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1、第四课 干涸后传热,尚智上海交通大学 核工系,Chapter 5. Lesson FourPost-CHF Heat Transfer,Shang ZhiDepartment of nuclear science and system engineeringShanghai Jiaotong University,Introduction and Objectives,In some situations, heat exchanger equipment is operated in a two-phase region beyond the critical heat flux point
2、; e.g., fossil fueled boilers or steam generators, under steady conditions and nuclear reactor cores under transient conditions. Because of these and other similar situations there exists a continuing need for more accurate information about heat transfer coefficients in post-CHF flow regimes. Signi
3、ficant improvements in understanding have been made over the past few years. In particular, the development of experimental techniques has allowed the determination of the complete forced convection boiling curve including the transition boiling region for particular values of local vapor quality an
4、d mass velocity.,The objective of this section is to review the subject of Post-CHF heat transfer and indicate the state-of-the-art. Although much of this work is specifically relevant to aqueous fluids the techniques used are beneficial in the design of other heat exchange equipment in the power an
5、d process industries.,Post-CHF Heat Transfer Models and Correlations,Three types of modelling approaches have been attempted: 1. Correlations of an empirical nature which made no assumption whatever about the mechanism involved in post-CHF heat transfer, but solely attempts a functional relationship
6、 between the heat transfer coefficient and the independent variables. This assumes that the vapor and liquid are at the saturation temperature and in thermodynamic equilibrium.,2. Correlations which recognize that departure from thermodynamic equilibrium condition can occur and attempt to calculate
7、the true vapor quality and vapor temperature, different from Tsat. A conventional single-phase heat transfer correlation for the vapor is then used to calculate the heated wall temperature.,3. Semi-theoretical models where attempts have been made to examine and derive equations for the various indiv
8、idual hydrodynamic and heat transfer processes occurring in the heated channel and relate these to the wall temperature (or heat flux depending on the boundary conditions).,Groeneveld (1973) has compiled a bank of carefully selected data drawn from a variety of experimental post-dryout studies in tu
9、bular, annular and rod bundle geometries for steam/water flows.,Empirical Correlations,This heat transfer coefficient is to be used for the determination of the wall temperature given the wall heat flux or vice versa for a known wall temperature. A considerable number of empirical equations have bee
10、n presented by various investigators for the estimation of heat transfer rates in the post-dryout region. Almost all of these equations are modifications of the well-known Dittus-Boelter type relationship for single-phase flow and take no account of the non-equilibrium effects discussed above. Rathe
11、r thermodynamic equilibrium is assumed between the vapor and liquid. Various definitions of the two-phase velocity and physical properties are used in these empirically modified forms and a number of correlations result. Each of these correlations is based on only a limited amount of experimental da
12、ta and Groeneveld, therefore, proposed a new correlation for each geometry optimized using his bank of selected data. This is the recommended correlation if one is to use the simplified equilibrium approach to a post-CHF analysis.,Non-Equilibrium Empirical Models,Consider the case of a constant heat
13、 flux. Now in this non-equilibrium approach we assume that the heat flux can be divided into two portions; one directly heating the vapor, qg , and one directly into the liquid, qf , causing it to evaporate.,where we define e as Groeneveld has suggested such a correlation for e given by,Semi-Theoret
14、ical Models,A comprehensive theoretical model of heat transfer in the post-CHF region must take into account the various paths by which it transferred from the surface to the bulk vapor phase. Six separate mechanisms may be identified: 1. heat transfer from surface to liquid droplets which impact wi
15、th the wall (wet collisions) 2. heat transfer form the surface to liquid droplets which enter the thermal boundary layer but which do not wet the surface (dry collisions). 3. convection heat transfer from the surface to the bulk vapor,4. convective heat transfer from the bulk vapor to suspended drop
16、lets in the vapor core 5. radiation heat transfer from the surface to the liquid droplets 6. radiation heat transfer from the surface to the bulk vapor.,One of the first semi-theoretical models proposed was that of Bennett et al., (1964) which is a one-dimensional model starting from known equilibri
17、um conditions at the point of CHF. It was originally assumed that there is negligible pressure drop along the channel by Groeneveld (1974) revised the equations to allow for pressure gradient and flashing effects. It was also assumed that droplets could no longer approach the surface.,Transition Boi
18、ling,Various attempts have been made to produce correlations for the transition film boiling region corresponding to the left of the minimum in the boiling curve (Figure). These correlations have usually been combined with expressions for the stable film boiling region. The difficulty with this appr
19、oach is that in flow boiling the physical meaning of this minimum is difficult to understand.,Tong (1974) suggested the following equation for combined transition and stable film boiling at 2000 psia with wall temperatures less than 800oF, and Tw-Tsat300oF.,Transition to Film Boiling,Consider the ca
20、se when the mass velocity decreases to very low value. As G decreases the flow situation becomes more like pool boiling where the post-CHF regime becomes film boiling. Because the liquid is displaced from the heating surface by a vapor film and the uncertainties associated with bubble nucleation are
21、 removed, film boiling is very amenable to analytical solution. This is similar to condensation where stratified flow occur with a liquid film. In general, the problem is treated as an analogue of film-wise condensation and solutions are available for horizontal and vertical flat surfaces, and also
22、inside tubes under both laminar and turbulent conditions with and without interfacial shear. It is not, however, proposed to review all these solutions here and the reader is referred to the comprehensive reviews of Clements and Colver (1970), of Hsu (1972) and of Bressler (1972).,Observations,The p
23、ost-CHF heat transfer regime has been summarized with three different approaches to estimate the heat transfer coefficient. We would recommend the empirical equilibrium or non-equilibrium approach for an estimate of post-CHF heat transfer rates. However, if the wall temperature and degree of superhe
24、at (or heat flux for the other boundary condition) are of crucial importance of semi-theoretical approach might be considered for detail analysis. This latter recommendation will require new data for specific geometries to determine the droplet-gas and droplet-wall heat transfer rates accurately.,The End本次课结束！,
