Effect of venting the periodic reverse vapor flow on the performance of a microchannel evaporator in air-conditioning systems Hanfei Tuo a , Pega Hrnjak a,b,⇑ a Air Conditioning and Refrigeration Center, Department of Mechanical Engineering, University of Illinois at Urbana Champaign, 1206 West Green Street, Urbana, IL 61801, USA b CTS, 2209 Willow Rd., Urbana, IL, USA article info Article history: Received 22 April 2013 Received in revised form 23 August 2013 Accepted 26 August 2013 Available online 29 October 2013 Keywords: Periodic reverse flow Boiling fluctuation Microchannel evaporator abstract This paper proposes a novel solution to reduce the impacts of periodic reverse flow and induced boiling fluctuations on the performance of a microchannel evaporator used in an air-conditioning or other sim- ilar refrigeration system. Reverse vapor trapped within the inlet header is continuously vented bypassing microchannels. Simultaneous flow visualizations and measurements quantified the effects of venting the reverse vapor on the evaporator performance. The vapor–liquid interface in the inlet header is elevated above all the microchannel inlets, resulting in more uniform liquid distribution among channels. Evapo- rator surface temperatures measured by an infrared camera oscillate with reduced amplitude (less than 1 °C) compared to the situation without venting, indicating more stable boiling heat transfer and two- phase flow within the microchannels. The evaporator pressure drop is reduced up to about 15% due to removal of the reversed vapor flow. In addition, the reverse vapor flow is characterized for the first time through this method. Both its average flow rate and oscillation amplitude increase with average heat flux, while the oscillation period is reduced. Compared to the total refrigerant flow rate supplied to the evap- orator, the average reverse vapor flow is in the range of 2–8% at the conditions explored. Published by Elsevier Ltd. 1. Introduction Due to increased heat flux densities and reduced channel size, reverse vapor flow and boiling oscillation is a very noticeable prob- lem in microscale heat exchangers with parallel flow. Extensive studies in past decades have identified several typical modes and underlying mechanisms, such as rapid bubble growth, parallel channel instabilities, and upstream compressible flow instability [1–5]. Nevertheless, to the authors’ best knowledge there was no study or even indication of such phenomena in refrigeration and A/C microchannel evaporators with the exception of the informa- tion from CTS [6]. In microchannels, nucleation bubble can easily grow up to a size comparable to the channel hydraulic diameter. Further bubble growth becomes confined by the channel walls, only expanding in a longitudinal direction along the channel in a form of an elon- gated bubble. Such rapid bubble nucleation as well as confined expansion may introduce a pressure spike in the flow which may overcome the inertia of the incoming flow and the pressure in the inlet header, causing the trailing edge of the bubble expanding upstream, i.e. a reverse flow [7]. However, this phenomenon sel- dom occurs in a large conventional tube, since the local high pres- sure generated by a single bubble is confined to a small region of the channel and can hardly affect the global pressure distribution and the bulk flow dynamics [4]. Kew and Cornwell [8] proposed a threshold of the bubble growth confinement based on the param- eter, that is, Confinement number Co, and set the threshold at Co = 0.5. Meanwhile, the inlet header serves as a buffer tank, pro- viding significant compressible volume upstream of the heated microchannel tubes as well as an opportunity to effectively tempo- rarily stop the flow in some channels because others are available for increased flow rates. Such volume may be able to intermittently retain and discharge the backflow vapor, and thus dynamic boiling instability will be sustained by interactions between tube vapor generation and the upstream compressible volume [9]. Reverse va- por flow in parallel microchannels will cause the flow maldistribu- tion, as the vapor–liquid interface in each channel may temporally extend into different directions, either forward or backward [10– 11]. Bogojevic et al. [12] deduced similar behavior in a group of 40 channels in which some had subcooled wall temperatures indi- cating single-phase flow while others were superheated indicating boiling. Related work is summarized in Table 1. Wu and Cheng [2] found the temporal alternating appearance of two-phase flow and 0017-9310/$ - see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2013.08.095 ⇑ Corresponding author at: Air Conditioning and Refrigeration Center, Depart- ment of Mechanical Engineering, University of Illinois at Urbana Champaign, 1206 West Green Street, Urbana, IL 61801, USA. Tel.: +1 217 244 6377; fax: +1 217 244 6534. E-mail address: pega@illinois.edu (P. Hrnjak). International Journal of Heat and Mass Transfer 69 (2014) 66–76 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt