# Subcooled Flow Boiling Heat Flux Enhancement Using High Porosity Sintered Fiber
## 1 Introduction  
Several researchers have studied the Boiling heat transfer phenomena due to the large applications on the cooling systems of several industries. In particular, researchers have focused on flow boiling since it presents a high heat transfer rate higher than the pool boiling, promoted by the forced convection. The flow boiling phenomenon is also classified into saturated flow boiling and subcooled flow boiling, where the subcooled flow boiling is characterized by the bulk flow temperature lower than its saturation temperature [1, 2, 3]. The heat transfer on the subcooled flow boiling is a combination of the single-phase convective and the nucleate boiling. At high heat flux, the heat transfer is essentially attributed by the nucleate boiling and little contribution of the single-phase. The nucleate boiling is identified by the bubble formation [4, 5, 6, 7, 8]. In the last decade, the heat flux of new microprocessors, power reactors, fossil boilers, etc., has increased exponentially. High heat flux of between 2 and 3 MW/m\({}^{2}\) have been reported, and, in particular, in the case of \"on-chip hot spot,\" a heat flux of 15 MW/m\({}^{2}\) was reported [9]. Due to this increment, researchers have proposed new passive technologies to increase the heat flux on the cooling systems. These techniques include; treated surfaces, rough surfaces, extended surfaces, displaced enhancement devices, swirl flow devices, coiled tubes, surface tension devices, additive for liquids, and additive for gases [10, 11]. Copper is the most used material to fabricate the microchannel heat sink due to its higher thermal conductivity, easy machinability, and relatively low production cost [12]. In particular, treated surfaces havebeen reported to increase the flow boiling heat flux between 20% and 100% in comparison with non-treated surfaces [11; 13; 14]. [PERSON] et al. [15] studied the influence of the channel height, mass flux, and the heated surface roughness, using deionized water as the working fluid. They analyzed the heat transfer rate on an aged copper surface and a polished copper surface, concluding that the heat transfer rate reduces by 10% on the aged copper after 24 h of operation in comparison with the polished surface. They also reported an enhancement between 15-20% on the 3 mm channel height in comparison with the 10 mm channel height at a mass flux of 330 kg/m\({}^{2}\)s. They attributed the enhancement to the bubble dynamics on the aging surface, the hydrophilic nature on the polished surface and the bubble formation density, and the thermal resistance of the oxidized layer on the aged surface. In 2017, [PERSON] et al. [16] conducted experimental research using a downward face porous honeycomb plate under saturated flow boiling and compared the results with a bare surface and a solid honeycomb plate. Water at saturation temperature under four different flow rates was considered as parameters. The honeycomb is fabricated with stainless steel by a sintering process having a final porosity of 35%. They reported an enhancement of around 2.4 times the critical heat flux (CHF) at a low flow rate (200 kg/m\({}^{2}\)s) using the porous honeycomb in comparison with the solid honeycomb. However, increasing the flow rate, the critical heat flux tends to reduce. At a flow rate of 1300 kg/m\({}^{2}\)s, the enhancement was reduced to 1.2 times. The CHF enhancement is attributed to the continuous water supply on the porous honeycomb. On the other hand, on the solid honeycomb, bubbles are generated, and they cover the boiling surface and CHF occurs faster due to the lack of water. At low flow rate conditions, the bubbles generated on the surface coalesce and cover the surface, thus a vapor film would be produced. In general terms, the porous media presents physical characteristics that improve the heat flux on subcooled flow boiling, and three main characteristics are defined.