Experimental investigations on pressure loss and heat transfer of two-phase carbon dioxide flow in a horizontal circular pipe of 0.4 mm diameter

The miniaturisation process in electronics, mechanics and other disciplines requires efficient, light and small cooling devices. Especially in the case of high thermal loads, mini-channels have been found to be appropriate candidates to remove heat efficiently from heated structures. The associated convective heat transfer is usually limited by the pressure loss which complicates the increase of flow rates and the achievement of an improved turbulent heat transfer. In this regard, increasing heat transfer of boiling flow in mini-channels by using CO2 as refrigerant opens an interesting new approach for such applications. The inner diameter (I.D.) of minichannels ranges from 0.2 mm to 3 mm. Two-phase carbon dioxide flow in mini-channels with I.D. >1 mm has been extensively experimentally and numerically investigated. Because advanced technologies now allow for the precise manufacture of smooth mini-channels with I.D. <0.5 mm, new trends intend to implement thinner tubes in order to further reduce hardware size. The necessary instrumentation of such thin tubes for the experimental investigation of two-phase flow carbon dioxide becomes delicate. In the present contribution, a modified 2-Phase Accumulator Loop (2-PACL) was used in order to measure heat transfer coefficients and pressure loss of boiling CO2 flowing through a horizontal circular pipe with an I.D. of 0.4 mm and a length of 140 mm. A specific part of the pipe was electrically heated and the resulting heat transfer coefficients were measured. Inlet conditions have been selected to be single phase with a saturation pressure of about 60 bar and a saturation temperature of about 22 °C. Experiments have been performed under high flow and heat load conditions. Mass flux and heat flux density have been systematically increased from 1100 to 4400 kgm2s and from 119.4 to 397.9 kWm2, respectively. Furthermore the measurements were compared to some of the latest published correlations to get an impression on how accurate two-phase flow with heat transfer using CO2 can be predicted for the design of mini-channel based cooling devices.