Numerical and Experimental ......(13)

Two main methods are often used to solve the coupled fluid flow and heat transfer problem: experimental investigation and theoretical calculation. Reliable information about a physical process is often given by the experimental measurement. However, full-scale test sometimes is not possible or too expensive. The alternative small-scale test may not reproduce all the features of the full-scale equipment, such as the turbulence. Also, some variables such as the convective heat transfer coefficients are not possible to measure directly and have to be calculated from other variables.

 绿色建筑博客M]~ s [l

A theoretical prediction is based on a mathematical model which mainly consists of a set of differential equations. A look at the classical text on heat transfer or fluid mechanics leads to the conclusion that only a tiny fraction of the range of practical problems can be solved in closed form. Further, these solutions often contain infinite series, special functions, transcendental equations, etc so that their numerical evaluation may present a formidable task (Patankar 1980).绿色建筑博客{rd@$V7m

 绿色建筑博客5D'`!T{!Za8]

Computational fluid dynamics (CFD) techniques and models have the advantages of low cost, high speed, flexibility to sensitivity analysis, ability to simulate realistic conditions and ability to simulate ideal conditions. The fluid flow domain is divided into many tiny cells and the properties of the fluid are thus assumed constant in one cell. The continuity equation, conservation of momentum equation, and conservation of energy equation are then solved in every single cell and information is transferred through the convection flux and diffusion flux between the cells, until the whole domain reaches stable and all equations become converged.

One promising use of CFD programs is to couple them with thermal modeling programs. Thermal modeling programs provide energy analyses for an entire building, providing space averaged indoor environmental conditions, cooling/heating loads, coil loads, and energy consumption. A drawback to these programs is that they cannot make detailed predictions of thermal comfort or predict the distributions of air velocity, and temperature like a CFD program could. Convective heat transfer coefficients used in thermal modeling programs generally come from empirical formulas that may or may not be accurate. It would be beneficial to use a CFD program to calculate these coefficients in order to improve their accuracy. Boundary conditions are a requisite for CFD programs and may be obtained from a thermal modeling program, demonstrating one of the benefits of coupling thermal modeling and CFD programs together. Coupling the two programs would allow both to acquire the necessary information in an iterative process (Zhai 2002; Bartak 2002).绿色建筑博客l5M5B?f.};v5`

Various models and investigations to study BIPV systems or double facades are reported in the literature. A 2-D finite element code was employed in a CFD study by Moshfeg (1996) and Sandberg (1999) with idealized boundary conditions. In 1996, a 2-D laminar flow model was investigated using control volume based finite difference method by Mootz and Bezian (1996). In the same year, detailed CFD studies were carried out by B. Moshfegh and M. Sandberg (1996) for both laminar flow and turbulent flows. The BIPV CFD model is further improved by introducing the radiation heat transfer in terms of the local surface temperature by Brinkworth (2002). However, this study was mainly focused on the laminar flow. Zolner et al. (2002) performed detailed experimental investigations and found that the flow was turbulent mixed convection. They report some measurements of the low velocities including different inlet sizes. In 2002, Mei et al. (2003) presented a dynamic thermal model integrated to the TRNSYS to study how the BIPV system can help to reduce the building load. However, none of the above studies used the real condition encountered in BIPV/T systems, which is nonuniform temperature profile for the PV panel.