NONINTRUSIVE MEASUREMENT OF THE MASS FLOW RATE INSIDE A CLOSED LOOP TWO-PHASE THERMOSYPHON

Abstract

Two-phase cooling is a promising technology for electronics cooling. It allows using dielectric fluids in passive systems and benefit from high heat transfer coefficients. Thermosyphons are a particularly interesting technology in the field of power electronics being entirely passive and simple equipments. Their performances are strongly related to the flow rate of the fluid inside the thermosyphon. In mini-channels thermosyphons liquid entrainment occurs so that flow rate is difficult to evaluate, hence the need for a non intrusive measurement method. For this purpose a mini-channel thermosyphon was manufactured out of borosilicate glass and equipped with a semi-transparent ITO layer as a direct current heating evaporator. It was illuminated by two lasers through the glass tube and the voltage from two photodiodes placed on the opposite side was measured. With an appropriate signal processing the maximal velocity was determined together with length and frequency of the vapor bubbles, the frequency of the thermosyphon cycle for several filling ratios and for various heating powers. From these measurements the total mass flow rate, the liquid mass flow rate, the gas mass flow rate, the void fraction, the vapor quality and the flow cycle frequency were calculated and compared to a thermosyphon simulation model developed at ABB.