena is discussed.

By using computational fluid dynamics (CFD) modeling and simulation, a predictive method was proposed to explore the erosion failure of reactor effluent air cooler (REAC) pipes with liner under multiphase flow. A theoretical model based on the erosion-corrosion effects of REAC on mixture turbulent flow was proposed for multiphase flow. Effects of various working conditions, liner shapes, and structures, as well as flow parameters on numerical simulations were investigated. Besides, the pipe’s erosion-corrosion rules under multiphase flow and the relationship between multiphase flow and erosion-corrosion under dangerous working conditions were studied. By CFD numerical simulations, the exact position where some typical pipes thinned and failed rapidly by erosion was found and the main factors causing erosion-corrosion failure were discussed. Finally, numerical results obtained by using the proposed method were compared with experimental results.

In this paper, we continue to study the mechanisms of the receptivity of the supersonic boundary layer to free-stream disturbances by using both direct numerical simulation and linear stability theory. Specifically, the receptivity of a Mach 4.5 flow over a flat plate to free-stream fast acoustic waves is studied. The receptivity to free-stream slow acoustic waves, entropy waves and vorticity waves will be studied in the future. The oblique shock wave induced by the boundary-layer displacement plays an important role in the receptivity because the free-stream disturbance waves first pass through the shock before entering the boundary layer. A high-order shock-fitting scheme is used in the numerical simulations in order to account for the effects of interactions between free-stream disturbance waves and the oblique shock wave. The results show that the receptivity of the flat-plate boundary layer to free-stream fast acoustic waves leads to the excitation of both Mack modes and a family of stable modes, i.e. mode I, mode II, etc. It is found that the forcing fast acoustic waves do not interact directly with the unstable Mack modes. Instead, the stable mode I waves play an important role in the receptivity process because they interact with both the forcing acoustic waves and the unstable Mack-mode waves. Through the interactions, the stable mode I waves transfer wave energy from the forcing fast acoustic waves to the second Mack-mode waves. The effects of incident wave angles, forcing wave frequencies, and wall temperature perturbation conditions on the receptivity are studied. The results show that the receptivity mechanisms of the second mode are very different from those of modes I and II, which leads to very different receptivity properties of these discrete wave modes to free-stream fast acoustic waves with different incident wave angles, frequencies, and different wall boundary conditions. The maximum receptivities of the second mode, mode I and mode II to planar free-stream fast acoustic waves are obtained when incident wave angles approximately equal 26 ◦ ,4 5 ◦ , and 18 ◦ ,r espectively. The results of receptivity to a beam of free-stream fast acoustic waves show that the leading edge is one of the most efficient regions for receptivity.

This paper is the first part of a two-part study on the mechanisms of the receptivity to disturbances of a Mach 4.5 flow over a flat plate by using both direct numerical simulations (DNS) and linear stability theory (LST). The main objective of the current paper is to study the linear stability characteristics of the boundary-layer wave modes and their mutual resonant interactions. The numerical solutions of both steady base flow and unsteady flow induced by forcing disturbances are obtained by using a fifth-order shock-fitting method. Meanwhile, the LST results are used to study the supersonic boundary-layer stability characteristics relevant to the receptivity study. It is found that, in addition to the conventional first and second modes, there exist a family of stable wave modes in the supersonic boundary layer. These modes play a very important role in the receptivity process of excitation of the unstable Mack modes, especially the second mode. These stable modes are termed mode I, mode II, etc., in this paper. Though mode I and mode II waves are linearly stable, they can have resonant (synchronization) interactions with both acoustic waves and the Mack-mode waves. Therefore, the stable wave modes such as mode I and mode II are critical in transferring wave energy between the acoustic waves and the unstable second mode. The effects of frequencies and wall boundary conditions for the temperature perturbations on the boundary-layer stability and receptivity are also studied.

Nondiffusive heat transfer has attracted intensive research interests in last 50 years because of its importance in fundamental physics and engineering applications. It has unique features that cannot be described by the Fourier law. However, current studies of nondiffusive heat transfer still focus on studying the effective thermal conductivity within the framework of the Fourier law due to a lack of a well-accepted replacement. Here, we show that nondiffusive heat conduction can be characterized by two inherent material properties: a diffusive thermal conductivity and a ballistic transport length. We also present a two-parameter heat conduction model and demonstrate its validity in different pump-probe experiments. This model not only offers new insights of nondiffusive heat conduction but also opens up new avenues for the studies of nondiffusive heat transfer outside the framework of the Fourier law.

It is of both fundamental and practical interest to study the flow physics in the manipulation of droplets. In this paper, we investigate complex flow in liquid droplets actuated by a linear gradient of wettability using dissipative particle dynamics simulation. The wetting property of the substrate ranging from hydrophilic to hydrophobic is achieved by adjusting the conservative solid-liquid interactions which results in a variation of solid-liquid surface tension. The internal three-dimensional velocity field with transverse flow in droplet is revealed and analyzed in detail. When the substrate is hydrophobic, it is found that there is slight deformation but strong flow circulation inside the droplet, and the droplet rolling is the dominant mechanism for the movement. However, large deformation of the droplet is generated after the droplet reaches the hydrophilic surface, and a mechanism combining rolling and sliding dominates the transportation of the droplet. Another interesting finding is that the thermal fluctuation can accelerate the spontaneous motion of a liquid droplet under a wetting gradient. The effects of the steepness of wetting gradient and the size of droplet on the translation speed are studied as well.

Based on a phonon hydrodynamic equation, we derive an analytical model to predict the effective thermal conductivity along dielectric thin films or nanowires with smooth wall surface. The model contains only two parameters: bulk thermal conductivity and Knudsen number, no other fitting parameter included. The predictions from the simple model agree approximately with available experimental data in silicon nanostructures and it outperforms other analytical models based on the Boltzmann transport equation.

The receptivity phenomenon, which is the process of environmental disturbances initially entering the boundary layers and generating disturbance waves, is one of the important but not well understood mechanisms involving laminar–turbulent transition of hypersonic flows. This paper presents a numerical simulation study of the receptivity to weak free-stream fast acoustic waves for a Mach 7.99 axisymmetric flow over a half-angle blunt cone. In hypersonic boundary-layer flow over a blunt cone, the process of receptivity to free-stream disturbances is altered considerably by the presence of a bow shock and an entropy layer. In the present study, both steady and unsteady flow solutions are obtained by computing the full Navier–Stokes equations with a fifth-order shock-fitting finite-difference scheme, which is able to account for the effects of bow-shock/free-stream-disturbance interaction accurately. The current numerical results for the steady base flow are compared with previous experimental and numerical results. In addition, a normal-mode linear stability analysis is used to identify the main components of boundary-layer disturbances generated by forcing free-stream fast acoustic waves. It is found that neither the first mode nor the second-mode instability waves are excited by free-stream fast acoustic waves in the early region along the cone surface, although the Mack modes can be unstable there. Instead, the second mode is excited downstream of the second-mode Branch I neutral stability point. The delay of the second-mode excitation is because the hypersonic boundary-layer receptivity is governed by a two-step resonant interaction process: (i) resonant interactions between the forcing waves and a stable boundary-layer wave mode I near the leading-edge region; and (ii) resonant interactions between the induced stable mode I and the unstable second Mack mode downstream. The same receptivity mechanism also explains the results that no first Mack mode components are generated by the current receptivity process because there is no resonant interaction between fast acoustic waves and the first Mack mode.