A horizontal mantle heat exchanger for a thermosyphon-driven SDHW(solar domestic hot water) was numerically simulated and fluid flow and heat transfer in the annulus of the mantle heat exchanger were quantitatively investigated. The Reynolds number, the location of the inlet, and the gap of the annulus were selected as the important design variables. The effects of the design variables on the heat transfer characteristics were thoroughly studied. Based on the numerical results, a correlation for predicting the heat transfer coefficient was suggested as the conclusion of this study.