Fuel surface temperature

Superheated steam far enough from critical conditions for property-averaging to be meaningful. The gas properties correspond to 35 atm (500 psi) and 535°C (1(XX)°F). Gas inlet and outlet temperature and pressure, and maximum fuel surface temperature are all fixed. 

This type of fuel element has the advantage that the structural parts of the element are unaffected by fission recoil damage, and, hence, the fuel burnup is not limited by impairment of the structural integrity of the fuel element. Furthermore, since these fuel elements consist only of graphite and the fuel materials themselves, both of which can withstand very high temperatures, a large improvement in the maximum allowable fuel surface temperature becomes possible—1000°C (1800°F) or higher. 

In-pile loop tests of this type of element were successfully carried out for 5700 hr at a maximum fuel surface temperature of 1500° to 1700°F and for appropriate burnup conditions (45). A full core loading of these fuel elements was manufactured for the EBOR reactor. 

With the fuel surface temperature and internal heat generation rate known, the fuel centerline temperature is calculated from ... 

Reactor Temperature -900-1150K Fuel Surface Temperature to 1400 K... 

Once the boundary conditions are fixed (i.e., local decay heat generation rate, inlet coolant temperature, channel coolant flow rate, plenum pressure, assembly pressure drop and inlet air void fraction), FLOWTRAN-TF iterates between cells/nodes to obtain an axial temperature distribution for the fuel assembly subchannel surfaces as a function of time. In the calculation for power limits, the assembly power is progressively increased in increments until one of the subchannel surface nodes equals or exceeds the ECS T/H criterion (i. e., the fuel surface temperature exceeds the coolant saturation/boiling temperature for the static pressure conditions at that axial location (T... 

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. 

Fuel surface temperature (T ) remains equal to or less than the local... 

Development of molybdenum electrodes in the 1950s permitted the use of electrically assisted melting in regenerative furnaces (81). In the 1990s, approximately one-half of all regenerative tanks ate electrically boosted. Operating practice has shown that effective use of electricity near the back end of the furnace, where the batch is added, can reduce fossil fuel needs. This lowers surface temperature and reduces batch volatilisation. 

Electrode Walls. Development of durable electrode wads, one of the most critical issues for MHD generators, has proceeded in two basic directions ceramic electrodes operating at very high surface temperatures (>2000 K) for use in channels operating with clean fuels such as natural gas, and cooled metal electrodes with surface temperatures in the range 500—800 K for channels operating with slag or ash-laden flows. 

Most theories of droplet combustion assume a spherical, symmetrical droplet surrounded by a spherical flame, for which the radii of the droplet and the flame are denoted by and respectively. The flame is supported by the fuel diffusing from the droplet surface and the oxidant from the outside. The heat produced in the combustion zone ensures evaporation of the droplet and consequently the fuel supply. Other assumptions that further restrict the model include (/) the rate of chemical reaction is much higher than the rate of diffusion and hence the reaction is completed in a flame front of infinitesimal thickness (2) the droplet is made up of pure Hquid fuel (J) the composition of the ambient atmosphere far away from the droplet is constant and does not depend on the combustion process (4) combustion occurs under steady-state conditions (5) the surface temperature of the droplet is close or equal to the boiling point of the Hquid and (6) the effects of radiation, thermodiffusion, and radial pressure changes are negligible. 

Surface Temperatures. At low temperatures, the oxidation reaetions on the eatalyst are kinetieally eontrolled, and the eatalyst aetivity is an important parameter. As the temperature inereases, the build-up of heat on the eatalyst surfaee due to the exothermie surfaee reaetions produees ignition and the eatalyst surfaee temperature jumps rapidly to the adiabatie flame temperature of the fuel/air mixture on ignition. Figure 10-26 shows a... 

Two limiting cases for gasification at the fuel surface were considered. In case 1, the fuel concentration was assumed constant and independent of time, i.e., f(Cf) = Cf and in case 2, it was assumed that the fuel mass flux was constant and independent of time or pressure, i.e.,/(Cy) = — D 8Cf/ dx = rfi. Case 1 was identified with a condensed phase behaving as a boiling liquid or subliming solid, and case 2 with a polymer undergoing irreversible decomposition at constant temperature. 

The maximum heat flux achievable with nucleate boiling is known as the critical heat flux. In a system where the surface temperature is not self-limiting, such as a nuclear reactor fuel element, operation above the critical flux will result in a rapid increase in the surface temperature, and in the extreme situation the surface will melt. This phenomenon is known as burn-out . The heating media used for process plant are normally self-limiting for example, with steam the surface temperature can never exceed the saturation temperature. Care must be taken in the design of electrically heated vaporisers to ensure that the critical flux can never be exceeded. 

I apply these computational methods to various aspects of the Earth system, including the responses of ocean and atmosphere to the combustion of fossil fuels, the influence of biological activity on the variation of seawater composition between ocean basins, the oxidation-reduction balance of the deep sea, perturbations of the climate system and their effect on surface temperatures, carbon isotopes and the influence of fossil fuel combustion, the effect of evaporation on the composition of seawater, and diagenesis in carbonate sediments. These applications have not been fully developed as research studies rather, they are presented as potentially interesting applications of the computational methods. 

The liquid temperature (Tl) corresponding to Xl is measured for practical purposes in two apparatuses known as either the closed or open cup flashpoint test, e.g. ASTM D56 and D1310. These are illustrated in Figure 6.3. The surface concentration (Xs) will be shown to be a unique function of temperature for a pure liquid fuel. This temperature is known as the saturation temperature, denoting the state of thermodynamic equilibrium... 

This should be compared to the measured open or closed cups of 285 and 289 K. We expect the computed value to be less, since the ignitor is located above the surface and mixing affects the ignitor concentration of fuel. Thus, a higher liquid surface temperature is needed to achieve A) at the ignitor. 

Let us just consider the piloted ignition case. Then, at Tpy a sufficient fuel mass flux is released at the surface. Under typical fire conditions, the fuel vapor will diffuse by turbulent natural convection to meet incoming air within the boundary layer. This will take some increment of time to reach the pilot, whereby the surface temperature has continued to rise. 

Equation (8.6) demonstrates that as the face weight, pd, decreases the spread rate increases. Moreover, if a material undergoing spread is heated far away from the flame, such as would happen from smoke radiation in a room fire, Ts will increase over time. As Ts - Tig, an asymptotic infinite speed is suggested. This cannot physically happen. Instead, the surface temperature will reach a pyrolysis temperature sufficient to cause fuel vapor at the lower flammable concentration. Then the speed of the visible flame along the surface will equal the premixed speed. This speed in the gas phase starts at about 0.5 m/s... 

Let us recall from the discussion of liquid evaporation that thermodynamically we have a property relationship between fuel vapor concentration and surface temperature,... 

We do expect Y0l (0) to be zero at the surface for combustion within the boundary layer since the flame reaction is fast and no oxygen is left. This must be clearly true even if the chemistry is not so fast. Moreover, since we are heating the surface with a nearby flame that approaches an adiabatic flame temperature, we would expect a high surface temperature. For a liquid fuel, we must have... 

PMMA bums in air at an average m" — 15 g/m2 s. Air temperature is 20 °C and the heat transfer coefficient is 15 W/m2 K. Determine the net radiative heat flux to the fuel surface. What amount comes from the flame ... 

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