# Experimental Study on Cooling Performance of a Hybrid Microchannel and Jet Impingement Heat Sink
## 1 Introduction  
High-power lasers have attracted extensive attention with the development of laser technology. A high-power diode array (HPLDA) is an effective way to convert electronic energy into a laser beam [1]. The temperature of HPLDAs affects their efficiency and reliability, and the cooling of HPLDAs is still a challenge. Microchannel heat sink (MCH) is an effective approach for high heat flux applications [2]. Compared with macrochannel heat sinks, microchannel heat sinks have a better cooling performance and a compact structure [3]. [PERSON] and [PERSON] [4] tested microchannel heat sinks for cooling electronic devices in 1980, which showed a cooling capacity of 750 W/cm\({}^{2}\). Water-cooled microchannel heat sinks were designed and tested by [PERSON] et al. where they cooled diode lasers with a thermal resistance of 0.29 K/W [5]. Compared with single-phase cooling, the flow boiling heat transfer can achieve cooling with a higher heat flux and more uniform temperature on the heating surface. However, there are issues of premature critical heat flux (CHF) and heat transfer deterioration owing to various instabilities in microchannels [6]. Major attention has been paid to the characteristics of instability and its suppression methods. Geometrical modification is an effective method to suppress stability [7]. In addition to flow instability, bubble clogging and flow reversal are also great challenges with microchannel heat sinks [8, 9]. The effects of inlet restrictors (IRs) on heat transfer characteristics were analyzed by [PERSON] et al. [10]. Results showed that IRs improved heat transfer characteristics effectively. A prototype of the copper heat sink with micro-pipes in the microchannel was designed by [PERSON] et al. [11], and a smaller dimension in the direction perpendicular to the junction was proposed to improve operability in that direction. [PERSON] et al. [12] investigated a multilayered microchannel heat sink which showed a thermal resistance of 5.6 \(\times\) 10\({}^{-6}\) m\({}^{2}\) K/W with a cooling capacity of 910 W/cm\({}^{2}\). A laser 3D printing heat sinkwas tested and optimized with a low smile value [13]. [PERSON] et al. [14] tested and analyzed a hybrid microchannel heat sink that could reduce thermal resistance by 15%. Microchannel heat sinks filled with metal foam were investigated numerically by [PERSON] et al. [15]. Results showed that microchannel heat sinks filled with copper foam could achieve better temperature uniformity. [PERSON] et al. [16] investigated variable structures of microchannel heat sinks. It was shown that the sinusoidal structure could increase the diode lifetime by 44% with an 18% increase of pressure drop. [PERSON] et al. [17] compared different slot-jet structures and found the hybrid module with plate fin could reduce thermal resistance by 30% with a wall temperature decrease of 70%. A hybrid microchannel heat sink was investigated numerically by [PERSON] et al. [18]. It was indicated that a hybrid microjet-microchannel could have a relatively low temperature at \(q=1000\) W/cm\({}^{2}\). Doubled-layered microchannel heat sinks have advantages in cooling performance and flow characteristics [19; 20]. Micro pin fins can increase turbulence disturbance and enhance the heat transfer in microchannels [21; 22; 23]. Cross-linked microchannels disrupt the boundary layers to enhance the heat transfer coefficient [24; 25; 26]. Re-entrant cavity structures can create artificial nucleation sites in microchannels to enhance boiling heat transfer [27; 28; 29]. Porous structures and nano-structures can improve the wall wettability and extend the heat transfer area to enhance the boiling heat transfer [30; 31; 32; 33; 34; 35]. In this work, a heat sink combining a microchannel and jet impingement with a highly-dense micro-jet array (\(\sim\)100 jets per cm\({}^{2}\) with jet width of 0.5 mm) was designed and fabricated for cooling devices with a heat flux of up to 400 W/cm\({}^{2}\).