Igor O. Golosnoy

Porous materials for thermal management under extreme conditions

A brief analysis is presented of how heat transfer takes place in porous materials of various types. The emphasis is on materials able to withstand extremes of temperature, gas pressure, irradiation, etc., i.e. metals and ceramics, rather than polymers. A primary aim is commonly to maximize either the thermal resistance (i.e. provide insulation) or the rate of thermal equilibration between the material and a fluid passing through it (i.e. to facilitate heat exchange). The main structural characteristics concern porosity (void content), anisotropy, pore connectivity and scale. The effect of scale is complex, since the permeability decreases as the structure is refined, but the interfacial area for fluid–solid heat exchange is, thereby, raised. The durability of the pore structure may also be an issue, with a possible disadvantage of finer scale structures being poor microstructural stability under service conditions. Finally, good mechanical properties may be required, since the development of thermal gradients, high fluid fluxes, etc. can generate substantial levels of stress. There are, thus, some complex interplays between service conditions, pore architecture/scale, fluid permeation characteristics, convective heat flow, thermal conduction and radiative heat transfer. Such interplays are illustrated with reference to three examples: (i) a thermal barrier coating in a gas turbine engine; (ii) a Space Shuttle tile; and (iii) a Stirling engine heat exchanger. Highly porous, permeable materials are often made by bonding fibres together into a network structure and much of the analysis presented here is oriented towards such materials.

An Analytical Model for Simulation of Heat Flow in Plasma-Sprayed Thermal Barrier Coatings

Numerical (finite difference) and analytical models have been developed for the simulation of heat flow through plasma-sprayed coatings, allowing the effective thermal conductivity to be predicted as a function of microstructural parameters. The structure is assumed to be composed of lamellar material (splats), separated by (thin) pores, within which there are areas of contact (bridges). The analytical model is based on dividing the material into two regimes, within which the heat flow occurs either by unidirectional serial flow through lamellae and pores or by being funneled through the regions of the lamellae above and below the bridges. The validity of this model is demonstrated by a comparison of the predictions obtained from it and those obtained from the numerical model. The effects of pore geometry on conductive and radiative heat transfer within the coating have been investigated over a range of temperatures and gas pressures. It is shown that the main factor controlling the conductivity is the intersplat bridge area. Comparisons are also presented with experimental conductivity data, for cases in which some attempt has been made to characterize the key microstructural features. The study is oriented toward thermal barrier coatings, based on zirconiayttria top coats. It is noted that the effect of microstructural sintering, which tends to occur in these coatings under service conditions, can be predicted using this model.

Numerical modelling of negative discharges in air with experimental validation

Axisymmetric finite element models have been developed for the simulation of negative discharges in air without and with the presence of dielectrics. The models are based on the hydrodynamic drift-diffusion approximation. A set of continuity equations accounting for the movement, generation and loss of charge carriers (electrons, positive and negative ions) is coupled with Poissons equation to take into account the effect of space and surface charges on the electric field. The model of a negative corona discharge (without dielectric barriers) in a needle-plane geometry is analysed first. The results obtained show good agreement with experimental observations for various Trichel pulse characteristics. With dielectric barriers introduced into the discharge system, the surface discharge exhibits some similarities and differences to the corona case. The model studies the dynamics of volume charge generation, electric field variations and charge accumulation over the dielectric surface. The predicted surface charge density is consistent with experimental results obtained from the Pockels experiment in terms of distribution form and magnitude.

The Effect of a High Thermal Gradient on Sintering and Stiffening in the Top Coat of a Thermal Barrier Coating System

Superalloy substrates coated with plasma-sprayed CoNiCrAlY bond coats and yttria-stabilized zirconia top coats (TCs) have been subjected to a high heat flux under a controlled atmosphere. The sintering exhibited by the TC under these conditions has been studied and compared with the behavior observed during isothermal heating. Sintering has been characterized by (a) microstructural examinations, (b) dilatometry, in both the in-plane and through-thickness directions, and (c) stiffness measurements, using both cantilever bending and nanoindentation. A numerical model has been used to explore the stress state under isothermal and thermal gradient conditions. Dilatometry data indicate significant linear contractions during holding at elevated temperatures, particularly in the through-thickness direction. This is largely attributed to microstructural changes associated with sintering, with any volume changes due to phase transformations making relatively small contributions. Sintering proceeds faster at higher temperatures but is retarded by the presence of tensile stresses (from differential thermal expansion between the coating and substrate) within the TC. Thus, it occurs preferentially near the free surface of the TC under gradient conditions, not only due to the higher temperature, but also because the in-plane stress is more compressive in that region.

