Transient heat transfer processes in a single rock fracture at high flow rates

•Transient heat transfer experiments in a sandstone fracture at velocities > 90 mm/s.•Increased flow velocities reduce water outflow temperature and transferred heat.•Rough surface estimation and contact area reduce heat transfer area by 20%.•Thermal breakthrough times drop non-linearly with increasing flow velocity.•Heat transfer is sufficient to heat water to max temperature for up to 40 seconds.