Thermal Feedback Effect Research Articles

Superphénix Reactivity and Feedback Coefficients

The different thermal feedback effects occurring in the Superphenix reactor are described. The experiments carried out to measure these effects are also described, and the corresponding results are...

Inherent reactor power controller for a metal-fueled ALMR

Inherent power control for metal-fueled ALMR (advanced liquid metal reactor) designs involves using reactivity thermal feedback effects to control reactor power. Using classical control design techniques, a control system for normal load following maneuvers was designed for a pool-type ALMR. This design provides active control of power removal in the balance of the plant, direct control of selected primary and intermediate loop temperatures, and passive control of reactor power. The inherent stability of the strong, fast reactivity feedback effects brings heat production in the core into balance with the heat removal system temperatures, which are controlled to meet power demand. A simulation of the control system successfully responded to a 10% step change in power demand by charging power at an acceptable rate without causing large temperature fluctuations or exceeding thermal limits.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Stomatal regulation in greenhouse crops: analysis and simulation

The impact of the stomatal opening variations on the transpiration of the entire canopy in protected crops cultivation is studied. Three effects contribute to a reduction of this impact: a damping effect due to the boundary layer adhering to the leaf, a thermal negative feedback effect linked to the energy balance and a hydrologic negative feedback effect linked to the water balance. Summing up these effects gives a coupling factor between the relative stomatal conductance changes and the relative transpiration rate modifications. Detailed expression of the coupling factor is established and a sensitivity analysis is performed. It shows that the coupling factor is practically insensitive to the exterior climate but depends mainly on the greenhouse and crop characteristics. Simulations are also carried out whose results validate the theory and confirm the sensitivity analysis. In addition they give an order of magnitude for the C factor which is in agreement with experimental measurements. They also show that large stomatal conductances are poorly effective in the control of the canopy transpiration ( C≅0.1). Significant values of C are only obtained for very small values of stomatal conductances.

Output conductance frequency dispersion and low-frequency noise in HEMTs and MESFETs

It is reported that the high level of low-frequency noise observed in MESFETs and HEMTs in the saturation regime usually scales with the device output conductance frequency dispersion (in MESFETs) or with parallel conduction through the GaAlAs (in HEMTs). It does not seem to be directly related to output resistance frequency dispersion. The fundamental cause of output resistance dispersion in HEMTs is still unknown. Thermal feedback effects, low-frequency variations of series resistances, and trap-related effects are possibilities that require investigation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Quadratic autocatalysis in a non-isothermal CSTR

Typical analyses of chemical systems have so far been carried out on either isothermal autocatalytic systems or deceleratory reactions with thermal feedback. In real chemical systems, however, heat may often be released by an autocatalytic reaction. The quadratic autocatalator with kinetic scheme ▪ is now studied as an example of a non-isothermal, non-adiabatic reaction in a CSTR. The techniques of singularity theory and Hopf theory are used to study the effects of thermal feedback on the steady-state and the dynamic behaviour of this four parameter system. An interesting variety of steady-state behaviour is found including breaking wave, isola and mushroom patterns. Hopf bifurcations and sustained oscillations also exist for large ranges of parameter values with a wide combination of stable and unstable limit cycles being found.

Stratospheric chemical and thermal response to long‐term variability in solar UV irradiance

A theoretical analysis of the chemical response of the stratosphere to possible long‐term variability of solar ultraviolet irradiance has been performed, taking into account the thermal feedback effect on the reaction rates. Numerical values of ultraviolet and visible irradiation fluxes used in this work are given for aeronomic modeling purposes and a possible variability related to the 11‐year solar cycle is suggested on the basis of recent and reliable observations of solar ultraviolet irradiance. This variability has been introduced in a stratospheric two‐dimensional model which simulates the zonally averaged distribution of the chemical species related to the oxygen, hydrogen, nitrogen and chlorine families. The results lead to a total ozone variation of the order of 3% from the minimum to the maximum solar activity, with a maximum of about 10% in the upper stratosphere. At these heights, the calculated temperature change is close to 2–4 degrees. The N2O concentration appears to be one of the most sensitive to long‐term solar variability and a monitoring of this constituent would be useful to give information on the solar variability in the ultraviolet.

