Study On Flow And Heat Transfer Characteristics In Helically Coiled Tubes With Non-uniform Heat Flux Condition

Helical pipes are widely used in a variety of practical applications, such as in power, refrigeration and cryogenics, chemical industry, food and medicine, due to their high efficiency in heat transfer, simple making, and compactness in volume. Compared with straight tube, it is well known that the secondary flow in the cross section of the helically coiled tube, due to centrifugal force and torsion force, is a significant factor affecting the flow and heat transfer. To date, many of the research activities have been conducted on both the single-phase and the two-phase flow and heat transfer characteristics in the helical pipes. However, the focuses are mainly pertinent to the conditions of uniform heat flux in the outside surface of the helical pipe. The study on Flow and Heat Transfer Characteristics which in the conditions of non-uniform heat flux in helically coiled tubes has seldom been involved. This thesis is not only to establish an experimental setup, which using refrigerant R22, for investigating the two-phase flow and heat transfer characteristics with the non-uniform heat flux conditions in the horizontal helical pipes, but also to analyze the single-phase turbulent heat transfer process under non-uniform heat flux condition in the helically pipe.Experiments were carried out to investigate the two-phase flow and heat transfer characteristics of R22 in horizontal helically-coiled tube under three different heat flux conditions which are the inside of coiled tube heated and the outside insulated, circumferentially uniform heat flux condition, and the inside of the tube insulated and the outside heated. The experimental parameters are heat fluxes of q=3.88-19.4k W/m2, mass fluxes of G=182-303kg/(m2s), and mass quality x=0.07-0.53. The heat transfer coefficients increased with both heat flux and mass flux under three different heat fluxes conditions in the helically-coiled tube. In the mass quality region of this thesis, the local heat transfer coefficients increased with the increase of quality. It is found that when the same heat flux is exerted to the helically coiled tube surface, the local heat transfer coefficient of the tube is maximum when the inside of coiled tube heated and the outside insulated, followed by that when inside and outside heated evenly, while that is minimum when the inside of the tube is insulated and the outside heated.Numerical simulation is carried out by establishing three different geometrical structure models of helically coiled tubes to analyzing the single-phase turbulent flow and heat transfer characteristics affected by different inlet Re numbers and different heat flux conditions. Based on the analysis of the data, it was found that effect on temperature distributions, velocity distributions, and the secondary flow of fluid in tube were taken by different heat flux conditions. The single-phase turbulent heat transfer coefficient of the helically-coiled tube under three different heat flux conditions is maximum when the inside of coiled tube heated and the outside insulated, followed by that when inside and outside heated evenly, while that is minimum when the inside of the tube is insulated and the outside heated.In summary, this thesis investigates flow and heat transfer characteristics of helically-coiled tube under non-uniform heat flux condition, which is a base about design and basic research of helically-coiled tube under non-uniform heat flux condition.