Characteristics of Boiling Flow in Helically Coiled Heat-Transfer Tube

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  • MATSUI Goichi
    Department of Mechanical Engineering and Biomimetics, Kinki University
  • SUZUKI Masatomi
    Graduate school of Mechanical Engineering and Biomimetics, Kinki University
  • KAJI Masuo
    Department of Mechanical Engineering and Biomimetics, Kinki University

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Abstract

This study deals with specific phenomena of boiling two phase flow in a helically coiled tube submerged in a bath held at a constant temperature. The two phase flow in a coiled tube under a low quality condition is strongly influenced by buoyancy and centrifugal forces. DNB (departure from nucleate boiling), CHF (critical heat flux) and dry-out on the tube wall are important characteristics for boiling flow. There are many studies conducted under a high quality condition but few under a low quality condition. Therefore, the investigations on detailed phenomena under a low quality condition are required for designing structure and safety operation of helically coiled tube heat exchangers.<BR>The characteristics of boiling phenomena were experimentally investigated for an apparatus of scale model using water or nitrogen gas-water as working fluids. The helically coiled test section is made from a stainless steel tube with an internal diameter of 10mm, thickness of 1mm, a coil diameter of 200mm, an inclined angle of 5 degrees and 2.5 turns. The test section was submerged in an oil bath heated at a constant temperature of about 200°C. The temperatures of the external wall of tube were measured by K-type thermocouples at 10 cross-sections along the tube and 4 points around the circumference at each cross-section.<BR>The temperature distributions in both the cross-section and the flow direction of the tube and the time variations were strongly influenced by flow rate conditions. Especially, the temperature changes like a saw-toothed wave were observed under a low flow rate conditions. That is, after the temperature on the external wall of the tube increased, it rapidly decreased. This fact suggests that the internal wall of the tube becomes in dry-out condition with boiling of water, then it is wetted again owing to arrival of liquid plug. Thus, DNB was supposed to occur repeatedly.

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