Local heat transfer coefficients in scraped-film heat exchanger.

Abstract

The instantaneous heat transfer coefficientsin a scraped-film heat exchanger have been determined using the electrochemical technique for a two-bladed, 78.7 mmI.D. exchanger. The time-averaged local heat transfer coefficients were found to agree fairly well with previous work of Azoory and Bott, though some variation in liquid flow rate was detected. The timeaveraged local Nusselt number was found to be represented by Nu =0.15(Rer à" Pr)inRel with a=(l-3.74 x 10"W)/9. The variation of heat transfer coefficient with time was observed to be very different from that predicted by the accepted model for the phenomena based upon unsteady state conduction.In a scraped-film heat exchanger, the liquid film is subjected to gravitational and centrifugal forces as the blades rotate and scrape it from the outer heat transfer surface. At the same time, fresh material is exposed to a clean heat transfer surface. The fluid mechanical and heat transfer phenomenaoccurring in such a system are very complex and hence any analysis of the heat transfer mechanisms must be based on a simplified model for these phenomena. The object of this study was to measure indirectly the instantaneous heat transfer rate at points in the exchanger in order to elucidate the actual behavior in such systems. This was done using the electrochemical method developed for such studies10\ The main advantage of such a system is that it provides an instantaneous mass transfer rate uncomplicated by thermal or mass capacity effects.The analogy between heat and mass transfer allows translation of the mass transfer rate to a heat transfer rate (or coefficient).Many investigators have studied the scraped-film heat exchanger. For instance, Kern and Karakas7) and Lustenader et al.9) have postulated that heat is transferred through the film by steady-state molecular conduction. Kool8) and Harriot5) suggest that the rate of heat transfer depends more on the rate of transient conduction in a frequently renewed layer than on the rate of steady-state conduction through Received February 16, 1978. Correspondence concerning this article should be addressed to H. Miyashita.

Heat Transfer in a Vertical, Liquid‐full Scraped‐surface Heat Exchanger. Application of the Penetration Theory and Wilson Plots Models

“…It may be concluded then that any theoretical approach for describi n g the h e a t transfer phenomenon in scraped-surface h e a t exchangers should explicitly include both V, and V,, the axial and the rotational components of the velocity field. Similar conclusions were arrived at by Ramdas et al (1977) for liquid-full SSHE and by Miyashita and Hoffman (1978) for film-type SSHE units. In both caBes, the axial velocity effects, accounted for in the axial Reynold's number, were found to be important.…”

Scraped Surface Heat Exchangers

“…Scraped surface film evaporators are not further discussed in this survey, but more information can be found elsewhere (Kern and Karakas 1959;Lustenander et al 1959;Bott and co-workers, 1963Bott and co-workers, , 1966aBott and co-workers, ,b, 1968Bott and co-workers, , 1968aBott and co-workers, ,b, 1969Azoory and Bott 1970;Miyashita and Hoffman 1978;Rosabal et al 1982a,b;Kohli and Sarma 1983).…”