Heat transfer enhancement in pool boiling of a refrigerant fluid with wire nets structures

Among many methods to enhance boiling heat transfer by means of porous coatings, the use of the wire nets covering the heated surface demonstrates some advantages due to its simplicity and low cost. Easiness of a wire net mounting at the heated wall allows testing many different nets and their combinations. This paper presents the experimental results on heat transfer and crisis using the dielectric refrigerant R141b pool boiling under atmospheric pressure. The analysis carried out in the present paper demonstrates that success in boiling heat transfer enhancement is provided not only by the nucleation sites density increase, but also by designing the channels in the porous structure to escape the generated vapor. Unsatisfactory results in relation to CHF value have been obtained with rather thick porous coating. In this case the problem is to provide vapor evacuation from the wall. Rather essential heat transfer coefficient and CHF increase has been observed while the porous structure has been formed with 3–7 layers of nets using wire of 0.25–0.40 mm diameter and mesh aperture of approximately 2 mm. The nets of stainless steel gave the greater effect with respect to copper, aluminium and brass. It is demonstrated that the presence of porous coatings brings about a considerable enhancement of the thermal stability of the system. An approximate model of crisis in boiling at the surface coated with metallic nets has been developed

Influence of pressure on film boiling of subcooled liquid

Film boiling of subcooled liquid as the most available and use-proven way of quick chilling of hardening pieces, ensuring required microstructure of metal is widely used in quenching technology. A vapour explosion is the other important process, in which this boiling regime is surely observed. Under high subcooling of liquid the cooling process of high temperature bodies is featured with very high intensity and can be considered as a particular heat transfer regime. This was revealed first in 1986 by G. Hewitt and D. Kenning, who used for this regime a term “microbubble boiling” [1]. Mechanisms of very intensive heat removal from the metal surface to subcooled liquid are not understood till now, because the surface temperature in this process is much higher than the temperature of homogeneous nucleation. Some specialists do not accept even an existence of the problem itself. For revealing regularities and mechanisms governing intense transfer of energy in this process, the present authors conduct systematic investigations of cooling of high temperature balls made of different metals in water with a temperature range from 20 to 100°C [2]. It has been determined that temperature field in the balls of diameter higher than 30 mm in the intense cooling modes loses its spherical symmetry. An approximate procedure for solving the inverse thermal conductivity problem for calculating heat flux density on the ball surface is developed. During film boiling, when the ball surface temperature is well above the critical level for water, so that the liquid cannot come in direct contact with the wall, the calculated heat fluxes reach 3–7 MW/m2. The main aim of this study was analysis of excess pressure effect on heat transfer during film boiling of different subcooled liquids. The experiments have been performed in the pressure range 0.1–1.0MPa at the coolant temperature -15 - +90°C for all the liquids used. The primary results of the research are the experimental thermograms of cooling the spheres under the elevated pressures.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016