Design and Analysis of a Radio-Frequency Moisture Sensor for Grain Based on the Difference Method

Grain moisture is one of the key indexes of grain quality, and acquiring an accurate moisture value is critical for grain storage security. However, the sensors used in the traditional methods for testing grain moisture are based on capacitance, microwave, or radio-frequency methods and still exhibit low accuracy and instability because they are susceptible to the temperature, moisture, and micro gas flow of the air in the granary. In this study, we employed a new design for a radio-frequency moisture sensor for grain. The structure of the sensor is based on the difference method and consists of two parallel probe units. These units are at different distances to the tested grain, resulting in different sensitivities in the moisture measurements. Through a phase difference operation on the test signals, the disturbance variable was reduced. The specific size of the two parallel probes was confirmed by calculation and simulation using High Frequency Structure Simulator (HFSS) software. The simulated and measured parameters of a prototype sensor agreed well. The linear relationship yielded a correlation coefficient of 0.9904, and the average error of the moisture testing was within ±0.3% under the conditions where the VSWR (voltage standing wave ratio) value and return losses were 1.5896 and −20 dB, respectively, at a measured central frequency of 100 MHz. The results indicate that the performance of the sensor was excellent.

Investigating and Modeling the Correlation of Pressurization and Electron Acceleration Inside a Micro Arc-Jet Thruster

Fuel mass is one of the main economical and technical restrictions while designing space propulsion systems. Given the high costs related to the transport of mass into space, the necessary fuel mass for accomplishment of the mission should be minimized. For an optimum thrust/fuel consumption ratio the gas exit velocity must be maximized. In this research this is achieved through the heating of the micro gas flow by an electrical arc inside the sub-sonic region of the propulsion system. The electrical arc induces a partial ionization of the propellant gas. Because of the very low mass flow the gap of the plasma channel has a width of just a few hundred microns. The electrical arc consists of electrons being accelerated through this small gap by the charged walls of the microchannel. The electrons move in a cross flow compared to the propellant gas. In order to study this approach an experimental rig is built up inside a vacuum chamber. The relation between electrical power and mechanical pressurization is investigated experimentally. The won data are compared with computational results of the electrodynamic behavior inside the micro gap. The computational model consists of the coupling of the micro gas flow in the trans-sonic thruster application (e.g. [1]) with the heating mechanism of the electron motion including the partial ionization of the subsonic flow of the electric propulsion system. The computational results are validated with the experimental data. Through this investigation a very efficient form of electrodynamic heating-modeling is developed. The very good results show the quality of the present method and encourage further utilization and development. For this reason this model will be used for the optimization and the computational engineering pre-development of future thermo-electric propulsion systems.

Two-Phase Flows in Mini-/Micro-Gas Flow Channels of PEM Fuel Cells

In this paper, two-phase flow dynamics in a micro channel with various wall conditions are both experimentally and theoretically investigated. Annulus, wavy and slug flow patterns are observed and location of liquid phase on different wall condition is visualized. The impact of flow structure on two-phase pressure drop is explained. Two-phase pressure drop is compared to a two-fluid model with relative permeability correlation. Optimization of correlation is conducted for each experimental case and theoretical solution for the flows in a circular channel is developed for annulus flow pattern showing a good match with experimental data in homogeneous channel case.