Fluid state inside the heat exchanger

As explained in Principles of Chemical Engineering, the heat transfer coefficient is higher when the velocity is within a certain range

In this state, the heat exchange efficiency is high

In a turbulent state, the liquid mixes well; there is no area that is hot while another area remains cold, resulting in higher efficiency

It is stated in the principles of chemical engineering. The flow regimes of single-phase flow are divided into laminar and turbulent flow. In laminar flow, there is no mass transfer in the radial direction; therefore, heat transfer relies mainly on conduction. In turbulent flow, however, mass transfer does occur, with both conduction and convection playing a role. Convection-based heat transfer is far superior to conduction, so turbulent heat transfer is more effective than laminar flow. Therefore, it is necessary to maintain a turbulent state. According to boundary layer theory, in any flow regime, there is a laminar sublayer near the wall surface. The thickness of this laminar sublayer affects the heat transfer efficiency; in simple terms, the higher the degree of turbulence, the thinner the laminar sublayer, and thus the better the heat transfer. Therefore, it is not only necessary to ensure turbulence but also to maintain a high level of turbulence.