First International scientific conference of Al-Ayen University (ISCAU-2019) Numerical investigation of the slip flow effect in counter flow microchannel Heat exchanger with different channels geometries Ahmed A. Ali Mushtaq I. Hasan Ghassan Adnan Mechanical Engineering Department, College of Engineering, Thi-Qar University, Iraq Abstract: The aim of this paper is to study the effects of slip flow assumption on the performance of counter flow microchannel heat exchanger (CFMCHE). Different shapes (square, triangle, trapezoidal, circle) of microchannel heat exchanger have been studied. The effect of channel size at constant cross sectional area has been also studied in this work. The results obtained indicated that, the slip condition cause a slight increase in thermal performance and decrease in hydraulic performance. The study focused on the relation between slip flow assumption and the shape of channel and the results showed, there is a weak relation between channel shape and slip flow and the effect of slip condition is clearly appeared in the triangular channel compared with other shapes, also to reduce time and effects of researchers in this field developed correlations to calculate the parameters of fluid flow and heat transfer with considering slip flow or without it. Key words: slip flow, Microchannel heat exchanger, Laminar flow, Numerical investigation, Thermal performance, CFD, counter flow. * Corresponding author: Mushtaq I. Hasan (mushtaq76h@Gmail.com). eng.ahmedameen91@gmail.com (Ahmed A. Ali) NOMENCLATURE: CFMCHE counter flow microchannel heat exchanger C p specific heat (J\kg.K) D h hydraulic diameter (m) H height (m) F v , momentum accommodation coefficient F T thermal accommodation coefficient Kn Kundusn number