Peter Woodfield

Graphite on paper as material for sensitive thermoresistive sensors

This paper reports on the thermoresistive properties of graphite on paper (GOP). A negative temperature coefficient of resistance (TCR) from −2900 to −4400 ppm K−1 was observed for the GOP. This negative and large TCR is attributed to an increase in the thermionic emission current over a low potential barrier with increasing temperature. The potential barrier was found to be 33 meV between the graphite grains. The paper also demonstrates the use of the GOP in a highly sensitive (0.83 mV (m s−1)−0.8 mW−1) GOP-based anemometer, indicating strong feasibility of using this material for low-cost and sensitive thermal sensing applications.

Thermoresistive Effect for Advanced Thermal Sensors: Fundamentals, Design Considerations, and Applications

Microelectromechanical systems sensors have been intensively developed utilizing various physical concepts, such as piezoresistive, piezoelectric, and thermoresistive effects. Among these sensing concepts, the thermoresistive effect is of interest for a wide range of thermal sensors and devices, thanks to its simplicity in implementation and high sensitivity. The effect of temperature on the electrical resistance of some metals and semiconductors has been thoroughly investigated, leading to the significant growth and successful demonstration of thermal-based sensors, such as temperature sensors, convective accelerometers and gyroscopes, and thermal flow sensors. In this paper, we review the fundamentals of the thermoresistive effect in metals and semiconductors. We also discuss the influence of design and fabrication parameters on the thermoresistive sensitivity. This paper includes several desirable features of thermoresistive sensors and recent developments in these sensors are summarized. This review provides insights into how it is affected by various parameters, and useful guidance for industrial designers in terms of high sensitivity and linearity and fast response. [2017-0022]

Estimation of Uncertainty in an Analytical Inverse Heat Conduction Solution

The method of sequential perturbations is applied to find the uncertainty in estimated surface temperature and heat flux from a two-dimensional analytical inverse heat conduction problem related to impinging jet quenching experiments. It is shown that for meaningful uncertainty estimates, the inverse solution itself must be formulated such that it can be interpreted as giving average surface conditions over a small period of time and space. A procedure for estimating the time and space resolution limits of the solution is proposed.

Characteristics of Heat Transfer for Hydrogen and Wall During Filling Hydrogen Into Actual Tank at High Pressure

An experiment has been made to measure the rise in temperature of hydrogen and tank wall during filling of actual tanks to 35 and 70 MPa. Three different tank configurations are used, having volumes of 205, 130 and 39 liters. The filling time is 5 to 20 minutes. A governing equation for the filling process is proposed, which includes unknown values for heat transfer coefficients between the hydrogen and the wall and the wall and surrounding air. The values are tentatively assumed to be 500 W/(m2 K) during filling and 250 W/(m2 K) after filling for the inside tank wall and 4.5 W/(m2 K) for the outside tank wall. The measured temperatures of the hydrogen gas and the wall are in good agreement with the calculated ones.Copyright

Thermomagnetic Convection Around a Current-Carrying Wire in Ferrofluid

Thermomagnetic convection of a ferrofluid flow induced by the internal magnetic field around a vertical current-carrying wire was theoretically analyzed and experimentally validated for the first time. The Nusselt number for a heated 50-μm diameter wire in a ferrofluid was measured for different electrical currents and fluid temperatures. The experimental results are in a good agreement with the proposed scaling analysis. We found that increasing the current will increase the Nusselt number nonlinearly and ultimately enhances the heat transfer capability of the induced ferrofluid flow. We observed that the thermomagnetic convection becomes dominant, if large enough currents are applied.

