Boiling/evaporative heat transfer from spheres in packed-bed thermal energy storage units

An experimental study was conducted to study boiling/evaporative heat transfer from heated spheres in vertical packed beds with downward liquid-vapor flow of Refrigerant-113. Surface superheats of 1 to 50°C, mass flow rates of 1.7, 2.7 and 5.6 Kg/min, sphere diameters of 1.59 and 2.54 cm, quality (i.e., mass fraction of vapor) of the inlet flow of 2 to 100 %, and two surface roughness conditions were considered. To determine heat transfer coefficients, smooth and roughened aluminum spheres of the same diameter as the other spheres in the bed were instrumented with two thermocouples each for measuring the surface temperatures and a tiny electrical resistance heater for input power. The heat transfer measurements were made under steady state conditions. Heat transfer coefficients were independently determined for each sphere at three values of surface superheat.

The quantitative results axe presented a.s a correlation for the boiling heat transfer coefficients in terms of a homogeneous model. The equation correlates very effectively with the dimensionless temperature difference. The correlation may be used in the development of numerical models to simulate the transient thermal performance of packed bed thermal energy storage unit while operating as an evaporator.

The boiling of the liquid-vapor flow around the spheres in the packed bed was visually observed with a fiber-optic boroscope and recorded on a video tape. The visualization results showed qualitatively the presence of four distinct flow regimes. One of these occurs under sub-cooled inlet conditions and is referred as the Sub-cooled Regime. The other three occur under saturated inlet conditions and are referred to as the Low-quality, Medium-quality, and High-quality Regimes. The regimes are discussed in detail in this thesis.