A design study of a proposed fiber‐optic flow sensor is presented. The concept used is that the temperature‐dependent fluorescent decay time of a ruby crystal may be measured to determine the temperature of the crystal. The heat induced by light from a laser shone on the crystal is dissipated through it and may be conducted away by a flowing fluid. The measurement of crystal temperature can therefore be used as a means of measurement of fluid flow. In this work, the transient heat distribution in the crystal is modeled; as the basis of the design study, the results produced by the model indicate that this is a method that has potential as a practical system.