Performance evaluation of ultra-long lithium heat pipe using an improved lumped parameter model

Lithium heat pipes have broad applications in heat pipe cooling reactors and hypersonic vehicles owing to their ultra-high working temperature.In particular,when the length of the lithium heat pipe is ultra-long,the flow and heat transfer characteristics are more complex.In this study,an improved lumped parameter model that considers the Marangoni effect,bending effect,and different vapor flow patterns and Mach numbers was developed.Thereafter,the proposed model was verified using the University of New Mexico’s Heat Pipe and HTPIPE models.Finally,the verified model was applied to simulate the steady-state operation of an ultra-long lithium heat pipe in a Heat PipeSegmented Thermoelectric Module Converters space reactor.Based on the results:(1)Vapor thermal resistance was dominant at low heating power and decreased with increasing heating power.The vapor flow inside the heat pipe developed from the laminar to the turbulent phase,whereas the liquid phase in the heat pipe was always laminar.(2)The vapor pressure drop caused by bending was approximately 22–23%of the total,and the bending effect on the liquid pressure drop could be ignored.(3)The Marangoni effect reduced the capillary limit by hindering the liquid reflux,especially at low vapor temperatures.Without considering the Marangoni effect,the capillary limit of the lithium heat pipe was overestimated by 9%when the vapor temperature was 1400 K.(4)The total thermal resistance of the heat pipe significantly increased with increasing adiabatic length when the vapor temperature was low.Further,the wick dryness increased with increasing adiabatic length at any vapor temperature.Such findings improve on current knowledge for the optimal design and safety analysis of a heat pipe reactor,which adopts ultra-long lithium heat pipes.

Wideband Signal Detection Based on MWC Discrete Compressed Sampling Structure

In order to solve the cross-channel signal problem caused by the uniform channelized wideband digital receiver when processing wideband signal and the problem that the sensitivity of the system greatly decreases when the bandwidth of wideband digital receiver increases,which both decrease the wideband radar signal detection performance,a new wideband digital receiver based on the modulated wideband converter(MWC)discrete compressed sampling structure and an energy detection method based on the new receiver are proposed.Firstly,the proposed receiver utilizes periodic pseudo-random sequences to mix wideband signals with baseband and other sub-bands.Then the mixed signals are low-pass filtered and downsampled to obtain the baseband compressed sampling data,which can increase the sensitivity of the system.Meanwhile,the cross-channel signal will all appear in any subbands,so the cross-channel signal problem can be solved easily by processing the baseband compressed sampling data.Secondly,we establish the signal detection model and formulate the criterion of the energy detection method.And we directly utilize the baseband compressed sampling data to carry out signal detection without signal reconstruction,which decreases the complexity of the algorithm and reduces the computational burden.Finally,simulation experiments demonstrate the effectiveness of the proposed receiver and show that the proposed signal detection method is effective in low signal-to-noise ratio(SNR)compared with the conventional energy detection and the probability of detection increases significantly when SNR increases.

Modeling and Control of an Isolated Module Multilevel DC/DC Converter for DC Grid

This paper presents an isolated DC/AC/DC converter using a middle frequency transformer coupling two modular multilevel converters(MMC),suitable for interconnecting DC transmission lines of different voltage levels in high voltage direct current(HVDC)system.The basic operational principle of the isolated module multilevel DC/DC converter(IMMDCC)is analyzed.The dynamic model of IMMDCC is studied in detail and the transient relationship between DC side and AC side of IMMDCC is revealed,which is physically straightforward for understanding the power transfer in IMMDCC.The control strategy in D-Q coordinate system is put forward,and the fault characteristic and corresponding protection method is analyzed.Finally,computer simulation using Matlab/Simulink is performed to verify the dynamic model and the proposed control strategy.The simulation results show good performances and the quick response ability of the proposed control strategy.

