This paper presents the modeling and validation of a counter-crossflow heat exchanger used to extract thermal energy from Heavy-Duty Diesel (HDD) engine exhaust. The finite volume evaporator modeling methodology is enhanced for both accurate working fluid temperature and phase length estimation, facilitating improved offline waste heat recovery simulation and accurate control-oriented model development. Transient model calibration and validation experiments were performed on a stand-alone flow bench. Heated gas was passed through the evaporator, replicating different engine exhaust gas conditions. In contrast to other studies, thermal imaging data served to identify the working fluid liquid, mixed and vapor phase lengths within the evaporator. The FVM modeling methodology was enhanced based on the thermal imaging data to accurately predict the working fluid phase lengths. Once calibrated, working fluid phase lengths predicted by the proposed model were validated against thermal imaging from additional transient experimental flow bench data sets. In comparison to the baseline finite volume evaporator model, the enhancements proposed herein showed a 43% mean improvement in predicting the vapor phase boundary during transient operation. (C) 2018 Elsevier Ltd. All rights reserved.