Numerical study of flow and heat transfer characteristics in hot water stores

The flow and heat transfer characteristics in a cylindrical hot water store during the charging process under adiabatic thermal boundary conditions were studied numerically in the present paper. The charging efficiency was used to evaluate the thermal stratification. The emphasis was put on the effects of charging temperature differences, charging velocities, charging flow rates and length to diameter ratios on the charging efficiency. The results were summarized both in dimensional and dimensionless forms. They indicate that the charging efficiency depends mainly on the modified Richardson number Ri(H,f) and Peclet number Pe(H,f) which present the combined effects of charging temperature difference and charging velocity on the charging efficiency. If Ri(H,f) is larger than 0.25, the charging efficiency is above 97%. At a given Richardson number the increase of Peclet number leads to a higher charging efficiency. For H/D less than 4, the increase of the height to diameter ratio H/D can improve the charging efficiency as well. The effect of the Fourier number (or charging flow rate) on the charging efficiency, however, is relatively small. A correlation of the numerical results was obtained for the design of effective hot water stores. (C) 1998 Elsevier Science Ltd. All rights reserved.