A Simplified Equivalent-Solid Conductivity Model for Thermosiphons in CFD Analysis

Thermosiphons are used extensively in many modern technologies, from industry to engineering applications. For such utilizations, the thermal performance estimation is highly important to reach their highest possible efficiencies. In general aspect, the computational fluid dynamics (CFD) has enabled researchers to calculate the complex two-phase fluid flow phenomena in thermosiphons; however, the CFD simulation is not computationally cost-effective in large domains, where the thermosiphon is a small part of that. This restriction also becomes serious in small domains though with so many inserted thermosiphons. In such cases, it is necessary to develop simple models, which help CFD to avoid simulating a full model of two-phase fluid flow phenomenon in thermosiphons. The present study develops an inclusive one-dimensional thermosiphon model, which can be readily utilized in large and complex solution domains. In this regard, the two-phase flow in a thermosiphon is carefully substituted with a single solid frame imposing suitable conductivity variations along the flow axis. Then, the proposed model is validated by comparing the achieved results with experimental data. The results indicate that the thermal resistance decreases with increasing thermal power; at thermal powers higher than 172.87 W, this effect can be neglected, and a constant conductivity magnitude can be attributed to the entire thermosiphon. Moreover, the discrepancy between the experimental and numerical results increases at lower thermal power inputs, while the model exhibits improved accuracy at higher thermal powers. The proposed approach can also be applied to the thermal analysis of large thermosiphon arrays in heat exchanger systems with significantly reduced computational cost.