Spinning heat transfer coefficient

In the 1950s Hickman developed a centrifugal vapor compression evaporator for seawater desalination (53). This device consisted of multiple spinning discs. Seawater sprayed on one side of the disc evaporated, while the centrifugal force removed the residue from the plate surface. The vapor was compressed and returned to the opposite side of the plate, where condensation provided the heat for evaporation and the desired freshwater for recovery. Overall heat transfer coefficients of 18 kW/m2-K are about three times higher than those achieved in steam turbine condensers. 

In order to improve the heat transfer in a reactor, use can be made of gravitational forces. This concept is used in the spinning disc reactor (SDR) as developed at Newcastle University. The reaction mixture flows in a thin layer in axial direction over a rotating disc. A typical heat transfer coefficient is 10 kW/m2K. This reactor however is dedicated for liquid-liquid reactions. Especially condensation reactions can be enhanced by removing the gaseous by-products thus shifting the chemical equilibrium to the right. 

The proper representation of macroscopic transport properties, particularly the heat transfer coefficient, is a major problem in the predictive modeling of spinning and other free-surface processing flows. Heat transfer coefficients are typically obtained from experiments on nondeforming wires, and the extension to a deforming surface with a variable cross section is not obvious. Data obtained on real spinlines require either infrared or intrusive contact temperatme... 

Another centrifugal evaporator concept was proposed by Porter and Ramshaw (1988). Unlike the unit described above, this uses a large number of plates located normal to the rotating shaft, and is designed to accommodate a wider range of separations than that of the above apparatus. As described in Chapter 4 and in the context of spinning disc reactors in Chapter 5, the enhanced evaporation (and condensation) heat transfer coefficients on spinning disc surfaces are of considerable benefit. 

Figure 3.10 The structure of the spinning disc heat transfer surface. Overall heat transfer coefficient of 10 kW/m K for low viscosity process fluids and water or glycol as heat/cool fluid. A 0.5 m dia disc with deltaT 20K will transfer 39kW, thanks to patented double spiral disc design, copper construction (with chrome plating for corrosion resistance).

The blowup ratio, BUR, is the ratio of the final bubble radius to the initial radius and is an analog of the draw ratio in spinning. Primes denote differentiation with respect to the dimensionless axial length, which is normalized with respect to Ro. The differential equations are more complex for viscoelastic liquids, and the stress equations must be solved in parallel with the momentum equations, r = w = 1 at the exit from the die, and r = 0 at the freezeline. The last boundary condition assumes that the viscosity becomes infinite at the freezehne and that there is no further deformation. (This condition is approximate at best and need not be used when a solidification model in which the sohd phase evolves and locks in structure is employed.) Heat transfer is very important, although it has usually been handled with rather simplistic assumptions about the heat transfer coefficient. 

Heat Transfer Coefficient and the Melt-Spinning Process... 

The heat transferred by radiation is strongly dependent on the temperature of the spinning filament. The apparent heat transfer coefficient for radiation can be expressed by ... 

One of the attractive features of the SDR is that its high fluid dynamic intensity favours the rapid transmission of heat, mass and momentum, thereby making it an ideal vehicle for performing fast endothermic reactions which usually also benefit from an intense mixing environment. It must be noted, however, that heat transfer from the process liquid to any cooling/heating fluid behind the disc involves a second film coefficient which may severely limit the overall heat transfer rate (this is discussed later). Some of the more relevant recent experimental studies of spinning disc performance may now be considered. 

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