Experimental investigations on pressure loss and heat transfer of two- phase carbon dioxide flow in a horizontal circular pipe of 0.4 mm diameter A. Kneer a,d,⇑ , M. Wirtz d , T. Laufer c , B. Nestler a,e , S. Barbe b a Karlsruhe University of Applied Sciences, Institute of Materials and Processes, Moltkestrasse 30, 76133 Karlsruhe, Germany b TH Köln, Faculty of Applied Natural Sciences, Chempark Leverkusen E39, 51368 Leverkusen, Germany c Carl Zeiss SMT GmbH, Rudolf-Eber-Str. 2, 73447 Oberkochen, Germany d TinniT Technologies GmbH, Essenweinstr. 25, 76131 Karlsruhe, Germany e Karlsruhe Institute of Technology, Institute for Applied Materials, Engelbert-Arnold-Strasse 4, 76131 Karlsruhe, Germany article info Article history: Received 4 April 2017 Received in revised form 19 October 2017 Accepted 28 November 2017 Keywords: Circular pipe Carbon dioxide Evaporation Two-phase flow Experiment Correlation 2-PACL abstract 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 CO 2 as refrigerant opens an interesting new approach for such applica- tions. 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 investi- gated. 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 CO 2 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 con- ditions 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 kg m 2 s and from 119.4 to 397.9 kW m 2 , 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 CO 2 can be predicted for the design of mini-channel based cooling devices. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction Refrigeration using two-phase carbon dioxide (CO 2 or R744 according to the ASHRAE Standard Designation) is gaining increas- ing interest in nuclear physics [1]. During the last years, it has been successfully implemented for the cooling of the high energy parti- cle physics detectors at Large Hadron Collider (LHC, CERN, Switzer- land). The LHCb VELO at LHC run 1 followed by the ATLAS Insertable B-Layer detector and the CMS Pixel detector (both oper- ational in 2017) have been upgraded with two-phase carbon diox- ide refrigeration systems. The CERN team is now planning a new CO 2 based cooling device in the frame of the VELO’s upgrade [2]. For the miniaturisation process in electronics and mechanics, it becomes crucial to minimize hardware size and mass. Evaporative cooling using CO 2 allows for high heat transfer coefficients and is a great improvement for this particular field of technology. CO 2 is non-toxic, environment friendly, non-combustible and currently regarded as an attractive alternative boiling refrigerant. In the field of nuclear physics, boiling CO 2 typically flows through circular tubes, so called macrochannels, with an inner diameter in the range of 1–10 mm. The management of the saturation condition https://doi.org/10.1016/j.ijheatmasstransfer.2017.11.146 0017-9310/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: aron.kneer@hs-karlsruhe.de (A. Kneer). International Journal of Heat and Mass Transfer 119 (2018) 828–840 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt