A new theory is presented, termed the vapor surfactant theory, which explains the heat and mass transfer enhancement mechanism of additives in absorption chillers. The theory holds that the enhancement additives are surfactants delivered to the surface of the solution film from the vapor, and that they produce enhancement by generating intense secondary flows on the film. The secondary flows, driven by surface tension gradients (Marangoni convection), augment the transfer process by providing mixing of the film. The primary variable in the enhancement is the surface concentration distribution of the surfactant additive because the surface tension gradients arise due to non-uniformities in the surface concentration. The vapor surfactant theory is found to explain all observations from several types of experiments. The new theory is a significant departure from past theories in its emphasis on the primary role of the vapor in the process.

Theory of Heat/Mass Transfer Additives in Absorption Chillers

In this article, experimental analysis was performed for ammonia–water falling film absorption process in a plate heat exchanger with enhanced surfaces such as offset strip fin. This article examined the effects of liquid and vapor flow characteristics, inlet subcooling of the liquid flow and inlet concentration difference on heat and mass transfer performance. The inlet liquid concentration was selected as 5%, 10% and 15% of ammonia by mass while the inlet vapor concentration was varied from 64.7% to 79.7%. It was found that before absorption started, there was a rectification process at the top of the test section by the inlet subcooling effect. Water desorption phenomenon was found near the bottom of the test section. It was found that the lower inlet liquid temperature and the higher inlet vapor temperature, the higher Nusselt and Sherwood numbers are obtained. Nusselt and Sherwood number correlations were developed as functions of falling film Reynolds Re 1 , vapor Reynolds number Re v , inlet subcooling and inlet concentration difference with ±15% and ±20% error bands, respectively.

Experimental correlation of combined heat and mass transfer for NH3–H2O falling film absorption

Condensation heat transfer on patterned surfaces

The objectives of this paper are to quantify the effect of Marangini convection on the absorption performance for the ammonia–water absorption process, and to visualize Marangoni convection that is induced by adding a heat transfer additive, n -octanol. A real-time single-wavelength holographic interferometer is used for the visualization using a He–Ne gas laser. The interface temperature is always the highest due to the absorption heat release near the interface. It was found that the thermal boundary layer (TBL) increased faster than the diffusion boundary layer (DBL), and the DBL thickness increased by adding the heat transfer additive. At 5 s after absorption started, the DBL thickness for 5 mass% NH 3 without and with the heat transfer additive was 3.0 and 4.5 mm, respectively. Marangoni convection was observed near the interface only in the cases with heat transfer additive. The Marangoni convection was very strong just after the absorption started and it weakened as time elapsed. It was concluded that the absorption performance could be improved by increasing the absorption driving potential ( x vb − x vi ) and by increasing the heat transfer additive concentration. The absorption heat transfer was enhanced as high as 3.0–4.6 times by adding the heat transfer additive that generated Marangoni convection.

J.N.A. Morrison

Papers

Augmentation of steam condensation heat transfer by addition of ammonia

Technical NoteAugmentation of steam condensation heat transfer by addition of methylamine