论文部分内容阅读
在热流密度q=0~25 kW/m2、质量流速G=10~262 kg/(m2·s)及入口压力Pin=8~9 MPa的实验参数范围内,研究超临界压力CO2在螺旋管中上升流动的传热特性,分析质量流速、热流密度及入口压力对换热系数的影响规律。结果表明,沿程换热系数总体呈先上升后下降的趋势,极大值发生在主流平均温度小于准临界温度而壁温大于准临界温度条件下;在换热系数上升段,沿程近壁区流体比热容增加引起的单位体积流体换热能力增强以及粘度减小引起的热边界层减薄是传热强化的主要因素;当近壁区CO2发生类液态到类气态的转变时,其比热容和导热系数减小是换热系数下降的主导因素。对于物性变化剧烈的超临界流体传热,Nu数仅作为对流与导热相对大小的度量,其数值大小不能客观反映实际换热能力的强弱。
Under the experimental parameters of heat flux density q = 0-25 kW / m2, mass flow rate G = 10-262 kg / (m2 · s) and inlet pressure Pin = 8-9 MPa, the effects of supercritical pressure CO2 Rising heat transfer characteristics of the flow, analysis of mass flow rate, heat flux and inlet pressure on the heat transfer coefficient of law. The results show that the total heat transfer coefficient increases first and then decreases, and the maximum value occurs when the average mainstream temperature is lower than the quasi-critical temperature and the wall temperature is greater than the quasi-critical temperature. During the rising period of the heat transfer coefficient, The heat transfer enhancement per unit volume of fluid caused by the increase of specific heat capacity and the thinning of the thermal boundary layer caused by the decrease of viscosity are the main factors of heat transfer enhancement. When the liquid-like liquid-like transition in the near wall region occurs, the specific heat capacity and thermal conductivity Reduction is the dominant factor in the decline of the heat transfer coefficient. Nu number is only a measure of the relative size of convection and thermal conductivity for the supercritical fluid heat transfer with drastic physical changes. The numerical value of Nu number can not objectively reflect the actual heat exchange capacity.