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Heat dissipation problem is not new for engineers; with the growth of technology over time, conventional heat exchangers have been replaced by compact heat exchangers. With the increasing demand for an effective cooling and heating system, researchers have explored various active and passive heat transfer enhancement techniques like surface modification, geometry modification, surface or fluid vibrations and using phase change materials. The study of curved tubes like helical coils has always been an area of interest as because of its geometry; it offers different flow behavior when compared to conventional straight tubes. Helical coils provide better mixing of fluid because of an additional secondary flow in them, which is the primary reason for enhanced heat transfer in them. Helical coils are excellent vibration dampers, provides more heat transfer area in a given volume, and can withstand more thermal stresses in comparison to the straight tube. Nowadays, helical coils have been used widely in heat exchangers, and because of the higher heat exchange area for a fixed volume of space, they are compact and provide many mechanical advantages. These coiled tubes are widely used in industries like power plants, refrigeration, food processing, cryogenics, space applications, polymer industries, etc. These coils are used in industrial processes like steam generation, condensation, mixing of miscible and immiscible fluids, homogenization, solid-liquid mixing, etc. The operation requirement of these processes can range from very high system pressure to vacuum pressure.Boiling is a superior heat transfer phenomenon in which the fluid absorbs latent heat of vaporization in addition to sensible heat and is accompanied by change in phase of the fluid. It is one of the most preferred method to dissipate high amount heat fluxes, its industrial applications are many ranging from macro systems like steam production for power plant to micro applications like heat dissipation of electronics. Similarly, low pressure systems have wide industrial applications like cryogenics, condensers, thermosyphons, adsorption chillers, etc. It is well known that there is always a loss in pressure in a fluid flow system, and the study of pressure drop is crucial as it helps to determine the pumping power required, which helps us to build an energy-efficient system.Literature survey on two-phase flow pressure drop apprises that very few studies have been performed on a system under sub-atmospheric pressure conditions. It has also been found that the pressure loss in a helical coil is much more when compared to a straight tube. This work presents a pioneer experimental investigation of a two-phase flow pressure drop on boiling water under sub-atmospheric pressure conditions on the helical coil. The test section is a helical coil made of SS-304, length 5330 mm, coil diameter 282.70 mm, and tube diameter 9.5 mm. The experiments are performed in the range of 0.15 bar to 1 bar absolute pressure at the exit of the coil. The mass flux varying from 100 to 800 kg/m2, the steam quality between 0 to 0.95, and power between 0 to 50 kW. The effect of system pressure on two-phase pressure drop has been studied accompanying mass flux, quality, and heat flux.It has been observed that the two-phase pressure drop across the helical coil is affected by the change in system pressure, the pressure drop is increased as the system pressure reduced for fixed mass flux and quality. The pressure drop increases with the increase in quality for particular mass flux and with an increase in mass flux for a fixed quality. Available correlations for two-phase flow pressure drop are compared with the experimental data. It is found that none of the considered correlation fits well with the present data bank. A correlation for pressure drop during two-phase flow has been developed based on the available experimental points. The experimental data bank is presented, and the interested readers are encouraged to extend the investigation further.<br/> |