Integration and verification case of IP-core based system on chip design

In this paper, the design and verification process of an automobile-engine-fan control system on chip （SoC） are introduced. The SoC system, SHU-MV08, reuses four new intellectual property （IP） cores and the design flow is accomplished with 0.35 btm chartered CMOS technology. Some special functions of IP cores, the detailed integration scheme of four IP cores, and the verification method of the entire SoC are presented. To settle the verification problems brought by analog IP cores, NanoSim based chip-level mixed-signal verification method is introduced. The verification time is greatly reduced and the first tape-out achieves success which proves the validity of our design.

Spectral Calculation of the Output Voltage of an Inverter with Bipolar Pulse Width Modulation

Converters with pulse width modulation are used for connections between the direct current （DC） and alternating current （AC） networks, e.g., in uninterrupted power supply systems, AC electromotor drives, for powering induction furnaces, in audio technique. Spectrum of signals sampled by pulse amplitude modulation and output voltage spectrum of the converter with pulse width modulation have similar properties. Spectrum of signals sampled by pulse amplitude modulation contains a harmonic of frequency equal to the frequency of the modulating signal and the harmonics of frequencies equal to the sum of frequency of the modulating signal and multiples of the sampling frequency. The output voltage spectrum of the converter with bipolar pulse width modulation contains harmonic of frequency equal to the frequency of the modulating signal and harmonics of frequencies equal to sum of the frequency of the modulating signal and multiples of the frequency of the carrier signal. It also contains harmonics of frequencies equal to the sum of the multiples of the frequency of the modulating signal and the multiples of the carrier signal. The comparison analysis was carried out for the harmonics of the output voltage of the converter with bipolar pulse width modulation in time domain. The dependency of the amplitudes and frequency spectrum on the wave forms of the carrier signal and modulating signal was shown. Similarity of the output voltage spectrum of the converter and signal spectrum sampled by the pulse width modulation was also shown. Key words： Output voltage converter with bipolar pulse width modulation, spectral analysis, Fourier series, carrier signal, reference signal.

Switching response of dual-output Mach–Zehnder modulator in channel-interleaved photonic analog-to-digital converter

This Letter theoretically and experimentally studies the response of photonic switching in a channel-interleaved photonic analog-to-digital converter(PADC) with high sampling rate and wide input frequency range. A figure of merit(FoM) is introduced to evaluate the switching response of the PADC when a dual-output Mach–Zehnder modulator(MZM) serves as the photonic switch to parallelize the sampled pulse train into two channels. After the optimization of the FoM and utilization of the channel-mismatch compensation algorithm,the system bandwidth of PADC is expanded and the signal-to-distortion ratio is enhanced.

Simulation of grid tracking control of power system connection and maximum power brushless doubly-fed generator in point wind

In this paper, based on the analysis of the mathematical model in a common synchronous reference frame of the brushless doubly-fed generator （BDFG）, the grid connection strategy and maximum energy extraction control were both analyzed. Besides, the transient simula- tion of no-load model and generation model of the BDFG have been developed on the MATLAB/Simulink platform. The test results during cutting-in grid confirmed the good dynamic performance of grid synchronization and effective power control approach for the BDFG-based variable speed wind turbines.

A new technique to measure the differential XAFS spectrum

A new technique has been developed for direct measurement of the differential X-ray absorption fine structure（XAFS） spectrum by the energy-modulation method. To acquire the energy-oscillating incident X-ray beam, a piezoelectric actuator is used to control the double-crystal monochromator. A logarithmic converter circuit and a lock-in amplifier are used to extract the modulated signals. The normal and differential XAFS spectra of the Mn K-edge of Li2 Mn O3 have been collected. The X-ray-absorption near-edge-structure（XANES） spectra verify that the signal-to-noise ratio has been greatly improved by the new technique, and the extended X-ray absorption fine structure（EXAFS） spectra demonstrate that this new technique can efficiently enhance the signals of the backscattering atoms.