# Numerical Heat Transfer Simulation of Oil Shale Large-Size Downhole Heater
## 1 Introduction  
Oil shale [1], a sedimentary rock containing hydrocarbon substances, has the potential to produce hydrocarbons with varying carbon content through accelerated thermal development of internal hydrocarbon compounds by external energy input [2, 3, 4]. Surface retorting and in situ cracking are the fundamental methods for processing oil shale [5, 6]. In alignment with the national low-carbon policy and the imperative for sustainable development [7], in situ pyrolysis is poised to become the predominant method for extracting oil shale in the future. In prior studies, oil shale reservoirs were typically discovered at depths exceeding 50 m [8, 9]. The downhole heater has emerged as the optimal choice for mitigating heat loss attributed to conductive heat from the surface. Downhole heaters are categorized into two types based on their heat generation mechanisms: combustion heaters and electric heaters [10, 11, 12, 13]. Concerning downhole stability, combustion heaters significantly lag behind electric heaters [14, 15]. Consequently, electric heaters are the more prevalent choice in the in situ conversion process [16, 17]. Royal Dutch Shell conducted an oil shale pilot project to experiment with in situ electric heating extraction technology (E-ICP) and developed a Y-type electric heater forheating the oil shale layer through heat conduction [18; 19]. This type of heater operates on three-phase electricity, providing high heating power, but it necessitates a high-quality oil shale reservoir. [PERSON] pioneered U-type and LU-type electric heaters. In contrast to Y-type electric heaters, U-type electric heaters are typically oriented horizontally, rendering them more suitable for the development of thin layers of oil shale [8]. Nevertheless, their construction can pose challenges. Due to the limited heat transfer area of the exposed electrode, the thermal conductivity efficiency of the reservoir is diminished, leading to prolonged heating times [20; 21]. Certain researchers advocate utilizing electric heating rods as the energy source and optimizing heat transfer structure to achieve an ideal thermal state in the flow. This approach has emerged as the primary focus in the current design of heaters [22; 23; 24]. Based on a heater's shell flow pattern, it can be divided into a segmental baffle board structure and a helical baffle structure [25; 26; 27; 28]. The segmental baffle structure is commonly used because it is simple, but the presence of a flow dead zone and a local high-temperature zone limits the usage of electric heaters [29; 30; 31]. On the contrary, the fluid in the shell has a helical baffle board structure similar to plunger flow with great heat transfer efficiency, flow uniformity, and no flow dead zone. As a result, the heat transmission performance of a helical baffle is superior to that of a segmental baffle, while the former generates and loses less entropy than the latter under the same Reynolds number situation. [PERSON] conducted an investigation into the performance of continuous helical baffle heat exchangers featuring varying dip angles. In comparison to the heat exchanger with a 21\({}^{\circ}\) dip angle, the heat transfer coefficient and pressure drop for the heat exchanger with an 11\({}^{\circ}\) dip angle experienced reductions of 86% and 52%, respectively [32]. [PERSON] introduced the concept of a multiple-shell tube heat exchanger, wherein fluid undergoes a sequential flow through both inner and outer shell processes [33]. The investigation focused on a continuous helical baffle combined multi-shell tube heat exchanger. While the multi-shell tube heat exchanger exhibits higher heat transfer coefficients and pressure drops compared to the segmental baffle shell tube heat exchanger, the former attains a superior heat transfer coefficient at an equivalent pressure drop. [PERSON] conducted a numerical analysis on the performance of a combined multi-shell continuous helical baffle heat exchanger.