Indian Society for Heat and Mass Transfer
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REVIEW OF CRITICAL HEAT FLUX IN A VERTICALLY DOWNWARD TWO PHASE FLOW

Rajeshwar Sripada
University of South Africa, Johannesburg, South Africa

Veeredhi Vasudeva Rao
University of South Africa, Johannesburg, South Africa

DOI: 10.1615/IHMTC-2017.2940
pages 2097-2102

摘要

Critical heat flux (CHF) or post burnout refers to the sudden decrease in heat transfer coefficient for a surface on which evaporation or boiling occurred. Exceeding this heat flux causes the replacement of liquid adjacent to the heat transfer surface with a vapor blanket. This blanket acts as a barrier to heat flow from the heat dissipating body, resulting in possible catastrophic failure. Two-phase flows were encountered in various applications including boilers and heat exchangers used in power plants, chemical plants and nuclear industry. The current day boiler industry application dictates for operation of boilers and heat exchangers over a wide range of operating conditions, starting from low pressure to very high pressure and high temperatures. The operation in harsh environment driven by high temperature corrosive gases prompts for usage of expensive materials and thermal coatings to protect from tube burnout and thereby improve the life. On the other hand, the efficiency of the heat exchanger strongly depends upon the surface area. The transportation limits put several constraints on making the heat exchanger fatter or taller, thereby adding to the complexity. To make these heat exchangers compact without compromising on its efficiency, one should look for alternative ways. One of the ways of doing it was to increase the number of passes by routing tubes in such a way that the two phase mixture travels downwards and upwards in a periodical manner. This brings in additional risks of premature tube burnout, 2 phase flow instabilities and other potential risks, especially in vertically downward flow. Accurate estimation of CHF risk was a mandate not only from performance or life perspective, but also more importantly from safety perspective.

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