The KWU PCI modelling approach

Since PCI fuel rod failures are generally attributed to stress corrosion cracking of the clad, mechanical as well as chemical aspects must be considered in PCI modelling. Transient fission gas release, amplified by a thermal feedback effect, is expected to be very important with regard to both aspects. In view of the limited possibilities to describe analytically the complex mechanisms contributing to PCI failures, KWU uses a semiempirical PCI approach to evaluate the experimental results from ramp tests and power reactor experience and to develop preliminary PCI failure criteria. In parallel to the empirical approach, systematic PCI investigations are performed. To calculate stresses and strains during ramps, including fuel and clad plasticity and creep, the code SENATOR has been developed. The ANSYS code is used for the calculation of local stress and strain effects.

Results and analysis of KWU power ramp investigations

KWU, in cooperation with others, is conducting extensive power ramping investigations. The results from the Petten experiments show that a lower limit for the failure range for fast ramps can be defined. This limit is confirmed by ramping experiments in power reactors, and is in agreement with observations from production fuel in power reactors. The post-irradiation investigations revealed that just around these power levels many characteristic phenomena change rapidly with further increase in power. Modified in-situ ramps have shown that the threshold can be safely exceeded if the ramp rate in the upper power range is limited. The experimental results can well be interpreted using the interference approach which calculates the hoop strain in the clad considering design parameters and the previous power history. Prepressurization is obviously an effective remedy against PCI by suppressing the thermal feed-back effect. Additional remedies are under investigation.

The effect of hot spots on the noise characteristic of large-area bipolar transistors

Measurements indicate that when hot spots are formed in large-area silicon power transistors, the 1 f noise increases and a new noise component with a 1 f 2 spectrum shows up. The increase in 1 f noise is attributed to the decrease in effective device area when the hot spot is formed. The noise with a 1 f 2 spectrum is attributed to a thermal feedback effect in the hot spot.

The monolithic op amp: a tutorial study

A study is made of the integrated circuit operational amplifier (IC op amp) to explain details of its behavior in a simplified and understandable manner. Included are analyses of thermal feedback effects on gain, basic relationships for bandwidth and slew rate, and a discussion of pole-splitting frequency compensation. Sources of second-order bandlimiting in the amplifier are also identified and some approaches to speed and bandwidth improvement are developed. Brief sections are included on new JFET-bipolar circuitry and die area reduction techniques using transconductance reduction.

A five-terminal plus or minus 15-V monolithic voltage regulator

A five-terminal /spl plusmn/15-V monolithic voltage regulator has been developed that incorporates internal frequency compensation and internally provides a /spl plusmn/1 percent output voltage tolerance. In addition, a thermally symmetric layout design of the chip has been used to eliminate the detrimental effects of thermal feedback on the die and ensure that the complementary tracking output voltages will be independent of the power dissipation in the series pass power transistors. Complete fault protection is accomplished by providing the power transistors with good dc safe operating area, internally limiting the short circuit output currents, and accurately limiting the junction temperature to within 10/spl deg/C of the specified maximum limit. Also, a new Zener diode geometry is employed that significantly reduces the noise associated with the reference voltage.

A Critical Evaluation of the Effect of Thermal Feedback in Liquid Flow

In a recent series of articles Gruntfest has obtained numerical solutions of the equations relating the effects of a dissipative process like viscous flow on the observable flow patterns of a heat-sensitive material. Gruntfest has implied that non-Newtonian flow is at least partially due to this thermal feedback, and he has demonstrated its significance in the flow of polymer-modified oils reported on by Philippoff. This raises the question of how important these effects are in general, i.e., is non-Newtonian flow an experimental artifact. In this paper the qualitative and quantitative predictions of Grantfest's analysis are examined. These predictions are then compared to some experimental data obtained with simple fluids, polymer solutions, polymer melts, and colloidal dispersions. It is shown that non-Newtonian flow usually appears well before the rates of shear at which thermal effects would be noticed. Hence, non-Newtonian flow is not an experimental artifact brought about by the inability to transfer thermal energy away from heat-sensitive materials.