Collisional heat transfer

Of course all surfaces that are at such low temperatures must be kept out of contact with the ambient environment. This is achieved by a detachable and rotable vacuum shroud that surrounds the two expander stages and the sample, all of which must be kept under high vacuum while they are cold to avoid collisional heat transfer. By default, evacuation of the assembly occurs through vacuum ports mounted on the main body of the expander, but in some cases it is advantageous to have extra ports on the vacuum shroud itself. Furthermore, the first expansion stage of closed-cycle cryostats, where a temperature of 35 10 K is attained, is usually fitted with... 

In gas-solid suspensions and fluidized beds, the heat transfer between particles and the wall surface or between a particle at one temperature and a group of other particles at another temperature is largely due to particle impacts. Thus, the average rate of heat transfer may be expressed in terms of a collisional heat transfer coefficient hc, which is defined by... 

Example 4.1 Determine the collisional heat transfer coefficients under each of the following conditions (1) collisions of a cloud of hot particles with a cold particle (2) collisions of a cloud of cold particles with a hot wall. Assume the particles are in random motion with the average impact velocity of 0.1 m/s. All the particles are spherical and of the same diameter of 100 fim. The particles and wall are made of steel with v = 0.3, E = 2 x 105 MPa, Pp = 7,000 kg/m3, Kp = 30 W/m K, and c = 500 J/kg K. The particle volume fraction is 0.4. 

Therefore, from Fig. 4.2, the correction factor C is found to be 5. The collisional heat transfer coefficient by interparticle collisions is thus given by Eq. (4.34) as... 

Cm Overall molar concentration hc Collisional heat transfer... 

For particle—wall static or collision contact, a wall can be treated as a particle with an infinite diameter and mass, as commonly used in the DEM. For two coUiding particles, if only collisional heat transfer... 

Of a special astronomical interest is the absorption due to pairs of H2 molecules which is an important opacity source in the atmospheres of various types of cool stars, such as late stars, low-mass stars, brown dwarfs, certain white dwarfs, population III stars, etc., and in the atmospheres of the outer planets. In short absorption of infrared or visible radiation by molecular complexes is important in dense, essentially neutral atmospheres composed of non-polar gases such as hydrogen. For a treatment of such atmospheres, the absorption of pairs like H-He, H2-He, H2-H2, etc., must be known. Furthermore, it has been pointed out that for technical applications, for example in gas-core nuclear rockets, a knowledge of induced spectra is required for estimates of heat transfer [307, 308]. The transport properties of gases at high temperatures depend on collisional induction. Collision-induced absorption may be an important loss mechanism in gas lasers. Non-linear interactions of a supermolecular nature become important at high laser powers, especially at high gas densities. 

Theologos and Markatos (1992) used the PHOENICS program to model the flow and heat transfer in fluidized catalytic cracking (FCC) riser-type reactors. They did not account for collisional particle-particle and particle-wall interactions and therefore it seems unlikely that this type of simulation will produce the correct flow structure in the riser reactor. Nevertheless it is one of the first attempts to integrate multiphase hydrodynamics and heat transfer. 

Arrowsmith et al used the crossed beam reaction F+Na— NaF+Na (3 P) to study radiative transfer and electronic energy transfer (E — E, V) in the Na (3 P)-1-NajCX S ) system. Previous studies of the Na2 system have utilized high-pressure cells or heat pipes in which radiation trapping is strong and Na + Na2 collisional energy transfer dominates. Time-resolved emission, following pulsed dye-laser excitation, has been used by Husain and his coworkers in a systematic survey of the excited-state behaviour of Mg(3 Pj), Ca(4 P,), and Sr(5 Pj). Dye-laser excitation of Mg vapour at 457.1 nm resulted in the observation of slow spontaneous emission from Mg(3 P,) which... 

These steps couple the reacting system A with the heat reservoir M by intermolecular energy transfer. Molecules A, which have gained sufficient energy through collisional energy transfer, are denoted by A . These can decompose by intramolecular processes ... 

The ideal diode thermionic converter model corresponds to a thermionic converter in which the emitter and collector are spaced so closely that no collisional or space charge effects take place. To reduce the complexity of the equations, ion emission effects will also be neglected. Although these assumptions do not strictly correspond to any thermionic converter, they do approach those of a very closely spaced diode operating in the vacuum mode. The ideal diode model defines the performance limit imposed by essential electron emission and heat transfer and provides a basis for comparison with practical converters. 

We consider an ensemble of reactant molecules with quantized energy levels to be immersed in a large excess of (chemically) inert gas which acts as a constant temperature heat bath throughout the reaction. The requirement of a constant temperature T of the heat bath implies that the concentration of reactant molecules is very small compared to the concentration of the heat bath molecules. The reactant molecules are initially in a MaxweD-Boltzmann distribution appropriate to a temperature T0 such that T0 < T. By collision with the heat bath molecules the reactants are excited in a stepwise processs into their higher-energy levels until they reach "level (2V+1) where they are removed irreversibly from the reaction system. The collisional transition probabilities per unit time Wmn which govern the rate of transfer of the reactant molecules between levels with energies En and Em are functions of the quantum numbers n and m and can, in principle, be calculated in terms of the interaction of the reactant molecules with the heat bath. 

---