Fuels thermal conductivity

The analyses were conducted for a singe rod, which is assumed to have the highest bum-up in the RMWR core. The models or material properties applied to the analysis, such as fuel thermal conductivity, fission product (FP) gas diffusion and release and creep rate are derived or extrapolated from those in the usual analysis of LWR fuel rods. For the first analysis, particular focus was on the thermal behaviour, such as FP gas release and internal pressure increase induced by the fuel temperature rise. 

X-9] BARON, D., GOUTY, J.C., A proposal for a unified fuel thermal conductivity model available for UO2 (U-PuO)2, and U02-Gd203 PWR fuel. Water Reactor Fuel Element Modelling at High Burnup and its Experimental Support, lAEA-TECDOC 957, Vienna (1997). 

D. Baron and J. C. Couty, A Proposal for Unified Fuel Thermal Conductivity Model Available for UO2, (U-Pu)02, and UOz-GdzOj PWR Fuel, Proc. the IAEA TCM on Water Reactor Fuel Element Modeling at High Burnup and Its Experimental Support, Windermere, UK (1994)... 

The third characteristic of interest grows directly from the first, ie, the high thermal conductance of the heat pipe can make possible the physical separation of the heat source and the heat consumer (heat sink). Heat pipes >100 m in length have been constmcted and shown to behave predictably (3). Separation of source and sink is especially important in those appHcations in which chemical incompatibilities exist. For example, it may be necessary to inject heat into a reaction vessel. The lowest cost source of heat may be combustion of hydrocarbon fuels. However, contact with an open flame or with the combustion products might jeopardize the desired reaction process. In such a case it might be feasible to carry heat from the flame through the wall of the reaction vessel by use of a heat pipe. 

As an alternative option, the insulation should meet the recommendations of BS 5422 1977. This Standard tabulates thicknesses of insulation too numerous to mention here, according to whether (1) the pipes carry central heating or domestic hot water, (2) the system is heated by gas and oil or solid fuel, (3) the water temperature is 75°C, 100°C or 150°C and (4) the thermal conductivity of the insulant is 0.04, 0.55 or 0.70 W/mK at the appropriate mean temperature. 

A fuel channel in a natural uranium reactor is 5 m long and has a heat release of 0.25 MW. If the thermal conductivity of the uranium is 33 W/mK, what is the temperature difference between the surface and the centre of the uranium element, assuming that the heat release is uniform along the rod ... 

Richardson, J.F. Fuel 28 (1949) 265. Spread of fire by thermal conduction. 

Yet thermal conductivity alone is not sufficient for the characterization of gaseous mixtures. Given a fixed air ratio and temperature, thermal conductivity of flue gases, resulting from the combustion of different fuels, does not vary by more than 1%. A changing air ratio has a smaller effect than a small rise in the flue gas temperature. Therefore the thermal conductivity alone is not suitable as reference value. Further information is required to identify fuel gases. 

A fire occurs in a 3 m cubical compartment made of 2 cm thick. Assume steady state heat loss through the concrete whose thermal conductivity is 0.2 W/m2 K. By experiments, it is found that the mass loss rate, m, of the fuel depends on the gas temperature rise, AT, of the compartment upper smoke layer ... 

A fire in a ship compartment bums steadily for a period of time. The average smoke layer achieves a temperature of 420 °C with the ambient temperature being 20 °C. The compartment is constructed of 1 cm thick steel having a thermal conductivity of 10 W/m2 K. Its open doorway hatch is 2.2 m high and 1.5 m wide. The compartment has an interior surface area of 60 m2. The fuel stoichiometric air to fuel mass ratio is 8 and its heat of combustion is 30 kJ/g. 

