Study on heat transfer enhancement by multiple leaf-like insert in a heating tube under oscillating flow

The heater performance is extremely important to the operation efficiency of Stirling engine. Herein, multiple leaf-like inserts were developed to improve the heat transfer in a heating tube under oscillating flow for a Stirling engine heater by using three-dimensional unsteady numerical simulation. Opposite guide paths to the working gas by the leaf-like inserts were achieved at the entry and return phases. Meanwhile, multi-longitudinal vortices were formed and changed with time. The cycle-averaged and cycle-maximum Nusselt numbers all showed an approximately linear pattern with the maximum Reynolds number. The overall performance metric of the enhanced tube varied non-monotonically with the insert geometrical parameters. The highest cycle-averaged PEC of 1.93 was acquired with the n = 3, p = 40 mm, l = 3.3 mm, δ = 45° and forward-facing for the leaf-like inserts. Specially, the leaf-like inserts were concave shape in the entry phase and convex shape in the return phase, which can satisfy the different heat transfer requirement in the two directions of oscillating flow in Stirling engine. The study not only enhances the real-world applicability of Stirling engine but also presents an intriguing reference to heat transfer enhancement of complex transient flow.