Abstract
Heat transfer from solid into fluid channels is found in many natural and artificial systems involving heating and cooling processes. It is highly desirable to have a general and simple equation for predicting the transient fluid temperature in narrow flow channels. This study fills the gap by developing a semi‐analytical model. The model involves heat conduction between solid and fluid and heat convection along the channel. The result of parameter analysis with the model for a rock/water system indicates that fluid temperature changes sharply during the early time. In the practical range of rock thermal conductivity, the fluid temperature change is not significantly affected by the rock thermal conductivity. The effect of fluid temperature at the entrance on the fluid temperature at the exit of the channel is almost linearly additive. Increasing channel width and length and reducing fluid flow rate improve fluid retention time in the channel, enhance the total heat transfer, and thus promote fluid temperature change over the flow channel. This work provides engineers with a closed‐form equation for predicting the transient fluid temperatures at the exits of flow channels in their heat transfer systems.