The frequency dependent regenerator cold section and hot section positional reversal in a coaxial type thermoacoustic Stirling heat pump

We have constructed and tested two travelling wave thermoacoustic heat pumps using a coaxial configuration with the regenerator positioned in the annulus. We discovered a frequency dependent positional reversal of the cold section and hot section of the regenerator within the test frequency range. By decomposing the measured pressure wave within the annulus, we obtained the positive (w₊) and negative (w₋) propagating travelling waves. It has been revealed the change of frequency is accompanied by a change in magnitudes of w₊ and w₋ which is in part influenced by the presence of travelling wave attenuation through the regenerator. The resulting change of dominant travelling wave on a given end of the regenerator will then change the direction of thermoacoustic heat pumping at that end. This will alter the regenerator temperature distribution and may reverse the cold and hot sections of the regenerator. As the reversal does not require additional moving parts, merely a change in frequency, this feature in coaxial travelling wave devices has tremendous potential for applications which require both heating and cooling operation.     

Study on a wire-type Joule Thomson microcooler with a concentric heat exchanger

This study examines the performance of a wire-type Joule Thomson microcooler utilizing a flexible concentric counterflow heat exchanger. Three gases: C₂H₄, CO₂ and N₂ were used separately for trials conducted at inlet pressures ranging from 0.5 MPa to 5 MPa with C₂H₄ having the best performance. During unloaded tests at an inlet pressure of 2.0 MPa, C₂H₄ obtained a minimum temperature of 225 K while CO₂ obtained a minimum temperature of 232 K. Using CO₂ the microcooler was able to maintain a temperature of 273 K at 100 mW heat input and 2 MPa inlet pressure. An inlet pressure of 3 MPa allowed a 550 mW heat input at 273 K. Theoretical performance calculations were conducted and compared to experimental results revealing considerable reduction of microcooler performance due to the presence of heat in-leak. Results have displayed that the JT coefficient of the coolant gas is a more dominant factor than heat transfer properties in determining the performance of the coolant. Due to the microscale of the device, relevant scaling effects were evaluated, particularly entrance effects, surface roughness and axial conduction.     

Bioavailability of Heavy Metals in Soil: Impact on Microbial Biodegradation of Organic Compounds and Possible Improvement Strategies

Co-contamination of the environment with toxic chlorinated organic and heavy metal pollutants is one of the major problems facing industrialized nations today. Heavy metals may inhibit biodegradation of chlorinated organics by interacting with enzymes directly involved in biodegradation or those involved in general metabolism. Predictions of metal toxicity effects on organic pollutant biodegradation in co-contaminated soil and water environments is difficult since heavy metals may be present in a variety of chemical and physical forms. Recent advances in bioremediation of co-contaminated environments have focussed on the use of metal-resistant bacteria (cell and gene bioaugmentation), treatment amendments, clay minerals and chelating agents to reduce bioavailable heavy metal concentrations. Phytoremediation has also shown promise as an emerging alternative clean-up technology for co-contaminated environments. However, despite various investigations, in both aerobic and anaerobic systems, demonstrating that metal toxicity hampers the biodegradation of the organic component, a paucity of information exists in this area of research. Therefore, in this review, we discuss the problems associated with the degradation of chlorinated organics in co-contaminated environments, owing to metal toxicity and shed light on possible improvement strategies for effective bioremediation of sites co-contaminated with chlorinated organic compounds and heavy metals.     

A collision-free formation reconfiguration control approach for Unmanned Aerial Vehicles

Formation flying of Unmanned Aerial Vehicles (UAVs) has gained a lot of interest due to its many potential advantages. Flying in formation allows wider sensing coverage area and in effect, this leads to improved surveillance and enhanced situational awareness. Also flying in formation eases coordination and data fusion. This paper presents the control architecture for fixed-wing UAV reconfiguration control using a novel combination of known techniques. The current premise is for the UAVs to assume their final target states within a specified time interval while avoiding collisions with one another or with an obstacle during the process. Some simulations are performed to assess the performance of the reconfiguration control scheme. The effects of the control parameters on the reconfiguration trajectories are also examined.