Due to the merits of high heat transfer coefficient and thermal expansion adaption, helical tube bundles are usually used for steam generators (SGs) and intermediate heat exchangers (IHXs) of High Temperature Gas-cooled Reactors (HTGRs). Helical tube bundles with small pitch to diameter ratio (P/D) are preferable for further improving the compactness. However, characteristics of cross flow over tube bundles with small P/D is more complicated. With the decreasing of P/D from 1.88 to 1.25, the time averaged flow field is no longer symmetric. Such complicated flow characteristics do have impact on the convection, but few researchers focused on it. In the present investigation, the cross flow over tube bundles with P/D =1.25 and 1.88 are simulated using Direct Numerical Simulation (DNS) method for different Reynolds numbers (Re) and Pr numbers (Pr). The time averaged flow and heat transfer characteristics are discussed in detail. Based on Zukauskas correlation, two corrected correlations for tube bundles with P/D=1.88 and 1.25 are proposed with the aid of the DNS results.