An experimental study of tube side boiling heat transfer of HFC-134a has been conducted in a single-pass, countercurrent flow heat exchanger under an electric field. By applying 0-8 kV to a concentric inner electrode, the mechanics of EHD induced flow and heat transfer augmentation/suppression have been investigated for flow conditions with inlet qualities of 0% to 60%, mass fluxes from 100 kg/m(2) s to 500 kg/m(2) s, and heat flux levels between 10 kW/m(2) and 20 kW/m(2). A theoretical Steiner type two-phase flow pattern map for flow boiling in the annular channel under applied DC high voltage is also developed. The flow regimes encountered in the convective boiling process have been reconstructed experimentally and compared with the proposed EHD flow regime map. The results show that when the proposed dimensionless criterion M-d similar to Re-2 is satisfied, EHD interfacial forces have a strong influence on the flow pattern which is considered to be the primary mechanism affecting the increase in pressure drop and the augmentation or even suppression of heat transfer. (c) 2005 Elsevier Ltd. All rights reserved.