헬리컬형 열교환기의 관외 적하 및 침전 열전달 특성에 관한 실험적 연구

Abstract

An experimental study is performed to investigate the characteristics of heat transfer of outside helical tubes. Many research works for heat transfer of inside helical tube have been established, but few has concerned about outside heat transfer phenomena.

The main heat exchanger consists of twelve curved columns of 300mm diameter and the total length of 1.2m copper tube having an outer diameter of 19.05mm and 1.5mm thickness. Water flows down the outside of helical tubes and refrigerant-11 flows the inside of the tube counter currently against the outside flow. Three types of outside flow patterns as vertical film falling flow, immerged flow, and mixed-flow which is partly combined by film falling flow and immerged flow are examined.

Experiments are carried out with respect to three different factors, i.e. water flow rate, refrigerant flow rate and immersion rate of outside helical tubes. The experimental range of the inside flow rate is 1.7∼3.2ℓ/min, and its outside flow rate, 21∼33ℓ/min. Temperatures of 24 points along the outside of tubes are continuously measured.

The experimental results shows that the heat transfer rates of the mixed flow are 8 to 56% higher than those of film falling flow and immerged flow, respectively. The temperature difference between the wall and the bulk of immerged flow is 61% higher than that of film flow. The results are presented as dimensionless Nusselt numbers with the corresponding Reynolds numbers for various outside and inside fluid flow rates. Interpretation of these results is given on the basis of physical reasoning and the correlation equations.

This thesis shows that the mixed flow composed of film falling flow and immerged flow can be one of the most efficient flow patterns for helically coiled tubes to obtain higher heat transfer rates.