Gap conductance model

The gap conductance model for conforming rough surfaces was developed, modified, and verified by Yovanovich and co-workers [35,73,100-104]. The gap contact model is based on surfaces having Gaussian height distributions. It also accounts for the mechanical deformation of the contacting surface asperities. The development of the gap conductance model appears in several papers [139,143,147]. 

Wesley and Yovanovich [127] compared the predictions of the proposed gap conductance model and experimental measurements of gaseous gap conductance between the fuel and clad of a nuclear fuel rod. The agreement was very good and the model was recommended for fuel pin analysis codes. 

Similar energy balance equation can be written for heat transfer between fuel element center and the cladding using the gap conductance model and the fuel radial conductivity. Using these expressions, the fuel central line temperature is obtained as... 

Thermal contact, gap, and joint conductance models developed by many researchers over the past five decades are reviewed and summarized in several articles [20,23,50,58,143,147,148] and in the recent text of Madhusudana [59]. The models are, in general, based on the assumption that the contacting surfaces are conforming (or flat) and that the surface asperities have particular height and asperity slope distributions [26, 116, 125]. The models assume either plastic or elastic deformation of the contacting asperities, and require the thermal spreading (constriction) resistance results presented above. 

M. M. Yovanovich, J. DeVaal, and A. A. Hegazy, A Statistical Model to Predict Thermal Gap Conductance Between Conforming Rough Surfaces, AIAA-82-0888, AlAAIASME Third Joint Thermophysics, Fluids, Plasma and Heat Transfer Conference, St. Louis, MO, June 1982. 

These events have been modelled [7.33], and a specific programme of in-pile experiments, so-called JOG 1 and JOG 2 tests, have been performed in the CABRI reactor [7.34] for further investigation of some parameters. The thermal conductivity of the JOG material is of particular interest since it is the main input parameter in any fuel-clad gap conductance calculation. Initial results tend to confirm the potential importance of this JOG layer in fuel behaviour at high bum-up its presence may result in a stabilization or even lowering of fuel temperatures and provide a joint with beneficial plastic properties which could ease fuel-clad mechanical interaction effects. 

Conduction and convection heat transfer through the air gap is modeled as a resistance to heat flow between two surfaces ... 

WSRC has developed a special code, FLOWTRAN-TF, based on the conservation of mass, energy, and momentum to account for two-phase flow, heat transfer effects, and cross-rib gap flows in assembly subchannels. The heat conduction models developed for FLOWTRAN-FI have been incorporated in FLOWTRAN-TF. Each subchannel coolant node has radially adjacent fuel surface temperature nodes to accommodate the heat transfer in the cell.. Rib fin effects are also handled in the same manner as they are in FLOWTRAN-FI. In order to initiate the computation, an air void fraction must be assigned to the computational cell above the fuel. This is done by assuming an air void volume in the first (top) axial cell as adjusted by an experimentally determined partitioning factor. Results from FLOWTRAN-TF have been shown to be relatively insensitive to the value assigned. Two-phase flow across the ribs is modeled by the application of an assumed partition factor based on values given in the literature. 

Contact Drying. Contact drying occurs when wet material contacts a warm surface in an indirect-heat dryer (15—18). A sphere resting on a flat heated surface is a simple model. The heat-transfer mechanisms across the gap between the surface and the sphere are conduction and radiation. Conduction heat transfer is calculated, approximately, by recognizing that the effective conductivity of a gas approaches 0, as the gap width approaches 0. The gas is no longer a continuum and the rarified gas effect is accounted for in a formula that also defines the conduction heat-transfer coefficient ... 

Hash-fire modeling is largely underdeveloped in the literature there are large gaps in the information base. Hardly any information is available concerning flash-fire radiation the only data available have resulted from experiments conducted to meet other objectives. Many items have not yet received sufficient attention. 

Here, W is a cut-off of the order of the 7t-band width, introduced because the right-hand side of Eq. (3.13) is formally divergent. As in the discrete model, the spectrum of eigenstates of Hct for A(a)= Au has a gap between -Ao and +Alh separating the empty conduction band from the completely filled valence band. 

Another famous hopping model is Mott s variable range hopping [23], in which it is assumed that the localized sites are spread over the entire gap. At low temperatures, the probability to find a phonon of sufficient energy to induce a jump to the nearest neighbor is low, and hops over larger distances may be more favorable. In that case, the conductivity is given by... 

The percolation theory [5, 20-23] is the most adequate for the description of an abstract model of the CPCM. As the majority of polymers are typical insulators, the probability of transfer of current carriers between two conductive points isolated from each other by an interlayer of the polymer decreases exponentially with the growth of gap lg (the tunnel effect) and is other than zero only for lg < 100 A. For this reason, the transfer of current through macroscopic (compared to the sample size) distances can be effected via the contacting-particles chains. Calculation of the probability of the formation of such chains is the subject of the percolation theory. It should be noted that the concept of contact is not just for the particles in direct contact with each other but, apparently, implies convergence of the particles to distances at which the probability of transfer of current carriers between them becomes other than zero. 

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

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