Diagnostic Protocol for Thermal Performance of District Heating Pipes in Operation. Part 1: Estimation of Supply Pipe Temperature by Measuring Temperature at Valves after Shutdown

This study evaluates temperatures measured at district heating (DH) valves in manholes and their usability for non-destructively assessing the thermal performance of buried DH pipes. The study was conducted as a field test in which part of a DH network was shut down and the temperature decline in the valves was analysed in terms of absolute temperature and thermal response time from the DH pipe to the top of the valve. The calculated and measured supply pipe temperatures by the drainage valves were in good agreement, with 1% deviation. The valve measurement analysis from this study shows that the drainage valve has good potential to serve as a measurement point for assessing the thermal status of a DH network. However, the shutdown valve measurements were greatly affected by the manhole environment.

Reduction of residual stress in polymorphous silicon germanium films and their evaluation in microbolometers

Hydrogenated polymorphous silicon germanium (pm-SixGe1–x:H) thin films were deposited by the PECVD technique at 200 °C. Three compositions were investigated by changing the silane/germane gas mixture. It was found that the temperature coefficient of resistance (TCR) varies from 2.25% K−1 to 4.26% K−1 while the electrical conductivity ranges from 9.1 × 10−6 S cm−1 to 3.7 × 10−3 S cm−1. On the other hand, the residual stress of as-deposited films was highly compressive reaching values of nearly 700 MPa. After a thermal annealing of 3 hours, it was observed an acceptable reduction and a slight change towards tensile stress. A thin film with low residual stress and high TCR was chosen to manufacture test microbolometers in order to assess if the thermosensing properties of pm-SixGe1–x:H were not affected. After fabricating the microbolometers, their structural conditions were evaluated by scanning electron microscopy and it was found that the reduction of stress significantly improved their mechanical stability and reduced the warping of the membranes. Finally, test structures were characterized at a chopper frequency of 30 Hz, with a DC current of 2.5 μA in a vacuum environment of 20 mTorr. Voltage responsivity of 1.9 × 106 V/W, detectivity of 4.4 × 108 cm ∙ Hz1/2/W, NEP of 1 × 10−11 W/Hz1/2, NETD of 18 mK and 2 ms of thermal response time were measured. In summary, we have studied different process conditions to obtain better pm-SixGe1–x:H films in terms of their electrical and mechanical properties. In this sense, the results obtained with microbolometers show that pm-SixGe1–x:H is a very attractive material to develop infrared vision systems with high sensitivity.

Multiscale fluid–particle thermal interaction in isotropic turbulence

We use direct numerical simulations to investigate the interaction between the temperature field of a fluid and the temperature of small particles suspended in the flow, employing both one- and two-way thermal coupling, in a statistically stationary, isotropic turbulent flow. Using statistical analysis, we investigate this variegated interaction at the different scales of the flow. We find that the variance of the carrier flow temperature gradients decreases as the thermal response time of the suspended particles is increased. The probability density function (PDF) of the carrier flow temperature gradients scales with its variance, while the PDF of the rate of change of the particle temperature, whose variance is associated with the thermal dissipation due to the particles, does not scale in such a self-similar way. The modification of the fluid temperature field due to the particles is examined by computing the particle concentration and particle heat fluxes conditioned on the magnitude of the local fluid temperature gradient. These statistics highlight that the particles cluster on the fluid temperature fronts, and the important role played by the alignments of the particle velocity and the local fluid temperature gradient. The temperature structure functions, which characterize the temperature fluctuations across the scales of the flow, clearly show that the fluctuations of the carrier flow temperature increments are monotonically suppressed in the two-way coupled regime as the particle thermal response time is increased. Thermal caustics dominate the particle temperature increments at small scales, that is, particles that come into contact are likely to have very large differences in their temperatures. This is caused by the non-local thermal dynamics of the particles: the scaling exponents of the inertial particle temperature structure functions in the dissipation range reveal very strong multifractal behaviour. Further insight is provided by the flux of temperature increments across the scales. Altogether, these results reveal a number of non-trivial effects, with a number of important practical consequences.