Thermal Conductivity Measurement of Gases by the Transient Short-Hot-Wire Method

Measurements of the thermal conductivity of helium and hydrogen are performed using the transient short-hot-wire method. The short hot wire is made of platinum and has a diameter of about 10 μm and a length of about 15 mm. It is attached by spot welding to platinum terminals with a diameter of 1.5 mm. The probe is inserted into the sample vessel that has a volume of 35 cm3 and an inner diameter of 30 mm. The thermal conductivity is evaluated by comparing a numerical solution of the heat conduction in and around the short wire with the experimentally obtained temperature rise of the wire. The measured thermal conductivities show good reproducibility. Also, the measured thermal conductivities agree with the reference equations within a deviation of ± 1%.

The mechanical properties of flax fibre reinforced poly(lactic acid) bio-composites exposed to wet, freezing and humid environments

Bio-composites are increasingly being perceived as a green alternative to synthetic composites in many applications. However, the overall long-term durability of bio-composites is a major concern, particularly their ability for sustained performance under harsh and changing environmental conditions. This paper reports a detailed study on the effect of environmental conditions on the performance of flax/poly(lactic acid) bio-composites. Neat poly(lactic acid) and bio-composite samples were exposed to environments similar to those found outdoors: wet, freezing and humid. Moisture absorption and physical changes of specimens were periodically examined. Flexural and tensile properties were evaluated periodically to determine the detrimental effect of each exposure condition on the mechanical performance of bio-composites. Direct contact with liquid water is the most deteriorating environment for bio-composites. A drying process can partially restore the mechanical performance of these materials. Bio-composites can survive reliably in warm humid environments and in those that could create freeze and thaw cycles for short-term outdoor applications. The mechanisms and reasons involved in the degradation of the properties of green composites are discussed.

A fluid density sensor based on a resonant tube

A fluid density sensor based on resonance frequency change of a metallic tube is presented. The sensor has been developed without using a complex micro-fabrication process. The sensor is able to identify fluid types/contaminations and improve the performance by reducing testing time, decreasing complexity of testing equipment and reducing sample sizes. The sensor can measure the resonance frequency of its own structure and determine the change in resonance frequency due to the subsequent sample inside the tube. Numerical modelling, analytical modelling and physical testing of a prototype sensor showed comparable results for both the magnitude and resonance frequency shift. The modelling results yielded a resonance frequency shift of 200 Hz from 9.87 kHz to 9.67 kHz after the water was filled into the tube. The actual testing illustrated a resonance frequency change of 280 Hz from 9.11 kHz to 8.83 kHz. The ultimate aim of the work is to determine resonance frequencies of desired samples at a level that could detect genetic disease on a cellular level.

Significance of buoyancy in turbulence closure for computational fluid dynamics modelling of ultraviolet disinfection in maturation ponds

Buoyancy-driven turbulent dispersion in a maturation pond is studied using a combination of field measurements and computational fluid dynamics. Modelling flow in maturation ponds requires turbulent closure models because of the large physical size and the need to model on diurnal timescales. Simulation results are shown to be more sensitive to the inclusion of a buoyancy production term appearing in the turbulent transport equations than to the model choice. Comparisons with experimental thermal profiles show that without this term, thermal mixing is over-predicted. When including the term, stratification occurs but thermal mixing is under-predicted in the lower water column. In terms of pond performance, the effect of this term is to cause increased surface die-off of Escherichia coli during sunlight hours due to the generation of stratification. It is recommended that future modelling consider and implement this term.

Effect of Time-Dependent Process Temperature Variation During Manufacture of Natural-Fibre Composites

One of the key issues in compression molding of natural fibre reinforced polymer (NFRP) bio-composites is the thermochemical degradation of the fibre and matrix during manufacture. In our previous work, models of thermal penetration, melt infusion, thermal degradation and chemical degradation of flax/PLA bio-composite were used to propose the degradation boundaries for bio-composite manufacturing. This study proposes a thermal degradation model which accounts for effect of time-dependent process temperature variation during manufacture of green composites. Kinetic data are used to calculate degradation progress parameters, defining experiment process maps for identifying the effect of the temperature history on the degradation progress and effects on the tensile strength. The model also can express the tensile strength changes in comparison with other degradation parameters.