Heat transfer enhancement in a microtube with an in-line package of spherical and inclined oval-trench dimples during transformer-oil pumping

The vortex-induced intensification of the laminar $($at $\rm Re = 308)$ flow of transformer oil and heat transfer in a microtube with an in-line package of dimples with a depth of $0.26$ of the tube diameter is calculated based on the Navier–Stokes equations and the energy equation. The heat-transfer increased by nearly $20$ times for eight inclined oval-trench dimples spaced around the circumference and by $13$ times for spherical dimples equal in area in comparison with that for a smooth tube; moreover, the relative hydraulic losses grow by approximately $30$ and $60\%$, respectively.