Study On Onset Of Subcooled Flow Boiling And Wall Temperature Distribution Characteristics In Horizontal Helically-coiled Tubes

Subcooled flow boiling has attracted much interest in the study of steam-liquid two-phase flow, it is widely applied in many engineering fields such as pressurized water reactor fuel rods channel heat transfer, high heat flux integrated circuit board cooling system and huge internal combustion engine forced circulation water cooling system, etc. Onset of nucleate boiling determination and its characteristics are the main research contents in subcooled flow boiling study. For example, in pressurized water reactor, onset of nucleate boiling has an important influence on neutron moderation performance and coolant flow instability characteristics.Helically-coiled tube is an important kind of enhanced heat transfer tube with compact structure, less space is needed when it is used in heat exchanger and it is easy to manufacture; it has a spring-like structure which is easy to extend and shorten, resulting in small thermal stress; secondary flow of working medium in it results in a high heat transfer coefficient and a low fouling heat resistance. Those advantages make it widely studied and applied in many engineering fields such as heat exchangers of refrigeration and cryogenic engineering, steam generator of high temperature gas cooled reactor, supercritical boiler spirally water tube and petrochemical industry. Study of wall temperature distribution characteristics of flow and heat transfer inside helically-coiled tube can deeply reveal the influence of centrifugal force and secondary flow on the flow and heat transfer.In this paper, transformation of steam-liquid two-phase flow test system for flow and heat transfer experiment was carried out, based on which the following two aspects were explored.Tube wall temperature distribution characteristics of four selected cross sections, including the bottom, the rising part, the top and the falling part cross section, was investigated when heat transfers by means of single-phase convection and subcooled flow boiling, under the system parameters of p=0A56MPa,△TSUb=8.14K, G=185kg m-2s-1, the main work focuses on exploring the wall temperature difference between inner point and outer point. In the meaning time, wall temperature distribution of front point and behind point was investigated. By comparing those two research results, reasons were given by revealing changes of centrifugal force and gravity from time to time.Influence of parameters such as system pressure, inlet subcooling and mass flux on the ONB heat flux were explored in detailes, under the system parameters of p=0.414-0.632MPa,△Tsub=5.0-11.0K, G=749.8-1249.6kg m-2s-1, q=115-5763Wm-2, the results show that influence of system pressure on the ONB heat flux may be considered negligible; in the low pressure range, qONB increases with the increasing of pressure; in the high pressure range,qONB decreases with the increasing of pressure. However, both of the two trends are not obvious. Mass flux has a large effect on ONB heat flux, generally speaking,qONB increases with the increasing of mass flux. However, increase of mass flux will affect the detachment and reattachment of bubbles in some conditions, resulting in opposite trend. Inlet subcooling also has a large effect on ONB heat flux,qONB increases with the increasing of inlet subcooling.