Thermal dispersion

Thermal Dispersion. Thermal dispersion level switches are used on appHcations where multiple shifts inhquid characteristics are present. The unit is responsive only to a change in the thermal conductivity of the Hquid and ignores shifts in specific gravity, dielectric, density, temperature, and pressure. Units are used for alarm signal however, pump control maybe obtained using two units with a latching relay. 

The microwave technique has also been found to be a potential method for the preparation of the catalysts containing highly dispersed metal compounds on high-porosity materials. The process is based on thermal dispersion of active species, facilitated by microwave energy, into the internal pore surface of a microporous support. Dealuminated Y zeolite-supported CuO and CuCl sorbents were prepared by this method and used for S02 removal and industrial gas separation, respectively [5], The results demonstrated the effective preparation of supported sorbents by micro-wave heating. The method was simple, fast, and energy-efficient, because the synthesis of both sorbents required a much lower temperature and much less time compared with conventional thermal dispersion. 

Yu et al. (1982, 1983) give an example of a moving-bed coal gasifier in which axial thermal dispersion must be included in the model. 

Comparison of steady-state profiles shows that neglecting axial mass diffusion has very little effect on the temperature and concentration profiles even though the axial gradients are significant. However, Figure 16 shows that neglecting the axial thermal dispersion in the gas does affect the solution... 

Fig. 16. Steady-state profiles neglecting axial thermal dispersion, type I conditions.

The dependency of melt properties upon temperature has not been taken into account in all the above-mentioned publications. An attempt to calculate the temperature field and the rate of establishment of equilibrium temperature profile determined by thermal dispersion and thermal conductivity was made by H. Winter25,26. ... 

For a monatomic gas, where the heat capacity involves only translational energy, V is independent of sound oscillation frequency (except at ultra-high frequencies, where a classical visco-thermal dispersion sets in). For a relaxing polyatomic gas this is no longer so. At sound frequencies, where the period of the oscillation becomes comparable with the relaxation time for one of the forms of internal energy, the internal temperature lags behind the translational temperature throughout the compression-rarefaction cycle, and the effective values of CT and V in equation (3) become frequency dependent. This phenomenon occurs at medium ultrasonic frequencies, and is known as ultrasonic dispersion. It is accompanied by... 

Pinjala V, Chen YC, Luss D. Wrong-way behavior of packed-bed reactors. II. Impact of thermal dispersion. AIChE J 1988 34 1663-1672. 

Fuel flow control in low-temperature cells is relatively simple. The control valves used are servomotor-driven quarter-turn valves, and the flow sensors can be the types used on gas furnaces, such as thermal dispersion type mass flow meters. 

Controlling the water feed to these boilers is a challenge in itself as the flow rates are low. For a 40 kW fuel cell system, the required water flow range is 0.25 to 10 g/s. Thermal dispersion flow meters can be considered. If conventional control valves do not prove adequate, pulsed solenoids can be used, with the flow being averaged to match the required ratio to the fuel gas. 

Most frequently, com or potato starches are used, but there are also applications of wheat starch, rice starch, tapioca starch and others. Recently, waxy maize starch has found commercial application in the manufacture of paper. Thermally dispersed or pregelatinized unmodified starches are used for paper strength improvement by addition to the furnish or by spraying onto the papermaker s wire. 

Cationic starch must be thermally dispersed before it is added to the papermaking furnish. In batch cooking, a slurry with up to 4% starch content is heated to a temperature of 93-96°C (200-205°F) for 20-30 minutes. In jet cooking, the solids can be raised to 6% and the temperature to 102-116°C (215-240°F) with short retention. For waxy maize starch, the cooking temperature should not exceed 107°C (225°F). The starch cook must be diluted to <1% solids before addition to the paper stock. 

Jang, Jiin Yuh and Chen, Jiing Lin. Thermal Dispersion and Inertia Effects on Vortex Instability of a Horizontal Mixed Convection Flow in a Saturated Porous Medium . Int. J. Heat and Mass Transfer. Vol. 36. No. 2, pp. 383-389. 1993. 

Pinjala, V., Chen. Y. C., and Luss, D. Wrong-Way Behavior of Packed-Bed Reactors II. Impact of Thermal Dispersion. AIChE J., 34, 1663-1672 (1988). 

In studies involving human exposure (Rengstorff and Mershon, 1969a, b), CS (0.1% or 0.25% in water 1.0% in triocyl phosphate) sprayed or administered as ophthalmic drops onto the eyes, caused apraxia of eyelid opening with blepharospasm upon eyelid closure for 10 to 135 s. It also caused a transient conjunctivitis but no comeal damage upon further inspection with a slit lamp. Rabbit eyes contaminated with CS as a solution (0.5-10% in polyethylene glycol), as a solid, or thermally dispersed as a smoke (15 min at 6,000 mg/m ) showed a greater toxicity with solution. CS in solution caused profuse lacrimation, conjunctivitis, iritis, chemosis, keratitis, and corneal vascularization at concentrations at or above 1%. 

Keith, J.M. Leighton, D.T. Chang, H.-C. A new design of reverse-flow reactors with enhanced thermal dispersion. Ind. Eng. Chem. Res. 1999,38,667-682. 

Flow through the porous bed enhances the radial effective or apparent thermal conductivity of packed beds [10, 26]. Winterberg andTsotsas [26] developed models and heat transfer coefficients for packed spherical particle reactors that are invariant with the bed-to-particle diameter ratio. The radial effective thermal conductivity is defined as the summation of the thermal transport of the packed bed and the thermal dispersion caused by fluid flow, or ... 

The symbols are defined as follows e is porosity p is gas density u is gas velocity t is time Cg is specific heat of the gas T is temperature of the gas x is distance Vj, Vi, Cgi, Wi, hi, and Wi are the mass fraction, diffusion velocity, specific heat, molar rate of production, molar enthalpy, and molecular mass of species i, respectively kg is gas thermal conductivity ) is the thermal disperse coefficient /i is the volumetric heat-transfer coefficient between the porous medium and the gas T, Cg, and kg are the temperature, specific heat, and thermal conductivity of the porous medium, respectively and q is the radiative heat flux in the x-direction. 

Since in two-phase flow and heat transfer in porous media for any direction, the bulk effective thermal conductivity is generally much smaller than the bulk thermal dispersion, the available studies on Ke are limited. In the following, we briefly discuss the anisotropy of K, and then review the available treatments. 

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