Convection media

Advances in high-throughput HPLC media such as perfusive and convective media enable purification of antibodies in a much shorter time than with the conventional media. Not only will this encourage a more systematic method development and optimization process that can lead to a better product, it will probably also have a positive economic impact in downstream processing. 

However, if the rate of heat transfer is not infinite the thermal shock-induced stresses will gradually build up and after some time reach a peak value that will be a fraction of the value given by equation (15.10). The solution requires transient stress analysis such as those of Cheng (1951) and Manson (1966) with the assumption of the plate of Fig. 15.1 being infinite. Following Fu and Fleck (1998), the plate is initially held at temperature T0 and at time t= 0 its top and bottom faces (at /= H) are suddenly exposed to a convective medium of temperature T, . The surface heat flow is assumed to satisfy... 

The precuring of the specimens were accomplished by using the RMS environmental chamber as the curing oven. Compressed air or vaporized liquid nitrogen was used as the convection medium. 

In capillary gel electrophoresis (CGE), the capillary is fllled with a gel rather than with a free solution. The separation in CGE works on the same principle as slab gel electrophoresis The analytes migrate depending on their electrophoretic mobility but they are retarded by the gel pores depending on their size. The gel acts as an anti-convective medium, reducing band broadening and as a molecular sieving material. 

If the initial moisture content of the textile material is high enough, the surface is covered with a continuous layer of See water and evaporation takes place mainly at the surface. Internal moisture transfer is mainly attributable to capillary flow of free water through the pores. Therefore, the drying rate is determined by external conditions only, i.e., the temperature, humidity and flow rate of the convective medium, and a constant drying rate period will be observed. As drying proceeds, the fraction of wet area decreases with decreasing surface moisture content, so that the mass transfer coefficient decreases. In order to predict how the wet area fraction varies with surface moisture content, it is necessary to introduce percolation theory. 

Free circulation of the coolant from the machine to the surrounding medium 0 Free convection No external power source is essential. Fleat dissipation is achieved through natural convection like a surface cooled motor... 

Contact temperature measurement is based on a sensor or a probe, which is in direct contact with the fluid or material. A basic factor to understand is that in using the contact measurement principle, the result of measurement is the temperature of the measurement sensor itself. In unfavorable situations, the sensor temperature is not necessarily close to the fluid or material temperature, which is the point of interest. The reason for this is that the sensor usually has a heat transfer connection with other surrounding temperatures by radiation, conduction, or convection, or a combination of these. As a consequence, heat flow to or from the sensor will influence the sensor temperature. The sensor temperature will stabilize to a level different from the measured medium temperature. The expressions radiation error and conduction error relate to the mode of heat transfer involved. Careful planning of the measurements will assist in avoiding these errors. 

We have so far assumed that the atoms deposited from the vapor phase or from dilute solution strike randomly and balHstically on the crystal surface. However, the material to be crystallized would normally be transported through another medium. Even if this is achieved by hydrodynamic convection, it must nevertheless overcome the last displacement for incorporation by a random diffusion process. Therefore, diffusion of material (as well as of heat) is the most important transport mechanism during crystal growth. An exception, to some extent, is molecular beam epitaxy (MBE) (see [3,12-14] and [15-19]) where the atoms may arrive non-thermalized at supersonic speeds on the crystal surface. But again, after their deposition, surface diffusion then comes into play. 

Zone electrophoresis is defined as the differential migration of a molecule having a net charge through a medium under the influence of an electric field (1). This technique was first used in the 1930s, when it was discovered that moving boundary electrophoresis yielded incomplete separations of analytes (2). The separations were incomplete due to Joule heating within the system, which caused convection which was detrimental to the separation. 

Radiative heat transfer is perhaps the most difficult of the heat transfer mechanisms to understand because so many factors influence this heat transfer mode. Radiative heat transfer does not require a medium through which the heat is transferred, unlike both conduction and convection. The most apparent example of radiative heat transfer is the solar energy we receive from the Sun. The sunlight comes to Earth across 150,000,000 km (93,000,000 miles) through the vacuum of space. FIcat transfer by radiation is also not a linear function of temperature, as are both conduction and convection. Radiative energy emission is proportional to the fourth power of the absolute temperature of a body, and radiative heat transfer occurs in proportion to the difference between the fourth power of the absolute temperatures of the two surfaces. In equation form, q/A is defined as ... 

Thermal conductivity increases with temperature. The insulating medium (the air or gas within the voids) becomes more excited as its temperature is raised, and this enhances convection within or between the voids, thus increasing heat flow. This increase in thermal conductivity is generally continuous for air-filled products and can be mathematically modeled (see Figure 11.3). Those insulants that employ inert gases as their insulating medium may show sharp changes in thermal conductivity, which may occur because of gas condensation. However, this tends to take place at sub-zero temperatures. 

Loop Tests Loop test installations vary widely in size and complexity, but they may be divided into two major categories (c) thermal-convection loops and (b) forced-convection loops. In both types, the liquid medium flows through a continuous loop or harp mounted vertically, one leg being heated whilst the other is cooled to maintain a constant temperature across the system. In the former type, flow is induced by thermal convection, and the flow rate is dependent on the relative heights of the heated and cooled sections, on the temperature gradient and on the physical properties of the liquid. The principle of the thermal convective loop is illustrated in Fig. 19.26. This method was used by De Van and Sessions to study mass transfer of niobium-based alloys in flowing lithium, and by De Van and Jansen to determine the transport rates of nitrogen and carbon between vanadium alloys and stainless steels in liquid sodium. 

In a number of these processes, a liquid heat- and mass-transfer medium is in direct contact with the catalyst and the reaction mixture. The main function of the liquid is to act as a heat sink and as a medium for convective heat transfer. However, since the liquid may be assumed to cover the solid particles and in this way act as a barrier between the gaseous and the solid phases, it must therefore also function as a mass-transfer medium. 

The estimation of the diffusional flux to a clean surface of a single spherical bubble moving with a constant velocity relative to a liquid medium requires the solution of the equation for convective diffusion for the component that dissolves in the continuous phase. For steady-state incompressible axisym-metric flow, the equation for convective diffusion in spherical coordinates is approximated by... 

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