Microjet array single-phase and flow boiling heat transfer with R134a Eric A. Browne a , Gregory J. Michna b , Michael K. Jensen a , Yoav Peles a, * a Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA b Department of Mechanical Engineering, South Dakota State University, Brookings, SD 57007, USA article info Article history: Received 15 March 2010 Received in revised form 10 July 2010 Accepted 10 July 2010 Keywords: Microjet Jet array Impingement Flow boiling abstract An experimental study of single-phase and flow boiling heat transfer of a submerged microjet array was conducted with R134a. The staggered array of seventeen 112-lm diameter orifices impinged onto a 1  1-mm heater. Single-phase data were gathered over the range 3050 6 Re d 6 10,600 with 53:6 6 Nu d 6 128. Boiling experiments were conducted with liquid subcoolings of 10, 20, and 30 °C at jet velocities of 4, 7, and 10 m/s. Boiling was generally found to enhance heat transfer with a maximum heat flux of 590 W/cm 2 . However, temperature excursions occurred at low superheats and were found to be related to non-condensable gas content. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Electronic devices are dissipating increasing heat loads from shrinking footprints, so cooling techniques more effective than forced air convection will be required. Single-phase flow and flow boiling in microchannels have been extensively studied [1–3] to address this cooling need. However, one potentially more effective method that has not been extensively studied to date is jet impingement cooling. Macroscale jets have been studied [4–10] with a variety of fluids and flow schemes and have been used to cool turbine blades and quench metals. Many of these practical uses of jets employ an array of jets rather than a single impinging jet, because arrays of jets typically maintain a more consistent sur- face temperature and can cool larger areas than a single jet. Jet ar- rays have been studied [7,8,11–17] and require several parameters to define their geometry, which affect heat transfer performance. Limited microscale studies have recently been conducted [18– 23] to investigate the effects of scale on performance, including a potential scale enhancement. A previous study by the authors [24] has shown that high (400,000 W/m 2 K) area-averaged heat transfer coefficients and heat fluxes exceeding 1100 W/cm 2 can be attained with an array of single-phase water microjets. Regardless of this high performance, water is generally consid- ered incompatible with electronics; therefore, a fluid that can come into contact with electronics without causing damage is required for use in electronics cooling. Various refrigerants are often se- lected for this purpose, with a popular choice being R134a. Unfor- tunately, the heat transfer properties of R134a are inferior to water. As a result, area-averaged heat transfer coefficients are low- er than those for water under the same flow conditions. However, boiling has been used to enhance heat transfer and may allow high heat removal rates while still using an electronics-compatible fluid. Boiling jet studies have been performed [25–30] but mostly with single jets. This study employs an array of submerged circular microjets of diameter 112 lm on a 1  1 mm heater to investigate single-phase and boiling heat transfer with R134a. 2. Experimental apparatus and method 2.1. Experimental apparatus A closed-loop system (Fig. 1) with a custom built fixture (Fig. 2), which housed a microdevice (Fig. 3) with an array of microjets and a thin-film heater, was used to experimentally study heat transfer with impinging microjets. The flow rate in the system was set with a gear pump and a bypass with a needle valve, which provided fine flow rate control; a turbine flow meter measured the flow rate. A bladder accumulator controlled the pressure in the loop. Pre- scribed inlet temperatures were achieved with a constant temper- ature bath pumping coolant through a concentric tube heat exchanger and with a separate electrical pre-heater. Inlet temper- ature and pressure were measured just prior to the fixture, and pressure was measured in the microdevice. The fixture was placed under a microscope equipped with a high-speed camera for flow visualization. The microdevice was seated into a pocket that was machined into the top face of the fixture. In that pocket were seats for o-rings, 0017-9310/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2010.07.062 * Corresponding author. Tel.: +1 518 276 2886; fax: +1 518 276 2623. E-mail address: pelesy@rpi.edu (Y. Peles). International Journal of Heat and Mass Transfer 53 (2010) 5027–5034 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt