Water soluble polymer added high mass flux spray: A novel approach for the attainment of enhanced heat transfer rate in transition boiling regime

Abstract

Although in the literature, the polyethylene glycol (PEG) added water dropwise evaporative cooling at very low temperature and single droplet evaporation mechanisms are revealed, the role of polyethylene glycol in spray evaporative quenching at very high initial surface temperature (900°C) has not been revealed so far. According to the reported information, PEG added water droplet evaporation favors high heat flux and therefore, in the current work, it is expected that by performing high mass flux polymer spray cooling at very high initial temperature, heat transfer rate can be augmented. The enhancement mechanism is based on the alteration of the surface properties in the favorable direction of heat transfer. The surface modification is achieved by adding water soluble polymer (Polyethylene Glycol) of different molecular weights (PEG 200 and PEG 600) as additives, which make the surface hydrophilic due to the deposition on the hot plate during evaporation. Before experimentation, the conditions and properties leading to the formation of coating were investigated by analyzing the polymer droplet wall interaction. This study depicts that due to the interaction of polymer with wall and very slow diffusive nature of polymer in water, the deposition of polymer is expected to occur during evaporation. The hydrophilic nature of the surface increases the spreading factor of the impinging droplet and this phenomenon increases heat transfer area, which in turn favors higher heat transfer rate. Spray evaporative cooling study was carried out on a hot AISI 304 steel plate using polymer solution of different concentrations as coolant and the enhancement intensity was identified by comparing with the result of pure water. The comparison illustrates significant enhancement i.e. 30% in CHF and 28% in IHF for 20 ppm concentration of polymer type I. For the verification of heat transfer mechanism and identification of the reason behind enhancement, different dimensionless numbers such as Reynold's number, Weber and Prandtl numbers were analyzed. The surface morphology of heat treated steel plate clearly indicates the deposition of the polymer over surface of the plate and the characteristics of the deposition confirm the superhydrophilic nature. In comparison with the literature, the developed coating on the surface of the plate is thermally, mechanically and chemically more stable.