Heat Transfer—Asian Res. 2020;49:406–423. wileyonlinelibrary.com/journal/htj 406 | © 2019 Wiley Periodicals, Inc. Received: 24 July 2019 | Revised: 5 October 2019 | Accepted: 15 October 2019 DOI: 10.1002/htj.21618 RESEARCH ARTICLE Heat transfer inside a horizontal channel with an open trapezoidal enclosure subjected to a heat source of different lengths Hanane Laouira 1 | Fateh Mebarek‐Oudina 1 | Ahmed K. Hussein 2 | Lioua Kolsi 3,4 | Amine Merah 1 | Obai Younis 5,6 1 Department of Physics, Faculty of Sciences, University of 20 août 1955‐Skikda, Skikda, Algeria 2 Mechanical Engineering Department, College of Engineering, University of Babylon, Babylon City, Iraq 3 Mechanical Engineering Department, College of Engineering, Haïl University, Haïl City, Saudi Arabia 4 Unité de Métrologie en Mécanique des Fluides et Thermique, Ecole Nationale d’Ingénieurs, Monastir, Tunisia 5 Department of Mechanical Engineering, College of Engineering at Wadi Addwaser, Prince Sattam Bin Abdulaziz University, Kharj, Saudi Arabia 6 Department of Mechanical Engineering, Faculty of Engineering, University of Khartoum, Khartoum, Sudan Correspondence Fateh Mebarek‐Oudina, Department of Physics, Faculty of Sciences, University 20 août 1955‐Skikda, B.P 26 Route El‐Hadaiek, Skikda 21000, Algeria. Email: oudina2003@yahoo.fr and f.mebarek_oudina@univ-skikda.dz Abstract The heat transfer phenomena inside a horizontal channel with an open trapezoidal enclosure subjected to a heat source of different lengths was investigated numerically in the present work. The heat source is considered as a local heating element of varying length, which is embedded at the bottom wall of the enclosure and maintained at a constant temperature. The air flow enters the channel horizontally at a constant cold temperature and a fixed velocity. The other walls of the enclosure and the channel are kept thermally insulated. The flow is assumed laminar, incompressible, and two‐dimensional, whereas the fluid is considered Newtonian. The results are presented in the form of the contours of velocity, isotherms, and Nusselt numbers profiles for various values of the dimensionless heat source lengths (0.16 ≤ ε ≤ 1). while, both Prandtl and Reynolds numbers are kept constant at (Pr = 0.71) and (Re = 100), respectively. The results indicated that the distribution of the isotherms depends significantly on the length of the heat source. Also, it