From solid rods to hollow tubes: Enhancing convective cooling and reducing surface temperature in cluster downhole electric heaters for tar‑rich coal in‑situ conversion

To address the critical issue of high surface temperatures in downhole electric heaters used for the in‑situ conversion of tar‑rich coal, this study proposes a novel cluster downhole electric heater featuring hollow electric heating tubes (CH). The aim of this study is to compare the flow and heat transfer characteristics of three heater configurations: the full‑channel electric heater with solid rods (FR), the cluster heater with solid rods (CR), and the proposed cluster heater with hollow tubes (CH). Using numerical simulations, this research systematically analyzes the velocity and temperature fields, heat transfer coefficients, pressure drops, and comprehensive performance under different mass flow rates, heat fluxes, and geometric parameters. The results show that the CH design significantly enhances airflow velocity on the heating tube surface, achieving a maximum increase of 41.96% compared with CR‑1. It also reduces the maximum surface temperature by up to 9.81% compared with FR. Under identical heat flux conditions, increasing the tube diameter from 16 mm to 24 mm for CH‑type heaters reduces the maximum surface temperature by 11.26%, despite a 50% increase in heating power. The heat transfer coefficient of CH‑type heaters is up to 6.58 times that of CR‑2. Based on a newly proposed comprehensive performance index integrating heat transfer and tube surface temperature, CH‑4 is identified as the superior configuration in terms of thermal performance. This work provides a basis for the design of high‑efficiency, long‑lifespan downhole electric heaters for unconventional resource conversion.