The form of Boq and Blq presented in Eq. (6.120) is based on the assumption that the fuel droplet has infinite thermal conductivity, that is, the temperature of the droplet is Ts throughout. But in an actual porous sphere experiment, the fuel enters the center of the sphere at some temperature 7) and reaches Ts at the sphere surface. For a large sphere, the enthalpy required to raise the cool entering liquid to the surface temperature is cpi(Ts — 7)) where cpi is the specific heat of the liquid fuel. To obtain an estimate of B that gives a conservative (lower) result of the burning rate for this type of condition, one could replace Lv by... 

Latent heat of vaporization (cal/gm) Density (gm/cm3) Thermal conductivity (cal/s-1/ cirr /KT1) Stoichiometric heat evolution in air per unit weight of fuel (cal/gm) Heat capacity (cal/gm-1/ K->)... 

Due to their high electrical and thermal conductivity, materials made out of metal have been considered for fuel cells, especially for components such as current collectors, flow field bipolar plates, and diffusion layers. Only a very small amount of work has been presented on the use of metal materials as diffusion layers in PEM and DLFCs because most of the research has been focused on using metal plates as bipolar plates [24] and current collectors. The diffusion layers have to be thin and porous and have high thermal and electrical conductivity. They also have to be strong enough to be able to support the catalyst layers and the membrane. In addition, the fibers of these metal materials cannot puncture the thin proton electrolyte membrane. Thus, any possible metal materials to be considered for use as DLs must have an advantage over other conventional materials. 

To design the optimal diffusion layer for a specific fuel cell system, it is important to be able to measure and understand all the parameters and characteristics that have a direct influence on the performance of the diffusion layers. This section will discuss in detail some of the most important properties that affect the diffusion layers, such as thickness, hydrophobicity and hydrophilicity, porosity and permeability (for both gas and liquids), electrical and thermal conductivity, mechanical properties, durability, and flow... 

Electrical and thermal conductivity are important diffusion layer properties that affect the fuel cell s overall performance. The maferial chosen to be the DL in a fuel cell must have a good electrical conductivity in order for the electron flow from the FF plates to the CLs (and vice versa) to have the least possible resistance. Similarly, the DL material must have good thermal properties so that heat generated in the active zones can be removed efficiently. Therefore, in order to choose an optimal material it is critical to be able to measure the electrical and thermal conductivity. In this section, a number of procedures used fo measure fhese paramefers will be discussed. 

As mentioned by Mathias et al. [9], reliable methods to measure the thermal conductivity of diffusion layers as a function of compression pressures are very scarce in the open literature. Khandelwal and Mench [112] designed an ex situ method to measure accurately the thermal conductivities of different components used in a fuel cell. In their apparatus, the sample materials were placed between two cylindrical rods made out of aluminum bronze (see Figure 4.28). Three thermocouples were located equidistantly in each of the upper and lower cylinders to monitor the temperatures along these components. Two plates located at each end compressed both cylinders together. The temperatures of each plate were maintained by flowing coolant fluids at a high flow rate through channels located inside each of the plates. A load cell was located between two plates at one end so that the compression pressure could be measured. 

It is important to note that Vie and Kjelstrup [250] designed a method of measuring fhe fhermal conductivities of different components of a fuel cell while fhe cell was rurming (i.e., in situ tests). They added four thermocouples inside an MEA (i.e., an invasive method) one on each side of the membrane and one on each diffusion layer (on the surface facing the FF channels). The temperature values from the thermocouples near the membrane and in the DL were used to calculate the average thermal conductivity of the DL and CL using Fourier s law. Unfortunately, the thermal conductivity values presented in their work were given for both the DL and CL combined. Therefore, these values are useful for mathematical models but not to determine the exact thermal characteristics of different DLs. 

R. A. Mercuri, T. W. Weber, and M. L. Warddrip. Flexible graphite article and fuel cell electrode with enhanced electrical and thermal conductivity. WO Patent 0178179 (2001). 

M. Khandelwal and M. M. Mench. Direct measurement of through-plane thermal conductivity and contact resistance in fuel cell materials. Journal of Power Sources 161 (2006) 1106-1115. 

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