Experimental studies of influence of DC and AC electric fields on bridging in contaminated transformer oil

Analysis of real operating condition revealed that HVDC transformers experience combined effect of DC biased AC electric field. The dynamics of pressboard particle in contaminated transformer oil under the influence of DC, AC and DC biased AC electric field has been investigated in this paper. Different levels of particle concentrations are tested at different applied voltages. Optical images of the particles accumulation together with conduction current have been recorded during the experiments. A complete bridge between the electrodes of cellulose particles were observed for all the tests carried out under DC and DC biased AC electric field. Opposite to that, for AC experiments, pressboard particles accumulated on surfaces of both electrodes but did not create a full bridge between the electrodes. It is concluded that a combination of DC and AC voltages in a HVDC transformer could lead to a bridge formation within the equipment which could cause failure.

Bridging phenomenon in contaminated transformer oil

The effect of cellulose particles contamination on transformer oil has been investigated. Different sizes and concentration levels of cellulose fibres were used to contaminate the transformer oil for the experiments. The conduction current was measured using electrodes of 10 mm diameter with a 10 mm gap and the optical microscopic images of bridge creation between the electrodes were also recorded. The bridge was thickened with increasing concentration level and the bridging time was shortened as the voltage increased. The conduction current increased with concentration level and voltage. Smaller size particles had stronger bond while creating the bridge and higher conduction current.

Locating partial discharge sources in high voltage transformer windings

The analysis of partial discharge (PD) measurement data obtained using radio frequency current transducers (RFCT) incorporated into the external earthing connections (i.e bushing tap points and neutral to earth connections) offers the possibility of on-line condition monitoring of large transformers. A method for locating PD sources in windings has been developed, based on analysis of differential equations that model the propagation of PD signals from the source to the measurement point. Analysis reveals that if the discharge at the source is assumed to be impulse-like, then the form of the response at the measurement points can be simulated as a function of source location along the winding. If all parameters of the winding are known then it is possible to model the response, however, in the field this is often not the case. So the method developed here uses information from the PD measurements themselves to estimate the unknown coefficients required to simulate the likely discharge currents. With these estimates it is then possible to compare PD measurement information with simulation data in order to identify the most likely location of a PD source. An experiment based on a section of high voltage winding has been used to produce PD measurement data and to validate the proposed approach.

Two dimensional studies of Trichel pulses in air using the finite element method

The formation of Trichel pulses using two dimensional axisymmetric finite element (FE) simulations is presented in this paper. The hydrodynamic drift-diffusion approximation is used to model the phenomenon and the simulation is based on solving the coupled system of partial differential equations accounting for the transport of charged particles and Poissons equation for the electric field. A needle-plane geometry with an air gap and constant negative step voltage is analysed at atmospheric pressure. The charge distributions and electric field variations during the formation of the first pulse are studied in detail. Factors influencing the first pulse shape, frequency and magnitude of subsequent Trichel pulses, namely the secondary emission coefficient and applied voltage, are also investigated.

Bridging in contaminated transformer oil under AC, DC and DC biased AC electric field

Cellulose particle accumulation under AC, DC and DC biased AC electric field in transformer oil has been investigated in this paper. Different levels of particle concentrations tested with spherical electrode system. Optical microscopic images and conduction current of the particle accumulation process have been recorded during the experiments. A complete bridge of cellulose particles only observed under DC and DC biased AC electric field.

Bridging in contaminated transformer oil under DC and AC electric field.

There are two experimental set up have been established to investigate the bridging effect between two electrodes with different potentials under ac and dc voltages. This paper will explain the full details of the experimental setup and the results. These experiments carried out on pressboard particles under three different levels of ac and dc voltages. Several contamination levels have been investigated. Optical images of particle accumulation have been recorded along with associated conduction current measurement during experiments. At higher voltages the rate of bridge formation is increased along with an associated current increase. The major differences between ac and dc bridges are the shapes of the particle accumulation. The bridge formation rate for ac is slower than dc.