Natural convection in an enclosure under time periodic heating: an experimental study

摘要

The effect of the heating period on the heat transfer across a water filled rectangular enclosure is examined experimentally. The enclosure is designed such that periodic heat by Joule effect is delivered at one wall while the opposite wall is maintained at an approximately constant temperature. All other surfaces are insulated. Several heating power levels are chosen with equivalent time averaged heat-flux based Rayleigh number varying from 2.5X10~8 to 1.0X10~9. The heating period varies from 32 seconds to 1600 seconds. For the highest Rayleigh number, Ra = 1.0X10~9, the oscillatory heating process leads to a cycle averaged heat transfer coefficient 20 percent higher than obtained with steady averagedheat flux heating. Results, presented in terms of temperature time series, phase-plane portrait, and time evolution of cooling and heating wall temperatures, indicate that for low heating frequencies the heat transfer across the enclosure is hindered, with the time averaged heat transfer coefficient decreasing by as much as 13 percent of the equivalent steady heating value. A plausible explantation for th phenomenon is offered and discussed. The instantaneous and cycle averaged heat transfer coefficients, and the corresponding Nusselt numbers, are also presented for various heating frequencies.