| Marine compact heat exchangers are mainly composed of an array of rectangular narrow channels,the flow rate and local thermal parameters of the rectangular narrow channel all fluctuate periodically with rolling motion due to the additional inertia force and spatial position of heat transfer equipment of marine power systems fluctuate periodically under rolling motion conditions.The fluctuations of flow rate and thermal parameters will effect the operation and safety of marine power systems,therefore,the experimental research and theoretical analysis of flow and heat transfer characteristics of boiling flow in a rectangular narrow channel under rolling motion conditions are performed in this paper.The mechanical rolling thermal-hydraulic experimental facility is used to simulate the rolling motion of a ship,the ranges of rolling angle and period are 10°-20° and 10-20s,the deionized water is used as the test fluid,and the rectangular narrow channel with a cross section of 2 X 40mm.The ranges of system pressure and heat flux are 0.1-2.5MPa and 10-700kW/m~2,the range of vapor quality is 0-0.8.The thermal equilibriums of the test section are all above 95%both in forced and natural circulation experimental studies.The wall temperature and heat flux fluctuate with the fluctuation of mass flow rate under rolling motion,the amplitude and phase of the fluctuations of inside temperature,outside temperature and heat flux are difference.Based on the theoretical calculation of inside wall temperature and heat flux,the results show that the fluctuation amplitudes of wall temperature and heat flux are the functions of rolling period and fluctuation amplitude of inside wall temperature.The phase differences between heat flux and wall temperature fluctuations are only affected by the rolling period.The experimental analysis of local boiling heat transfer coefficient in rolling motion is conducted base on the theoretical calculations of inside wall temperature and heat flux.The results show that the fluctuation amplitudes of boiling heat transfer coefficients increase with the increase of rolling angle,rolling period,heat flux and mass flux,it will decrease with increasing system pressure.The local thermal parameters of rectangular narrow channel will fluctuate with rolling motion due to the system pressure fluctuates periodically with the rolling motion.In boiling flow regions the system pressures are mainly affected by the spatial position of the experimental facility and block degree of steam-water mixture in the inlet region of condenser.The phase difference between the fluctuations of system pressure and local pressure drop is ? under the short rolling period conditions.The main mechanism of boiling heat transfer in rectangular narrow channel is two-phase forced convection under steady state,base on the superposition concept a new correlation for predicting the boiling heat transfer coefficient of the channel is developed,and the mean absolute error of boiling heat transfer coefficients between predicted value and experimental data is 14.4%.The experimental research and theoretical analysis of flow and heat transfer characteristics of natural circulation in the rectangular narrow channel under steady and rolling motions are performed.The results show that in single-phase flow region the mass flux and heat transfer coefficient increase linearly with the increase of heat flux,in subcooled boiling region the gradient of heat transfer coefficient is bigger than the gradient of mass flux,in saturate boiling region the gradient of heat transfer coefficient is smaller than the gradient of mass flux,in the flow region of mass flux decrease the heat transfer coefficient will increase rapidly with increasing heat flux.With the increase of rolling angle and frequency the time average mass flux of single-phase natural circulation will decrease but the time average heat transfer coefficient will increase.The flow characteristics of natural circulation and forced circulation are compared;the results show that in subcooled boiling flow region the mass flux of forced circulation will decrease with the increase of heat flux,and in saturate boiling flow region the mass flux is mainly affected by the drive head of natural circulation.Based on the mass and momentum conservation equations of liquid film and momentum conservation equation of vapor core,a theoretical model of annular flow is developed.The results show that the mean absolute errors of heat transfer coefficient and pressure gradient between predicted value and experimental data are 16.4%and 14.3%.The liquid film thickness will decrease with the increase of mass flux,inlet temperature and axial distance,while decreasing with the increase of mass flux and channel height.The liquid film velocity gradient of laminar boundary layer is bigger than that of turbulent boundary layer and the velocity gradient increases obviously near the liquid/vapor interface. |
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