Two-phase operational maps, pressure drop, and heat transfer for flow boiling of R236fa in a micro-pin fin evaporator

Cooling the new generation of 3D high power electronic chips is one of the leading challenges in microelectronics, as it is a key to achieve high computational performance at lower cooling system power consumption, thus reducing the operating cost. Two-phase flow boiling in two micro-pin fin heat sinks is studied here for this cooling process. Each micro-evaporator has a heated area of 1 cm(2) and contains 66 rows of cylindrical micro-pin fins with an in-line configuration and diameter, height and pitch of respectively 50 mu m,100 gm and 91.7 mu m. The fluid tested is refrigerant R236fa. Channel entrances with and without inlet restrictions are tested in order to evaluate their relative effect on the stability of the flow. The inlet restrictions consist of an extra row of micro-pin fins with a larger diameter of 100 mu m placed at the inlet of the heated area. The present study investigates operational stable and unstable flow regimes, pressure drop and heat transfer performance for the two micro-evaporators (one with and one without inlet restrictions) by coupling high speed visualization of the micro-pin fins flow area with fine resolution infrared temperature measurements of the micro-evaporator base, which yields a 2D map of local heat transfer coefficients of the whole heated area. Working conditions tested have mass flux varying from 500 kg m(-2) S-1 to 2500 kg m(-2) S-1, heat flux ranging from 20 W cm(-2) to 48 W cm(-2), and a constant outlet saturation temperature of 30.5 degrees C. In agreement with previous studies for multi-microchannel evaporators, it is here observed that inlet restrictions extend the map of stable operational regimes, in particular toward lower values of the mass flux, without any appreciable increase of the pressure drop. Unlike evaporation through parallel microchannels, the heat transfer coefficient trends and magnitudes vary dramatically with the flow conditions (mass flux and heat flux), thus suggesting that micro-pin fins have a strong impact on the two-phase flow pattern development, in particular delaying the transition to annular flow. (C) 2016 Elsevier Ltd. All rights reserved.

Experimental investigation on flow boiling pressure drop and heat transfer of R1233zd(E) in a multi-microchannel evaporator

Navid Borhani, Houxue Huang, John Richard Thome

An experimental study on flow boiling pressure drop and heat transfer of a new environmentally friendly refrigerant R1233zd(E), in a parallel multi-microchannel evaporator was carried out. The silicon microchannels evaporator was 10 mm long and 10 mm wide, having 67 parallel channels, each 100 x 100 mu m(2), separated by a fin with a thickness of 50 mu m. Upstream of each channel, a micro orifice was placed to stabilize the two-phase flow and to obtain good flow distribution. The operating conditions for flow boiling tests were: mass fluxes from 500 to 2750 kg m(-2) S-1, heat fluxes from 6 to 50 W cm(-2), inlet subcooling of 5.8 K, and a nominal outlet saturation temperature of 35 degrees C for stable flow boiling. The test section's backside base temperatures were measured by an infrared (IR) camera. The stable flow boiling data without back flow was selected through flow visualization recorded by a highspeed camera coupled with a microscope. These data were then used to assess the applicability of existing two-phase pressure drop models, and to further develop a new empirical model suitable for the high mass flux operating conditions. This new pressure drop model was used to predict the local fluid temperature for the further heat transfer data identification. The fine-resolution local heat transfer coefficients were obtained by solving the three-dimensional inverse heat conduction problem. The experimental results showed that in the saturated flow boiling region the local heat transfer coefficient first decreased moderately in the very low vapor quality region (x < 0.05), then increased significantly but monotonically along the flow direction. The fine-resolution local heat transfer data at the saturated flow boiling region were compared with two groups of heat transfer correlations. The first one considered the flow boiling mechanism occurring in muliti-microchannels as a combination of nucleate boiling and forced convection boiling, while the other one associated this mechanism to liquid thin film evaporation, thus indicating a controversy. It is found that the flow pattern based model belonging to the second group yielded the best agreement with the experimental data, predicting 92.0% of this new database within 30%. (C) 2016 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2016

Cooling High Heat Flux Micro-Electronic Systems using Refrigerants in High Aspect Ratio Multi-Microchannel Evaporators

Etienne Costa-Patry

Improving the energy efficiency of cooling systems can contribute to reduce the emission of greenhouse gases. Currently, most microelectronic applications are air-cooled. Switching to two-phase cooling systems would decrease power consumption and allow for the reuse of the extracted heat. For this type of application, multi-microchannel evaporators are thought to be well adapted. However, such devices have not been tested for a wide range of operating conditions, such that their thermal response to the high non-uniform power map typically generated by microelectronics has not been studied. This research project aims at clarifying these gray areas by investigating the behavior of the two-phase flow of different refrigerants in silicon and copper multi-microchannel evaporators under uniform, non-uniform and transient heat fluxes operating conditions. The test elements use as a heat source a pseudo-chip able to mimic the behavior of a CPU. It is formed by 35 independent sub-heaters, each having its own temperature sensor, such that 35 temperature and 35 heat flux measurements can be made simultaneously. Careful measurements of each pressure drop component (inlet, microchannels and outlet) found in the micro-evaporators showed the importance of the inlet and outlet restriction pressure losses. The overall pressure drop levels found in the copper test section were low enough to possibly be driven by a thermosyphon system. The heat transfer coefficients measured for uniform heat flux conditions were very high and typically followed a V-shape curve. The first branch was associated to the slug flow regime and the second to the annular flow regime. By tracking the minimum level of heat transfer, a transition criteria between the regimes was established, which included the effect of heat flux on the transition. Then for each branch, a different prediction method was used to form the first flow pattern-based prediction method for two-phase heat transfer in microchannels. A non-uniform heat flux creates important temperature gradients in the evaporator, such that the data reduction procedure needs to be adapted to include heat spreading within the evaporator. To do so, a robust multi-dimensional thermal conduction scheme was developed. Once these effects were taken into consideration, the local heat transfer coefficients provided by two-phase flow were found to be the same for uniform and non-uniform heat fluxes, allowing the flow pattern-based method to be extended to non-uniform heat flux conditions. Lastly, with proper control of the mass flow, transient heat flux situations were well handled by the micro-evaporators.

EPFL2011

Flow Boiling of R134a in a Multi-Microchannel Heat Sink with Hotspot Heaters for Energy-Efficient Microelectronic CPU Cooling Applications

Etienne Costa-Patry, Yassir Madhour, Bruno Michel, Jonathan Olivier, John Richard Thome

This paper focuses on two-phase flow boiling of refrigerant R134a inside a copper multi-microchannel heat sink for microelectronic central processing unit cooling applications. The heat sink is composed of 100 parallel microchannels, 100 mu m wide, 680 mu m high, and 15 mm long, with 72-mu m-thick fins separating the channels. The base heat flux was varied from 2.57 to 189 W/cm(2) and the mass flux from 205 to 1000 kg/m(2)s, at a nominal saturation temperature of 63 degrees C. Over 40 000 local heat transfer coefficients were measured at 35 locations using local heaters and temperature sensors, for which different heat transfer trends were identified. The main ones were that the heat transfer coefficient increased with heat flux and was independent of mass flow rate. Heat transfer coefficients as high as 270 000 W/m(2)K (relative to the base area) were reached, keeping the chip under 85 degrees C with a maximum of 94 kPa of pressure drop, for no inlet subcooling and a coolant flow rate of 1000 kg/m(2)s.