This study investigates experimentally and theoretically the preignition state of a nonpremixed corrugated counterflow of nitrogen-diluted n-heptane versus heated air under atmospheric pressure. Instantaneous images of Rayleigh scattering and laser-induced fluorescence (LIF) of formaldehyde were collected simultaneously. Mixture fraction and temperature images were calculated from the Rayleigh images using flamelet profiles. From the resulting instantaneous mixture fraction fields, the local curvature of the isomixture fraction contours was determined. Elevated relative concentrations of formaldehyde are observed mainly in the mixing zone of the two flows at locations that are concave toward the air side. As formaldehyde is a typical intermediate species that is formed in cool flames and under low-temperature oxidation conditions, these results indicate that preignition chemistry depends significantly on the curvature of the mixture fraction field. Based on the experimental findings, a flamelet formulation was derived that explicitly accounts for curvature effects. Numerical calculations using a detailed mechanism with 72 elementary reactions among 37 species were performed to predict the influence of curvature. The results confirm the experimentally observed influence of this parameter. They also are consistent with previous direct numerical simulation calculations for H-2/O-2 ignition that predict development of ignition kernels at such distinct locations. (c) 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.