Core Temperature Profiles

Core analysis includes determination of reactor core neutron flux profiles, determination of the conditions for reactor criticality, the effects of fuel burnup and fission product poisons on core flux and core temperature profiles based upon heat generation rate, coolant flow rate, and core inlet temperature. 

Describe typical core temperature profiles and the basis for their shapes,... 

To keep the core heat transfer design straightforward, simple fins made of stainless steel were placed around the active nuclear core. The actual fin length, width, and pitch (i.e., distance between fins) will be optimized for maximal heat transfer at a minimal parasitic pressure drop. The best location of the cooling fins is between the core and the reflector due to a carbon/clad interface material compatibility concern. These fin parameters will affect the neutron leakage out the ends and the core temperature profile. The fin parameters will also affect the performance of the closed Brayton power conversion cycle because the parasitic pressure drop through the firmed section will penalize the cycle efficiency. 

Along with a constant velocity zone (Zone 1), there is a constant temperature zone in a jet. Heat diffusion in a jet is more intense than momentum diffusion therefore the core of constant temperatures fades away faster than that of constant velocities and the temperature profile is flatter than the velocity profile. Thus the length of the zone with constant temperature (Fig. 7.23) is shorter than the length of the constant velocity zone (Zone I... 

Salamatin, A. N., Lipenkov, V. Y., Barkov, N. I. et al. (1998). Ice core age dating and paleothermometer calibration based on isotope and temperature profiles from deep boreholes at Vostok Station (East Antarctica). /. Geophys. Res. 103(D8), 8963-8977. 

Salamatin, A. N., V. Y. Lipenkov, N. I. Barkov, J. Jouzel, J. R. Petit, and D. Raynaud, Ice Core Age Dating and Paleothermometer Calibration Based on Isotope and Temperature Profiles from... 

If the same amount of source energy were delivered by, say, an electric current over a time larger than the time of development of a minimal flame, the temperature at the core would drop below the flame temperature, the heat liberation in the reaction zone would not attain a balance with the outflow of heat into the preheat zone, and the flame would become extinct. On the other hand, if the current flow were continued for a longer period, the temperature profile ultimately would become sufficiently broad, and the temperature in the core sufficiently high, so that heat liberation within the reaction zone overbalances the outflow of heat and ignition occurs... 

The transfer of heat and/or mass in turbulent flow occurs mainly by eddy activity, namely the motion of gross fluid elements that carry heat and/or mass. Transfer by heat conduction and/or molecular diffusion is much smaller compared to that by eddy activity. In contrast, heat and/or mass transfer across the laminar sublayer near a wall, in which no velocity component normal to the wall exists, occurs solely by conduction and/or molecular diffusion. A similar statement holds for momentum transfer. Figure 2.5 shows the temperature profile for the case of heat transfer from a metal wall to a fluid flowing along the wall in turbulent flow. The temperature gradient in the laminar sublayer is linear and steep, because heat transfer across the laminar sublayer is solely by conduction and the thermal conductivities of fluids are much smaller those of metals. The temperature gradient in the turbulent core is much smaller, as heat transfer occurs mainly by convection - that is, by... 

Many of the models demand that the temperature at some mass cut above the helium core be in excess of the ignition temperature of hydrogen, Th, and hence, the structure is self-inconsistent. For a helium core mass of 4 M , models with total mass in the range 4.05 to 14 M , are excluded. This conclusion does not depend sensitively on the specific value adopted for Th, since the temperature profiles tend to rise so steeply in the excluded range. Similar conclusions follow for models with larger luminosities and... 

In practice, there is always some degree of departure from the ideal plug flow condition of uniform velocity, temperature, and composition profiles. If the reactor is not packed and the flow is turbulent, the velocity profile is reasonably flat in the region of the turbulent core (Volume 1, Chapter 3), but in laminar flow, the velocity profile is parabolic. More serious however than departures from a uniform velocity profile are departures from a uniform temperature profile. If there are variations in temperature across the reactor, there will be local variations in reaction rate and therefore in the composition of the reaction mixture. These transverse variations in temperature may be particularly serious in the case of strongly exothermic catalytic reactions which are cooled at the wall (Chapter 3, Section 3.6.1). An excellent discussion on how deviations from plug flow arise is given by DENBIGH and TURNER 5 . 

Fig. 4.19 Temperature-concentration diagram for a binary mixture as well as the temperature and concentration profiles in the vapour and the condensate. Indices 0 cold wall, I interface, G core flow of vapour (G Gas), a boiling and dew point lines b condensate and vapour boundary layer c temperature profile d concentration profile...

As long as the wall temperature stays below that required for the formation of vapour bubbles, heat will be transferred by single-phase, forced flow. If the wall is adequately superheated, vapour bubbles can form even though the core liquid is still subcooled. This is a region of subcooled boiling. In this area, the wall temperature is virtually constant and lies a few Kelvin above the saturation temperature. The transition to nucleate boiling, is, by definition, at the point where the liquid reaches the saturation temperature at its centre, and with that the thermodynamic quality is r h = 0. In reality, as Fig. 4.53 indicates, the liquid at the core is still subcooled due to the radial temperature profile, whilst at the same time vapour bubbles form at the wall, so that the mean enthalpy is the same as that of the saturated liquid. As explained in the previous section, the... 

The temperature-profile data in Prob. 13-3 are to be represented by a constant kg (across the tube diameter) and a wall heat-transfer coefficient h . Estimate the values of kg and which best fit the temperature data. Note that at the boundary layer between the wall and the central core of the bed the following relation must apply ... 

Temperature profile. Let us discuss qualitative specific features of convective heat and mass transfer in turbulent flow past a flat plate. Experimental evidence indicates that several characteristic regions with different temperature profiles can be distinguished in the thermal boundary layer on a flat plate. At moderate Prandtl numbers (0.5 < Pr < 2.0), it can be assumed for rough estimates that the characteristic sizes of these regions are of the same order of magnitude as those of the wall layer and the core of the turbulent stream, see Section 1.7. 

---